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Dissolved Oxygen, Chlorophyll a andNutrients: Seasonal Cycles in Waters ofthe Gulf of Aquaba, Red SeaMohammad Ismail BadranPublished online: 30 Nov 2010.

To cite this article: Mohammad Ismail Badran (2001) Dissolved Oxygen, Chlorophyll a andNutrients: Seasonal Cycles in Waters of the Gulf of Aquaba, Red Sea, Aquatic Ecosystem Health &Management, 4:2, 139-150

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Dissolved Oxygen, Chlorophyll a and Nutrients: Seasonal Cycles in Waters of the Gulf of Aquaba,

Red Sea

Mohammad Ismail BadranUniversity of Jordan, Marine Science Station, P.O. Box 1072. Aqaba, Jordan. e-mail: abuadam@ju.edu.jo

Abstract

Seawater samples were collected weekly over fourteen months at 25 m intervals between the surface and 200 mfrom the offshore waters of the Gulf of Aqaba, Red Sea. Water temperature and dissolved oxygen concentrationand percent saturation were measured in situ. Within two hours of sampling, samples were analysed for chloro-phyll a, ammonia, nitrate, nitrite, phosphate and silicate. The temperature field depicted a well defined seasonalpattern of winter mixing from December to April and summer stratification from May to November. All otheranalysed parameters were intimately related to this pattern. Dissolved oxygen showed a maximum and homoge-neous distribution in winter and a minimum in summer. Chlorophyll a showed a seasonal pattern close to that ofdissolved oxygen, but with a distinct summer peak between 50 and 75 m. Ammonia was absent from the entirewater column during summer and relatively abundant and homogeneously distributed in winter. Nitrite had aseasonal pattern similar to that of chlorophyll a and exhibited a summer subsurface maximum just below that ofchlorophyll a. Nitrate, phosphate and silicate had similar seasonal patterns characterised by high concentrationsin deeper water during summer overlaid by vanishingly low concentrations of nitrate and phosphate and relativelylow in the case of silicate. In winter the three nutrients exhibited relatively high concentrations homogeneouslydistributed in the entire water column. These findings are analysed and discussed with reference to previousrecords from the Gulf of Aqaba and other oligotrophic water bodies.

Keywords: oligotrophic, primary productivity, mixing-stratification, thermocline

1. Introduction

High resolution long-term time series studies are prob-ably the only reliable approach to understanding theoceanic processes that control nutrient recycling andprimary productivity in the euphotic zone. Such stud-ies, however, face the serious problem of high frequencysampling of distant oceanic water, which is highly labourintensive and extremely expensive.

The Gulf of Aqaba is a semi-enclosed water basinattached to the semi-enclosed Red Sea. It is a morph-

tectonic trough that originated in late Cenozoic times inthe Syrian - African rift system. It has extremely steepsubmarine slopes, no coastal plain or true shelf. Thelength of the Gulf of Aqaba is about 170 km and theaverage width is about 15 km. It is totally surroundedby desert; Senai from the west and the Jordanian Saudidesert from the east. The Gulf is very deep with a maxi-mum water depth close to that of the Red Sea proper,~1800 m, representing a virtual oceanic basin. The cli-mate is arid with high temperatures reaching a maximumduring July and August and a minimum during Decem-ber and January. The annual range of variation in airtemperature is extremely high, ~5 to 40o C and is typical

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of the desert climate, the diurnal change in air tempera-ture is also high, 10 to 15o C. Mean annual rainfall is 35mm y-1. All rainfall occurs between October and Mayand 61% of the total rainfall occurs between Decemberand February. The Gulf receives no river runoff. Withthese unique geophysical characteristics the Gulf quali-fies to serve as an oceanic model and allows the studyof oceanic processes in relative convenience at a rea-sonable cost.

Dissolved oxygen (DO) cycles involve exchange atthe air-water interface, oxygen production via photo-synthesis in the euphotic zone, and consumption viarespiration and organic matter oxidation in the entirewater column. Consequently, the DO concentration isusually high in the upper waters and decreases withdepth in varying degrees depending on the intensity ofbiological productivity and the organic matter concen-tration and composition. Decrease of oxygen concen-tration with depth in the Gulf of Aqaba, however, is notsharp. Klinker et al. 1976 reported a relatively high DOconcentration on the western side of the Gulf (3.75 ml l-

1 = 5.36 mg l-1) at 800m. The authors attributed varia-tions in the DO concentration primarily to variations insalinity. From the eastern side of the Gulf of Aqaba, theJordanian-Saudi side, no previous records on oxygenconcentration or percent saturation in the offshore wa-ters exist.

Phytoplankton and chlorophyll a concentrations inthe Gulf of Aqaba have been studied by Kimor andGolandsky, 1977; Levanon-Spanier et al., 1979; Legerand Artiges, 1978; Natour and Nienhuis, 1980;Mahasneh, 1984; Wahbeh and Badran, 1991 and Lindelland Post, 1995. These studies were either restricted tocoastal waters or included incomplete and coarsely re-solved temporal cycles. However, the common denomi-nator among all studies is that phytoplankton and chlo-rophyll a concentrations were typically oligotrophicboth in magnitude and seasonality.

