Remote sensing of phytoplankton distribution in the Red Sea and Gulf of Aden

Remote sensing of phytoplankton distribution in the Red Sea and Gulf of AdenALKAWRI Abdulsalam1*, GAMOYO Majambo2

1 Department of Marine Biology and Fisheries, Faculty of Marine Science and Environment, Hodiedah University, Hodiedah,Yemen

2 Coastal Oceans Research and Development in the Indian Ocean, P.O. Box 10135, Mombasa 80101, Kenya

Received 28 April 2013; accepted 13 May 2014

©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014

AbstractAnalysis of high-resolution 4 km sea surface temperature, Chlorophyll a (Chl a), and wind datasets provides a detailed description of the spatio-temporal seasonal succession of phytoplankton biomass in the Red Sea and Gulf of Aden. Based on Moderate Resolution Imaging Spectroradiometer on-board aqua platform (MO-DIS Aqua) data andsynoptic observations in the Red Sea, Chl a varies from north to south, with the northern part appearing to be oligotrophic. This is likely due to the absence of strong mixing and low nutrient intru-sion in comparison to the southern part during winter. In the Gulf of Aden, the emergence of upwelling cell is clearly evident along the coast of Yemen, and is only distinct from the summer-autumn seasons. Most notable is the pulsating nature of the upwelling, with warm and cold events clearly distinguished with phy-toplankton response to this physical forcing also evident. The phytoplankton biomass distribution varies considerably between the two regions of study. In both study areas, water temperature and prevailing winds control nutrient concentrations.Key words: Red Sea, Gulf of Aden, Phytoplankton, chlorophyll a, SST

Citation: Alkawri Abdulsalam, Gamoyo Majambo. 2014. Remote sensing of phytoplankton distribution in the Red Sea and Gulf of Aden. Acta Oceanologica Sinica, 33(9): 93–99, doi: 10.1007/s13131-014-0527-1

1 IntroductionThe Red Sea and Gulf of Aden region is a complex and unique

tropical marine ecosystem, with extraordinary biological di-versity and a remarkably high degree of endemism. It is one of the most important repositories of marine biodiversity on a global scale and supports a particularly high diversity of corals (Pilcher, 2003). Unfortunately, limited scientific literature exists for describing the biological dynamics of the Red Sea (Alkershi and Menon, 2011). Some research has been performed on the biological and physiological oceanography of the Gulf of Aqaba (Eilat), the eastern extension of the northern Red Sea. This work indicates that the Gulf of Aqaba exhibits a strong spring bloom and a weaker fall bloom, both of which exhibit considerable in-terannual variability (e.g., Labiosa et al., 2003).

The Gulf of Aden is one of the least well known areas of the Indian Ocean in terms of its biology. It holds fishery resources of international importance due to the upwelling of cool, nutri-ent-rich water during the southwest and northeast monsoons and is characterized by a prevailing high-energy climate (Sand-ers, 1981a, b; Sanders and Morgan, 1989; MEP, 1998). Along the Gulf of Aden, the coast of Yemen, studies have included ocean-ographic research (Wilson and Klaus, 2000), fishery investiga-tions (Sanders, 1981a, 1981b; Sanders and Morgan, 1989; MEP, 1998), and various biological assessments (Wilson and Klaus, 2000). Little is known about the coastal and marine resources of the Gulf of Aden off Somalia. Compared with the extensive research on phytoplankton productivity carried out in the off-shore oceanic waters of the Arabian Sea during the Internation-al Joint Global Fluxes program (Smith, 2001; Marra and Barber, 2005), phytoplankton variability in the coastal waters of Yemen

and Oman is poorly known. There is a lack of a dedicated pro-gram with systematic sampling along the coast at regular time intervals: no information is available on seasonal and interan-nual variability of phytoplankton communities.

One important factor distinguishing the Red Sea and the Gulf of Aden is the influence of the Arabian Ocean monsoon. In general, the region south of 20°N latitude is subject to the influ-ence of both the spring and fall monsoon, which affect the pre-vailing wind direction over the southern Red Sea, and also the transport of dust from adjacent desert regions (Edwards, 1987). The northern Red Sea is not notably affected by the monsoon (Sheppard et al., 1992). Along the coast in summer, strong sea breezes build up and strike the coast obliquely due to the influ-ence of the prevailing winds (Sheppard et al., 1992).

