- Email: [email protected]
Effects of the multiple stressors copper and reduced salinity on the metabolism of the hermatypic coral Porites lutea
Marine Environmental Research 52 (2001) 289–299 www.elsevier.com/locate/marenvrev
Effects of the multiple stressors copper and reduced salinity on the metabolism of the hermatypic coral Porites lutea S. Alutoin, J. Boberg, M. Nystro¨m*, M. Tedengren Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden Received 10 January 2001; received in revised form 24 April 2001; accepted 2 May 2001
Abstract This study investigates the physiological responses in the hermatypic coral Porites lutea when exposed to a combination of reduced salinity (from ambient 30 psu to 20 psu) and two concentrations of copper (CuSO4), 10 mg l1 and 30 mg l1. Corals were exposed for 14 h and changes in metabolism in terms of primary production rate per chlorophyll a and respiration per surface area (cm2) were used as measures of stress. The results showed no significant changes in respiration rate in any of the treatments compared with controls, or between treatments. The primary production rate, however, displayed a more complex pattern. Corals exposed to reduced salinity, 30 mg l1 copper, and the combination of the two stressors significantly reduced the production rate, whereas corals exposed to 10 mg l1 only, remained unaffected. However, adding 10 mg l1 copper to reduced salinity did not affect the production rate thus indicating an antagonistic effect. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Corals; Copper; Salinity; Disturbance; Metabolism; Multiple stress pollution; Porites lutea; Thailand
1. Introduction Coral reefs have always been subjected to a wide variety of disturbances (Grigg & Dollar, 1990). However, increasing pressure on the tropical marine environment from human use (e.g. fishing and tourism), and abuse (e.g. destructive fishing and * Corresponding author. Fax: +46-8-15-84-17. E-mail address: [email protected] (M. Nystro¨m). 0141-1136/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-1136(01)00105-2
290
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
pollution) are likely to alter natural disturbance regimes (Nystro¨m, Folke, & Moberg, 2000) and generate compounded perturbations (Hughes & Connell, 1999; Paine, Tegner, & Johnson, 1998). While a single stress can severely damage corals and other reef organisms, multiple stressors may magnify the negative impact by generating synergistic effects (Maragos, Crosby, & McManus, 1996). For example, studies of coral communities in Hawaii exposed to reduced salinity after freshwater flooding showed that corals can recover quickly from natural disturbances, but not under polluted conditions (Jokiel, Hunter, Taguchi, & Watarai, 1993). Moreover, Loya (1990) demonstrated that coral recovery after an extreme low tide event was severely hampered by chronic oil pollution affecting reproduction and settlement. Thus, chronic background disturbances may delay, or even halt, recovery following disturbance (Brown, 1997; Hughes & Connell, 1999). Since reef-building corals rely on the symbiosis between the coral animal (the polyp) and its unicellular photosynthesising dinoflagellates (zooxanthellae), any disruption of this cellular symbiotic relationship is likely to generate effects on a community level. Corals lack the capability of osmoregulation (Hoegh-Goldberg & Smith, 1989; Ranklin & Davenport, 1981) and even short-term (hours) salinity stress can induce changes in animal respiration and symbiont photosynthesis. If the exposure is prolonged (days or more) it can affect growth, reproduction and may eventually lead to mortality (Coles & Jokiel, 1992). Several studies have demonstrated the effects of reduced salinity on coral metabolism. For example, in Porites lutea and Pocillopora damicornis photosynthetic rates were lowered proportionally to sudden drops (hours) in salinity from ambient 30 psu to 20 and 10 psu (Moberg, Nystro¨m, Kautsky, Tedengren, & Jarayabhand, 1997). Furthermore, Jokiel et al. (1993) investigated the effects of storm floods in 1987 that reduced salinity to 15 psu in the surface waters of Kaneohe Bay, Hawaii. They found that all colonies of Pocillopora damicornis and Montipora verrucosa suffered total mortality when the salinity remained below 20 psu for more than 5 days. The heavy metal copper is an essential element, but it is also known as a common marine pollutant (Jones, 1997a) where coastal run-offs, mining and smelting operations and disposal of public solid waste incinerator ash are known sources (Dubinsky & Stambler, 1996; Jones, 1997a). Copper affects corals by inhibiting the electron transport in photosystem II (Samson, Morisette, & Popovic, 1988). Heavy metals can also be incorporated in the skeleton making the coral fragile and thus more sensitive to physical actions (Howard & Brown, 1984). However, the symbiotic algae have a high tolerance for heavy metals (Mu¨ller-Parker & D’Elia, 1997) and are thought to be the site for metal uptake in cnidarians (Buddemeier, Schneider, & Smith, 1981). For example, large amounts of the metals copper, cadmium and zinc have been found in symbiotic algae of the anemone Anemonia viridis (Harland & Nganro, 1990). Howard and Brown (1984) state that ‘‘the soluble metals in seawater probably represents the most obvious and direct route of metal uptake to corals’’. Other pathways comprise the feeding activities, which may contribute to the metal intake, especially by tentacular capture of zooplankton that may be metal enriched (Howard & Brown, 1984). Results from a study by Evans in 1977 (cf. Howard & Brown, 1984) where Pocillopora damicornis and Montipora verrucosa were exposed
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
291
to solutions of copper sulphate, indicate toxic effects of copper at lower concentrations than 10 mg l1. Further evidence of copper’s toxicity are provided by Nystro¨m, Moberg, and Tedengren (1997), who found that the metabolism of Pocillopora damicornis and Porites lutea were significantly affected when exposed to 10 mg l1 copper. The inner Gulf of Thailand is exposed to reduced salinity (Moberg et al., 1997) and increasing pollution from domestic, industrial and agricultural sources (Menasveta & Hongskul, 1988). Large amounts of freshwater enter the gulf via four major rivers and can cause occasional drops in salinity from the normal 27.1–29.8 psu in the surface waters of the inner gulf (Menasveta & Hongskul, 1988) to salinity as low as 10 psu (Moberg et al., 1997). Measurements of the heavy metal levels in the inner Gulf of Thailand have shown copper concentrations ranging between 0.33 and 14.16 mg l1 copper (cf. Johnston, 1998). Synergistic effects from multiple stressors are frequently discussed in literature but few studies have experimentally investigated the potential effects (but see Nystro¨m, Nordemar, & Tedengren, 2001). Porter, Lewis, and Porter (1999) suggested that the effects of combined disturbances might not simply be additive. They showed that elevated temperature and salinity was less stressful in combination compared with the sum of the effects generated when acting separately. The objective of this study was to determine the physiological effects of copper and reduced salinity acting simultaneously on the hermatypic coral Porites lutea. The combinations of the stressors are ecologically relevant since they co-occur in the study area at levels that are within the range of this study. The physiological response is estimated by measuring changes in net production and respiration, which is a sublethal approach where stress responses can be detected within hours.
