Effects of suspended sediment and colored dissolved organic matter on response of phytoplankton to nutrient loading in an urban, freshwater estuary

Background/Question/Methods

The availability of light and nutrients are important controls on the structure and function of shallow aquatic ecosystems.  In contrast to shallow lakes, where nutrient loading fuels phytoplankton growth which in turn diminishes light availability, coastal rivers may have high concentrations of suspended sediments or colored dissolved organic matter (CDOM) that diminish light and constrain the potential for phytoplankton production to respond to nutrient loading.  The St. Louis River Estuary, on the western edge of Lake Superior, is characterized by high spatial heterogeneity of nutrient inputs, CDOM, and suspended sediment.  We report the results of a series of field observations and bottle incubation experiments that quantify the effects of CDOM and suspended sediment concentrations on the response of phytoplankton to nutrient loading. 

Results/Conclusions

Preliminary results show that gross primary production (GPP) rates are related to both chlorophyll a and pycocyanin concentrations, and are highest at the upstream-most sites within the estuary, which have a relatively low suspended sediment load but have high CDOM concentrations.  The station with the highest suspended sediment load and the highest light extinction coefficient (~10 m^-1) had intermediate GPP levels.  The lowest observed GPP levels were from stations in the industrialized lower estuary, which also receive the highest rates of nutrient loading from urban runoff and a regional wastewater treatment plant.  These results suggest that, despite low light availability due to CDOM and suspended sediments, other factors are limiting GPP in this estuary, and there is a limited capacity for phytoplankton production within the estuary to remove anthropogenic nutrients from the lower estuary.  These results will be combined with a 3-dimensional hydrodynamic model of the estuary to produce a better understanding of the capacity of this ecosystem to process anthropogenic nutrients.