COS 26-6 - A Change is Gonna Come… but will resilience remain?

Background/Question/Methods

Climate warming and changes to Earth’s natural biogeochemical cycles have already caused a notable increase in lake browning and eutrophication. In turn, these events may influence the resilience and health of lake ecosystems by, for instance, shifting the phytoplankton community towards a cyanobacteria-dominated state and increasing the importance of the microbial pathway in the transfer of food to higher trophic levels. Studies examining the synergistic effects of these stressors on food webs remain either limited to short time frames, to a single driver or to drivers with narrow range of variation. Indeed, the combined effect of seasonality, browning and eutrophication on aquatic food webs (from microbes to higher trophic levels) remains unclear. To address this gap, we conducted a nine-week mesocosm experiment from Summer to Fall 2021 at Lake Hertel (Qc, Canada). Here, a fully factorial experimental design was applied, crossing treatments of dissolved organic carbon (DOC) with phosphorus + nitrogen (TP + TN), to examine how each alone and together altered the structure of two trophic levels (i.e., phytoplankton and zooplankton). Physico-chemical data (oxygen, pH, specific conductivity, and temperature), phytoplankton groups and zooplankton genera were measured every four weeks.

Results/Conclusions

DOC and TN+TP concentrations were highest at the start of experiment and decreased thereafter. Temperature likewise progressively decreased from week 1 (27^oC) to week 9 (11^oC). In contrast, pH, conductivity and oxygen concentrations peaked midway through the experiment (week 5) in nutrient enriched mesocosms. To examine relationship among phytoplankton, zooplankton and environmental gradients, we applied a multiple factor analysis (MFA). This multivariate correlation allowed us to determine which taxa were most correlated to high vs low treatment conditions. At the start and end of the experiment, there was a clear separation of the communities among levels of DOC. By week 5, however, we noted a change in community dissimilarity among mesocosms and a reduced association to DOC. In general, cyanobacteria, diatoms and cryptophytes dominated in high DOC treatments, while green algae were more strongly associated with low DOC (3.5 mg/L). With respect to zooplankton, Bosminid responded positively to low DOC, Ceriodaphnia to intermediate treatments, and Chydorus were correlated to highest nutrient level. Upcoming analyses of environmental DNA will help shed some light on dynamics of the bacterial community and whether these could have become a greater food source under elevated DOC levels.