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In recent decades, extensive coral mortality throughout the Persian/Arabian Gulf (PAG) from thermal stress events has led to increasing reef degradation and the loss of biodiversity across the region. To quantify these dynamics, water temperatures and benthic cover were monitored on ten reefs spanning about 350 km in the Southeastern PAG (Abu Dhabi) over a 10-year period. Water temperatures measured on the reefs fell into cooler (2010–2015) and warmer (2015–2020) periods. 2010–2015 had lower mean winter minimum (19 °C vs. 19.56 °C) and lower mean summer maximum (35.3 °C vs. 35.8 °C, coral bleaching threshold is 35.7 °C). Over the decade, mass coral bleaching occurred in seven years with four bleaching years after 2015. Coral cover decreased by 78% while turf and coralline algae strongly increased (66% and 154%, respectively), and fleshy macroalgae also collapsed in cover (− 83%). Cyanobacteria increased by 980% from 2017–20 with concurrent coral bleaching but without becoming spatially dominant. Despite the decline in coral cover, no shift into a macroalgae-dominated system occurred.
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Vol.:(0123456789)
Coral Reefs
https://doi.org/10.1007/s00338-024-02562-0
REPORT
A decade ofbenthic changes oncoral reefs intheSouthern
Persian/Arabian Gulf (2010–2020)
BernhardRieglJr1· AndrewBauman2·
JeneenHadj‑Hamou3· JohnA.Burt3
Received: 23 June 2024 / Accepted: 9 September 2024
© The Author(s), under exclusive licence to International Coral Reef Society (ICRS) 2024
Abstract In recent decades, extensive coral mortality
throughout the Persian/Arabian Gulf (PAG) from thermal
stress events has led to increasing reef degradation and
the loss of biodiversity across the region. To quantify
these dynamics, water temperatures and benthic cover
were monitored on ten reefs spanning about 350km in
the Southeastern PAG (Abu Dhabi) over a 10-year period.
Water temperatures measured on the reefs fell into cooler
(2010–2015) and warmer (2015–2020) periods. 2010–2015
had lower mean winter minimum (19°C vs. 19.56°C) and
lower mean summer maximum (35.3°C vs. 35.8°C, coral
bleaching threshold is 35.7°C). Over the decade, mass
coral bleaching occurred in seven years with four bleaching
years after 2015. Coral cover decreased by 78% while turf
and coralline algae strongly increased (66% and 154%,
respectively), and fleshy macroalgae also collapsed in cover
(− 83%). Cyanobacteria increased by 980% from 2017–20
with concurrent coral bleaching but without becoming
spatially dominant. Despite the decline in coral cover, no
shift into a macroalgae-dominated system occurred.
Keywords Persian/Arabian Gulf· Coral benthos·
Monitoring· Bleaching· Mortality· Climate change
Introduction
In many areas, reef building corals are in crisis. Climate-
driven thermal stress causes widespread mass coral
bleaching, threatening coral reefs globally (Gardner etal.
2003; Bruno and Selig 2007; Hughes etal. 2018a). At local
scales, land-use changes and other human pressures are
widespread and just as damaging (Hoegh-Guldberg etal.
2007; Carpenter etal. 2008; Hughes etal. 2018a; Riegl and
Glynn 2020; Jones and Gilliam 2024). However, different
levels of reef degradation are observed globally (Cinner
etal. 2016). High-latitude reefs were proposed as potential
refugia (Glynn 1996; Riegl and Piller 2003; Beger etal.
2014), and it is also believed that they could serve as a
springboard for coral range expansion in a warmer climate
(Precht etal. 2002; Yamano etal. 2011; Baird etal. 2012).
This highlights the importance of high-latitude reefs. While
relatively healthy in Japan, South Africa, Australia and the
Eastern Pacific (Schleyer etal. 2018; Denis etal. 2013; Abdo
etal. 2012; Glynn etal. 2018), other high-latitude reefs, for
example in the Caribbean and Persian/Arabian Gulf (PAG),
are degraded (Weil 2004; Weil and Rogers 2010; Burt etal.
2011, 2019; Riegl etal. 2018). In general, the health of
tropical reefs is better documented than that of high-latitude
reefs (Abrego etal. 2021).
