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Running head: Monitoring of maerl beds
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Mapping and monitoring the maerl beds in the Fal & Helford Special Area of Conservation:
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a review of techniques
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Katie Sambrook*, Trudy Russell
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Falmouth Marine School, Killigrew Street, Falmouth, Cornwall, TR11 3QS
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Abstract
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Maerl is a generic term for several species of unattached, non-geniculated coralline red
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algae. Maerl beds are considered to be of conservation importance due to their rarity,
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environmental sensitivities and high biodiversity and are subject to both UK and European
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legislation. The EU Habitats Directive resulted in the creation of a network of Special Areas
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of Conservation, established to provide a degree of governance for key habitats and species.
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The Fal & Helford Special Area of Conservation contains the largest live maerl bed in
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southwest Britain. Conservation targets for maintaining the favourable condition of these
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maerl beds include monitoring the extent, distribution, percentage of live maerl and species
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composition. It is a requirement that these attributes are assessed every six years. This
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review evaluates existing techniques used for subtidal benthic habitat mapping and
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biodiversity surveys recommending suitable methods for monitoring maerl in the Fal &
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Helford Special Area of Conservation.
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Keywords
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Maerl, habitat mapping, special area of conservation, biodiversity, monitoring, surveys *Corresponding author. Email: katie.sambrook@hotmail.co.uk
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Introduction
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Maerl is the generic term for unattached, non-geniculated coralline red algae (Birkett et al.
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1998, Foster 2001). It has a hard calcium carbonate skeleton which research indicates
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makes a significant contribution to carbon sequestration in the oceans (Canals & Ballesteros
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1997, Birkett et al. 1998). Maerl beds form when living and dead maerl thalli accumulate,
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with accretion occurring over long periods due to the slow growth rate of maerl, usually
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between 0.05 and 1.0mm y-1 (Birkett et al. 1998, Foster 2001, Bosence & Wilson 2003, Grall
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& Hall-Spencer 2003). Found in polar, temperate and tropical waters, maerl-forming species
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are patchily distributed due to narrow environmental tolerances (Barbera et al. 2003, Grall
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& Hall-Spencer 2003).
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distribution of maerl as it is intolerant to smothering and burial (Birkett et al. 1998, Barbera
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et al. 2003). Light, temperature and salinity also contribute to its spatiotemporal presence
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(Birkett et al. 1998, Barbera et al. 2003, Wilson et al. 2004, Sciberras et al. 2009). Three
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species of maerl are found to contribute to the majority of maerl beds in the UK,
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Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum, the latter
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being predominant (Birkett et al. 1998).
Current flow is the primary abiotic factor that influences the
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The nodular structure of maerl creates an interlocking matrix providing a habitat for a wide
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range of infauna and epifauna (Birkett et al. 1998, Barbera et al. 2003). This lattice
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formation is responsible for the high biodiversity found on maerl beds which is comparable
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to other algal biotopes such as sea grass beds and kelp forests (Birkett et al. 1998, Kamenos 2
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et al. 2004a). As well as exhibiting high biodiversity, maerl beds have been found to harbour
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juveniles of commercially important species such as gadoids and the queen scallop
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Aequipecten opercularis (Kamenos et al. 2004b, c, d). Studies in Scotland found that the
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highest densities of juvenile cod (Gadus morhua), saithe (Pollachius virens) and pollack
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(Pollachius pollachius) were observed between September and November (Kamenos et al.
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2004b); and for A. opercularis between October and December (Kamenos et al. 2004b).
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Although pristine live maerl beds (PLM) exhibit higher biodiversity, the long-term
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accumulation of dead maerl deposits also represent an important habitat, particularly for
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burrowing species (Birkett et al. 1998). Functional diversity on maerl beds is high with Grall
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& Glémarec (1997) identifying eight trophic groups on a maerl bed in France.
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Recognising the ecological importance of maerl, both L. corallioides and P. calcareum are
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included in Annex V of the European Union’s Habitats Directive as species whose
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exploitation is subject to management and are listed under the UK Biodiversity Action Plan
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as priority species (Council Directive 92/43/EEC 1992). Maerl beds ares also protected under
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Annex I of the Habitats Directive and appears on the Convention for the Protection of the
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Marine Environment of the North-East Atlantic (OSPAR) list for threatened or declining
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habitats and species. The Habitats Directive resulted in the creation of a number of Special
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Areas of Conservation (SACs) established to provide a degree of governance and protection
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to key habitats and species.
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The Fal & Helford SAC is located in Cornwall and covers 6387.8 hectares (JNCC 2011). It
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includes a range of Annex I habitats including ‘sandbanks which are slightly covered by
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seawater all the time’ which contains the sub feature of maerl beds (JNCC 2011). The SAC 3
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has the largest live maerl bed in southwest Britain, found on St Mawes Bank, and also
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contains deep deposits of dead maerl gravel indicating that during the past live maerl was
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much more prevalent than today (Birkett et al. 1998).
