Diel vertical distribution of barnacle larvae in the nearshore waters of La Jolla, CA.

Larval transport in nearshore waters influences larval dispersal and connectivity of intertidal benthic organisms. While some horizontal transport of larvae can be attributed to advection, behavioral responses, like vertical swimming and buoyancy control, allows larvae to position themselves at depths where flow direction can be exploited. Thus, knowledge on how vertical larval distribution relates to physical processes can be fundamental to better understand larval transport. Recent work at our study site in Bird Rock (La Jolla), California, USA suggests that late-stage barnacle larvae accumulate at a mid-depth in a shallow (4m) station when offshore waters are stratified. However, it remains unknown how the water column structure (temperature and salinity) varies at this site, and the consequences to the vertical distribution and abundance of larvae. We collected high-resolution physical measurements (temperature, salinity and currents) and high-frequency measurements of barnacle larvae (Chthalamus fissus) within 300 m from shore. Larvae were sampled hourly for a period of 24-hours on five instances during the summers of 2017 and 2018. Samples were collected from a single 4m deep station using a semi vortex pump from distinct 1m depth intervals (0-1m, 1-2m, 2-3m, 3m-bottom), and by filtering water through a 118 µm mesh net. Barnacle cyprids were more abundant than naupliar larvae in most samples. Cyprid depth distribution fluctuated, likely as a consequence of the hydrodynamic and hydrographic conditions. The implication on nearshore larval transport will be discussed.