Ethic statement

All work was conducted according to relevant national and international guidelines; therefore investigations were done with the official admission and the respective research permit of the National Research Council of Thailand in concert with the Phuket Marine Biological Centre for the Similan Islands (NRCT, permit numbers: TS0907.1/12593 and NRCT002.3/03231). We did our best to keep animal (Porites lutea) and algae sampling (microphytobenthos and turf algae) at a minimum and to avoid any lasting impact on the investigated reefs. As no corals were transported out of the country, no CITES permit was necessary.

Study site and background parameters

Samplings and measurements were mainly carried out on the east (E: 8°33´57.97´´N 97°38´24.49´´E) and west (W: 8°34´15.24´´N 97°37´57.42´´E) sides of the central Similan Island # 4 (Ko Miang) from January to March 2008. Two water depths (shallow: 6-9 m; deep: 19-21 m) were selected and are referred to as sampling ‘sites’ E shallow, E deep, W shallow, W deep. A map of the Similan Islands showing their location in the Andaman Sea and the orientation of Ko Miang within the island group is given in Figure 1. Large amplitude internal waves (LAIW) occur in the Andaman Sea throughout most of the year (except August and September, [21]), and they are most pronounced at the Similan Islands from February until April ([27]). Our study was therefore conducted during LAIW high season, when they have the main impact.

Water temperature was recorded at all 4 sites using temperature (C°, TidbiT v2, Onset) loggers with a logging interval of 1 minute. Supplementary temperature loggers (TidbiT v2, Onset) with a logging interval of 4 minutes were deployed at three additional islands at 4 locations situated along the Similan Island chain (# 2, # 7, # 8 south, # 8 north, Figure 1) at the same depths and orientations as at Ko Miang (7 and 20 m, on E and W; n = 16). At all of these locations, the temperature characteristics were consistent with those at Ko Miang (same time period 02.02.-16.03.2008) and daily temperature ranges (DTR), maximum temperature – minimum temperature, provided a measure for LAIW impact at each site (Table 1 Ko Miang, Table S1 all islands). The DTR revealed increasing LAIW impact from E shallow, E deep, W shallow to W deep; therefore, sampling sites were aligned accordingly throughout this manuscript to reflect the increasing influence of LAIW. Furthermore, the consistent temperature record along the islands suggests that east (E) and west (W) faces of Ko Miang, located centrally within the Similan island chain, can be considered representative for LAIW-exposed and LAIW-sheltered island sides, respectively.

Light intensity was recorded with light (lx, Pendant, Onset) loggers at 1 minute intervals at the 4 study sites at Ko Miang and computed as daily mean around noon (11:00-14:00, 02.02.-16.03.2008). To assess the suitability of the Pendant logger data as rough proxies for the photosynthetic active radiation (PAR), a calibration experiment was carried out during 6 diurnal cycles in shallow (7 m, n = 4) and deep (20 m, n = 2) water. The light intensity (I [lx]) was measured with Pendant light loggers (as above), and photosynthetic active radiation (PAR, µmol quanta m^-2 s^-1) was recorded concomitantly with the internal light sensor of a submersible pulse amplitude modulated fluorometer (Diving PAM, Heinz Walz GmbH, Germany). In spite of the different optical spectra recorded by the Pendant and Diving PAM sensors, we found a fair correlation [I (lx) = 0.0101x PAR (µmol quanta m^-2 s^-1), R² = 0.52, Figure S1]. It has to be taken into account that this is only an estimated correlation as different wave lengths penetrate increasing water depth with varying success, but in the context of this ecological study it provides sufficient resolution to compare all sites. Additional factors causing measured deviation may be a general scattering of light due to wave movements, or a diverging sensitivity of the sensors (bio fouling could be excluded due to meticulous cleaning).

