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The effect of irradiance on estuarine phytoplankton productivity and nitrogen uptake
This study measured the effect of photosynthetically active radiation (PAR) on carbon, ammonium, and nitrate uptake by natural assemblages of phytoplankton from the northern San Francisco Estuary (nSFE). The first part of the study was conducted in the fall of 2017 along a downstream transect in the low salinity zone — characterized by increasing turbidity and decreasing PAR. The second part was done in 2018 at one station (Rio Vista) from the transect. Rates were measured using stable isotope tracers over 4 or 24 hours at six or eight light levels. Carbon and NO3' uptake mostly showed a hyperbolic response to PAR, whereas NH4+ uptake showed little response to increased PAR. All uptake rates were maximal at low irradiances. These results provide direct measurements of primary productivity and nitrogen uptake for the SFE and show that light is not as important for phytoplankton growing on NHC as it is when they are growing on NO3'. This study is relevant to aquatic scientists by helping to explain why the turbid SFE does not exhibit the effects of eutrophication and shows that light is not as important of a constraining factor on these phytoplankton as previously thought.