Abstract
Theory predicts and recent study revealed that depth of the thermocline can strongly influence the nutrient availability and composition of plankton communities. We are focused on the effect of metalimnetic gradients on water chemistry and plankton communities in three stratified lakes with different trophic conditions. Vertical changes in water chemistry revealed significant increase of macroelement concentrations in the metalimnion of all studied lakes. However, there was no significant increase of nutrient concentrations in the thermocline of lakes with smoother metalimnetic gradient, whereas sharp and deep thermocline zone caused higher concentration of orthophosphates and dissolved inorganic nitrogen in the metalimnion. The maximum concentrations of phytoplankton were observed just below the thermocline and were caused mostly by the abundance of diatoms and cryptophytes. Vertical distribution of the crustacean zooplankton was similar to the distribution of phytoplankton. Especially, Daphnia cucullata was strongly related with the phytoplankton distribution and reached maximum densities in deep layers with high chlorophyll concentrations, and, conversely, smaller crustacean species and rotifers were not affected by the vertical distribution of phytoplankton.
Highlights
It is well known that metalimnetic gradients are key factors that influence nutrient cycling and structure of plankton communities in stratified lakes the during summer stagnation period (Gliwicz 1979; Cantin et al 2011; Gauthier et al 2014)
Most of the studies describe the thermocline as a nutrient-rich layer (Fee et al 1996; Wetzel 2001) but there was no significant increase of nutrient concentrations in the metalimnion of Lake Szurpiły and Jaczno
Increased algal densities below the thermocline often were related to nutrient availability (Fee et al 1996; Fasham et al 1985), but in our study there was no significant increase of nutrient concentration in the metalimnion
Summary
It is well known that metalimnetic gradients are key factors that influence nutrient cycling and structure of plankton communities in stratified lakes the during summer stagnation period (Gliwicz 1979; Cantin et al 2011; Gauthier et al 2014). Nutrient depletion often occurs in the epilimnion of oligotrophic and mesotrophic lakes (Margalef 1983; Christensen et al 1995; Padisak et al 1998). Sharp vertical gradient of temperature in the metalimnion results in increased water density and viscosity, which decreases sinking speed of falling particles. These nutrient-rich waters of the metalimnion may play an important role in the functioning of phytoplankton in lakes of low trophy. The role of zooplankton is complex: they may enhance hypolimnetic algal growth by grazing in the epilimnion, which increases light penetration; but they may feed on the hypolimnetic algal layer and reduce the biomass (Christensen et al 1995). Phosphorus uptake by algae is directly affected by the rate of supply
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