Figure 1.
Relationship between an organism’s respiration rate and the environmental oxygen partial pressure.
In this conceptual diagram, the solid line represents the linear relationship between environmental oxygen partial pressure and an organism’s theoretical maximum respiration rate, as predicted by Fick’s Law First of Diffusion (Eq. 1). Critical oxygen partial pressure (Pcrit) is the environmental oxygen level below which an organism’s theoretical maximum respiration rate is lower than its normal, or target respiration rate (TRR, dotted line). Respiration rate is oxygen limited when environmental oxygen partial pressure is below this critical partial pressure. Above the critical partial pressure respiration rate is generally independent of environmental oxygen (indicated by gray shading). Lethal oxygen partial pressure (Pleth) is the environmental oxygen level below which an organism’s theoretical maximum respiration rate is lower than its minimum survivable (non-lethal) respiration rate (MRR, dashed line). Hypoxia-induced mortality will increase when environmental oxygen partial pressure is below this lethal oxygen partial pressure.
Table 1.
Factors used in the conversion of Acartia tonsa egg production, somatic growth, and ingestion rates to analogous respiration rates (ARR, µg O2 mg dry wt−1 d−1).
Figure 2.
Feeding by Acartia tonsa in normal and low oxygen water.
Ingestion rates of adult female Acartia tonsa under incubation conditions of hypoxia (mean oxygen 5.0 kPa = 2.1 mg L−1 = 1.6 mL L−1) and normoxia (mean oxygen 19.9 kPa = 8.3 mg L−1 = 6.2 mL L−1). Data are graphed as means of three separate laboratory feeding experiments where animals were fed Rhodomonas sp. and incubated for 1–2 d. Error bars are+SD.
Figure 3.
Relationship between Acartia tonsa respiration rate and environmental oxygen partial pressure.
Using the same concepts illustrated in Fig. 1, analogous respiration rates (ARR) are plotted against associated environmental oxygen partial pressures. Theoretical maximum respiration rate (solid black line) is the linear regression through all data <8 kPa oxygen partial pressure. Target respiration rate (TRR, dotted line) is the average of normal respiration rates for A. tonsa (from 10 studies reviewed in [29]). Minimum survivable respiration rate (MRR, dashed line) is the average of respiration rates of persistently starved A. tonsa (reported in [20], [30]). A. tonsa critical oxygen partial pressure (Pcrit, based on TRR) and lethal oxygen partial pressure (Pleth, based on MRR) are also indicated. Gray shading indicates the region of oxygen independent respiration. Each type of symbol indicates a specific data source (citations on graph) and whether the measurement was of egg production rate (EPR), somatic growth rate (G), or ingestion rate (I). Error bars are+SD. ARR values are means at each distinct oxygen partial pressure and within each study, and the linear regression is based on 17 mean ARR values from a total of 153 individual measurements (see text for details). Regression statistics: y = 34.9×–20.5; R2 = 0.735; slope p-value <0.0005; n = 17.
Table 2.
Summary of the model predicting effects of hypoxia on Acartia tonsa vital rates.
Figure 4.
Model predicting the influence of hypoxia on Acartia tonsa applied to data from Chesapeake Bay.
Each data point is a measurement of temperature and oxygen partial pressure between June and September in the sub-pycnocline Chesapeake Bay. Points are coded to indicate predicted lethal, sub-lethal, or no effect on Acartia tonsa. The temperature and oxygen measurements used were taken monthly from 1986 to 2011 as part of Chesapeake Bay Program (CBP) monitoring (http://www.chesapeakebay.net/data). Thresholds for critical (Pcrit) and lethal (Pcrit) oxygen partial pressures are also indicated, as is the traditionally hypoxic 2 mg L−1 oxygen facet. Data are from (a) a region with severe summer hypoxia (CBP station CB4.3C), and (b) a region outside of the extent of severe hypoxia (CBP station CB6.1C).