Variability in thermocline depth and strength, and relationships with vertical distributions of fish larvae and mesozooplankton in dynamic coastal waters

Abstract

In this paper the dynamic nature of thermoclines is documented, and their influences on the vertical distributions of larval fishes and mesozooplankton in coastal waters off Sydney, southeastern Australia are tested. Significant small-scale spatial and temporal variability in thermocline depth and strength was observed, and even though there were strong depth-related differences in abundances of fish larvae and mesozooplankton, there were no predictable effects of thermoclines on their vertical distributions. Peak concentrations of some fish larvae were observed in thermoclines, but these patterns were not consistent among sampling locations and time periods. Fish larvae and mesozooplankton were most concentrated in the upper 30 m of the water column, regardless of thermocline position, and therefore trophic interactions among fish larvae and zooplankton would be strongest in this depth strata. Consequently, thermoclines were not considered the most important interface for trophic interactions in this dynamic coastal zone. We argue that thermoclines are probably not critical to the survival of fish larvae in relatively shallow coastal waters characterised by dynamic oceanographies where perturbations in the position and intensity of thermoclines are frequent.