Water currents were measured with 2 autonomous upward-looking Acoustic Doppler Current Profilers (ADCP, RDI Teledyne Workhorse Sentinel, 600 kHz and 300 kHz) deployed at E and W of Ko Miang in 25 m depth . The currents in the water column were measured above the transducer at 1 m vertical and 1 min temporal resolution with an accuracy of 0.3 to 0.5 % of the water velocity and a precision of ± 0.3 to 0.5 cm s^-1. Data were imported into Matlab (RDADCP by Rich Pawlowicz, U. of British Columbia, http://www2.ocgy.ubc.ca/~rich/) and current speeds were determined by averaging the 3-D current measurements from 3 vertical bins for deep (20 m : 20 to 17 m depth) and shallow (7 m: 9 to 6 m depth).

Water samples for inorganic nutrient analyses (ammonium, nitrate + nitrite, and phosphate) were collected during the sampling period using SCUBA (in total: E shallow, n = 11; E deep, n = 12; W shallow n = 8; W deep n = 9 samples). Water samples were filtered (GF/F filters, Ø 25 mm, nominal pore size: 0.7 µm), and the filtrate was conserved with HgCl₂ ([42]).

Benthic community composition

Line point intercept transects (LPI, 50 m length, [43]), were conducted along isobaths at the E and W of Ko Miang, at 7 m and 20 m. Two (deep) to 3 (shallow) replicate transects were carried out spaced 10 m apart. Within each transect, the benthic community was sampled every 0.5 m (comprising 101 data points each). Hard substrate (live and dead coral cover and rock), sediment and turf algae cover (on all substrates and only on hard substrate), and coral morphologies (used for the 2D to 3D factors, see below) were determined.

Benthos Sampling and Processing

Only the top layer of the sediment was considered as photosynthetically active, as light can only penetrate a few mm into the sediment ([44]). Surface cores of 1.2 cm thickness (less was not practicable) and 1.3 cm diameter (i.e. 6.4 ml volume and 3.9 cm² surface area) were taken for investigation of sediment composition and photosynthesis-related parameters. The samples were processed within 1 h after collection and used for incubation experiments (E shallow, n = 15; E deep, n = 15; W shallow, n = 14, W deep, n = 20), pigment determinations of chl-a and pheophytin content (n = 10 for each site), and analysis of sediment grain sizes (12 individual samples pooled for each site).

Additionally cores of 5.6 cm sediment thickness (33.4 ml) were taken for pore water nutrient analysis (n = 4 for each site). Within 1 h after sampling, the cores were washed with 10 ml distilled water, centrifuged and the supernatant filtered through pre-combusted GF/F filters (pore size ~ 0.7 µm). This procedure was repeated two more times and HgCl₂ was added to the final filtrate for conservation ([42]). Filters were frozen for later determination of particulate organic carbon (POC) and particulate total nitrogen (PN).

Turf algae samples were defined as a conglomerate of diverse unidentified filamentous algae growing up to a height of about 1 cm. They occasionally appeared on sediment, but only the ones on hard substrate were collected for analyses: Small pieces of coral rubble and rock were collected and the associated algae removed within 1 hour after sampling using a scalpel and an airbrush filled with filtered seawater. The algae in seawater were used for incubations and later filtered (GF/F filters) and frozen for pigment analysis (E shallow, n = 5; E deep, n = 15; W shallow, n = 10; W deep, n = 15). Surface area was calculated using the best fitting geometrical form(s) of each scraped rubble piece. Only the light-exposed surfaces of the rubble fragments were considered. To determine algal growth rates, microscope slides (later referred to as algae tiles) were fixed on holders and deployed at each site. Algae tiles were sampled and displaced in a random order after two to three weeks (E shallow, n = 15; E deep, n = 15; W shallow, n = 10; W deep, n = 17). They were scraped with a scalpel and airbrushed with filtered seawater. The algae-seawater suspension was treated as above for pigment analysis. Daily growth rates were calculated as amount of chl-a (µg, see pigment determination below) per algae tile area (cm^-2) and unit time (d^-1).

