Suisun Bay zooplankton are important prey for Bay-Delta fish populations, including smelt and juvenile salmon. During the recent 2012-2016 drought, the State Water Board issued temporary urgent change orders (TUCOs) that allowed lower Delta outflow than would normally be required in Critically Dry water years. Specifically, those Board orders allowed lower outflow in spring of 2014 and 2015. To evaluate the effects of these orders, I looked at June outflow versus June density of cladoceran and calanoid copepods, the primary fish prey in Suisun Bay and Marsh, the prime east Bay nursery area.
Cladocerans
Cladocerans, sometimes called “water fleas,” are generally most prevalent in freshwater portions of the estuary, and would be expected to have lower density as brackish water encroaches in Suisun Bay with lower outflow. Cladoceran density was near zero in June 2014 and 2015, when outflow fell below 5000 cfs under the TUCOs (Figures 1 and 2) and EC reached an unprecedented 13-16 millimhos in June at Mallard Island in Suisun Bay (CDEC data). Cladoceran densities were highly variable in the outflow range of 6000-8000 cfs.
Calanoid Copepods
Calanoid copepods are another important prey of juvenile fish in Suisun Bay/Marsh. They reside in fresh and brackish waters of the Bay-Delta estuary. Calanoid copepod adult density was near zero in June 2014 and 2015, when outflow fell below 5000 cfs under the TUCOs (Figures 3 and 4) and EC reached an unprecedented 13-16 millimhos in June at Mallard Island in Suisun Bay (CDEC data). Calanoid copepod adult densities were highly variable in the outflow range of 6000-8000 cfs.
Conclusion
Maintaining Delta outflow in spring at least as great as is required by existing water quality standards is important in maintaining the productivity of zooplankton in Suisun Bay and Suisun Marsh. June outflows greater than existing requirements generally increased the productivity of these important prey for fish.