Effects of variable irradiance on phytoplankton productivity in shallow estuaries

Abstract

Primary productivity in estuaries is profoundly affected by water‐column mixing which can be attributed to currents, tides, and wind. Mixing in shallow systems has the dual effect of increasing light attenuation in the water column through resuspension of particulate matter, while allowing entrained phytoplankton cells to experience a constantly varying light regime. We investigated the latter effect during a 7‐month period in the lower Neuse River, North Carolina, a well‐mixed, coastal plain estuary. A rotating light‐field simulator was devised to assess phytoplankton productivity while considering the problems of mixing and light attenuation. This technique was compared to a fixed‐light level (static) incubation method. We found that constantly varying irradiance stimulated phytoplankton productivity by both reducing photoinhibition and mitigating the growth‐limiting effects of turbidity‐induced light limitation found under static incubation conditions. Our results demonstrate that static incubations may, at times, significantly underestimate true phytoplankton productivity in shallow, well‐mixed aquatic systems.