Some high-latitude reefs are clearly in deep trouble. Coral
cover on high-latitude Caribbean reefs (i.e., Florida) was
decimated by bleaching, disease outbreaks and hurricanes
(Jackson etal. 2014; Lirman etal. 2014; Jones etal. 2020;
Jones and Gilliam 2024). At a similar latitude, reefs in
the Persian/Arabian Gulf (PAG) also have significantly
declined in coral health (Burt etal. 2011; 2019). Since 1996,
bleaching events have recurred with increasing frequency
and impacted reefs throughout the region (Sheppard and
Loughland 2002; Burt etal. 2008, 2019; Riegl etal. 2018).
* Bernhard Riegl Jr
riegl58@gmail.com
1 Department ofBiology, Halmos College ofArts
andSciences, Nova Southeastern University, Davie,
FL33328, USA
2 National Coral Reef Institute, Halmos College ofArts
andSciences, Nova Southeastern University, 8000 N Ocean
Drive, DaniaBeach, FL33004, USA
3 Mubadala Arabian Center forClimate andEnvironmental
Sciences (ACCESS), New York University Abu Dhabi,
AbuDhabi, UnitedArabEmirates
Coral Reefs
Coral cover, fish abundance and biomass have declined,
and changes in the algae flora may have occurred (George
and John 2005; Schils and Wilson 2006; Sheppard etal.
2010; Grandcourt 2012). Seasonal temperature variations
and greater daily fluctuations likely modify the growth
conditions for corals, microalgae and macroalgae, affecting
production efficiency (Ras etal. 2013) and community
composition (Bauman etal. 2013; Burt and Bauman 2020).
Shifts from coral dominance to communities
characterized by other benthos, most frequently algae, have
become increasingly common (Hughes etal. 2003; Gardner
etal. 2003; Hoegh-Guldberg etal. 2007; Wilkinson and
Souter 2008). In the Gulf, severe, recurrent bleaching events
have disadvantaged corals (Sheppard and Loughland 2002;
Bento etal. 2016; Riegl etal. 2018; Burt and Bauman 2020)
but the magnitude of coral decline and trajectories in other
benthic communities remains to be clearly documented.
Since temperatures in PAG are comparable to what can be
expected across the tropics (Riegl and Purkis 2012; Paparella
and Burt 2024), it is important to have a more holistic
understanding of the reaction of entire reef communities to
closely spaced heat events.
In this study, a decade-long time series of benthic
community data recording cover by corals, macro-,
turf- and coralline algae (CCA), sessile invertebrates in
ten monitoring sites within the southern Gulf (United
Arab Emirates) is presented. Using image analysis of
photoquadrats, trajectories were examined to describe:
(i) variability of benthic community composition, (ii) the
stability of reef-builders (corals and CCA) relative to algal
turfs and macroalgae through time and (iii) how these
functional groups changed with thermal stress. Relationships
between 1) temperature, and 2) benthic community cover
and composition from 2010–2020 are presented.
Methods
Monitoring sites
Ten sites were monitored across the United Arab Emirates
in the southern PAG for 10years (2010–2011, 2013–2020).
In these sites (Mukasab, Ras Ghanada, Delma, Al Dhabiya,
Saadiyat, Al Hiel, Bu Tinah, Al Saada, Al Yasat, Hawksbill,
Fig.1), temperature data and phototransects for benthic
community analysis were obtained. Not all sites were
consistently revisited every year (in 2018, only 3 sites were
visited).
Temperature data collection
Hourly seawater-bottom temperature was recorded at each
site using HOBO v2 temperature loggers, attached directly to
the reef. All loggers were set to record at one-hour intervals
until replaced. However, over the ten years, temperature
data were not recorded consistently at each location causing
data gaps at some sites. Ras Ghanada had continuous
data collection from January 2011 to December 2020. Al
Dhabiyah had similar exposure and had continuous data
collection from July 2010 to 2020. The other sites provided
additional data of variable lengths but no continuous record.
Therefore, the mean temperature across data from all
available sites at any period was used to obtain a complete
average hourly temperature time series that was relevant to
the region and to explain trends of benthic cover. While this
regional mean data resulted in a loss of granularity at local
scales, gradual and region-wide temperature trends were still
accurately represented.