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A number of studies have been conducted on the extent and biodiversity of maerl in the Fal
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& Helford SAC. Blundel et al. (1981) recorded that live maerl was found at depths of 0-10m
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in the Fal & Helford SAC; Davies & Sotheran (1995) mapped the extent of live and dead
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maerl as part of the BioMar project; Perrins et al. (1995) compared the percentage of live
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and dead maerl on St Mawes Bank between 1982 and 1992; Frau-Ruiz et al. (2007) surveyed
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Falmouth Bay to establish the presence of maerl to inform decision-making on scallop
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dredging and ship anchoring and Axelsson et al. (2008) undertook an ecological survey as
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part of an appropriate assessment for the Port of Falmouth Development Initiative which is
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currently seeking to dredge a part of the SAC in order to deepen the channel and increase
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ship access.
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The morphology and community structure of the maerl is likely to have changed since
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Davies & Sotheran (1995) mapped the biotopes in the estuary. At this time both aggregate
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dredging and scallop dredging were allowed within the site, although both activities were
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banned in 2005, with the scallop dredging ban extending into Falmouth Bay in 2008 (Hall-
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Spencer 2005, legislation.gov.uk 2008). Both practices are known to be destructive with
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research into maerl beds in other regions showing that scallop dredging can lead to >70%
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reduction in live maerl coverage with no indication of recovery during the subsequent four
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years (Hall-Spencer & Moore 2000). Due to its slow growth rates and intolerance to
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activities that create sediment perturbation and siltation, maerl is now recognised as a non-
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renewable resource (Barbera et al. 2003).
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In the UK to date, most surveys on maerl have been in relation to disturbance activities with
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limited time-series data collected (OSPAR 2010). However, in order to maintain the integrity
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of the SAC designation, it is fundamental that a monitoring programme is implemented to
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conduct regular assessments of the protected habitats or species enabling early detection of
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changes such as a decline in live maerl, issues with invasives, effects of climate change or
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regime shifts (Birkett et al. 1998, Hiscock 1998, Sciberras et al. 2009). Conservation targets
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under OSPAR and the Habitats Directive for maintaining the favourable condition of maerl
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beds include monitoring the extent, distribution, percentage of live maerl, species
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composition as well as recording the abiotic factors that could impact the health of the
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maerl (OSPAR 2010). These checks are required at six-yearly intervals (Malthus & Karpouzli
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2003, OSPAR 2010).
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This paper seeks to evaluate current methodologies for obtaining this information, review
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examples of good practice and make recommendations on the techniques that would work
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best for the collection of data pertaining to the maerl in the Fal & Helford SAC. It does not
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aim to provide detailed methodologies on how the surveys should be carried out. While
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each site should be reviewed separately to assess potential threats specific to its location
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and activity levels, it is a long-term aim that a standard protocol could be implemented
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across other SACs containing maerl to establish a comprehensive network of data
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contributing to its long-term sustainability.
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Methods
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Mapping the extent and distribution of maerl
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The Fal & Helford SAC contains a busy port, has many recreational users and is home to the
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only oyster fishery worldwide still fished under sail using traditional methods of dredging
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(Challinor et al. 2009). An understanding of the extent and distribution of the live and dead
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maerl is essential for informing decision-making surrounding the management and
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conservation of this site.
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Remote sensing
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Recent technological advances have seen the development of remote sensing techniques
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such as airborne sensing (aerial photography or video and hyperspectral data), satellite
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imagery and acoustic sensing to support biotope mapping (Held et al. 2003, Diaz et al. 2004,
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Godet et al. 2009). The use of remote sensing in the marine environment is advantageous
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over traditional approaches such as grab sampling or dredging as it allows large areas to be
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mapped over short timescales (Brown et al. 2002, Sciberras et al. 2009, Simons & Snellen
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2009; Brown et al. 2011); provides more accurate spatial discrimination due to virtually
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continuous sampling (Brown et al. 2002, Godet et al. 2009) and is non-destructive which is
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important when surveying protected habitats and species (De Backer et al. 2009).
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High costs and limited depth range restrict the use of both airborne sensing and satellite
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imagery in marine habitat mapping at present although advances in hyperspectral mapping
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should be reviewed in the future (Kenny et al. 2003, Brown et al. 2011). Acoustic sensing
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has been used extensively in subtidal environments as a predictive tool, validated by
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rigorous groundtruthing to confirm the results (Birkett et al. 1998, Hiscock 1998, Brown et
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al. 2005, Ehrhold et al. 2006). There are four broad categories of acoustic mapping device:
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(i) broad acoustic beam systems such as side scan sonar (SSS); (ii) acoustic ground-
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discrimination systems such as RoxAnnTM and QTC-ViewTM (iii) multiple narrow-beam swath
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bathymetric systems and (iv) multiple-beam side scan sonar (Kenny et al. 2003).
