Monthly sampling of phytoplankton, chlorophyll a, temperature, salinity and nutrients was carried out in the Sea of Oman and in the Arabian Sea, from 2004 and onwards. In addition to time series data, historical data from 1976 to 2003 were collected from the Ministry of Agriculture and Fisheries Wealth reports. Annual averages of algal blooms occurrence along the coast of Oman showed three major components of seasonal and interannual fluctuations- the seasonal cycle, interannual fluctuations with approximate period of 8 years, and the interannual rising tendency. The dominant algal blooms species comprise both diatoms and dinoflagellates, with obvious dominance of Noctiluca scintillans followed by Cochlodinium polykrikoides and Prorocentrum sp.
The impacts of algal blooms have been on the rise in many coastal regions worldwide (Anderson, 1989; Hallegraeff, 1993). The marine ecosystem effects associated with algal blooms range from pelagic and benthic community mortalities to fish and shellfish aquaculture mortalities and include toxic blooms which lead to illness and death in humans, marine mammals, and other marine life (Anderson et al., 1994; Heil et al., 2005). In addition, non-toxic blooms can cause damage to marine ecosystems, fisheries resources, tourism and recreational facilities (Anderson, 1989; Hallegraeff, 1993). Environmental changes may affect algal blooms expansion and their impact to marine life in several ways. For instance, change in some environmental parameters such as temperature, wind stress and salinity could lead to their expansion, while other environmental changes could lead to the persistence of some algal blooms species and cause significant damages to marine life (Moore et al., 2008).
The increase in frequency and diversity of algal blooms and their impacts presents a significant challenge to those responsible for the management of coastal waters resources. Abundance and distribution of algal blooms in coastal water of Oman is poorly investigated. Although the data sets recorded along the coast of Oman provide valuable information on phytoplankton abundance and composition, such data lack continuity and are limited to basic parameters related to phytoplankton community. In order to achieve conservation and long-term sustainable use of marine living resources, we need to build a fundamental understanding of the coast through continuous monitoring and research that will allow us to differentiate between anthropogenic and natural variability.
In this study we examine changes in algal blooms events in Omani waters and assess the impacts of environmental changes driven by the seasonal monsoonal cycle and possible impact of anthropogenic activities on algal blooms outbreaks.
The oceanography of the coastal water of Oman is driven by the reversal monsoon wind system and the geostrophic features of the Arabian Sea and the Gulf Oman (currently Sea of Oman) (Coles and Seapy, 1998; Flagg and Kim 1998). The two components of the monsoon system are referred to as the North East Monsoon (NEM) and the South West Monsoon (SWM). The NEM winter season extends from November-February, during which sea surface winds over the Sea of Oman are predominantly northeasterly (Schottt et al., 1990) and the SWM season which extends from June-mid-September when sea surface winds over the region are predominantly from the southwest (Brock and McClain, 1992) and stronger than during the NEM (Burkill et al., 1993). During SWM coastal upwelling persists along the coast of Oman with stronger impacts in the southern part of the coast (Dhofar region) (Savidge et al., 1990, Coles and Seapy, 1998). The effects of upwelling can extend to about 750km offshore of Oman and can also be observed in the Sea of Oman through the injection of cool water which strongly affects temperatures profiles during summer (Coles and Seapy, 1998). The extent of intrusion of upwelled water into the Sea of Oman can be strongly impacted by high air temperature and the inflow of the water from the Persian Gulf, both of which lead to strong vertical stratification in the coastal areas (Quin and Johnson 1996). The seasonally monsoon wind plays a significant role in the productivity and dispersal of phytoplankton blooms along the coast of Oman (Al-Azri et al., 2007; Gomes et al., 2008; Al-Azri et al., 2010).
In situ and remote sensing data
The data presented in this study is a combination of the oceanographic time series established since 2004 along the coast of Oman and the historical data provided by the Ministry of Agriculture and Fisheries Wealth. Details of stations, methodology and some of the data used in this paper could be found in Al-Azri et al., 2007, 2010 and Piontkovski et al., 2011.