The massive stony coral Porites lutea ([45]) is the most common coral species at the Similan Islands, and one of the most abundant species in the Andaman Sea ([29]). Five colonies were sampled at each site, and two fragments were chiselled from each colony. The fragments were left at the appropriate site on racks to heal for two weeks. One replicate of each colony was used for light and the other for dark incubations, five fragments for each incubation chamber (light or dark). Coral fragments were measured with a calliper and surface areas were calculated according to Naumann et al. ([46]) with the best fitting geometric shape (half-ellipsoid). After the incubation experiments coral tissue was removed with an airbrush and filtered seawater, and homogenized with an Ultra Turrax (IKA). A subsample was filtered (GF/F filters) and frozen for pigment analysis. For the incubation procedures see below.

Oxygen fluxes

Sample analyses

Primary production budget

Daily net and gross primary production (P_net and P_gross) and respiration (R) of the sedimentary microphytobenthos and the turf algae were calculated using the results of the short-term incubations (maximum rates around noon and dark incubations respectively; see above). Start and end time points of photosynthetic activity by corals were obtained via the 2^nd order polynomial curves (see above) and used to conduct quadratic extrapolations. P_gross = P_net + R, for each incubation, with all values given in µg cm^-2 h^-1. For all primary producers, the integral areas of the daily curves were calculated (Mathematica 5.2, Wolfram Research, Inc. 2005), resulting in the calculated daily respiration, net and gross primary production rates. To scale daily rates to area, they were multiplied with the relative proportion of each primary producer at each site (projected area) and corrected by conversion factors (CF) between projected and actual area for turf algae and corals, respectively. For turf algae we obtained a CF value of 1.5 (i.e. a 50% increase of the actual surface relative to the projected area). For corals the in situ CF after Alcala and Vogt ([53]) were used (massive: 2.31, branching: 6.88, folios: 4.43). A coral 2D to 3D factor for each site was estimated using the mean of all factors for each coral growth form (branching, massive and encrusting or foliose) and weighted by the percentage cover of each coral growth form found. Gross photosynthesis is given over the period of PAR per day (12 h of sun light; see also above) and respiration for 24 h.

Statistical analyses

The software Statistica v 9 was used for statistical analyses. Data were tested for normal distribution and homogeneity of variances with Kolmogorov-Smirnov and Levene’s test and log-transformed if necessary. Based on the data characteristics, both, 2-factorial analysis of variance (ANOVA) with posthoc, pairwise comparisons of the adjusted group means via Tukey HSD-tests, as well as non-parametric one-factorial Kruskal-Wallis ANOVA and median test followed by multiple comparisons of mean ranks were used to test the effect of side (W and E) and depth (shallow, deep) on environmental parameters such as temperature (DTRs), light (PAR), water currents, nutrient concentrations and sediment characteristics (PN, POC), as well as on photosynthesis related characteristics (algal growth rates, pigment content, photosynthesis and respiration) of the main primary producers (microphytobenthos, algal turf, corals). Side and depth (parametric ANOVA) or site (e.g. W 7 m, non-parametric Kruskal-Wallis ANOVA) were used as treatment factors. To test whether environmental parameters or biological responses, or both, are related to DTRs (LAIW impact) the non-parametric Spearman correlation was used, setting DTRs as ordinal ranks. If not stated otherwise values are given as mean ± SE.

Environmental background

Synchronous temperature conditions and events (temperature variations in differing strengths) each at west (W) and east (E), deep and shallow sites along the whole Similan island chain (Table S1; [27]) confirm the central Similan Island Ko Miang (Ko #4) as a representative study area for the Similan Islands. Despite the almost identical temperature mode and mean values measured at Ko Miang, daily temperature ranges (DTRs, daily maximum – minimum) were highest at W and in deep when compared to E and shallow (Table 1, ANOVA results Table S2 A). Figure S3 exemplary shows a diurnal cycle (24h, 15.02.2008) of the temperature regime at all Ko Miang sites to visualize the strengthening of temperature drops towards W and deeper waters.

