Live and dead zooplankton distributions within the Humboldt current system during upwelling and downwelling events

Abstract

Traditional field sampling assumes all zooplankton collected are originally alive, but ignoring even a small percentage of dead zooplankton can result in large errors in predicting population dynamics and secondary production. We tested a protocol for Neutral Red staining and live/dead sorting of zooplankton samples from upwelling zones within the Humboldt Current System. In laboratory tests, live individuals of diverse copepod species including eggs, naupliar and advanced stages, as well as several gelatinous zooplankton groups, were stained readily and the staining efficiency was better than 94.6 %. Larval fish and crabs were only weakly stained. Once validated, we applied the staining method in field sampling in Mejillones Bay and Concepcion during upwelling and downwelling periods. Both abundances and percentages of zooplankton carcasses were significantly higher during upwelling events, and carcasses were mostly concentrated within the Oxygen Minimum Zone, suggesting that intrusion of oxygen-poor water during upwelling led to higher non-predatory mortality especially among species sensitive to hypoxia. Carcass abundances also varied between developmental stages, likely a result of differences in low-oxygen tolerance as well as in situ abundances between stages. Incorporation of Neutral Red staining method into routine sampling will generate live/dead zooplankton distribution data that will improve understanding of zooplankton population dynamics, secondary production, trophic link to fisheries, and zooplankton responses to environmental changes.