A comparison of the mesozooplankton response to hypoxia in Chesapeake Bay and the northern Gulf of Mexico using the biomass size spectrum

Hypoxic (dissolved oxygen concentration < 2 mg L− 1) zones are becoming more prevalent in coastal and estuarine systems worldwide. We compared the response of mesozooplankton (> 200 µm) to hypoxia in Chesapeake Bay and the northern Gulf of Mexico using an optical plankton counter (OPC) and pump collections. We predicted that hypoxia would restructure the vertical biomass and biomass size spectrum of mesozooplankton similarly in both ecosystems. Our results showed that the mesozooplankton response to hypoxia may be dependent on the amount of the water column that is hypoxic. In both systems, mesozooplankton appeared to avoid hypoxic water. The Chesapeake Bay biomass size spectrum of mesozooplankton biomass was altered when > 45% of the OPC measurements was made in hypoxic water. Oxic surface water contained smaller-sized mesozooplankton than hypoxic bottom water. There was no consistent shift in size distribution of northern Gulf of Mexico or Chesapeake Bay mesozooplankton biomass in hypoxic water when < 25% of OPC measurements was made in hypoxic water. Pump counts for both systems showed that smaller-sized mesozooplankton were more abundant in surface water. Bottom (hypoxic) pump counts also showed a prevalence of smaller-sized mesozooplankton, but also had a secondary abundance peak at larger sized mesozooplankton. Using a vital staining technique, we found that less than 40% of mesozooplankton collected from hypoxic water in the northern Gulf of Mexico were alive, compared to 70% alive in surface water. This suggests that the OPC may be overestimating mesozooplankton biomass in hypoxic water. It is not possible to estimate from our data whether lower mesozooplankton biomass in hypoxic water is a function of active avoidance or differential predation/mortality. Whatever the case, it is clear that mesozooplankton distribution becomes altered when the majority of the water column becomes hypoxic. The alteration in mesozooplankton distribution has important ramifications for trophic dynamics and the transfer of carbon to higher organisms.