Variation in iron and its potential relevance to phytoplankton ecology in Oklahoma reservoirs

Abstract

The effects of nitrogen (N) and phosphorus (P) on productivity have been well characterized in freshwater ecosystems, including in the abundance of diazotrophic cyanobacteria that cause harmful algal blooms. However, there is much residual variation in these relationships. Because iron (Fe) is required for N-fixation, we tested whether Fe impacts productivity and cyanobacterial abundance using in situ P and Fe addition experiments. While P had the largest effect on total primary productivity, Fe increased cyanobacterial abundance at nearly the same rate as P. Moreover, algal Fe content was strongly correlated with cyanobacterial abundance. These results indicate that cyanobacterial abundance is positively related to total Fe in reservoirs, although there is little known about the variability in Fe among freshwater ecosystems. We surveyed 25 Oklahoma, U.S.A, reservoirs to capture variation in total dissolved Fe, and conducted nutrient addition bioassays to understand the role of Fe in productivity. We found three orders of magnitude variation in total Fe among reservoirs, although Fe alone did not limit primary production. Together, these results indicate that attention to heterogeneity in the supply of elements beyond those typically quantified in freshwaters (i.e. N and P) could be useful in understanding harmful algal blooms.