Benthic community surveys
Coral reef benthic communities were surveyed at the above-
mentioned ten reefs. At each site, surveys were conducted
along six 30m line transects at 6–8m depth. Along each
transect, benthic communities were surveyed within eleven
0.25 m2 quadrats photographed at 3m placement intervals
(Bauman etal. 2013). Composition and percent cover of
benthic communities were quantified within each quadrat
using 50 randomly distributed points using CoralNet image
analysis software (Williams etal. 2019). Benthic cover
was classified into five major categories: (i) hard corals,
(ii) algal turfs (< 10mm in height), (iii) fleshy macroalgae
(> 10mm in height), (iv) coralline crustose algae (CCA),
(v) cyanobacteria, (vi) bivalves and (vii) other sessile
invertebrates. Reefs were surveyed three times per year
Fig. 1 Monitoring sites in the SE Persian/Arabian Gulf in Abu
Dhabi, United Arab Emirates. Sample sites are indicated in red; other
known reef sites are shown by empty circles (Riegl and Purkis 2012)
Coral Reefs
during winter, summer and fall, during the monitoring
period.
Data analysis
All data were analyzed using R version 4.2.2 (R Core Team
2022), variable in its base version and in the tidyverse
(Wickham and Grolemund 2017; Wickham etal. 2019)
utilizing a variety of libraries like stringr (Wickham 2023),
tidyverse (Wickham etal. 2019), ggpubr (Kassambara 2023)
and vegan (Oksanen etal. 2022).
Average hourly mean temperatures were calculated over
the entire monitoring period, from whichever site data were
available. From this dataset, absolute and annual maximum
and minimum temperatures were calculated. Since coral
bleaching thresholds in PAG are published (Riegl et al.
2011; Burt etal. 2019), bleaching periods could be analyzed.
The number of hours and days above stress and lethal
temperature levels (Riegl etal. 2011; Riegl and Purkis 2012)
were calculated.
Point count-cover data were summarized to show intra-
and inter-annual patterns. Annual data were plotted against
sampling years by generalized linear models and a trendline
(GLMs, Crawley 2014). Temperature data were evaluated
for annual and season-specific variability (averages, maxima
and minima) also for timing and frequency of bleaching
levels. When consistent differences among monitoring years
were detected, they were stratified into a warmer and cooler
period. This monitoring information provided an accurate
but static view of benthic cover changes and one of their
most important potential environmental drivers (heat) in
retrospect.
To visualize how the community had changed with time,
non-metric multidimensional scaling based on a Bray–Curtis
dissimilarity matrix (Borcard etal. 2011) plotted sample
years grouped by benthic categories as well as vice versa
within the same ordination diagram. Arrows between
the MDS axis 1 and axis 2 scores were used to illustrate
trajectory.
Results
Temperature data
Sea-bottom temperatures ranged from 17.7°C (2017) to
36.4°C (2018), giving an absolute range of 18.8°C across
the decade (Fig.2). Temperatures were strongly seasonal,
with an average annual range of 16.2°C. Two temperature
periods, 2010 (spring)—2015 and 2015 (summer)—2020,
could be separated (Fig.3). The period 2010–15 differed
from that of 2015–2020 by lower mean winter minimum
(19.0 °C vs. 19.6 °C), lower mean summer maximum
(35.3°C vs. 35.8°C) and lower annual average (28.2°C vs.
28.3°C). While not significant (t = − 1.5854, p = 0.1526 for
maxima, t = − 0.7785, p = 0.4623 for minima, t = − 0.1144,
p = 0.9142 for average), a half degree °C difference in
mean maximum temperatures can mean life or death for
the thermo-sensitive PAG benthos, especially corals, which
exists near the uppermost physiological limit of adaptability.
Corals are the most temperature sensitive of the measured
benthic community. Their PAG bleaching and mortality
threshold (35.7°C; Riegl etal. 2011) were reached in 2012
and exceeded in 2017 and 2018 (Table1), suggesting mass
coral bleaching and mortality in those years. The years
2015 and 2020 came very close to the mortality threshold.
This identifies 4years of significant coral mortality, and a
generally heat-stressed period beginning in 2015 lasting to
2020.
Variability inthebenthic community
Pair plots of data summarized by sampling years and sites
(Fig.4) suggested that clear trends existed across time and
across the sampling region. Each is further analyzed below.
