Human activities and coastal development driven by economic growth in countries bordering the Arabian Gulf have increased marine pollution, including eutrophication. In response to growing concerns, the Environment Agency- Abu Dhabi initiated a long-term monitoring survey (2006 to 2019) to study the marine water quality of Abu Dhabi, including the Mussafah South Channel (MSC). This confined area is of eutrophic nature, under pressure due to receiving approximately 400,000 m3 of treated sewage per day and continuous algal blooms incidents with maximum concentration of 18 x 107 cells l−1. Observations made on physical, chemical, and biological parameters, such as temperature (18 -35 °C), salinity (28 – 70 psu), pH (5 to 9), and dissolved oxygen (0.02 – 13 mg l−1) revealed abnormal conditions and a stratified water column. The nutrient values, particularly nitrate (2.90 – 866.06 Mol l−1) and phosphate (1.68 -98.24 Mol l−1), were many times higher than Abu Dhabi’s reference values (3.55 Mol l−1 and 1.58 Mol l−1, respectively) which confirmed its eutrophic nature. Algal blooms were frequently recorded, and were associated with widespread harmful impacts, including hypoxic events (oxygen levels of 0.02 mg l−1), finfish kills (Nematalosa nasus) and subsequent loss of benthic organisms. The deterioration of water quality in the MSC is probably due to the anthropogenic activities, insufficient treatment of municipal and industrial wastewater discharges. Extensive sediment load may also have exacerbated the situation and contributed to eutrophication and subsequent alteration of the ecosystem. The sources, composition and consequences of nutrient enrichment along with the management actions are also discussed in detail.

Introduction

The coastal areas of Abu Dhabi are important for both recreation and food. However, the environment is under increasing stress due to intensive usage and large nutrient supplies from anthropogenic activities. Excess concentrations of these elements can significantly degrade water quality by increasing turbidity, decreasing dissolved oxygen, degrading the habitat, altering foodweb relationships and biodiversity, and even causing toxic algal species to bloom, posing risks to fish, shellfish, and human health. Nixon (1995), Jørgensen and Richardson (1996), Cloern (2001), Boesch (2002), de Jonge et al. (2001), and Wassmann and Olli (2004) have all studied the impact of nutrients on the marine environment. Abu Dhabi waters, particularly those in confined areas, are experiencing the adverse effects of eutrophication through continuous algal blooms and their subsequent fish kills and loss of benthic fauna (EAD, 2017). The Mussafah South Channel is a confined area that receives high nutrients through treated sewage effluents, and it has logged continuous algal blooms and fish kills. As a part of marine water quality management efforts, the Environmental Agency - Abu Dhabi (EAD) initiated a monitoring program in 2006, and it continues to develop practices to help protect public and ecosystem health. The monitoring provides support to make effect-based decisions on coastal environmental permit applications and other coastal zone issues such as wastewater disposal and dredging. This paper presents a case study and detailed discussion of the sources, composition, and consequences of nutrient enrichment as well as the management actions in the Mussafah South Channel.

Methodology

Study area

Abu Dhabi is located in the Arabian Gulf and has an arid subtropical climate with an average annual rainfall of <82 mm (SCAD, 2018) along the coast. Mussafah is an industrial area situated at the seashore and characterized by the presence of many industries, marinas, dry docks for boat and ship repair, gas stations, oil storage facilities, and municipal drainage areas. Consequently, many different activities occur in the area, including painting, metal working, and dredging. The Mussafah South Channel is a dredged channel, with one end connected to the main Mussafah Channel and the other end closed. It is nearly 6 km long and 0.5 km wide with an average of depth of 7m. The channel is subject to wastewater discharge through 5 outlets, mainly from sewage treatment plants (400 000 m3·day−1 Rajan et al.) in the industrial area. Furthermore, the sides of the channel are lined with dry docks, oil storage units, and boat repair facilities (Figure 1).

Figure 1.

Study Area.

Figure 1.

Study Area.