2. Materials and methods This study was carried out during October and November 1999, at Sichang Marine Research and Training station (SMaRT) in the inner Gulf of Thailand. The study area is situated south of Sichang Island on the southeast side of Khang Kao Island, 13 090 N, 100 480 E. The coral community around Khang Kao Island is relatively low diverse and the scleractinian coral Porites lutea is the overwhelmingly dominant coral species (Sakai, Yeemin, Snidvongs, Yamazato, & Nishihara, 1986). 2.1. Experimental design Corals were collected at 4-m depth and transported to an outdoor facility where epiphytes and boring fauna were carefully removed immediately after arrival. The corals were kept in a large flow-through tank (300 dm3) between 1 and 4 days with continuous aeration before exposure. Specimens (approximately 1 dm2 in size) were placed in exposure tanks (60 dm3) for 14 h over night before the start of the physiological measurements. However, due to logistic restraints only one or two treatments were studied per day. The different treatments were A: 30 psu (ambient), B: 20
292
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
psu, C: 10 mg l1 copper in 30 psu, D: 10 mg l1 in 20 psu, E: 30 mg l1 in 30 psu and F: 30 mg l1 in 20 psu. Copper ions were derived from a stock solution of copper sulphate (CuSO4). The number of replicates for each exposure varied between 8 and 11, a total of 57 replicates (Table 1) for the production and chlorophyll measurements. The respiration results are based on 74 replicates. After exposure the specimens were put in individual transparent airtight experimental containers (4.7 dm3), equipped with submersible pumps (138 mm, capacity 10 l min1), containing water from the exposure tank. Changes in dissolved oxygen levels, oxygen saturation and temperature were measured using a microprocessor oxymeter (WTW, OXY196) in light and darkness. Measurements were taken initially, after 30 min and after 1 h. The pumps were run during measurements and for 5 s every 15 min to create water movement in order to prevent oxygen gradients from building up in the containers (Moberg et al., 1997). The oxygen saturation in the experimental containers never dropped below 87% or reached above 124%. 2.2. Physiological measurements Net production was measured outdoors between 10:00 and 14:00 on clear sunny days. The experimental containers were placed in a cooling tank in order to avoid temperature increase during production measurements. Observation showed an average increase of 2 C during the measurements. A light sensor (LI-COR 192A) positioned close to the cooling tank showed that mean light intensities never reached below 1100 mE m2 s1 during production measurements which ensured maximum net production (Chalker, Dunlap, & Oliver, 1983). This light intensity is approximately what the corals receive in field at 4 m depth (Moberg et al., 1997). For respiration measurements the experimental containers were placed in non-transparent tanks.
Table 1 Summary table of means and statistical comparisons of net production/chlorophyll a between the different treatmentsa Treatment mg Cu2+ : psu
chl a (SE) mg chl a cm2
Net prod/chl. a (SE) mg O2 (mg chl a)1 h1
n
0:30 0:20 10:30 10:20 30:30 30:20
27.8 32.5 30.8 25.9 31.8 30.7
1.3 0.7 1.1 1.2 0.5 0.5
8 9 9 10 11 9
(2.5) (1.6) (1.7) (2.0) (1.7) (1.8)
(0.2) (0.1) (0.1) (0.1) (0.1) (0.1)
A B C D E F
A
B
C
D
E
F
– * ns ns ** **
* – ns * ns ns
ns ns – ns ** **
ns * ns – ** **
** ns ** ** – ns
** ns ** ** ns –
a The net production/chlorophyll a and chlorophyll a concentration data from the different treatments (S.E. within parenthesis) and aposteriori test results. n, Number of replicates. ns, no significance. *P <0.05. **P <0.001.
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
293
In order to examine any potential effect of the laboratory environment on coral’s physiological performance a study was performed where net production and respiration were measured on corals (n=4) the day after arrival, and again after 5 days. 2.3. Chlorophyll analysis The living polyp surface area of each replicate was determined using the aluminium foil method (Marsh, 1970). A small fragment (3 cm2) was removed from each coral replicate and mortared. Chlorophyll a was extracted in 7 ml of 90% acetone for 24 h. The samples were centrifuged at 3500 rpm for 10 min and acclimatised to room temperature for 30 min to avoid condensation in the spectrophotometer (single beam, spectronic genesys 5, Milton Roy). Absorption was measured at 750, 664 and 630 nm with a 90% acetone solution serving as blank (Jeffrey & Humphrey, 1975; Parsons, Maita, & Lalli, 1984). Concentration of chlorophyll a was calculated according to the equations of Jeffrey and Humphrey (1975). In order to make sure that all chlorophyll a had been extracted a second extraction was performed on three samples showing that less than 4% of the chlorophyll a was unextracted. Thus, one extraction was sufficiently accurate. 2.4. Statistical analysis To ensure that the data within each treatment did not differ between days a oneway ANOVA (Statistica’99 edition) was carried out. Analysis of variance (two-way ANOVA) was performed in order to evaluate the observed differences in net production, respiration and chlorophyll levels between treatments. The net production per chlorophyll a data was transformed with the square root, as it did not meet the assumptions of ANOVA (Cochran’s test). Aposteriori tests were made with Tukey HSD for unequal sample size.
3. Results The investigation of potential physiological effects on corals while kept in the laboratory showed that there were no differences in primary production (P=0.45) and respiration (P=0.25) rates between the 1st day and the 6th day on lab. Coral replicates exposed to 30 mg l1 copper in both 20 psu and ambient salinity displayed a discoloration from normal brown colour to lighter brown after 14 h exposure. The colour loss was uniform across the surface area of all replicates. However, analysis of chlorophyll a displayed no significant differences between treatments. 3.1. Effects of copper and salinity on respiration rate No significant changes in respiration rate per surface area were detected in any of the treatments compared with controls, or between treatments (Fig. 1).
294
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
3.2. Effects of copper and salinity on net production/chlorophyll a Net production differed significantly between treatments (Fig. 2, Table 1). The post hoc tests showed that net production rate was significantly reduced in corals exposed to 20 psu (P=0.002), 30 mg l1 copper in 30 psu (P=0.0002), and 30 mg l1 in 20 psu (P=0.0001) compared with the controls. Corals exposed to 30 mg l1 copper in 30 psu had a significantly lower production rate compared with those exposed to 10 mg l1 in 30 psu (P=0.001). Corals exposed to 10 mg l1 in 20 psu, however, had higher production rate than those exposed to 30 mg l1 in 20 psu (P=0.0002) and 20 psu only (P=0.018).