Statistically significant negative correlations (calculated
as sum of squared differences) in trends among benthic
categories were observed between coral cover and
calcareous algae (R2
= − 0.409, p < 0.001), fleshy macroalgae
(R2 = − 0.281, p < 0.01), turf algae (R2
= − 0.694, p < 0.01)
and bivalves (R2 = − 0.266, p < 0.05). Further positive
Fig. 2 Mean bottom temperatures on 10 reef sites across reef sites in
the southern Persian/Arabian Gulf. Dotted red line is set at 35.7°C,
the coral mortality threshold temperature in the Persian/Arabian
Gulf. Red bars represent the approximate duration of bleaching days
in years where bleaching threshold was exceeded and mass coral
mortality encountered. Bleaching continues after the temperature
threshold is reached; therefore, the red bars extend for a variable
period after peak stress. The period is longer after more severe
heat stress. Red dot (2010, 2011) represents years where bleaching
thresholds were reached (red dotted line) but mortality was low
Coral Reefs
significant correlations existed between the cover of
calcareous algae and bivalves (R2
= 0.392, p < 0.001).
Variability acrossspace (sampling sites)
No clear pattern with regard to an East–West gradient in
mean cover data grouped by sampling site across all years
was evident (Fig.5). Data showed scatter in cover values
among the sites and years (Fig.5), as is also evidenced in
Fig.5 by the large standard deviations around the means.
The absence of pronounced patterns (W–E) suggested that
region-wide trends could be considered by pooling sites
every year.
Variability acrosstime—seasonal patterns
Algae are known to exhibit annual variability in PAG
(John 2012). Any variability in the coralline algae, coral
and bivalve data were ignored since they could only have
been based on sample placement. The slower life cycles of
Fig. 3 Annual summary statistics of sea temperatures across all
sampling sites in the southern PAG. 2010 is not shown since data
series begins in July. Cooler and hotter years (that cause biotic stress)
are readily identified by their mean (green dots) and maximum
temperatures (red dots). It is readily seen that the period from 2015 to
2020 is characterized by generally higher means and maxima
Table 1 Coral bleaching and
mortality thresholds in the Gulf
occur after 22days at 35°C,
1day at 35.7°C (Riegl etal.
2011, 2012)
**Coral mortality threshold exceeded, *coral mortality threshold reached. 2010 monitoring began in July
at the onset of the hottest period (July)
Calendar year Days ≤ 35.0°C Days ≥ 35.7°C Maximum temp Total
annual temp
variability
2010 6.9 0 35.4 14.19
2011 7.5 0 35.6 16.69
2012* 21.1 0.83 36.0 18.38
2013 0 0 34.9 16.30
2014 0 0 34.7 15.96
2015 13.08 0.75 35.9 15.59
2016 2.45 0 35.2 15.04
2017** 25.66 2.2 36.0 17.29
2018** 28.75 3.8 36.4 16.69
2019 10.50 0 35.5 14.96
2020 11.91 0.25 35.8 17.23
Coral Reefs
these organisms do not allow intra-annual cycles. Seasonal
data were pooled over all sites. Fleshy macroalgae showed
a peak in the spring season (April, May) that rapidly abated
in summer (Fig.6). Turf algae and cyanobacteria showed
a peak in late summer/autumn (September/October).
Fleshy algae peaked at 4.8% space cover and cyanobacteria
at 0.75% while the lowest values of turf algae in winter
were 27% with a peak of 60.6% in autumn. Thus, despite
wide variances, mean cover across all months of the algae
categories clearly differed (fleshy macroalgae 0.9 ± 1.6%;
turf algae 40.8 ± 11.9%; cyanobacteria 0.2 ± 0.3% mean
cover per month). Also, sessile benthic invertebrates (other
than corals and bivalves) increased in summer over the
warmer period of the year to a cover of 1.25% (mean cover
per month 0.6 ± 0.4%; Fig.6).
