Biological Iron Limitation within Lake Superior's Watershed: Oligotrophic Algal Blooms Along a Land to Lake Gradient

Abstract

Growing amounts of freshwater literature highlight phytoplankton iron limitation in systems of varying hydrology and trophic status. Furthermore, global trends of increased dissolved iron in lakes could play an important role in relieving iron limitation. Concurrent rises in oligotrophic harmful algal blooms frequency could be influenced by recently relieved iron limitations. Lake Superior, while typically considered an environment resistant to harmful algal blooms, has begun to develop them along its southwest shore. This study investigated how iron limitation impacts algal communities from a land to lake transect on the south shore of Lake Superior, including an upland lake and stream. Four-day long, trace metal clean, nutrient addition bioassays with iron chelator (EDTA), phosphorus, and iron treatments using whole water were performed to measure growth differences among sites, times, and treatments with concurrent, environmental, elemental characterization. Results revealed that algal communities experienced iron limitation, once phosphorus was added, at least once in every site, irrespective of initial, dissolved iron concentration. Taxa responded differently as well, with diatoms demonstrating stronger Fe limitation effects compared to green algae. Finally, slight spatial differences indicate that algal communities from sites with previously recorded blooms seem to have experienced less iron stress than those from sites without blooms.