Linking larval abundance, onshore supply and settlement using instantaneous versus integrated methods

Understanding larval transport mechanisms is important to coastal ecosystem dynamics. However, the relationships between nearshore larval abundance, onshore supply and settlement remain poorly understood. We investigated instantaneous versus integrated methods to quantify barnacle and mussel larval distributions as larvae traverse the last 100s of m of water, are delivered and eventually settle onshore. Using the shore's edge at low tide as a horizontal reference (similar to 0 m), we sampled at the following distances: >100s of m (offshore; Stn 1), >1 to 5 m (nearshore; Stn 2) and -10 to -15 m (onshore; Stn 3). Offshore we used integrated collectors on moorings, nearshore instantaneous pump samples were taken and onshore integrated plankton traps and collectors were deployed. All sampling methods captured spatiotemporal variability but each sampled a different part of the water column and potentially, larval life history. As larvae moved across the shore, distributions varied by taxon, depth and sampling method. Offshore, mussel settlement varied between sites and depth. Nearshore, mussel and barnacle larval abundances were coherent between sites. Mussel larvae were well mixed vertically while barnacle larvae were stratified (higher abundances at 1 m). Onshore (Stn 3) larval supply (traps) differed between sites for mussels only and settlement differed for barnacles. Temporal coherence was high for the settlement, larval abundance and onshore supply of mussels, as well as onshore barnacle settlement. Each sampling method was useful for examining larval distributions in a different part of the environment. Used in concert, these methods provide a powerful approach for linking oceanographic processes with nearshore larval abundance, onshore delivery and replenishment of benthic communities.