Investigations of nutrient concentrations in the Gulfof Aqaba are mainly those of Mohammed, 1940; Reiss,1977; Sournia, 1977; Freemantle et al., 1978; Klinker etal., 1978; Levanon-Spanier et al., 1979; Hulings and AbuHilal, 1983; Dor and Levy, 1984; Mahasneh, 1984; Jaberand Kamal, 1985; Jebrein, 1986; Genin et al., 1995 andBadran and Foster, 1998. The broad and detailed stud-ies, on the western side of the Gulf are those of Klinkeret al. (1978) and Levanon-Spanier et al. (1979), in theframework of the DCPE (data collection programme be-tween 1973 and 1976) and on the eastern side those ofHulings and Abu Hilal, (1983) and Badran and Foster,(1998). Appreciable differences between the data sets

from the western and eastern sides of the Gulf do exist,yet all records confirm that waters of the Gulf of Aqabaare typically oligotrophic.

The objective of the present investigation was toimprove our understanding of the oceanic processes inthe euphotic and near aphotic zone of the oligotrophicoceanic waters of the Gulf of Aqaba by generating adetailed appreciation of the temporal and vertical cyclesof DO, chlorophyll a and the inorganic nutrients ammo-nia, nitrate, nitrite, phosphate and silicate between thesurface and 200 m depth of the offshore waters.

2. Materials and methods

Seawater samples were collected weekly, at a fixedtime of around 10:00 am, during the period of March 21,1994 to May 15, 1995, from one station about 3 km off-shore of the Marine Science Station, Aqaba, where thewater column is about 600 m deep (Fig. 1). Samples werecollected from the upper 200 m at a vertical resolution of25 m in a 10l Niskin bottle equipped with a reversingthermometer, onboard a 12-foot boat with a manualwinch.

Dissolved oxygen concentration and % saturationwere recorded on board using a YSI oxygen meter. Forthe remaining analyses samples were kept on ice in anicebox and analysed soon upon return to the laboratoryfollowing methods developed from Strickland and Par-sons (1972) and IOC Manuals and Guides No. 12 (1983).For determination of ammonia, nitrate, nitrite and phos-phate, samples were filtered through pre-washed GF/Cfilters before analysis, while for the determination ofsilicate, samples were filtered through pre-washedWhatman No. 2 filter paper. Chlorophyll a was mea-sured spectrophotometrically after extraction in 90%acetone. Five liters of seawater were filtered through a0.45um cellulose membrane filter, which was then dis-solved in 5 ml of 90% acetone and left to soak in arefrigerator for 12 to 15 h. After centrifugation, extinc-tion was measured at 750, 663, 645 and 630 nm. Chloro-phyll a concentration was calculated according to theSCOR/UNESCO equations.

3. Results

Records of seawater temperature, dissolved oxygenconcentration, percent saturation and chlorophyll aconcentration are shown Figure 2. In late April the wa-

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Figure 1 Gulf of Aqaba, Red Sea and location of the sampling station.

Figure 2 Records of seawater temperature, dissolved oxygen concentration, percent saturation and chlorophyll a concentrationbetween the surface and 200 m of waters of the Gulf of Aqaba during the period March 21 1995 to May 15 1996.

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ter column warmed slowly resulting in the appearanceof a weak thermocline, which by late May had a magni-tude between the surface and 200 m, of 1.7o C. Thereaf-ter, the warming rate increased substantially and non-uniformly with respect to depth. The thermocline inten-sified and reached a maximum in September and Octo-ber with a magnitude of 5.7o C. In November a seriousshift towards erosion of the thermocline occurred andin January isothermal conditions characterised the up-per 200m of the water column. Mixed water conditionspersisted till April.

Dissolved oxygen concentrations, both temporallyand vertically, exhibited only small variations, whichwere clearly temperature dependent. During May, Juneand July DO concentration in the upper 200 m decreasedsuch that by mid July the concentration ranged from6.52 to 6.58 mg l-1. For the period between August andOctober, a DO concentration minimum developed be-tween the surface and 50 m. From November onwardsthe concentration increased progressively until Febru-

ary, when the maximum monthly mean DO concentra-tion in the upper 200 m was recorded.

Chlorophyll a concentrations were relatively high(~0.40 µg l-1) and uniformly distributed in the upper 200m during winter. In March and April chlorophyll a con-centrations were distinguishably high , reaching 1.2 µgl-1 at a sampling event in the upper 50 m in March. Dur-ing summer (June-September) a concentration peak(~0.30 µg l-1) developed between 50 to 75 m. In lateOctober the concentration peak occurred at 50 m andsubsequently chlorophyll a concentrations increasedrapidly in the entire water column.

