This doctoral dissertation improvement project will study factors causing algal blooms in lakes and reservoirs. An experimental approach in which the absolute concentration of phosphorus, the amount of phosphorus relative to nitrogen, and the amount of light are manipulated will be used to assess how these factors affect the occurrence and abundance of algal species. The study will also assess what combination of factors lead to toxin production by blue-green algae.
Many inland waters are impacted by excessive nutrient input, which can lead to surface scums of algae. This cultural eutrophication can lead to decreases in property values and recreational opportunities, and in some cases can result in disruptions of drinking water supplies. Understanding the factors leading to algal blooms is important in developing strategies to mitigate the effects of nutrient pollution in surface waters.
Algae are a diverse group of organisms and a subset of these algae, called cyanobacteria, are capable of producing toxins that contaminate recreational and drinking water. Human activities (such as row-crop agriculture and pastured animals) increase the amount of nitrogen and phosphorus in lakes and rivers. Surplus nutrients, in turn, lead to excessive algae growth. To date, the scientific consensus has been that phosphorus supply determines the amount of algae while the ratio of nitrogen to phosphorus determines how much of the algae is cyanobacteria. My research adds to a growing body of literature suggesting that these relationships are less straightforward, and that complex interactions of nutrients and light determine which lakes will have elevated cyanobacteria and toxins. The goal of my project was to establish which combination of phosphorus level, nitrogen to phosphorus ratio, and light availability led to abundant toxin-producing cyanobacteria. I performed large-scale experiment in which I manipulated the amount of phosphorus, the ratio of nitrogen to phosphorus, and light availability. I monitored the abundance of cyanobacteria and measured the concentration of cyanobacterial toxins present in the water. I found that the proportion of cyanobacteria was greatest in treatments with high phosphorus, but that the total amount of cyanobacteria was highest in treatments with high nitrogen and phosphorus. This is counter to expectations based on previous research. I also found that the amount of toxins was greatest in treatments with abundant nitrogen and phosphorus. Current efforts to manage cyanobacteria focus solely on controlling phosphorus pollution. My research joins a growing body of evidence that implicates total nitrogen, in addition to total phosphorus, in the growth of large amounts of toxic algae. Future efforts to keep lakes and reservoirs clean should consider the roles of both nitrogen and phosphorus in creating toxic algae blooms.
