Coral bleaching events due to elevated temperatures are increasing in both frequency and magnitude worldwide. Mass bleaching was recorded at five sites in the northern Persian Gulf during August and September 2012. Based on available seawater temperature data from field, satellite and previous studies, we suggest that the coral bleaching threshold temperature in the northern Persian Gulf is between 33.5 and 34°C, which is about 1.5 to 2.5°C lower than that in the southern part. To assess the bleaching effects, coral genera counted during 60-minute dives were categorized into four groups including healthy, slightly bleached (<50% bleached tissue), mostly bleached (>50% bleached tissue) and fully bleached colonies. The anomalously high sea surface temperature resulted in massive coral bleaching (~84% coral colonies affected).
Los eventos de blanqueamiento de corales están aumentando en frecuencia y magnitud debido al aumento de la temperatura en los océanos. En este trabajo se describe un evento de blanqueamiento masivo observado en seis localidades del norte del Golfo Pérsico entre Agosto y Septiembre de 2012. Los datos de temperatura disponibles, medidas in situ y observaciones de satélites de la temperatura de la superficie del mar, sugieren que la temperatura umbral que desencadena el blanqueo de coral en el norte del Golfo Pérsico corresponde a una temperatura de entre 33,5-34°C que es una temperatura entre 1.5 a 2.5°C inferior a la observada en las zonas más meridionales del Golfo Persico (35-36°C). Para la evaluación de los efectos de blanqueo sobre las comunidades de coral, se realizaron transectos de 60 minutos entre 0 y 6 m de profundidad. Durante las inmersiones cada colonia de coral se identificó a nivel de género y su estado se asignó a una de las siguientes categorías: no afectada, ligeramente afectada (<50% del tejido blanqueado), severamente afectada (>50% del tejido blanqueado) y colonias totalmente afectadas (100% del tejido blanqueado). El análisis de los datos de temperatura permitió identificar una anomalía térmica durante el periodo de blanqueamiento que afectó a un 84% de las colonias censadas. Contrariamente a otros eventos de blanqueamiento analizados, las colonias de
Coral bleaching, loss of symbiotic algae (zooxanthellae) and/or their photosynthetic pigments by zooxanthellate reef-builders is considered a major threat for coral reefs worldwide (
Coral bleaching events have increasingly affected coral reefs on a global scale in recent years (
The most destructive global coral bleaching events reported have occurred worldwide in the last three decades (
The Persian Gulf, a semi-closed sea (
This paper studies the spatial variation in bleaching susceptibility of corals from some Iranian islands in the northern and central Persian Gulf during a coral bleaching event observed in 2012. To this end, we examined the responses of coral communities and reefs based on the bleaching status of individual colonies (i.e. fully bleached, mostly bleached, partially bleached or healthy). We also compared genus-specific responses. Moreover, from the analysis of temperature data sets available, we suggest 33.5 to 34°C as the coral bleaching threshold temperature in the northern Persian Gulf, which is about 1.5 to 2.5°C lower than the threshold suggested for the southern part of the Gulf (35 to 36°C). This work, which is the first quantitative assessment of a bleaching event in the northern Persian Gulf and Bu-Musa Island, provides valuable data which can be used as a baseline for studies on ecological and physiological changes in the coral communities and their responses to future bleaching events in this region.
Coral reefs at five sites around three Iranian islands of the northern Persian Gulf, Larak, Hormuz and Qeshm Islands, were assessed for effects of the high temperature anomalies recorded in late August and early September 2012. Bu-Musa Island, located in the central Persian Gulf, was investigated on October 6 and 7 (
Two sites were chosen: L-NE in the northeast (26°53’N, 56°23’E) and L-SW in the southwest (26°49’N, 56°18’E) of the island’s main reefs. The site L-NE had 77.35% coral cover with
Two sites were sampled: H-RS to the south (27°01’N, 56°27’E) and H-E (27°03’N, 56°30’E) to the east of Hormuz Island. The depth range of corals at both sites is the same, less than 5 m with no clear slope. Therefore, the survey depth was similar to the depth range of the corals. These two sites display different coral cover and are subject to differential environmental conditions. Site H-RS is next to an iron mine, so its benthic communities are continuously exposed to fine organic sediment deposition. Benthic communities at this site are usually surrounded and/or covered by red iron particles, but are still viable. Horizontal visibility in some parts of this site ranges from less than 30 to 220 cm. Dominant benthic communities are zoanthids, with 44±29% (mean±SD) cover, and isolated small coral colonies, with 4±6% (mean±SD) (Kavousi et al. unpublished data).
