Elevated Springtime Macronutrient Concentrations near Freshwater Inputs of Humboldt Bay, California and their Effects on Primary Productivity

The number of coastal sites reporting hypoxia is slowly increasing each year as a consequence of eutrophication, characterized by excessive nutrients resulting in increased primary productivity. This research focuses on potential eutrophication in Humboldt Bay (HB), California, due to its several freshwater inputs subjected to agricultural runoff. This inflow of external nutrients may enhance the riverine concentrations of macronutrients. We hypothesized that macronutrient concentrations in HB near river mouths would be elevated above background levels during springtime high water flow and result in elevated primary productivity. Sampling occurred throughout HB during an ebb tide in April 2019 to assess nutrient concentrations (nitrate, nitrite, phosphate, ammonia and silicate), hydrographic parameters, chlorophyll concentration, density of phytoplankton cells, and phytoplankton community structure at each location. Station EB-1 (at the HB mouth) had the highest salinity, 1 psu greater than the highest value of any of the other stations within HB. Therefore, EB-1 serves as an ocean “reference location” to which we can compare HB conditions. The warmest (12.98-13.94℃) and least saline (30.74-32.10 psu) waters were observed in the North Bay, potentially indicating more significant freshwater influence. North Bay also had higher concentrations of chlorophyll and phytoplankton cells. Phytoplankton communities within North Bay were >90% similar to each other, and stations at the Elk River mouth (ERM) and within South Bay (SB-2) were <30% similar to other samples. Preliminary nutrient results show elevated silicate concentrations at station ERM, relative to station EB-1. These preliminary data suggests that North Bay experienced increased macronutrient concentrations from external sources that elevated primary productivity.