Warmer climates boost cyanobacterial dominance in shallow lakes

April 10th, 2012

Dr. S. Kosten (Wageningen University) & Prof. V. Huszar (Universidade Federal do Rio de Janeiro)

Worldwide many standing waters, ranging from watering ponds for cattle to natural lakes and man-made reservoirs, suffer from nutrient over-enrichment (eutrophication). The underlying cause of eutrophication varies regionally, but urban, agricultural and industrial development are often responsible. Eutrophication of surface water has many undesirable effects and has become the major water quality issue in many freshwater and coastal systems1.

Eye-catching symptoms of eutrophication are harmful phytoplankton blooms and scums. Phytoplankton consists of planktonic organisms that derive their energy from photosynthesis, the major groups are algae and cyanobacteria. Phytoplankton blooms increase the turbidity of the water and threaten the growth of submerged plants and – subsequently – the habitat of different fish species. Die-off of blooms may also deplete oxygen concentrations leading to fish kills and bad smells.When the blooms consist of cyanobacteria, an additional threat is composed of the potent toxins they can produce.

Some of the toxins poison aquatic animals and may make freshwaters unusable by humans. Cyanobacterial problems tend to increase with cyanobacterial abundance, and the frequency and extent of such blooms is increasing. It is therefore important to understand what determines their abundance.

Obviously nutrients are important. Cyanobacteria can only reach high densities when nutrients are plentiful. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria2,3, but evidence from global scale field data has so far been scarce. Climatic extremes are important as well, as recent cyanobacterial blooms during heat waves have shown4. For many regions in the world both average temperatures and the frequency of occurrence of extreme climate events are predicted to rise5.

Besides temperature, precipitation regimes are also expected to change. These changes are expected to lead to higher nutrient concentrations in inland waters in different parts of the world. Regional increases in extreme rainfall events, for example, may lead to higher nutrient loadings from terrestrial areas to surface waters. Alternatively, in warm arid regions, an increase in nutrient concentrations may occur due to increased evaporation and subsequent higher nutrient concentrations in the remaining water6,7. The expected changes in nutrient levels and temperature may strongly influence the aquatic ecosystem (see Figure 1). How excessive nutrient loading and changes in temperature interact and affect cyanobacteria on a global scale has so far not been studied.

Our study, based on data from shallow lakes ranging from subarctic Europe (60 lakes) to southern South America (83 lakes), now reveals a global pattern, covering a wide climatic range8. Our results show that while warmer climates do not result in more phytoplankton biomass, the percentage of cyanobacteria in shallow lakes increases steeply with temperature. This is especially so when nutrient availability is high. Further, climate change and enhanced nutrient loading seem to work synergistically. In other words, the proportion of cyanobacteria relative to the total phytoplankton community increases with warming as well as with eutrophication.

Our results also reveal that the percentage of cyanobacteria is greater in turbid lakes where light availability for phytoplankton is low. This points to a positive feedback effect because decreased light availability is often a consequence of high phytoplankton densities, which in turn may be driven by nutrient loading. Climatic changes may, in various ways, enhance nutrient loading to surface waters. Cyanobacteria can profit from this and create circumstances that are especially favorable for themselves. Cyanobacterial species can sometimes be combated by forced mixing of the water column and can, in some situations, be eliminated from lakes by flushing, but nutrient control has long been identified as an effective way of reducing cyanobacterial blooms.

Our findings suggest that when temperatures rise, substantially lower nutrient loadings might be needed to reduce the risk of cyanobacterial dominance. Our findings could furthermore be interpreted as suggesting that, in order to compensate for the effects of a 1 °C increase in temperature, total nitrogen levels should be reduced by as much as one third. While the demonstration of the different synergistic effects of temperature and nutrients on such a large scale is of scientific interest, the practical implications of this study is that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.

The measures necessary to reduce nutrient loadings to surface waters vary locally. In some regions highest reductions may be achieved by eliminating untreated industrial and household waste water inputs. In other regions implementing more sustainable agriculture with less loss of nutrients to surface waters may be necessary. In arid regions restrictions of human water use are required. Yet another method to reduce nutrient inputs is the (re-)establishment of wetlands. These have a high capacity for nitrogen removal.

References:

1. Smith, V. H. and D. W. Schindler (2009). “Eutrophication science: where do we go from here?”, Trends in Ecology & Evolution, 24(4): 201-207.

2. Paerl, H. W. and J. Huisman (2008). “Blooms like it hot.” Science, (5872): 57-58.

3. Carey, C. C., B. W. Ibelings, et al. (2012). “Eco-physiological adaptations that favour freshwater cyanobacteria in a changing climate.” Water Research, 46(5): 1394-1407.

4. Jöhnk, K. D., J. Huisman, et al. (2008). “Summer heatwaves promote blooms of harmful cyanobacteria.” Global Change Biology, 14(3): 495-512.

5. IPCC (2007). “Climate Change 2007: Synthesis Report.” 52.

6. Jeppesen, E., B. Kronvang, et al. (2009). “Climate change effects on runoff, catchment phosphorus loading and lake ecological state, and potential adaptations.” Journal of Environmental Quality, 38: 1930-1941.

7. Jeppesen, E., B. Kronvang, et al. (2011). “Climate change effects on nitrogen loading from cultivated catchments in Europe: implications for nitrogen retention, ecological state of lakes and adaptation.” Hydrobiologia, 663(1): 1-21.

8. Kosten, S., V. L. M. Huszar, et al. (2012). “Warmer climate boosts cyanobacterial dominance in lakes.” Global Change Biology, 18(1): 118-126.

9. Moss, B., S. Kosten, et al. (2011). “Allied attack: climate change and nutrient pollution.” Inland waters(1): 101-105.

Dr. Kosten is a specialist on climate change effects on inland waters. She works as a researcher at the Aquatic Ecology & Water Quality Management Group, Department of Environmental Sciences of the Wageningen University and Research Centre (The Netherlands) and as a guest researcher at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin and Neuglobsow (Germany). Prof. Huszar is a Phytoplankton Ecology specialist. She works as a full professor at the Museu Nacional, Universidade Federal do Rio de Janeiro (Brasil). The article is based on an original piece of research published in Global Change Biology titled ‘Warmer climes boost cyanobacterial dominance in shallow lakes‘. The authors can be contacted at: Sarian.kosten@wur.nl and vhuszar@gbl.com.br.

This short article is the outcome of a cooperation between many South American and European universities and institutes. It is largely based on the cited Global Change Biology paper co-authored by Eloy Bécares, Luciana S. Costa, Ellen van Donk, Lars-Anders Hansson, Erik Jeppesen, Carla Kruk, Gissell Lacerot, Néstor Mazzeo, Luc de Meester, Brian Moss, Miquel Lürling, Tiina Nõges, Susana Romo and Marten Scheffer. Without them and other researchers participating in the umbrella projects “SALGA” and “ECOFRAME” this work would not have been possible. The study in Europe was partially financed by the European Community (ECOFRAME EVK1-CT-1999-00039), and in South America (SALGA) by Conselho Nacional de Desenvolvimento Cient?´fico e Tecnolo´gico (CNPq) grants 311427, 480122, 490487, Brazil; The Netherlands Organization for Scientific Research (NWO) grants W84-549 and WB84-586, The National Geographic Society grant 7864-5; PEDECIBA, Maestr?´a en Ciencias Ambientales, Donacio´n de Aguas de la Costa S.A. and Banco de Seguros del Estado, Uruguay. Individual researchers were sponsored by different sources (see GCB paper).

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.