J.C. Ho, A.M. Michalak and N. Pahlevan
Toxic phytoplankton blooms in lakes can have adverse effects on public health and ecosystem services provided by lakes. While reports of toxic phytoplankton bloom incidents have increased recently, it remains unclear if blooms are significantly increasing globally. Using three decades of high resolution satellite data we investigated whether or not intense summertime phytoplankton blooms are increasing globally. We found that the majority of lakes studied had increases in peak summertime bloom intensity, while lakes that experienced significant decreases in bloom intensity were rare. The lakes with significantly decreased bloom intensity were also found to have experienced less warming relative to lakes that decreased in bloom intensity before subsequently increasing. This result suggests that lake warming may already be counteracting management efforts to prevent eutrophication and that an understanding of global trends and drivers of phytoplankton blooms is necessary for effective management regimes.
Figure: Global distribution of lake bloom intensity trends shows that the peak summertime bloom intensity has increased since the 1980s. The map shows bloom intensity trends for all 71 study lakes for the period 1984–2012 (Supplementary Table 1). Colours and symbols indicate whether the bloom intensity decreased (blue) or increased (red), and whether the trend is statistically significant (triangles for P < 0.1; circles for P > 0.1). The base map was generated using Generic Mapping Tools.
Freshwater blooms of phytoplankton affect public health and ecosystem services globally. Harmful effects of such blooms occur when the intensity of a bloom is too high, or when toxin-producing phytoplankton species are present. Freshwater blooms result in economic losses of more than US$4 billion annually in the United States alone, primarily from harm to aquatic food production, recreation and tourism, and drinking-water supplies. Studies that document bloom conditions in lakes have either focused only on individual or regional subsets of lakes, or have been limited by a lack of long-term observations. Here we use three decades of high-resolution Landsat 5 satellite imagery to investigate long-term trends in intense summertime near-surface phytoplankton blooms for 71 large lakes globally. We find that peak summertime bloom intensity has increased in most (68 per cent) of the lakes studied, revealing a global exacerbation of bloom conditions. Lakes that have experienced a significant (P < 0.1) decrease in bloom intensity are rare (8 per cent). The reason behind the increase in phytoplankton bloom intensity remains unclear, however, as temporal trends do not track consistently with temperature, precipitation, fertilizer-use trends or other previously hypothesized drivers. We do find, however, that lakes with a decrease in bloom intensity warmed less compared to other lakes, suggesting that lake warming may already be counteracting management efforts to ameliorate eutrophication. Our findings support calls for water quality management efforts to better account for the interactions between climate change and local hydrological conditions.