DISTRIBUTION OF ESTUARINE BENTHIC DIATOM SPECIES ALONG SALINITY AND NUTRIENT GRADIENTS

Changes in relative abundance of estuarine epipelic microalgae along a n estuarine gradient were investigated, and population densities of di fferent species along a small-scale nutrient gradient generated by a s ewage treatment outfall and in laboratory mesocosms enriched with ammo nium were also studied. The relative abundance of certain species of e pipelic diatoms was related to location along the estuarine salinity a nd nutrient gradient: Navicula gregaria and N, phyllepta were abundant at oligo- and mesohaline sites respectively, and Pleurosigma angulatu m and Plagiotropis vitrea were abundant at polyhaline sites. On a smal ler spatial scale, though there were no significant patterns in microa lgal biomass in relation to nutrient enrichment, there were significan t differences in the population densities of different epipelic specie s along a multivariate nutrient gradient (decreasing concentrations of ammonium, nitrite, silicate, organic content, and increasing salinity and pore-water nitrate concentrations) away from a sewage outfall alo ng a saltmarsh creek. The diatoms Nitzschia sigma and Gyrosigma limosu m and the cyanobacteria Oscillatoria limosa and O. princeps had signif icantly higher population densities near the outfall, and Navicula phy llepta, N. pargemina, Nitzschia frustulum, Cylindrotheca signata and P leurosigma angulatum were significantly more abundant at the seaward e nd of the gradient. In laboratory tidal mesocosms, sediment cores from along the gradient had their pore-water ammonium concentrations incre ased to 380-450 mu M NH4+. After 26 days, the population densities of Gyrosigma fasciola, G. littorale, P. angulatum, N. phyllepta, Cylindro theca signata, C. closterium and Niztschia apiculata were significantl y reduced, while those of G. limosum, N. sigma, Scolioneis tumida and O. limosa were unaffected, or were significantly higher compared with control cores. Laboratory manipulations of ammonium concentrations sup ported the observed field distributions, indicating that epipelic spec ies do have different trophic preferences and ammonium concentration m ay be a significant factor in determining estuarine species compositio n of epipelic algae.