RESPONSE OF MICROALGAL EPIPHYTE COMMUNITIES TO NITRATE ENRICHMENT IN AN EELGRASS (ZOSTERA-MARINA) MEADOW

The community structure and productivity of epiphytic microalgae on fi eld populations of eelgrass (Zostera marina L.) from a high flour regi me were characterized under water-column nitrate enrichment over a 30- d period during the autumn growing season for the macrophyte. Epiphyte communities in replicate low-nitrogen sites (LOW-N, median water-colu mn nitrate concentrations below detection) were compared to communitie s in replicate N-enriched sites wherein nitrate was leached from clay pots filled with enriched agar (N-ENRICH, median concentration ca. 6 m u M NO3-N; Pots replaced at 8- to 12-d intervals). In experimental cha mbers, total epiphyte community productivity as C-14-bicarbonate uptak e was determined from short-term (3-h) laboratory assays. Track light microscope-autoradiography enabled estimates of species-specific produ ctivity for abundant algal taxa. After 6 d in the LOW-N and N-ENRICH c ommunities, the crustose adnate red alga Sahlingia subintegra (Rosenvi nge) Kornmann was dominant in terms of cell number and codominant in b iovolume. Photosynthetic dinoflagellates, not previously reported as a bundant in eelgrass epiphyte communities, were dominant in biovolume c ontribution after both 6 and 30 d in LOW-N communities. Nitrate enrich ment stimulated the adnate monoraphid diatom Cocconeis placentula Ehr. but apparently inhibited dinoflagellates and the diatom Melosira sp. Total productivity of the epiphyte communities remained comparable in both the LOW-N and N-ENRICH sites. Shifts in community structure and s pecies-specific productivity, however, indicated a controlling influen ce of nitrate supply on microalgal epiphytes in the field eelgrass com munity.