ZOOPLANKTON RESPONSES TO HYPOXIA - BEHAVIORAL-PATTERNS AND SURVIVAL OF 3 SPECIES OF CALANOID COPEPODS

Seasonally recurrent and persistent hypoxic events in semi-enclosed co astal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of <2.0 ml l(-1) Shifts in the distribution patterns o f zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investig ated. This study assessed interspecific differences in responses to hy poxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), A cartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of m oderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l(-1), sharp declines in survival at d.o. = 0.9 to 0.6 ml l(-1), and 100% mortality with d.o. = 0.5 ml l(-1). Ad ults oft. aestiva and C. hamatus were more sensitive to oxygen depleti on: both species experienced significant decreases in survival for d.o . = 1.0 ml l(-1). Nauplii of L. aestiva and A. tonsa showed no signifi cant mortality with d.o. = 1.1 to 1.5 ml l(-1) and d.o. = 0.24 to 0.5 ml l(-1), respectively. In addition, experiments investigating behavio ral avoidance of moderate to severe hypoxia were carried out for adult s of all three species. None of the three species effectively avoided either severely hypoxic (d.o. <0.5 ml l(-1)) or moderately hypoxic (d. o. approximate to 1.0 ml l(-1)) bottom layers in stratified columns. T hese results suggest that in nearshore areas where development of zone s of d.o. <1.0 ml l(-1) may be sudden, widespread, or unpredictable, p atterns of reduced copepod abundance in bottom waters may be due prima rily to mortality rather than avoidance.