4. Discussion 4.1. Effects of copper and reduced salinity on respiration rate The physiological measurements showed that salinity had no significant effect on the respiration rate. This result agrees with a short-term study (1.5 h exposure) by Moberg et al. (1997) who found no effect on respiration per biomass when salinity was reduced from 30 to 20 psu. Long-term exposure, however, may lead to a
Fig. 1. Respiration rate per surface area (cm2) shows no significant differences between the six treatments (meanS.E.).
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
295
decrease in respiration, which is a normal response to salinity stress as demonstrated by Vernberg and Vernberg (1972) where corals through reduced activity decrease the metabolic costs for maintenance, which in turn increases the ability to withstand disturbances (Moberg et al., 1997). For example, a 3-week study by Ferrier-Page`s, Gattuso, and Jaubert (1999) on Stylophora pistillata showed reduced respiration rates even after minor changes in salinity. The present study showed that neither of the two copper concentrations had any significant effect on the respiration rate in Porites lutea. This was not surprising as copper primarily affects the photosynthesis. 4.2. Effects of copper and reduced salinity on chlorophyll a concentration Zooxanthellae have been found to accumulate heavy metals to a larger extent than their hosts (Peters, Gassman, Firman, & Richmond, 1997). Expulsion of zooxanthellae is a common response to heavy metals in corals and has even been suggested as a mechanism to control heavy metal concentrations in symbiotic animals (Harland & Nganro, 1990; cf. Peters et al., 1997). In the present study a discoloration was observed during the experiments, particularly in those exposed to a combination of 30 mg l1 copper and lowered salinity, but also in the corals exposed to 30 mg l1 in ambient salinity. Since no pronounced loss of color was observed for corals exposed to decreased salinity only or to the lower copper concentration, it
Fig. 2. Net primary production rate per chlorophyll a for the six treatment (meanS.E.). Treatment A display a significantly higher production rate than B, E, and F, and treatment C is significantly higher than E and F. Treatment D has significantly higher production rate than B, E and F.
296
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
seems that the higher dose of copper is causing this discoloration. Our results, however, show no significant loss of chlorophyll a despite the observed color loss in corals exposed to copper. These results are consistent with a study by Jones (1997a) on branch tips from colonies of the staghorn coral Acropora formosa that were exposed to elevated concentrations of copper, 5–80 mg l1. Jones (1997a) found no significant differences in chlorophyll a levels except at the two highest concentrations (40 and 80 mg l1) of copper. The results obtained from the present study suggest that the discoloration of corals exposed to copper could be due to a contraction of coral polyps exposing the white naked skeleton (Brown, Le Tissier, & Dunne, 1994). An alternative explaination could be that even if zooxanthellae are lost, remaining symbionts might increase their production of zooxanthellar chlorophyll a (Jones, 1997b). 4.3. Effects of copper and reduced salinity on net production per chlorophyll a Since hermatypic corals lack osmoregulatory mechanisms they may only be able to cope with reduced salinity by behavioural responses, such as retraction of polyps, in order to minimize exposure with surrounding water (Muthiga & Szmant, 1987). This, however, implies hampered photosynthesis due to less light available for the symbiotic algae. Despite that no discoloration was observed in corals exposed to 20 psu there was a significant reduction in production rate. This could be due to cell disruption (Muthiga & Szmant, 1987) or a contraction of polyps that could not be recorded by visual inspection. While decreased salinity indirectly reduce production, copper acts directly on the photosynthesis by inhibition of the electron transport on the oxidizing side of photosystem II (Samson et al., 1988). The present study showed that exposure to the higher copper concentration significantly reduced net production per chlorophyll a, whereas corals exposed to the lower dose remained unaffected. Since this was a relatively short-term study, corals may still be affected at lower concentrations of copper when exposed to continuous pollution. However, Porites lutea has been shown to be tolerant to a wide range of stressors in general, including salinity fluctuations, which may explain its dominance in the study area (Moberg et al., 1997; Nystro¨m et al., 1997). For example, Harland and Brown (1989) showed that the physiological response to elevated iron concentrations in Porites lutea was less severe for corals occurring in iron contaminated areas. They also found that corals in the polluted area had higher levels of iron in their tissues. We suggest that corals found in our study area might have developed similar means for accumulation of copper. This, however, is purely speculative and needs to be investigated further. The combination of shallow coastal waters highly influenced by rivers and heavy run-off and the many sources of heavy metals have resulted in a periodically low saline environment with relatively high background levels of copper (Nystro¨m et al., 1997). The treatment of 10 mg l1 copper in ambient salinity did not generate any change in net production while reduced salinity and 30 mg l1 in ambient salinity led to significantly decreased production rate. An unexpected result was that the com-
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
297
bination of 10 mg l1 and 20 psu did not affect the production rate indicating an antagonistic effect. Similar results were obtained by Porter et al. (1999) who showed that the combination of increased salinity and temperature was less stressful to corals han exposure to elevated temperature alone. Since the colonies of Porites lutea used in this study are found in an area with continuous copper exposure and re-occurring dramatic salinity fluctuations it is likely that they may have adapted to the prevailing conditions and that the effects, therefore, might be site-specific. This calls for studies comparing different regions in order to understand the disturbance complexity of these stressors. Moreover, we suggest that future studies should also aim at investigating long-term low-level stress in order to simulate in situ conditions more accurately.
Acknowledgements We thank Fredrik Moberg, Hans Kautsky and Nils Kautsky for helpful comments on the manuscript. Dr. Padermsak Jarayabhand and Sompop Rungsupa at Chulalongkorn University, for their support. The Swedish International Development and co-operation Agency (SIDA/SAREC) for funding.
