Hartman, R. (2019). 24 Hour Bugs: Tidal and diel changes in zooplankton distribution in the Sacramento River. Interagency Ecological Program for the San Francisco Estuary. IEP Newsletter. 34(1):11-22.

480820

Hartman, R.

2019

34(1):11-22.

Publication

https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=184974

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The California Department of Fish and Wildlife’s Fish Restoration Program Monitoring Team (FRP) is tasked with monitoring tidal wetland restoration sites of the upper San Francisco Estuary for the benefits they provide to at-risk fishes, particularly the Delta Smelt (Hypomesus transpacificus), Longfin Smelt (Spirinchus thaleichthys), and Chinook Salmon (Oncorhynchus tshawytscha). One of the major hypothesized benefits of these sites is export of production in the form of phytoplankton and zooplankton (Sherman et al. 2017, and references therein). In particular, wetland restoration may increase abundance of mesozooplankton, such as copepods and cladocera, which are recognized as the largest component of Delta Smelt diets (Slater and Baxter 2014) and a significant component of salmon diets (Sommer et al. 2001).However, many zooplankton migrate through the water column in response to tidal and diurnal changes to avoid predators and/or maintain position (Kimmerer et al. 1998, 2002, Burks et al. 2002). Vertical migration patterns make characterizing the contribution of tidal wetlands to zooplankton in fish diets difficult, because a sample collected at a single time point may not represent the abundance of zooplankton at other times of day. FRP must choose a scheme for sampling zooplankton that provides the best data on fish food resources given limited staff time. By sampling during times when fish of concern are most active, FRP can characterize zooplankton most available for fish consumption. However, salmon are most active either at night (Wilder and Ingram 2006, Plumb et al. 2016), or at dawn and dusk (Clark and Levy 1988), while Delta Smelt are most active during the day (Young et al. 2004, Hasenbein et al. 2013). Though Delta Smelt feed chiefly during daylight hours, they have been documented feeding on more adult Pseudodiaptomus forbesi than would be expected based on the relative abundance of this species in zooplankton samples taken during the day (Slater and Baxter 2014). Additional feeding by Delta Smelt during dawn and dusk, when adult P. forbesi and other copepods have migrated to the surface (as seen in Kimmerer et al. 2002, Kimmerer and Slaughter 2016), may explain part of this discrepancy. Furthermore, when trying to characterize export of production from the wetland, daytime sampling may miss important components of the community that enter the pelagic food web of the surrounding channel at different times of day (as suggested in Dean et al. 2005, Kimmerer et al. 2014).

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