Phytoplankton and zooplankton species richness along a tidal estuarine gradient

Background/Question/Methods

Understanding patterns of species richness along gradients of environmental factors (e.g. latitude, altitude, water depth) is a major focus in much ecological research.  In estuaries and transitional waters the salinity gradient formed as riverine water transitions to marine water strongly influences the distribution of species and subsequently the level of diversity observed.  In general, species richness is higher at freshwater and marine salinities and lowest at intermediate levels.  This species minimum model of Remane (1934), typically referred to as the artenminimum, has been subsequently observed in multiple systems.  This model has recently been challenged to describe the distribution of plankton diversity, with highest protistan species richness reported at intermediate salinities.  Additionally, further studies have suggested that the artenminimum pattern observed may not be due to absolute salinity levels, but a gradient in the range of salinities observed at different sites.  Using 20 years of phytoplankton and zooplankton composition, primary productivity and water quality data, collected from fixed stations along a salinity gradient in Chesapeake Bay, several relationships were identified. 

Results/Conclusions

Plankton diversity was high, with over 1000 phytoplankton and over 500 mesozooplankton taxa identified.  Several water quality parameters were highly correlated with salinity, most notably total nitrogen (p<0.000, R=-.983), total phosphorus (p=0.026, R=.729) and secchi depth (p=0.001, R=.917).  Chlorophyll, productivity, and phytoplankton/zooplankton abundance generally had a unimodal distribution peaking at mesohaline salinities.  Phytoplankton species richness, calculated as the total number of species observed per station per year was minimal within the mesohaline with increased diversity at fresher and more saline stations, resulting in a significant unimodal (quadratic) relationship with both average salinity (p<0.000, R²=.546) and total nitrogen (p<0.000, R²=.612).  There was a significant positive correlation between phytoplankton species richness and zooplankton species richness (p<0.000, R=.826), with zooplankton diversity also minimal at intermediate salinities.  Range per station per year in salinity and total nitrogen had no significant correlation with either phytoplankton or zooplankton species richness.  These results provide further evidence for the artenminimum model in planktonic organisms in a large tidal estuary and illustrate the confounding factors (ie. nutrient concentrations) that vary along salinity gradients.  Additionally, the results show that within the confines of this study, phytoplankton richness and productivity are inversely related when examined spatially in an estuary.