References Brown, B. E. (1997). Disturbances to reefs in recent times. In C. Birkeland (Ed.), Life and death of coral reefs (pp. 354–379). Chapman and Hall. Brown, B. E., Le Tissier, M. D. A., & Dunne, R. P. (1984). Tissue retraction in the scleractinian coral Coeloseris mayen, its effects upon coral pigmentation, and preliminary implications for heat balance. Marine Ecology Progress Series, 105, 209–218. Buddemeier, R. W., Schneider, R. C., & Smith, S. V. (1981). The alkaline earth chemistry of corals. Proceedings of the 4th International Coral Reef Symposium, Philippines, 2, 81–85. Chalker, B. E., Dunlap, W. C., & Oliver, J. K. (1983). Bathymetric adaptation of reef-building corals at Davies Reef, Great Barrier Reef, Australia II. Light saturation curves for photosynthesis and respiration. Journal of Experimental Marine Biology and Ecology, 73, 37–56. Coles, S. L., & Jokiel, P. L. (1977). Effects of temperature on photosynthesis and respiration in hermatypic corals. Marine Biology, 43, 209–216. Dubinsky, Z., & Stambler, N. (1996). Marine pollution and coral reefs. Global Change Biology, 2, 511– 526. Ferrier-Page`s, C., Gattuso, J. P., & Jaubert, J. (1999). Effect of small variations in salinity on the rates of photosynthesis and respiration of the zooxanthellate coral Stylophora pistillata. Marine Ecology Progress Series, 181, 309–314. Grigg, R. W., & Dollar, S. J. (1990). Natural and anthropogenic disturbance on coral reefs. In Z. Dubinsky (Ed.), Ecosystems of the world, Vol. 25 (pp. 439–452). New York: Elsevier. Harland, A. D., & Brown, B. E. (1989). Metal tolerance in the scleractinian coral Porites lutea. Marine Pollution Bulletin, 20, 353–357. Harland, A. D., & Nganro, N. R. (1990). Copper uptake by the sea anemone Anemonia viridis and the role of zooxanthellae in metal regulation. Marine Biology, 104, 297–301. Hoegh-Goldberg, O., & Smith, G. J. (1989). The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata
298
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
Esper and Seriatopora hystrix Dana. Journal of Experimental Marine Biology and Ecology, 129, 279– 303. Howard, L. S., & Brown, B. E. (1984). Heavy metals and reef corals. Oceanography and marine biology: an annual review, 22, 195–210. Hughes, T. P., & Connell, J. H. (1999). Multiple stressors on coral reefs: a long-term perspective. Limnology and Oceanography, 44, 932–940. Jeffrey, S. W., & Humphrey, G. F. (1975). New spectrophotometric equations for determining chlorophylls a, b, c1 and c2 in higher plants, algae and natural phytoplankton. Biochemistry and Physiolology Pflanzen, 167, 191–194. Johnston, D. M. (1998). SEAPOL integrated studies of the Gulf of Thailand. Southeast Asian Program on Ocean Law, Policy and Management (Vol. 1). Jokiel, P. L., Hunter, C. L., Taguchi, S., & Watarai, L. (1993). Ecological impact of a freshwater ‘‘reef kill’’ in Kaneohe Bay, Oahu, Hawaii. Coral Reefs, 12, 177–184. Jones, R. J. (1997a). Zooxanthellae loss as a bioassay for assessing stress in corals. Marine Ecology Progress Series, 149, 163–171. Jones, R. J. (1997b). Changes in zooxanthellar densities and chlorophyll concentrations in corals during and after a bleaching event. Marine Ecology Progress Series, 158, 51–59. Loya, Y. (1990). Changes in a Red Sea coral community structure: a long-term case history study. In G. M. Woodwell (Ed.), The earth in transition: patterns and processes of biotic impoverishment (pp. 369– 384 Cambridge. Maragos, J. E., Crosby, M., & McManus, J. W. (1996). Coral reefs and biodiversity: a critical and threatened relationship. Oceanography, 1, 83–99. Marsh, J. A. (1970). Primary productivity of reef-building calcareous red algae. Ecology, 51, 255–263. Menasveta, D., & Hongskul, V. (1988). The Gulf of Thailand. In H. Postma, & J. J. Zijlstra (Eds.), Ecosystems of the world, Vol. 27 (pp. 363–383). New York: Elsevier. Moberg, F., Nystro¨m, M., Kautsky, N., Tedengren, M., & Jarayabhand, P. (1997). Effects of reduced salinity on the rates of photsynthesis and respiration in the hermatypic corals Porites lutea and Pocillopora damicornis. Marine Ecology Progress Series, 157, 53–59. Mu¨ller-Parker, G., & D’Elia, C. F. (1997). Interactions between corals and their symbiotic algae. In C. Birkeland (Ed.), life death of coral reefs (pp. 96–113). New York: Chapman and Hall. Muthiga, N. A., & Szmant, A. M. (1987). The effects of salinity stress on the rates of aerobic respiration and photosynthesis in the hermatypic coral Siderastrea siderea. Biological Bulletin, 173, 539–551. Nystro¨m, M., Folke, C., & Moberg, F. (2000). Coral reef disturbance and resilience in a human-dominated environment. Trends in Ecology and Evolution, 15, 413–417. Nystro¨m, M., Moberg, F., & Tedengren, M. (1997). Natural and anthropogenic disturbance on reef corals in the inner Gulf of Thailand: physiological effects of reduced salinity, copper and siltation. Proceedings of the 8th International Coral Reef Symposium, 2, 1893–1898. Nystro¨m, M., Nordemar, I., & Tedengren, M. (2001). Simultaneous and sequential stress from increased temperature and copper on the metabolism of the hermatypic coral Porites cylindrica. Marine Biology (in press). Paine, R. T., Tegner, M. J., & Johnson, E. A. (1998). Compounded perturbations yield ecological surprises. Ecosystems, 1, 535–545. Parsons, T. R., Maita, Y., & Lalli, C.M. (1984). A manual of chemical and biological methods for seawater analysis (pp. 101–104). Oxford: Pergamon Press. Peters, E. C., Gassman, N. J., Firman, J. C., Richmond, R. H., & Power, E. A. (1997). Ecotoxicology of tropical marine ecosystems. Environmental Toxicology and Chemistry, 16, 12–40. Porter, J. W., Lewis, S. K., & Porter, K. G. (1999). The effect of multiple stressors on the Florida Keys coral reef ecosystem: a landscape hypothesis and a physiological test. Limnology and Oceanography, 44, 941–949. Ranklin, J. C., & Davenport, J. (1981). Animal osmoregulation. New York: Halsted Press. Sakai, K., Yeemin, T., Snidvongs, A., Yamazato, K., & Nishihara, M. (1986). Distribution and community structure of hermatypic corals in the Sichang Islands, inner part of the Gulf of Thailand. Galaxea, 5, 27–74.
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
299
Samson, G., Morisette, J., & Popovic, R. (1988). Copper quenching of the variable fluorescence in Dunaliella tertiolecta. New evidence for a copper inhibition effect on PSII photochemistry. Photochemistry and Photobiology, 48, 329–332. Vernberg, F. J., & Vernberg, W. B. (1972) Environmental physiology at marine animals. New York: Springer Verlag.
