Hypoxia and seasonal temperature: Short-term effects and long-term implications for Acartia tonsa dana

Far more attention has been given to the short-term lethal impacts of reduced dissolved oxygen on commercially important fish and crabs than to the long-term sublethal impacts on these same species, or on lower trophic levels. This study demonstrates that chronic, sublethal effects of hypoxia on the copepod Acartia tonsa, a critical component of many pelagic coastal food webs, can lead to significant decreases in population growth. The results of laboratory experiments conducted at 15 °C (winter) and 25 °C (summer), under conditions of normoxia (Controls), sublethal hypoxia (1.5 ml l^− 1) and lethal hypoxia (0.7 ml l^− 1) show that egg production female^− 1 day^− 1 was significantly lower at 0.7 ml l^− 1 compared to Controls at both temperatures, while egg production female^− 1 day^− 1 was significantly lower at 1.5 ml l^− 1 compared to controls in both summer experiments and in one of the two winter experiments. Survival was significantly decreased in the 0.7 ml l^− 1 treatment compared to Controls and the 1.5 ml l^− 1 treatment. Copepods developed more slowly and matured at smaller adult body sizes at both temperatures under both lethal and sublethal hypoxia compared to normoxia. Under summer temperatures, egg production was reduced by hypoxia exposure on two counts: (1) exposure to hypoxia during development resulted in smaller adults, which translated into lower egg production, and (2) egg production was still significantly lower in hypoxia treatments compared to Controls even when differences in body size were taken into account. While copepods collected in winter and exposed to winter temperatures and hypoxia also matured at smaller body sizes than copepods exposed to normoxia, egg production in winter was almost entirely attributable to this reduction in body size. These results suggest that coastal hypoxia may have a significantly greater impact in the summer months, when copepod populations are most abundant and growing at their most rapid rate of the year. With the anticipated increases in global temperatures, hypoxia may have even greater impacts on pelagic food webs.