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.
To understand dissolved oxygen deficiency in Chesapeake Bay and its direct impact on zooplankton and planktivorous fish communities, six research cruises were conducted at two sites in the Chesapeake Bay from spring to autumn in 2010 and 2011. Temperature, salinity, and dissolved oxygen were measured from hourly conductivity, temperature, and depth (CTD) casts, and crustacean zooplankton, planktivorous fish and gelatinous zooplankton were collected with nets and trawls. CTD data were grouped into three temperature groups and two dissolved oxygen-level subgroups using principal component analysis (PCA). Species concentrations and copepod nonpredatory mortalities were compared between oxygenated conditions within each temperature group. Under hypoxic conditions, there usually were significantly fewer copepods Acartia tonsa and bay anchovies Anchoa mitchilli, but more bay nettles Chyrsaora chesapeakei and lobate ctenophores Mnemiopsis leidyi. Neutral red staining of copepod samples confirmed that copepod nonpredatory mortalities were higher under hypoxic conditions than under normoxia, indicating that the sudden decline in copepod concentration in summer was directly associated with hypoxia. Because comparisons were made within each temperature group, the effects of temperature were isolated, and hypoxia was clearly shown to have contributed to copepod decreases, planktivorous fish decreases, and gelatinous zooplankton increases. This research quantified the direct effects of hypoxia and explained the interactions between seasonality and hypoxia on the zooplankton population.
Abstract. The objectives of the Dead Zone Zooplankton research project (NSF OCE-0961942) were to study the effects of 5 the onset, development, and dissipation of hypoxia (DO < 2 mg L -1 ) on the plankton food web in the Chesapeake Bay. Here, we present the hydrologic and meteorology data from CTD, Scanfish, and the Safety Measurement System (SMS) of the research vessel to describe the environmental conditions of the bay during the project. We collected data from the mesohaline portion of Chesapeake Bay from 37.5-38.5 掳N and from 76-76.5 掳W during six research cruises in 2010 and 2011. We analyzed the temperature, salinity, and dissolved oxygen from hourly CTD casts using principal component 10 analysis (PCA) to understand variations in hydrography among depths, stations, seasons, and years. In addition to using the commonly accepted standard of hypoxia (DO < 2mg L -1 ), we also estimated the oxygen supply and demand of the copepod Acartia tonsa according to the surrounding temperature and salinity. The hypoxia in the bay began in the late spring, developed from the bottom layer upstream, progressed toward the sea, became fully established in summer, and gradually dissipated in autumn beginning in the downstream regions. However, we observed that extreme weather could interrupt this 15 succession and reignite hypoxia events after summer. Our PCA results indicated that temperature was the major driver of environmental conditions, and dissolved oxygen in the bottom layer was the second most important driver. Within each temperature group, we found that samples from 2011 and the north station were less oxygenated than samples from 2010 and the south station. Comparing the two metrics of oxygen deficiency, we found that the duration, distribution, and severity of environmental oxygen deficiency could be underestimated using the traditional metric, especially under warm and salty 20 conditions. We recommend that temperature and species-specific metrics be considered along with dissolved oxygen concentration when setting water quality goals for management. We uploaded the CTD data to the Biological and Chemical Oceanography Data Management Office (DOI:10.1575/1912/bco-dmo.687991; hyphen is part of the DOI), and we stored the Scanfish and SMS system data in the Rolling Deck to Repository
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