Biomass variation and autotrophic production of an epiphyte-macrophyte community in a coastal Danish area: II. Epiphyte species composition, biomass and production

A study done at Hvidøre in the Øresund, Denmark, in 1979 showed that algae and bacteria occurred on eelgrass leaves throughout the year. Algal species composition, biomass and productivity varied markedly with season, depending on both environmental conditions and growth pattern of the host macrophyte. Bacteria and the diatom Cocconeis scutellum Ehrenberg were pioneer colonizers on young eelgrass leaves. With increasing leaf age several algal species appeared and, in total, 24 algal species were recorded. Species diversity reached a maximum in spring when green and brown macroalgae dominated, while during summer and autumn, microalgae and bacteria dominated. Epiphyte biomass and productivity followed a bimodal pattern with maximum values in April-May and small, secondary maxima in August. The life time of eelgrass leaves ranged between 50 days (summer) and 200 days (winter) and, thus, exposure of the leaf substratum to epiphyte colonization and biomass accumulation varied substantially with season. Optimal environmental conditions to algal growth, reflected by high species diversity and productivity, occurred in early spring when the ice-cover disappeared and light conditions improved. During summer, epiphyte productivity was suppressed, most likely, due to limitation in nutrient availability. Grazing by herbivorous invertebrates was not quantified, but the results suggest that during late summer and autumn, grazing is an important biomass-regulating factor, which needs further study. The annual epiphyte production was 70 g C • m^-2 of eelgrass bed and it contributed 8% to combined eelgrass and epiphyte production. Light attenuation by epiphyte layers occasionally exceeded 90% on old leaves, but on an annual basis it averaged less than 10% and did not appear to affect eelgrass production.