Benthic community variability throughtime
Variation in the composition of benthic communities on
an annual basis throughout the decade showed a pattern
of shifts in the dominant benthic taxa. Coral cover showed
a decline in 2013 in response to the bleaching year 2012,
followed by regeneration until summer 2015. After summer
2015, there was no more regeneration, until coral cover had
declined by 78% at the end of the decade. Turf algae in 2010
covered roughly equal space as corals and showed a similar
weakly declining trend until 2015. In stark contrast to corals,
they began increasing strongly over the warmer period
between 2015 and 2020, occupying much more space by the
end of the monitoring period (Table1, Fig.7C). This mirrors
intra-annual trends where turf algae have a peak during the
warmest period of the year. Macroalgae increased until 2017
and were apparently not disadvantaged by the hot summer of
2015, but then abruptly declined as of 2017 (Fig.7D) to end
up with dramatically lower cover in 2020 than in 2010 (83%
less, Table1), declining to roughly 1% of cover on the reefs
(Fig.7B). In 2017, temperatures exceeded the acceptable
threshold for macroalgae (Fig.3). Also, this mirrors the
intra-annual trends where macroalgae decline after a spring
bloom over the hotter summer period. Cyanobacteria
covered essentially 0% of the reef in 2010. While individual,
small patches of cyanobacteria were indeed encountered
on surveys, these were so small that they were missed by
the photo quadrats. From virtually zero cover (0.001%)
throughout the cooler 2010–2015 period, they expanded by
980% through the warmer period to cover about 1% (0.98%)
of the reef in 2020 (Table2, Fig.7D). While their overall
cover on reefs was not significant, their relative increases
were. Cyanobacteria also favored the summer in their intra-
annual variability, which suggests that an overall warmer
temperature regime did not harm them.
Fig. 4 Pair plot showing correlations among benthic categories.
Data are mean values in each of the 10 sampled location in each of
the 10 sampled year. The first two rows show histograms of values
across years (first row) and across sites (second row). The diagonal
shows a density plot of values in each variable. The lower triangular
section consists of pairwise scatterplots of data (row vs. column),
and the upper triangular section shows correlation coefficients.
Stars denote statistical significance level (* = 0.5, *** = 0.001),
Cy = Cyanobacteria etc.
Coral Reefs
Starting the decade with roughly 4% (3.71%) cover of
reef space, coralline algae experienced a series of increases
and decreases. They reached about 10% (9.44%) reef cover
in both 2013 and 2020. The sudden increase from ~ 4 to
~ 10% in 2013 followed a mass coral bleaching and mortality
event in that year. Some of the gained space seems to have
been subsequently lost to competition with other algae and
invertebrates. From 2015, coralline algae show consistent
increase throughout the warm period, in step with increased
space availability due to coral mortality.
The pattern of variable declines and increases in benthic
taxa influenced community structure (Fig.8). The ordination
plot of overall community structure in each sampling year
(arrangement of years based on dissimilarities in benthic
categories) showed a pattern of degradation, regeneration
and renewed further degradation. The underlying
community shift can be read from the ordination plot of the
dominant species (arrangement of benthic categories based
on dissimilarities between years). The community begins
in a setting with high coral cover. The shift away after 2012
to more coralline and turf algae dominance followed mild
bleaching in 2010 and 2011 and severe bleaching and coral
mortality in 2012 (Figs.2, 3). A regeneration trajectory
toward higher coral cover (see also Fig.7) was observed
until 2015, when another strong bleaching and mortality
event was observed (Figs.2, 3). The community moved
toward higher macroalgal cover and further away from the
original coral-dominated state (Figs.7A, D, 8). Two strong
heat events in 2017 and 2019 removed the community from
the macroalgal stage (collapse of macroalgae cover, Fig.7D)
and were characterized by bleached corals. The ongoing
coral mortality reduced coral cover (Fig.7A) and moved into
a community state characterized by the increase of bivalves
and cyanobacteria (Figs.7E, G, 8).
Discussion
There has been argument over what state will succeed that
dominated by corals on degraded reefs, and whether any
Fig. 5 The overall mean coverage values of benthic categories, pooled over all sampling years, did not show patterns along a West–East gradient
Coral Reefs
stable or unstable alternate states exist. Of particular interest
is what type of algae may dominate after coral loss (Fong
etal. 2006, 2017; Bruno etal. 2009; Johns etal. 2018; Precht
etal. 2020). The reefs of southeastern PAG serve as yet
another example, in a hitherto not so well-documented area,
demonstrating the fate of a continuously degrading coral reef
benthic community.