Sampling

Sampling was performed in the center of the channel from 2006 to 2017. Water quality parameters such as salinity, temperature, dissolved oxygen, pH and chlorophyll a were measured from both the surface and bottom of the channel using a Hydrolab Surveyor III and DS5 (Hydrolab Corp., Lincoln, NE, USA), whereas the nutrients, biochemical oxygen demand, and total suspended solids were measured only at the surface using following methods accordingly APHA4500, APHA5210 and APHA2540. Phytoplankton samples were collected using a Niskin water sampler for the quantitative analysis. For the qualitative analysis, the phytoplankton were trawled horizontally during early morning hours using nets with a 20-micron mesh size (Hydro-Bios). The samples were immediately fixed using Lugol’s solution and stored in dark bottles for later identification of the phytoplankton species and diversity in the laboratory. A quantitative analysis of the phytoplankton was also performed in the laboratory and is represented as the number of cells per liter. The species identification and taxonomy were based on Carmelo (1997) and Cupp (1943).

Statistics

Pearson correlations and analyses of variance (ANOVA) were used to perform the statistical analyses (SYSTAT software) The study area was distinguished by classifications (Livingston, 1997a) for salinity (salinity stratification indices) and dissolved oxygen (dissolved oxygen indices). The eutrophication index for Abu Dhabi was calculated based on a formula originated by the Canadian Council of Ministers of the Environment (CCME; 2001) using the following parameters: nutrients (nitrate, phosphate, and ammonia), dissolved oxygen, and chlorophyll a. An index score between 0 and 100 was generated (where 0 represents the extreme eutrophic state) based on three factors: 1) the number of parameters that exceeded their respective maximum allowable concentrations (i.e. the scope); 2) how frequently the measured parameters exceeded the maximum allowable concentrations (i.e. the frequency); and 3) by how much the maximum allowable concentrations were exceeded (i.e. the amplitude). The Ambient Abu Dhabi Marine Waters and Sediments Specifications were used to specify the maximum allowable concentrations (Abu Dhabi Quality and Conformity Council (ADQCC), 2017b)

Results and discussion

Marine ecosystem eutrophication is one of the pressing problems in the Arabian Gulf, including the waters of Abu Dhabi (Rajan and Al Abdesalaam, 2008b). The problems of nutrient enrichment are apparent and the key parameters involved are nitrogen, phosphorous and chlorophyll a. The first sign of a eutrophication problem noted in Abu Dhabi was in the Mussafah South Channel during 2003 with an incident of algal blooms and corresponding fish kills (Rajan and Al Abdesalaam, 2006). The results of this current study are a part of a long-term marine water quality monitoring program by the EAD that began in 2006 and continues today.

The physicochemical parameters of the study area were recorded for 144 months, from January 2006 to December 2017. Because Abu Dhabi is an arid region transitional between tropical and subtropical, the water quality parameters were at extreme levels (Subba Rao and Al-Yamani, 1998). The summer and winter seasons are the governing authority of these waters. The ‘Shamal’, northwesterly winds found year around (summer Shamal and winter Shamal), brings the strongest winds, which also influence the hydrography and other ecological parameters of the area.

The surface water temperatures recorded in the Mussafah South Channel fluctuated from 18.93 °C to 35.09 °C, while the bottom waters showed slightly lower values (18.58 °C to 34.71 °C). The differences between the surface and bottom temperatures were minimal due to the shallow (7 m) nature of the area. However, the temperature showed very clear seasonal variations (Figure 2). The statistical analysis (ANOVA) showed significant differences between the months and seasons, but not between years (Table.1).

Figure 2.

Number of algal bloom incidents.

Figure 2.

Number of algal bloom incidents.

Table 1.

ANOVA to test the variation between months, seasons and years.

