What drives variability in nitrate concentration off the Oregon Coast?

Nitrate is a key nutrient for phytoplankton productivity. In general, phytoplankton uptake nutrients in the Redfield ratio (C:N:P = 106:16:1). Here we examine nitrate variability off the coast of Oregon, where wind-driven coastal upwelling brings cold, nutrient rich water to the surface, and the Columbia River brings fresh water into the offshore environment.

The data presented here was taken from the Ocean Observatories Initiative (OOI) Endurance Array. The OOI Endurance Array maintains surface moorings at three sites off central Oregon. The inshore mooring is in 25 m water depth and is approximately 2 km from the coast. The shelf mooring is in 80 m water depth and is approximately 18 km from the coast. The offshore mooring is in 475 m of water over the continental slope and is approximately 70 km from the coast.

All three moorings have Sea-Bird SUNA nitrate sensors 7 m beneath the surface buoy in combination with other measurements. Here we describe nitrate variability and its relationship to temperature, salinity, chlorophyll-a fluorescence, and wind stress. The data were hourly averaged and quality controlled. To see the spatial variability around the moorings, we also present satellite sea surface temperature and chlorophyll data from NOAA CoastWatch.

Nitrate was much higher at the inshore mooring, occasionally exceeding 30 μM. Here time series shows strong trends consistent with coastal upwelling. The satellite images confirm cold water along the coast during high nitrate events. Trends at the shelf site were similar to the inshore site, with lower nitrate peaks (20 μM). At the offshore sites, nitrate was much lower (<4 μM) and the connection to upwelling was much weaker. Some of the highest nitrate observed there was during a period of low salinity, consistent with the presence of the Columbia River plume.