Records of ammonia, nitrate, nitrite, phosphate andsilicate concentrations are shown in Figure 3. Ammoniaconcentrations showed a distinctive seasonal pattern,characterised by ammonia depletion from the entire watercolumn during summer and moderate abundance eitherin the entire water column or parts of it during otherseasons. Nitrate concentrations displayed a well de-fined pattern of seasonal variation. During summer the

Figure 3 Records of ammonia, nitrate, nitrite, phosphate and silicate concentrations between the surface and 200 m of waters ofthe Gulf of Aqaba during the period March 21 1995 to May 15 1996.

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concentration was segregated at around 100 m into twolayers. In the upper (above 100 m) the concentrationwas remarkably low, in the range of 0.02 to 0.20 µM andin the lower the concentration increased steadily withtime until it reached a maximum at 200 m in October.With the erosion of the thermocline in November, ni-trate concentration became more uniformly distributedin the water column until it exhibited complete mixingdown to 200 m in January. Nitrite was relatively abun-dant and uniformly distributed down the water columnduring winter. In summer nitrite concentration exhibiteda subsurface maximum similar to that of chlorophyll a orjust below it, at 100 m. Phosphate and silicate concen-trations exhibited seasonal patterns similar to those ofnitrate.

4. Discussion

4.1 Dissolved oxygen

Dissolved oxygen concentrations recorded hereinare relatively low as compared to values in open oceanwaters or other oceanic basins, such as the Mediterra-nean Sea (7.1-8.6 mg l-1; Souvermezolou et al., 1992).This is attributed to the high temperature and salinity ofthe waters of the Gulf of Aqaba (Wiess, 1970). Oxygenpercent saturation records prove that the surface wateris never below 100% DO saturation and that oxygensaturation decreases only slightly over the year withinthe upper 200 m of the water column. Compared to pre-vious records from the Gulf of Aqaba, oxygen concen-trations of the present study are in good agreementwith concentrations reported by Klinker et al. (1976).

The high oxygen percent saturation in the entirewater column can be attributed to two main factors: acontinuous dynamic equilibrium between oxygen con-sumption and reproduction in the euphotic zone and tothe annual oxygenation of deep waters through wintermixing. This is well supported by the temperature fieldcharacteristics and the dissolved oxygen concentrationrecord in February, when the entire upper 200 m wereabove 100% saturated.

Jenkins and Goldman (1985) reported on the oxygenseasonal cycle in the Sargasso Sea. The major charac-teristics of the Sargasso Sea waters are: 1) during sum-mer, oxygen concentrations in the 50 and 75 m waterswere higher than in the surface and 25 m waters, whichwere at a minimum; 2) minimum oxygen anomalies (satu-ration in ml l-1) and intermediate oxygen concentrations

occurred during the winter thermal mixing; and 3) nega-tive anomalies down to 75 m occurred only in winter.There is good agreement in the summer DO concentra-tion profiles of the two basins, which can simply beattributed to temperature. Both basins have similar sum-mer maximum surface water temperatures (~27o C) andsimilar summer minimum oxygen concentration (~6.5 mgl-1). Minimum concentration recorded herein is ~0.2 mgl-1 lower than the minimum reported by Jenkins andGoldman (1985). This according to the Weiss (1970)equations can be quantitatively related to the highersalinity and the slightly higher temperature of waters ofthe Gulf of Aqaba. With respect to the second and thirdcharacteristics, the Gulf of Aqaba and the Sargasso Seaare completely in contrast. In the Gulf of Aqaba maxi-mum DO percent saturation and maximum concentra-tions occurred during the winter mixed water conditions.In addition, saturation above 100% (positive anoma-lies) in waters deeper than 50 m occurred mainly duringwinter (December to March) and negative anomaliesdominated the summer time. Jenkins and Goldman (1985)attributed the negative winter oxygen anomalies in theSargasso Sea to vertical mixing of oxygen depleted wa-ters from below. On the contrary the positive winteroxygen anomalies in the Gulf of Aqaba can be attrib-uted to the vertical mixing of relatively oxygen-rich wa-ter from the distinguishably deep euphotic zone(Levanon-Spanier et al., 1979). Besides, the verticallymixed deep waters in the Gulf of Aqaba are never oxy-gen-depleted (Klinker et al., 1976). On the other handthe summer positive oxygen anomalies in the SargassoSea were attributed by Jenkins and Goldman (1985) tosurplus oxygen production within the water column overdemand. On the contrary the negative oxygen summeranomalies recorded at the bottom of the euphotic zone,herein can be attributed to a shortage of oxygen pro-duction with respect to demand. This can be inferredfrom the summer subsurface nitrite concentration maxi-mum that occurred at 100m. It is noteworthy also thatthe seasonally recyclable section of the water columnin the Sargasso Sea extends to about 100 m, while in theGulf of Aqaba it exceeds 400 m (Genen et al., 1995;Manasrah, 1998)

