Effects of eelgrass density on filter feeder biomass and condition index in a multi-habitat living shoreline

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Brandon A. Quintana, Mayra Silva and Danielle C. Zacherl, Biological Science, California State University, Fullerton, Fullerton, CA

Background/Question/Methods
Over two decades, estuarine ecosystems have suffered 50% habitat loss. Implementation of multi-habitat living shorelines (MHLS) can restore lost habitat function. Researchers constructed a MHLS in 2016 in Newport Bay, California by restoring eelgrass and oyster beds together and in isolation at four sites. Increased sedimentation from eelgrass could threaten oysters and other filter feeders. This study will investigate how eelgrass impacts filter feeder biomass and condition index. To compare short-term and longer-term success of oysters restored with varying densities of adjacent eelgrass, I will calculate the total biomass and per capita condition indices of native and non-native filter feeders on restored oyster beds one-year (2018) and three-years (2020) post-restoration. I hypothesize that increased eelgrass density will increase sedimentation rates onto adjacent oyster beds, causing declines in filter feeder biomass and per capita condition indices, biomass will increase, and condition index will decline with increasing filter feeder density, and there will be no interannual variation in the effect eelgrass density has on response factors. I will use previously collected data on these factors. I will excavate filter feeders from quadrats on oyster beds, identify them by species, and weigh to determine wet and dry tissue and shell weight, and quantify condition index. I will quantify sedimentation by measuring mud deposition on oyster beds via point-contact and will quantify filter feeder and eelgrass densities using quadrat surveys.
Results/Conclusions
O. lurida had the highest biomass relative to other bivalves on oyster beds restored alone; nonnative M. senhousia dominated biomass on oyster beds restored adjacent to eelgrass. C. spinosum, M. galloprovincialis, and O. lurida had 26, 6, and 7X higher biomass, respectively, on oyster beds restored alone vs. with eelgrass. O. lurida had 2X higher condition index on oyster beds restored alone vs. with eelgrass. A two-way ANOVA showed that across all years the oyster/eelgrass treatment at one site had significantly higher mud deposition. The ANOVA also showed that mud deposition on both treatments was significantly higher in 2020 than 2018. Increasing our understanding of the relationship filter feeders share with eelgrass will inform future management decisions about whether to restore MHLS together or in isolation.