Seasonal and synoptic oceanographic changes influence the larval biodiversity of a retentive upwelling shadow

Understanding sources of variability in larval supply and transport is integral to the dynamics, structure and effective management of marine populations and communities. Yet, a barrier to this understanding is the high variability in the supply and transport of marine larvae, especially in upwelling regions where wind forcing causes dynamic circulation. Since larvae of many species complete development close to shore, resolving the relationship between oceanographic processes and nearshore larval assemblages is essential to better understand larval transport in highly productive upwelling regions. The goal of our study was to examine the effects of variation in upwelling and relaxation dynamics on the nearshore larval assemblage in northern Monterey Bay. To determine how seasonal and daily upwelling and relaxation dynamics influence the nearshore larval assemblage, we surveyed distributions of marine larvae and physical, environmental factors along a cross-shelf transect in northern Monterey Bay, USA, during August and October of 2013. Conditions in August and October differed in temperature, salinity, stratification, and chlorophyll-a fluorescence. Richness and diversity of the larval assemblage did not change appreciably, but the abundance and composition of species shifted after the influx of offshore waters. Specifically, nearshore taxa were more abundant during August, which was characterized by strong upwelling conditions, and especially more abundant with increased wind forcing leading to a retentive upwelling shadow in the northern bay. Conversely, offshore taxa were more abundant during October, which was characterized by weakened upwelling and the persistent influx of offshore water. Our study suggests that relationships between larval taxa, life history characteristics, and water types provide insights into water mass history, circulation and larval recruitment in highly dynamic upwelling regions