Interacting effects of simulated eutrophication, temperature increase, and microplastic exposure on Daphnia

The effects of multiple stressors are difficult to separate in field studies, and their interactions may be hard to predict if studied in isolation. We studied the effects of decreasing food quality (increase in cyanobacteria from 5 to 95% simulating eutrophication), temperature increase (by 3 degrees C), and microplastic exposure (1% of the diet) on survival, size, reproduction, and fatty acid composition of the model freshwater cladoceran Daphnia magna. We found that food quality was the major driver of Daphnia responses. When the amount of cyanobacteria increased from 5 to 95% of the diet, there was a drastic decrease in Daphnia survival (from 81 +/- 15% to 24 +/- 21%), juvenile size (from 1.8 +/- 0.2 mm to 1.0 +/- 0.1 mm), adult size (from 2.7 +/- 0.1 mm to 1.1 +/- 0.1 mm), and reproduction (from 13 +/- 5 neonates per surviving adult to 0), but the decrease was not always linear. This was most likely due to lower availability of lipids, eicosapentaenoic acid (EPA), and sterols from the diet. Micro plastic exposure did not affect Daphnia survival, size, or reproduction. Food quality had an interactive effect with temperature on fatty acid content of Daphnia. Total fatty acid content of Daphnia was almost 2-fold higher at 20 degrees C than at 23 degrees C when fed 50% cyanobacteria. This may have implications for higher trophic level consumers, such as fish, that depend on zooplankton for energy and essential lipids. Our findings suggest that as proportions of cyanobacteria increase, in tandem with water temperatures due to climate change, fish may encounter fewer and smaller Daphnia with lower lipid and EPA content.

Automated summary

The quality of food is the major driver of responses in Daphnia, with an increase in cyanobacteria negatively impacting survival, size, and reproduction. Microplastic exposure did not affect Daphnia, but food quality interacted with temperature to influence fatty acid content, with potential implications for higher trophic level consumers.