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Assessing the Effects of Stress on Reef Corals
B. E. Brown and L. S. Howard
Department of Zoology, University of Newcastle upon Tyne Newcastle upon Tyne, England
I. Introduction . . . . . . . . . . . . . . . . . . . . 1 11. Natural Fluctuations and Man-made Influences . . . . . . . . . . 3
A. Assessing changes on coral reefs . . . . . . . . . . . . 3 B. Interpreting temporal changes on coral reefs . . . . . . . . 5 C. 9 D. Predicting recovery of reefs . . . . . . . . . . . . . . 17
111. Experimental Studies on Effects of Pollutants on Corals . . . . . . 20 A. Growth rate . . . . . . . . . . . . . . . . . . 20 B. Metabolism . . . . . . . . . . . . . . . . . . 27 C. Loss of zooxanthellae . . . . . . . . . . . . . . . . 29 D. Behavioural responses . . . . . . . . . . . . . . . . 35 E. Reproductive biology . . . . . . . . . . . . . . . . 46 F. Histopathology . . . . . . . . . . . . . . . . . . 48 G. Biochemical and cytochemical indexes . . . . . . . . . . 50
IV. Discussion and Future Research Needs . . . . . . . . . . . . 51 References . . . . . . . . . . . . . . . . . . . . 55
Effects and apparent lack of effects of pollution on coral reefs . . . .
Some years ago Johannes (1975) published the first major literature re- view on the effects of marine pollutants on coral reefs. At that time he
ADVANCES I N MARlNh BIOLOGY. VOL 22 Copyright (0 19x7. by Academic Press I n i (London) Ltd All rights of reproduction in any form reserved
2 B . E. BROWN A N D L. S. HOWARD
highlighted the paucity of knowledge in many areas of the subjects. Al- though research efforts in the field have increased, particularly with re- spect to potential effects of pollution by oil (Loya and Rinkevich, 1980) and drilling muds (Dodge and Szmant-Froelich, 1984) there are still enor- mous gaps in our knowledge and serious contradictions in the existing literature. Much of this lack of information may be attributed to our limited understanding of the physiology of corals, although recent papers have contributed valuable data on growth (Highsmith, 1979), reproduction (Highsmith, 1982; Kojis and Quinn, 1981, 1982; van Moorsel, 1983), behavior patterns (Lasker, 1979), carbon turnover (Crossland ef ul., 1980b), calcification (Barnes and Crossland, 1978, 1982; Gladfelter 1982a), mucus production (Crossland et al., 1980a), and associated bacterial populations on living corals (Ducklow and Mitchell, 1979).
The aims of the present article are to consider long-term ecological studies in the light of known effects of disturbances on coral reefs and to ask whether the effects of disturbances can be distinguished from long- term fluctuations on the reef and also where other difficulties lie in assess- ment of pollution in the field. In addition, in an attempt to improve under- standing of the overall susceptibility of reef corals to marine pollution, an assessment is made of the responses of corals to stress and methods by which these responses have been monitored to date.
The definition of stress has been much discussed in the literature (Grime, 1979; Pickering, 1981; Stebbing, 1981; Rosen, 1982), particularly with reference to problems involved in identifying and quantifying such a condition. Working with plants, Grime (1979) defined stress as the exter- nal constraints limiting dry matter production by all or part of the vegeta- tion, while Rosen (l982), considering corals, described stressful condi- tions as those resulting in restricted growth and reproduction. Such specific definitions as these have not always been applied in many of the publications referred to in this article. For our purposes Rosens broader view of stress as a gradient between ideal conditions and the ultimate limits of survival will be adopted. As noted by Stebbing (1981), the term stress may be used as a cause or as an effect. In agreement with Stebbing and most other authors, we choose to view stress as an external force or stimulus.
The article is divided into three sections, the first section dealing with observations in the field, the second with laboratory assessment of pollu- tant effects, and the third section incorporating a general discussion of the validity of generalizations made to date on the overall vulnerability of coral reefs to man-made disturbance.