The primary production in the entire Red Sea increases from north to south, consistent with distribution of nutrients (Koblentz-Mishke et al., 1970; Weikert, 1987). Productivity in the range of 170 mg·m−2·d−1 in the central Red Sea is cited by Do-widar (1983) and Weikert (1987). Koblentz-Mishke et al. (1970) indicated that average annual production north of 17° N ranges from 250–500 mg·m−2·d−1. The existing literature provides little detailed information regarding annual cycles and seasonal pro-ductivity in the northern Red Sea. The concentration of chloro-phyll in general increases in the southern region of the Red Sea relative to the northern region (Acker et al., 2008).

Using satellite data, we have made an attempt to address some of these concerns, mainly investigating chlorophyll a (Chl a) distribution, the fundamental element of primary pro-duction, and a proxy for phytoplankton biomass. In general, the primary goal of the present study was to understand the sea-

Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–99

DOI: 10.1007/s13131-014-0527-1

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*Corresponding author, E-mail: [email protected]

ALKAWRI Abdulsalam et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 9, P. 93–9994

sonal cycle of phytoplankton and the extent of coastal physi-cal–biological coupling in the Red Sea and Gulf of Aden, which supports a large fisheries industry (Al-Jufaili et al., 2006).

2 Data and methods

2.1 Study areaThe study focused on the Red Sea and Gulf of Aden (Fig. 1).

The Red Sea, which is also a large marine ecosystem (LME), is an important economic and environmental asset. It’s a narrow oceanic basin which is meridionally elongated about 2 250 km, bordered by the African and Asian continental shelves. At its northern end, the Red Sea connects with the Mediterranean Sea through the Suez Canal, while the southern region exchanges its waters with the open ocean (the Gulf of Aden and the Ara-bian Sea via the strait of Bab-el-Mandeb (Sofianos and Johns, 2002). The southern region of the Red Sea is considered to be among the hottest regions in the world. The air temperature in the northern region is slightly lower than in the southern region. Rainfall in the Red Sea region is extremely sparse and is usu-ally localized in the form of short showers. The entire stretch of the Red Sea lacks inputs from rivers or stream sources. The Gulf of Aden is located in the Arabian Sea between Yemen, on the south coast of the Arabian Peninsula and Somalia, in the Horn of Africa.

2.2 Satellite remote sensing dataFor this study, monthly L3 daytime-derived chlorophyll a

and SST data at 4 km were used, covering the 2003–2011 pe-riod. The seasonal time windows in this study were defined as Winter: December–March; Spring: March–May; Summer: May–September; Autumn: September–December. The dataset came from Moderate Resolution Imaging Spectroradiometer (MODIS) on board Aqua platform. Level 3 data are the derived

geophysical variables mapped to uniform grid scale. To display spatial distribution and analysis of Chl a and SST, we used an online tool known as Giovanni. Geovanni is a web application tool developed by the Goddard Earth Sciences Data and Infor-mation Services Centre (GES DISC) that provides a simple and intuitive way to visualize, analyse, and access vast amount of Earth science remote sensing data without having to download the data. Giovanni is an acronym for the Geospatial interac-tive Online Visualization and Analysis Infrastracture (Acker and Leptoukh, 2007).

Besides temperatuere, wind plays a crucial role in determin-ing the spatial and temporal variability of Chl a by advection and Ekman pumping. To describe the air–sea interaction in the study area, we generated sea surface wind fields covering the period from 2003–2011 over the study area using QuikSCAT monthly data available at 0.25 degree spatial resolution.

3 Results

3.1 Chlorophyll a distributionGenerally speaking, coastal regions tend to have higher

chlorophyll a than the open oceans/seas. The spatial distri-bution of chlorophyll a in the Gulf of Aden and Red sea are il-lustrated by the annual chlorophyll a climatology (2003–2011) (Fig. 2). Chlorophyll a concentration was relatively high (>2.5 mg·m-3) south of the Red Sea while the northern part exhibits oligotrophic conditions, splitting the red sea almost in half. In the Gulf of Aden area, chlorophyll a is relatively high nearshore along the Yemen coast, and is lower in the open sea.

The succession of chlorophyll a for the two regions (Red sea and Gulf of Aden) can be seen from the seasonal plots. Overall, the two study areas depict a strong seasonal pattern with maxi-mum concentrations seen during the winter time and mini-mum during spring and summer.

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3.2 Sea surface temperature (SST)SST climatology (2003–2011) clearly shows the distribution

of water temperature in the study area (Fig. 3). It is evident from the climatology plot that the southern part of the Red Sea and the Gulf of Aden exhibit high SSTs of 27–30°C, with the upwell-ing cell along the coast of Oman also being evident. SST is high-est (>28°C) during summer, and is distributed evenly across the whole gulf of Aden. During the winter monsoon, SSTs are low (<26°C) in the gulf of Aden and along the strait.

