Spatiotemporal Variation in Dissolved, Bioavailable, and Particulate Elements and the Abundance of Harmful Algae in Grand Lake

Harmful algal blooms (HABs) are often linked to the increased loading of limiting nutrients such as nitrogen and phosphorus. Little is known about the relevance of other biogenic elements, the supplies of which are spatiotemporally heterogeneous, on HABs. We measured the dissolved, bioavailable, and particulate concentrations of 26 elements at four locations draining different catchments of a large reservoir during three seasons, in addition to the total abundance of phytoplankton and % of cyanobacteria. Finally, we manipulated a key element (Fe) in microcosms to test its effect on the community. Phytoplankton abundance and community structure varied spatiotemporally, with minimal variation in N/P. The variation in environmental supplies of several other elements was correlated with phytoplankton abundance, as well as up to 3 orders of magnitude differences in cyanobacterial yield. Bioassays manipulating Fe impacted total phytoplankton as well as the abundance of cyanobacteria, with Fe-chelated treatments resulting in a significant decline in phytoplankton as well as cyanobacterial yield. In summary, we found substantial heterogeneity in elemental supplies that are relevant to the phytoplankton community. Exploring the relevance of the entire system of elements in the context of HABs may be more rewarding than studies emphasizing a subset of elements.