Does tidal marsh restoration lead to the recovery of trophic pathways that support estuarine fishes?

Evaluation of tidal marsh restoration success is typically based on the recovery of habitat acreage and target species. However, food-web structure may provide valuable insight into ecosystem functioning trajectories. Here, we studied restored tidal marshes of different ages (new, young, old; spanning 1 to 150 years) in comparison to nearby reference sites along the San Francisco Estuary. We asked: (i) How does restoration help recover energy pathways that support fish? (ii) Do fishes rely more on algal vs. detrital pathways in restored sites?; and (iii) How does food-web structure vary as a function of species origin and life history? To answer these questions, we sampled fish (n=806) and basal resources (emergent vegetation and phytoplankton; n=109) seasonally over two hydrologically contrasting years. Using stable isotopes (𝛿13C, 𝛿15N, and 𝛿34S), we calculated fish isotopic niche volumes, food chain lengths, and the relative importance of algal vs. detrital energy pathways. We found that food chains in restored sites were 8% shorter than in their paired reference site. Additionally, the young and old restored sites had 37% smaller niche volumes than their references, but the opposite was true for the new restored site (11% larger), illustrating the characteristic trophic surge of early succession. Fishes found in restored sites relied significantly less on detrital energy (7% less) than fishes found in reference sites, and resident fishes showed 12% higher reliance on the detrital pathway than transient species. Finally, most of the native niche volume overlapped with that of introduced fish, which was in turn 38% larger, and a similar pattern was observed when comparing resident to transient fish. Our findings demonstrate that food-web structure does not immediately recover with tidal marsh restoration, even if fish assemblages are species-rich, and show that transient trophic surges may complicate restoration success assessments of newly restored marshes. We contend that incorporating recovery of energy pathways as an indicator of performance may help strengthen monitoring and design of wetland ecosystem restoration projects.

Methods
Using the stable isotopes carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S), we characterize and compare food webs at six sites (three reference, three restored) spanning the low salinity zone of the San Francisco Bay-Delta: upstream of the Carquinez Strait and downstream of the Sacramento‑San Joaquin confluence. These sites represent a range of restoration ages (2-50+ years since breaching) and include Tule Red Island (breached in October 2019) and Chipps Island (slated for interior restoration). We reconstruct food webs by calculating food chain length, isotopic niche volume, and energy pathways based on isotopic signatures of basal resources (e.g., phytoplankton, emergent vegetation) and fish. All field data were collected in Winter and Summer 2020 and 2021.