Site H-E has stronger tidal currents than site H-RS. H-E comprises the main coral reefs and communities of Hormuz Island and has 60±16% (mean±SD) and 8.68±7.0% (mean±SD) of the substratum covered by zoanthids and scleractinian corals, respectively (
The sedimentation rate was 0.05±0.01 g cm–2 day–1 at H-E (
At Qeshm Island the site Shib-Deraz (26°41’N, 55°56’E; site Q-SD) harbouring one of its main coral community sites was studied. This site has 14.03±4.8% live coral cover but no true coral reefs are developed; instead, individual colonies of a few species are found (
Bu-Musa Island (25°52’N, 55°02’E; site BM) is located in the central Persian Gulf. Diving and scientific activities are controlled around this island. Data presented here are integrated from two sites (55°1.3’47”E, 25°53’22N and 55°2’40.32”E, 25°53’47.17”N) and were collected by snorkeling. Coral communities with low diversity and density are mainly distributed at 2-8 m depth at both sites. However, only corals at depths of less than 6 m were studied. Soft corals and zoanthids are rare at these sites.
Colonies of reef-building corals were recorded using a timed swim method (
Biweekly sea surface temperature (SST) data from 2009-2012 were derived from the NOAA Satellite and Information Service (
Contingency table and proportions were used to describe the frequency distribution of each bleaching category for each study site. Moreover, to search for potential differences in bleaching impact across sites and between coral genera we compared proportions of colonies affected by bleaching with a Z test. This test was applied because the number of replications (number of coral colonies) was not similar in comparisons. All statistical analyses were done using SPSS version 16.
The multi-year plot of the SSTs in the study area (
2012 | 2011 | 2010 | 2009 | 2008 | |
---|---|---|---|---|---|
July | 33.7±0.5 | 31.9±1.3 | 32.1±1.1 | 32.1±0.7 | 31.3±1.2 |
August | 33.8±0.4 | 32.4±0.8 | 33.6±0.7 | 32.6±0.8 | 32.6±1.0 |
September | 32.5±0.5 | 32.5±0.5 | 33.1±0.6 | 32.5±0.9 | 31.9±1.2 |
Scleractinian corals were the dominant benthic organisms at all sites except for sites H-RS and H-S, which were covered by zoanthids. On the other hand, soft coral Sinularia spp. had 31±7% coverage of the substratum in some parts (not all) of site L-NE (based on three 50-m line intercept transects), whereas it was rare or absent at other sites. There was no sign of bleaching on the zoanthids and soft corals from Hormuz and Larak Islands. Only four fully bleached colonies of soft corals were observed at Bu-Musa Island.
Although coral diversity at the study sites was low, differences in coral communities and their distribution at the study sites were clear (
Genus | H-RS | H-E | L-NE | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
PS | F | M | S | H | PS | F | M | S | H | PS | F | M | S | H | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | 9.4 | 0 | 2.4 | 66.6 | 31 | |
2.5 | 0 | 0 | 100 | 0 | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | 0.4 | 0 | 0 | 100 | 0 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
24.1 | 100 | 0 | 0 | 0 | 1.9 | 100 | 0 | 0 | 0 | 7.2 | 9.4 | 6.3 | 68.7 | 15.6 | |
63 | 81.4 | 0 | 13.7 | 4.9 | - | - | - | - | - | 1.1 | 0 | 20 | 60 | 20 | |
- | - | - | - | - | - | - | - | - | - | 7.4 | 3 | 0 | 97 | 0 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
1.2 | 0 | 0 | 0 | 100 | - | - | - | - | - | 13.4 | 13.3 | 26.7 | 50 | 10 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | 6.5 | 100 | 0 | 0 | 0 | 6.9 | 0 | 6.5 | 71 | 22.5 | |
- | - | - | - | - | - | - | - | - | - | 6.3 | 25 | 53.6 | 10.7 | 10.7 | |
9.2 | 46.7 | 53.3 | 0 | 0 | 85.1 | 5.1 | 25 | 69.9 | 0 | 33.3 | 7.4 | 10.7 | 73.8 | 8.1 | |
- | - | - | - | - | - | - | - | - | - | 0.4 | 0 | 0 | 50 | 50 | |
