Seasonal fluctuations in environmental factors and variations in symbiotic algae and chlorophyll pigments in four Indo-Pacific coral species

Abstract

Marked seasonality is observed in sea-surface temperature (SST) and photosynthetically active radiation (PAR) received by shallow water corals at Phuket, Thailand (7" N latitude). This seasonality is reflected in both algal densities and algal chlorophylls (a and C?) in 4 massive species of scleractinian corals, namely Coeloseris rnayeri, Goniastrea retiformis, Porites lutea, and G. aspera. Algal numbers and algal chlorophylls are generally maximal at the end of the wet season (November) and minimal at the end of the dry season (March to May). Algal densities are reduced by up to 53 % in May compared to November in some species, whde algal chlorophylls may be up to 4-fold greater in November. Concomitant with these changes are differences in algal cell volume in 3 out of the 4 species studied and alterations in host tissue depth, with smaller algal volume and greater tissue depth at the end of the wet compared with the dry season. Over the period 1993 to 1996 algal numbers showed a significant negative correlation with monthly (30 d) and short-term (1 to 2 d) values of both SST and PAR dose, while algal chlorophylls displayed a significant negative correlation with PAR dose in the 5 d preceding collection. Rising SST and increasing PAR in the dry season are paralleled by decreasing algal numbers and algal chlorophylls on an annual basis. Recovery of algal densities and a build-up in algal chlorophylls follow reductions in SST and PAR at the end of the wet season. In one of the years, when coral 'bleaching' was widespread at the study site (May 1995), the interaction between SST and PAR resulted in an 86 % reduction in algal densities in bleached corals compared to their annual maximum density. This study highlights the importance of seasonal fluctuations in physical environmental parameters and concomitant changes in the coravalgal symbiosis. Furthermore, it emphasises the need for greater understanding of the potential effects of seasonal and inter-annual variability on the status of algal symbionts, as well as identification of critical times of the year when visible bleaching may not be obvious but when algal numbers and algal chlorophylls are low and effects on coral physiology may be far-reaching.

Stress response of two coral species in the Kavaratti atoll of the Lakshadweep Archipelago, India

“…Some studies have not endorsed losses in chlorophyll a and c and have even suggested an increase of these pigments in stressed and bleached corals (Szmant and Gassman 1990;Le Tissier and Brown 1996). This has been ascribed to increased nutrient availability to the remnant algae in the host or the greater loss of apical algae leaving the subsurface algae with higher chlorophyll concentrations below (Brown et al 1999). Therefore, chlorophyll a concentrations and zooxanthellae densities may not be universally applicable as early stress indicators.…”

“…Reduction in zooxanthellae numbers during natural bleaching event was reported in six species of corals (Brown et al 1995) and in solar bleached Goniastrea aspera (Le Tissier and Brown 1996). Increased seasurface temperature (SST) and photosynthetically active radiation (PAR) experienced by shallow water corals at Phuket in Thailand also showed decreased zooxanthallae numbers on an annual basis (Brown et al 1999). In…”

The physiological response of reef corals to diel fluctuations in seawater temperature

“…4). Although we cannot exclude the possibility that the corals were compromised by stress-related bleaching in trial 2, in part because coral color and Symbiodinium density can be uncoupled in the early stages of bleaching (Fitt et al, 2001), we suspect that the declines in Symbiodinium density reflected the onset of routine seasonal changes in algal populations (Brown et al, 1999;Fitt et al, 2000). Such changes can be rapid, and in the Caribbean coral Montastrea faveolata, for example, seasonal declines in Symbiodinium density can occur at 2.2 × 10 6 cells cm 2 month − 1 in a non-bleaching year (i.e., when corals do not appear pale), which corresponds to a 44% reduction month − 1 from the winter maximum ( Fig.…”

“…The results of the two trials suggest that the response of back reef corals to oscillatory thermal environment may differ between periods when their tissues are well populated with, versus depleted of, Symbiodinium. When Symbiodinium densities are high, for example in the winter and spring (Brown et al, 1999;Fitt et al, 2000), corals are able to respond by reducing their Symbiodinium population in response to diurnal variations in temperature. This responsiveness may degrade, however, as Symbiodinium populations decline through seasonal effects concurrent with the end of summer and start of autumn (Brown et al, 1999;Fitt et al, 2000).…”

Bioindicators of changes in water quality on coral reefs: review and recommendations for monitoring programmes

“…Concentrations of chlorophyll a (and hence colour brightness) increase in response to exposure to elevated nutrients (Hoegh-Guldberg and Smith 1989; Table 2) and reduced irradiance (Falkowski and Dubinsky 1981;Dubinsky et al 1984) whereas symbiont density may decrease in response to sedimentation (Nugues and Roberts 2003) and exposure to pollutants, such as cyanide (Cervino et al 2003). However, symbiont density also varies with season (Stimson 1997;Brown et al 1999;Fagoonee et al 1999) and seawater temperature, indicating a moderate specificity to changes in water quality. Correlating colony brightness to changes in water quality also requires large spatial and temporal replication in monitoring programmes because photo-acclimatory responses occur on short timescales (Anthony and Hoegh-Guldberg 2003).…”