Zooplankton Dynamics in the Cache Slough Complex of the Upper San Francisco Estuary

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.

Production of the Copepod Pseudodiaptomus forbesi Is Not Enhanced by Ingestion of the Diatom Aulacoseira granulata During a Bloom

“…S1 b). Prior work in the Cache Slough Complex (stations 4-6) has shown high levels of cyanobacteria DNA and negligible levels of cryptophyte DNA in the guts of P. forbesi, the opposite of what we expected based on their respective abundance and presumed nutritional value (Holmes 2018;Kimmerer et al 2018). Other sequencing-based studies have found cyanobacteria (Synechococcus) to be highly abundant in the guts of C. finmarchicus (Yeh et al 2020).…”

“…This species is also demersal in shallow, clear waters (Kimmerer and Slaughter 2016). A previous study at station 5, the shallowest station, showed day/night differences in abundance of adult females in the 4-m water column (Kimmerer et al 2018), but the mean proportions of P. forbesi females that were ovigerous did not differ between day and night during the three surveys of that study (Kimmerer unpublished). Thus, we are confident that our daytime sampling was unbiased with regard to the proportion of females that were ovigerous and, therefore, the egg production rate.…”

Hydrodynamics drive pelagic communities and food web structure in a tidal environment

“…Site differences were largely driven by greater density of cladocerans at the upstream site relative to downstream. Seasonal differences were driven largely by typical species turnover within the San Francisco Estuary (Kimmerer et al, 2018), with calanoid ( Pseudodiaptomus forbesi ) and cyclopoid ( Limnoithona tetraspina ) copepods dominating summer communities and other taxa dominating other seasons (Table 2).…”

“…The distribution of zooplankton and nekton within the channel was strongly correlated with the hydrodynamically driven habitat zones described above. The upstream zone characterized by no exchange with downstream habitats supported higher zooplankton densities, and higher densities of Cladocera, a group more abundant in low‐velocity, freshwater habitats regionally (Frantzich et al, 2018; Kimmerer et al, 2018). The downstream, high exchange zone supported the lowest densities of zooplankton, which could reflect low phytoplankton abundance observed in the study or simply the relatively low densities of zooplankton present in many nearby SFE pelagic habitats (Kimmerer et al, 2018).…”

“…The downstream, high exchange zone supported the lowest densities of zooplankton, which could reflect low phytoplankton abundance observed in the study or simply the relatively low densities of F I G U R E 5 Biplot of primary producer and consumer isotope values. Light gray boxes represent phytoplankton estimates and dark gray boxes represent pooled vascular vegetation (see Table 3) zooplankton present in many nearby SFE pelagic habitats (Kimmerer et al, 2018). Nekton were generally more abundant in the intermediate, low exchange zone.…”