COMMUNITY STRUCTURE AND PRODUCTIVITY OF EPIPHYTIC MICROALGAE ON EELGRASS (ZOSTERA-MARINA L) UNDER WATER-COLUMN NITRATE ENRICHMENT

In experimental mesocosms the community structure, total community pro ductivity, an species-specific productivity of loosely attached epiphy tic microalgae were compared across a gradient of water-column nitrate enrichment during the fall season of high microalgal growth. Eelgrass (Zostera marina L.) and its epiphytes were maintained for 6 wk under low water exchange (10% day-1) to simulate conditions in poorly flushe d lagoons and upper embayments, in control (ambient nitrate at < 1 muM NO3- N) and treated mesocosms (enriched with pulsed daily water-colum n additions to 3.5 muM and 7.0 muM NO3- N, respectively, for low [LOW] and moderate [MOD] treatments). In experimental chambers productivity was estimated as C-14-bicarbonate uptake for the total epiphyte commu nity from liquid scintillation counts, and for abundant microalgal spe cies using track light microscope-autoradiography. After 3 wk MOD nitr ate enrichment stimulated total epiphyte community productivity, mostl y as blue-green algae, whereas LOW enrichment was insufficient to caus e a stimulatory effect. After 6 wk, however, community productivity wa s enhanced in both LOW and MOD enrichment, mostly contributed by small diatoms such as Nitzschia closterium (biovolume 70-560 mum3). At the population level, the response of larger diatom taxa such as Nitzschia longissima and Bacillaria paradoxa (biovolume > 3000 mum3) varied dep ending on the degree of nitrate enrichment. The data indicate a contro lling influence of nitrate supply on community structure and species d ominance of the microalgal epiphytes.