Can small zooplankton enhance turbulence in a lake during vertical migration?
Danielle Wain, University of Bath, Architecture and Civil Engineering, Bath, BA2, United Kingdom, Stefano Simoncelli, University of Bath, Architecture and Civil Engineering, Bath, United Kingdom and Stephen Thackeray, Centre for Ecology and Hydrology, Lake Ecosystems Group, Lancaster, United Kingdom
Abstract:
Recent research in both oceanic and freshwater systems suggests that the Diel Vertical Migration (DVM), a predator-avoidance mechanism adopted by many zooplankton, may be an underrepresented source of turbulence and mixing. In particular, the migration can play a crucial role when organisms cross the thermocline; this could be particularly important in enhancing the mixing in lakes, where the pelagic zone is often quiescent, with a consequent impact on lake ecosystem functioning. A field experiment was performed to directly measure the temperature fluctuations and kinetic energy dissipation rate generated by DVM of Daphnia spp., a 1 mm crustacean zooplankton genus. Profiles of turbulence were acquired with a temperature microstructure profiler in Vobster Quay (UK), a small quarry with small wind fetch, steep sides, and with a maximum depth of approximately 25 m. Sixteen profiles were measured over the course of two hours during sunset on 16 July 2015, during which there was no wind. Backscatter strength from bottom-mounted ADCP was used as a proxy to assess DVM. Zooplankton vertical distribution was also quantified by sampling with a 100 μm mesh net before and after the turbulence profiling in 8 layers to verify the distribution of Daphnia spp. before and after the migration. Zooplankton tows show higher abundance (450 ind./L) of Daphnia at 9m and near the bottom before sunset (8PM). Samples after dusk (11.20PM) showed an increase in the surface layer, from 0 up to 250 ind./L. However, migration also appears to happen horizontally. Ensemble-averaged profiles show a great variation of the dissipation rates over the course of the time series with a peak of 10-7 W/kg between 6m and 12m where the DVM is happening and with respect to profiles before sunset. Given the uncertainty in measuring the length scales of turbulence associated with small zooplankton, further analysis is required to determine if the observed turbulence during the time of migration was due the migration or due to other causes, such as the onset of penetrative convection associated with night-time cooling. Three further datasets were collected during sunset in August and September 2015 and will be used to determine if turbulence is always present during the migrations.
