Examining the relationship of biochemically-derived oyster larval food supply metrics with concurrent optically-derived seston properties in Mississippi Sound

Abstract

The capacity for oyster (Crassostrea virginica) larvae to successfully develop, grow, and survive is dependent on the quantity and quality of available food. Therefore, examining food supply based upon its biochemical composition of lipid, protein, and labile carbohydrate offers critical insight into oyster larval performance and settlement potential. Biochemical analyses, however, are time-intensive, requiring that such studies be retrospective, generating a need to investigate alternative, real-time techniques to characterize food supply, such as evaluating inherent optical properties. To better understand the food assemblage available to oyster larvae, water samples from seven oyster reefs in Mississippi Sound over two years (May through October) were analyzed for temperature, salinity, particulate organic matter, biochemical properties (lipid, protein, carbohydrate), and in situ optical properties used to infer plankton abundance by size class (pico-, nano-, microplankton), total chlorophyll content, and the magnitude of absorption for colored detrital material. A subset of observations, considered to reflect conditions facilitative for oyster larval survival, clustered into four statistically distinct groups characterized by: high-microplankton, low-microplankton, high-protein, and high-salinity. Total chlorophyll content changed with abiotic conditions, with relatively high concentrations during oligohaline and mesohaline regimes, but declined during periods of changing salinity. Further, transitioning salinities co-occurred with reductions in microplankton concentration and increases in picoplankton concentration. Results of a Spearman's rank analysis, principal components analysis, and stepwise linear regressions revealed that optical properties were not strongly associated with biochemical properties, preventing these optical data from providing an effective index of oyster larval food supply. Instead, optically-derived microplankton abundance recapitulated total chlorophyll, both of which poorly corresponded to biochemical properties. Picoplankton, colored detrital material, and particulate organic material all exhibited a similarly weak correspondence to biochemical food properties, corroborating that measurements of chlorophyll and particulates do not accurately reflect the food available to oyster larvae, and that biochemical metrics remain as superior food supply indicators.