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Main types of acoustic survey method for benthic habitat mapping
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Side scan sonar
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Broad beam swath systems such as side scan sonar can produce an almost photographic
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representation of the seabed (JNCC 2001, Kenny et al. 2003, Georgiadis et al. 2009).
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Comprising of an underwater transducer connected by a cable to the towing vessel where
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data is recorded, an acoustic signal is emitted from a single beam or multiple beams rapidly
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returning echoes which are transmitted to the recording device for later analysis (JNCC
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2001). As the sonar is towed at a fixed height above the seabed, it casts relatively large
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acoustic shadows enabling the detection of variations in sediment structure (Kenny et al.
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2003). Side scan sonar provides continuous coverage of the area being surveyed (Brown et
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al. 2011). The introduction of digital side scan sonar devices has increased mapping
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capabilities with object detection possible down to tens of centimetres (JNCC 2001). To
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some extent, the quality of the images generated can be evaluated manually with features
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such as sand ripples identifiable by eye (JNCC 2001). The REBENT monitoring network in
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France has adopted side scan sonar as part of its benthic mapping programme which
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includes maerl beds (Ehrhold et al. 2006). Georgiadis et al. (2009) used side scan sonar to
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map coralline algae in the southern Aegean Sea.
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studies by Brown et al. (2002, 2005) and Freitas et al. (2006) to identify different seabed
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assemblages alongside groundtruthing exercises and OSPAR recommends its application
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where maerl beds are thick and extensive (OSPAR 2010).
Side scan sonar has also been used in
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Acoustic ground-discrimination systems (AGDS)
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Acoustic ground-discrimination systems (AGDS) operate using a hull-mounted single-beam
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echosounder which detects differences in the seabed using acoustic reflection properties
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(Greenstreet et al. 1997, Kenny et al. 2003). A single pulse is emitted from the device which
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is directed straight down towards the seabed producing a footprint of the area directly
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below the vessel (Brown et al. 2005). On reflection, the echo is returned to a transducer
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(JNCC 2001).
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discrimination systems do not produce continuous coverage maps like side scan sonar, but
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instead produce a track covering the area beneath the vessel (Wilding et al. 2003). Gaps
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between the tracks must be interpolated which can result in inaccurate assumptions about
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the seabed habitat (Kenny et al. 2003, Brown et al. 2005). This means that AGDS should be
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considered a predictive tool rather than a definitive view of the seabed (JNCC 2001, Wilding
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et al. 2003). There are two systems commonly in use in the UK - RoxAnnTM and QTC-ViewTM
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(JNCC 2001, Foster-Smith & Sotheran 2003). RoxAnnTM derives its values from the
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interpretation of two elements of the returning echo: the first echo (E1) is related to
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seafloor roughness, while the second multiple return echo (E2) is related to the hardness of
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the seabed (Kenny et al. 2003, Wilding et al. 2003). Supplemented by groundtruthing, the
Due to the focused pulse emitted from the sounder, acoustic ground-
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returned responses are classified into seabed characteristics (Kenny et al. 2003, Wilding et
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al. 2003, Brown et al. 2005). QTC-ViewTM converts the echo into a digital form using the first
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returning signal only and organises the seabed into acoustic classes (JNCC 2001, Ellingsen et
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al. 2002).
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considerable groundtruthing using calibration sites, the second relying on post-processing
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analysis (Ellingsen et al. 2002). The use of AGDS has become widespread in SACs in the UK
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(Brown et al. 2005) with Davies & Sotheran (1998) adopting this approach when evaluating
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biotopes in Falmouth Bay and the lower Fal Ruan Estuary. The JNCC Marine Monitoring
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Handbook (2001) recommends the use of AGDS where broad-scale surveys are required and
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as a tool for locating sites of particular interest, thus reducing survey time. The cost of AGDS
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in comparison to other acoustic devices is relatively cheap and integration with other
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onboard systems such as GPS is straightforward (Birkett et al. 1998, Kenny et al. 2003,
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Wilding et al. 2003). However, concerns have been raised around the unpredictability of
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responses, resolution at varying depths, levels of interpolation required and accuracy when
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determining habitat boundaries (Kenny et al. 2003, Wilding et al. 2003, Brown et al. 2005;
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Freitas et al. 2011).
