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Fig. 2. Actual fluctuating h and constant d growth response and adjusted estimated TWA response based on 72.6 of the constant, as calculated using Eq. 3.
based on the results from these tests, which more closely represents the actual fluctuating response.
4. Discussion
When trying to assess the growth effects from episodic or cyclic hypoxia on biota, the number of possible cyclic variations and complexity of cyclic exposure testing makes
extensive laboratory testing impractical. The universe of possible scenarios mandates that the problem be addressed from a modeling perspective. Estimations have to be made
using a finite set of useful data. A reasonable first step to address the issue of fluctuating exposure to hypoxia is to describe the effects of selected common D.O. cycles, such as
tidal and diel fluctuations, and then to look for a relationship between the effects of the cyclic exposures and more easily studied continuous-exposure responses to hypoxia. If a
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consistent relationship is established, then continuous-exposure data could be used to estimate effects in natural systems experiencing fluctuating D.O.
There are likely several approaches that can be considered when trying to estimate effects of non-constant hypoxia using laboratory-derived continuous-exposure data. Two
of the simplest are as follows. The first approach is to assume that the effect of a persistent cyclic exposure is the same as a constant exposure to hypoxic conditions of
equal total duration. For example, a diurnal cycle of 12 h at high-D.O. and 12 h at low-D.O. would be assessed as if it had been a 24 h low-D.O. exposure. This approach
only applies to the minimum concentration and therefore it is likely this would overestimate the response. The second option is to use a time-weighted approach where
the estimated fluctuating effect is related in a time-proportioned manner to the actual duration of hypoxia during the cycle. Time-weighted averaging assumes nonlethal
effects, such as reduced growth, are synchronous with the stress and do not extend beyond the period of exposure. While time-weighting may seem to be an appropriate
approach to relate intermittent exposure to continuous hypoxia, and would provide a method to generalize for all types of exposures, this study has demonstrated that it
underestimates actual growth reduction, at least for diurnal and semidiurnal cycles.
For the cyclic durations and D.O. exposures used in the tests presented here, the TWA estimated growth impairment under fluctuating D.O. conditions would be 50 of that
under constant low-D.O. conditions. Yet, the observed effect of cyclic hypoxia with larval P
. vulgaris was closer to 73 of the constant response. In work with freshwater juvenile largemouth bass, Stewart et al. 1967 saw a similar response pattern with
cyclic hypoxia. In their experiment, the fish in the cyclic treatments were exposed to low-D.O. for eight or 16 h of the day. Under those conditions, growth impairment was
almost always more than would have been estimated had the animals been held continuously at a concentration equal to the mean of the fluctuating exposure. Stewart’s
data were reanalyzed using the TWA method to see if the results corresponded with those seen here when assessed in a similar manner. Using our method, growth
impairment was usually more severe than the estimated TWA. Growth impairment greater than expected at the mean concentration of the fluctuation was also seen by
Whitworth 1968 with brook trout and by Fisher 1963 with underyearling coho salmon. Revaluation of Fisher’s results again showed growth impairment that was more
severe than the estimated TWA.
Why did the diurnal and semidiurnal cycles used in the tests presented here result in growth impairment that was almost 1.5 times greater than the TWA? A plausible
explanation may be that the recovery following exposure to stress should not be expected to be instantaneous, but that hypoxia continues to affect the organism for a
period after the D.O. returns to saturation, resulting in a lag in recovery.
Recovery time is an important component of low-D.O. exposure. If an aquatic system incurs only a few short term low-D.O. events, and if those events are above the lethal
threshold for the biota, there will probably be little to no long term effect. However, if low-D.O. persists, either as constant hypoxia, as it does in Long Island Sound Welsh et
al., 1994, or cyclic hypoxia, as in Chesapeake Bay Sanford et al., 1990; Diaz et al., 1992, its cumulative effect may extend to multiple levels of organization within the
aquatic community.
L .L. Coiro et al. J. Exp. Mar. Biol. Ecol. 247 2000 243 –255
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Since there are numerous factors which may be influencing an animal’s ability to recover from low-D.O. stress, determining the major influences would allow for a more
accurate application of TWA in estimating responses associated with natural fluctuating exposures. Little data exists on the factors associated with fluctuating D.O. which exert
the strongest influences on this response. Factors which should be evaluated include the absolute degree of change in D.O., the slope of the transition between the minimum and
maximum concentrations of exposure, duration of hypoxia within each cycle, and the amount of time at no-effect conditions between hypoxic periods of exposure. There are
several possibilities for how these factors could influence growth-related recovery. The data from this study shows that over the entire sublethal D.O. range for growth of larval
P
. vulgaris 1.4–3.2 mg l there was almost always more severe growth impairment associated with cyclic exposure as compared to the time-weighted response. This
suggests that the absolute change in D.O., i.e., amplitude of the cycle, may not be the most critical factor affecting subsequent recovery and growth. This study did not,
however, address many of the other possible influences on D.O. exposure and recovery. For example, gradual changes in D.O., or other coexisting abiotic factors, may permit
acclimation, resulting in incremental changes to respiration rate, feeding behavior, and metabolic activities which, in turn, may reduce impairment effects and allow quicker
recovery. This was demonstrated by Cech et al. 1990, showing a relationship between temperature acclimation and changes in metabolic rates as they related to hypoxia. In
general, those animals that were acclimated to temperature before experiencing hypoxic conditions had fewer significant changes in metabolic rates compared with those that
experienced an abrupt temperature change immediately before hypoxic exposure.
The influence of duration of exposure and amount of time at non-stress conditions can affect recovery in many ways. One possibility is that there is a proportional relationship
between the length of exposure and the amount of time needed to reach complete recovery and return to a normal growth rate. Another is that once the no-effect threshold
has been exceeded, there is a discrete amount of time needed for recovery regardless of exposure duration.
All of the fluctuating exposures in this study had equal time under hypoxic and saturated conditions 50:50 and therefore only partially address the issue of the
proportional relationship between exposure duration and recovery time. For larval P .
vulgaris, when there are equally proportionate exposure and recovery durations, growth is impaired by nearly 1.5 times the expected amount. The two cycle durations 6 h low:6
h high or 12 h low:12 h high and the different lengths of the tests 4 days, 7 days, or 8 days did not appear to influence growth differently, possibly since the ratio of hypoxic
exposure to saturated exposure is the same in all treatments, although there were not sufficient data to establish this point statistically. To better address which parameters are
influencing recovery, additional testing with cycles of different regimes is required to determine the shape of the recovery curve for diurnal or semidiurnal patterns, as well as
other patterns of fluctuation.
Adjusted time-weighted-averaging is a method to estimate responses to fluctuating D.O. exposure which is neither overly conservative nor overly liberal. Our results show
that calculated estimates of growth effects based on time-weighted averages of constant- exposure responses underestimated the observed laboratory effects for this species, but
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by a fairly consistent amount. The results presented here for P . vulgaris larvae and
juveniles, D . sayi larvae, and juvenile P. dentatus, along with the work of Stewart et al.
1967, Whitworth 1968 and Fisher 1963, suggests that this pattern of enhanced growth impairment may occur in fishes as well as crustaceans. If the observed
relationship between constant low-D.O. exposure response and fluctuating exposure response remains consistent across additional species, it will be reasonable to use an
adjusted time-weighted average to assess potential hypoxia-induced stress on the biota in ecosystems experiencing diurnal and semidiurnal cycles.
5. Disclaimer
