Nutrient Loading, Temperature and Heat Wave Effects on Nutrients, Oxygen and Metabolism in Shallow Lake Mesocosms Pre-Adapted for 11 Years

Erik Jeppesen,Joachim Audet,Søren Larsen,Meryem Beklioğlu,Yu Cao,Martin Søndergaard,Torben Lauridsen,Érika Neif,Nur Filiz,Thomas Davidson,Tserenpil Sh

doi:10.3390/w13020127

Abstract

Global changes (e.g., warming and population growth) affect nutrient loadings and temperatures, but global warming also results in more frequent extreme events, such as heat waves. Using data from the world’s longest-running shallow lake experimental mesocosm facility, we studied the effects of different levels of nutrient loadings combined with varying temperatures, which also included a simulated 1-month summer heat wave (HW), on nutrient and oxygen concentrations, gross ecosystem primary production (GPP), ecosystem respiration (ER), net ecosystem production (NEP) and bacterioplankton production (BACPR). The mesocosms had two nutrient levels (high (HN) and low (LN)) combined with three different temperatures according to the IPCC 2007 warming scenarios (unheated, A2 and A2 + 50%) that were applied for 11 years prior to the present experiment. The simulated HW consisted of 5 °C extra temperature increases only in the A2 and A2 + 50% treatments applied from 1 July to 1 August 2014. Linear mixed effect modeling revealed a strong effect of nutrient treatment on the concentration of chlorophyll a (Chl-a), on various forms of phosphorus and nitrogen as well as on oxygen concentration and oxygen percentage (24 h means). Applying the full dataset, we also found a significant positive effect of nutrient loading on GPP, ER, NEP and BACPR, and of temperature on ER and BACPR. The HW had a significant positive effect on GPP and ER. When dividing the data into LN and HN, temperature also had a significant positive effect on Chl-a in LN and on orthophosphate in HN. Linear mixed models revealed differential effects of nutrients, Chl-a and macrophyte abundance (PVI) on the metabolism variables, with PVI being particularly important in the LN mesocosms. All metabolism variables also responded strongly to a cooling-low irradiance event in the middle of the HW, resulting in a severe drop in oxygen concentrations, not least in the HN heated mesocosms. Our results demonstrate strong effects of nutrients as well as an overall rapid response in oxygen metabolism and BACPR to changes in temperature, including HWs, making them sensitive ecosystem indicators of climate warming.

Highlights

The Earth is facing a rapid climate warming and population growth [1]
Oxygen metabolism and bacterioplankton production before, during and after a simulated 1-month summer heat wave (HW) in shallow lake mesocosms run at contrasting nutrient concentrations and temperatures
We found a significant positive effect of temperature on ecosystem respiration (ER) and bacterioplankton production (BACPR), both when the dataset was analyzed in full and when it was divided into LN and HN, but no effect was found on gross ecosystem primary production (GPP) and net ecosystem production (NEP)

Summary

Introduction

The Earth is facing a rapid climate warming and population growth [1] This has implications for lake ecosystem functioning and entails an increased risk of eutrophication, reflecting both increasing external and internal nutrient loading and a shift in top-down control that reduces the zooplankton grazing pressure on phytoplankton [2,3,4]. This is especially the case for shallow lakes that are presumed to be the most common and vulnerable lake type to global warming [2,5,6]. We may expect an increasing degree of heterotrophy in shallow lake ecosystems with increasing temperatures [9,11], as demonstrated experimentally in a recent cross-European shallow lakes experiment [12]

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Results

Discussion

Conclusion