Wind‐driven thermocline movements affect the colonisation and growth of Achnanthidium minutissimum, a ubiquitous benthic diatom in lakes

Abstract

Summary Benthic algae form a major component of primary production in shallow waters and are an important component of lake food webs. I tested the effects of thermocline movements and bathymetric slope on the colonisation and population development of a ubiquitous and often dominant benthic diatom, Achnanthidium minutissimum. Sampling sites were positioned along the upwind and downwind shorelines of an elongated 22 km2 lake basin, on a range of bathymetric slopes (1–16%). A first set of substrates was deployed at eight sites during early stratification (June 19, 2004) and were sampled after 4, 11 and 20 days and 5.5 and 11 weeks to compare colonisation, establishment, early growth rate and density of mature populations. The second set of substrates was deployed at 16 sites in mid‐summer (July 26) and was sampled after 5.5 weeks. The density of A. minutissimum and their average cell length were measured in all samples. Thermocline movements were calculated using a 3D hydrodynamic model, which was calibrated offshore in the lake basin and validated at each sampling site. Achnanthidium colonised the open substrates very rapidly. Initial cell densities increased with increasing thermocline movement, especially along shallow bathymetric slopes. These results suggest an inverse relationship between thermocline‐induced nearshore turbulence and bathymetric slope. The density of early colonisers was reduced by 30–95% during the establishment period (4–11 days after substrate deployment), suggesting that Achnanthidium needs time to attach firmly to the substrate and is vulnerable to disturbances during that period. Achnanthidium established most efficiently at upwind nearshore sites exposed to more thermocline movement. The early growth rate of Achnanthidium in nearshore areas was negatively related to mean water temperature, a likely surrogate for nutrient availability at the sediment–water interface. The density of Achnanthidium in mature populations was positively related to thermocline movements and showed a negative interaction with bathymetric slope early in the stratification period, but not later in summer. These results suggest that physical forces associated with thermocline movements interact differently with the bottom in nearshore areas as the season progresses and as the water‐column stabilises. The size of Achnanthidium cells increased during settlement and early population growth, peaked in early July at most sites and decreased over the rest of the summer. This common pattern of seasonal change in Achnanthidium cell sizes suggests a strong factor synchronising its life cycle across the whole lake basin. Thermocline movements over nearshore substrates clearly affect the colonisation and population development of A. minutissimum. These results support the idea that wind‐driven physical forces are a major factor structuring nearshore habitats in lakes, even in the lower littoral zone, and that benthic organisms should distribute themselves in a predictable way around lake basins.