4.2 Nutrient comparison with previous recordsfrom the Gulf of Aqaba

Nutrient concentrations recorded in the present in-vestigation clearly characterise a typical oligotrophicwater body. Previous data on nutrients in the Gulf ofAqaba are scarce. Comparison with the limited avail-

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able data however, reveals a good agreement in sea-sonal patterns and concentrations of nitrate concentra-tions reported by Klinker et al. (1978) and Levanon-Spanier et al. (1979), but not with the values reported byHulings and Abu Hilal (1983). The latter authors re-ported constant nitrate concentrations in the Jordanianoffshore surface waters of ~2.0 uM throughout the year.The summer values are disproportionately higher thanall other data from the Gulf of Aqaba or any other olig-otrophic water body. They are most likely attributed toanalysis.

Regarding phosphate, apart from a few sporadic highvalues (~1.0 uM) the concentration ranges reported byKlinker et al. (1978) and Levanon-Spanier et al. (1979)are not far from the concentration ranges recordedherein. Previous studies however, did not show sea-sonality in the phosphate concentrations as clearly asshown herein. This can be attributed to the low phos-phate concentrations (0.01-0.16 uM) and the samplingfrequency. At such low concentrations, experimentalerror in a course temporally resolved samplingprogramme could create considerable bias.

Apart from the present investigation, silicate con-centration records exist only in the report of Klinker etal. (1978). The concentrations range reported by theauthors for the northern section of the Gulf of Aqabaare in good agreement with the range recorded in thepresent study. As in the case of phosphate, Klinker etal. (1978) detected no seasonality throughout the yearor variability in the concentration with depth duringsummer. Indeed the coefficients of variation in silicateconcentrations with both depth and time are lower thanthose of any other nutrient, but the ranges of variationare the second highest after those of the nitrate con-centrations. As for ammonia, no previous records exist.

4.3 Ammonia and nitrite

Ammonia concentrations in seawater are shaped bythe interaction of a set of sources (bacterial and zoop-lankton excretion + decomposition of dead organic mat-ter + nitrogen fixation) and a set of sinks (bacterial andphytoplankton consumption plus nitrification). Al-though all ammonia sources and sinks in sea water arebiogenic, physical agents such as ambient water tem-perature, light availability and the state of oxygenationare of considerable importance in determining the am-monia concentration. In most oceanic basins, deep wa-ters are cold and poorly oxygenated, favouring denitri-fication and nitrate reduction. Therefore, nitrogen thatenters the system as a decomposition product is stored

in the deep water mainly in the ammonium form (Chen etal., 1988). In the Gulf of Aqaba deep waters down to1000 m are warm and well oxygenated (~20o C and ~5 mgl-1 oxygen; Klinker et al., 1976). Such conditions en-hance nitrification and prevent the formation of a deepwater ammonium reservoir. This implies that the mainsource of ammonia in the water column exists in theeuphotic zone.

Ammonia production in the euphotic zone is stronglysequestered by the rapid nitrification and photosyn-thetic consumption. This makes the assessment of am-monium production in the water column in the Gulf ofAqaba, or even detecting it for prolonged periods dur-ing the annual cycle extremely difficult. Results of thepresent investigation are in good agreement with thisscenario. Ammonia starts to appear in the upper sec-tion of the water column in late summer and its concen-tration increases notably only during winter, when otherforms of nitrogen are available and nitrogen is well inexcess of the requirements of primary production.Fasham et al. (1990) used an annual-cycle model of plank-ton and nitrogen dynamics in the mixed layer and pre-dicted ammonia concentrations in the Sargasso Sea torange between ~0.00 to 0.25 uM. The authors statedthat they were not aware of any measured annual cycleof ammonia concentrations in any oligotrophic waterbody in the world with which to compare their predic-tions. In their model, ammonia concentrations startedto increase in late October, reached a maximum in Janu-ary and vanished around mid April. This prediction,which compares well with the results recorded hereinwas based on a 1 m d-1 sinking rate. Fasham et al. (1990)used the same model for the prediction of ammonia con-centrations assuming a 10 m d-1 sinking rate. Resultsfrom such runs were much lower. Brzezinski (1988) re-ported ammonia concentrations in the upper 200 m ofthe Sargasso Sea during June 1987. The results are ingood agreement with the values reported herein for thesame month. In both studies ammonia concentrationswere close to 0.10 uM below 100 m and less than 0.06uM above.

Nitrite like ammonia does not have a deep water res-ervoir. The seasonal patterns of these two nitrogen spe-cies are, however, considerably different. Contrary toammonia, nitrite is never depleted from the water col-umn. Before the winter nitrite surplus is consumed, anew supply starts to build up around 100 m in earlysummer. The differences recorded between the nitriteand ammonia seasonal cycles can be related to eitherone or more of the following factors: 1) ammonia andnitrite may have different sources and unequal inputs.

