Estuarine ecology and environmental Indicators The ecological health of Rangataua Bay, Tauranga Harbour
Introduc<on • • • • •
Estuarine ecology Te Tahuna o Rangataua; history of the area Ecological survey methods Preliminary results of sediment characteris<cs Ecological indicators to assess environmental health
Estuarine Ecology • Estuaries provide a buffer between land and open ocean, crea<ng uniquely dis<nct habitats for many organisms • Flora and fauna found within estuaries have adapted to live in harsh physical and chemical condi<ons • Estuaries are vulnerable to nega<ve impacts from a variety of human ac<vi<es including de-‐foresta<on, agricultural prac<ces, urbanisa<on and discharge of pollutants and waste to marine environments • Nega<ve impacts associated with wastewater discharge and terrestrial run-‐off can include sedimenta<on, eutrophica<on and toxicity to sediments, organisms and the water column.
Area of study Te Tāhuna ō Rangataua is a bay in the Welcome Bay area which is located adjacent to the Te Maunga Wastewater Treatment Plant and its treatment ponds • Reclama<on of the bay for the si<ng of oxida<on ponds occurred in 1974. • Bioresearches (1974) conducted an ecological study of the area before reclama<on occurred • The Mangatawa Drain was built some years before 1974, which discharges storm water into the bay from state highway 2 and the Papamoa catchment
Bioresearches (1974) •
Ecosystems within the area of reclama<on that were considered ecologically important included marshland and seagrass beds.
Marshland played important ecological roles -‐buffering the shoreline -‐protec<ng from erosion -‐serving as biochemical filters •
•
A dense popula<on of Ti<ko were a dominant biological feature in the marsh area
•
The marsh supplied a stable niche and food to many species both locally and with <dal transport, to the Welcome Bay area in general.
Bioresearches (1974) • The area to be reclaimed was considered to have high ecological and biological value, which was expected to increase in the future. • Reclama<on within Rangataua Bay was expected to “destroy the present ecology”, reducing the marsh area by 25% and the <<ko popula<on by 33%. • The 1974 report states if pond construc<on was done correctly with “impervious walls and stone capping material” pollu<on from the ponds would not be an issue, though seepages have occurred for many years.
Seepages As a condi<on of the TCC Te Maunga Waste Water consent, annual assessment of treated waste water seepages into the bay is conducted • One seepage from the old oxida<on ponds has been present since 1987 and was iden<fied again during the 2013 assessment • The total area that was affected from a visual assessment was 63m2. Within this area, abundance and diversity of organisms is reported to be limited
Image taken from TCC Annual <<ko and seepage monitoring report (Gibbons-‐Davies, 2013) showing seepage site W6.
Ecological survey methods Methods follow Standardised Estuary Monitoring Protocol by Robertson et al (2002) based on benthic community and sediment proper?es
Sampling was undertaken in January and February 2014 30 Sites were selected along three transect lines at varying intervals (5, 15, 30, 50, 100, 200, 300, 400, 500 & 600m) At each site a 25 by 10 m grid of ten plots was marked out with 10 replicates in each grid.
Variables sampled • Sediment samples were analysed for organic maeer (ash free dry weight), total recoverable Arsenic, Copper, Lead, Mercury, Phosphorus, Zinc, Total Nitrogen, chlorophyll a and percentage of mud content • Core samples of benthic invertebrates were collected (10 replicates per site) to assess benthic community composi<on. • Overall 300 samples of benthic invertebrates were collected to be analysed
Benthic invertebrate core sample
Preliminary sediment results • Mud percentage can indicate rate of fine silt sedimenta<on • Percentage of mud content is moderate across sites, indica<ng some sedimenta<on is occurring in the area Sedimenta<on can suffocate organisms and reduce heterogeneity of ecosystems within an area In New Zealand, estuaries are being recognised as the coastal environments most at risk Within the Rangataua area sedimenta<on may be coming from several sources, including soil erosion, the Mangatawa Drain and freshwater streams
14 12 10 8 6 4
Percentage of mud content
2 0
The graph shows the average percentage of mud content across sites (transect A, B and C) at each distance (metres) from the WWT ponds.
Freshwater inputs • Two freshwater streams enter the Rangataua area; the Rocky stream and the Waitao stream • Catchment use comprises of forest and pasture with some hor<culture Waitao Stream
• An ac<ve quarry and urban and road construc<on would have influenced sediment inputs to streams
Heavy trace metals Heavy metals can provide a preliminary assessment of toxic contamina<on throughout food webs
• From the heavy metal analysis, none of the heavy trace metals exceed ANZECC sediment quality guidelines • A sediment quality study was undertaken between 2001-‐2003 (Parks 2003) with 2 sites located in Rangataua Bay. These sites had high heavy metal concentra<ons for copper, lead and zinc. • Cu, Pb and Zc contamina<on can be aeributed to storm water discharges and urban run off
20 18 16 14 12 10 8 6 4 2
Pb mg/kg average Hg mg/kg average Zn mg/kg average
0
The graph shows the average heavy metal concentra?ons for Lead (Pb), Mercury (Hg) and Zinc (Zn) across sites (transect A, B and C) at each distance (metres) from the WWT ponds.
