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Seasonal and interannual variabilityof chlorophyll-a in the Gulf of Omancompared to the open Arabian SearegionsSergey Piontkovski a , Adnan Al-Azri a & Khalid Al-Hashmi aa Department of Marine Science and Fisheries, College ofAgricultural and Marine Sciences, Sultan Qaboos University, CAMS,P.O. 34, Al-Khod, 123, Sultanate of OmanPublished online: 15 Aug 2011.

To cite this article: Sergey Piontkovski , Adnan Al-Azri & Khalid Al-Hashmi (2011) Seasonaland interannual variability of chlorophyll-a in the Gulf of Oman compared to the openArabian Sea regions, International Journal of Remote Sensing, 32:22, 7703-7715, DOI:10.1080/01431161.2010.527393

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International Journal of Remote SensingVol. 32, No. 22, 20 November 2011, 7703–7715

Seasonal and interannual variability of chlorophyll-a in the Gulfof Oman compared to the open Arabian Sea regions

SERGEY PIONTKOVSKI*, ADNAN AL-AZRI and KHALID AL-HASHMIDepartment of Marine Science and Fisheries, College of Agricultural and Marine

Sciences, Sultan Qaboos University, CAMS, P.O. 34, Al-Khod 123, Sultanate of Oman

(Received 15 September 2009; in final form 12 February 2010)

Field sampling, remote sensing and modelling were employed to understand theseasonal and interannual changes of chlorophyll-a concentrations in the Gulf ofOman in comparison to open sea regions. In these regions, maximal chlorophyllconcentrations were reported during the summer monsoon (with peaks in Juneand August), while in the Gulf of Oman, the chlorophyll maximum was observedduring the winter monsoon (February–March). From 1997 through to 2008, theinterannual variability in chlorophyll-a concentrations in the Gulf of Oman has notexhibited pronounced trends and neither have the other two (oceanic) regions in thewestern Arabian Sea. However, an increase of the annual variation in chlorophyllconcentrations over the years was noticed. The diatom biomass decreased two-foldfrom 1997 to 2007. Nitrate concentration and mixed-layer depth also declined. Incomparison to the seasonal blooms driven in the Gulf of Oman by the dinoflagel-late Noctiluca scintillans, the year 2008 was markedly different. The summer bloomwas shifted to September; it was gradually extended in time and formed by theother species. An applicability of the concept of ecosystem regime shift is discussed.

1. Introduction

Numerous national and international expeditions (such as the International IndianOcean Expedition (IIOE), the Joint Global Ocean Flux Studies (JGOFS), theNetherlands Indian Ocean Programme (NIOP), the North Arabian Sea Environmentand Ecosystem Research Programme of Pakistan (NASEER) and many others) havebeen carried out in the western Arabian Sea over the past three decades (Burkill et al.1993, Smith 2001, Marra and Barber 2005, Banse and Piontkovski 2006). In the back-ground of these expeditions (mainly in open oceanic waters and southern regions ofthe sea), the hydrology and biology of the Gulf of Oman has been much less inves-tigated. Existing research has implied the gradual differences that this region has incomparison to the physical–chemical dynamics and physical–biological coupling inthe western Arabian Sea (Banse 1997, Al-Azri et al. 2009).

In terms of physical–chemical dynamics, the Findlater Jet in the atmosphere(Findlater 1969), along with the Ras Al Hadd frontal zone in the water mass, both setup a boundary which makes the Gulf of Oman dynamically isolated from the westernArabian Sea (Lee et al. 2000). This isolation has seasonal changes mediated by the

*Corresponding author. Email: spiontkovski@gmail.com

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online © 2011 Taylor & Francis

http://www.tandf.co.uk/journalshttp://dx.doi.org/10.1080/01431161.2010.527393

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reversal of monsoonal winds. In winter, the air over land becomes cooler and denserthan air over ocean, which creates a high atmospheric pressure over the continent anda low pressure over the ocean. The resulting pressure gradient leads to northeasterlywind flow, the northeast monsoon (November–March), directed from continent toocean south of the equator. As the year progresses, increased heating reduces the highpressure over the continent. By summer, a high atmospheric pressure is formed overthe ocean and the wind blow direction is reversed to a southwesterly wind flow, thesouthwest monsoon (May–September), directed from ocean to continent.

