Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000
Acclimatization/adaptation of coral reefs in a marginal environment R. P. M. Bak1 and E. H. Meesters1 ABSTRACT We studied coral reefs in the marginal, high turbidity, high sedimentation environment of the Bay of Banten, NW Java, Indonesia. Coral cover increased and coral partial mortality decreased along an inshore-offshore gradient. Asexual recruitment was dominant, but sexual recruitment was still occurring (~10%). High turbidity (k' = 0.17-1.26) and sedimentation (2.5-63 mg cm-2day-1) limit reef development inshore but levels of sedimentation did not correlate with reef development. Resuspension of bottom sediment (75.3%) was important, preventing direct negative effects of sedimentation. At the level of the coral organism we found RNA/DNA ratios in coral tissue, presumably reflecting tissue growth characteristics, to be negatively related with depth over the reef slope. Also, RNA/DNA ratios were consistently higher in corals living in turbid environments, except for the most extremely turbid conditions. This may be genetically based variation and suggests that these corals are adapted sensu stricto to higher turbidity levels.
Keywords Coral distribution, SPM, Sedimentation, Resuspension, RNA/DNA, Recruits, Partial mortality Introduction Coral reefs are known to flourish in clear, oligotrophic waters. However, many reefs occur in turbid conditions (e.g. Kleypas 1996, Woolfe and Larcombe 1998). Marginal conditions for reefs have been defined on the basis of physio-chemical tolerance boundaries for temperature, salinity, nutrients, and light (Kleypas et al. 1999). These environmental factors are often strongly correlated, e.g. light attenuation is influenced by suspended and dissolved matter. Kleypas et al. (1999) found that light penetration varied more than all other environmental variables considered. Low levels of light on reefs are generally caused by increased turbidity. Suspended particles scatter and absorb light, changing depth penetration, and increased sediment load may lead to increased sedimentation on coral reefs. Such environmental characteristics have biological consequences (Rogers 1990, Babcock 1991, Meesters et al. 1992, Anthony 1999). Because the depth distribution of corals is primarily limited by the ambient light regime, coral reefs become 'compressed' as water clarity decreases (Acevedo 1988, Tomascik et al. 1993). Reduced irradiance also affects the energy balance of the coralsymbiont association. Corals may acclimatize/adapt to, biochemical and physiological adaptations (Brown 1997). Turbidity and increased sedimentation are known to affect recruitment by decreasing the amount of substratum that is available for settlement of larvae (Babcock 1991). A high sediment load may lead to increased mortality of coral colonies (e.g. Bak 1978). Increased turbidity does not necessary lead to increased sediment deposition (Larcombe 1999). Sediment on the bottom may be resuspended by water movement. Sedimentation, as measured by tube-like traps (Rogers 1990), represents a mixture of primary and secondary (i.e. due to resuspended sediment) sedimentation. The problem of resuspension appears not to have been addressed in reef studies. We 1
used traps with different dimensions (Flower 1991) to investigate the importance of sedimentation and resuspension. We studied coral reefs in the Bay of Banten (Teluk Banten) at the north-western coast of Java, Indonesia. Our objective was to describe relevant biological characteristics of reefs in the context of the marginal environmental conditions (Kleypas et al. 1999). We were especially interested in effects of turbidity and sedimentation. These are two factors of world-wide importance in degrading reef environments to marginal conditions. This is a consequence of increasing pressure from human populations in the coastal zone (Wilkinson 1997). To investigate a possible effect of adverse environmental conditions on the coral organisms we looked for an indicator of coral metabolic functioning. Methods proposed earlier were not suitable or inadequate for our purpose and we decided to explore the potential of using RNA/DNA ratios to relate coral condition to environmental condition. The concentration of RNA is supposed to reflect the rate of protein synthesis. RNA/DNA ratio would provide an index of protein synthetic capacity per cell and provide an estimate of growth rate and metabolic status (e.g. Frantzis et al. 1992, Buckley et al. 1999, McNamara et al. 1999). We decided to measure RNA/DNA ratios in corals along two gradients: in relation with depth over the reef slope and with increased turbidity between sites. Methods We studied 4 reefs in the Bay of Banten (Teluk Banten), Java, Indonesia (Fig. 1), March 1998 to May 1999. The bay is approximately 150 km2. It has a muddy floor, a depth mostly < 10 m and a diurnal tidal range of 0.2 to 0.9 m (neap tide, spring tide, Hoitink and Hoekstra unpubl.) During Southeast and Northwest Monsoon wind speed rarely exceeds 10 m sec–1, and surface currents are typically 15 cm sec-1 with peaks up to 65 cm sec–1 (Hoitink and Hoekstra unpubl.). We measured turbidity,
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sedimentation, resuspension and assessed coral communities in 4 islands: Gosong Dadapan, Pulau Kubur Pulau Pamujan Kecil, and Pulau Pamujan Besar, situated along an inshore to offshore gradient through the bay.
