Determining the environmental preferences of four estuarine epipelic diatom taxa: growth across a range of salinity, nitrate and ammonium conditions

Intertidal cohesive estuarine sediments frequently support extensive microp hytobenthic biofilms, which are usually dominated by epipelic (motile) diat oms. The distribution of different diatom taxa Varies along estuarine gradi ents, but it is not clear what the causative factors are in determining the se patterns. In many estuaries, salinity covaries with other environmental gradients, particularly with nutrient concentrations when the river is the main nutrient source. Autecological information is required to aid interpre tation of these field distributions. In this study the maximum specific gro wth rate (mu) and stationary phase biomass of four common taxa of estuarine epipelic diatoms (Navicula phyllepta, N. perminuta, N, salinarum and Cylin drotheca closterium) were measured in axenic unialgal cultures grown under a range of nitrogen and salinity regimes. All three Navicula species and tw o different clones of C. closterium showed significant differences in their response to a range of ammonium (10-4000 mu M) and nitrate (10-2000 mu M) concentrations across a salinity range of 10-35%. Growth rates of N. phylle pta were significantly lower at ammonium concentrations >400 mu M, but the other three taxa showed little evidence of ammonium toxicity, even at conce ntrations >1 mM. On the basis of the response of each species, the followin g putative ranking of preferences for inorganic nitrogen concentrations (lo west to highest, with salinity optima in parentheses) was constructed: N. p hyllepta (10-20%,) - N. perminuta (10-30%) - N. salinarum (20-35%)/C. clost erium (clone 1, 10-25%, done 2, 25-35%). Such a ranking should be interpret ed cautiously, as interclonal differences and adaptation to local environme ntal conditions can alter the preferences of taxa. These autecological data are partially supported by previously published data on field distribution , and suggest that certain taxa may have value as trophic indicator species .