Multpile Drivers of Hypoxia in a Central California Coastal Ecosystem Influence Fish Diversity and Estuarine Nursery Function

Along the west coast of the United States, some coastal ecosystems experience hypoxia from both land-based (nutrient loading) and ocean-based (upwelling) drivers. Here we report on drivers of hypoxia from both the land and sea and the climatic factors that regulate hypoxia severity by combining multiple data sets of water quality, climate, and fish that span > 40 years in an estuarine ecosystem in central California, Elkhorn Slough. The combination of these multiple hypoxia drivers ultimately had consequences for flatfish habitat quality and nursery function of the estuary. Remarkably, despite intense and increasing nutrient loading from a watershed characterized by agriculture, the estuary displayed periods of resilience. Variation in dissolved oxygen in the lower part of the estuary, where nutrient loading is most severe, was driven not by nutrient loading, but by annual intensity in upwelling. Yet, variation in dissolved oxygen conditions in the upper part of the estuary was driven by climate, specifically El Niño years increased precipitation which alleviated poor dissolved oxygen conditions, likely due to increased flushing of organic material, a product of eutrophication. Areas of the estuary behind water control structures were characterized by persistent hypoxia, and were devoid of flatfish. In fully tidal sites of the estuary, fish diversity and juvenile flatfish abundance were driven by variation in hypoxia. Finally, hypoxic conditions in the flatfish nursery correlated with lag effects to recruitment and fishery landings for a model flatfish species, English sole (Parophrys vetulus), where > 50% of adults in the offshore fishery have been documented to use Elkhorn Slough as a nursery. These results highlight the importance of understanding the drivers and consequences of hypoxia from multiple sources.