It can used in ‘supervised’ or ‘unsupervised’ modes, the first requiring
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Multibeam echosounders
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Multibeam echosounders (MBES) were originally an extension of single-beam
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echosounders, transmitting multiple beams which are capable of covering a broad swath
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either side of the vessels track (Brown & Blondel 2009, Simons & Snellen 2009). Multibeam
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echosounding is becoming increasingly utilised within the field of seabed habitat mapping
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due to its collective ability to obtain bathymetry and backscatter data concurrently (Brown
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& Blondel 2009, Brown et al. 2011). The return signal can provide details of the geoacoustic 9
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properties of the sea bed including grain size and porosity, both of which could be useful
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features for identifying maerl beds (Brown & Blondel 2009). With the introduction of faster
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processing capabilities and quality improvements to the backscatter images which are now
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comparable to those collected through side scan sonar, studies using multibeam
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echosounders have rapidly increased (Brown et al. 2011). Like side scan sonar, continuous
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coverage maps can be created in conjunction with groundtruthing (Simons & Snellen 2009).
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Brown & Blondel (2009) observed that multibeam echosounders are now able to provide as
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much if not more information than side scan sonar. However, a high level of understanding
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is required to establish the most appropriate analytical approach and significant expertise is
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needed to interpret the data (Brown et al. 2011). To date no studies have been published
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demonstrating the successful application of multibeam echosounders to identify maerl.
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A more comprehensive review of acoustic techniques used for seabed habitat mapping can
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be found in Brown et al. (2011).
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Groundtruthing
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While acoustic surveys are increasingly used to map and classify seabed habitats,
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groundtruthing is still necessary either for calibration with acoustic analytical tools or to
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enable classification of the discrete characteristics identified through acoustic imaging (JNCC
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2001, Brown et al. 2011). Grab samples, towed or drop-down video and remote operated
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vehicles (ROVs) are common methods of groundtruthing.
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Grab samples
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Grab sampling involves the use of a two shelled steel device that is lowered open to the
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seabed and digs into the sediment, bringing the two shells together to retain a sediment
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sample which can be analysed at the surface. There are a number of grab samples in
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standard use: the van Veen which is most appropriate for soft sediments; the Day Grab
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which can be used on a variety of sediment types and the Hamon Grab which can be used to
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collect samples on coarser sediments including cobbles (JNCC 2001). Both the van Veen and
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Day Grab devices can be used from a small vessel with two operators where the Hamon
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Grab sampler can only be launched from a larger vessel and requires a minimum of three
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operators (JNCC 2001).
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consideration should be given to the method of sampling arrays.
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Monitoring Handbook (2001) has details on random, stratified random and systematic
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approaches to sampling. Analysis of the contents of the grab can be used to assist with the
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classification of seabed characteristics found during acoustic surveys (Greenstreet et al.
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1997, Davies & Sotheran 1998, Brown et al. 2002, Foster-Smith & Sotheran 2003, Freitas et
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al. 2003, Ehrhold et al. 2006, Georgiadis et al. 2009). The number of samples and replicates
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collected should take into consideration the area of the survey and the variations detected
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from the acoustic mapping exercise (JNCC 2001).
When using grab sampling to groundtruth acoustic data, The JNCC Marine
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Underwater video imaging
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Boat operated underwater video imaging systems include drop-down video, towed sledge
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video and remotely operated vehicles. Drop-down video involves the use of a video camera
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attached to an umbilical cord being lowered over the side of the survey vessel either when
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stationary or at low speed (JNCC 2001). This method gives the operator a limited degree of
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control about the location studied and can be easily deployed to survey as many areas as 11
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required (JNCC 2001). Brown et al. (2002) and Ehrhold et al. (2006) used drop-down video
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to validate acoustic survey data collected for biotope mapping. Towed sledge video involves
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a camera and lights mounted onto a rigid sled supported by buoyancy devices to keep the
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image stable as the gear is towed (JNCC 2001). Large areas can be surveyed rapidly using
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this approach and the clarity of the image enables accurate identification of biotopes (JNCC
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2001). To reduce positional errors when comparing acoustic data to the video footage, a
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transponder should be ideally fitted to the sledge to pinpoint its location (JNCC 2001).
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Without a transponder, manual calculations will have to be performed prior to comparison.
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Davies & Sotheran (1998) and Ruiz-Frau et al. (2007) used towed video when mapping
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biotopes in the Fal. Remotely operated vehicles (ROVs) are also linked to the research vessel
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by an umbilical cord but can be manoeuvred remotely by an operator onboard. ROVs can
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survey large areas and focus on precise points of interest and are useful at sites with rapid
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depth changes (Moore & Bunker 2001). However they are expensive and difficulties can be
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encountered with recording the devices position. Georgiadis et al. (2009) used ROV drops in
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conjunction with side scan sonar to map coralligène formations in the eastern
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Mediterranean Sea and Thomson (2003) used an ROV as part of project to develop
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autonomous sensors for marine resource mapping. All of these approaches provide a
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permanent record of the seabed which can be stored, edited and reused. They are also
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non-intrusive methods for identifying different substrate types. Successful use of any image
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recording device is highly dependent on underwater visibility.