Effects of the multiple stressors copper and reduced salinity on the metabolism of the hermatypic coral Porites lutea S. Alutoin, J. Boberg, M. Nystro¨m*, M. Tedengren Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden Received 10 January 2001; received in revised form 24 April 2001; accepted 2 May 2001
Abstract This study investigates the physiological responses in the hermatypic coral Porites lutea when exposed to a combination of reduced salinity (from ambient 30 psu to 20 psu) and two concentrations of copper (CuSO4), 10 mg l1 and 30 mg l1. Corals were exposed for 14 h and changes in metabolism in terms of primary production rate per chlorophyll a and respiration per surface area (cm2) were used as measures of stress. The results showed no significant changes in respiration rate in any of the treatments compared with controls, or between treatments. The primary production rate, however, displayed a more complex pattern. Corals exposed to reduced salinity, 30 mg l1 copper, and the combination of the two stressors significantly reduced the production rate, whereas corals exposed to 10 mg l1 only, remained unaffected. However, adding 10 mg l1 copper to reduced salinity did not affect the production rate thus indicating an antagonistic effect. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Corals; Copper; Salinity; Disturbance; Metabolism; Multiple stress pollution; Porites lutea; Thailand
1. Introduction Coral reefs have always been subjected to a wide variety of disturbances (Grigg & Dollar, 1990). However, increasing pressure on the tropical marine environment from human use (e.g. fishing and tourism), and abuse (e.g. destructive fishing and * Corresponding author. Fax: +46-8-15-84-17. E-mail address: [email protected] (M. Nystro¨m). 0141-1136/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-1136(01)00105-2
290
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
pollution) are likely to alter natural disturbance regimes (Nystro¨m, Folke, & Moberg, 2000) and generate compounded perturbations (Hughes & Connell, 1999; Paine, Tegner, & Johnson, 1998). While a single stress can severely damage corals and other reef organisms, multiple stressors may magnify the negative impact by generating synergistic effects (Maragos, Crosby, & McManus, 1996). For example, studies of coral communities in Hawaii exposed to reduced salinity after freshwater flooding showed that corals can recover quickly from natural disturbances, but not under polluted conditions (Jokiel, Hunter, Taguchi, & Watarai, 1993). Moreover, Loya (1990) demonstrated that coral recovery after an extreme low tide event was severely hampered by chronic oil pollution affecting reproduction and settlement. Thus, chronic background disturbances may delay, or even halt, recovery following disturbance (Brown, 1997; Hughes & Connell, 1999). Since reef-building corals rely on the symbiosis between the coral animal (the polyp) and its unicellular photosynthesising dinoflagellates (zooxanthellae), any disruption of this cellular symbiotic relationship is likely to generate effects on a community level. Corals lack the capability of osmoregulation (Hoegh-Goldberg & Smith, 1989; Ranklin & Davenport, 1981) and even short-term (hours) salinity stress can induce changes in animal respiration and symbiont photosynthesis. If the exposure is prolonged (days or more) it can affect growth, reproduction and may eventually lead to mortality (Coles & Jokiel, 1992). Several studies have demonstrated the effects of reduced salinity on coral metabolism. For example, in Porites lutea and Pocillopora damicornis photosynthetic rates were lowered proportionally to sudden drops (hours) in salinity from ambient 30 psu to 20 and 10 psu (Moberg, Nystro¨m, Kautsky, Tedengren, & Jarayabhand, 1997). Furthermore, Jokiel et al. (1993) investigated the effects of storm floods in 1987 that reduced salinity to 15 psu in the surface waters of Kaneohe Bay, Hawaii. They found that all colonies of Pocillopora damicornis and Montipora verrucosa suffered total mortality when the salinity remained below 20 psu for more than 5 days. The heavy metal copper is an essential element, but it is also known as a common marine pollutant (Jones, 1997a) where coastal run-offs, mining and smelting operations and disposal of public solid waste incinerator ash are known sources (Dubinsky & Stambler, 1996; Jones, 1997a). Copper affects corals by inhibiting the electron transport in photosystem II (Samson, Morisette, & Popovic, 1988). Heavy metals can also be incorporated in the skeleton making the coral fragile and thus more sensitive to physical actions (Howard & Brown, 1984). However, the symbiotic algae have a high tolerance for heavy metals (Mu¨ller-Parker & D’Elia, 1997) and are thought to be the site for metal uptake in cnidarians (Buddemeier, Schneider, & Smith, 1981). For example, large amounts of the metals copper, cadmium and zinc have been found in symbiotic algae of the anemone Anemonia viridis (Harland & Nganro, 1990). Howard and Brown (1984) state that ‘‘the soluble metals in seawater probably represents the most obvious and direct route of metal uptake to corals’’. Other pathways comprise the feeding activities, which may contribute to the metal intake, especially by tentacular capture of zooplankton that may be metal enriched (Howard & Brown, 1984). Results from a study by Evans in 1977 (cf. Howard & Brown, 1984) where Pocillopora damicornis and Montipora verrucosa were exposed
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
291
to solutions of copper sulphate, indicate toxic effects of copper at lower concentrations than 10 mg l1. Further evidence of copper’s toxicity are provided by Nystro¨m, Moberg, and Tedengren (1997), who found that the metabolism of Pocillopora damicornis and Porites lutea were significantly affected when exposed to 10 mg l1 copper. The inner Gulf of Thailand is exposed to reduced salinity (Moberg et al., 1997) and increasing pollution from domestic, industrial and agricultural sources (Menasveta & Hongskul, 1988). Large amounts of freshwater enter the gulf via four major rivers and can cause occasional drops in salinity from the normal 27.1–29.8 psu in the surface waters of the inner gulf (Menasveta & Hongskul, 1988) to salinity as low as 10 psu (Moberg et al., 1997). Measurements of the heavy metal levels in the inner Gulf of Thailand have shown copper concentrations ranging between 0.33 and 14.16 mg l1 copper (cf. Johnston, 1998). Synergistic effects from multiple stressors are frequently discussed in literature but few studies have experimentally investigated the potential effects (but see Nystro¨m, Nordemar, & Tedengren, 2001). Porter, Lewis, and Porter (1999) suggested that the effects of combined disturbances might not simply be additive. They showed that elevated temperature and salinity was less stressful in combination compared with the sum of the effects generated when acting separately. The objective of this study was to determine the physiological effects of copper and reduced salinity acting simultaneously on the hermatypic coral Porites lutea. The combinations of the stressors are ecologically relevant since they co-occur in the study area at levels that are within the range of this study. The physiological response is estimated by measuring changes in net production and respiration, which is a sublethal approach where stress responses can be detected within hours.
