Inferring internal bore larval transport from barnacle settlement and high resolution physical measurements

Resolving larval transport by internal bores and by other mechanisms is challenging, since larval distributions are patchy, abundance varies with time, and larval concentration measurements must be obtained during episodic larval transport events. Ecologists use larval settlement and physical time series instead of larval distributions to infer larval transport, and whereas obtaining these time-series is relatively less challenging than measuring larval transport, interpretation of settlement measurements can be difficult. We measured barnacle Chthamalus spp., and Balanus glandula settlement daily in a Southern California rocky shore from April to July, and from September to December, both in 2014 and in 2015. Time series of water temperature, circulation, and pressure were also obtained in the nearshore zone. As in previous studies, we found large variability in settlement. In one large event, settlement increased sharply, coinciding with a change in the structure of the water column and the appearance of the internal tide. The large peak in settlement may be related to the internal tide or to other processes. The internal tide was often detected at a 8 m water depth location 700 m distant from the shore. There was little evidence of high-frequency internal waves associated with the internal tide at the 8 m site, and temperature variability patterns at the 8 m and at shallower sites (< 5 m water depth) were different. Previous observations in a location ~15 km distant away, with a more abrupt bottom slope, indicate different physical and biological patterns, and we speculate that local processes might influence spatial variability in settlement and in the shoaling of the internal tide.