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Journal of Experimental Marine Biology and Ecology 247 2000 243–255
www.elsevier.nl locate jembe
Hypoxic effects on growth of Palaemonetes vulgaris larvae and other species:
Using constant exposure data to estimate cyclic exposure response
a , b
a
Laura L. Coiro , Sherry L. Poucher , Don C. Miller
a
U .S. Environmental Protection Agency, Atlantic Ecology Division, 27 Tarzwell Drive, Narragansett,
RI 02882, USA
b
Science Applications International Corporation , 221 Third Street, Newport, RI 02840, USA
Received 1 December 1999; received in revised form 18 December 1999; accepted 3 January 2000
Abstract
First stage larval marsh grass shrimp, Palaemonetes vulgaris, were exposed to patterns of diurnal, semidiurnal, and constant hypoxia to evaluate effects on growth and to determine if there
was a consistent relationship between exposures. A comparison of growth with cyclic exposures versus constant low dissolved oxygen D.O. concentrations equivalent to the minima of the cycles
showed the cyclic exposures resulted in less growth impairment than constant low-D.O. exposures, when compared to saturated controls. The mean extent of growth impairment of cyclic hypoxia
was, however, almost 1.5 times more than would be estimated using an arithmetic time-weighted- average of the hypoxic portion of the cycle. Additional testing with other life stages and species
resulted in similar patterns of response. Based on this relationship, an adjusted time-weighted- average could be used to estimate field related responses to cyclic dissolved oxygen from
laboratory-derived constant exposure data.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Cyclic exposure; Dyspanopeus sayi; Growth; Hypoxia; Marsh Grass Shrimp; Palaemonetes
vulgaris; Paralichthys dentatus; Say Mud Crab; Summer Flounder
1. Introduction
Dissolved oxygen D.O. conditions in aquatic systems are rarely stable. Episodes of low-D.O. are driven by biotic and abiotic factors such as photosynthesis and respiration,
Corresponding author. Tel.: 11-401-782-3000; fax: 11-401-782-3030. E-mail address
: coiro.lauraepamail.epa.gov L.L. Coiro 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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.L. Coiro et al. J. Exp. Mar. Biol. Ecol. 247 2000 243 –255
tidal cycles, and weather patterns. The variability associated with episodes of hypoxia dissolved oxygen below air-saturation is unique to each site and season. For example,
in the northeastern United States, low dissolved oxygen can persist over much of the summer, as is often experienced in the waters of western Long Island Sound Welsh et
al., 1994, or can cycle daily with the tide, as is characteristic of Chesapeake Bay and its tributaries Sanford et al., 1990; Diaz et al., 1992. How low-D.O. affects the biota of
aquatic systems is related to duration and frequency as well as intensity of the event. Although hypoxic events can eventually lead to mortality, more commonly they lead to
sublethal effects such as reduced growth, changed metabolism, and delayed develop- ment. Compared to conditions that cause mortality, sublethal levels of D.O. are generally
less severe, longer-lasting and more widespread.
Research on the oxygen requirements of aquatic animals has focused primarily on the lethal and sublethal effects of continuous exposure to reduced oxygen. To better evaluate
risk from intermittent or cyclic hypoxia, research is needed that reflects naturally occurring patterns of low dissolved oxygen. However, given the complexity of cyclic
exposure testing, there has been little research addressing the effects of this phenom- enon. Because cyclic hypoxia is so variable, it is preferable to use a modeling approach
to assess the possible effects on a system. In a system that experiences time-variable hypoxia, the degree to which an individual organism will be affected by low-D.O.
induced stress may be best described by the cumulative effect of exposures over the annual growth period. The intent of this study was to establish a method for estimating
the effects associated with multiple exposures to cyclic hypoxia using easily-derived constant-exposure growth data as the foundation. In laboratory tests, growth of an
organism is often the sublethal endpoint used and has been shown to be a more sensitive indicator of low dissolved oxygen than survival e.g., Morrison, 1971; U.S. EPA, 1986;
Das and Stickle, 1993; Thursby et al., 1997. One model for estimating this cumulative effect would be to prorate known laboratory responses from constant-D.O. exposure
testing to reflect the actual field exposure, but the accuracy of this approach has never been systematically tested. In application, having estimated response patterns which
relate effects under non-constant conditions to known responses from constant exposure testing would allow for more meaningful assessments of risk, without requiring the
difficulty of cyclic exposure testing.
In this study, we measured the growth of a larval estuarine crustacean, Palaemonetes vulgaris in five tests involving both constant and fluctuating low-D.O. conditions. Tests
which support the findings of this study were also conducted with juvenile P . vulgaris,
larval Say mud crab Dyspanopeous sayi , and juvenile summer flounder Paralichthys dentatus, but these results will not be discussed at length here because the data sets are
small and the treatments were not as tightly paired as in the tests presented. Though natural cycles of hypoxia are sinusoidal, we chose to use a square wave exposure regime
in an attempt to elicit the most extreme possible response to fluctuating dissolved oxygen. Growth data for effects at fluctuating D.O. regimes were then compared to
estimates of responses derived by time-weighted-averaging the effects from constant- exposure conditions. This comparison was used to evaluate the utility of using constant
exposure data to estimate effects from non-constant exposures. As will be demonstrated below, time-weighted averaging underestimated actual response to cyclic hypoxia, as
L .L. Coiro et al. J. Exp. Mar. Biol. Ecol. 247 2000 243 –255
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was expected, but the underestimation was fairly consistent and could therefore be adjusted to provide a reasonable estimate of effect which was still related to the time
component of exposure.
2. Materials and methods
