Intra‐annual variation of phytoplankton community responses to factorial N, P, and CO<sub>2</sub> enrichment in a temperate mesotrophic lake
Egor Katkov, Étienne Low‐Décarie, Gregor F. Fussmann
Abstract
Abstract Across primary producer communities in different lakes, nitrogen (N) and phosphorus (P) can exhibit many different patterns of limitation. Here, we look at the intra‐annual variability of these patterns in a single lake. Furthermore, we investigate whether a third resource, carbon dioxide (CO 2 ) can have significant effects on phytoplankton biomass and community composition. We performed five in situ lacustrine mesocosm experiments at different times of the year. In each experiment, we had a factorial design with two levels of N, P and CO 2 enrichment (no enrichment or double lake concentrations for N and P and atmospheric (400 ppm) and c. 1,000 ppm for CO 2 ) resulting in a total of eight treatments. Mesocosms of c. 1,600 L were suspended in a temperate, mesotrophic lake (Lac Hertel, Canada). Each experiment lasted 2 weeks and chlorophyll a biomass, coarse chemotaxonomic community composition (measured using fluorometry), and several environmental variables were recorded at a minimum of four time points. We found that the limiting, synergistic, and community composition effects of N and P varied between experiments. TN:TP ratios explained, in part, some of this variability, along with insolation and water temperature. Despite relatively high levels of CO 2 in the control mesocosms, we found a constant synergistic effect of CO 2 with N. In combination with the synergistic effect of P with N found in some experiments, this provides support for CO 2 as one of the multiple limiting resources in nutrient‐enriched systems. This finding could have implications for eutrophic lakes exposed to increasing concentrations of CO 2 . We also found that the effects of CO 2 on community composition varied intra‐annually. Thus, we conclude that generalised predictions about the effect of CO 2 on community composition at a coarse chemotaxonomic scale are unlikely to hold, but predictions specific to season and system are likely to be reliable.