Small-scale spatiotemporal thermal regimes drive patterns of zooplankton abundance in a kelp forest

Abstract

During internal tidal forcing, high-frequency water column temperatures and zooplankton abundances were measured within and at the outer edge of a Giant Kelp Forest at two depths. Although the sampling sites were only separated by 300m, differences occurred in their thermal regimes and in their zooplankton abundance, distribution, and temperature relationships. Dominant organisms at both sites included copepods and bryozoan cyphonautes. Outside of the kelp forest, the water column was more stratified, with distinct alternating cold and warm periods defined by abrupt thermal fronts indicative of internal wave phases. Lower zooplankton abundances and fewer taxa (only 31) were detected, with more cyphonautes and copepods found in the deeper strata and during the cold phase of internal waves. Inside the kelp forest, on the other hand, the water column was less stratified and less variable over time, with no clear alternating internal wave phases. An order of magnitude greater zooplankton abundance occurred with more taxa (54) identified and more cyphonatues and copepods found in the surface strata. At both sites, abundances of copepods increased with high frequency cooling in the deeper strata, whereas near the surface they showed the reverse pattern, increasing during warming. Across sites, cyphonautes decreased outside of the kelp forest while they increased within the kelp forest. The patterns detected across depths, across sites, and related with high frequency thermal variability suggest complex forcing of zooplankton abundances by alternating shoaling and sinking of the thermocline related with internal waves. These results emphasize the small-scale spatiotemporal biophysical complexity in nearshore waters and contribute to the understanding of nearshore plankton dynamics related with internal wave forcing mechanisms in Giant Kelp Forest environments.