Parameters Source Sum of Squares df Mean Square F-ratio 
Temperature Year 43.926 11 3.993 0.176 0.999 
 Month 2830.956 11 257.36 168.169 
 Season 2406.857 802.286 179.394 
Salinity Year 1186.16 11 107.833 13.11 
 Month 121.969 11 11.088 0.681 0.755 
 Season 65.969 21.99 1.396 0.247 
pH Year 7.436 11 0.676 8.471 
 Month 0.933 11 0.085 0.657 0.776 
 Season 0.314 0.105 0.83 0.48 
Dissolved Oxygen Year 585.737 11 53.249 14.757 
 Month 69.57 11 6.325 0.841 0.599 
 Season 43.945 14.648 2.014 0.115 
Chlorophyll-a Year 11319.911 10 1131.991 10.633 
 Month 568.696 11 51.7 0.256 0.992 
 Season 122.778 40.926 0.213 0.887 
Nitrite Year 1.2655 10 1.2655 10.528 
 Month 1.64164 11 1492401.621 0.685 0.75 
 Season 2519148.462 839716.154 0.391 0.76 
Nitrate Year 7.07858 10 7.07858 11.185 
 Month 8.7514 11 7.95582 0.673 0.762 
 Season 3.44285 1.14762 0.99 0.396 
Ammonia Year 8.56988 10 8569875.065 29.916 
 Month 2414707.175 11 219518.834 0.214 0.996 
 Season 194224.203 64741.401 0.066 0.978 
Phosphate Year 1.71529 10 1.71529 16.967 
 Month 9199997.997 11 836363.454 0.342 0.974 
 Season 1052223.201 350741.067 0.149 0.93 
Silicate Year 2.85041 10 2.85041 13.054 
 Month 2.33322 11 2121105.427 0.472 0.917 
 Season 1.43992 4799739.339 1.122 0.343 
BOD Year 1373.232 10 137.323 5.912 
 Month 115.361 11 10.487 0.304 0.984 
 Season 34.057 11.352 0.345 0.793 
TSS Year 2396.389 10 239.639 7.513 
 Month 439.048 11 39.913 0.806 0.634 
 Season 150.813 50.271 1.034 0.38 
Parameters Source Sum of Squares df Mean Square F-ratio 
Temperature Year 43.926 11 3.993 0.176 0.999 
 Month 2830.956 11 257.36 168.169 
 Season 2406.857 802.286 179.394 
Salinity Year 1186.16 11 107.833 13.11 
 Month 121.969 11 11.088 0.681 0.755 
 Season 65.969 21.99 1.396 0.247 
pH Year 7.436 11 0.676 8.471 
 Month 0.933 11 0.085 0.657 0.776 
 Season 0.314 0.105 0.83 0.48 
Dissolved Oxygen Year 585.737 11 53.249 14.757 
 Month 69.57 11 6.325 0.841 0.599 
 Season 43.945 14.648 2.014 0.115 
Chlorophyll-a Year 11319.911 10 1131.991 10.633 
 Month 568.696 11 51.7 0.256 0.992 
 Season 122.778 40.926 0.213 0.887 
Nitrite Year 1.2655 10 1.2655 10.528 
 Month 1.64164 11 1492401.621 0.685 0.75 
 Season 2519148.462 839716.154 0.391 0.76 
Nitrate Year 7.07858 10 7.07858 11.185 
 Month 8.7514 11 7.95582 0.673 0.762 
 Season 3.44285 1.14762 0.99 0.396 
Ammonia Year 8.56988 10 8569875.065 29.916 
 Month 2414707.175 11 219518.834 0.214 0.996 
 Season 194224.203 64741.401 0.066 0.978 
Phosphate Year 1.71529 10 1.71529 16.967 
 Month 9199997.997 11 836363.454 0.342 0.974 
 Season 1052223.201 350741.067 0.149 0.93 
Silicate Year 2.85041 10 2.85041 13.054 
 Month 2.33322 11 2121105.427 0.472 0.917 
 Season 1.43992 4799739.339 1.122 0.343 
BOD Year 1373.232 10 137.323 5.912 
 Month 115.361 11 10.487 0.304 0.984 
 Season 34.057 11.352 0.345 0.793 
TSS Year 2396.389 10 239.639 7.513 
 Month 439.048 11 39.913 0.806 0.634 
 Season 150.813 50.271 1.034 0.38 

The surface water salinity fluctuated between 28.9 and 47.04 psu but the bottom waters were always high, with values above 70 psu. An increasing salinity trend was noted from the surface to the bottom, i.e. lower salinity values were recorded for the surface waters and increased with depth. This variation could be due to a higher concentration of nutrients and other salts deposited in the bottom sediments. The shallow nature of the area, low water circulation, high evaporation, minimum precipitation, and limited tidal movement (Rajan and Al Abdesalaam, 2008a). are other reasons for the high salinity concentrations. Because the salinity variation between the surface and bottom waters was more than 10 psu, the area has been classified as highly stratified.

The pH values of the Mussafah South Channel fluctuated between 7.05 and 9.08 in the surface waters and between 5 and 8.36 in the bottom waters. The mean values showed a minimum pH of 7.82 in 2011 and a maximum pH of 8.58 in 2016.The minimum was associated with the minimum dissolved oxygen values and anthropogenic activities such as land reclamation and filling activities in the channel. A range of potential water quality problems can cause changes in pH, and even small shifts in water pH in the marine environment can affect phytoplankton photosynthetic activity (Hinga, 2002). In addition, Ruttner (1963) has pointed out that high photosynthetic activity would cause pH values to increase due to bicarbonate degradation by carbonic anhydrase, which occurs concurrently with photosynthesis.

