SEAGRASS BEDS: CHARACTERISTICS, FACTORS, FLORA AND FAUNA - BIOLOGY - 2024

The seagrass or seagrasses are flowering formations (flowering plants) that live completely submerged in seawater. They are also known as marine grasslands, a name that is due to the fact that they resemble terrestrial grasslands.

These environments are considered one of the most productive ecosystems on earth. They grow on sandy and sandy bottoms. The most common and abundant species belong to the genus Zostera, species known as eelgrass.

Thalassia meadow. Taken and edited from Dry Tortugas NPS, via Wikimedia Commons

Other species of seagrass include Thalassia testudinum (North Atlantic), Possidonia oceanica (Mediterranean) or Ruppia maritima (South Atlantic). Seagrasses host a great diversity of macroalgae. Some of these macroalgae are seasonal, others are permanent residents of the phanerogam meadows.

There is also a complex community of animals with which various associations are established. Some species live buried among plant rhizomes, others live attached to leaves, and others simply roam between or on plants.

characteristics

Seagrasses are made up of higher plants. They have organs and tissues similar to other flowering plants. In almost all of them an upper and a lower part can be differentiated.

The lower part of the plant is made up of roots and rhizomes and the upper part of shoots with several leaves. The flowers are generally very small in size.

Some seeds may have a thick protective coating and are dormant. Others have thin protective covers and do not have dormancy.

They generally develop below 10 meters in depth.

Taxonomic aspects

The term seagrass was used in the scientific literature for the first time in 1871. This term defines an ecological group and lacks taxonomic validity. All the plants that are part of the seagrass belong to the monocots.

Seagrasses belong to four families. The families Zosteraceae, Cymodoceaceae, and Posidoniaceae are represented only by marine species. The Hydrocharitaceae family is made up of 17 genera, of which only 3 are considered seagrasses.

Abiotic and biotic factors

Abiotic factors

Abiotic factors are the non-living components of an ecosystem. Among the factors that determine seagrass beds are:

Temperature

Seagrass meadows are distributed in warm and temperate waters. They are not found in polar waters. They tolerate large temperature variations because many of them must withstand periods of desiccation during low tides.

The seeds of some species can also withstand desiccation.

Light

Seagrasses require abundant light for photosynthesis. Due to this, they must be located in waters with low turbidity.

Depth

Seagrasses have higher light requirements than algae. Due to this, they are almost restricted to waters with depths less than ten meters.

Only two species can be found at greater depths, Halophila decipiens and Thalassodendron ciliatum, which can live deeper than 50 meters.

Salinity

In general, seagrasses are euryhaline, which means that they tolerate large ranges of salinity. However, tolerance levels to this factor vary depending on the species.

Species of the genus Thalassia, for example, live in environments with salinities between 35 and 45 ups (practical units of salinity). Posidonia tolerates broader ranges (35 - 55 ups). Ruppia, on the other hand, can live both in hypersaline lagoons and in permanently fresh waters.

Substratum

Seagrass meadows develop on sandy or muddy substrates. They require this type of substrate to be able to take root. Additionally, these grasses help stabilize the substrate and are soil-forming. A single genus (Phyllospadix) inhabits rocky shorelines.

Biotic factors

Biotic factors are the living factors of the ecosystem. They are represented by flora, fauna (in its broad sense) and microorganisms.

The flora is made up of algae, phanerogams and even fungi. The fauna is represented by both invertebrates and vertebrates.

Algae

They are abundant and diverse in seagrass beds. They can grow on grass, substrate, or on rocks that are always scattered in the meadows. Ulvaceae are common among green algae.

There are also algae of the genera Codium and Acetabularia, among others. Common among brown algae are the genera Padina, Dyctiota, and Sargassum. Additionally, some species of red algae are common.

Phanerogams

They are the main component of this type of ecosystem. They are the forming plants of grasslands.

Depending on the geographical location there will be phanerogams of different species. Thalassia, Halophila, Syringodium, and Halodule have species in the Atlantic and Indo-Pacific. Zostera and Posidonia, meanwhile, are distributed on all the temperate beaches of the world.

Zostera is the most diverse and widely distributed genus. It has about 60 described species and is found in both the northern and southern hemispheres.

Microorganisms

A complex community of microorganisms is found in seagrass soils that break down organic matter in sediments.

Sulfate-using bacteria predominate under anoxic conditions. However, there are also species that use iron and manganese.

Invertebrates

Species of different taxa inhabit buried among the rhizoids of the seagrasses. These make up the so-called infauna. Among them are some species of bivalve mollusks.

Nematodes and polychaetes are also frequent. Crustaceans are represented by stomatopods and some species of crabs and shrimp.

Other species develop on the leaves of the phanerogams. They are known as epibionts. Sponges, anemones, sea squirts and nudibranchs predominate among them.

The epifauna that lives free on the substrate and between the leaves of the grasses is the most diverse. Gastropods predominate among mollusks. Echinoderms are represented by sea cucumbers, some species of stars, starfish and by sea urchins.

Sea star Oreaster reticulatus in seagrass meadows. Taken and edited from Jstuby at en.wikipedia, from Wikimedia Commons.

Crustaceans are diverse, ranging from small isopods and amphipods, to lobsters, crabs, hermit crabs and shrimp.

The rocks that are scattered in the prairies are also strongly colonized by invertebrates, such as sponges, sea squirts, polychaetes, among others.

Vertebrates

They are dominated by fish, some of them benthic, such as toad fish, and other pelagic fish that come to the meadow in search of food.

Zostera is known as eel grass, as these fish spend part of their lives in these environments. Thalassia testudinum is known as turtle grass, as it serves as food for sea turtles.

References

1. M. Díaz-Piferrer (1972). The higher algae and seagrasses. In: J. Castelvi (Ed.), Marine Ecology. La Salle Foundation for Natural Sciences. Editorial Dossat.
2. P. Castro & ME Huber (2003). Marine Biology. 4th Edition, McGraw-Hill Co.
3. C. den Hartog & J. Kuo (2006). Taxonomy and biogeography of seagrasses. In: AWD Larkum, RJ Orth & CM Duarte. Seagrasses: Biology, Ecology and Conservation. Springer.
4. J. Kuo & C. den Hartog (2006). Seagrass morphology, anatomy, and ultrastructure. In: AWD Larkum, RJ Orth & CM Duarte. Seagrasses: Biology, Ecology and Conservation. Springer.
5. C. Lira (1997). Thalassia Prairie Fauna. In. MARNR. Basic Atlas of the Nueva Esparta State. Special Edition for the VII Summit of Heads of State and Government. Margarita Island.
6. R. Bitter (1993). Structure and function of the Thalassia field as an ecosystem. Ecotropic.