Nutrient Inputs in Mesocosms of an Oligotrophic Lake Fail to Sustain an Algal Bloom, Lake George, NY, 2019

Harmful algal blooms (HABs) pose a significant threat to aquatic ecosystems, and their frequency in oligotrophic lakes—previously thought resistant to such blooms due to their low nutrient levels—has been increasing. This challenges the traditional view that nutrient loading, a major driver of HABs, is less relevant in low-nutrient systems. To investigate whether nutrient enrichment plays a role in HAB development in oligotrophic lakes, we used in-lake mesocosms to assess the effects of varying nitrogen and phosphorus levels on pelagic communities over a 4-week period. The experiment utilized 20 mesocosms (~2,000 L each), with varying phosphorus (P) and nitrogen (N) concentrations, all maintaining a 30:1 N:P ratio, representing ten distinct nutrient levels (235:8 µg/L [control], 600:20 µg/L, 1200:40 µg/L, 1800:60 µg/L, 2400:80 µg/L, 3000:100 µg/L, 3600:120 µg/L, 4200:140 µg/L, 4800:160 µg/L, and 5400:180 µg/L). Nutrients were added twice a week (Tuesdays and Fridays) to sustain the concentrations, with two replicates for each of the ten treatments, totaling 20 experimental units. Water samples were collected to assess water chemistry at two points during the experiment (days 12 and 26). The water chemistry variables measured included total phosphorus (TP), total dissolved phosphorus (TDP), dissolved reactive phosphorus (DRP), total nitrogen (TN), nitrate, nitrite, ammonium, and chlorophyll a. Phytoplankton abundance and abiotic conditions (dissolved oxygen, temperature, pH, and turbidity) were monitored on days 5, 7, 10, 13, 17, 19, 21. Grab samples were also collected on day 12 and 26 to identify phytoplankton species/abundance and zooplankton abundance.  In this experiment, nutrient additions enhanced the fluorescence of chlorophyll a (indicative of total phytoplankton) and phycocyanin (a proxy for cyanobacteria), these increases plateaued at low nutrient levels and were transient. Phytoplankton species identification and enumeration revealed no significant changes in community composition, and the abundance of the dominant zooplankton group, cladocerans, remained unaffected. Nutrient additions also maintained elevated concentrations of total phosphorus and nitrogen, with readily available dissolved phosphorus and nitrate throughout the experiment.