Evidence for Phosphorus, Nitrogen, and Iron Colimitation of Phytoplankton Communities in Lake Erie

Three nutrient enrichment experiments involving the addition and removal of iron (Fe) alone, as well as in combination with phosphorus (P) and/or nitrogen (N), were conducted in the offshore and nearshore waters of the eastern basin of Lake Erie. Lake Erie phytoplankton are at times colimited by P, N, and Fe. This was most clearly demonstrated in the offshore, strongly stratified waters, where the Fe concentration was below detection (DL = 2.0 nmol L⁻¹), and nutrient limitation indicators (APA, P debt, C : P, N : P, N debt, and C : N) indicated strong P and moderate N limitation. Enrichment with Fe alone did not result in a significant increase in phytoplankton biomass, but the combined addition of Fe, P, and N yielded greater biomass increases than the addition of P and N alone. Phosphorus and Fe were both required to stimulate biomass increases. Nutrient limitation indicators and dissolved nutrient measurements showed that P and Fe additions facilitated the uptake of nitrate (NO $({\rm NO}^-_3),$) which is the most abundant source of N because of the low ammonium (NH $({\rm NH}^+_4)$) concentrations in Lake Erie. Our results support a colimitation hypothesis wherein the addition of Fe reduces N limitation by allowing the phytoplankton communities to use NO ${\rm NO}^-_3,$ take up more P, and become more strongly P limited. Phytoplankton communities of the eastern basin of Lake Erie and numerous other lakes experience colimitation by P, N, and Fe during the summer period of thermal stratification.