Effects of nutrients and physical lake characteristics on bacterial and phytoplankton production: A meta-analysis

We performed a meta-analysis comprising field (300 studies) and experimental data (249 studies) from a wide range of lake trophic states and locations. We examined the effects of nitrogen (N), phosphorus (P), carbon (dissolved organic matter [DOM]), temperature, latitude, and lake morphometry on the absolute and relative rates of phytoplankton primary production (PPr) and secondary bacterial production (BP). Areal and volumetric rates of PPr, BP, and BP : PPr were compared, and we analyzed differences between experimental and natural systems. Both field studies and experimental results showed agreement with regard to N and P as predictors of volumetric PPr and BP, respectively, despite the large variation in study duration, size, and nutrient addition rates in experimental systems. This indicates that bacteria and phytoplankton do not seem to be competing for the same nutrients. Areal measurements were more difficult to predict and were more dependent on physical lake characteristics than nutrients. Temperature was positively correlated with PPr, but not with BP. BP : PPr was stable across experiments regardless of N, P, DOM, or glucose additions. In contrast, BP : PPr ratios varied greatly in the field data set and were highest in systems with low total N and at high latitudes. This pattern was driven by reduced PPr, not BP; therefore, experimenters may need to manipulate PPr to change BP : PPr. Collectively, our results indicate that increased temperatures and N availability will lead to higher PPr and lower BP : PPr, potentially decreasing the importance of energy mobilized through the microbial food web on a global scale.