3.3 Wind pattern The prevailing winds over the northern region of the Red Sea

do not exhibit significant seasonal variations. However, there is seasonal variations both in wind direction and speed over the southern half of the Red Sea. Winds over the southern Red Sea are in a northward direction during the winter monsoon at about 18°N, where a strong convergence zone exists (Fig. 4). This convergence zone marks the boundary between the mon-soon-dominated atmosphere in the south, and continental at-

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Fig.2. Modis aqua Chl a concentration of the study area for the period 2003–2011. The upper single plot (a) represents annual mean/climatology while the double plots below represents the seasons (b. winter, c. spring, d. summer and e. autumn).

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mosphere in the north. During summer (May–September), the convergent wind pattern is replaced by weaker northweaterly winds over the entire Red Sea basin. During summer, there are strong southwesterly winds that appear in the Gulf of Aden and over the Arabian Sea. These southwest monsoon winds results induce the Ekman transport, which causes upwelling along the coasts of Yemen and Oman.

4 DiscussionThe southern part of Red Sea and the Gulf of Aden have

characteristics of mesotrophic waters receiving relatively high energy from solar radiation, which creates favorable conditions for the photosynthesis of phytoplankton, and also experience a seasonal reversal monsoon.

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4.1 Seasonality of Chl a in the Red SeaThe annual Chl a climatology (2003–2011) reveal distinct

distribution patterns with the northern part exhibiting oligo-trophic conditions (Chl a concentrations <1 mg/m3), and the southern part being mesotrophic (Chl a concentrations > 1 mg/m3), splitting the area into almost two (Fig. 2a). The seasonality of phytoplankton biomass can be seen from the monthly means of Chl a for the 2003–2011 period (Figs 2b, c and d). The Red Sea Chl a depicts a strong seasonal pattern with high concen-trations seen during winter time, reaching their lowest during summer, with autumn being the initiation period, and spring being the termination period. The high Chl a concentrations in the southern region of the Red Sea during winter time might be attributed to vertical mixing and intrusion of nutrient-rich waters from the Gulf of Aden (Salem et al., 2001). The seasonal changes of the Red Sea exchange through the strait is driven primarily by the seasonal change in winds (monsoon) over the southern Red Sea and Gulf of Aden (Fig. 4).

The highest Chl a values occuring in the winter season in-dicates that Chl a in the Red Sea exhibits a significant negative correlation with Sea surface temperature (SST). Fahmy (2003) indicates that productivity in the Red Sea is controlled by phos-phate concentrations and is also related to temperature and sa-linity variability.

In summer, there appears to be a seasonal pattern in which slightly increased Chl a occurs near the southern boundary of the Red Sea. Acker et al., 2008 linked this phenomenon to the strength and timing of the monsoon in the Arabian Sea. It is therefore possible that effects of the monsoon could extend into the Red Sea. On the other hand, during the summer in the Red Sea region, sea breezes caused by marked temperature dif-ferences between the land and ocean waters strengthen during the day. These sea breezes strike the coast obliquely due to the influence of prevailing winds (Sheppard et al., 1992). Elevated wind events have been shown to cause sediment resuspension and transport on carbonate banks (Acker et al., 2002), so it is likely that elevated wind speeds would also cause sediment re-

suspension in the Red Sea reef complexes. Sediment resuspen-sion increases bottom reflectance (Acker et al., 2002) and resus-pended sediments can also be a significant source of nutrients to the water column (Fanning et al., 1982). Red Sea coral reef waters have significantly higher nutrient concentrations than the open Red Sea waters, due primarily to bacterial respiration of organic matter in the sediments (Rasheed et al., 2002).

SST in the Red Sea increases from north to south, and de-creases in the far southern Red Sea to the straits of Bab al Man-deb. Winter to summer SST variability is greater in the northern Red Sea than in the southern Red Sea, and SST distribution in the southern Red Sea is markedly influenced by the wind direc-tion induced by the monsoon. The water column throughout the Red Sea is highly stratified, with a thermocline extending from 50 mto 250–300 m (Edwards, 1987). The stratification is generally weaker in the winter than in the summer. Below 250–300 m, the entire Red Sea basin is comprised of water with extremely uniform salinity of 40.6‰ (Edwards, 1987). The ho-mogeneity of the deep water of the Red Sea, forming a persis-tent thermocline and halocline, is the primary reason nutrient renewal from deeper waters is suppressed, particularly in the northern Red Sea. The biological productivity of surface waters is therefore dependent on the nutrient concentrations found in the upper 200–300 m of the Red Sea water column (Acker et al., 2008).

