Tropical sea-grasses

Tropical sea-grasses

Marianne Holmer, Ph.D., Institute of Biology, University of Southern  Denmark

The distributionrange of tropical sea-grasses and their interaction with carbon and nutrientnutritive salt pools in carbonate sediments

This project resumes a line of research from the earlier Galathea expeditions by focusing on the biological di versity of the Caribbean. Sea-grass meadows are highly productive and function as habitat and hiding place of a large number of organisms, including juvenile fish fry that later migrate to coral reefs and open oceans. The focus of this project will be on the distributionrange and population dynamics of sea grasses in relation to the access to nutritive saltsnutrient availability. In tropical regions, the  access to nutritive saltsnutrient availability is often low, and this applies in particular to phosphor, which is bound hard in the carbonate-containing seabedsediment. Unlike other plants (seaweeds, plankton), the sea grasses benefit from being anchored with their roots into  the seabed, from which they are able to take upderive nutritive saltsnutrients. The  diversityrange of sea grasses is, however, therefore often far greater in tropical waters compared to temperate waters. We still have limited knowledge of the factors that allowenable sea grasses to release phosphate  the bound in the sedimentsphosphor, and to throw light on this we will studyshall research the underlying mechanisms for the phosphor dynamics in sea grass beds. Sea grasses are able to release oxygen to the sediment, whereby sulphide is oxidizedseabed, which produces sulphide-oxidation. This acid-generating process will cause the dissolution of carbonates, and thus the release of phosphate, which can then subsequently be taken upabsorbed by the plants. Another important purpose of this study is to studyresearch  the tolerance limits of sea grasses to  increasingrising concentrations of nutritive salts nutrients and sulphides in the seabedsediments. We expect the highly diverse tropical sea-grasses to show different tolerance levels, and believe that, for instance,  regime shifts may occur due to shifts in regime caused by increased levels of nutrients salts. Changes to the sea-grass community may significantlyacquire major significance for affect the structure and function of the  coastal ecosystems close to the coasts, which may also influence life in the oceans, for instance through changes to the living conditions for juvenile fish in seagrass meadows. to emerging fry, parr and smolt of fish.