EFFECTS O F STRESS ON REEF CORALS 3
II. Natural Fluctuations and Man-made Influences
A. Assessing Changes on Coral Reefs
Assessing change necessarily implies that reefs are monitored regularly by standard, repeatable methods. It is only in recent years, however, that quantitative methods have become routinely employed on permanent transects over time intervals. The first review of field methods applied on coral reefs was published by Stoddart in 1972. Subsequently other work- ers (Loya, 1972, 1978; Done, 1977; Bouchon, 1983; Dodge er al., 1982) have successfully tested various quantitative and semiquantitative meth- ods on the reef. Strictly quantitative techniques vary from plotless (Loya, 1978) to quadrat methods (Bak and Engel, 1979), and more recently work- ers have compared both approaches at the same sites in an attempt to gauge their relative efficiencies (Dodge et al., 1982; Bouchon, 1983). Al- though there appear to be no major differences in the results obtained by either methods, there are variations in the quantity and type of informa- tion generated and the time required for use; the line transect or inter- sected length method generally makes the most efficient use of the time spent underwater.
Measurements made on coral reefs using these techniques include coral cover, diversity, evenness (Loya, 1972, 1976a; Brown and Holley, 1982; Dodge et af . , 1982; Bouchon, 1983), colony number and colony size (Loya, 1972; Fishelson, 1977), and more recently spatial complexity (Rog- ers er af . , 1982, 1983) and spatial arrangement of both living and dead substratum components (Bak and Luckhurst, 1980).
Although monitoring of coral cover and diversity may yield fundamen- tal information about coral assemblages, few studies incorporate mea- surements of the cover and diversity of other components of the coral community, such as soft corals, zoanthids, algae, sponges, and ascidians. Recently, the importance of monitoring these groups has been highlighted by the publications of Benayahu and Loya (1977), Bak er af. (1981), and Tursch and Tursch (1982). Invasion and/or overgrowth of scleractinian corals by many species of soft corals (Nishihira, 1981; Tursch and Tursch, 1982) and ascidians (Bak er al., 1981; Sammarco er al., 1983; Syarani, 1983) has frequently been observed in both the Indo-Pacific and the Carib- bean provinces.
In studies where dominant components of the coral community have been considered (Rogers er af . , 1982), the effect of disturbance on coral reef diversity may be complex. Diversity of scleractinian corals as a result of hurricane damage in St. Croix was shown to decrease in shallow reef
4 B . E. BROWN A N D L. S. HOWARD
zones, whereas diversity of the community as a whole actually increased because of colonization of new substrata by a wide variety of reef organ- isms, e.g., algae, sponges, tunicates, bryozoa, and hydroids. Clearly quantitative measurements on coral reefs affected by disturbance should include some account of all major components of the reef community.
Such measurements are also improved by an appreciation of the struc- tural complexity of the coral reef environment. Rogers et al. (1982) used a modified transect method, with a linked chain following the contours of the reef, to obtain an index of reef topography or structural complexity. Done (1981) has applied the use of stereophotography to permanent tran- sects on the Great Barrier Reef. A stereo pair of photographs provides a great resolution of detail, a means of determining the three-dimensional coordinates of colonies and substratum, and a means of determining true dimensions and shapes of benthic organisms at any depth in the photo- graph. With automated stereoanalysis it should be possible to accurately map three-dimensional growth patterns of living corals and/or surface area of other substratum components. So complex are the interactions on coral reefs (Bak et al., 1982; Porter et al., 1982) that standard measure- ments of areal coverage, diversity, and abundance may not always be sensitive to changes in interactions which would be detected in a three- dimensional approach to community analysis.
Such an analysis should also consider the nonliving components of the substratum. Bak and Luckhurst (1980) have highlighted the importance of monitoring not only living cover but also nonliving substrata such as rock and sediments. Their study showed that alteration of spatial arrange- ment through dislodgement and collapse of substrata and changes in sedi- ment flow were of paramount importance in describing the community, particularly in shallow-water (10- and 20-m) quadrats. As the authors note, a continuous change in the cover of nonliving components must have serious implications for the settlement and survival of juvenile ben- thos.
One further factor should be considered when assessing changes on coral reefs, and that is the measurement of colony size. This parameter has been used by various workers (Loya, 1972, 1967a; Fishelson, 1977) involved in monitoring the effects of disturbance on coral reefs. It has recently been recognized (Hughes and Jackson, 1980) that partial colony mortality, colony fission, and colony fusion may affect any simple rela- tionship between the size and age of reef corals. Following known corals in photographs for successive years demonstrated that, in foliose Carib- bean corals, size a