Climate change effects on shallow lakes: design and preliminary results of a cross-European climate gradient mesocosm experiment

Abstract

Climate change is expected to profoundly affect both temperature and net precipitation, with implications for lake water level. We describe the design of a harmonized, simultaneous, cross-European mesocosm experiment to elucidate the effects of climate change on community structure, functioning, and metabolism in shallow lakes at low and high nutrient levels with contrasting depths (1 and 2 m). We used cylindrical (D = 1.2 m) tanks that were either 1.2 or 2.2 m high, each having a 10-cm sediment layer. We inoculated the mesocosms with a mixed sample of sediment and plankton from lakes with contrasting nutrient concentrations and added macrophytes and planktivorous fish. Sediment was pre-equilibrated to the required experimental nutrient concentration. During the experiment the water level decreased with increasing temperature (up to 90 cm in the Mediterranean mesocosms) while conductivity increased. The average chlorophyll a concentration increased with temperature in the deep mesocosms but was more variable in the shallow mesocosms. Macrophyte F. Landkildehus et al. 72abundance increased with temperature, while the oxygen data suggest that net primary production peaked at intermediate temperatures. We conclude that our experimental design has the potential for tracking the interacting effects of global warming and eutrophication in shallow lakes.

Macroecological Patterns of Resilience Inferred from a Multinational, Synchronized Experiment

“…A thorough description of the experimental design can be found in Landkildehus et al [51]. Briefly, one lake from each of the six participating countries (Sweden, Estonia, Germany, Czech Republic, Turkey, Greece) was selected based on water depth (<5 m), alkalinity (<4 meq/L), water color (<20 mg Pt/L), electrical conductivity (<500 mS/m) and nutrient concentration (<25 µg/L of total phosphorus (TP) to accommodate the experiment.…”

“…At the start of the experiment, the water depth was adjusted to 1 and 2 m in the shallow and deep water level treatments, respectively. Once the pontoon bridges were in place, a combination of sand and mud from a nearby mesotrophic lake was added as bottom substrate and equilibrated beforehand regarding TP fluxes between the water column and the sediment [51]. Subsequently, the mesocosms were filled with filtered nutrient poor water to reach 1020 and 2150 L, respectively corresponding to the shallow and deep water levels.…”

“…These conditions reflect natural abiotic settings of shallow, nutrient-poor lakes across Europe, increasing the generality of the mesocosm study. The experiment ran simultaneously in the respective locations (Table 1) using standardized sampling methods and lasted for approximately six months (May to beginning of November 2011), corresponding to the ice free period in the northern countries [51]. …”

“…Following water addition, suspended solids were left to settle for 96 h. Thereafter the mesocosms received inocula of plankton (phytoplankton and zooplankton) from five nearby lakes as a biotic inoculum, to enable the development of a diverse flora and fauna and to cover regional communities. Plankton inocula were collected by performing five vertical net hauls (plankton net with a mesh size of 50 µm and a diameter of 20 cm), over the entire water column from the five nearby lakes (from each respective countries) and pooled in 5 L barrels (Landkildehus et al 2014). The contents of the barrels (5 L) from the five lakes were subsequently carefully mixed and subsamples of 1 L were added to each mesocosm [51].…”

“…Plankton inocula were collected by performing five vertical net hauls (plankton net with a mesh size of 50 µm and a diameter of 20 cm), over the entire water column from the five nearby lakes (from each respective countries) and pooled in 5 L barrels (Landkildehus et al 2014). The contents of the barrels (5 L) from the five lakes were subsequently carefully mixed and subsamples of 1 L were added to each mesocosm [51]. From the same five lakes sediment was collected and sieved through a 10 mm sieve to avoid introducing fish eggs or large mussels.…”