Daily temperature variation explains zooplankton community differences in freshwater mesocosms recovering from a natural drought

Abstract

Drought events are typically studied as a single stressor. However, multiple environmental variables are altered under drought conditions, including water level and daily temperature variation. Here, we experimentally manipulated both variables in 16 outdoor mesocosms to disentangle how they impact freshwater zooplankton communities. During a natural drought in summer 2022, we maintained water level in eight mesocosms while allowing eight to almost dry out. We reduced daily temperature variation using insulation in half of these outdoor mesocosms to give a fully factorial design with four replicates for each of the four treatments: Control, Low water level, Insulated, Insulated + Low water level. We found a lasting effect of low water level over the 150‐day study period; community recovery was incomplete despite natural precipitation restoring the Low water level treatment to a similar depth as Controls. However, our results consistently identified daily temperature variation as more important than water depth in explaining drought effects, with amplified daily temperature variation strongly associated with the initial divergence of zooplankton community structure and reductions in net biomass. Since daily temperature variation may be higher in small outdoor mesocosms than larger natural freshwaters, we recommend that this is considered in the design of mesocosm experiments. By taking a multiple stressor perspective, our findings indicate that temperature variation can be more important than water level in structuring freshwater invertebrate communities.