Overall current velocities were higher at W compared to E (Table 1, Kruskal Wallis test: χ² = 14.23, df = 3, p < 0.03), reflecting an enhanced water flux there. The higher current speed may cause higher turbidity and accordingly light intensity as photosynthetic active radiation (PAR) was lower at W (Table 1, ANOVA results Table S2 B, Figure S4). The enhanced turbidity may also be reflected by a higher particle load as both, total particulate nitrogen (PN) and particulate organic carbon (POC), accumulated within the sediment and were higher at W than E (Table 1, ANOVA results Table S2 C, D). The sediments themselves showed deviating grain size characteristics at each site (Table 1). Almost no clay and silt was found at W deep and little fine sand for both W sites (fewest at W shallow) contrasting with finer sediments at E. More coarse sand could be found at W due to an elevated current regime. Accordingly, water content within sediments at W was higher than in ones at E. Carbonate content was one-tenth lower at W than E (Table 1). ANOVA results revealed that ammonia and phosphate concentrations did not show significant side or depth effects (Table S3 A, B) in the water column or in the sediment pore water. But pooled nitrate + nitrite concentrations revealed highest concentrations in W deep and lowest in E shallow (Table 1, ANOVA results Table S2 C). The rank correlations however, showed that ammonia and pooled nitrate + nitrite concentrations increased with greater LAIW impact (DTR), whereas phosphate concentrations failed to show any correlation (Table 1). Nutrient concentrations in sedimentary pore water revealed different characteristics. Both ammonia and phosphate concentrations were coupled to LAIW impact (DTR), whereas nitrate + nitrite concentrations did not show any correlation (Table 1).

Benthic community composition

Benthic community composition (Figure 2, all ANOVA results: Table S4) assigned microphytobenthos (sediment associated micro-algae), turf algae and corals to be the main primary producers at Ko Miang. Sediment cover was highest at E shallow due to extensive sandy patches, medium at deep sites (E and W) and lowest at W shallow. Hard substrate (live coral cover, dead coral cover and rock) was mainly determined by the presence of rocks (granite boulders). Highest rock cover was found at W shallow, a little at W deep and none at the E sites. The W side showed higher turf algae cover on any substrate and reduced coral cover as a fraction of hard substrate especially in W deep compared to E.

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Figure 2. Benthic cover.

Line transects on benthic cover at Ko Miang (shallow: 7m, deep: 20m) in %; live coral cover as a fraction of hard substrate; values are given as mean ± SE. The right panel shows the comparison of coral and turf algae cover between E and W to visualize the differences between sides (turf: p < 0.01, live coral: p < 0.05); values are shown as median ± 95% CI.

https://doi.org/10.1371/journal.pone.0081834.g002

Growth rate of turf algae and pigment content of main primary producers

Algal growth rates determined as chlorophyll-a increase (Figure S5) were higher at W and lowest at E deep (ANOVA results: Table S5). Growth rates were positively correlated to LAIW impact (DTR) (Table 2).

Pigment concentrations of turf algae on hard substrate and sediment showed similar values at all sites (ANOVA results: Table S6). Only pheophytin was positively coupled to increasing LAIW impact (DTR) (Table 2; Figure S6, right panel; Figure S7, right panel).

Coral chlorophyll-a content exhibited lowest values at E shallow, medium values at E deep and W shallow, and highest values at W deep (Table S5, Figure S8) and was therefore higher with more pronounced DTR (Table 2).

Photosynthesis and respiration

The microphytobenthos revealed net and gross photosynthetic rates negatively correlated to DTR with highest values at E shallow and lowest at W deep (Table 2). Respiration was reduced at W. Net photosynthesis and respiration were generally higher at shallow sites compared to deep ones (Figure 3, left panel; ANOVA results: Table S7).