Organic maeer Sediment organic content is an important variable as high levels of organic content can result in anoxic sediments, excessive nutrients and other nega<ve impacts
• Organic maeer is found to be higher across sites closest to the treatment ponds
3 2.5 2 1.5
• Percentage of organic maeer is moderate to low across sites, though organic enrichment, along with sedimenta<on and pollu<on, will be further assessed once we have macro invertebrate data
1
Organic Maeer Average
0.5 0 5m 15m 30m 50m 100m 200m 300m 400m 500m 600m
The graph shows the average percentage organic maTer across sites (transect A, B and C) at each distance from the WWT ponds.
Chlorophyll a Chlorophyll a concentra<ons are the measure of micro algal growth in an area and they are also indicators which reflect organic enrichment and eutrophica<on. 45
Increase in chlorophyll a may be related to: • nutrient concentra<ons, • decreased flow or change in hydrodynamics • change in water clarity The treatment ponds may influence these variables
40 35 30
Chll a* mg/kg average
25 20 15 10 5 0 5m 15m 30m 50m 100m 200m 300m 400m 500m 600m
The graph shows the average chlorophyll a values across sites (transect A, B and C) at each distance (metres) from the WWT ponds.
Nutrients In shallow estuaries such as Rangataua Bay nutrients can accumulate within the sediment
• Total phosphorus and nitrogen was found to be low across all sites, though concentra<ons are highest closest to the ponds and then decrease at short distances from the ponds
0.1 0.09 0.08 0.07
• Nutrient levels could be elevated due to the ponds but appear only to be localised in the immediate area
250 200
0.06 0.05
• The decrease in values reflects the trend found for chlorophyll a
300
150
TN g/100g average
100
TP mg/kg dry wt average
0.04 0.03 0.02
50
0.01 0
0
The graph shows the average nutrient values across sites (transect A, B and C) at each distance (metres) from the WWT ponds.
Environmental Indicators • Benthic invertebrates are a common tool used in environmental management. • Invertebrates are omen sessile and responses to pollu<on can be measured over months to years • Presence and absence of macro invertebrates, in par<cular polychaete worms, can give an indica<on of a pollutants presence over <me
Polychaete worms
Family: Spionidae Species: Prionospio aucklandica Family: Neriedidae
Family: Capitallidae Species: Heteromastus filliformus
Conclusion • There has been an observable loss of biodiversity in Rangataua Bay since 1974 • Rangataua Bay is influenced by the treatment ponds, Mangatawa drain and the surrounding catchment area • Once taxonomy data is complete, we may assess change in community composi<on based on our sediment data and form some strong conclusions about biodiversity and ecological health of the area
References
Carpenter, S. R., Caraco, N. F., Correll, D. L., & Howarth, R. W. (1988). Nonpoint Pollu<on of Surface Waters with Phosphorus and Nitrogen. Ecological Applica8ons, 559-‐568. Dean, H. K. (2008). The use of polychaete (Annelida) as indicator species of marine pollu<on: a review. Revista de Biología Tropical, (56) 11-‐38. Gibbons-‐Davies, J. (2013). Consent 62881 Annual Report of Ti8ko Monitoring and Seepages into Rangataua Bay 2013. Tauranga: Tauranga City Council. Gibbs, M., & Hewie, J. (2004). Effects of Sedimenta8on on macrofaunal communi8es: a synthesis of research studies for ARC. Hamilton: Na<onal Ins<eue of Water and Atmospheric Research Ltd. Hume, T., & Swales, A. (2003). How esturaies grow old. Water and Atmosphere 11 (1), 14-‐15. Larcombe, M. F. (1974). Ecological Impact Report-‐ si8ng of Oxida8on Ponds, Welcome Bay. Auckland: Bioresearches Ltd; Steven and Fitzmaurice Consul<ng and Sanitary Engineers. Madarasz, A. (2006). Ahuriri Estuary: Environmental Assesment and Monitoring. Napier: Hawke's Bay Regional Council. Nybakken, J. W., & Bertness, M. D. (2005). Marine Biology An Ecological Approach 6th ed. San Francisco: Pearson Educa<on; Benjamin Cummings. NZ Transport Agency and Tauranga City Council. (2009). Mangatawa Drain Flood Bypass and TEM Stormwater AEE. Tauranga: NZTA and TCC. Park, S.G. (2003): Marine Sediment and Contaminants Survey (2001-‐03) of Tauranga Harbour. Environment BOP Environmental Report 2003/20. Environment Bay of Plenty, PO Box 364, Whakatane Tay, H. W., Bryan, K. R., & Pilditch, C. A. (2013). The hydrodynamics of the Southern basin of Tauranga Harbour. New Zealand Journal of Marine and Freshwater Research, 47: 249-‐274.