Food Web Fuel Differs Across Habitats and Seasons of a Tidal Freshwater Estuary

Abstract

Estuarine food webs are fueled by multiple different primary producers. However, identifying the relative importance of each producer to consumers is difficult, particularly for fishes that utilize multiple food sources due to both their mobility and their generally high trophic levels. Previous studies have documented broad spatial differences in the importance of primary producers to fishes within the Upper San Francisco Estuary, California, including separation between pelagic and littoral food webs. In this study, we evaluated the importance of primary producers to adult fishes in three closely spaced subregions that represented disparate habitat types (a tidal wetland channel, a turbid backwater channel, and a deep open-water channel), each a potential outcome of local restoration projects. Using stable isotope analysis coupled with a Bayesian mixing model, we identified significant differences in primary-producer contribution to fishes and invertebrates across habitats and seasons, especially in the relative contribution of submersed aquatic vegetation and phytoplankton. Most fishes utilized multiple primary producers and showed little segregation between pelagic and littoral food webs among habitats. Availability of primary producers differs seasonally and across multiple spatial scales, helping to buffer environmental variability and thus enhancing food web resilience. Ecosystem restoration may improve with emphasis on restoring a wide variety of primary producers to support consumers.

Complex Tidal Marsh Dynamics Structure Fish Foraging Patterns in the San Francisco Estuary

“…The absence of microzooplankton (e.g., copepods) from gut contents may have been an artifact of the size selectivity of gill nets for larger planktivores. The dominance of macrozooplankton (mysids) and benthic macroinvertebrates (especially amphipods) across feeding guilds highlights the potential of tidal marshes to support robust benthic and pelagic food webs (Durand 2015;Schroeter et al 2015;Young et al 2020). The consumption of gammarid, corophiid, and talitrid amphipods is noteworthy because they are associated with complex benthic and epibenthic habitats such as soft-bottom sediments, root wads, and submerged aquatic vegetation such as Stuckenia spp.…”

“…The contribution of tidal marshes to estuarine food webs has more recently become a topic of interest in the SFE (Herbold et al 2014;Brown et al 2016). Recent evidence suggests that benthic and pelagic food web pathways (e.g., marsh detritus and phytoplankton) may support diverse fish assemblages (Durand 2015;Schroeter et al 2015;Young et al 2020). Consumer resource use often varies among species, life histories, seasons, and marsh structural characteristics [e.g., channel order, amount of edge or vegetation; (Visintainer et al 2006;Gewant and Bollens 2012;Whitley and Bollens 2014;Montgomery 2017)].…”

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

“…The term detritus here is meant to incorporate the actual detrital organic material and associated bacteria and microzooplankton essential for detrital nutrient cycling (Azam et al, 1983). Zooplankton and pelagic nekton in the San Francisco Estuary are ultimately fueled by multiple carbon sources contingent on availability, including phytoplankton (Grimaldo et al, 2009; Müller‐Solger et al, 2002) and detrital‐derived sources (Harfmann et al, 2019; Howe & Simenstad, 2011; Young et al, 2020), and so further understanding of hydrodynamic mechanisms that drive the availability of these two different trophic pathways can inform habitat management for Delta Smelt and species‐specific habitat management.…”

Climate Change Implications for Tidal Marshes and Food Web Linkages to Estuarine and Coastal Nekton

“…Tidal Marsh Food Webs Tidal marsh integration of complex physical, biogeochemical, and ecological processes is reflected in the diversity of primary producers, such as emergent vegetation, phytoplankton, benthic algae, or aquatic macrophytes, which support marsh resident and transient nekton (Currin et al 1995). Due to the fact that marshes are embedded in seascape mosaics linked by an overlying water column that integrates multiple interacting processes (Childers et al 2000), consumers that are able to exploit different resources in space and time may be more resilient to change (Young et al 2020). However, if consumers are reliant on marsh-derived organic matter, a reduction in its availability resulting from marsh plant loss could have cascading effects on nekton growth and recruitment (Litvin and Weinstein 2004;Litvin et al 2018), foraging success (Colombano et al 2021), energy reserves (Litvin et al 2014), and trophic relays (Childers et al 2000;Deegan et al 2000;Kneib 2000).…”