2. Materials and methods This study was carried out during October and November 1999, at Sichang Marine Research and Training station (SMaRT) in the inner Gulf of Thailand. The study area is situated south of Sichang Island on the southeast side of Khang Kao Island, 13 090 N, 100 480 E. The coral community around Khang Kao Island is relatively low diverse and the scleractinian coral Porites lutea is the overwhelmingly dominant coral species (Sakai, Yeemin, Snidvongs, Yamazato, & Nishihara, 1986). 2.1. Experimental design Corals were collected at 4-m depth and transported to an outdoor facility where epiphytes and boring fauna were carefully removed immediately after arrival. The corals were kept in a large flow-through tank (300 dm3) between 1 and 4 days with continuous aeration before exposure. Specimens (approximately 1 dm2 in size) were placed in exposure tanks (60 dm3) for 14 h over night before the start of the physiological measurements. However, due to logistic restraints only one or two treatments were studied per day. The different treatments were A: 30 psu (ambient), B: 20
292
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
psu, C: 10 mg l1 copper in 30 psu, D: 10 mg l1 in 20 psu, E: 30 mg l1 in 30 psu and F: 30 mg l1 in 20 psu. Copper ions were derived from a stock solution of copper sulphate (CuSO4). The number of replicates for each exposure varied between 8 and 11, a total of 57 replicates (Table 1) for the production and chlorophyll measurements. The respiration results are based on 74 replicates. After exposure the specimens were put in individual transparent airtight experimental containers (4.7 dm3), equipped with submersible pumps (138 mm, capacity 10 l min1), containing water from the exposure tank. Changes in dissolved oxygen levels, oxygen saturation and temperature were measured using a microprocessor oxymeter (WTW, OXY196) in light and darkness. Measurements were taken initially, after 30 min and after 1 h. The pumps were run during measurements and for 5 s every 15 min to create water movement in order to prevent oxygen gradients from building up in the containers (Moberg et al., 1997). The oxygen saturation in the experimental containers never dropped below 87% or reached above 124%. 2.2. Physiological measurements Net production was measured outdoors between 10:00 and 14:00 on clear sunny days. The experimental containers were placed in a cooling tank in order to avoid temperature increase during production measurements. Observation showed an average increase of 2 C during the measurements. A light sensor (LI-COR 192A) positioned close to the cooling tank showed that mean light intensities never reached below 1100 mE m2 s1 during production measurements which ensured maximum net production (Chalker, Dunlap, & Oliver, 1983). This light intensity is approximately what the corals receive in field at 4 m depth (Moberg et al., 1997). For respiration measurements the experimental containers were placed in non-transparent tanks.
Table 1 Summary table of means and statistical comparisons of net production/chlorophyll a between the different treatmentsa Treatment mg Cu2+ : psu
chl a (SE) mg chl a cm2
Net prod/chl. a (SE) mg O2 (mg chl a)1 h1
n
0:30 0:20 10:30 10:20 30:30 30:20
27.8 32.5 30.8 25.9 31.8 30.7
1.3 0.7 1.1 1.2 0.5 0.5
8 9 9 10 11 9
(2.5) (1.6) (1.7) (2.0) (1.7) (1.8)
(0.2) (0.1) (0.1) (0.1) (0.1) (0.1)
A B C D E F
A
B
C
D
E
F
– * ns ns ** **
* – ns * ns ns
ns ns – ns ** **
ns * ns – ** **
** ns ** ** – ns
** ns ** ** ns –
a The net production/chlorophyll a and chlorophyll a concentration data from the different treatments (S.E. within parenthesis) and aposteriori test results. n, Number of replicates. ns, no significance. *P <0.05. **P <0.001.
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
293
In order to examine any potential effect of the laboratory environment on coral’s physiological performance a study was performed where net production and respiration were measured on corals (n=4) the day after arrival, and again after 5 days. 2.3. Chlorophyll analysis The living polyp surface area of each replicate was determined using the aluminium foil method (Marsh, 1970). A small fragment (3 cm2) was removed from each coral replicate and mortared. Chlorophyll a was extracted in 7 ml of 90% acetone for 24 h. The samples were centrifuged at 3500 rpm for 10 min and acclimatised to room temperature for 30 min to avoid condensation in the spectrophotometer (single beam, spectronic genesys 5, Milton Roy). Absorption was measured at 750, 664 and 630 nm with a 90% acetone solution serving as blank (Jeffrey & Humphrey, 1975; Parsons, Maita, & Lalli, 1984). Concentration of chlorophyll a was calculated according to the equations of Jeffrey and Humphrey (1975). In order to make sure that all chlorophyll a had been extracted a second extraction was performed on three samples showing that less than 4% of the chlorophyll a was unextracted. Thus, one extraction was sufficiently accurate. 2.4. Statistical analysis To ensure that the data within each treatment did not differ between days a oneway ANOVA (Statistica’99 edition) was carried out. Analysis of variance (two-way ANOVA) was performed in order to evaluate the observed differences in net production, respiration and chlorophyll levels between treatments. The net production per chlorophyll a data was transformed with the square root, as it did not meet the assumptions of ANOVA (Cochran’s test). Aposteriori tests were made with Tukey HSD for unequal sample size.
3. Results The investigation of potential physiological effects on corals while kept in the laboratory showed that there were no differences in primary production (P=0.45) and respiration (P=0.25) rates between the 1st day and the 6th day on lab. Coral replicates exposed to 30 mg l1 copper in both 20 psu and ambient salinity displayed a discoloration from normal brown colour to lighter brown after 14 h exposure. The colour loss was uniform across the surface area of all replicates. However, analysis of chlorophyll a displayed no significant differences between treatments. 3.1. Effects of copper and salinity on respiration rate No significant changes in respiration rate per surface area were detected in any of the treatments compared with controls, or between treatments (Fig. 1).
294
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
3.2. Effects of copper and salinity on net production/chlorophyll a Net production differed significantly between treatments (Fig. 2, Table 1). The post hoc tests showed that net production rate was significantly reduced in corals exposed to 20 psu (P=0.002), 30 mg l1 copper in 30 psu (P=0.0002), and 30 mg l1 in 20 psu (P=0.0001) compared with the controls. Corals exposed to 30 mg l1 copper in 30 psu had a significantly lower production rate compared with those exposed to 10 mg l1 in 30 psu (P=0.001). Corals exposed to 10 mg l1 in 20 psu, however, had higher production rate than those exposed to 30 mg l1 in 20 psu (P=0.0002) and 20 psu only (P=0.018).