The dissolved oxygen values varied from 0.16 to 13.43 mg l−1 in the surface waters and from 0.02 to 6.59 mg l−1 in the bottom waters. Due to the area being filled with algal blooms, the dissolved oxygen concentrations were abnormal, particularly in the bottom waters, which were always hypoxic, with benthic organisms rarely present. The mean dissolved oxygen values recorded for the different years ranged between 2.15 and 9.34 mg l−1. Over the 12-year study period, comparatively low values were recorded during 2010 and 2011, possibly due to the filling activities in the channel performed during those years, which created further problems such as algal blooms and fish kills. Abnormal low dissolved oxygen conditions were noted throughout the study period, possibly due to the higher salinity values, higher temperatures, algal blooms, and more biochemical oxidation of organic matter. Redfield (1948) reported that oxygen solubility is higher at low salinities and temperatures. Similarly, Untawale and Parulekar (1976) found low oxygen levels when the salinity was high. Boto and Bunt (1981) related decreases in dissolved oxygen to increased dissolved organic matter in the water, which consists mainly of polyphenolic compounds (weak acids) that undergo direct oxidation, thereby lowering the dissolved oxygen levels. Furthermore, increased particulate and dissolved organic matter in the environment likely enhanced bacterial production and respiration, which also decreases dissolved oxygen levels (Teixeira et al., 1969; Jukubiec et al., 1971). Given these conditions, the Mussafah South Channel has been recognized as a hypoxic area.

The biochemical oxygen demand of the surface waters of the Mussafah South Channel varied from 0 to 33 mg l−1, with the yearly mean fluctuating between 3.24 (2015) and 12.33 (2011) mg l−1. The total suspended solids also varied between years, with the maximum (14.18 mg l−1) mean value recorded in 2011 and minimum (1.32 mg l−1) in 2007. The water transparency values also changed between periods, with a maximum values of 4 feet (depth) and minimum of 1.5 feet. The mean values between the years varied from 2.05 to 3.41 feet.

In the marine environment, nitrogen occurs in several different chemical forms such as dissolved inorganic nitrogen, which is the sum of dissolved nitrates (NO3) and nitrites (NO2), and ammonia (NH3). Other forms include dissolved organic nitrogen and particulate nitrogen. These different fractions of the nitrogen pool can be combined into a measure of total nitrogen. According to the observations in this study, the nutrient concentrations were never at normal levels during the sampling period (Figure 2). Nitrite values fluctuated between 0.004 and 199.98 Mol/L, with a maximum mean value of 60.72 Mol/L in 2011 and a minimum value of 1.18 Mol/L in 2007. The nitrate values oscillated between 2.90 and 866.06 Mol/L, and the phosphate concentrations varied between 1.68 and 98.24 Mol/L. The minimum mean phosphate concentration of 10.29 Mol/L was recorded in 2006, while the maximum of 44.29 Mol/L was in 2011. The ammonia concentrations varied from 0 to 285.36 Mol/L.

The silicate concentrations ranged between 2.15 and 105.60 Mol/L, with concentrations always higher in the channel and with limited fluctuations. This clearly shows that silica utilization in the environment was less than that of the nitrogen forms. This may be due the higher presence of dinoflagellates and cyanobacteria (blooms) in the area and a lack of diatoms, which require significant amounts of silica for the construction of their external cells. Most other plankton groups have minimal silica requirements. There have been several reports on the role of silica in overall primary production in near-shore waters. In general, silica availability can have important effects on phytoplankton composition such as leading to changes in the algal community from diatom dominance to other classes of algae (Egge et al., 1992).

The water quality parameters, particularly the nutrient levels (Table 3), observed in the reference area were lower than those recorded in the eutrophic Mussafah South Channel and comparable to values reported in other areas of the Arabian Gulf (Al-Ansi et al., 2002; Devlin, 2015. CPMR, 2009 and Al-Zahed, 2008). However, the nutrient levels in the Mussafah South Channel were many-fold higher than those in the reference area. One reason for the high nutrient concentrations in the study area may be that it also experienced frequent sand storms. This would agree with the findings of Paerl (1993, 1995), Nixon (1995), and Valigura et al. (1996), who suggested that the atmospheric supply of nutrients in marine environments is also an important source of nutrient enrichment. The anaerobic microbial decomposition of the deposited organic material would also increase nitrogen concentration in the channel. This scenario cannot be ruled out, because the bottom waters of the channel were hypoxic/anoxic throughout the study period (Table 3).