The Ras Al Hadd frontal zone (which has a thickness of about 150–400 m) is aprominent feature of the East Arabian Current, an intense offshore jet formed at theeastern tip of the Arabian Peninsula known as Ras Al Hadd (Böhm et al. 1999). To thenorth, the front demarcates the boundary between the high-salinity waters partiallyoriginated in the Persian Gulf and occupying the Gulf of Oman (Kumar and Li 1996)and the northern part of the Arabian Sea. The Ras Al Hadd frontal zone is poorlypronounced or decays entirely during the northeast (winter) monsoon season, whenthe northern part of the Gulf is occupied by a current penetrating the Gulf from thenortheast.

From the west, the hydrological regime of the Gulf is mediated by the high-salinitywaters from the shallow Persian Gulf through the Strait of Hormuz (Banse 1997,Senjyu et al. 1998, Prasad et al. 2001) and propagating towards the open sea, to theeast and southeast. The Persian Gulf Water Mass moves along the Omani coast, whilethe Indian Ocean Surface Water Mass is centred near the Iranian coast. These watermasses are covered by a homogeneous surface layer (Pous et al. 2004).

By using field sampling, remote sensing and modelling data, we have aimed atunderstanding the seasonal and interannual changes in chlorophyll-a in the Gulf ofOman, from 1997 through to 2008.

Due to distinct deflection of the Omani coast to the west (at almost 90o to the Gulfentrance) and impact of the Findlater Jet and the Ras Al Hadd front, the seasonaland interannual changes in the Gulf of Oman versus the open Arabian Sea shouldbe different. Offshore Ekman transport and positive wind stress curl reportedly causepronounced seasonal upwelling along the coast facing the western Arabian Sea (Böhmet al. 1999). In the Gulf of Oman, however, the coastline deviates to the west at about90o. This means that the Findlater Jet should not induce intensive Ekman transport inthis region in summer; therefore, seasonal and interannual changes of phytoplanktoncommunities in the western Arabian Sea and the Gulf of Oman might be different.

In this article, we will investigate the predominant trends, leaving the discussion ofthe deviations from the trends for the next publication.

2. Methods

To compare seasonal and interannual dynamics throughout the regions, data wereaveraged within three, 3o quadrates (figure 1). The first region (57◦–60◦ E, 23◦–26◦ N)incorporated almost all the area of the Gulf of Oman. The second one (62◦–65◦ E, 22◦–25◦ N,) was picked up to characterize the processes taking place in the open sea (north-eastern region). The third one (57◦–60◦ E, 19◦–22◦ N) was picked up to characterizethe processes in the southwestern part of the Arabian Sea (near Massirah Island).

Satellite-derived (9 km spatial resolution Sea-viewing Wide Field-of-view Sensor(SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua)monthly Level-3 data for sea surface temperature (SST) and chlorophyll-a

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Figure 1. Location of the three regions selected for data analysis. Background image: three-dimensional bathymetric map of the region (www.earth.google.com). Dashed and dotted linedemarcates the location of the Ras Al Hadd frontal zone. Dashed lines indicate the direction ofthe main current flows (in summer through to autumn).

concentration were used to assemble time series (1997–2008). The SST andchlorophyll-a products are available from the National Aeronautics and SpaceAdministration (NASA) Ocean Color Group (http://oceancolor.gsfc.nasa.gov).Monthly time series of these parameters were acquired using the Goddard EarthSciences (GES) Data and Information Services Center (DISC) Interactive OnlineVisualization and Analysis Infrastructure software (GIOVANNI) of NASA.

Monthly data on atmospheric pressure (hPa), wind speed (m s−1) and air temper-ature (◦C) were obtained from the nearest meteorological station, about 60 km fromMuscat.

Temperature, salinity and dissolved oxygen were measured using a Conductivity,Temperature, Depth (CTD) Sea-Bird probe.