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Evaluating proportions of live and dead maerl
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Both live and dead maerl are considered important habitats for a diverse range of organisms
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although the biodiversity on live maerl beds is considered greater (Birkett et al. 1998). The
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slow growth rate of maerl means that any detrimental influences such as eutrophication,
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invasive species, dredging or pollution is likely to have long-term implications for maerl and
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its inhabitants (Hall-Spencer & Moore 2000, Barbera et al. 2003, Grall & Hall-Spencer 2003).
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By establishing and monitoring the location and extent of live maerl beds, managers of the
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Fal & Helford SAC will be able to observe any reduction in area.
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There are a number of methods that could be used to collect this information. Grabs or
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core samples (discussed above) could be used but would only provide data on a small
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proportion of the survey area. They would also be unable to provide a quantitative
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measurement as the grabs take a significant ‘bite’ from the seabed and the proportions of
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sediment may not represent the actual quantity of live maerl present (Ruiz-Frau et al. 2007).
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The use of drop-down or towed sledge video means that large areas can be surveyed
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relatively quick to assess the extent of a live maerl bed but quantification of results would
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again be difficult (Davies & Sotheran 1995, Ruiz-Frau et al. 2007). Thomson (2003) found, as
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part of the SUMARE programme (Survey of Marine Resources) to establish autonomous
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sensors for identifying maerl, that the use of a greylevel histogram applied to video footage
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identified unique signatures for live and dead maerl. Although time consuming, direct diver
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observation using quadrats or transect lines could be employed and would provide
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quantifiable data on percentages of dead and live maerl (Axelsson et al. 2008). In the
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Republic of Ireland, SCUBA divers use direct propulsion vehicles to rapidly assess
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percentages (OSPAR 2010). All these methods could be interpreted and used in mapping.
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Assessing community structure / biodiversity
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The complex structure of maerl brings a number of challenges when surveying the
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biodiversity of maerl beds. Steller et al. (2003) classified the type of organisms associated
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with maerl into three main categories:
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Epibenthic: motile or sessile organisms living on the seabed.
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Cryptofaunal: those organisms living within the natural cavities created by the lattice network of maerl.
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Infaunal: those individuals living buried in the substrate.
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The small size of many organisms found on maerl beds has implications for their
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classification and any detailed studies require a high level of taxonomic expertise. Birkett et
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al. (1998) recognised this as an issue and observed that identifying down to genus level
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could still provide enough information to assess the health of the biotope. The size of
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organisms also affects the types of survey that can be selected to gain sufficient quantitative
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information on biodiversity. Diver surveys, hand-held video and still photographs can
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provide data on conspicuous species but may miss smaller organisms. These methods
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cannot be used to survey the cryptofauna and infauna. In order to obtain results for these,
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samples must be collected and analysed ex situ. As maerl is slow growing, the quantity of
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samples required when employing any intrusive survey techniques such as grab samples and
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cores needs to be carefully considered. Maggs (1983) suggests that the minimum sub-
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sample size should be where a 10% increase in the number of species in the sub-sample is
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derived from a 10% increase in the area. Processing of samples must be appropriately
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designed to ensure the preservation of organisms for analysis and successful capture across
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the size range of organisms.
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In situ diving surveys
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Diver quadrat surveys can be used for recording epibenthic species found on maerl.
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Quadrats come in a variety of sizes, commonly 1m2, 0.25m2 or 0.1m2 (JNCC 2001). For maerl,
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a 0.25m2 grid quadrat would be a sensible choice for the size of organisms under survey. A
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minimum of two suitably qualified divers descend a weighted shot line, which can be used
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by the survey vessel to record location, and lay a transect in a pre-agreed direction. This
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avoids potential bias by the divers once underwater to select areas that look ‘most
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interesting’ (Eleftheriou & McIntyre 2005). The quadrat and transect line should be slightly
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negatively buoyant so that it can rest on the seabed while the divers complete the survey
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(Axelsson et al. 2008). Divers should have a standard survey form to record species and
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abundance. The JNCC Marine Monitoring Handbook (2001) recommends conducting a pilot
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survey of the location to familiarise the divers with the species likely to be encountered
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during the survey. Once the survey is complete, the dive team should use a delayed surface
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marker buoy to inform the survey vessel of the end location. This will enable the results to
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be mapped. Collecting data in this way is a non-destructive technique; provides quantitative
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data which allows changes to species composition to be monitored over time; is a simple
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method for recording conspicuous species without damaging the maerl and is easily
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repeatable (Eleftheriou & McIntyre 2005). The divers need to be confident of their buoyancy
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control to avoid kicking up sediment or causing damage to the surrounding maerl; should
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have an understanding of the biotope and potential species and have the appropriate
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qualifications for conducting underwater surveys (JNCC 2001). Percentages of live and dead
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maerl could also be recorded as part of this survey. Steller et al. (2003) used transect
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surveys to estimate species richness and abundance on maerl beds in the Gulf of California.
