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

Abstract

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.

Evaluation oxygen deficiency in the Chesapeake Bay

“…Although the oxygen deficiency is temporary and localized, many studies have found negative effects of summer bottom hypoxia on species distributions (Breitburg, 1992;Keister et al, 2000), abundance (Roman et al, 1993), and diversity (Cooper and Brush, 1993). Previous studies found copepods show different diel migration patterns under seasonal hypoxia (Pierson et al, 2017), and their non-predatory mortality was higher in summer (Elliott et al, 2013a). (Fig 9).…”

“…4) and the lethal oxygen partial pressure (P leth , Eq. 5) (Elliott et al, 2013b;Pierson et al, 2017). By comparing pO 2 with p crit and P leth , we estimated the oxygen supply and the oxygen demands of copepod A. tonsa under different temperature and salinity conditions.…”

“…Compared with other seasonally hypoxic ecosystems, the Chesapeake Bay has moderate 35 temperature and salinity, but it could be warm and salty in the southern region of the Bay during summer, where hypoxia is often overlooked compared to the northern region of the Bay. For example, in the 2011-Summer, our South station had similar DO as the North station at the same depth, but a much higher percentage of the vertical water column was biologically hypoxic (76% at South vs. 52% at North , Fig 2 & 6, more detailed analysis in Pierson et al, 2017) the DO was similar or higher, and therefore the hypoxic habitat under warm and salty conditions were likely to be overlooked when using a fixed standard for defining hypoxia.…”

“…In addition to the regular survey path, we extended the Scanfish survey to the upper bay during the 2011 Autumn cruise to observe the post-hurricane conditions (Fig 1f). More cruise and sampling details were described at Pierson et al (2017). During each cruise, meteorology and sea surface data were collected 10 every 10 minutes with the Surface Mapping System (SMS) on the R/V Sharp during the cruises.…”

“…Both the Scanfish and SMS data were processed in Matlab, and CTD data were first processed in Sea-Bird data processing software and then in Matlab (details described in Pierson et al 2017). For each CTD cast, pycnocline depths were chosen as the depth of maximum density gradient.…”