Abstract
Author(s): Kimmerer, Wim; Ignoffo, Toni R.; Bemowski, Brooke; Moderan, Julien; Holmes, Ann; Bergamaschi, Brian | Abstract: We studied abundance and dynamics of zooplankton in the tidal freshwater Cache Slough Complex (CSC) in the northern Delta of the San Francisco Estuary during June, July, and October 2015. We asked whether the CSC was an area of high zooplankton production that could act as a source region for open waters of the estuary. Abundance of the copepod Pseudodiaptomus forbesi was similar to that in freshwater reaches of the central and eastern Delta and higher than that in the adjacent Sacramento River. Growth rate of P. forbesi was higher than previously measured in large estuarine channels because of higher temperature and phytoplankton biomass in the CSC. Samples of P. forbesi examined with molecular techniques contained an unexpectedly high proportion of DNA from cyanobacteria and little DNA from more nutritious phytoplankton. We also examined tidal exchanges of phytoplankton biomass and copepods between Liberty Island, a shallow tidal lake within the CSC, and the adjacent southern Cache Slough, which links the CSC to the Sacramento River. We calculated zero net flux of phytoplankton over 127 days between June and October. The tidal flux of copepods, calculated using tidal flow from an in situ flow station and half-hourly sampling over three 24.8-hr tidal cycles, varied a great deal because of temporal patchiness and day/night variation in abundance. Overall, the tidal flux was indistinguishable from zero, while the tidally-averaged water flow (and therefore the net copepod flux) was always into the wetland. Our results show some promise for the CSC as a productive habitat for planktivorous fishes and as a laboratory for learning how to design future wetland restoration. However, we remain cautious about whether wetlands such as the CSC may export large quantities of food organisms that can support fishes in other regions of the estuary.
Highlights
Wetlands are productive elements of natural landscapes with many attributes valued by human populations
Plans for restoration or creation of wetlands in the freshwater reaches of the Delta list among their goals the enhancement of food web productivity to support Delta Smelt and other native fishes (e.g., California Natural Resources Agency, undated). This productivity could be made available directly if these fishes occupy the wetlands, or indirect if it is exported in usable form to the open waters of the estuary
We evaluate the roles of the Cache Slough Complex (CSC) in supporting Delta Smelt directly, and in exporting food for pelagic fishes to the open waters of the estuary
Summary
Wetlands are productive elements of natural landscapes with many attributes valued by human populations. They can support productive food webs and species of concern, improve water quality, sequester carbon, protect shorelines, and provide opportunities for active and passive recreation (Zedler and Kercher 2005). The habitat value of wetlands arises from the shelter provided by complex bathymetry, turbidity, and aquatic vegetation, and from the high productivity of food sources for fish. Food sources within wetlands may include insects, benthic fauna, fauna resident on surfaces such as vegetation, and zooplankton, which are typically consumed mainly by small forms, including larval and early juvenile fishes
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