J.C. Ho and A.M. Michalak
Harmful algal blooms are expected to increase globally due to climate change. Various mechanisms have been observed explaining how climate change will impact the prevalence of harmful algal blooms, but most of these observations have been based on studies of individual lakes or a small subset of lakes. In this study, we investigate the association between climatic variables (temperature and precipitation) and total phytoplankton abundance, species dominance, and toxicity by using summertime lake observations from over twelve hundred lakes across the continental United States. We found that increased temperature was associated with increased total phosphorus concentrations and cyanobacterial growth, suggesting that temperature may influence nutrient availability in addition to creating conditions for harmful algal bloom growth. Precipitation events were found to both increase nutrient runoff and also reduce nutrient concentrations through greater flushing. Overall, nutrient concentrations explained most of the variability in harmful algal blooms, suggesting that the impacts of climate change on harmful algal blooms may be mitigated through reducing nutrient inputs into lakes. This study highlights the usefulness in identifying mechanisms that are widespread throughout a geographically diverse set of lakesfor understanding the impacts of climate change on harmful algal blooms broadly.
Figure: Geographic distribution of harmful algal blooms (HAB) indicators in green, including (a) Chl a concentration (Cchla), (b) cyanobacterial biovolume (Bcyano) and (c) microcystin concentration when above the detection limit (Cmc), and in situ (d) total nitrogen (CTN) and (e) total phosphorus (CTP) concentrations in maroon. Each circle represents one measurement from either the NLA 2007 or NLA 2012 data sets, up to 1260 in total (see “Methods” section). The size of the circle represents the magnitude of the observation, indicated by the legend.
Climate change is expected to impact the severity of harmful algal blooms in lakes and reservoirs through a number of mechanisms related to the influence of warming temperatures and changes to precipitation patterns. Evidence on the prevalence of individual mechanisms is lacking, however, with knowledge of many mechanisms restricted to studies of individual or small subsets of lakes. Here, we leverage over twelve hundred summertime lake observations from across the continental U.S. to explore evidence for the hypothesized risks from climate change attributable to specific mechanisms. Using a statistical model selection approach, we examine associations between temperature and precipitation variables and indicators of total phytoplankton abundance, species dominance, and toxicity. We find evidence in support of the hypotheses that summer temperatures drive total abundance, that the length of the summer drives cyanobacterial abundance, and that increased temperatures may reduce the observed toxicity of blooms in some cases. We find that nutrient concentrations are also likely to be impacted by lake warming, as increased temperatures are robustly associated with increased total phosphorus concentrations. Evidence for the impact of precipitation is mixed, however, as there is evidence to support that increased nutrient runoff from precipitation could support blooms but also that nutrient concentrations could be reduced through greater flushing due to precipitation. While statistical associations are not definitive evidence of formal mechanistic links, the geographic scale of the results is useful for identifying hypothesized mechanisms that are widespread across the continental U.S., and therefore for informing understanding of the influence of climate change.