The chlorophyll a concentration recorded in the Mussafah South Channel fluctuated between 0.48 and 62.69 l−1, with the highest yearly mean value during 2015 along with the nitrate concentration (Figure 2). Among the nutrients, nitrate showed a relatively high positive correlation with the chlorophyll a levels. The statistical analysis (ANOVA) showed significant differences between the months and seasons, but not between years (Table.1).

The eutrophication index calculated using the CCME (2001) method indicated the Mussafah South Channel was eutrophic throughout the sampling period. All index values recorded since 2006 have been below 10, meaning the channel is highly eutrophic (Figure 3). However, recent values (2016 and 2017) have shown marginal improvement. This may be due to current restrictions on wastewater quality and reductions in the discharge of treated sewage into the channel.

Figure 3.

Eutrophication Index scores of Mussafah South Channel.

Figure 3.

Eutrophication Index scores of Mussafah South Channel.

Figure 4.

Variation of marine water quality parameters (mean values) during different years.

Figure 4.

Variation of marine water quality parameters (mean values) during different years.

Eutrophic ecosystems stimulate the growth of cyanobacteria (Oscillatoria sp.) and dinoflagellates, both of which form blooms that were apparent throughout the study period. The diatom population was very limited and present only a few times. In general, the channel was teeming with high biomass and few species throughout the study period, with only a total of 30 species recorded. The dinoflagellate population was represented by Prorocentrum micans, Prorocentrum sigmoides, Prorocentrum minimum, Prorocentrum triestinum, Gymnodinium sp., Gyrodinium sp., Protoperidinium sp., Protoperidinium steinii, Gonyaulax grindleyi, and Scripcella sp. Alexandrium sp. and Karena sp. were also recorded, but only a few times. The diatom population was represented by only a few species, Nitzschia sp., Cylindrotheca closterium, Thalassiosira sp., and Thalassionema nitzschioides. The quantitative analysis revealed that the phytoplankton biomass fluctuated between years, but not between the months and seasons. This could be due to variation in the distribution of nutrients and the nature of the bloom-forming species. It is worth mentioning that the nutrient concentrations in the channel also significantly varied between years (Table 1).

Harmful algal blooms (HABs) in the confined areas and coastal regions of the Arabian Gulf have been increasing in frequency, and many have been linked to eutrophication (Rajan and Al Abdessalaam, 2008a; Glibert (2007). Continuous monitoring of the Mussafah South Channel has indicated at least 111 blooms during the past 12 years. Most were caused by Oscillatoria sp., followed by P. minimum and P. triestinum, and once by Gyrodinium instriatum. Oscillatoria sp. dominated the summer blooms and Prorocentrum spp. dominated the winter blooms.

The presence of a continuous bloom in the study area demonstrates the deteriorated condition of the channel and the role of nutrients and other anthropogenic activities. This study’s results agree with the findings of many others, who have proposed that the worldwide increase in HABs is related to nutrient enrichment (e.g. Smayda, 1990 and Davidson, 2014). It is clear that nutrient inputs are leading to the occurrence of higher biomass algal blooms, and several clear examples have linked eutrophication to HABs (Anderson et al., 2002).

However, Glibert et al. (2005a) and Glibert (2007) have pointed out that it is difficult to identify the link between eutrophication and nutrient enrichment because not all eutrophic waters support HABs, and not all HABs occur in nutrient-rich water. In this study, the statistical analysis made between the nutrients and chlorophyll a concentrations showed no significant positive correlations (Table 2), although nitrate had a relatively high positive correlation with chlorophyll a (Table 2). In addition, it is important to note that the monitoring was performed not only in the Mussafah South Channel, but also in another 20 ecologically important areas. The nutrient and chlorophyll a concentrations recorded in these areas showed a linkage between the nutrients and chlorophyll a, because areas other than the Mussafah South Channel had lower nutrient and chlorophyll a concentrations and no HABs (EAD, 2017). In contrast, in the Mussafah South Channel, the nutrient and chlorophyll a concentrations were high and visually obvious blooms were recorded continuously.

