Dynamics of zooplankton in the Cache Slough Complex

Science Center Objects

Our purpose is to investigate what controls the distribution and abundance of fish prey within the Cache Slough Complex (CSC).

We assume that the principal fish of concern there is Delta Smelt, which feeds mainly on zooplankton. Our key question is therefore: What controls the abundance and species composition of zooplankton within the Cache Slough Complex (CSC)? The answer to this question is directly relevant to looming decisions about restoration, and to our understanding of why the CSC seems to be such a special place as habitat for Delta Smelt (Hypomesus transpacificus) and other fishes.

To answer this question we must determine how food limitation, hydrodynamic gains and losses, and mortality limit zooplankton populations within this region. Our objectives are therefore:

  1. To quantify rates of reproduction, growth, and mortality of zooplankton in the CSC.
  2. To characterize the feeding environment for zooplankton by linking methods previously applied in the Kimmerer and Bergamaschi laboratories.
  3. To estimate zooplankton exchange between shallow environments and the channels that link the CSC to the broader estuary.
  4. Ultimately to assess the distribution and the characteristics of environments that support high zooplankton production and the potential for expanding those environments to support recovery of endangered fish species.

The central hypothesis for this study is: The availability of food for Delta Smelt and other fishes in the CSC is predictable from fundamental measurements such as sources and supply rates of phytoplankton and other organic matter, zooplankton mortality rates, and water residence time.

Our approach is to identify and quantify the relationships between zooplankton production and more readily measured attributes of the habitat. This will provide insights into attributes that support high zooplankton production, allow spatial and temporal monitoring, and support models to predict food resources across broader spatial and temporal domains than typically possible.

Conceptual diagram of zooplankton production

Figure 1.

(Public domain.)

Conceptual diagram of zooplankton production

Figure 2.

(Public domain.)

Above: Conceptual diagrams of zooplankton production in a shallow area like the CSC and its export to the broader estuary. Figure 1 illustrates the case where a combination of moderate residence time and low benthic grazing allows for high phytoplankton production based on a high nutrient flux due to high river nutrients and moderate exchange rate. Conditions are ripe here for large phytoplankton cells (green) to develop, stimulating high zooplankton growth and development rate. This productivity is then available for consumption by predators including fish, and for subsidizing the zooplankton of surrounding waters.

In figure 2, the situation has been reversed. Long residence time retains production within the area, and the lower nutrient supply rate (because of low exchange rate) supports low phytoplankton productivity. Grazing by clams contributes to low productivity and also to dominance by small cells that are poorly available to zooplankton, resulting in low zooplankton growth rate and poor capacity to support predators or to provide a subsidy of zooplankton to other regions of the estuary.