Phytoplankton are vital primary producers in both marine and freshwater communities. Rising lake temperatures caused by global climate change will alter phytoplankton community diversity. In addition, changing nutrient inputs and invasive phytoplankton may also impact community structure.  How will phytoplankton communities reorganize in response to interacting global change stressors such as rising temperatures, altered nutrients and invasive species? Answering these questions can bring insight into the future structure, functioning and overall health of aquatic ecosystems.

We assembled a phytoplankton community consisting of ten different species from major taxonomic groups, diatoms, green algae, cryptophytes, and cyanobacteria typical of temperate lakes.  We then subjected these replicate communities to treatments designed to reflect the projected warmer conditions of temperate lakes due to global warming along with the changing nutrients and competitive pressure of an invasive harmful nitrogen-fixing cyanobacterium (Cylindrospermopsis raciborskii). The treatments were the current peak summer average lake temperature (25˚C), projected peak summer average lake temperature (30˚C), low and high nutrient (phosphorus) availability.
Results/Conclusions

At lower temperature (25˚C) C. raciborski successfully outcompetes the other species of phytoplankton, but only under phosphorus limiting conditions. At higher temperature (30˚C), C. raciborskii dominates at all levels of nutrient availability. Green algae (Chlamydomonas, Scenedesmus and Ankistrodesmus) are better competitors in phosphorus-rich conditions in low temperatures. The projected increases in temperature will likely promote C. raciborskii invasions into temperate regions.