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Figure 3. Oxygen fluxes of sediment and turf algae.

Microphytobenthos and turf algae were incubated to determine oxygen fluxes (net photosynthesis and respiration); values were normalised to surface area of sediment or turf algal growth substrate (µg cm^-2 min^-1) and are given as means ± SE. Sedimentary net and gross photosynthesis as well as algae respiration are correlated with daily temperature ranges (DTR).

https://doi.org/10.1371/journal.pone.0081834.g003

Turf algae showed a positive coupling of net photosynthesis, but a negative correlation of respiration rates to DTR (Table 2), with respiration rates that were lower at W compared to E. Gross photosynthetic rates were constant, irrespective of side, depth or DTR (Figure 3, right panel, Table S7).

Corals’ net photosynthesis over the day seemed to be relatively similar at all sites, but with more productive photosynthesis at the shallow sites (Figure 4), regardless if E or W(Table S7), despite lower light intensities at W (Table 2, Figure S4). Gross photosynthetic rates of corals around noon (net and gross, 11:00- 14:00 h) were positively correlated to DTR (Table 2), still higher in shallow. Respiration rates were similar at most sites, except for E shallow, which was double that of the other sites (Figure S9). Photosynthetic rates of corals around noon related to pigment content (chl-a^-1) were negatively correlated to DTR, suggesting a more costly photosynthesis at W (Spearman rank correlation, p <0.001 and R_sp = -0.792, p <0.002 and R_sp = -0.745, respectively).

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Figure 4. Oxygen fluxes of corals.

Incubations of P. lutea in situ with an automated respirometer during one day at each site (shallow: 7m, deep. 20m) to determine oxygen fluxes, i.e. net and gross photosynthesis; values were normalised to surface area (µg cm^-2). Maximum rates (at midday) of gross photosynthesis are correlated with daily temperature ranges (DTR).

https://doi.org/10.1371/journal.pone.0081834.g004

Primary production budgets

Calculated net and gross primary production rates, i.e. rates attributed to photosynthetic active surface area (d^-1), showed highest values for the microphytobenthos, followed by turf algae, whereas corals only achieved a comparatively low productivity (Table 3). The microphytobenthos showed by far the highest photosynthetic activity at E shallow, with almost 1.5-times higher photosynthetic rates than algae or corals from any other site. Taking into account the different proportions of benthic cover and the conversion factors (2D to 3D) for each site, the sediments were the major primary producers at E, especially at the shallow site (Table 3). By contrast, at W turf algae were the major primary producers. Corals contributed very little to the primary production at W deep, but a relatively large share at all other sites.

Benthic community response

Primary production

Primary production budgets related to reef area for all sites were estimated by combining incubation with transect data, i.e. photosynthetic rates with benthic cover, taking into account the 2D to 3D conversion factors (Table 3). It should be noted that no light respiration could be measured. Light respiration can be considerably higher than dark respiration ([69,76]), partly due to the required reparation of reversible photodamage as mentioned above ([72,74,75]). Gross primary production may therefore be underestimated and this may explain why the overall photosynthesis - respiration ratios per site are < 1. Both sediments and algae exceeded the productivity of corals, which was mainly caused by higher absolute cover, but also to some extent by a higher photosynthetic activity. Corals contributed a rather small amount to primary production, especially at W deep. This low coral productivity is supported by a study of Rogers and Salesky ([35]) that identified turf algae as more photosynthetically productive than Acropora palmata. Total gross primary production budgets for each site yielded similar outputs―somewhat higher at shallow―leading to the conclusion that overall gross primary production was relatively independent of the factor side, but the contributions of the key primary producers varied under different environmental conditions.

