Quantifying changes in the vertical distribution of mesozooplankton in response to hypoxic bottom waters

Changes in the vertical distribution of mesozooplankton in coastal and estuarine systems due to hypoxia (dissolved oxygen concentration <2.0 mg l−1) could lead to changes in trophic dynamics and nutrient flux in pelagic systems. We analyzed data collected with an Optical Plankton Counter from the Chesapeake Bay and the Gulf of Mexico to determine how mesozooplankton vertical distribution changes under different hypoxic conditions. Our goal was to determine if there were significant differences between the vertical distribution of mesozooplankton size classes and the amount of hypoxia. The mesozooplankton median depth (depth at which 50% of the biovolume was found above and below) was used as a metric of vertical distribution. This metric, along with the oxycline depth, was normalized to the depth of the water column to allow comparisons between systems. In the Gulf of Mexico there was no significant relationship between the mesozooplankton median depth and the amount of hypoxia for any size class, but in the Chesapeake Bay the median depth was positively related to the oxycline depth for all sizes. In addition, the oxycline depth in the Gulf of Mexico was always deeper than the pycnocline depth, but in the Chesapeake the two clines usually co-occurred. In general, mesozooplankton median depth was greater than the oxycline depth when hypoxia accounted for more than 40% of the water column. These results suggest that mesozooplankton are found more often in low oxygen waters as the amount of hypoxia increases. Regardless of the mechanism, our results show that there are quantitative, size-based relationships between mesozooplankton vertical distribution and the amount of hypoxia, because the available habitat for the mesozooplankton is modified by the presence of bottom water hypoxia. These findings should be considered in ecosystem-based modeling and management efforts in regions of persistent or seasonal hypoxia. Habitat compression due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey, and for mesozooplankton mediated nutrient flux by affecting the vertical movement of mesozooplankton.