Seagrass meadows Seagrass meadows

What are seagrasses?

Seagrasses (marine phanerogams) are flowering and seeding fruit-bearing plants (angiosperms) which are adapted to life in salt-water or brackish water and on sandy or muddy soils rich in organic material and calcium carbonate. These grasses commonly thrive in the infra-li oral zone (Figure 1), permanently underwater, although they may also be found in the li oral zone, on soils periodically covered by water due to the changing des (Figure 1).

Mean high water mark

Mean low water mark

Seagrasses o en grow in patches on the sea floor, but they can extend to form large, dense meadows. Seagrass meadows may be composed of a single species (monospecific) or several different species (mul specific) (Figure 2.)

Seagrass meadows: Teacher’s Guide

Seagrasses, like mangrove trees, belong to the marine angiosperm family. These plants evolved during the Cretaceous period, about 145 million years ago, from terrestrial angiosperms that adapted to life in an aqua c environment (Figure 3). Seagrasses are able to live fully submerged in seawater. Their strong root system anchors them to the substrate and helps them withstand wave and current ac on. Like land-based plants, seagrasses produce seeds and pollen. Seagrasses are monocotyledons and, although they are similar to land-based grasses, they are not related to them.

Angiosperms: Angiosperms are plants which produce flowers, seeds and fruits. Some examples include passionfruit, avocado and mangoes, among many others.

Brackish: Water featuring more dissolved salts than freshwater, but less salt that seawater.

Carbon sink: natural or ar ficial environment viewed in terms of its ability to absorb carbon dioxide.

Cretaceous: The Cretaceous is a division in the geological metable. It is the third and last period of the Mesozoic Era; it began approximately 145.5 million years ago, and ended approximately 65.5 million years ago.

Infra-li oral zone: Part of the coast which is permanently submerged.

Li oral zone: Coastal zone under the influence of the shi ing des.

Monocotyledons: Group of vascular plants (phanerogams) that produce flowers (angiosperms), in which the seed embryos present a single ini al leaf (a single cotyledon). Monocotyledons include corn, most cereals, tulips and onions, among others.

Phanerogams: Flower-producing, vascular (featuring xylem and phloem) plants.

Importance of the seagrass meadows ecosystem

A seagrass meadow is a fundamentally important ecosystem, necessary for our planet’s ecological balance. It provides habitat, food and shelter for several marine species. It also serves as a carbon sink, and helps prevent coastal erosion. These and other factors that make seagrass meadows crucial to the ecosystem are explored in this sec on.

1. Interac ons with mangrove forests and coral reefs

Mangrove forests, seagrass meadows and coral reefs are all connected to one another, and they interact and stabilize the coastal environment. Seagrasses capture sediments and suspended par cles with their roots and leaves, keeping the waters clear. Water clarity is important to seagrass meadows and coral reefs, since both these ecosystems depend on photosynthesis for their survival. The oxygen produced by photosynthesis aids the aerobic decomposi on and nutrient recycling occurring in coastal zones. Moreover, seagrass meadows absorb large amounts of nutrients and func on as natural filters for chemical substances in the marine environment. This is important when maintaining the balance between corals and macro-algae inhabi ng the reefs, since an excessive amount of nutrients in the water would cause uncontrolled algal growth that would overgrow and suffocate the coral. Finally, seagrass meadows are highly produc ve systems that export nutrients and biomass to other ecosystems. For instance, some fish and crustacean species live in seagrass meadows during their juvenile life-stages, and eventually make their way to other marine ecosystems such as the coral reef.

2. Primary produc vity and biological diversity

Seagrass meadows are highly produc ve ecosystems that serve as a carbon sink by absorbing large quan es of carbon dioxide and produce oxygen through their photosynthe c processes. This ecosystem, along with tropical and temperate forests, is classified as the most produc ve in the world. The high produc vity and structural complexity of seagrass meadows increase habitat diversity and therefore marine biodiversity.

Aerobic decomposi on: The process that uses oxygen to breakdown organic material and wastes in order to recycle nutrients.

Biomass: The total organic mass from all the creatures living in a par cular place.

Carbon dioxide: This is a gas present in Earth’s atmosphere. It is created as a byproduct of living organisms’ respira on, organic material decomposi on, or the combus on of carbon-based fuels. This gas is vital for photosynthesis.

Detritus: Residue from organic material fragmenta on.

