The Distribution of Two Marine Cladocerans During Upwelling and Relaxation Events Off the Oregon Coast
Offshore surface currents that appear during upwelling events have the potential to carry nearshore coastal zooplankton offshore. I tested the hypothesis that the marine cladocerans Podon leuckarti and Evadne nordmanni are pushed offshore during summer upwelling. Additionally, stratified horizontal currents during upwelling, with surface regions moving offshore and deeper layers moving onshore, could differentially distribute reproductive stages or sizes that have different vertical distributions. I tested the hypothesis that reproductive stages and sizes of cladocerans would be distributed differentially from one another during upwelling and relaxation events. The fecundity of marine cladocerans off Oregon was also investigated. A vertically stratified transect of 7 stations located from 0.5 to 28 km offshore near Coos Bay, Oregon was sampled for zooplankton on four days in the summer of 2007. Two of the sample dates (27 June and 14 August) were characterized by upwelling conditions, and two (3 and 18 July) were characterized by relaxation or weak downwelling conditions. Preserved cladocerans were counted, measured, and assigned a reproductive stage. In females the number of embryos was counted. Neither cladoceran species showed evidence of offshore surface transport during upwelling, and both showed densest concentrations nearshore, although a deeper offshore concentration of E. nordmanni may have been affected by deep onshore upwelling currents. Parthenogenic P. leuckarti with early embryos were distributed similarly to those with advanced embryos, and males tended to stay near the bottom or surface nearshore. All stages of E. nordmanni stayed primarily near the bottom or surface nearshore. Smaller cladocerans and parthenogenic females with early embryos were found in larger proportions closer to shore, while larger cladocerans and parthenogenic females with advanced embryos were found farther offshore. Cladoceran fecundity in Oregon was high, with a mean and 95% confidence interval of 5.42±0.37 embryos per parthenogenic P. leuckarti and 10.21±0.247 embryos per parthenogenic E. nordmanni. Complex interactions between physical oceanography and cladoceran behavior may account for observed cladoceran distributions off the Oregon coast.