What is killing fish in the Pescadero Marsh?
Studies indicate that during long periods of sandbar closure when waters start to back up and jump low levees and flow through rusted open tidal gates within the Pescadero Marsh, a deadly natural process begins to happen. This natural process causes some waters within the Pescadero Marsh to become anoxic. Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. Anoxic conditions result from several factors; for example, stagnation conditions, density stratification, inputs of organic material, and strong thermoclines; all of which are dangers present in the Pescadero Marsh. When oxygen is depleted, bacteria first turn to the second-best electron acceptor, which in sea water is nitrate. Denitrification occurs, then after reducing some other minor elements, the bacteria will turn its attention to reducing sulfate. The bacterial production of sulfide starts in the sediments, where the bacteria find suitable substrates, and then expands into the water column. This is what causes that strong “rotten egg” odor emanating from the marsh. It is so strong, that you can smell it as you drive along the marsh on Hwy 1 and along Pescadero Road. If the wind is right it could be smelled all the way into the town of Pescadero, which is [two] miles inland. When the sandbar breaks and these conditions exist all that anoxic water starts to funnel out into the ocean and when the bottom matter is stirred up it creates a very toxic environment. The fish are suffocated and die immediately.
According to geologic record, anoxic events happening in the past may have caused mass extinctions.
Studies have also shown that when State Parks projects were done in the 1990’s, it changed the natural way the sandbar opens and closes. Many levees were altered and even built by State Parks. They also installed a series of tidal gates allowing salt water to enter the north pond, which after only one year became inoperable because of rust. The tidal gates were never fixed and are still inoperable to this day as they remain rusted open. These modifications created new areas for water to fill within the system and have contributed to the anoxic conditions and have changed the natural opening and closing of the sandbar.
Below is a report of the 2007 studies made by the Geological Society of America (GSA)
These studies were done 4 years ago and still nothing has been done to fix it.
You can read the full report on their website here Geological Society of America (GSA)
BIOGEOCHEMICAL CAUSES OF FISH KILL EVENTS IN PESCADERO MARSH
SAN MATEO COUNTY, CALIFORNIA
BRADSHAW, Samantha 1, ALLEN, Christin M.2, LEROY, Sverre L.2, FAUL, Kristina L.2, and RADEMACHER, Laura K.1, (1) Dept of Geosciences, Univ of the Pacific, 3601 Pacific Ave, Stockton, CA 95211, email@example.com, (2) Dept of Chemistry and Physics, Mills College, 5000 MacArthur Blvd, Oakland, CA 94613
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.
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