Epiphytes: This refers to any organism growing over or on top of several different surfaces (living and nonliving), using them as support but not harming them in any direct way (that is to say, they are not parasi c).

Estuarine zones: Coastal areas in which large bodies of freshwater (such as rivers) meet the sea. They receive dal change ac vity, but are protected from direct wave and wind ac on.

Euryhaline: Organisms that can tolerate a wide range of salt concentra ons. In this case, it refers to organisms that can tolerate changes in salinity within the aqua c environment in which they live.

Foraminifera: Small, unicellular organisms belonging to the protozoa family. Their bodies are protected by a delicate shell or test.

Planktonic organisms: Small, o en microscopic organisms that live floa ng in fresh or salt water (they are weak swimmers).

Sedimenta on: Process of sediment accumula on and deposi on.

Substrate: Surface on which an organism lives.

3. Shelter, nursery and food for many species

Seagrass meadows are home to many organisms such as sea stars, sea urchins and clams, which find protec on from wave and current ac on. Similarly, the meadows shelter the larval and juvenile life-stages of fishes, mollusks, lobsters, and other commercially important species. Seagrass leaf surfaces func on as substrate for several epiphy c algae, sponges and foraminifera, among others. This ecosystem is also home to many medically important species (Mar nez Daranas, 2007).

Seagrass leaves are an important food source for a great variety of marine organisms, including manatees and green sea turtles, both animals currently listed as endangered species. Furthermore, the detritus generated by plant fragmenta on is a source of food for other creatures living on the sea floor and on sandy shorelines, such as sea cucumbers, crabs and anemones. Seagrass decomposi on liberates nutrients into the water which are then reabsorbed by the seagrasses themselves as well as by certain planktonic organisms. Seabirds such as seagulls and pelicans fly over the seagrass meadows searching for food.

4. Erosion preven on

The extensive system of roots and rhizomes that anchor seagrasses in the seafloor helps prevent sea floor erosion since it increases the sedimenta on process (sediment accumula on and deposi on) and soil compac on and stabiliza on. Furthermore, seagrass leaves help protect the coasts from erosion by absorbing and dissipa ng the energy from waves and currents. The ability of seagrasses to trap and retain sediments helps keep the water clear.

Differences between seagrasses and seaweeds

Seagrasses are o en mistaken for seaweeds. However, there are clear morphological, ecological and physiological differences between the two, some of which are detailed in the following table (Table 1).

Table 1. Differences between seagrasses and seaweeds. Taken from: Seagrass-watch. h p://www.seagrasswatch.org/seagrass.html

SEAGRASSESSEAWEEDS

They are more complex plants with an internal vascular system (xylem and phloem) through which all the minerals and nutrients are transported to every cell in the organism.

Non-vascular organisms. Nutrients reach the organism’s cells through diffusion

Feature true roots, stems and leaves. Feature rhizoids, pedicels (thallus) and laminae (fronds). They lack true roots, stems and leaves.

Possess roots and rhizomes that allow them to anchor themselves to the substrate and absorb nutrients and minerals.

Possess rhizoids that allow them to anchor themselves to the substrate, but absorb all nutrients through all of their surface cells using diffusion.

They perform photosynthesis only through the cells in their leaves. Photosynthesis is performed in all of their cells.

Their reproduc on can be sexual (flowers fruits and seeds) or asexual (rhizomes, roots or shoots).

Their reproduc on is chiefly asexual, achieved through spores, fragmenta on or shoots, but may also reproduce sexually through gametes

Di ffusion: Process through which molecules flow from an area with higher concentra on to one with lower concentra on.

Gametes: Cells par cipa ng in sexual reproduc on. They only have a single copy of each chromosome (they are haploid). In many animals, we know them as ovum and sperm. When they unite, they create a cell (zygote) with two copies of each chromosome (diploid), such as humans.

Laminae: Generally fla ened structure found in algae, which resembles a plant’s leaves.

Phloem: A phanerogam’s vascular ssue, which transports sugars and nutrients in ascending and descending direc ons, from the organs that produce them to those which consume and store them.

Photosynthesis: The process that uses solar energy, water, nutrients, plant chlorophyll and carbon dioxide are used to form vegetal ssue (for instance, in the form of sugars, fats or proteins).

Rhizoid: Structure similar to a plant’s roots, used by algae to anchor themselves to the ground and stabilize themselves. Unlike true roots, rhizomes do not absorb and transport nutrients.

