Abstract
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.
Highlights
IntroductionDiversity 2020, 12, 35 of seasonal flushing, which strengthens stratification and impedes full circulation [4]
Our objectives were to determine if the concentrations and distributions of A. tonsa and its predators A. mitchilli, M. leidyi, and C. chesapeakei vary with respect to levels of dissolved oxygen, and to estimate the direct impact of hypoxia on A. tonsa populations by quantifying the non-predation mortality rates of A. tonsa under different DO conditions
These two stations were selected because both stations were at the mainstem of Chesapeake Bay with comparatively deeper water columns that allow persistent stratification to form, and the North was expected to experience more severe oxygen deficiency over a longer duration compared with the South
Summary
Diversity 2020, 12, 35 of seasonal flushing, which strengthens stratification and impedes full circulation [4]. In addition to these natural causes of hypoxia, anthropogenic drivers, such as eutrophication and warming, have contributed to hypoxia in the bay [5]. The volume of hypoxic water has been increasing, and the seasonal onset has been earlier since the 1950s [6,7,8] With both temperature and human population (the major source of eutrophication) projected to increase, the hypoxic volume of the bay could increase in the future [1,9,10,11]
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