4. Discussion 4.1. Effects of copper and reduced salinity on respiration rate The physiological measurements showed that salinity had no significant effect on the respiration rate. This result agrees with a short-term study (1.5 h exposure) by Moberg et al. (1997) who found no effect on respiration per biomass when salinity was reduced from 30 to 20 psu. Long-term exposure, however, may lead to a
Fig. 1. Respiration rate per surface area (cm2) shows no significant differences between the six treatments (meanS.E.).
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
295
decrease in respiration, which is a normal response to salinity stress as demonstrated by Vernberg and Vernberg (1972) where corals through reduced activity decrease the metabolic costs for maintenance, which in turn increases the ability to withstand disturbances (Moberg et al., 1997). For example, a 3-week study by Ferrier-Page`s, Gattuso, and Jaubert (1999) on Stylophora pistillata showed reduced respiration rates even after minor changes in salinity. The present study showed that neither of the two copper concentrations had any significant effect on the respiration rate in Porites lutea. This was not surprising as copper primarily affects the photosynthesis. 4.2. Effects of copper and reduced salinity on chlorophyll a concentration Zooxanthellae have been found to accumulate heavy metals to a larger extent than their hosts (Peters, Gassman, Firman, & Richmond, 1997). Expulsion of zooxanthellae is a common response to heavy metals in corals and has even been suggested as a mechanism to control heavy metal concentrations in symbiotic animals (Harland & Nganro, 1990; cf. Peters et al., 1997). In the present study a discoloration was observed during the experiments, particularly in those exposed to a combination of 30 mg l1 copper and lowered salinity, but also in the corals exposed to 30 mg l1 in ambient salinity. Since no pronounced loss of color was observed for corals exposed to decreased salinity only or to the lower copper concentration, it
Fig. 2. Net primary production rate per chlorophyll a for the six treatment (meanS.E.). Treatment A display a significantly higher production rate than B, E, and F, and treatment C is significantly higher than E and F. Treatment D has significantly higher production rate than B, E and F.
296
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
seems that the higher dose of copper is causing this discoloration. Our results, however, show no significant loss of chlorophyll a despite the observed color loss in corals exposed to copper. These results are consistent with a study by Jones (1997a) on branch tips from colonies of the staghorn coral Acropora formosa that were exposed to elevated concentrations of copper, 5–80 mg l1. Jones (1997a) found no significant differences in chlorophyll a levels except at the two highest concentrations (40 and 80 mg l1) of copper. The results obtained from the present study suggest that the discoloration of corals exposed to copper could be due to a contraction of coral polyps exposing the white naked skeleton (Brown, Le Tissier, & Dunne, 1994). An alternative explaination could be that even if zooxanthellae are lost, remaining symbionts might increase their production of zooxanthellar chlorophyll a (Jones, 1997b). 4.3. Effects of copper and reduced salinity on net production per chlorophyll a Since hermatypic corals lack osmoregulatory mechanisms they may only be able to cope with reduced salinity by behavioural responses, such as retraction of polyps, in order to minimize exposure with surrounding water (Muthiga & Szmant, 1987). This, however, implies hampered photosynthesis due to less light available for the symbiotic algae. Despite that no discoloration was observed in corals exposed to 20 psu there was a significant reduction in production rate. This could be due to cell disruption (Muthiga & Szmant, 1987) or a contraction of polyps that could not be recorded by visual inspection. While decreased salinity indirectly reduce production, copper acts directly on the photosynthesis by inhibition of the electron transport on the oxidizing side of photosystem II (Samson et al., 1988). The present study showed that exposure to the higher copper concentration significantly reduced net production per chlorophyll a, whereas corals exposed to the lower dose remained unaffected. Since this was a relatively short-term study, corals may still be affected at lower concentrations of copper when exposed to continuous pollution. However, Porites lutea has been shown to be tolerant to a wide range of stressors in general, including salinity fluctuations, which may explain its dominance in the study area (Moberg et al., 1997; Nystro¨m et al., 1997). For example, Harland and Brown (1989) showed that the physiological response to elevated iron concentrations in Porites lutea was less severe for corals occurring in iron contaminated areas. They also found that corals in the polluted area had higher levels of iron in their tissues. We suggest that corals found in our study area might have developed similar means for accumulation of copper. This, however, is purely speculative and needs to be investigated further. The combination of shallow coastal waters highly influenced by rivers and heavy run-off and the many sources of heavy metals have resulted in a periodically low saline environment with relatively high background levels of copper (Nystro¨m et al., 1997). The treatment of 10 mg l1 copper in ambient salinity did not generate any change in net production while reduced salinity and 30 mg l1 in ambient salinity led to significantly decreased production rate. An unexpected result was that the com-
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
297
bination of 10 mg l1 and 20 psu did not affect the production rate indicating an antagonistic effect. Similar results were obtained by Porter et al. (1999) who showed that the combination of increased salinity and temperature was less stressful to corals han exposure to elevated temperature alone. Since the colonies of Porites lutea used in this study are found in an area with continuous copper exposure and re-occurring dramatic salinity fluctuations it is likely that they may have adapted to the prevailing conditions and that the effects, therefore, might be site-specific. This calls for studies comparing different regions in order to understand the disturbance complexity of these stressors. Moreover, we suggest that future studies should also aim at investigating long-term low-level stress in order to simulate in situ conditions more accurately.
Acknowledgements We thank Fredrik Moberg, Hans Kautsky and Nils Kautsky for helpful comments on the manuscript. Dr. Padermsak Jarayabhand and Sompop Rungsupa at Chulalongkorn University, for their support. The Swedish International Development and co-operation Agency (SIDA/SAREC) for funding.
