Pulsed Zooplankton Production in Seasonally Managed Wetlands Could Supplement Food Availability for a Threatened Fishery in the San Francisco Estuary

Seasonal managed wetlands in the San Francisco Estuary are novel habitats that can support aquatic food webs. However, their potential to benefit fish in the estuary is poorly understood. We quantified wetland zooplankton production dynamics to assess whether outflows from wetlands might be used to subsidize fishes. Our study goals were to (1) compare seasonal zooplankton trends between wetlands and adjacent waters, (2) identify differences in zooplankton composition between habitat types, (3) quantify zooplankton trends in managed wetlands with relation to season and hydroperiod, and (4) compare traditional zooplankton sampling with a new rapid-assessment technique. We collected zooplankton via tow-net and rapid-assessment samples, between autumn 2018 and autumn 2022, from six managed wetlands and eight tidal waterbodies across Suisun Marsh, located in the brackish zone of the estuary. Across seasons, managed wetlands supported 22.3 times more zooplankton biomass on average than tidal habitats, including tidal restoration sites. Managed wetlands favored large-bodied zooplankton, such as Daphnia magna, Acanthocyclops spp., and Eurytemora affinis. Within managed wetlands, peak copepod abundances (~ 204 copepods L− 1) occurred two months after initial flooding, while peak cladoceran abundances (~ 31.4 cladocerans L− 1) occurred after four months. The modeled relationship between rapid zooplankton assessments and tow-net samples was 0.89 (near 1:1), indicating the usefulness of rapid zooplankton assessments in coarsely and quickly characterizing zooplankton abundance. However, we recommend rapid zooplankton assessments continue to be accompanied with traditional tow-net samples for more detailed analyses. Findings show that managed wetlands support pulsed zooplankton production in response to controlled floods, which could be timed to subsidize plankton stocks for fish.