Rhizomes: Subterranean plant stalk that grows horizontally and from which roots and new shoots may form. Rhizomes aid a plant’s rapid spread throughout an area.

Shoots: A developing plant sprout that may include stalks and leaves.

Vascular plants: Plants presen ng a vascular system (xylem and phloem) that distributes water and nutrients throughout the plant.

Xylem: Vascular plant ssue that transports water and nutrients upward from the roots, and also provide support to the plant.

Simple root

Parts of a seagrass plant

Leaf

Ver cal shoot

Rhizome or horizontal stem

Parts of a seaweed specimen

Laminae Rhizoid

Necessary condi ons for seagrass growth

There are several climate zones around the world, including polar, temperate and tropical zones. The polar zones are on the northernmost and southernmost parts of Earth (from the polar circles to 90º la tude in each hemisphere). There, temperatures are very cold and precipita on is scarce. These are inhospitable places. The temperate zones, on the other hand, are found between 30º and 60º la tudes in each hemisphere, and there are dis nct differences in temperature and precipita on as prescribed by the seasons throughout the year. These regions are life-sustaining. Lastly, the tropical regions are warm throughout the year, and although the zone has four seasons, precipita on pa erns set apart two dis nct seasons: winter and summer.

Most seagrasses are found in the tropical (0° to 30° N and S) and subtropical zones (found between 25° and 40° la tude in each hemisphere). However, some seagrass species live in temperate zones (Figure 5). Their distribu on is governed by a series of parameters, such as:

1. Temperature: Seagrasses primarily thrive in waters with temperatures higher than 75°F (24°C) (Figure 5). However, some species may be found in considerably colder waters (as low as 39°F; or 4°C).

2. Salinity: Most seagrasses tolerate a high range of salinity, which means they can survive in estuarine zones (areas with low salinity in which sea water mixes with fresh water) as well as in more saline environments. Among the Caribbean species, shoalgrass (Halodule wrigh i) is the most resistant to extreme saline condi ons (i.e., the most euryhaline) (Phillips and Meñez, 1988, cited by Mar nez Daranas, 2007).

3. Wave ac on: Although they feature strong structures that anchor them to the substrate, seagrasses tend to inhabit areas protected from strong surf and current condi ons: bays, lagoons and estuaries.

4. Availability of light and depth: Seagrasses use photosynthesis which requires sunlight, therefore, they live in rela vely clear, shallow waters with good sunlight availability at the bo om. Sunlight penetra on through the water column is what determines the depth to which these seagrass species can grow. Sediment and nutrient discharge from the coasts increase water turbidity and reduces light availability in these environments.

Puerto Rico’s seagrasses

Although seagrasses can be found in all la tudes except the polar areas, most seagrasses thrive in tropical regions. The most well-developed seagrass meadows in Puerto Rico can be found in the southwestern, southern, and eastern regions of the island, where the insular pla orm is wider and shallower than along the northern coast. These zones are also characterized by having gentler surf and current condi ons than in the northern coast (Figure 7), which aids the establishment and development of healthy seagrass meadows.

There are around 60 seagrass species around the world. In the Caribbean and Puerto Rican region, there are five commonly found species: a. Turtle grass (Thalassia testudinum) b. Manatee grass (Syringodium filiforme) c. Shoal grass (Halodule wrigh i) d. Paddle grass (Halophila decipiens) e. Widgeon grass (Ruppia mari ma).

Note: Common names for seagrasses can vary depending on the area in which they are found. Some of the names men oned in this guide are those used in Puerto Rico, while others are a direct transla on from their Spanish names.

Generally speaking, seagrass meadows in Puerto Rico have a ver cal zona on (spa al distribu on) in which shoalgrass predominates in the shallowest area, followed by turtle grass or a mixture of both seagrasses, then by manatee grass and lastly, and less frequently, by paddle grass in the deeper waters (Figure 7). Widgeon grass, on the other hand, lives in brackish or low-salinity waters and does not necessarily share spa al distribu on with the other seagrasses.

These five seagrass species present morphological differences such as the size and shape of their leaves, their flowers and their fruits. Their tolerance of salinity and depth also vary according to the place in which they develop.

Shoalgrass Halodule wrigh

Turtle grass

Thalassia testudinum liforme

Manatee grass Syringodium

Paddle grass

Halophila decipiens