Thousands of acres of land have been restored to tidal marsh in the San Francisco Bay and Delta (Bay-Delta) over the last two decades, often by breaching old dikes. Currently, several more large-scale, multiagency marsh restoration projects are planned or underway. Nevertheless, the extent to which dike breaching yields robust and diverse aquatic food webs remains unclear.
This study compared samples from various restoration sites in the Bay-Delta, with a particular focus on their food webs. The samples, which were collected in 2019 and 2020, came from three paired sets of reference and restored sites. The structure of the food web at each site was characterized using information gained from three stable isotopes: carbon (δ13C), which tells us whether plant-eaters are consuming the aquatic or terrestrial vegetation; nitrogen (δ15N), which identifies whether an animal is a top predator or their prey; and finally sulfur (δ34S), which differs in animal and plant tissues depending if they reside in freshwater or marine environments.
The results indicated that, among other outcomes, restored sites tended to have smaller food-chain lengths — fewer steps between the primary producer and the top predator. Further, fish at the restored sites relied more on a “brown” food web than a “green” food web; the organisms get their energy from consuming decaying biomass, rather than from eating vegetation or algae. This suggests that food-web structure is not immediately recovered with tidal marsh restoration. That fact highlights that successful ecosystem restoration must include the recovery of energy pathways as a crucial measure of success. The current study is specifically relevant to the understanding and assessment of the recent restoration of Tule Red in Grizzly Bay.