Over the monitored decade (2010–2020), thermal stress
events in southeastern PAG have increased in frequency
and magnitude with a concurrent shift away from coral
dominance. A warmer period (2015–2020) had 0.5 °C
higher mean, minima and maxima than the preceding years
(2010–2015). Mass coral bleaching was frequently recorded
(2010, 2011, 2012, 2017, 2019 and 2020), making it the
highest documented bleaching recurrence rate on any reef
system and identifying temperature-related bleaching and
mortality as the single most important environmental driver
of community structure on the sampled reefs. Absence of
significant regeneration caused a decline in Abu Dhabi coral
cover from 38.7% in 2010 to 8.5% in 2020. As a result, corals
are no longer the dominant benthic taxa on these reefs.
On PAG reefs, also macroalgae decreased in cover during
the study period (− 83%, Table2), unlike observed in studies
from elsewhere (tropical Eastern Pacific: Fong etal. 2006,
2017; Great Barrier Reef: Johns etal. 2018; Red Sea: Anton
etal. (2020); Caribbean: Hughes 1994; Jones etal. 2020).
Seasonal dynamics within PAG benthos align with observed
annual variability of macroalgal dynamics (John 2012), and
the present study showed a peak of macroalgae cover in
spring and early summer. However, fleshy macroalgae never
dominated the southeastern PAG reefs from 2010–2020 but
only briefly benefitted from coral death and exhibited a short
spike in cover (1.18%), then collapsing in 2017, likely due
to heat sensitivity (George and John 2005). They finally
occupied even less space by 2020 than in 2010 (0.05%
from 0.27%), and no macroalgae-dominated state was ever
observed on these reefs. This supports observations of
Bruno etal. (2009) and Precht etal. (2020) from the GBR
and the Caribbean that in the majority of reefs, shifts into
macroalgae dominance hardly occur.
At the same time, coralline algae (like Litophyton
kothschyanum), turf algae and cyanobacteria benefited from
the loss of corals as the primary space competitor, with each
gaining significant cover (+ 154%, + 66%, + 980%, Table2).
This is reminiscent of the situation in the Caribbean, where
turf algae became the most abundant benthic group in
Curacao (Vermeij etal. 2010; Fricke and Teichberg 2011).
Coralline and turf algae showed resilience to increased
temperatures and increased their coverage on the PAG
reefs, with turf algae dominating space cover (35.1–58.3%)
and coralline algae cover more than doubling in cover
(3.71–9.44%). While initially present as small tufts,
Fig. 6 Intra-annual variability of mean monthly space cover of algae, cyanobacteria and sessile invertebrates averaged across all sites between
2010 and 2020
Coral Reefs
cyanobacteria only increased during the multiyear heat
events from 2017 onwards and, although they did not cover
much space on the reefs (~ 1%), showed the most rapid
increase of any category across the entire dataset. They are
a typical crisis biota and reflect the increasing stress levels
in PAG reef ecosystems. Heat events enhance proliferation
(Ford etal. 2018; Beltram etal. 2019), as is the effect of
cyanotoxins (Ritson-Williams etal. 2016), and increased
cyanobacterial cover may be expected to cause community
shifts by reduction of coral larval survival and declines in
Fig. 7 Percent cover (mean and standard deviation) in benthic
categories pooled annually over all sites during the decade 2010–
2020. Data pooled across all sites in the southern Persian/Arabian
Gulf and dynamics in the cooler and warmer periods identified by
temperature measurements shown by trendlines obtained by glm
(generalized liner modeling). Y-axes differ in scale. Dotted lines
represent the separation of time series (2010–15 and 2015–20)
Table 2 Average cover by
benthic cover categories on
ten Abu Dhabi reefs in the SE
Persian/Arabian Gulf over the
entire investigation period
Columns show cover at the beginning of the studyin 2010 in the cooler period, in 2015 at the threshold to
the warmer period and in 2020 at the end of the warmer period
Category % cover in 2010 % cover in 2015 % cover in 2020 Total % gain/loss
Coral 38.7 37.04 8.5 − 78
Coralline Algae 3.71 4.3 9.44 + 154
Fleshy Algae 0.266 1.18 0.045 − 83
Turf Algae 35.1 33.5 58.3 + 66
Cyanobacteria 0.001 0.001 0.98 + 980
Coral Reefs
fleshy and coralline algae (Ford etal. 2018; Beltram etal.
