Elkhorn Slough is one of the most studied estuaries in California. It is also one of the most impaired. A year of monitoring data tells a complicated story.
Elkhorn Slough, a tidal estuary on the eastern shore of Monterey Bay, is one of the most intensively monitored estuaries in California. The Elkhorn Slough National Estuarine Research Reserve, established in 1979, maintains a network of water quality sensors that record salinity, temperature, dissolved oxygen, turbidity, and chlorophyll-a at fifteen-minute intervals. The data record extends back more than two decades.
The past year's data tells a story that is consistent with the longer record but contains some new elements. Dissolved oxygen levels in the upper slough — the portions farthest from the bay mouth, most influenced by agricultural drainage — continued to show seasonal hypoxic events, periods when oxygen levels drop below the threshold that most fish and invertebrates can tolerate. These events are not new. They have been documented since the monitoring program began.
What is new, or newly prominent, is the interaction between hypoxic events and the timing of otter foraging. Otters in the slough — a population of roughly 100 animals, one of the densest concentrations in California — have been observed shifting their foraging locations during hypoxic events, moving toward the bay mouth where oxygen levels are higher. The behavioral response is documented. Its long-term implications for otter condition and reproductive success are not yet quantified.
The primary driver of hypoxia in the upper slough is nutrient loading from agricultural drainage. The Elkhorn Slough watershed is dominated by strawberry and vegetable production, with high fertilizer inputs. Nitrate concentrations in drainage ditches feeding the slough regularly exceed state water quality standards. The pathway from fertilizer application to hypoxic event in the slough is well understood.
The Elkhorn Slough Foundation and the Reserve have been working with landowners in the watershed on nutrient management practices — cover cropping, reduced fertilizer application, constructed wetlands to intercept drainage. Progress has been made. The nutrient loading has not decreased to levels that would eliminate hypoxic events.
The slough is also experiencing the effects of sea level rise. Tidal prism — the volume of water exchanged between the slough and the bay with each tidal cycle — has increased as sea level has risen, which has in some ways improved flushing of the upper slough. The interaction between sea level rise, tidal dynamics, and nutrient loading is complex and not fully modeled.
A year of data does not resolve the long-term trajectory. It confirms that the slough is impaired, that the impairment is driven by identifiable and addressable causes, and that the otters living in it are adapting their behavior to conditions that are not optimal. Whether the adaptation is sufficient, and for how long, is a question the monitoring data will eventually answer.