Monthly time series on concentration of nitrate and mixed-layer depth wereretrieved from the NASA Ocean Biogeochemical Model, which is a coupled three-dimensional model incorporating general circulation, biogeochemical, radiative com-ponents and assimilating monthly global products (Gregg 2008).

The model is driven by wind stress, shortwave radiation and SST. It has 14 verti-cal layers, four groups of phytoplankton, nutrients (nitrate, regenerated ammonium,silica and iron), three pools of detritus and a group of herbivores. The assimilationprocess has an algorithm of constant readjustment of the model parameters, basedon the chlorophyll data acquired by satellite sensors (Nerger and Gregg 2007). It isassumed that oceanic radiation is driven by water scattering, absorption, dissolved

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organic matter and optical properties of the phytoplankton groups. The estimates ofthe sea surface nitrates were based on the algorithms linking the shipboard data withremotely sensed data (Goes et al. 2004).

In addition to remotely sensed data, direct measurements of SST, chlorophyll-aconcentration and phytoplankton sampling were used. Temperature, conductivity anddepth were measured with an Idronaut Ocean Seven 316 CTD probe. Water sampleswere collected from a depth of 1 m from January 2001 through to December 2008at two sites (station F: 58.5◦ E, 23.67◦ N and station BK: 58.72◦ E, 23.51◦ N) at thesouthern end of the Gulf of Oman with a depth range of 10–20 m over the samplingsites. Regular (bimonthly) sampling was carried out in 2001–2002 and 2004–2008.

Samples for chlorophyll-a (250–1000 ml) were filtered through 47 mm WhatmanGF/F filters and the phytopigments were extracted in 10 ml of 90% acetone solutionunder cold and dark conditions for 24 hours. Chlorophyll-a concentrations were esti-mated using a Turner 10 Designs Fluorometer in accordance with the method of Tett(1987).

Sub-samples for phytoplankton abundance were collected in dark glass bottles(50–100 ml) into which five to ten drops of acid Lugol’s solution (Throndsen 1978)were added and stored at 4◦C. The entire sample (50–100 ml) was transferred intograduated cylinders and allowed to settle overnight. The upper layer of seawater inthe measurement cylinder was carefully siphoned off using a tube, one tip of whichwas covered with a 15 µm mesh net to concentrate the sample to 20 ml. The concen-trated sample was then transferred into sedimentation chambers to settle overnightbefore counting using inverted microscopy. Cell density was counted by transferring1 ml replicates of the concentrated sample onto a Sedgewick Rafter counting chamber,and identification was based on standard keys, in most cases to the species level.

3. Results

Basic elements of the physical–biological coupling underlying seasonal patterns aregiven in figure 2, which exemplifies the 2 year fragment of data. In 2004, follow-ing a gradual decline of atmospheric pressure from February through to midsummer(figure 2(a)), winds achieve their maximal stable speed (6 m s−1) in July and markedlyaffect fluctuations of the air temperature and SST.

There was a clear coupling between the atmosphere and ocean as evidenced in thestatistically significant relationship between air temperature and atmospheric pressure(with a correlation coefficient r = −0.73, p < 0.01), as well as between air temperatureand SST (r = 0.84, p < 0.01). Average SST ranged between 24◦C and 32◦C, with amaximal value during summer and a minimum in winter.

During the summers of 2004 and 2005, the most significant decline (∼6◦–10◦C) inSST was in August and September of both years (figure 2(b)). Although this patternof decline was consistent during the two years of sampling, the southwest monsoonof 2004 was much cooler than that in 2005. The other component of the SST declinepertains to seasonal winter cooling and convective mixing, with minimal temperatureobserved in January (figure 2(b)). Seawater salinity showed seasonal changes that wereanalogous to that of SST (r = 0.68, p < 0.05).

