Abrupt stop of deep water turnover with lake warming: Drastic consequences for algal primary producers

Yana Yankova,Oliver Köster,Stefan Neuenschwander,Thomas Posch

doi:10.1038/s41598-017-13159-9

Yana Yankova, Oliver Köster + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-13159-9

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Abstract

After strong fertilization in the 20th century, many deep lakes in Central Europe are again nutrient poor due to long-lasting restoration (re-oligotrophication). In line with reduced phosphorus and nitrogen loadings, total organismic productivity decreased and lakes have now historically low nutrient and biomass concentrations. This caused speculations that restoration was overdone and intended fertilizations are needed to ensure ecological functionality. Here we show that recent re-oligotrophication processes indeed accelerated, however caused by lake warming. Rising air temperatures strengthen thermal stabilization of water columns which prevents thorough turnover (holomixis). Reduced mixis impedes down-welling of oxygen rich epilimnetic (surface) and up-welling of phosphorus and nitrogen rich hypolimnetic (deep) water. However, nutrient inputs are essential for algal spring blooms acting as boost for annual food web successions. We show that repeated lack (since 1977) and complete stop (since 2013) of holomixis caused drastic epilimnetic phosphorus depletions and an absence of phytoplankton spring blooms in Lake Zurich (Switzerland). By simulating holomixis in experiments, we could induce significant vernal algal blooms, confirming that there would be sufficient hypolimnetic phosphorus which presently accumulates due to reduced export. Thus, intended fertilizations are highly questionable, as hypolimnetic nutrients will become available during future natural or artificial turnovers.

Highlights

Global warming changes physical and chemical properties of lakes and catchments[1], as well as their biota[2]
Algal spring blooms are followed by high abundances of herbivores causing the breakdown of phototrophs, which is mirrored by high water transparency
Our study is based on a long-term monitoring of Lake Zurich which can be considered as a model ecosystem for temperate European lakes[3,17,18,19]

Summary

Introduction

Global warming changes physical and chemical properties of lakes and catchments[1], as well as their biota[2]. Nutrients in combination with higher irradiance, rising water temperature and reduced turbulence[10] enable photoautotrophic organisms (algae) to develop massive populations, so called spring blooms[11] This first boost in primary production is the basis for organisms’ successions within food webs[12,13]. The phototrophic spring bloom community, commonly formed by diatoms and cryptophytes in temperate lakes[15], is cut off from essential nutrients such as PO4-P and nitrogen (mainly nitrate NO3-N) This reduction in primary production will affect the entire food web as algae are the major source of substrates for bacteria and of food for consumers[13]. This phenomenon may propagate up to the level of top predators, causing drastic decreases in fish stocks[16]

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Conclusion