Convective circulation influences horizontal movement by planktonic crustaceans in the littoral zone of a mesotrophic lake
Authors: Razlutskij Vladimir I. , Buseva Zhanna F. , Yu. Feniova Irina , Semenchenko Vitaliy P. 
Published: December 2020

Crustacean zooplankton perform diel vertical and horizontal migrations in lakes to avoid predators and/or exploit food resources more efficiently. Crustaceans may also move passively with convective flows that are generated by the nocturnal cooling of the surface water. Water in the shallow littoral zone cools faster than deeper pelagic waters. Therefore, bottom water moves from inshore to the pelagic zone while surface water moves in the opposite direction.

The purpose of this study was to determine if crustaceans performed horizontal movements in the bottom and surface layers of a mesotrophic lake, Lake Obsterno in north‐west Belarus. We installed plankton nets horizontally in the bottom and surface layers of the lake at three sites located at different distances from the shore. We hypothesised that (1) if crustaceans performed diel horizontal migration, they would move in identical directions in the bottom and surface layers either from nearshore to open waters in the evening and back in the morning, or vice versa; (2) if crustaceans moved with convective flows, they would be carried to the shore in the surface layer and to the open waters in the bottom layers.

Day and night air temperatures differed by more than 10°C over 24 hr. We assumed that the rates of convective currents were similar to those reported for other lakes exposed to a similar range of air temperatures. Rates of convective currents were also measured in Lake Obsterno and varied between 0.6–0.7 cm/s. These rates of convective currents were shown to exceed the cruise speed of planktonic crustaceans.

The direction of movement of the most abundant taxa coincided with the direction of convective currents in more than 90% of the cases in the bottom layer and 70–80% of the cases in the surface layer. The abundance of invertebrate predators (mainly predatory cladocerans and boatmen (Hemiptera, Heteroptera) was negatively related to the abundance of crustaceans in the surface layer. Deviations in the distribution patterns of some crustacean species compared to distributions induced by convection currents in the upper layers may have been associated with the avoidance of invertebrate predators and/or predator grazing.

Abundances of crustaceans were positively correlated between the sites located at different distances from the shore, and individual taxa were positively related to each other indicating that they moved in the same direction. Based on the fact that the directions of crustacean movements and convective currents mostly coincided, we concluded that convective currents passively carried the crustaceans.

At low light intensities and low predation pressure (by visual fish predators), the direction of horizontal movement by zooplankton was mainly determined by convective circulation generated by cooling of the lake surface. Actively swimming planktonic invertebrate predators are likely to move in the same direction as herbivorous crustaceans. Therefore, predators could influence zooplankton distribution regulated by convective flows. We suggest that studies of horizontal migration and coupling between pelagic and littoral zones need to account for diurnal water currents in lakes.