The Challenges of Living in Hypoxic and Hypercapnic Aquatic Environments

Organisms living in coastal waters, and especially estuaries, have long been known to have behavioral or physiological mechanisms that enable them toexist in water containing low amounts of oxygen. However, the respiratory consumption of oxygen that generates hypoxia is also responsible for producing significant amounts of carbon dioxide. An elevation of carbon dioxide pressure in water will cause a significant acidosis in most aquatic organisms. Thus, the combination of low oxygen and elevated carbon dioxide that occurs in estuaries represents a significant environmental challenge to organisms living in this habitat. Organisms may maintain oxygen uptake in declining oxygen conditions by using a respiratory pigment and/or by making adjustments in the convective flow of water and blood past respiratory surfaces (i.e., increase cardiac output and ventilation rate). Severe hypoxia may result in an organism switching partially or completely to anaerobic biochemical pathways to sustain metabolic rate. There is also evidence to suggest that organisms lower their metabolism during hypoxic stress. Elevated water CO2 (hypercapnia) produces an acidosis in the tissues of organisms that breathe it. This acidosis may be wholly or partially compensated (i.e., mechanisms return pH to pre-exposure levels), or may be uncompensated. Some studies have examined the effects on organisms of exposure simultaneously to hypoxia and hypercapnia. This article reviews some of the specific adaptations and responses of organisms to low oxygen, to high carbon dioxide, and to the cooccurrence of low oxygen and high carbon dioxide