Challenges in derivation of water quality guidelines for dissolved oxygen, the alien

Eutrophication and hypoxia are now a wide spreading problem worldwide. Setting up water quality guidelines (WQG) for dissolved oxygen is inarguably important, and yet very difficult. First, dissolved oxygen (DO) is very different from other water quality parameters in many aspects. Both temporal and spatial variations of DO in a single water body are typically large, and this is further augmented by marked diurnal oscillations. This makes it extremely difficult to have a representative estimate of DO without extensive measurements (i.e., different days including both sunny and dull days, day and night, and different depths), which is often impractical. Jurisdictions adopting average DO in their WQG is obviously wrong and misleading. Second, DO levels may not necessarily be low enough to cause mortality. More often, low to moderate DO levels may lead to growth reduction, reproductive impairment and abnormal development, thereby posing a threat to sustainability of the aquatic species. While numerous studies have been carried out to determine the short term acute effects caused by extreme hypoxic conditions, there is an acute shortage of scientific data on chronic exposure and sublethal effects of hypoxia. The required scientific studies, albeit important, are understandably difficult, since: (a) the effective window for sublethal hypoxic effects to occur is typically narrow, and (b) the physiological response and adaptive strategy of most species is largely dependent upon the period of hypoxic exposure. Third, tolerance of aquatic organisms varies considerably among different species (particularly between warm-water and cold-water species), and different life stages (eggs, larvae, adults). Reproductive and developmental stages appear to be more sensitive. Very few information is available on the hypoxic tolerance of marine and estuarine species, especially for warm water species. Since most existing WQG for DO are derived from cold water and/or freshwater species, their applicability to warm water marine environments is questionable. The use of a field-based biological assessment approach for deriving WQG of DO will be discussed.