Influence of trace metal additions on the longevity of algal blooms across a productivity gradient.

Phytoplankton require specific nutrients in varying quantities for growth, defense, and proper cell functioning. When macronutrients are present in excess, trace metals can exhibit co-limitation on phytoplankton. This experiment aimed to understand how ambient nutrient concentrations impact phytoplankton community persistence (i.e., bloom longevity). To examine potential interactions, three ponds of escalating trophic states were selected. Pond water was transferred into sealed, clear, floating plastic containers inside a floating frame within each pond. The containers were treated with a full factorial design of additions of nitrogen, phosphorus, and/or a mixture of trace metals. In the oligotrophic pond, the containers treated with nitrogen, phosphorus, and trace metals had 39% more phytoplankton than those treated with just nitrogen and phosphorus, by the end of the experiment. In the mesotrophic pond, the containers treated with nitrogen and trace elements had 14% more phytoplankton than those with nitrogen alone, by the conclusion of the experiment. In the eutrophic pond, no combinations of added nutrients affected algal growth when compared to the control treatment. These results show that the addition of trace metals along with nitrogen and phosphorus allowed phytoplankton to resist the effects of nutrient starvation when compared with treatments that only provided nitrogen and phosphorus. These results indicate the utility of trace metals for sustaining rather than limiting algal growth.