MEPS 297:215-223 (2005)  -  doi:10.3354/meps297215

ABSTRACT: Low dissolved oxygen poses a major threat to coastal marine ecosystems worldwide. This study demonstrates that both growth and reproduction of the amphipod Melita longidactyla are impaired even by moderately low dissolved oxygen levels (3.5 to 4.5 mg O₂ l^–1), which are higher than levels considered to be hypoxic (2.8 mg O₂ l^–1). Negative growth and decreases in respiratory energy expenditure were observed after exposure to moderately low oxygen levels for 3 wk. The RNA:DNA ratio was most sensitive, and decreased significantly (by 50 to 86%) following exposure to 4.5 and 3.5 mg O₂ l^–1 for 1 wk. Amphipods exposed to 3.5 or 4.5 mg O₂ l^–1 for 1 wk followed by recovery in normoxia for a further week exhibited no significant change in growth, energy consumption, respiratory energy expenditure and scope for growth compared with individuals kept in normoxia for the whole period. Complete reproductive failure occurred when amphipods were exposed to 3.5 mg O₂ l^–1 for 1 mo; however, no significant difference was found in percentage copulation, number of broods and offspring or fecundity between the normoxic control and the 4.5 mg O₂ l^–1 treatment group, indicating that reproductive impairment occurs within a narrow range of oxygen decrease below 4.5 mg O₂ l^–1. M. longidactyla plays an important role in trophodynamics and nutrient recycling and is abundant along the Chinese coast, where dissolved oxygen is often low. Our results suggest that natural populations of this species are potentially under threat, and the present water quality objective of 4 mg O₂ l^–1 adopted in some countries/places may not afford protection for this ecologically important species.

KEY WORDS: Amphipod · Oxygen · Growth · Reproduction · RNA:DNA ratio