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Hand-held video surveys carried out by qualified divers can provide information on the
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conspicuous benthos in the chosen study site (JNCC 2001). A video camera, mounted in
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underwater housing and with lighting to improve image quality is the standard equipment
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required (JNCC 2001). It provides a permanent record of the site surveyed and analysis can
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be conducted ex situ. Poor underwater visibility; scaling of the images recorded; the ability
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to collect quantitative data that can be readily related to GPS points and consistency of
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recording means that this method may be more appropriate for collecting footage that can
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be used for educational purposes rather than monitoring. Axelsson et al. (2008) conducted
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diver video surveys in the Fal Estuary in conjunction with in situ diver observations.
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Still photography of maerl beds involves the use of an underwater high-resolution camera
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with lighting. In order to obtain quantitative data, the camera should be mounted onto a
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reference frame or a quadrat used to provide a scale to the photograph (JNCC 2001). In the
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same way as video, photographs provide a permanent record and with greater resolution
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capabilities than video, photographs can enable more detailed images to be collected.
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Analysis can be carried out ex situ meaning that the divers do not need to be taxonomists.
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Most commonly used for collecting fixed point data, there are currently no examples of this
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method being used for random sampling (JNCC 2001). However, this method could be used
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in conjunction with a diving quadrat survey to record unidentified specimens or get close up
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shots without removing specimens.
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Sample collection techniques
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Hand-held cores can be used to collect cryptic fauna, infauna and sediment samples
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(Bordehore et al. 2003, Moore et al. 2004, Axelsson et al. 2008, Sciberras et al. 2009). These
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cores are operated by divers who manually push the core into the sediment. Axelsson et al.
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(2008) found that the substrate in areas of the Fal made the extraction of sediment samples
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using hand-held cores quite challenging. On reaching the appropriate depth, the top and
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bottom of the corer are sealed with caps and it is placed into bags for secure storage and to
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prevent any organisms escaping (Axelsson et al. 2008, Sciberras et al. 2009). Ideally on
377
board the survey vessel and within 24 hours of collection, the samples should be sieved and
378
fixed in 10% formal saline to ensure the samples are preserved for laboratory analysis (JNCC
379
2001, Axelsson et al. 2008). The small nature of some species found on maerl means that
380
the mesh size should be below 1000Îźm to ensure robust analysis (Moore et al. 2004,
381
Axelsson et al. 2008).
382
To collect cryptic fauna, Steller et al. (2003) used divers to collect random samples of maerl
383
thalli by hand, which were then preserved and the cryptic and burrowing species were
384
extracted in the laboratory for analysis.
385
Grab samples (discussed in the mapping section) can be used for biological sampling as well
386
as groundtruthing (Sciberras et al. 2009). A combined approach to groundtruthing and
387
infaunal sample collection can involve skimming off the top layer of the contents of the grab
388
sample for sediment analysis and fixing the rest for biological analysis in the laboratory
389
(Moore et al. 2004). Ruiz-Frau et al. (2007) used a 0.1m2 Day Grab for collecting sediment
390
and biological samples in Falmouth Bay. Grab samplers usually require deployment from a
391
research vessel with space to winch the samples in. 17
392
393
Measuring abiotic factors that could impact the health of maerl beds
394
Environmental tolerances of maerl are still poorly understood (Birkett et al. 1998). However,
395
any robust monitoring programme should include abiotic measurements so they can be
396
factored in if any unexplained degradation of the maerl beds occurs.
397
Temperature is known to affect the geographic distribution of maerl beds with L.
398
corallioides absent in Scotland due to cooler temperatures but present in southwest Britain
399
(Birkett et al. 1998, Wilson et al. 2004). The species composition of maerl beds is influenced
400
by temperature, so monitoring the water temperature on maerl beds could track changes
401
that may occur as a result of climate change (Wilson et al. 2004). Maerl beds are normally
402
found in fully saline conditions but the impact of variable salinity conditions is not
403
understood (Birkett et al. 1998). Temperature and salinity can be measured together in situ
404
using a temperature-salinity probe (Sciberras et al. 2009); through the use of a CTD
405
(conductivity-temperature-depth) package or by using a niskin bottle to collect water
406
samples for analysis on the surface.
407
Maerl is a coralline algae and therefore needs to photosynthesise in order to grow, however
408
irradiance requirements are not understood for maerl (Birkett et al. 1998). In the UK, maerl
409
beds rarely exceed 30m (Wilson et al. 2004). Turbidity can affect the amount of light
410
penetrating to the maerl bed and can be measured simply using a secchi disk (Sciberras et
411
al. 2009).
