ABSTRACTThe community structure and productivity of epiphytic microalgae on field populations of eelgrass(Zostera marinaL.) from a high flow regime 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‐column nitrate concentrations below detection) were compared to communities in replicate N‐enriched sites wherein nitrate was leached from clay pots filled with enriched agar(N‐ENRICH,median concentration ca. 6 μM NO3−‐N; pots replaced at 8– to 12–d intervals). In experimental chambers, total epiphyte community productivity as14C‐bicarbonate uptake was determined from short‐term (3–h) laboratory assays. Track light microscope‐autoradiography enabled estimates of species‐specific productivity for abundant algal taxa. After 6 d in theLOW‐NandN‐ENRICHcommunities, the crustose adnate red algaSahlingia subintegra (Rosenvinge) Kornmann was dominant in terms of cell number and codominant in biovolume. Photosynthetic dinoflagellates, not previously reported as abundant in eelgrass epiphyte communities, were dominant in biovolume contribution after both 6 and 30 d inLOW‐Ncommunities. Nitrate enrichment stimulated the adnate monoraphid diatomCocconeis placentulaEhr. but apparently inhibited dinoflagellates and the diatomMelosirasp. Total productivity of the epiphyte communities remained comparable in both theLOW‐NandN‐ENRICHsites. Shifts in community structure and species‐specific productivity, however, indicated a controlling influence of nitrate supply on microalg