Report of base line distribution survey of Zostera and Ruppia in the Fleet. Whilst aquatic birds (including swans) may depend upon these grasses as a food resource, hence their importance, the populations of Zostera and Ruppia in the Fleet are of considerable significance in their own right. Such extensive mixed populations are virtually unknown from anywhere else in Britain [cf. recent surveys of Loch Maddy & other W. Isles/Orkney/Shetland lagoon sites]

Survey must be done in late July/August when weather & water clarity conditions are best, Zostera and Ruppia are growing healthily and at their greatest biomass, and flowers and fruit are at their most obvious to assist in identification, particularly of Ruppia.

The Abbotsbury basin was not surveyed rigorously, but appeared very sparsely colonised, and limited to Ruppia only. Algae such as Ulva, Cladophora and Chaetomorpha were abundant. Cladophoralean algae dominated the mud surface down to Rodden Hive Point, with their frequency diminishing to the east of this zone as Zostera spp. became more dominant. Rodden Hive is the most westerly point of the Fleet where a diverse flowering plant community was observed, and where the combined cover of these species far exceeded that of algae. Below Lynch Cove and the Narrows no aquatic flowering plants were recorded. The upper and lower limits of distribution of Zostera and Ruppia were the same in 1983 as in 1968 (in Whittaker 1968). The report contains lengthy discussion as to possible changes in seagrass and algal populations over time, and possible mechanisms for such changes, as well as recommendations for future work. Concerns expressed by local residents about decline in seagrass are mentioned, and attributed tentatively to the wasting disease of the 1930=s resulting in changes in species mix as well as decline in populations, and to subsequent declines in densities/extent due to hard winters. A recent increase in algae is reported, tentatively attributed to increases in nutrient inputs from domestic and agricultural sources, or to increases in the swan population over the previous decade.

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