How Global Change Shapes Our World: Nitrogen and Salt Addition Affect Phytoplankton Morphology in a Small Freshwater Lake

Abstract

Global environmental change has accelerated the deposition of nitrogen and salt into freshwaters. These changes affect the morphology of freshwater microbes, especially phytoplankton, which serve as primary producers for these ecosystems. Morphology reflects a microbe’s response to energy demands, selection, and environmental disturbance. In this pilot experiment, a handmade suspension device held samples from Cedar Bog Lake (East Bethel, MN) in three nitrogen conditions (17.6 mM NaNO3, 9 mM NaNO3, 0 mM NaNO3) crossed with two salt conditions (10 mM of NaCl or 0 mM of NaCl) plus a DI water control for BG11 media in triplicate on the surface of the lake for three weeks. I used flow cytometry to quantify community-level within-sample morphology using circle fit, area-based diameter (ABD) volume, aspect ratio, elongation, ABD area, compactness, particles/mL, and perimeter. A principal components analysis (PCA) revealed trait correlations between ABD area and ABD volume, and between circle fit, elongation, and compactness. A two-way analysis of variance (ANOVA) on the first three principal components (highly significant, accounting for 94.89% of the total variance) revealed a significant effect of nitrogen and a marginally significant effect of salt on principal component three, but no effect of treatment on the first two principal components. Nitrogen had a significant effect on particles/mL but treatment did not affect any of the other morphological traits individually. These results indicate that climate change has a complicated effect on freshwater microbial morphology at the community level. Future studies should focus on long-term changes in morphology in the field, focusing on the effects of nitrogen.