Variable flow effects on phyto- and zooplankton at the river-estuary transition in the San Joaquin River, CA

Background/Question/Methods

As part of a larger study investigating algal dynamics in the tidally-influenced San Joaquin River (SJR) of the Sacramento – San Joaquin Delta Estuary, algal and zooplankton abundances were analyzed during periods of zero, moderate, and high net flows into the Stockton Deep Water Ship Channel (DWSC) from June 2007 through June 2008.  High flow conditions resulted from reservoir releases or the installation of the Head of Old River Barrier (HORB) during October and early November 2007, which provides increased SJR flow to enhance fish habitat.  The purpose of this study is to elucidate planktonic behavior subject to flow and seasonal variation.  The study reach consisted of a 25-mile segment of the SJR including the dredged DWSC, which was sampled at approximately two mile intervals over two high and low slack tides.  Monitoring was performed at intervals ranging from one to five weeks.  Chlorophyll a and pheophytin a were quantified as surrogates of phytoplankton abundance and physiological condition.  Zooplankton population density and biomass were determined by microscopic examination of mid-depth collected samples.

Results/Conclusions

Spatial clusters of high zooplankton populations are strongly influenced by flow, with higher flows pushing zooplankton further downstream into the forty-foot deep DWSC; lower flows allow populations to develop further upstream in the shallower reaches of the SJR.  Under moderate flows zooplankton populations develop in an intermediate location, where the shallow SJR enters the deeper DWSC.  High flow drove zooplankton density into the DWSC during fall 2007, where they remained until June 2008, once flows subsided.  During net zero flows of July and August 2007, zooplankton proliferated upstream, with algal concentrations declining drastically as zooplankton concentrations increased.  Throughout the study reach in general, algal and zooplankton populations were negatively correlated under moderate flows and warmer weather.  With higher flows, which generally occurred during cooler months, population abundance of both algae and zooplankton were low with no clear abundance pattern.  The highest algal concentrations were observed in the uppermost study reach due to favorable light conditions associated with shallow river depths and low zooplankton populations.  Spatial distributions of algae exhibit a large decline near the SJR/DWSC transition, the specific location being flow dependent.  Algal loss at this transition is likely caused by zooplankton grazing and/or depth-induced light-reduction.