Spatio-temporal variability of dissolved oxygen within the shallow subsurface zone of a freshwater pearl mussel bed

Spatio-temporal variability of dissolved oxygen (DO) concentrations was assessed within the shallow subsurface zone of a river supporting an important population of the Freshwater Pearl Mussel Margaritifera margaritifera. Understanding of hyporheic DO dynamics at the depths relevant to juvenile mussels has been hindered by the ex-situ and spot sampling approaches used in previous studies. In the current study, a combination of in-situ logging and hand held optical sensors were used to characterise DO at 0.05 m depth in two river reaches. DO was generally high (>80% saturation) and similar to surface water at most locations for most of the twelve month monitoring period. There was considerable temporal variability in DO within monitoring locations, but this only corresponded to changes in discharge at a single location. There was also considerable spatial variation in DO, with values ranging from close to zero to 100% saturation during summer low flow conditions. In some instances DO concentrations differed markedly between sampling points located only a few meters apart. The study demonstrates how in-situ sampling approaches and sensor technologies offer great potential to improve understanding of the influence of DO on mussels and, consequently, the subsurface habitat conditions needed for their conservation. Even at shallow depths, dissolved oxygen can be much reduced relative to streamwater.