Seagrasses are marine angiosperms (flowering plants) that have physiology adapted to aquatic environments allowing for pollination, seed formation and germination in water. During the day seagrasses absorb carbon dioxide and release oxygen to the water through a thin outer leaf membrane. A slower reverse process (respiration) occurs at night.
Right: Seagrass leaves covered with algal epiphytes. Photo: WPSP.
Air is transported to the roots via air canals, assisting survival in anaerobic muds. Being dependent on light for photosynthesis, seagrasses usually occur at depths from 2 – 12 metres. Most species are sub-tidal but some inter-tidal species have adapted to exposure to the air at low tide. Water quality is a major limiting factor, with turbid waters restricting light availability.
High levels of dissolved nutrients in the water can cause excessive algal growth on the surface of leaves that also interfere with photosynthesis. Sediment deposits and sand movements can also physically smother seagrasses.
Hence factors like catchment run-off into embayments and in-bay dredging can determine the sustainability of seagrass communities. Seagrasses can form large mats or banks of plants (meadows) that effectively stabilize marine substrates and can influence local water quality and chemistry. Where seagrass losses occur, as in Western Port, muddy sediments are mobilized causing deteriorating water quality, flattened profiles and loss of channel profiles.
Seagrasses act as a nursery and refuge for many small marine organisms, including seahorses and juvenile whiting.
The leaves are home for algae and many small organisms called epiphytes. These epiphytes are a major food source for shellfish, crustaceans and fish. A few animals like garfish, leatherjackets and black swans are adapted to eating and digesting the tough seagrass leaves.
Seagrasses generate considerable standing biomass and represent sizable nutrient pools. Storms and autumn leaf drop results in large accumulations of detritus along shorelines. This detritus eventually breaks down and provides food and a steady release of nutrients into the water column and sediments.