Kelly H. Shrader Janet K. Thompson Francis Parchaso Karen Gehrts Elizabeth Wells Emily L. Zierdt Smith 2023 <p>Phytoplankton are an important and limiting food source in the Sacramento-San Joaquin Delta and San Francisco Bay. The decline of phytoplankton biomass is one potential factor in the decline of the protected <i>Hypomesus transpacificus</i> (delta smelt) and other pelagic organisms. The bivalves <i>Corbicula fluminea</i> and <i>Potamocorbula amurensis</i> (hereafter <i>C. fluminea</i> and <i>P. amurensis</i>, respectively) have been shown to control phytoplankton biomass in several locations throughout the San Francisco Bay and the Sacramento-San Joaquin Delta; therefore, knowledge of their distribution and population dynamics are of great interest.</p><p>Here, we describe the distribution and dynamics of bivalve biomass using samples collected by the California Department of Water Resources (DWR) as part of the benthic monitoring program in 2019. One element of DWR’s and the Bureau of Reclamation’s Environmental Monitoring Program—the Generalized Random Tessellation Stratified (GRTS) program—examines the spatial and temporal extent of <i>C. fluminea</i> and <i>P. amurensis</i> control on phytoplankton. Historically, the GRTS program sampled 175 benthic stations (50 stations that are monitored every year and 125 randomly selected new stations that are changed yearly) throughout the Sacramento-San Joaquin Delta and northern San Francisco Bay (San Pablo and Suisun Bays) during one week in May and October. In 2019, only the 50 annually replicated stations were sampled.</p><p><i>Corbicula fluminea</i> and <i>P. amurensis</i> biomass and grazing rates had similar trends; therefore, the conclusions regarding biomass are applied to grazing rate data as well. <i>Corbicula fluminea</i> biomass decreased from May to October, whereas <i>P. amurensis</i> average biomass (reported increased from May (1 g ash-free-dry-tissue mass/square meter (g AFDM/m²) to October (2 g AFDM/m²). Although <i>C. fluminea</i>’s average biomass was lower in October (10 gAFDM/m²) than in May (20 gAFDM/m²), the highest single biomass value was also observed in October (300 gAFDM/m²). In both May and October, most stations that recorded high <i>C. fluminea</i> biomass values were located in the deep water (≥3 m of depth between the surface of the water and the surface of the substrate on the bottom) and were sampled in either rivers or sloughs. A relation between depth and biomass was not observed for <i>P. amurensis</i>.</p><p>Both <i>C. fluminea</i> and <i>P. amurensis</i> recruitment (recruits are considered animals ≤2.5mm in length in this study and recruitment is the process of recruits successfully settled to the bottom) increased from May to October. The total number of <i>C. fluminea</i> recruits more than doubled from May to October, whereas <i>P. amurensis</i> total recruitment increased by 8-fold during the same period. Most <i>P. amurensis</i> recruits in May can be attributed to one station, whereas the recruits in October were found at 14 stations. A relation between number of recruits and station depth was not evident for either <i>C. fluminea</i> or <i>P. amurensis</i>.</p> application/pdf 10.3133/ofr20221102 en U.S. Geological Survey Bivalve effects on the food web supporting delta smelt—A spatially intensive study of bivalve recruitment, biomass, and grazing rate patterns with varying freshwater outflow in 2019 reports