Larval supply is a limited determinant of settlement at mesoscales across an anthropogenic seascape

Abstract

The relationship between larval supply and settlement is an integral part of the demographic processes of benthic marine organisms that determine their distribution at subsequent life stages. In ascidians, a strong positive relationship between larval supply and settlement has been previously documented, but only at small spatial scales (one location) and short time scales (less than one day). We investigated how this relationship might scale up by sampling larval abundance and settlement of a colonial ascidian (Botryllus schlosseri) across mesoscales (100 s of m) of the anthropogenic seascape of a 1.5-ha marina. Settlement varied vertically with a linear decrease in settlement with increasing depth. This pattern corresponded to the vertical distribution of larvae in the morning, when they were most abundant. However, larval abundance explained only 26% of the variation in settlement at the meso-spatial scale (i.e. the combined effect of the horizontal and vertical dimensions). This weakening of the relationship between larval supply and settlement suggests that the two processes may become decoupled at larger spatial and temporal scales. Our study underscores how changing the scales of sampling may affect our understanding of larval dispersal and settlement processes.