The vertical distribution and diel variability of mesozooplankton biomass, abundance and size in response to hypoxia in the northern Gulf of Mexico USA

The biological response of mesozooplankton (250-2000 m size range) to hypoxia in the northern Gulf of Mexico was investigated using an optical plankton counter (OPC) and a high-capacity pump. We sampled the water column in an area with the most persistent occurrence of hypoxia every 4 h for a 24-h period in both years. The amount of hypoxic bottom water differed between 2006 and 2007, with 2006 having more widespread bottom hypoxia than 2007. Large-sized mesozooplankton (>1000 m) were more abundant in 2006 and were found in greater abundance and biomass in hypoxic water. Small- (250-500 m) and mid-sized (500-1000 m) mesozooplankton showed diel variability, but did not appear to respond to hypoxia. The opposite pattern was observed in 2007, where smaller-sized mesozooplankton were dominant and diel variability in this size class was not detected, whereas large- and mid-sized mesozooplankton did show evidence of diel variability in 2007. Using a vital staining technique (neutral red), we found a significantly higher proportion of stained mesozooplankton in oxic, surface waters compared with deep, hypoxic waters. These findings show that mesozooplankton vertical and diel distributions differed between a year with widespread, bottom hypoxia and a year with a thin layer of hypoxic water. It remains unclear as to what is driving these size differences, the direct impact of hypoxia on mesozooplankton individual or egg mortality, differential predation in the water column or other factors such as more nutrient input related to increases in zooplankton production.