Phytoplankton production at the oligohaline station NAV appears to be limited principally by light. There is no generally accepted bioassay procedure to confirm light limitation. Light is constrained at NAV due to the inputs from blackwater tributaries as well as particulate turbidity. The river at NAV is deep and well mixed, likely keeping phytoplankton cells under aphotic conditions for extended periods. Farther downstream, the mesohaline area near Channel Marker 54 is a transition zone between light and nutrient limitation.
Meteorology plays an important role in the regulation of phytoplankton productivity in mid-estuary.
Upstream rainfall and consequent watershed runoff increase loading of nutrients, turbidity, and water color, leading to stronger light attenuation and consequent light limitation. Dry, low flow conditions lead to decreased nutrients, turbidity and light attenuation, and increased phytoplankton biomass, with phytoplankton production becoming nutrient limited. In the upper to middle estuary, winter nutrient levels are high, light penetration is poor in the turbid, highly colored waters, and chlorophyll a levels are low. Dredging and periodic rainfall and runoff episodes add turbidity pulses to the estuary, driving the system to periodic light limitation. This is most prevalent in winter, with permitted channel dredging activity and increased flow to the lower estuary because of reduced upstream evapotranspiration in cold weather.