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It is well established that ammonia is produced by ex-cretion of zooplankton and higher level organisms, whilenitrite is believed to be an excretion product of phy-toplankton when they are abruptly confronted with anexcess of available nitrate at low irradiance (Morcos,1970). This implies that ammonia can be excreted any-where in the water column, but nitrite excretion is re-stricted to the lower limit of the primary productivityzone. 2) irrespective of the sources and inputs, ammo-nia is consumed faster than nitrite and therefore getsdepleted from the water column due to preferential con-sumption (Dugdale and Goering, 1967; Jamart et al., 1977;Brezezinski, 1988). 3) nitrite is a nitrification intermedi-ate product, and there is evidence in the literature thatnitrification is inhibited by light (Olson, 1981; Garside,1985). It is therefore possible that nitrite accumulateswhen optimum light conditions for nitrification are en-countered, especially if such light conditions are pri-mary productivity limiting.

The behaviour of nitrite in the water column is poorlyresearched and poorly understood. According to King(1987) the sites and rates of nitrification in the watercolumn are not well defined. Some ambiguity also existsregarding the agents responsible for nitrification. Kaplan(1983) suggested that bacteria that oxidise ammonia tonitrite are different from bacteria that oxidise nitrite tonitrate. The role of nitrite in regenerated production isquite often ignored. Nevertheless the annual cycle ofnitrite concentrations recorded in the present investi-gation emphasises the importance of nitrite as a regen-erated nitrogen species. McCarthy et al. (1977) inferredsuch a significant role of nitrite in regenerated primaryproduction in the Chesapeake Bay, but could not relatethe abundant nitrite during August and October to aspecific source. The summer peak of nitrite concentra-tions at 100 m seems to be characteristic not only of theGulf of Aqaba but also of other oligotrophic water bod-ies. Brzezinski (1988) reported similar peaks in the Sar-gasso Sea and the Gulf Stream during June 1987.

4.4 Nitrate, phosphate and silicate

Nitrate, phosphate and silicate share in common closeseasonal patterns of variability. The three nutrients ex-ist in deep water reservoirs. Their injection into the mixedlayer is controlled by physical-biological processes,mainly the thermal stratification-destratification and pri-mary productivity. Consequently the three nutrientsdecrease in concentration above the thermocline dur-ing summer and increase below it. Taking nitrate as anexample, Figure 4. illustrates the variation of concentra-

tions with temperature in the upper waters (surface-25m) and the deeper waters (175-200 m).

With the erosion of thermal stratification, concentra-tions of the three nutrients in the upper 200 m attainuniformity. For the year 1994 uniform concentrationsoccurred in January, when the water temperature be-tween the surface and 200 m was uniform. Followingthese conditions the concentrations increased furtherand the temperature decreased indicating entrainmentof cooler, nitrate-, phosphate- and silicate-rich watersfrom below.

Despite the general similarity in the seasonal cyclesof the three nutrients, some differences between themdo exist. Nitrate concentrations exhibit the highest an-nual variability amongst the nutrients both in magni-tude and as a coefficient of the mean. Phosphate con-centrations exhibit the lowest annual variability in mag-nitude and silicate concentrations exhibit the lowestannual coefficient of variation (Table 1). Another char-acteristic difference between the nitrate and silicate con-centrations is that during summer, silicate concentra-tions exhibit a minimum around 50 m, while nitrate con-centrations are equal at the surface and 50 m. This canbe attributed to atmospheric deposits of silicate carriedby the desert winds. Al Fuqaha (1994) reported dustdeposits in coastal waters of the Gulf of Aqaba ~0.1 gm-2 d-1, of which more than 50% is silt. These depositscan be a major source of silicate in Aqaba sea water andmight be responsible for preventing silicate concentra-tions above the thermocline to drop sharply during sum-mer. In comparison with other oligotrophic waters, sili-cate concentrations in the euphotic zone in the Gulf ofAqaba during summer (~1.00 uM) are higher than theconcentrations reported by Menzel and Ryther (1960)for the Sargasso Sea (0.30 uM) and than the concentra-tions reported by Souvermezoglou et al. (1989, 1992) atBab el Mandab in the Red Sea, which also reach 0.30uM, very close to nitrate concentrations (0.4 uM).

4.5 Chlorophyll a summer subsurface maximum

A characteristic feature of the chlorophyll a annualcycle was the development of a subsurface chlorophylla maximum below 50 m during summer. Development ofsuch a chlorophyll a maximum is a well known phenom-enon in oligotrophic ocean waters. Venrick et al. (1973)reported a chlorophyll a maximum between April andSeptember 1969 in the 50 to 100 m waters at the westernedge of the California Current (30.00o N, 120.07o W) whichthey believed was a continuation of the conditions inthe Central Pacific. Shulenberger and Reid (1981) also

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Figure 4 Illustration of the nitrate concentration variation with temperature in the upper waters (surface-25 m) and the deeperwaters (175-200 m) of the Gulf of Aqaba during the period March 21 1995 to May 15 1996.