Table 2.

Pearson correlation matrix between different marine water quality parameters.

  Temperature °c Salinity (psu) pH D.Oxygen (mg l−1Chlorophyll-a (1−1Secchi Disc (feet) Nitrite (1−1Nitrate (1−1Ammonia (1−1Phosphate (1−1Silicate (1−1BOD (mg l−1TSS (mg l−1
Temperature °c 1.000                         
Salinity (psu) 0.133 1.000                       
pH −0.081 −0.181 1.000                     
Dissolved Oxygen (mg l−1−0.152 0.064 0.505 1.000                   
Chlorophyll-a (1−1−0.038 −0.381 0.225 0.183 1.000                 
Secchi Disc (feet) −0.071 0.320 0.116 0.115 −0.392 1.000               
Nitrite (1−10.027 0.071 −0.401 −0.306 0.125 −0.209 1.000             
Nitrate (1−1−0.151 −0.479 0.108 0.037 0.466 −0.195 −0.014 1.000           
Ammonia (1−10.046 −0.215 −0.203 −0.024 0.373 −0.241 0.256 0.248 1.0000         
Phosphate (1−10.079 −0.202 −0.257 −0.558 0.041 −0.230 0.348 0.156 −0.091 1.000       
Silicate (1−10.175 −0.219 −0.242 −0.300 0.346 −0.271 0.274 0.271 0.325 0.415 1.000     
BOD (mg l−10.011 0.143 −0.419 −0.339 −0.023 −0.119 0.188 −0.086 −0.016 0.237 0.260 1.000   
TSS (mg l−10.202 0.079 −0.267 −0.34 0.091 −0.034 0.35 0.019 0.146 0.316 0.202 0.236 1.000 
  Temperature °c Salinity (psu) pH D.Oxygen (mg l−1Chlorophyll-a (1−1Secchi Disc (feet) Nitrite (1−1Nitrate (1−1Ammonia (1−1Phosphate (1−1Silicate (1−1BOD (mg l−1TSS (mg l−1
Temperature °c 1.000                         
Salinity (psu) 0.133 1.000                       
pH −0.081 −0.181 1.000                     
Dissolved Oxygen (mg l−1−0.152 0.064 0.505 1.000                   
Chlorophyll-a (1−1−0.038 −0.381 0.225 0.183 1.000                 
Secchi Disc (feet) −0.071 0.320 0.116 0.115 −0.392 1.000               
Nitrite (1−10.027 0.071 −0.401 −0.306 0.125 −0.209 1.000             
Nitrate (1−1−0.151 −0.479 0.108 0.037 0.466 −0.195 −0.014 1.000           
Ammonia (1−10.046 −0.215 −0.203 −0.024 0.373 −0.241 0.256 0.248 1.0000         
Phosphate (1−10.079 −0.202 −0.257 −0.558 0.041 −0.230 0.348 0.156 −0.091 1.000       
Silicate (1−10.175 −0.219 −0.242 −0.300 0.346 −0.271 0.274 0.271 0.325 0.415 1.000     
BOD (mg l−10.011 0.143 −0.419 −0.339 −0.023 −0.119 0.188 −0.086 −0.016 0.237 0.260 1.000   
TSS (mg l−10.202 0.079 −0.267 −0.34 0.091 −0.034 0.35 0.019 0.146 0.316 0.202 0.236 1.000 

Algal blooms also result from multiple interactions between the various ecological parameters. It is widely accepted that phytoplankton blooms are influenced by various physical, chemical, and biological factors. Many studies have addressed the effects of habitat on phytoplankton (Harding, 1994; Cloern, 1999; Hallegraeff and Fraga, 1998; Sin et.al., 1999). Glibert (2007) has pointed out that the ultimate success of a given species is regulated by the interactions between nutrient discharges, temperature, and other physical factors.

The EAD has recorded many fish kills in the Mussafah South Channel since 1998. The incidents in 2003 and 2004 were large-scale single-species (Nematalosa nasus) events, presumably caused by oxygen depletion due to poor circulation and continuous algal blooms in the channel. The channel has since been widened to increase water circulation, which has improved, and the number of fish kills since then have been reduced. Nevertheless, the current study recorded many fish kills (Nematalosa nasus), and investigations have shown (EAD, 2017) that high nutrient concentrations combined with high algae and depleted oxygen concentrations (0.02–0.16 mg l−1) were the cause.