2019; Vizon etal. 2024).
Thermal stress events are increasing in frequency and
magnitude and have driven; over only a decade, dramatic
changes in the partitioning of space cover in southern PAG
coral reef benthos (coral, algae, cyanobacteria). Coral
cover declined by ~ 75%, while turf algae cover increased
by roughly the equivalent amount. Also, macroalgae,
which initially benefited from coral decline, collapsed in
cover during the hot summer of 2017. These trajectories
suggest significant changes in ecological dynamics, as
visualized by the trajectory in multidimensional scaling
(nMDS). Starting the decade in what could be considered a
relatively “healthy” coral-dominated system (2010–2012),
the trajectory headed toward states dominated by different
algal categories (coralline, macro- and turf algae). A short
return or regeneration trajectory period toward the healthy
coral state was observed in a period spared of bleaching for
several years (2014–2015) but ultimately it turned further to
a system dominated by algae, bivalves and cyanobacteria.
Conclusion
This study suggests notable changes in the reef
environment of the southern Persian Gulf over the
decade 2010–20. In 2012 and 2015 temperatures reached
coral bleaching thresholds (35.7°C). 2017, 2018, 2020
exceeded bleaching thresholds and caused mass coral
mortality. The changes in benthic composition, especially
in coral cover, can be attributed to temperatures extremes
with increasingly high recurrence. Corals in 2010 were
roughly at ~ 40% and in 2020 ~ 8% space cover. Also, the
cover of fleshy macroalgae collapsed after the very hot
2017. Concurrently, algae categories (turf-, macro- and
calcareous-) and cyanobacteria increased in cover and
began to dominate most of the reef substrate.
Author contributions BRJr curated and analyzed data and wrote the
paper; JAB oversaw and funded data collection, manuscript preparation
and corrected manuscript; AB and JHH participated in data collections,
manuscript preparation and corrected manuscript.
Fig. 8 Ordination by non-metric multidimensional scaling (nMDS)
of a Bray–Curtis dissimilarity matrix of the benthic cover data.
Ordination of years based on composition of the benthic categories is
connected by arrows to show the community’s trajectory. Also shown
is the ordination of benthic categories based on years
Coral Reefs
Funding JAB appreciates funding by NYUAD, ACCESS and WRC.
Data availability Data available upon request.
Declarations
Conflict of interest The authors declare that they have no conflict
of interest.
Ethical approval No approval of research ethics committees was
required to accomplish the goals of this study because it is a purely
observational study.
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Chapter
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Article
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Chapter
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Chapter
An unequivocal link exists between human population density and environmental degradation, both in the near field (local impacts) and far field (impacts due to teleconnections). Human population is most widely predicted to reach 9–11 billion by 2100, when the demographic transition is expected in all but a handful of countries. Strongest population growth is in the tropics, where coral reefs face dense human population and concomitant heavy usage. In most countries, > 50% will be urbanized but growth of rural population and need for food in urban centres will not alleviate pressure on reef resources. Aquaculture will alleviate some fishing pressure, but still utilizes reef surface and is also destructive. Denser coastal populations and greater wealth will lead to reef degradation by coastal construction. Denser populations inland will lead to more runoff and siltation. Effects of human perturbations can be explored with metapopulation theory since they translate to increases in patch-mortality and decreases in patch-colonization (= regeneration). All such changes will result in a habitat with overall fewer settled patches, so fewer live reefs. If rescue effects are included, bifurcations in system dynamics will allow for many empty patches and, depending on system state relative to stable and unstable equilibria, a part-empty system may either trend towards stability at higher patch occupancy or extinction. Thus, unless the disturbance history is known, it may be difficult to assess the direction of system trajectory—making management difficult. If habitat is decreased by destruction, rescue effects become even more important as extinction-debt, accumulated by efficient competitors with weaker dispersal ability, is realized. Easily visible trends in human population dynamics combined with well-established and tested ecological theory give a clear, intuitive, yet quantifiable guide to the severity of survival challenges faced by coral reefs. Management challenges and required actions can be clearly shown and, contrary to frequent claims, no scientific ambiguity exists with regards to the serious threat posed to coral reefs by humankind's continued numerical increase.