4.2 Seasonality of Chl a in the Gulf of AdenThe seasonal cycle of Chl a, in the Gulf of Aden and coast

of Oman, is dominated by two periods of high concentrations: January–March and August–October. These months correspond to the end of the southwest monsoon (SWM) and northeast monsoon (NEM) (Banse and English, 2000; Marra and Barber, 2005). It is unclear whether the distribution of this bloom ob-served by us is strictly regulated by seawater temperatures, or if factors such as water column stratification, nutrient availability and grazing play a part in this horizontal gradient.

In summer, Chl a content is relative high and uniform

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Fig.4. Red Sea and Gulf of Aden climatological QuikSCAT wind speed (m/s) winds for January (winter) and June (summer) at the height of the NE and SW monsoon seasons. The black arrows highlight the different wind direction between the two seasons, where-as the red arrow indicates the intrusion of water masses through the facilitation of northwards winds.

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in the whole Gulf (Fig. 2); very high Chl a concentrations are limited only to a narrow band along the coast. Because of the strong SWM that blows parallel to the coast, the Ekman trans-port produces a net movement of surface water 90 degrees to the right. Stratification vanishes and vertical mixing develops during this period. Upwelling brings nutrients from deep wa-ters to surface waters, and these nutrients then support phyto-plankton blooms. Also, the most distinct variations in SST are the sharp drops observed during this period. Coles and Seapy (1998), Claereboudt et al. (2001), and Al-Azri et al. (2010) attrib-uted this to the upwelling and cyclonic eddies as cause; deeper nutrient-rich waters to shoal may be responsible for the abrupt decreases in SST during this period. From March to May, with weak winds and large sea surface heating, water stratification is well developed, which results in relative uniform SSTs in the en-tire Gulf. This water stratification limits upwelling of nutrients and phytoplankton growth. High Chl a concentration along the coastal waters may also be due to nutrients from runoff from coastal cities and some small rivers that discharge into the gulf.

During SWM, water-masses are enriched with nutrients brought up by monsoon upwelling during June-September. At the end of September, diatom blooms are triggered. During December, nutrients (specially Nitrate) is depleted and dino-flagellate blooms appear (Gomes et al.,2008). The satellite Chla data indicate that the largest winter blooms in the southern Red Sea, the Gulf of Aden, which extend to the Gulf of Oman, were formed in January and persisted until March, where winter convective mixing is the strongest. Many studies reported that the Arabian Sea is a prominent site for the appearance of the blooms of Noctilucamiliaris (Devassy, 1987; Gomes et al., 2008; Matondkar et al., 2012). However, N. miliaris appears in coastal waters (Devassy 1987) as well as in the open ocean (Gomes et al., 2008). In the coastal waters along the West coast of India, bloom is recorded during SWM, and in the open ocean and north east Arabian Sea, during NEM (Feb-Mar period). More recent studies report the appearance of N. miliarisin the SWM (Sahayak et al., 2005) when colder, oxygen-poor, deep water is brought to the surface by coastal upwelling. Although we don’t have any in situ data, perhaps a similar event is taking place in the Gulf of Aden, which may extend to the southern region of the Red Sea.

Gomes et al. (2008) suggested from their satellite altimetry data a strong coupling between N.miliaris blooms and meso-scale eddy activity in the north Arabian Sea. They hypothesized that cyclonic cold eddies, through their ability to bring up nu-trients and oxygen-poor subsurface waters from the depths (100–300 m), are facilitating the genesis and evolution of N. miliaris blooms in the north Arabian Sea. In a recent review, Wiggert et al. (2005) have emphasized the need to ascertain the importance and role of mesoscale eddies in the biogeochem-istry of the Arabian Sea. A few observations from the Arabian Sea JGOFS program cite the enhancement of Chla during the passage of a cold eddy (Dickey et al., 1998), although this phy-toplankton bloom was not characterized taxonomically. A study similar to that of Crawford et al. (2005), which details the influ-ence of mesoscale eddies in the eastern Gulf of Alaska on pri-mary productivity and their role in advecting phytoplankton-rich coastal waters offshore into the deep-sea region, would be of immense benefit.

Remote-sensing data, such as that provided by MODIS-Aqua, provide a new capability for examination of the Red Sea

and Gulf of Aden, despite the lack of in situ data to support the satellite observations, and only a few references which discuss processes here. For that reason, inferences regarding the cau-sation of the patterns observed in the remote-sensing data are necessarily provisional. The observations in this paper can pro-vide an introductory framework for further investigation of this region, perhaps utilizing other types of remote-sensing data and data with increased spatial and temporal resolution.

AcknowledgementsChlorophyll a and sea surface temperatures analysis and

visualizations used in this paper are produced using the Geo-vanni online data system, and developed and maintained by the NASA GES DISC.

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