Will Climate Change Enhance Algal Blooms? The Individual and Interactive Effects of Temperature and Rain on the Macroalgae Ulva

Global climate change has led to increased sea surface temperatures and altered precipitation patterns worldwide. Concurrently, macroalgal blooms in coastal systems have been increasing in frequency and severity globally and are successful due to their fast growth rates and broad environmental tolerances. Here, we examine the responses of the bloom-forming green algae Ulva compressa and U. lacinulata to individual and interactive effects of increased temperature and rain (divided into salinity and nitrogen effects). We assessed these impacts on the growth, photosynthetic efficiency, and tissue carbon and nitrogen content of both species. We found that temperatures ranging from 15–27 °C had no significant effect on the growth rate of either U. compressa or U. lacinulata, although both species tended to have lower growth rates after exposure to 27 °C for 3 weeks. High nitrogen, defined as a pulse of 60 µM NO₃ and 20 µM NH₄, enhanced the growth rate of U. compressa but not U. lacinulata and did not impact the photosynthetic efficiency of either species, although the C:N ratio revealed significant N limitation in both species. Lastly, we found the effects of temperature and rain (via added nitrogen) were not additive. Our results suggest that direct deposition of nitrogen via rain will enhance the growth of U. compressa, while increased temperatures likely will not. Together, our findings indicate that the prevalence of macroalgal blooms in coastal systems will likely increase under global climate change.