Onshore transport of plankton by internal tides and upwelling-relaxation events

Abstract

Identifying biophysical mechanisms of larval transport is essential to understanding the delivery of larvae to adult habitats. In addition, harmful algal blooms (HABs) can be transported onshore from populations that form offshore. In summer 2011, we measured sea surface and bottom temperatures and daily phytoplankton abundance and intertidal cyprid (barnacle post larvae) settlement at Carmel River State Beach, California, USA. Using time-series analysis, we compared the abundance of Pseudo-nitzschia spp. and daily cyprid settlement to physical forcing mechanisms (e.g. internal tides and upwelling-relaxation events) that could generate onshore delivery. Minimum bottom water temperature was significantly cross-correlated with the spring−neap tidal cycle; minimum temperatures occurred between neap and spring tides, and maximum temperatures were recorded around neap tides. When the temperature data were transformed to remove the relationship between tides and temperature, we found significantly higher maximum sea surface temperatures during upwelling-relaxation events. We observed 4 pulses in Pseudo-nitzschia spp. abundance. Pseudo-nitzschia spp. chains were longest at the start of pulses and then decreased, suggesting that they had been transported to shore from a more productive site offshore, likely the upwelling front. Pulses occurred during periods of maximum sea surface temperature associated with upwelling-relaxation events. In contrast, cyprid settlement was significantly cross-correlated with the spring−neap tidal cycle, with settlement peaks occurring during fortnightly periods of cold bottom temperatures; onshore transport of cyprids appears to have been due to the internal tides.

Transport of larvae and detritus across the surf zone of a steep reflective pocket beach

“…(1) Concentrations of holoplankton and precompetent larvae in the surf zone were not correlated with their concentrations just offshore, and on many days were 1 to 2 orders of magnitude lower than their concentrations offshore. There was a similar relationship between the concentrations of phytoplankton in and out of the surf zone (Shanks et al 2014); these or ganisms somehow avoided or were prevented from entering the surf zone. (2) Detritus, organisms often associated with detritus, and competent larvae were also not correlated with offshore concentrations, but on many days their concentrations in the surf zone were 1 to 2 orders of magnitude higher than offshore.…”

“…How might holoplankters, precompetent larvae, and phytoplankton avoid entering the surf zone? Since the enumerated phytoplankton (diatoms; Shanks et al 2014) do not swim and most precompetent larvae and many holoplankters swim slowly, avoidance of the surf zone cannot be due to these organisms swimming away; rather, surf zone hydrodynamics must keep them out of the surf zone. The cross-shore flow measured at the outer edge of the surf zone by the EMCM array was shoreward near the surface and seaward throughout the water column (Fig.…”

“…As part of this study, we sampled phytoplankton in the surf zone and offshore, including at the northern end of the beach (NR in Fig. 1, Shanks et al 2014). The concentration of offshore phytoplankton taxa collected at the north rocks was only a small percentage (around 1%) of that offshore (A. L. Shanks et al unpubl.…”

“…(2) Over the course of the study, we measured the daily settlement of barnacle cyprids onto rocks at the northern end of the beach (labeled NR in Fig. 1) (Shanks et al 2014). These rocks occur where the hydrographic model (Fuji mura et al 2013) suggests that the onshore flow hits the shore and turns to become the alongshore current.…”

“…data). NR: north rocks where barnacle settlement was measured during a separate study (Shanks et al 2014). The current data are reported relative to the orientation of the beach as indicated by the x, y coordinate system.…”