References Brown, B. E. (1997). Disturbances to reefs in recent times. In C. Birkeland (Ed.), Life and death of coral reefs (pp. 354–379). Chapman and Hall. Brown, B. E., Le Tissier, M. D. A., & Dunne, R. P. (1984). Tissue retraction in the scleractinian coral Coeloseris mayen, its effects upon coral pigmentation, and preliminary implications for heat balance. Marine Ecology Progress Series, 105, 209–218. Buddemeier, R. W., Schneider, R. C., & Smith, S. V. (1981). The alkaline earth chemistry of corals. Proceedings of the 4th International Coral Reef Symposium, Philippines, 2, 81–85. Chalker, B. E., Dunlap, W. C., & Oliver, J. K. (1983). Bathymetric adaptation of reef-building corals at Davies Reef, Great Barrier Reef, Australia II. Light saturation curves for photosynthesis and respiration. Journal of Experimental Marine Biology and Ecology, 73, 37–56. Coles, S. L., & Jokiel, P. L. (1977). Effects of temperature on photosynthesis and respiration in hermatypic corals. Marine Biology, 43, 209–216. Dubinsky, Z., & Stambler, N. (1996). Marine pollution and coral reefs. Global Change Biology, 2, 511– 526. Ferrier-Page`s, C., Gattuso, J. P., & Jaubert, J. (1999). Effect of small variations in salinity on the rates of photosynthesis and respiration of the zooxanthellate coral Stylophora pistillata. Marine Ecology Progress Series, 181, 309–314. Grigg, R. W., & Dollar, S. J. (1990). Natural and anthropogenic disturbance on coral reefs. In Z. Dubinsky (Ed.), Ecosystems of the world, Vol. 25 (pp. 439–452). New York: Elsevier. Harland, A. D., & Brown, B. E. (1989). Metal tolerance in the scleractinian coral Porites lutea. Marine Pollution Bulletin, 20, 353–357. Harland, A. D., & Nganro, N. R. (1990). Copper uptake by the sea anemone Anemonia viridis and the role of zooxanthellae in metal regulation. Marine Biology, 104, 297–301. Hoegh-Goldberg, O., & Smith, G. J. (1989). The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata
298
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
Esper and Seriatopora hystrix Dana. Journal of Experimental Marine Biology and Ecology, 129, 279– 303. Howard, L. S., & Brown, B. E. (1984). Heavy metals and reef corals. Oceanography and marine biology: an annual review, 22, 195–210. Hughes, T. P., & Connell, J. H. (1999). Multiple stressors on coral reefs: a long-term perspective. Limnology and Oceanography, 44, 932–940. Jeffrey, S. W., & Humphrey, G. F. (1975). New spectrophotometric equations for determining chlorophylls a, b, c1 and c2 in higher plants, algae and natural phytoplankton. Biochemistry and Physiolology Pflanzen, 167, 191–194. Johnston, D. M. (1998). SEAPOL integrated studies of the Gulf of Thailand. Southeast Asian Program on Ocean Law, Policy and Management (Vol. 1). Jokiel, P. L., Hunter, C. L., Taguchi, S., & Watarai, L. (1993). Ecological impact of a freshwater ‘‘reef kill’’ in Kaneohe Bay, Oahu, Hawaii. Coral Reefs, 12, 177–184. Jones, R. J. (1997a). Zooxanthellae loss as a bioassay for assessing stress in corals. Marine Ecology Progress Series, 149, 163–171. Jones, R. J. (1997b). Changes in zooxanthellar densities and chlorophyll concentrations in corals during and after a bleaching event. Marine Ecology Progress Series, 158, 51–59. Loya, Y. (1990). Changes in a Red Sea coral community structure: a long-term case history study. In G. M. Woodwell (Ed.), The earth in transition: patterns and processes of biotic impoverishment (pp. 369– 384 Cambridge. Maragos, J. E., Crosby, M., & McManus, J. W. (1996). Coral reefs and biodiversity: a critical and threatened relationship. Oceanography, 1, 83–99. Marsh, J. A. (1970). Primary productivity of reef-building calcareous red algae. Ecology, 51, 255–263. Menasveta, D., & Hongskul, V. (1988). The Gulf of Thailand. In H. Postma, & J. J. Zijlstra (Eds.), Ecosystems of the world, Vol. 27 (pp. 363–383). New York: Elsevier. Moberg, F., Nystro¨m, M., Kautsky, N., Tedengren, M., & Jarayabhand, P. (1997). Effects of reduced salinity on the rates of photsynthesis and respiration in the hermatypic corals Porites lutea and Pocillopora damicornis. Marine Ecology Progress Series, 157, 53–59. Mu¨ller-Parker, G., & D’Elia, C. F. (1997). Interactions between corals and their symbiotic algae. In C. Birkeland (Ed.), life death of coral reefs (pp. 96–113). New York: Chapman and Hall. Muthiga, N. A., & Szmant, A. M. (1987). The effects of salinity stress on the rates of aerobic respiration and photosynthesis in the hermatypic coral Siderastrea siderea. Biological Bulletin, 173, 539–551. Nystro¨m, M., Folke, C., & Moberg, F. (2000). Coral reef disturbance and resilience in a human-dominated environment. Trends in Ecology and Evolution, 15, 413–417. Nystro¨m, M., Moberg, F., & Tedengren, M. (1997). Natural and anthropogenic disturbance on reef corals in the inner Gulf of Thailand: physiological effects of reduced salinity, copper and siltation. Proceedings of the 8th International Coral Reef Symposium, 2, 1893–1898. Nystro¨m, M., Nordemar, I., & Tedengren, M. (2001). Simultaneous and sequential stress from increased temperature and copper on the metabolism of the hermatypic coral Porites cylindrica. Marine Biology (in press). Paine, R. T., Tegner, M. J., & Johnson, E. A. (1998). Compounded perturbations yield ecological surprises. Ecosystems, 1, 535–545. Parsons, T. R., Maita, Y., & Lalli, C.M. (1984). A manual of chemical and biological methods for seawater analysis (pp. 101–104). Oxford: Pergamon Press. Peters, E. C., Gassman, N. J., Firman, J. C., Richmond, R. H., & Power, E. A. (1997). Ecotoxicology of tropical marine ecosystems. Environmental Toxicology and Chemistry, 16, 12–40. Porter, J. W., Lewis, S. K., & Porter, K. G. (1999). The effect of multiple stressors on the Florida Keys coral reef ecosystem: a landscape hypothesis and a physiological test. Limnology and Oceanography, 44, 941–949. Ranklin, J. C., & Davenport, J. (1981). Animal osmoregulation. New York: Halsted Press. Sakai, K., Yeemin, T., Snidvongs, A., Yamazato, K., & Nishihara, M. (1986). Distribution and community structure of hermatypic corals in the Sichang Islands, inner part of the Gulf of Thailand. Galaxea, 5, 27–74.
S. Alutoin et al. / Marine Environmental Research 52 (2001) 289–299
299
Samson, G., Morisette, J., & Popovic, R. (1988). Copper quenching of the variable fluorescence in Dunaliella tertiolecta. New evidence for a copper inhibition effect on PSII photochemistry. Photochemistry and Photobiology, 48, 329–332. Vernberg, F. J., & Vernberg, W. B. (1972) Environmental physiology at marine animals. New York: Springer Verlag.