412
The nutrient requirements of maerl are not known, however with a rigid skeleton of calcium
413
carbonate maerl does have a requirement for calcium (Birkett et al. 1998). In laboratory 18
414
experiments, King & Scramm (1982) found that the calcium ionic concentration affected
415
maerl growth, with an optimum uptake of 30 ‰. Under laboratory conditions, Martin &
416
Gatuzzo (2009) conducted experiments to assess the effects of ocean acidification on
417
coralline algae and found that based on current projections, net dissolution was likely to
418
exceed net calcification in Lithothamnion cabiochae by the end of the century. pH therefore
419
should be monitored and can be tested in situ using an underwater housed pH meter or on
420
the research vessel if a water sample has been collected.
421
422
Discussion
423
424
The three-dimensional structure of maerl brings additional complexities and considerations
425
on top of those encountered during other subtidal surveys. The current deficiency in
426
standardised protocols for assessing maerl communities combined with rapid advances in
427
the technology used for mapping benthic habitats makes difficult work for conservation
428
managers when establishing a monitoring programme. There is no single method that can
429
monitor all the conservation objectives set for maerl within the Fal & Helford SAC. Instead
430
the solution requires a suite of approaches that will satisfy the requirements and provide
431
the management team with a robust dataset that can inform decision-making.
432
recommendations discussed in the proceeding sections are based on the knowledge gained
433
through studies on maerl in other regions, the use of standardised methodologies where
434
possible and factor in resource and cost considerations.
435
sensitivities of maerl, the recommendations exhibit a preference towards non-destructive
436
techniques where feasible. Table 1 summarises the recommendations discussed below. 19
The
Due to the environmental
437
438
Recommendations for mapping
439
While the importance and potential application of acoustic devices for seabed habitat
440
mapping is widely recognised, the difficulties with accurately differentiating maerl habitats
441
from other mixed sediments is still problematic and can currently only be used if validated
442
with groundtruthing. All three forms of acoustic sensing discussed have been used to map
443
maerl biotopes but the most recent research shows a preference towards the use of side
444
scan sonar in conjunction with a multibeam echosounder (Kenny et al. 2003, Diaz et al.
445
2004, Ehrhold et al. 2006, Brown et al. 2011). Side scan sonar has the highest definition of
446
seabed features amongst the techniques in use and this combined with the bathymetric and
447
backscatter imaging provided by multibeam echosounders will generate a robust baseline
448
for the monitoring programme (Diaz et al. 2004). The two devices are also capable of
449
providing 100% coverage unlike AGDS which needs interpolation between the tracks. From
450
previous studies of maerl in the Fal Estuary, it is apparent that maerl occurs in fairly shallow
451
waters (Blundel et al. 1981, Hiscock 1998). Acoustic surveys therefore need to be carried
452
out during high tides for maximum accuracy. Provision will need to be made within the
453
budget for a mapping expert to advise on the survey design and interpret the outputs. Due
454
to the area the Fal & Helford SAC covers, the quickest and most cost-effective option for
455
groundtruthing is drop-down video. For deeper parts like the channel in Carrick Roads,
456
towed sledge video may be required but will give comparable results.
457 458
While these surveys can be carried out at any point during the year, it is recommended that
459
mapping is carried out during the summer months when visibility, light and weather
20
460
conditions are optimum. For consistency, future surveys should be carried out across the
461
same period. Surveys lasting longer than an hour should have tidal corrections applied
462
(Brown et al. 2005).
463 464
Recommendations for evaluating proportions of live and dead maerl
465
An initial baseline assessment should focus on establishing the location and dimensions of
466
the live maerl beds. Drop-down video is a practical way to collect this data and would work
467
well with the groundtruthing exercise. With drop-down video, GPS points can be easily
468
collected when live maerl is observed and later plotted onto a GIS map. Where live maerl
469
beds are identified surveys should be conducted to identify the perimeter of the bed so that
470
a true area can be calculated. While the primary goal is to obtain a map showing the
471
locations, another important measurement is to identify the proportion of live and dead
472
maerl present within the live maerl beds. The best method for obtaining quantitative data
473
is to conduct diver quadrat surveys along a series of transects. Procedural Guideline 3-7
474
from the JNCC Marine Monitoring Handbook (2001) covers the methodology in more detail.
475
Measurements should be taken as percentages not abundance scales to enable quantitative
476
analysis. It should be noted that these survey methods will only measure the surface layer
477
of the maerl.