On an interannual scale, data on chlorophyll-a sampling in the Gulf of Oman haveshown statistical correlation with the data on remote sensing for the same region(r = 0.54, p > 0.05). We noticed certain seasonal differences in chlorophyll val-ues that were maximal in August and March. As it was averaged for 1998–2008,

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the SeaWiFS chlorophyll-a exceeded the sampled chlorophyll-a by twice as much.Differences between the remotely sensed and sampled chlorophyll concentration basi-cally have a technical origin, which was the cloud mask typical for these (monsoon)periods.

Seasonal cycles of chlorophyll-a in the Gulf of Oman and the southwestern part ofthe Arabian Sea (the Massirah Island region) were quite different (figure 3). For the

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Figure 3. Seasonal cycles of chlorophyll-a (mg m−3; 1998–2008, SeaWiFS data). GOM, theGulf of Oman; OS, the northeastern (open sea) region.

southwestern region, maximal chlorophyll concentration was reported for the summermonsoon (with peaks in June and August), while in the Gulf of Oman, the chlorophyllmaximum was observed during the winter monsoon (in February and March).

The area-averaged chlorophyll-a in the Gulf of Oman (which is to the northwest ofthe Findlater Jet axis) and in the open sea region (to the east of the Jet axis) showedmore concordant seasonal changes, in comparison with the Massirah Island region.

As far as interannual changes are concerned, the chlorophyll concentration in theGulf of Oman has not exhibited pronounced trends, and neither did the other tworegions. However, a tendency of the annual variation coefficient to increase over theyears was noticed for the Gulf (figure 4). The same tendency was noticed for the opensea region, although it was less pronounced there. In addition, interannual changes ofdiatoms (retrieved from the model) showed a declining trend in which the biomass ofthis group decreased two fold from 1997 to 2007.

In order to understand the physical–chemical dynamics underlying the interan-nual increase of chlorophyll variation within the seasonal cycle, a set of key variableswas analysed. Interannual changes of nitrate concentration retrieved from the NASAOcean Biogeochemical Model exhibited a general decline from 1996 through to 2006in the Gulf of Oman and the open sea region (figure 5). Regression curves indi-cated different slopes of decline (over regions), mostly pronounced for the Gulf ofOman, in which the mixed-layer depth decreased from 13.0 ± 2.6 to 11.5 ± 2.4 m.Both parameters (nitrates versus mixed-layer depth) exhibited significant positivecorrelation (figure 6, r = 0.48, p < 0.01).

A sharp difference between regions was in the degree of midsummer cooling, whichis believed to be driven by the deepening and cooling of the surface layer due to wind-driven shear instabilities and entrainments (Weller et al. 2002). At this time of the year,the SST used to be 2◦–3◦C lower. The decline of SST was less pronounced in the Gulfof Oman, while the event picked up its strength towards the open sea region (figure 7).

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GOMOS1996 2000 2004 2008

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Figure 5. Nitrate concentration in the Gulf of Oman (GOM, bold line) and the north-eastern (open sea (OS)) region. Dashed lines = 95% confident intervals. For the GOM,y = 56.29 − 0.03x. For the OS region, y = 19.51 − 0.01x. Both equations fit the 95% confidentintervals.

In terms of deviations from the main (interannual) trend, a reference could bemade to the year 2008, as an example. A huge algae bloom started to develop inthe Gulf of Oman in September 2008, and lasted throughout the entire intermon-soon period (figure 7, table 1). In comparison to typical seasonal blooms driven in theregion by the dinoflagellate Noctiluca scintillans (Al-Azri et al. 2007), the 2008 bloom

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Figure 6. Relationship between nitrate concentration (µM) and mixed-layer depth (m) inthe Gulf of Oman. Dashed lines = 95% confidence intervals. y = −0.17 + 0.03x; r = 0.48,p = 0.0002.

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was unusually widespread and persisted until April 2009. This bloom was dominatedby the dinoflagellate Cochlodinium polykrikoides, which has never been a large-scalebloom contributor in the Gulf according to the 7 years of our sampling (2001–2008).From the Gulf of Oman, the bloom propagated along the coast to the east. Severalweeks later, the bloom was reported much further (about 1000 km) to the south, uptowards Massirah Island.

