Monterey Bay Aquarium Research Institute
Marine Botany
SEAGRASS ECOSYSTEMS Seagrass meadows are important ecosystems (Dawes, p.481):
  1. Contribution to primary productivity of oceans via photosynthesis.
  2. Efficient in removing nutrients from water column and replenishing nutrients and oxygen in soil via roots.
  3. Highly specialized root system traps sediments, improving water clarity. Also, important in bottom stabilization: There has been a lot of recent interest and research in their transplantation to coastal areas prone to erosion. Seagrasses can also function as a natural sewage filtration. A case was reported in Australia where the removal of an eelgrass meadow resulted in the poisoning of the benthic biota that had been previously unexposed to the raw sewage by the shelter of the dense vegetation (Phillips & McRoy, 300).
  4. Seagrasses are a direct food source to many herbivores, (ie. Strongylocentrotus purpuratus: sea urchin) and are also indirectly at the base of many food webs. In one study, over 340 animals analyzed showed some level of seagrass consumption (Fry and Parker, 1979).
  5. Dense vegetation offers shelter and different habitats to a myriad of organisms by creating a calm and shaded micro-environment. Seagrass beds serve as permanent and seasonal homes or forage arenas for many species of fish. The plants' protected bases are home for juvenile lobsters in southern California. The need for the conservation of seagrass meadows was not fully recognized until one study showed that a decline in an eelgrass bed was followed by a sharp decline in the numbers of juvenile and young adult stages of many commercially important fish (Dawson & Foster).
  6. Vegetative mass of the seagrass provides an abundant substrata for attachment, especially for algal epiphytes which themselves contribute to the primary productivity and are food for many small fish and invertebrates. Often seagrasses appear purplish or pink, this is due to the profuse growth of two of its most common obligate epiphytes: Smithora naiadum, a fleshy Bangiale and Melobesia, a crustous red algae (Dawson, p.178). Colonies of the bryozoan genus Membranipora are also common dwellers on the blades of seagrass. In the intertidal environment where space is a limiting factor, the importance of seagrass in providing substrata for other organisms to live on cannot be overstated. All parts of the plants are maximized by other algae and animals: the leaves, stems, rhizomes, and the leafage cover (Phillips and McRoy).

The Surfers

Living in water, especially where the waves are breaking with immense force, is not easy. Some waves are so powerful that they can crash a wooden boat and smash it to pieces, but many marine organisms are able to survive in the surf-zone.As marine angiosperms, sea grasses successfully fulfill the requirements for life at sea outlined by den Hartog (1970, p.12):
  1. They are adapted to life in a saline medium. Osmoregulation is achieved by specialized epidermal cells and the leaves have no need for stomates, as opposed to terrestrial plants, because all of the gas-exchange is accomplished through the epidermis.
  2. Seagrasses have the ability to grow when completely submerged.
  3. Surf grass have a successful anchoring system to withstand tidal currents and moderate wave action.
  4. Seagrasses are able to reproduce in an aquatic medium. This adaptation called hydrophilly, which is unique to aquatic plants, allows the surf grass to perform both surface and completely submerged pollination.
The leaves of surf grasses are especially adapted to living in a turbid aquatic medium.

As mentioned above, they have no stomates that open to the exterior so the leaves have no holes that might weaken them. The interior of the leaves is composed of specialized parenchymous tissue called aerenchyma that has regularly arranged air spaces or lacunae. These internal air spaces serve for flotation and gas exchange purposes (Arber, p.187). Furthermore, the fully aquatic angiosperms have chloroplasts in the leaf epidermis while terrestrial plants only have chloroplasts inside the leaf tissue.

The leaves are flat and supple.

The reduction of vascular bundles and the absence of lignification (woody or cork material) allows the leaves to remain erect in strong water action. The natives of the Pacific coast used to weave baskets with seagrass because it is so flexible and stiffens when dries. It also resists rotting and for this same reason it was used as stuffing material in the former U.S.S.R. (Phillips & McRoy, p.301).

The leaves are also very strong.

If you pull on a leaf of Phyllospadix, you will feel a lot of resistance. The stress force for a Phyllospadix leaf blade was measured to see how much force the blade could withstand before breaking. The expected tensile strength for macroalgae is between 0.7-10 MN/meter sq. (Denny et al, 1989) and that of Phyllospadix was calculated to be 10.2 which is highly beneficial in the turbulent zone where the plant grows.

A 1988 study by McRoy & Cooper described the anatomical adaptations that allow Phyllospadix to thrive on rocky substrates in the surf exposed zone in comparison to Zostera marina, more commonly found in sheltered areas of the coast. Phyllospadix shows significantly more root hair growth than Zostera (another closely related sea grass) which might provide extra attachment force to hold on to rocks in turbid water. The roots and rhizomes of Phyllospadix also have thicker outer epidermal walls, making it more able to withstand strong wave force. The lacunae (internal air-spaces) are reduced because the plants live in a highly oxic (oxygen-rich), well-mixed environment.

As would be expected for a plant that needs to be adapted to water motion in a turbulent surf-zone, Phyllospadix shows more flexible (non-lignified hypodermal) leaf tissues than does Zostera.

Phyllospadix pages copyright Erika Marin-Spiotta 1996.


AuthorErika Marín-Spiotta1996. The legal stuff. You gotta love the weeds. Check out the rest of the Hopkins algae pages. Gracias to people.
Last updated: Feb. 05, 2009