Large-Scale Flow Management Action Drives Estuarine Ecological Response

Resource managers rely on large-scale flow and habitat enhancement actions to support sensitive species in the San Francisco Bay-Delta. The effects of these actions on target species and ecosystems are largely unknown. In 2018, we implemented an ecosystem-scale experiment to reduce salinity and improve access to high-quality habitat in Suisun Marsh, California, for Endangered Species Act-listed Delta Smelt Hypomesus transpacificus. The action included a release of approximately 46 million m(3) of additional flow in conjunction with the operation of a novel water management facility, the Suisun Marsh Salinity Control Gates. This experimental action resulted in more low-salinity habitat in Suisun Marsh than would have been present as compared to similar historical low-flow summer conditions. We evaluated the effect of decreasing salinity on the species assemblage in Suisun Marsh using a combination of ordination analyses and circular statistics. We focused this analysis on long-term monitoring data collected within Suisun Marsh during July and August and found that the aquatic community changed significantly in small sloughs but not large-slough habitat in response to the Suisun Marsh action. The assemblage shift from July to August 2018 differed from historical trends in both slough sizes, albeit not significantly in large-slough habitat. Specifically, we observed (1) a shift in California bay shrimp Crangon franciscorum and Striped Bass Morone saxatilis abundance like that occurring in low-salinity, wet water year types; and (2) an uncharacteristic seasonal decline in Black Sea jellyfish Maeotias marginata. We posit that the observed community changes were driven by a combination of behavioral responses to lower-salinity conditions and physical displacement by directional flow resulting from the flow management action.