The nutrient values we found in the study area were many-fold higher than those in other Abu Dhabi waters (Table 3), although the Pearson correlation coefficients calculated for the different parameters indicated no significant correlations. The reduction in nitrate and phosphate levels in the Mussafah South Channel during the higher concentration periods indicate it was probably due to the blooms. According to Reddy and Venkateswarlu (1986), cyanophycean algae dominate in polluted areas, while Premila and Rao (1977) have shown that blue-green algae (Oscillatoria nigroviridis) are indicative of waters affected by sewage in less polluted areas. Chang (1988) has described a Prorocentrum micans bloom in a New Zealand estuary that was coincident with increased nitrogen from upwelling. Riegman (1995) has also related the occurrence of HABS to macronutrient dynamics, and Anderson and Garrison (1997) have extensively studied the effect of nutrient loading on individual phytoplankton populations.

Table 3.

Mean values of different water quality parameters recorded in Abu Dhabi and other Arabian Gulf Countries

 
 

The nutrient enrichment in the Mussafah South Channel has been through both point and nonpoint sources, mainly through the continuous release of nutrient-rich treated sewage from the sewage treatment plants located in Mafraq, Abu Dhabi, and other outlets on the channel. According to the EAD’s estimate, treated sewage effluent discharges into the channel amounted to approximately 400 000 m3·day−1. This is in addition to the unidentified load of industrial effluents discharged. The shamal (a local wind) also brought a lot of Aeolian dust, which increased the nutrients in the channel. Nitrogen released from vehicles also contributed a significant amount. Land reclamation and urbanization are other important activities in Abu Dhabi that increase nutrient levels in the coastal waters. Along with this continuous nutrient enrichment, restricted water circulation, low tidal influences, and developmental activities along the channel boundaries have enhanced eutrophication levels to the point where the water is now highly stratified, and the bottom is hypoxic. These abnormal conditions have led to continuous algal blooms with their subsequent fish kills and an absence of benthic fauna.

Phytoplankton proliferation in the marine environment is dependent on the concentrations and ratios of the nutrients. The basis of eutrophication management is to limit the nutrient concentrations. Hence, prevention or mitigation approaches should be focused on efforts to reduce nutrient (nitrogen and phosphorus) inputs and thus deal directly with the primary cause of eutrophication. In response to growing concerns about eutrophication, concerted efforts have been undertaken in Abu Dhabi to reduce both point and diffuse nitrate and phosphate inputs into the marine environment. These efforts have included reducing the entry of nutrients through environmental impact assessments for coastal developmental projects, permit programs for point sources, waste disposal regulations, dredging and dredging material disposal regulations, and finally, reducing nutrient inflow by utilizing treated sewage water for irrigation purposes. In addition, based on hydrodynamic modeling studies, the EAD widened the channel to increase water circulation and restricted developmental activities along the border. Furthermore, to control the nutrient inflow in the channel, standards have been developed for ambient marine water quality (ADQCC, 2017a) and wastewater discharge (ADQCC, 2017b) These management actions have considerably reduced the inflow of nutrients into the channel, the water quality has improved, and the number and intensity of harmful algal blooms have been reduced.

Conclusions

Population growth and economic development are the main driving forces for eutrophication in the Abu Dhabi Emirate. Long-term monitoring also indicates that nutrient over-enrichment is of concern across all Abu Dhabi waters, especially in the waters surrounding Abu Dhabi City, including the Mussafah South Channel. It is safer and generally more economical to take early preventive measures to control eutrophication than it is to develop curative strategies later, when the water quality has already deteriorated. The case of the Mussafah South Channel should be taken as a model of the detrimental effects that can occur as a result of eutrophication due to human activities. Eutrophication cannot be solved by a single technical intervention. It requires a group of social, economic, and technical actions, and transparent research and monitoring activities are prerequisite to decision making. Due to the multiple sources of nutrient loading, resolving these issues will require the active participation of different levels of government (federal, emirate, municipal), industry, and the public. Lastly, transboundary influences must also be considered to properly manage eutrophication.

Acknowledgements

Authors would like to express their sincere thanks to the management of Environment Agency- Abu Dhabi for their support of this study. We thank the Secretary General of EAD for her interest and support. We are especially grateful to our colleague Mr. Shahid Bashir Khan for his timely support on statistical analysis needed for this research.

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