The scope of extreme-environment diving defined within this work encompasses diving modes outside of the generally accepted no-decompression, open-circuit, compressed-air diving limits on selfcontained underwater breathing apparatus (scuba) in temperate or warmer waters. Extreme-environment diving is scientifically and politically interesting. The scientific diving operational safety and medical framework is the cornerstone from which diving takes place in the scientific community. From this effective baseline, as evidenced by decades of very low DCS incidence rates, the question of whether compressed air is the best breathing medium under pressure was addressed with findings indicating that in certain depth ranges a higher fraction of oxygen (while not exceeding a PC 2 of 1.6 ATA) and a lower fraction of nitrogen result in extended bottom times and a more efficient decompression. Extremeenvironment diving under ice presents a set of physiological. equipment, training and operational challenges beyond regular diving that have also been met through almost 50 years of experience as an underwater research tool. Diving modes such as mixed-gas, surface-supplied diving with helmets may mitigate risk factors that the diver incurs as a result of depth, inert gas narcosis or gas consumption. A close approximation of inert gas loading and decompression status monitoring is a function met by dive computers, a necessity in particular when the diver ventures outside of the single-dive profile into the realm of multi-level, multi-day repetitive diving or decompression diving. The monitoring of decompression status in extreme environments is now done exclusively through the use of dive computers and evaluations of the performance of regulators under ice have determined the characteristics of the next generation of life-support equipment for extreme-environment diving for science. These polar, deep and contaminated water environments require risk assessment that analyzes hazards such as cold stress, hydration, overheating, narcosis, equipment performance and decompression sickness. Scientific diving is a valuable research tool that has become an integral methodology in the pursuit of scientific questions in extreme environments of polar regions, in contaminated waters, and at depth.

Many of the world’s lakes are located at high latitudes in the northern hemisphere where seasonal ice cover is common. The ice restricts lake-atmosphere gas exchange, causing an under-ice accumulation of CO2. However, there are few spatial under-ice CO2 studies of river influence on lakes. Here, I examine the river plume of a river-influenced bay and compare it to a control bay without river influence in lake Örträsk, a humic of 7 km2 lake in boreal Sweden. There was no relationship between CO2 or DIC and distance from the Vargån river outlet in Vargån bay, even though the CO2 and DIC concentration of the river was found lower in Vargån river (median CO2: 53,2 µM; DIC: 178 µM) than in Vargån bay (median CO2: 84,7 µM; DIC: 301 µM). The median values of the control bay were CO2: 92,7 µM and DIC: 345 µM. There was a negative relationship in CO2 concentration with distance in the control bay, but not in DIC. The control bay had a higher concentration of CO2 at 0-60 m distance than Vargån bay, but there was no difference at 70-180 m. The DIC concentration was higher in the control bay than in Vargån bay. I have reported lower CO2 and DIC concentrations in the river than in the lake, which is not in accordance with literature and is probably due to an earlier ice-melt in the river than in the lake. The negative CO2 relationship in the control bay is likely due to sediment respiration.