In addition to the outstanding chlorophyll values featured in the 2008 bloom,figure 8 shows a general pattern that is a marked decline in the intensity of summerblooms (related to the southwest monsoon period), while the winter blooms (relatedto the northeast monsoon) are well pronounced throughout all 9 years of satellitemonitoring.

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Table 1. A list of most abundant phytoplankton species in the coastal watersof the Gulf of Oman in November 2008.

Phytoplankton species Abundance (cells l−1)

Cochlodinium polykrikoides 427 000Amphiprora sp. 80Thalassionema nitzschioides 2000Chaetacerous sp. 240Ceratium furca 280Ceratium symmetricum 80Ceratium punges 80Prorocentrum micans 160Prorocentrum gracile 240Centrodinium elongatum 80Protpredinium tuba 80Thalassiothrix frauenfeldii 320Pleurosigma sp. 160

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Figure 8. Interannual changes of the seasonal cycle of chlorophyll-a in the Gulf of Oman(SeaWiFS data). Vertical axis: chlorophyll-a (mg m−3).

4. Discussion

Böhm et al. (1999) noticed that seasonal changes in the Gulf of Oman were notaffected directly by upwelling during the southwest monsoon. The differences wereport for seasonal cycles of chlorophyll over three regions are consistent with thisnote. In the Gulf of Oman and in the open Arabian Sea (to the east of the FindlaterJet axis), seasonality is more similar in comparison with the Massirah Island region.This means that in the Gulf, winter cooling has a more pronounced effect on vari-ability of chlorophyll concentration than the impact of physical–biological coupling

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driven by the winds of the summer monsoon. This is consistent with the processestaking place in the eastern Arabian Sea (Prakash and Ramesh 2007).

Anomalies and time shifts in seasonal cycles of coastal plankton communities arestill poorly understood for the western Arabian Sea. Our sampling in the Gulf in2004–2006 showed that the seasonal pattern was dominated by diatoms in Januaryand February, with an annual diatom-to-dinoflagellate abundance ratio of 10. Bothdiatoms and dinoflagellates exhibited fluctuations with the dominance of diatoms inwinter, while dinoflagellates dominated during summer (Al-Azri et al. 2009). However,the ratio exhibited marked variation over the years.

The 2008 bloom (which started in September) was different. It has provided moreinsight into the issue of temporal shifts, as well as the variation in species diver-sity in the Gulf of Oman. The bloom was induced by a few dinoflagellate species inwhich Cochlodinium polykrikoides dominated over the next two months of the bloomdevelopment.

This dominant species has never been reported for the Gulf of Oman previously.Possibly, it was transported to the region with the ballast waters or coastal currentsalong the Indian and Iranian coasts. The species reportedly caused huge blooms alongthe Indian and South Korean coasts (Kim et al. 1999, Bhat and Matondkar 2004).

After formation in the western part of the Gulf, the bloom propagated with a narroweastward, then southward current far to the south. The persistence of the southwardcoastal current has been reported previously by the JGOFS expeditions (Morrisonet al. 1998, Shi et al. 2000).

The development of the bloom in the form of a relatively narrow belt of extremelyhigh chlorophyll concentration corresponded to the other reports which implied thatthe offshore flow of upwelled waters have a form of narrow jets, mostly pronounced inthe vicinity of the capes (Smith and Bottero 1977, Brink et al. 1998). The alongshorecoastal transport could have a geostrophic velocity of about 1 m s−1, a width of about100 km from the coast and an integrated alongshore transport of about 106 m3 s−1

with a depth of about 300 m (Elliot and Savidge 1990). During the autumn of 1994,for example, a dominant southward flow was reported for the Ras Al Hadd region(Böhm et al. 1999).

We noticed that the chlorophyll concentration in the Gulf of Oman has not exhib-ited pronounced interannual trends and neither did the other two regions. This doesnot support the concept of the productivity increase in the western Arabian Sea dueto the warming of the Eurasian land mass (Goes et al. 2005). Using satellite data,Goes et al. (2005) reported an increase of more than 350% in average summertimechlorophyll-a along the coast and over 300% offshore.