Although a different incubation method had to be used for corals compared to the sediment and turf algae, all measured oxygen fluxes were consistent with published rates from other reef areas (sediment: [77], [34], [31]; turf algae: [37], [78], [65]; corals: [79], [11], [69]). However, the metabolic characteristics of P. lutea may not be representative for scleractinian corals in general. In an earlier study comparing photosynthetic efficiency and productivity between corals ([80]), Porites (P. cylindrica) ranked among the lowest productive species; this would underestimate coral photosynthesis. P. lutea may also be a relatively robust species in general considering its resilience to bleaching ([81]). In situ huge P. lutea colonies were regularly observed to be covered with mucus sheets to get rid of the sedimentation, especially on the west side (personal observation), and Porites sp. is known to be abundant even in areas with high particle load ([82,83]). Generally, sedimentation of corals can induce a decrease in photosynthesis and respiration rates as well as a decline of several other metabolic features ([84,85]). This may cause some coral species to compete less successfully on the west side, which may lead to an overestimation of the photosynthetic performance there.

Different current regimes on E and W may further lead to changes in photosynthetic performance in situ. According to Roder et al. ([28]) a higher particle load can be found on W and may be due to increased suspension. This could result in elevated nutrient and matter fluxes. Corals for instance, take up more nutrients under higher velocities ([86]) and have a higher photosynthesis ([87]). Additionally, higher currents reduce the boundary layer above the coral tissue (and above any other interface), enabling a more rapid oxygen uptake ([88]), as well as a more easy release of waste products. This could lead to a more efficient metabolism on the W side, counteracting hampering conditions such as reduced light levels or sedimentation. Overall, higher water currents on W may increase photosynthesis of the investigated primary producers in situ as indicated by the studies of Carpenter et al. ([78]) on turf algae, Cook and Røy ([89]) on sedimentary microphytobenthos, and Dennison and Barnes ([90]) on corals.

It should be considered that in situ a plenitude of microhabitats can be found, caused by the 3D structure of the reef itself or by overgrowth (or neighbouring growth) of epiphytic organisms. This effect may lead to a variety of different small-scale light and current regimes (e.g. 91,92,76). Consequently, this may cause deviations from rates (growth of turf algae and oxygen fluxes by the main primary producers) measured during this study, as our investigations could not account for these kind of variations. Incubations were obtained under relatively constant conditions, either on land (incubations in water bath) or in situ (algae tiles on equalling holders and incubations within an in situ respirometer).

Ecological implications

The Similan Islands offer the unique opportunity to investigate coral reef ecosystem functioning and resilience in response to deviating environmental settings located in close vicinity. Our study focused on the impact of internal waves, determined as DTR, and the associated changes in water parameters on coral reef primary production. For this reason the study period was chosen when LAIW are most pronounced along the island chain ([27]). Short-lived benthic cover, e.g. turf algae cover, and metabolic performance, e.g. respiration of sediment, may change over the year according to the prevailing environmental conditions. Hence, our study may not give a general picture on the reefs’ community and metabolism throughout the year, but only of reefs impacted by short term fluctuations introduced into shallow reef areas by LAIW.

Enhanced nutrient levels play an important role in the competition between corals and algae and may lead to a competitive advantage for algae compared to corals ([39,40,41]). Our study showed that even short-term nutrient pulses are detectable in the water column and may impact an ecosystem subjected to these changes. This may affect benthic cover as demonstrated by the enhanced turf algae cover on the west side. Furthermore, an alteration in coral reef benthic community composition―as reported from many reef locations world-wide ([39,12,13,14])―may imply changes in ecosystem-wide primary productivity or productivity in general. Accordingly, a higher growth of turf algae combined with a reduced sedimentary metabolism was detected on the west side. Our study revealed a high plasticity of the overall benthic primary production under different environmental conditions, as the overall primary production budget was similar at all sites, with only the contribution of each main primary producer varying. It should be noted that even though the overall budget remained similar, functional groups (e.g. the sediments on the west side) may show a depressed metabolism and therefore an altered contribution to reef functioning. Further investigations are needed, particularly focusing on changing coral reef ecosystems and their performances including coherences in the trophic net.