Nutrient inputs in mesocosms of an oligotrophic lake fail to sustain an algal bloom

Abstract Harmful algal blooms (HABs) rapidly change and threaten aquatic ecosystems. HABs in oligotrophic lakes are becoming more commonly observed, despite the long‐held paradigm that algal blooms cannot happen in low‐nutrient systems. This raises the question of whether nutrient loading, understood as a major driver of HABs, plays an important role in the potential for HAB development in oligotrophic lakes. Using in‐lake mesocosms, we examined the effects of a gradient of nitrogen and phosphorus additions on the pelagic communities of an oligotrophic lake over 4 weeks. We hypothesized that increasing nutrients would result in an increased abundance of phytoplankton and an increased dominance by cyanobacteria. Although our nutrient additions caused an increase in the fluorescence of chlorophyll a (i.e., a proxy for total phytoplankton) and phycocyanin (i.e., a proxy for total cyanobacteria), these increases plateaued at low‐nutrient concentrations and the increases were short‐lived. The identification and enumeration of phytoplankton species confirmed that the composition of the phytoplankton also did not change with added nutrients, nor did the abundance of the dominant zooplankton group (i.e., cladocerans). Furthermore, our biweekly nutrient additions maintained elevated concentrations of total phosphorus and total nitrogen in the water column, with total dissolved phosphorus and nitrate both readily available throughout the experiment. Given that light is very abundant (Secchi depths of 9–10 m), this suggests that another nutrient, such as iron or carbon, may have limited phytoplankton growth. Our findings suggest that excess nutrients may not always drive HABs in oligotrophic lakes and further studies should examine the effects of other micronutrients using similar controlled seminatural mesocosms.