However, the regions analysed (52◦ E–57◦ E, 5◦ S−10◦ N; 47◦ E–55◦ E, 5◦ S−10◦ N)were far to the south of our regions. Investigations in the northeastern part of the sea(across the regions we analysed) did not show an increasing trend in chlorophyll-a,although if the hypothesis of Goes et al. (2005) is correct, the land–sea driven rela-tionships should have been more clearly pronounced in the northeastern Arabian Seabecause of its close proximity to the Himalaya compared to the southwestern ArabianSea (Prakash and Ramesh 2007).

Overall, an impact of the high-saline Persian Gulf waters (Kumar and Li 1996,Morrison et al. 1998), along with the different modes of the seasonal upwelling (incomparison to the southwestern Arabian Sea) and the isolation effect caused bythe Findlater Jet, all make physical–biological dynamics in the Gulf of Oman fairlydifferent from the other Arabian Sea regions. Perhaps, relative isolation serves as a

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proxy for the other event we reported, which was the increase of phytoplankton sea-sonal variability (amplitudes of seasonal cycles) over the past 10 years (figure 3). Thisincrease was based on a general decline of the diatom biomass on one hand and nitrateconcentration on the other (figure 5), which in turn, was associated with a decline of amixed-layer depth in the Gulf over the years.

The seasonality of nitrates, phosphates and ammonia in the Gulf of Oman was char-acterized with concentrations that were always above detection limits throughout theyear (Al-Azri et al. 2007, 2009). The transition to the northeast monsoon contributesto the largest influx of nutrients into the euphotic layer.

In our study, maximal concentrations of nitrates were associated with maximalvalues of the mixed-layer depth. The mechanism of this link could be based on thevariability of the wind field; an increase of the wind speed should lead to intensivemixing and deepening of the mixed upper layer. A well-developed mixed layer favoursa pronounced flux of nutrients from the depth. Therefore, it is no wonder that nitrateconcentration and the mixed-layer depth exhibited a positive correlation (figure 6).

Alternatively, an increase of solar radiation (and SST as a result), along with weakwinds, would lead to a decrease of the mixed-layer depth due to strong stratificationof the water mass (Narvekar and Kumar 2006).

On an interannual scale, the decline of diatom biomass and the increase of the con-tribution of dinoflagellates to the biomass of seasonal blooms were reported for thenortheastern Arabian Sea (Gomes et al. 2008). Perhaps, a similar event is taking placein the Gulf of Oman. Through the property of being less heavy but more abundant,the dinoflagellate cells contribute to the increased amplitude of the chlorophyll annualvariability over the years.

5. Summary

Seasonal cycles of chlorophyll-a in the Gulf of Oman and the southwestern part ofthe Arabian Sea were quite different. For the southwestern region, maximal chloro-phyll concentrations were recorded during the summer monsoon, while in the Gulf ofOman, the chlorophyll maximum was associated with the winter monsoon. The con-centration of chlorophyll-a in the Gulf of Oman and in the open sea region showedmore concordant seasonal changes, in comparison with the southwestern (MassirahIsland) region.

Interannual changes of the chlorophyll in the Gulf of Oman have not exhibited apronounced trend. However, a tendency of the variation coefficient to increase wasnoticed. In addition, interannual changes of diatoms showed a declining trend inwhich the biomass of this group decreased two fold from 1997 to 2007.

Reported properties of the temporal variability of chlorophyll concentration in theGulf of Oman bring us to the following issues. Once the variation within the annualcycle of phytoplankton biomass has increased, how much variability could the ecosys-tem afford? Is this a dangerous trend in terms of the ecosystem’s stability? Will it resultin a gradual shift of the structure of the coastal ecosystem? Could we interpret currentoutbursts of the Cochlodinium bloom as the first warning sign of the regime shift inthe Gulf of Oman ecosystem?

AcknowledgementsThe present work was supported by research grants from Sultan Qaboos University,The Research Council and Agricultural and Fisheries Development Fund (Sultanate

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of Oman). We are grateful to the editor and anonymous referees for their commentsthat have helped to improve the manuscript.

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