Synergistic effects of seasonal deoxygenation and temperature truncate copepod vertical migration and distribution

The vertical distribution of copepods in coastal and estuarine systems is altered by the concentration of dissolved oxygen in the water column. We studied the combined effects of temperature and dissolved oxygen on vertical distribution and migration behavior of the copepod Acartia tonsa in the Chesapeake Bay throughout the period of seasonal deoxygenation in 2010 and 2011. Vertically stratified net tows and hydrographic casts were conducted at 2 stations over diel cycles in spring, summer, and autumn. The partial pressure of dissolved oxygen (pO(2)) was used as a metric for the condition of copepod habitat during each cruise instead of oxygen concentration, because partial pressure incorporates the temperature dependence of dissolved oxygen solubility, making it a more useful indication of available oxygen supply than concentration. Habitat conditions were described as having pO(2) above limiting conditions, below the maximum respiratory demand but above potentially lethal conditions, or below the minimum basal respiratory demand. In general, adult males were found deeper in the water than females in most oxygen conditions. Regression tree analysis supported these findings and showed that pO(2) was a key predictor of the fraction of copepodites, adult females, and adult males above the pycnocline, and for copepodites and adult females below the pycnocline. Temperature was a strong predictor of the fraction of adult males below the pycnocline, with a smaller fraction found there at higher temperatures. These findings suggest sex-specific responses to deoxygenation, potentially as a result of different oxygen demands or behavior.