Relationship between environmental conditions and zooplankton community structure during summer hypoxia in the northern Gulf of Mexico

The environmental conditions and mesozooplankton community were monitored during summer cruises in 2003, 2004 and 2006–2008 in the northern Gulf of Mexico. Data were collected on temperature, salinity, dissolved oxygen, chlorophyll-a and zooplankton taxonomic abundances. Environmental conditions were largely related to the influence of the Mississippi River plume. The zooplankton community was numerically dominated by copepods, with various meroplankton and other non-copepod zooplankton present in lower abundances. Cluster analysis revealed four distinct groups of co-occurring zooplankton taxa. Two of these were numerically dominant, one characterized by the copepod Acartia spp. and a second consisting of several copepod taxa (including larger-bodied genera) and salps. Relative to the group containing Acartia spp., this second dominant group was more abundant at higher salinity, lower temperature and greater vertical extent of hypoxic water into the water column above the seafloor. Dissolved oxygen, as vertical extent of hypoxia, was of comparable importance to temperature and salinity in predicting the abundances of zooplankton taxa. Variability of the zooplankton community with environmental conditions, and particularly the greater abundance of larger taxa in association with hypoxia, has implications for the contribution of zooplankton-mediated vertical flux to oxygen drawdown, and for the quality of the food environment for larval and other planktivorous fish.