- | - | - | - | - | 6.5 | 71.4 | 28.6 | 0 | 0 | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | 14.2 | 0 | 0 | 28.6 | 71.4 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
L-SW | Q-SD | BM | |||||||||||||
PS | F | M | S | H | PS | F | M | S | H | PS | F | M | S | H | |
0.7 | 50 | 0 | 0 | 50 | - | - | - | - | - | - | - | - | - | - | |
28.5 | 9.1 | 19.4 | 41.6 | 29.9 | - | - | - | - | - | 37.1 | 0 | 50 | 50 | 0 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
3.7 | 0 | 20 | 30 | 50 | - | - | - | - | - | - | - | - | - | - | |
2.2 | 16.7 | 33.3 | 50 | 0 | - | - | - | - | - | - | - | - | - | - | |
16.3 | 22.7 | 22.7 | 31.8 | 22.8 | - | - | - | - | - | 7.6 | 25 | 0 | 0 | 75 | |
7 | 26.4 | 21.2 | 26.4 | 26 | 3.2 | 0 | 100 | 0 | 0 | 6.7 | 14.3 | 0 | 0 | 85.7 | |
5.6 | 53.3 | 13.3 | 33.4 | 0 | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | 1.8 | 0 | 0 | 0 | 100 | - | - | - | - | - | |
0.4 | 0 | 100 | 0 | 0 | - | - | - | - | - | - | - | - | - | - | |
0.7 | 0 | 50 | 0 | 50 | 0.9 | 0 | 0 | 0 | 100 | - | - | - | - | - | |
21.9 | 3.4 | 69.5 | 23.7 | 3.4 | 49.4 | 44.9 | 11.2 | 43.9 | 0 | 15.2 | 0 | 0 | 25 | 75 | |
1.1 | 100 | 0 | 0 | 0 | - | - | - | - | - | - | - | - | - | - | |
10.4 | 14.3 | 17.9 | 42.8 | 25 | - | - | - | - | - | 33.4 | 0 | 5.7 | 0 | 94.3 | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
- | - | - | - | - | - | - | - | - | - | - | - | - | - | - | |
1.1 | 0 | 0 | 0 | 100 | - | - | - | - | - | - | - | - | - | - | |
0.4 | 0 | 0 | 100 | 0 | - | - | - | - | - | - | - | - | - | - | |
Others* | - | - | - | - | - | 44.7 | 15.5 | 23.7 | 30.9 | 29.9 | - | - | - | - | - |
Overall data collected from the five sites of the northern Persian Gulf islands (Larak, Hormuz and Qeshm Islands,
Unlike the predominant genera, which were more affected by the bleaching, less common genera recorded in the area, including
The 2012 coral bleaching in the northern Persian Gulf, including Larak, Hormuz and Qeshm Islands, had a great incidence, affecting about 84% of the coral colonies surveyed. Monthly average SSTs were warmer in 2012 than in previous years (
Although
The 2012 bleaching event described in this study was concomitant with temperature highs of 34°C on many days over an approximate one-and-a-half-month period (
In contrast with 2012, monthly average SSTs at our study area in August and September 2010 were 1 and 0.6°C higher than the highest temperatures in the same months of other years (
A comparison among monthly average temperatures (
The 2012 coral bleaching event bleached more than 84% of coral colonies observed in this study in the northern Persian Gulf, but only 12% of corals recorded from Bu-Musa Island located in the central gulf (
In general, previous bleaching reports have indicated that massive corals tend to have higher survival from thermal stress, especially
Turbid waters have been noted to reduce coral mortality in severe bleaching events because solar irradiance acts to amplify thermal stress (
The highest incidence of bleaching colonies in this study was found in more turbid waters (about 3 m deep). The most turbid waters were around H-RS, where 80% of corals were fully bleached. Turbid waters can provide protection from light stress during high-temperature bleaching events (
The synergic effects of other factors with temperature conditions are also illustrated by the onset of diseases during and after bleaching events. For instance,
Dominant coral genera at a majority of our study sites were among those most affected in the 2012 coral bleaching event. The same results were reported by
We thank Dr. T. Goreau, Dr. T. Nakamura, Dr. J. Reimer and Dr. J. Burt for improving the manuscript with their useful comments. We are also grateful for the helpful comments of the anonymous reviewers. Thanks to Professor G. MacLean for final English editing.