Diving through the thermal window: implications for a warming world

Population decline and a shift in the geographical distribution of some ectothermic animals have been attributed to climatic warming. Here, we show that rises in water temperature of a few degrees, while within the thermal window for locomotor performance, may be detrimental to diving behaviour in air-breathing ectotherms (turtles, crocodilians, marine iguanas, amphibians, snakes and lizards). Submergence times and internal and external body temperature were remotely recorded from freshwater crocodiles (Crocodylus johnstoni) while they free-ranged throughout their natural habitat in summer and winter. During summer, the crocodiles' mean body temperature was 5.2±0.1°C higher than in winter and the largest proportion of total dive time was composed of dive durations approximately 15 min less than in winter. Diving beyond 40 min during summer required the crocodiles to exponentially increase the time they spent on the surface after the dive, presumably to clear anaerobic debt. The relationship was not as significant in winter, even though a greater proportion of dives were of a longer duration, suggesting that diving lactate threshold (DLT) was reduced in summer compared with winter. Additional evidence for a reduced DLT in summer was derived from the stronger influence body mass exerted upon dive duration, compared to winter. The results demonstrate that the higher summer body temperature increased oxygen demand during the dive, implying that thermal acclimatization of the diving metabolic rate was inadequate. If the study findings are common among air-breathing diving ectotherms, then long-term warming of the aquatic environment may be detrimental to behavioural function and survivorship.

Artificial destratification options for reservoir management

Reservoir stratification impacts reservoir and downstream water quality, creating complex management challenges driven by interactions between hydrodynamics, weather patterns, and nutrient dynamics. Artificial destratification is one technique used to ameliorate the impacts of stratified reservoirs, with bubble plumes or mechanical mixers being the primary methods employed. This global review assessed 138 bubble plume and mechanical mixer artificial destratification systems installed in 114 reservoirs to evaluate the comparative effectiveness of each method. Destratification systems were assessed in terms of their effectiveness in breaking thermal stratification and consequently mitigating cold water pollution, increasing dissolved oxygen concentrations throughout the water column, reducing the concentration of soluble metals, and reducing (potentially toxic) cyanobacteria populations. Bubble plume destratification was found to be more effective than mechanical mixing at mitigating the impacts of thermal stratification. Successful thermal destratification was closely linked to subsequent increases in dissolved oxygen concentrations and decreases in manganese and iron concentrations. Mixed results were observed for the reduction of cyanobacteria populations from artificial destratification; however, a correlation was observed between cyanobacteria control and successful thermal destratification in deeper reservoirs. Achieving thermal destratification was closely linked to the ratio of the reservoir capacity to the air flowrate used for destratification (the "volumetric destratification coefficient"). Failed thermal destratification was observed in reservoirs where the volumetric destratification coefficient was less than approximately 0.005 L/s/ML. This review identified the potential for scalability of bubble plume destratification across different reservoirs, but future research needs to provide more quantitative data that can be used to develop holistic design guidelines for bubble plume destratification systems for a wide range of reservoirs and operational conditions.