Response of zooplankton abundance to internal motions and a glacial lake outburst flood in a Patagonian fjord: Zooplankton abundance in Patagonian

Volume backscatter (Sv) derived from an acoustic profiler was used to explore the response of zooplankton abundance to a glacial lake outburst flood (GLOF) and to periodic deepening of the stratified layer in a Patagonia fjord. River discharge during the GLOF increased almost three-fold from 770 m³/s to 2200 m³/s. This pulse enhanced the gravitational circulation and affected the horizontal advective fluxes of plankton. The maximum backscatter was found both during the GLOF and in late winter to early spring of 2010. The backscatter increase in late winter and spring corresponded to multiple zooplankton species found from in-situ net sampling. In addition, diel vertical migrations were found during this seasonal increase both qualitatively and in spectral analysis. Concurrently with zooplankton increases, wind forcing produced a deepening/shallowing of the pycnocline. Zooplankton abundance was at its peak in the fjord during the pycnocline departures and throughout the entire spring season, indicating that mixing conditions could favor secondary production. Eddy diffusivity (kv = 0.8 × 10^-4 m²/s) was found in a shear layer (20 m depth) with a Richardson number of 0.8, denoting a potential for advective instabilities to occur during spring. Recently collected data (2014) shows a similar fjord response to another GLOF event.