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reported a subsurface chlorophyll a maximum that wasoverlaid by an oxygen maximum during summer in theCentral Pacific. Summer subsurface chlorophyll amaxima also exist in the Sargasso Sea (Menzel and Ryther,1960) and in the Mediterranean Sea (Kucuksezgin et al.,1995). Trying to explain the occurrence of chlorophyll asubsurface maximum, Venrick et al. (1973) suggestedthat the nutrient regime rather than the ambient light isthe ultimate determinant of the depth of the chlorophylla summer maximum. Alternatively, the chlorophyll asummer maximum can be a result of primary productiv-ity that is a photo-chemical reaction that cannot pro-ceed unless both the suitable substrate and energy areavailable. During summer the energy (light) and the sub-strate (nutrients) for oceanic primary productivity areprovided from different directions. Nutrients are pro-vided mainly from the deep water by diffusion acrossthe thermocline and light is provided from the sky. There-fore, nutrients keep diffusing upwards until they areconfronted by sufficient light to spark primary produc-tion. This light-nutrient front is the lower (deeper) limitof primary productivity and above it forms a layer wheremost diffusing nutrients are consumed. This layer is thesummer chlorophyll a maximum layer. It acts as a nutri-ent trap and varies from one place to another, depend-ing on the ambient irradiance, water dynamics and thenutrient diffusion rate.

According to Shulenberger and Reid (1981) andJenkins and Goldman (1985) in the Central Pacific andthe Sargasso Sea the summer sub surface chlorophyll amaximum was accompanied by positive oxygen anoma-lies. In the present case, the subsurface chlorophyll amaximum was accompanied by slightly negative oxy-gen anomalies. Quantitative assessment of the oxygenanomalies in relation to primary productivity is quitedifficult, because the state of oxygen saturation does

not depend on primary productivity alone, but also andprobably more significantly in oligotrophic waters dur-ing summer it depends on the sea water temperature(Platt, 1984). In the Gulf of Aqaba, despite the thermalstratification during summer all the waters in the upper200 m have similar levels of oxygen saturation, but theydiffer significantly in their temperatures. Therefore anywaters diffusing upwards, will rise in temperature andincrease in oxygen saturation even with no change inits oxygen concentration. Indeed, during the summer,thermal stratification vertical displacement of water isquite restricted, but it is not realistic to assume that novertical water transport at all occurs. Internal waves canplay a significant role in such transport. Therefore, de-spite thermal stratification during summer, the thermalsystem can still play a significant role in maintaininghigher oxygen saturation closer to the surface and loweroxygen saturation in deeper waters. Another phenom-enon that accompanies the subsurface chlorophyll amaximum in the records of the present investigation isthe summer nitrite peak, which occurs just below thechlorophyll a maximum. This nitrite peak is an oxygensink. It indicates the presence of two oxygen consum-ing processes: decomposition and nitrification. There-fore most of the oxygen produced within the chloro-phyll a maximum layer, if not all can be trapped in thenitrite maximum layer.

4.6 The question of the primary productivitylimiting nutrient

Both Levanon-Spanier et al. (1979) and Hulings andAbu Hilal (1983) addressed the question of the primaryproductivity limiting nutrient. Different answers to thequestion were given by different authors. Levanon-Spanier et al. (1979), based on the nitrogen-phosphoru s

Table 1 Annual ranges and coefficients of variation of nitrate, phosphate and silicate concentrations in oceanic waters of the Gulfof Aqaba during the year March 1994 to March 1995.

1 m25 m50 m75 m100 m125 m150 m175 m200 m

Mean

Nitrate0.02-1.19

0.02-1.190.02-1.200.03-1.200.05-1.220.29-1.260.43-1.900.51-1.920.58-1.960.22-1.45

Phosphate0.01-0.130.01-0.130.01-0.130.02-0.130.02-0.130.04-0.130.07-0.140.08-0.140.10-0.160.04-0.14

Silicate0.92-1.47

0.91-1.460.89-1.450.91-1.450.98-1.451.05-1.461.08-1.471.12-1.471.16-1.531.00-1.47

Nitrate0.27 ±1270.27±1310.27±1360.31±1110.46±690.65±411.20±321.28±301.44±300.68±79

Phosphate0.051±570.052±570.053±590.057±450.071±380.080±260.11±150.11±130.12±130.078±36

Silicate1.13±121.13±131.12±141.14±131.21±101.24±91.32±61.33±51.37±51.22±10

Range, uM Mean±Coefficient of variation

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Figure 5 Nitrogen-phosphorus molar ratios and regression relationship between the surface and 200 m waters of the Gulf of Aqabaduring the period March 21 1995 to May 15 1996.