478
479
Recommendations for biodiversity
480
Apart from the maerl species there are no key indicator species associated with maerl at
481
present so it is necessary to ensure a robust biodiversity survey is conducted. Collecting data
21
482
on the number and type of species found on maerl beds will require the application of
483
several methodologies and is likely to be the most time-consuming part of the monitoring
484
programme. Both live and dead maerl deposits should be surveyed. Multiple diver quadrat
485
surveys along a transect line are recommended for surveying the epifauna and flora
486
following the sample size recommendations proposed by Maggs (1983). This approach will
487
require at least one diver to be a trained biologist with an understanding of the taxonomy of
488
organisms associated with maerl. Organisms should be identified down to species level
489
where possible but genus is acceptable. The second diver should have a high-resolution
490
underwater camera with a macro setting and strobe lighting. Each quadrat should be
491
photographed with a unique id. Any unknown organisms should be photographed for
492
identification ex situ. The photograph should include a scale for reference. A standard form
493
should be generated for the survey team to complete including depth, temperature and
494
percentage of live and dead maerl. Any diving surveys must comply with the Diving at Work
495
Regulations 1997 and adhere to the Scientific and Archaeological Approved Code of
496
Practice.
497 498
There are currently no non-destructive methods for surveying the infauna. The use of a
499
diver hand-held corer is considered a suitable method for obtaining samples. A number of
500
objectives can be achieved through collecting core samples. The species of maerl will be
501
easier to identify in the laboratory, the underlying sediment can be measured and cryptic
502
fauna and infauna can be surveyed at the same time. Samples should be processed and
503
fixed within 24 hours of collection for later analysis in a laboratory. Comprehensive
504
methodologies for collecting, processing and analysing cryptic fauna and infauna can be
505
found in Steller et al. (2003) and Sciberras et al. (2009) respectively. 22
506 507
Due to the seasonal variations experienced on maerl beds, two sampling seasons are
508
recommended to gain a greater understanding of community change however this will be
509
constrained by the budget allotted to the monitoring programme. Consideration should
510
also be given to avoid extensive destructive survey techniques around any peak recruitment
511
periods. If only one survey is practicable, then July is advised when species richness is at its
512
peak yet recruitment and settlement of known commercially important species is low
513
(Kamenos et al. 2004b, c, PeĂąa & BĂĄrbara 2010).
514 515
Recommendations for measuring abiotic factors
516
Although OSPAR (2010) states that monitoring should be conducted as a minimum at six-
517
yearly intervals for SACs, it is recommended that abiotic factors should be collected twice
518
yearly during Spring and Autumn. Plankton levels are generally high in Spring while water
519
temperature is at its annual lowest, conversely in late Autumn, annual water temperature
520
reaches its maximum and plankton is on the decline (Miller 2004). Temperature, salinity,
521
pH, turbidity and dissolved oxygen levels should be recorded. These are relatively simple
522
and quick measurements to collect and may be important if any change to the extent and
523
distribution of the maerl is observed. It is important that the same equipment is used and
524
calibrated each time thus avoiding potential discrepancies between devices.
525
526
Further recommendations
23
527
Continuing advances in the technology surrounding benthic habitat mapping and our
528
increasing understanding of maerl biotopes mean that the techniques recommended in this
529
review will need to be reassessed at appropriate junctures.
530
While not within the scope of the review, it is worth emphasising the importance of the
531
supporting systems that will need to be established in order to manage and store the data
532
collected through the monitoring programme. A suitable format will need to be selected
533
which is likely to include GIS functionality and scope for statistical analysis of the data.
534
535
Conclusions
536
537
The high biodiversity associated with maerl beds has long been recognised. However for
538
many years maerl has been subjected to damaging anthropogenic activities such as
539
aggregate extraction and dredging. Investigations on maerl have largely focused on
540
assessing the implications of these destructive processes or sought to gain a greater
541
understanding of the biology and ecology associated with maerl. As such, there have been
542
few monitoring programmes established to look at the long-term dynamics of maerl bed
543
communities.
544
Valuable contributions have been made by the scientific community to expand our
545
understanding of maerl. These studies have shown that maerl grows exceptionally slowly,
546
has narrow environmental tolerances and is highly sensitive to disturbance resulting in the
547
acknowledgement that maerl is a non-renewable resource. They have also identified its role
548
in carbon sequestration and as a nursery ground for commercially important species. 24
549
As a consequence, a growing emphasis is being placed on the conservation importance of
550
maerl beds, with local and European protection designations at both species and habitat
551
level.
552
comprehensive baseline from which to monitor long-term trends.
553
Those parties involved in the management of the Fal & Helford SAC should find this review a
554
useful summary of the techniques most suitable for surveying maerl beds. While it shows
555
that establishing a maerl monitoring programme for the Fal & Helford SAC is a complex
556
process that will involve significant time, cost and planning, it also demonstrates that there
557
are multiple methods that can be adopted in order to achieve the primary conservation
558
objectives.
In order to manage maerl beds effectively, it is important to establish a
559
560
Acknowledgements
561
562
This review was kindly supported by the Falmouth Harbour Commissioners. This manuscript
563
benefited from comments by C. Eatock, A. O’Brien, T. Russell, and A. Thornton. Particular
564
thanks go to thank N. Woods for his assistance with testing ROV and multibeam
565
echosounder methodologies and the use of the research vessel RV Ann Kathleen.
566
567
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