Abstract
We describe a model predicting the effects of low environmental oxygen on vital rates (egg production, somatic growth, and mortality) of the coastal planktonic copepod Acartia tonsa. Hypoxic conditions can result in respiration rate being directly limited by oxygen availability. We hypothesized that A. tonsa egg production, somatic growth, and ingestion rates would all respond in a similar manner to low oxygen conditions, as a result of oxygen dependent changes in respiration rate. Rate data for A. tonsa egg production, somatic growth, and ingestion under low environmental oxygen were compiled from the literature and from supplementary experiments. The response of these rates to oxygen was compared by converting all to the analogous units in terms of oxygen utilization, which we termed analogous respiration rate. These analogous respiration rates, along with published measurements of respiration rates, were used to parameterize and evaluate the relationship between A. tonsa respiration rate and environmental oxygen. At 18°C, our results suggest that A. tonsa experiences sub-lethal effects of hypoxia below an oxygen partial pressure of 8.1 kPa (∼3.1 mg L−1 = 2.3 mL L−1). The results of this study can be used to predict the effects of hypoxia on A. tonsa growth and mortality as related to environmental temperature and oxygen partial pressure. Such predictions will be useful as a way to incorporate the effects of coastal hypoxia into population, community, or ecosystem level models that include A. tonsa. This approach can also be used to characterize the effects of hypoxia on other aquatic organisms.
Highlights
The occurrence and extent of coastal hypoxia has increased over the last several decades [1]
Supplementary Ingestion Experiments To supplement the available published data describing the effects of low oxygen on A. tonsa egg production and somatic growth, we conducted a series of feeding experiments in which ingestion rates were measured under normoxic and hypoxic conditions
Under low oxygen analogous respiration rate (ARR) showed a statistically significant linear increase with increasing oxygen (R2 = 0.735, t = 6.41, p,0.0005), as indicated by least squares regression through all data points,8 kPa oxygen (Fig. 3, solid black line). This regression was based on 17 mean ARR values, which in turn were calculated from a total of 153 individual measurements of egg production, somatic growth, and ingestion
Summary
The occurrence and extent of coastal hypoxia has increased over the last several decades [1]. Many zooplankton that reside in open ocean regions with persistent oxygen minimum zones have evolved specific mechanisms to survive daily excursions down into low-oxygen environments [7] These animals display several strategies for tolerating exposure to extremely low oxygen conditions, including enhanced adenosine triphosphate (ATP) production prior to and following exposure to hypoxia, increased anaerobic ATP production, and decreased energy consumption through metabolic suppression [7,8]. Coastal zooplankton are less likely to have evolved such specialized mechanisms to tolerate exposure to low oxygen, and behavioral, rather than physiological adaptations appear to occur in response to hypoxia [9] These coastal species may instead attempt to avoid hypoxic waters, and experimental evidence suggests that behavioral avoidance may be an adaptive trait in the common coastal copepod Acartia tonsa, occurring in some populations but not in others [10]. It is clear that zooplankton do occur in low oxygen coastal bottom waters [11,12,13], and that these animals may experience sub-lethal consequences or even mortality upon exposure to hypoxia [14,15,16]
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