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Climate warming and nutrient pollution may interact to alter future shallow lake ecosystems

December 23, 2016
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A shallow lake ecosystem, Langer Teich, in Germany. Image: Mathias Liebing | Flickr Creative Commons

Combined future effects of climate warming and nutrient enrichment may lead to increased variability in bacterioplankton communities in shallow lakes, according to a new study in ISME, the journal of the International Society for Microbial Ecology.

Bacterioplankton are bacterial plankton which drift in the water column (‘plankton’ is derived from the Greek word πλανκτος or planktos, meaning ‘wanderer’ or ‘drifter’). Bacterioplankton play a number of important roles in aquatic ecosystems, particularly the decomposition of organic matter and nitrogen fixation.

Bacterioplankton are the largely-invisible ‘engine room’ of aquatic systems, supporting numerous cycling and recycling processes which help support and maintain a wider ecosystem. As such, understanding how bacterioplankton communities are likely to respond to future climate change and other human pressures is a key research topic for aquatic scientists and managers.

A research team led by Lijuan Ren, from the Chinese Academy of Sciences, and including MARS team member Erik Jeppesen from Aarhus University in Denmark used a series of mesocosms – artificial micro-lake environments in which conditions can be closely controlled – to run experiments simulating possible future climate changes and nutrient enrichment scenarios.

The 24 outdoor mesocosm experiments – located in Central Jutland, Denmark – were run over eight and a half years, as part of the world’s longest running lake mesocosm experiment studying the impacts of climate change. Each mesocosm has inflows and outflows of water from the local environment, with a water ‘residence time’ of around two and a half months: mimicking natural lake systems.

The scientists found that neither climate warming (simulated under the IPCC A2 scenario) nor nutrient enrichment had significant effects on bacterioplankton diversity in the individual mesocosms.

However, where higher levels of climate warming (50% above the IPCC A2 scenario) were simulated together with nutrient enrichment, bacterioplankton beta diversity (that is, the diversity between different habitats) was increased. What this tells us is that combined climate warming and nutrient pollution of lake systems may cause increased variability in bacterioplankton communities between ecosystems in the future.

The composition of bacterioplankton communities also changed under combined high-warming-high-nutrient conditions. The abundance of some species such as Actinobacteria decreased, whilst the percentages of Cyanobacteria, and some rare and unclassified phyla increased.

The results indicate that significant future climate warming coupled with high levels of nutrient pollution are likely to significantly alter the diversity and composition of bacterioplankton communities in shallow lakes.

The implications of this finding for the health and diversity of shallow lake ecosystems is as yet uncertain. However, given the key role of bacterioplankton in cycling nutrients in aquatic systems, their responses to climate warming and nutrient enrichment observed in this study are likely to be significant in influencing wider shallow lake ecology.

More broadly, the study provides more evidence of the potential impacts of combined multiple stressors in freshwaters. As aquatic systems become increasingly pressurised, we are learning that the intricate ecological networks that support them are being increasingly threatened.

Ren, L et al (2016) Warming and nutrient enrichment in combination increase stochasticity and beta diversity of bacterioplankton assemblages across freshwater mesocosms, ISME, December 2016

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