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ratios concluded that nitrogen is the primary productiv-ity limiting nutrient in the Gulf of Aqaba. Hulings andAbu Hilal (1983), on the other hand found irregularlyfluctuating nitrogen-phosphorus ratios between 6:1 to21:1 with a mean of 11.8:1 in the offshore Jordaniansurface water. Yet the authors assumed a direct relation-ship to exist between low phosphorus concentrationsand high primary productivity, because nitrate concen-tration according to them was more or less constantthroughout the year. On this basis they concluded thatphosphorus was the primary productivity limiting nu-trient. Figure 5 shows the total inorganic nitrogen (am-monia, nitrate and nitrite)-phosphorus molar concen-tration ratios in the upper 200 m waters. It is obvious inFig. 5 that neither in the upper 100 m nor in the deepzone (175-200 m) does the nitrogen-phosphorus ratiosexceed the Redfield ratio (16:1). This leads to the con-clusion that the limiting nutrient in waters of the Gulf ofAqaba is nitrogen. Figure 5 also shows the total inor-ganic nitrogen-phosphorus regression relationship. Thecorrelation between total inorganic nitrogen and phos-phorus is highly significant and total inorganic nitro-gen is below 16 fold that of phosphorus .

Acknowledgments

This work has been sponsored by the British Coun-cil, Amman, Jordan

References

Al Fuqaha, F., 1994. A textural and geochemical study on thereefal sediments of the Gulf of Aqaba and the input of airbornedust to the area. M.sc. thesis. Yarmouk University. Irbid,Jordan.

Badran, M.I., Foster, P., 1998. Environmental quality of theJordanian coastal waters of the Gulf of Aqaba, Red Sea. Aquat.Ecosystem Health and Manag. 1, 83-97.

Brezezinski, M.A., 1988. Vertical distribution of ammonium instratified waters. Limnol. Oceanogr. 33, 1176-1182 .

Chen, D., Horigans, S.G., Wang, D.P., 1988. The late summervertical nutrient mixing in Long Island Sound. J. Mar. Res.46, 753-770.

Dor, I., Levy, I. 1984. Primary productivity of the benthic algaein the hard - bottom mangal of Senai. pp. 179-191. In: F.D.Por, I. Dor, (Eds.), Hydrobiology of the Mangal. Dr. W. JunkPubl.

Dugdale, R.C., Goering, J.J., 1967. Uptake of new and regeneratedforms of nitrogen in primary production. Limnol. Oceanogr.12, 196-206.

Fasham, M.J.R., Ducklow, W.H., McKelvie, S.M., 1990. Anitrogen-based model of plankton dynamics in the oceanic

mixed layer. J. Mar. Res. 48, 591-639.Freemantle, M.H., Hulings, N.C., Mulqi, M., Watton, E.C., 1978.

Calcium and phosphate in the Jordan Gulf of Aqaba. Mar.Pollut. Bull. 9, 79-80.

Garside, C., 1985. The vertical distribution of nitrate in openocean surface water. Deep-Sea Res. 32, 723-732.

Genin, A., Lazar, B., Brenner, S., 1995. Vertical mixing andcoral death in the Red Sea following the eruption of MountPinatubo. Nature 377, 507-510 .

Hulings, N.C., Abu Hilal, A., 1983. The temporal distribution ofnutrients in the surface waters of the Jordan Gulf of Aqaba.Dirasat 10, 91-105.

IOC. Manuals and guides, 1983. Chemical methods for use inmarine environmental monitoring. UNESCO, Paris. pp. 53.

Jaber, A.M.Y., Kamal, M.R., 1985. Major and trace elements inthe seawater of the Jordanian coast of the Gulf of Aqaba.Arabian J. Sci. Engng. 10, 282-289.

Jamart, B.M., Winter, D.F., Banse, K., Anderson, G.C., Lam,R.K., 1977. A theoretical study of phytoplankton growthand nutrient distribution in the Pacific Ocean of thenorthwestern U.S. coasts. Deep-Sea Res. 24, 753-773 .

Jebrein, A.O., 1986. Productivity and distributional patterns ofselected red, brown and green algal species in the Gulf ofAqaba. Masters thesis. University of Jordan. Amman, Jordan.

Jenkins, W.J., Goldman, J.C., 1985. Seasonal oxygen cyclingand primary production in the Sargasso Sea. J. Mar. Res. 43,465-491.

Kaplan, W.A., 1983. Nitrification. pp.139-190. In: E.J.Carpenter, D.G. Capone, (Eds.) Nitrogen in the MarineEnvironment Academic Press, New York.

Kimor, B., Golandsky, B., 1977. Microplankton of the Gulf ofElat: aspects of seasonal and bathymetric distribution. Mar.Biol. 42, 55-67.

King, F.D., 1987. Nitrogen recycling efficiency in steady-stateoceanic environments. Deep-Sea Res. 34, 843-856.

Klinker, J., Reiss, Z., Kropach, C., Levanon, I., Harpaz, H.,Halicz, E., 1976. Observations on the circulation patterns inthe Gulf of Elat (Aqaba), Red Sea. Israel J. Earth Sci. 25, 85-103.

