Hydrodynamics and habitat interact to structure fish communities within terminal channels of a tidal freshwater delta

Abstract

Terminal channels were historically a common feature of tidal delta ecosystems but have become increasingly rare as landscapes have been modified. Tidal hydrodynamics are a defining feature in tidal terminal channel ecosystems from which native aquatic communities have evolved.However, few studies have explored the relationship between fish community structure and hydrodynamics in these tidal terminal channel ecosystems. We sampled fish communities throughout a network of terminal channels within the northeasternmost region of the San Francisco Estuary to determine the relationship between fish community structure and hydrodynamics within these environments. We collected two years (2017 and 2018) of fish community samples using gill nets and analyzed data using multivariate community analyses and count models. We found metrics of fish diversity and counts of native fishes to be greatest upstream (farthest from tidal influence) of the tidal excursion within terminal channels. Counts of non-native fishes were less affected by this hydrodynamic feature of terminal channels and more tightly correlated to local habitat conditions (e.g., water temperature, depth). Our results suggest that channel hydrodynamics plays a role in structuring fish communities within terminal channels, particularly native fishes. These results indicate that hydrodynamics in tidal delta ecosystems may be able to be altered in ways that benefit native fishes without the cost of water pumping.K E Y W O R D S backwater slough, Cache Slough complex, multivariate, San Francisco Estuary, spatial scale, tidal excursion, tidal marsh † Deceased

Wetland geomorphology and tidal hydrodynamics drive fine-scale fish community composition and abundance

“…There was no difference in the conditional effects for otter trawl between the intertidal basin (Little Holland Tract) and the dendritic wetland (Liberty Island Conservation Bank), which is why only one smooth line was drawn for each parameter organic matter indicated higher water residence time within the dendritic wetland, consistent with general spatial patterns observed in the region (Downing et al 2016). Residence time, a major driver of biogeochemical processes, corresponds to geomorphic and hydrologic attributes (Downing et al 2016), adult fish distribution (Huntsman et al 2023), and trophic structure (Young et al 2021). In tidal wetlands, like those in this study, the timing of tides and the distance of the tidal excursion (maximum distance a parcel of water moves during a tidal cycle) interact with fixed geomorphic features to affect physical properties such as mixing, stratification, and water velocity.…”

“…Previous work has documented the influence of tidal processes on various ecosystem constituents in the region, including biogeochemical cycling (Downing et al 2016), primary productivity (Stumpner et al 2020), fish prey distribution (Montgomery 2017;Feyrer et al 2017;Young et al 2021), and adult fish distribution (Huntsman et al 2023). Our analysis underlines the importance of considering both physical habitat structure and hydrodynamics when attempting to understand the ecological processes and functions driving organismal abundance and distribution in complex systems, such as tidal wetlands.…”

“…1) consists of a network of tidal channels and wetlands with high primary productivity and turbidity relative to other freshwater regions of the Sacramento-San Joaquin Delta (Downing et al 2016;Stumpner et al 2020). This region has a diverse fish community that includes a relatively high proportion of native species (Young et al 2015(Young et al , 2020Huntsman et al 2023). We studied two freshwater tidal wetlands, Little Holland Tract and Liberty Island Conservation Bank (Fig.…”

“…Fishes often use different habitats across diel cycles, and by only sampling in daylight hours, we may miss fish that can move to or from sampled and unsampled habitats diurnally (Nemerson and Able 2020;Colombano et al 2021). Tidal variability is known to drive fish distributions in many systems (Boswell et al 2019;Nemerson and Able 2020;Young et al 2021;Colombano et al 2021;Huntsman et al 2023) and may have potentially influenced our observations. For example, our sampling efficiency in small channels may have changed during the highest tides when access to the inundated intertidal habitats (i.e., mudflat and marsh plain) may have provided fishes a refuge from capture.…”

“…In California's Sacramento-San Joaquin Delta, 90% of the historical tidal wetlands have been lost due to human modifications (Whipple et al 2012;Brophy et al 2019). In remaining wetlands, varying scales of physical habitat, hydrodynamics, and submerged aquatic vegetation are key drivers of fish distribution (Brown and Michniuk 2007;Feyrer et al 2015;Young et al 2018;Huntsman et al 2023). In brackish regions, channel configuration and marsh plain elevation drive the abundance and distribution of native and non-native fishes; native fishes are generally most abundant in natural dead-end channels with tidally activated marsh plains (Visintainer et al 2006;Colombano et al 2020).…”