Introduction

Natural convection in lentic water bodies can influence water quality (Naghib et al. 2018) and may have substantial ecological impacts (Mao et al. 2019). In a lake, mixing and transport of particles can be promoted by density currents driven by variations in water temperature (Mortimer 1974; Okely and Imberger 2007; Tsydenov et al. 2016; Mao et al. 2019). Horizontal differences (limnetic vs littoral areas) in temperature create density gradients, which promote the establishment of horizontal surface exchange flows (Farrow 2004; Okely and Imberger 2007). The faster heating of the water in shallower areas than in the deeper areas generates convective currents from the littoral to the open lake at the surface. Temperature differences as small as 0.5 °C lead to velocity magnitudes of the order of a few centimetres per second (Pálmarsson and Schladow 2008). Also, aquatic plants are able to induce convective motion by promoting differential shading and by reducing wind in shallower regions (Lovstedt and Bengtsson 2008; Lightbody et al. 2008; Zhang and Nepf 2009). Lovstedt and Bengtsson (2008) observed average temperature differences of 0.8 °C and mean velocities of 0.8 ± 0.5 cm/s in surface currents towards the vegetated littoral in a shallow lake in southern Sweden. These thermal flows can transport nutrients, chemicals and pollutants through the surface of water across lentic water bodies (Mao et al. 2019), and can influence zooplankton distribution patterns (Podsetchine and Schernewski 1999).

The distribution of zooplankters has been argued to be driven by abiotic and/or biotic factors (Viljanen and Karjalainen 1993; Pinel-Alloul 1995; Thackeray et al. 2004; Gabaldón et al. 2019; Rollwagen-Bollens et al. 2020), relating to two major zooplankton movement patterns: diel horizontal migration (DHM) and diel vertical migration (DVM) (Pinel-Alloul 1995; Hembre and Megard 2003; Pinel-Alloul et al. 2004; Emily et al. 2017; Ermolaeva et al. 2019). DVM describes the movement into deeper and darker sites in the water column during the light time (day) to avoid visual predators, but at night the opposite movement typically occurs towards the water surface for improved acquisition of food resources (O’Brien 2000; Adamczuk 2014), with higher abundance generally recorded within the Myriophyllum stand than the Schoenoplectus stand. Bosmina have a different feeding flexibility and locomotory behaviour than other cladocerans because they feed more like a raptorial predator than a passive collector, and they can select food items upon availability, which is an energy-efficient mechanism allowing them to share habitat with competitors without the need for costly spatial migration (DeMott and Kerfoot 1982).

Challenges to and inconsistencies with zooplanktonic DHM theory are well known and relate to the contrast between predation and prey refuge (Burks et al. 2002; Nurminen and Horppila 2002; Meerhoff et al. 2006; Castro et al. 2007b; Jensen et al. 2010; Arcifa et al. 2013; Antón-Pardo et al. 2021), as well as to the role of water transparency in moderating the relationship. Turbidity, which is high in Lake Vela, has a consistent negative effect on prey capture by visually oriented predators, and there is also evidence that high turbidity leads to reduced prey capture in non-visual predators (Ortega et al. 2020). The behaviour of dominant planktivorous fish in Lake Vela may also contribute to the inconsistencies, because young pumpkinseed sunfish tend to prey in the littoral (García-Berthou and Moreno-Amich 2000), as do mosquitofish, mostly upon littoral cladocerans (García-Berthou 1999). Unfortunately, there are no systematic records on the fish assemblage of Lake Vela at the time of the sampling, and mosquitofish were consistently observed near both vegetated areas during the sampling period, while pumpkinseed sunfish were more rarely observed. The role of wind and thermal currents in modulating the spatial heterogeneity of zooplankton distribution in lakes has been postulated (Okely and Imberger 2007), but these ideas have also been questioned as factors affecting zooplankton spatial distribution (Lévesque et al. 2010).

In a shallow lake with littoral regions populated by emergent vegetation, differential solar heating can produce near-surface temperature differences between vegetated and non-vegetated regions. During the day, especially on sunny days, the shadowing effect on the littoral areas should reduce surface water heating relative to the limnetic areas, leading to the generation of horizontal exchange flows towards the vegetated areas (Zhang and Nepf 2009). During the night, the cooling effect is expected to be more efficient in the limnetic areas, leading to a surface flow from littoral to lake open areas. Nevertheless, the water temperature measurements of the present work did not follow that expected pattern. In the Schoenoplectus axis, the surface water in the limnetic area was always warmer than in the littoral. In the Myriophyllum axis, the water was warmer in the littoral than in the limnetic area during the day, whereas the temperatures were similar in the littoral and limnetic areas during the night. This behaviour is likely related to the type of vegetation and its specific heat features. Myriophyllum aquaticum is characterized by very dense plant distributions with short canopies. A potential large heat absorption by the plants might contribute to a faster water heating process compared with the limnetic area.

The correspondence between the expected surface exchange flow based on the water temperature differences and the abundance of the whole and group-specific zooplankton was not definitive with respect to the role of thermally driven exchange flows on the zooplankton horizontal migration patterns. Lake circulation is prone to the influence of several factors leading to complex flow (Zhang and Nepf 2009; Mao et al. 2019; Naghib et al. 2018). In shallow lakes in Mediterranean regions with high temperatures and exposure to annually prevalent winds (North Atlantic Anticyclone combined with North Atlantic Oscillation), the wind pattern and intensity may have a significant influence on the surface exchange flow and, therefore, on the spatial distribution of zooplankton. The potential role of the wind in this process will require further analysis and additional study.