BIOGEOCHEMICAL CAUSES OF FISH KILL EVENTS IN PESCADERO MARSH; SAN MATEO COUNTY, CALIFORNIA

Estuaries lie at the terrestrial-marine interface, linking freshwater and ocean systems. These transitional environments are influenced by many parameters such as temperature, oxygen concentration, and biologically mediated processes over multiple timescales, including daily tidal cycles, intra-annual seasonal changes, and inter-annual climatic oscillations. Despite the transient nature of the geochemical conditions of these systems, estuaries provide critical habitat for many specialized species. While many studies of biogeochemical cycling have been conducted on large estuary systems, relatively little is known in the smaller estuaries along the west coast.

Located 60 km south of San Francisco, Pescadero Marsh is an estuary on the central coast of California that is formed by the mouths of Pescadero and Butano creeks. During summer months, a sandbar forms across the mouth of Pescadero estuary, separating the salt marsh from the ocean and causing the saline lagoon to become brackish. Runoff associated with the first significant winter rain erodes the sandbar, reconnecting the estuary with the ocean. First observed in 1995, fish kills occur every year in association with the breaching of the sandbar. To develop an understanding of how biogeochemical cycling (especially of O, C, N, and P) is related to these fish kill events, in situ observations of dissolved oxygen, conductivity, temperature, and pH were collected at multiple times during the 2007 water year. Water samples collected during the same trips were used to analyze anion, cation, and carbon content. Preliminary results indicate that isolated deep-water zones within the estuary system are anoxic during the late summer months. These anoxic waters are likely mixed into the estuary during breaching events and contribute to deterioration in environmental conditions required by coho salmon, steel head trout, and tidewater goby. Decreases in salinity and increases in conductivity and pH were also observed in the months leading up to the breaching event. The results of our analyses will provide a basis for understanding the causes of the Pescadero Marsh fish kill events and help in the development of management strategies that will protect the organisms in this ecosystem.