Klinker, J., Reiss, Z., Kropach, C., Levanon, I., Harpaz, H.,Shapiro, Y. 1978. Nutrients and biomass distribution in theGulf of Aqaba (Elat), Red Sea. Mar. Biol. 45, 53-64.

Kucuksezgin, F., Balci, A., Kontas, A., Altay, O., 1995.Distribution of nutrients and chlorophyll a in the Agean Sea.Oceanol. Acta 18, 343-352 .

Leger, G., Artiges, J.M., 1978. Etude Pliminaire du phytoplanktondes eaux Jordaniennes du Golfe d Aqaba. Ann. Rept. NieceUniv. Jordan Coop. Prog. 26 pp.

Levanon Spanier, I., Padan, E., Reiss, Z., 1979. Primaryproduction in a desert-enclosed sea- the Gulf of Elat (Aqaba),Red Sea. Deep-Sea Res. 26, 673-685.

Lindell, D., Post, A.F., 1995. Ultra phytoplankton succession istriggered by deepwinter mixing in the Gulf of Aqaba (Eilat),Red Sea. Limnol. Oceanogr. 40, 1130-1141 .

Mahasneh, I.A., 1984. The physiological ecology of the marinephytoplankton in the Jordanian Gulf of Aqaba. MSc thesis.Yarmouk University. Irbid, Jordan.

Manasrah, R.S., 1998. Circulation of the Jordanian waters of theGulf of Aqaba, Red Sea. Master thesis. Marine Science Station,Yarmouk University, Aqaba, Jordan.

McCarthy, J.J., Taylor, W.R., Taft, J.L., 1977. Nitrogenousnutrition of the plankton inthe Chesapeake Bay. 1. Nutrient

149Badran / Aquatic Ecosystem Health and Management 4(2001) 139-150

Dow

nloa

ded

by [

UO

V U

nive

rsity

of

Ovi

edo]

at 0

4:44

28

Oct

ober

201

4

availability and phytoplankton preference. Limnol. Oceanogr.22, 996-1011 .

Menzel, D., Ryther, J. 1960. The annual cycle of primaryproduction in the SargassoSea off Bermuda. Deep-Sea Res. 6,351-367.

Mohamed, A.F., 1940. The Egyptian exploration of the RedSea. Proc. R. Soc. Lond. 128, 306-316.

Morcos, S.A., 1970. Physical and chemical oceanography ofthe Red Sea. Oceangr. Mar. Biol. 8, 73-202.

Natour, R.M., Neinhuis, H., 1980. Some phytoplanktonic studiesin Aqaba Gulf of Jordan. Nova Hedwigia 33, 433-443.

Olson, R.J., 1981. Differential photoinhibition of marinenitrifying bacteria: a possible mechanism for the formationof the primary nitrite maximum. J. Mar. Res. 39, 227-238 .

Platt, T., 1984. Primary productivity in the central North Pacific:comparison of oxygen and carbon fluxes. Deep Sea Res. 31,1311-1319.

Reis, Z., 1977. Forminiferal research in the Gulf of Elat-Aqaba.Utrecht Micropaleont. Bull. 15, 7-26

Shulenberger, E., Ried, J.L., 1981. The Pacific shallow oxygenmaximum and primary productivity, reconsidered. Deep SeaRes. 28, 901-919.

Sournia, A., 1977. Notes on primary production of coastal watersin the Gulf of Elat (Red Sea). Int. Revue. ges. Hydrobiol. 62,813-819.

Souvermezoglou, E., Metzl, N., Poisson, A., 1989. Red Sea budgetsof salinity, nutrients and carbon, calculated in the Strait ofBab-El-Mandab during the summer and winter seasons. J.Mar. Res. 47, 441-456 .

Souvermezoglou, E., Hatzigeorgiou, E., Pampidis, I., Sipasali,K., 1992. Distribution and seasonal variability of nutrientsand dissolved oxygen in the northeastern Ionean Sea.Oceanologica Acta. 15, 585-594 .

Strickland, J.D.H., Parsons, T.R., 1972. A Practical Handbookof Seawater Analysis. Fisheries Res. Board of Canada. Ottawa,pp. 310.

Venrick, E.L., McGowan, J.A., Mantyla, A.W., 1973. Deepmaxima of photosynthetic chlorophyll in the Pacific Ocean.Fish Bull. 71, 41-52.

Wahbeh, M.I., Badran, M.I., 1991. Distribution of chlorophyll ,organic carbon, protein, biomass and trace elements in theMicroplankton of Aqaba (Jordan). Dirasat 17, 161-167 .

Weiss, R.F., 1970. The solubility of nitrogen, oxygen and argonin water and seawater. Deep-Sea Res. 17, 721-735.

150 Badran / Aquatic Ecosystem Health and Management 4(2001) 139-150

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by [

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