Large-Scale Flow Management Action Drives Estuarine Ecological Response

Abstract

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 m3 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.