Fig. 1 Map of Banten Bay, NW Java, with bay study sites Gosong Dadapan, Pulau Kubur, P. Pamujan Kecil, P. Pamujan Besar, and offshore reef site P. Tunda
Sedimentation (the downward flux of sediment) was measured in 30 cm long PVC tube traps (diameter 5.5 cm). To obtain an indication of resuspension and net sediment accumulation, we used dish traps (inner diameter 8.6 cm, height 1.4 cm, ratio 0.16). Next to each tube trap, a dish trap was fastened to the same iron stake 10 cm above the bottom. Measurements were at the bottom of each reef (all 4.6 m except 2.4 m at shallow Gosong). The collected sediment was filtered (9 cm diameter Whatmann GF/C filters) and dried at 60º C to constant weight. An indication of water turbidity (dry weight mg l-1) was obtained by an Optical Backscatter Sensor (OBS). We obtained very similar results in calibrating the OBS with bay suspended material (r2 = 0.81-0.98). A total of 307 profiles were taken on 21 days. We measured light irradiance over depth profiles 54 times during the period February 1998 till April 1999 using an IL 1400A light meter (International Light, Inc.) equipped with a cosine corrected, underwater sensor (PAR, 400-700nm). Irradiance values were used to calculate extinction value (k'), the slope of the exponential regression of the depth profiles. Coral characteristics were investigated at the northwest side of each island. Line transects (10 m length, n= 11 to 18,) were video-taped in situ at three depths: shallow (1-2 m), intermediate (2-3.5 m) and deep (4.5-5.5 m), representing reef terrace, upper reef slope and lower reef slope. At Gosong Dadapan, we surveyed 1.0, 2.0 and
3.0 m because of the limited depth range of the reef (1-3 m). We determined mean cover of scleractinian coral species, partial mortality (dead coral skeleton under the transect line connected by skeleton to living parts). Numbers of coral recruits were estimated in small quadrats (60 x 60 cm) positioned parallel to the transects. Recruits, defined as coral colonies of less than 4 cm diameter (Bak and Engel 1979), were assigned to one of three classes depending on their origin: sexually derived recruits, fragments broken from parent colony, living tissue parts separated by partial mortality. Fragments from fragile Montipora digitata were excluded from the recruitment counts, because fragment numbers varied from 0 to more than 150 fragments per quadrat, causing disproportionally large variation in recruitment numbers. At Gosong Dadapan, recruits were only measured at two depths (1.2 and 2.4 m) because of the short reef slope. For RNA/DNA determination we collected small cores of the colony tops of massive Porites species over the depth gradient of the reef at three sites: Gosong Dadapan, P. Kubur and, outside of the bay and far offshore in the Java Sea, the island Pulau Tunda. P. Tunda has relatively clear water (Fig. 2) and reefs extend to much greater depths (> 20 m) than inside the bay (< 5 m). Samples were stored immediately in liquid oxygen and not thawed before analyses (HPLC, see Coppela et al. (1987, Stoeck et al. 1998). Gradient flow rate and composition of effluents used for HPLC determination were as in Dell'anno et al. (1998). We used Systat 10, SPSS Inc. 2000, in all statistical analyses. Residuals were graphically examined to test assumptions and variables ln transformed if necessary. Probabilities of post-hoc comparisons adjusted following Bonferoni.
Fig. 2 Irradiance over the depth gradient. Mean values for Banten Bay; offshore P. Tunda; oceanic reef in Curaçao, Netherlands Antilles
Results Sedimentation in the dish taps was much lower than in the tube traps (Fig. 3). Net sedimentation differed between the two periods, being much higher at all sites during the second period, when stronger winds occurred (2-way ANOVA, F1,29 = 16.83, p = 0.001 for periods, p=0.093 for sites, p=0.473 interaction). From the difference between the dish traps and tube traps we calculated the percentage resuspension (Fig. 3). Resuspension of bottom sediment was important (mean 75.3 %, SE = 0.86) and similar among all islands and
between the two periods (2-way ANOVA, p-values respectively 0.76 and 0.36). Resuspension remained approximately the same even when gross sedimentation (Fig. 3 tube traps) was 2-3 times higher in the second period. Light extinction was largest at the reefs closest to shore (Gosong Dadapan, 0.78 ± 0.07, µ ± SE, Table 1). The differences between the four reefs were highly significant (1-way ANOVA F3,50 = 11.5, p < 0.0001) and post-hoc tests using Bonferroni probability adjustment indicated significant differences between Gosong Dadapan and the other reefs (p < 0.05).
Fig. 3 Sedimentation and resuspension in Banten Bay. Data (mean ± se) from two periods at four islands. Tube traps prevent resuspension, dish traps allow resuspension. %Res is percentage sediment resuspended
Average OBS profiles (Fig. 4, n=307) show a significant increase in suspended particle concentration with depth (Linear regression, p < 0.05). Average profiles are similar for all islands with the exception of Gosong Dadapan, which has higher levels of SPM (Fig. 4). Mean concentrations, averaged over the whole depth interval, were 6.0 (sd = 3.4), 3.8 (2.0), 3.8 (1.6), and 3.6 (1.9) for Gosong Dadapan, P. Kubur, P. Pamujan Kecil, P. Pamujan Besar (post-hoc comparisons p < 0.0001).
Fig. 4 Average concentration (± se) of suspended particulate matter with depth at each site Sediment in the bay is generally very fine grained (90 % smaller than 65 µm). There exists a clear relationship between SPM and average extinction (k' = 0.1034SPM +
0.1607, r2 = 0.89). Using this empirical relationship and the average SPM concentrations per island we calculated the average extinction coefficient (k') as 0.78, 0.55, 0.55, 0.53 for Gosong Dadapan, P. Kubur, P. Pamujan Kecil, P. Pamujan Besar. Living coral cover was high, especially in the shallow zones where it ranged between 16.7 and 48 % (Fig. 5). Coral cover decreased with depth but there were differences between the islands (interaction sites x depth, F3, 173 =.7.07, p
