- Elevated lake chloride concentration has been observed in many regions, due to human activities such as mining, agriculture, and urbanisation. Meanwhile, lakes are also experiencing increasing frequency and intensity of heatwaves. The combination of elevated salinity and heatwaves has not been thoroughly studied in freshwater communities, limiting our ability to predict outcomes of future disturbances.
- We conducted a mesocosm experiment to investigate the individual and interactive effects of increased salinity and heatwaves on a freshwater zooplankton community. The combined effects of the two stressors were examined in two scenarios: when they occurred simultaneously and when a heatwave was preceded by an 8-week increase in salinity. We expected to see a synergistic effect when the two stressors were applied simultaneously, as organisms might experience energy deficiency due to physiological changes caused by salinity stress and be overwhelmed by the heat treatment. When the two stressors were applied sequentially, we expected them to act independently as the two stressors trigger different physiological responses and physiological homeostasis may have already recovered from previous salt exposure and not influence an organism's response to a subsequent stressor.
- Individually, increased salinity and heatwave conditions both impaired zooplankton communities with largest effects on copepod nauplii and cladocerans. Together, these stressors caused antagonistic effects on total zooplankton abundance and biomass in both the simultaneous and sequential scenarios, with the combined effects being similar to the salt-only effects.
- Our experiment illustrates the potential for heatwaves to have hidden effects when they occur in lakes experiencing salinisation. The findings suggested that the two stressors negatively impacted some zooplankton taxonomic groups, and at the community level, they acted antagonistically such that the occurrence of a 3-day heatwave did not cause any additional loss of abundance or biomass regardless of whether the community was exposed to the sequential or simultaneous scenario. Our findings also illustrated that even when the two stressors were decoupled in time, the community could still be influenced by a previous stressor.
During summer 2019, we conducted an outdoor mesocosm experiment at Queen's University Biological Station (Ontario, Canada; 44.568, −76.324). We established two Cl− concentrations: 6 mg Cl−/L (no salt addition, ambient lake water concentration) and 350 mg Cl−/L, representing a common summer Cl− concentration in or near urban areas in Ontario (Lawson & Jackson, 2021). We added 103.2 g of laboratory-grade sodium chloride (NaCl) into mesocosms to achieve the high salinity treatment. Exposure time for the salt treatment was 8 weeks because salt can remain in lentic habitats for long periods forming a press stressor and our time span is long enough for the zooplankton community to respond. We simulated heatwave events by increasing water temperature 5.2 ± 0.4°C above ambient water temperature (average ambient water temperature was about 21.0°C) for 3 days (Figure S1), using two 50-W or one 100-W aquarium heaters (Pawfly HT-2100, Guangdong, China; Aqueon 6101/6100, Phoenix, USA) for each heated mesocosm. It took an additional 12 hr for the water temperature to reach the desired level. So far there is no universally accepted definition of heatwave conditions (Raha & Ghosh, 2020), and we chose 3 consecutive days as our heatwave exposure time as it is a common duration of heatwaves in meteorology studies (e.g., Korea in Lee et al., 2016; USA in Robinson, 2001; Europe in Schoetter et al., 2015).
We filled 42 tanks with 180 L of water from nearby Lake Opinicon that was filtered through 50-μm mesh to remove larger invertebrates and zooplankton but not smaller phytoplankton. We allowed phytoplankton to grow for 1 week before adding zooplankton. To create a diverse community, zooplankton were collected from six lakes: South Otter, Big Salmon, Round, Lindsay, Doe, and Opinicon (Table S1), using a 50-μm mesh net, pooled, and evenly distributed among mesocosms. Each mesocosm was covered with a 1-mm mesh screen to prevent colonisation by aerial insects. Initial zooplankton and chlorophyll-a samples were collected before adding salt.