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Capturing the Impact of Riverine Nutrient Delivery on Coastal Ocean Biogeochemistry


Rivers smaller than the Amazon tend to be excluded from earth system modeling efforts. Does it matter? Do sub-grid-scale rivers have significant impacts on offshore primary productivity? Using the Eel River in northern California, the river with the largest sediment yield per drainage area in the continental United States, as a test case, this question is explored using two approaches. First, a data-driven analysis of relevant time series taken on land, by buoy, and from space, demonstrates very little evidence of direct impact of Eel River discharge on contemporaneous coastal ocean primary productivity - but to the extent that that evidence exists, it seems to occur during years of greatest river discharge. To further analyze mechanistic drivers, a coupled mesoscale modeling framework unifying ocean, watershed and atmospheric representations is formulated and run in hindcast over the 2002-2010 period. Monthly average climatologies, interannual variabilities, and event-driven analysis of each year's largest river discharge are all examined for evidence of a river-ocean connection expressed through primary production. Storm event-generated turbulence appears to dominate the primary productivity during the winter months. The impact of the river seems to be largely independent of nutrient load, because its dissolved nitrate is less than that of the coastal ocean. There is no evidence that riverine delivery of gradually bioavailable detritus has a significant effect. Although a sufficiently super-nitrous river shows the ability to sustain a plume-nutrient-driven-bloom even at periods of extremely low flow, this is not currently a realistic scenario for the Eel River. The possibility remains that another micronutrient not studied in the modeling framework, such as iron, could be important to this system.

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