We collected data from three sampling sites, two of which were located in the Sea of Oman, Fahal (F) (23°40′N, 58°30′E) and Bandar Khairan (BK) (23°31′ N, 58°43′ E) and one in Arabian Sea at Masira Island (20°13′N, 58°45′E) from 2004 onwards (Figure 1). Data collected and processed have included phytoplankton taxonomy (identified using the following references: Cupp, 1947; Dodge, 1982 and 1985; Sournia, 1986; Ricard, 1987; Round, 1990; Tomas, 1997; Rita, 2005; Thonderson et al., 2007) temperature, salinity, and nutrients. Subsurface water samples representative of the mixed layer were collected at 1m depths with a 5-litre Niskin bottle for analysis of nutrients. Nutrient samples were frozen at −20°C and analyzed using a 5-channels SKALAR Flow Access nutrient auto-analyzer according to the procedure described in Strickland and Parsons, (1972) and modified by the manufacturer (Skalar analytical, 1996). Phytoplankton samples were collected in dark glass bottles, preserved with Lugol's iodine solution (Throndsen, 1978) and stored at 4°C. Samples were concentrated overnight where the whole sample were transferred into graduated cylinder for cells to settle and concentrated to 20 ml. 1 ml replicates of the concentrated samples were transferred onto a Sedgewick-Rafter counting chamber and observed under a Zeiss inverted light microscope at 200x and 400x magnification. The remotely sensed Chl-a time series from SeaWiFs scanner were used to analyse seasonal and interannual changes (http://oceancolor.gsfc.nasa.gov/ftp.html). Satellite derived (9-km spatial resolution SeaWIFS and MODIS-Aqua) weekly and monthly Level-3 products for sea surface temperature (SST) and Chl-a concentration were employed, to assemble time series (1997–2010). Time series of Chl-a and SST were acquired using the GES-DISC Interactive Online Visualization and Analysis Infrastructure (GIOVANNI) software as part of the NASA's Goddard Earth Sciences Data and Information Services Center.
Data on wind speed for the Sea of Oman were extracted from the NCEP Reanalysis database (Kalnay et al., 1996).
Annual algal bloom occurrences along the coast of Oman showed an increasing tendency from 1976 to 2011, with maximum occurrence in 2007, 2008 and 2010, respectively (Figure 2). Our observation shows 47 algal blooms events were reported from 1976 to 1999, while 76 algal blooms events were reported in the last decade (2000–2011). It was noticed that the increase in occurrence of algal blooms in the Sea of Oman was during the period of January to April and August to September (Figure 3), while in the Arabian Sea the increase in occurrence was during August to October. Major causative species of algal blooms (Figure 4) comprise both diatoms and dinoflagellates, with Noctiluca scintillans as the most dominant followed by Cochlodinium polykrikoides and Prorocentrum sp. Of the algal blooms recorded from 1976 to 2011, only 11 events were registered as harmful because of fish kill and impact on other marine life, while the rest did not cause any negative impact.
SeaWiFS data for surface chlorophyll concentrations downloaded from Giovanni GES DISC for the Sea of Oman exhibited seasonal fluctuations with a major peak in February–March, during the NEM (Figure 5). In remotely sensed time series, some data were missed for summer months due to cloud mask. For the whole time series, monthly changes were approximated by a cubic spline. In some ways, the remotely sensed peaks of Chl-a give a general picture of phytoplankton biomass fluctuations, but they do not resemble explicitly the algal blooms intensity and dynamics. In fact, Figure 5 shows the dynamics of the natural background biomass over seasons and years, and it is partially corresponding to the algal blooms. However, these dynamics are important as cases of intensive algal blooms can be defined as areas where chlorophyll concentrations significantly exceed the background biomasses.
In comparison to the Sea of Oman, the chlorophyll peaks of the Arabian Sea occur in August-September, during the SWM (Piontkovski et al., 2011). Monthly average of nutrients data shows seasonality of nitrate and phosphate (Figures 6a and b). During the NEM season nitrate showed high concentrations followed by a decline to lower concentrations. Phosphate concentrations were high during NEM season with slight decline during the SWM.
Temporal changes of algal blooms occurrences along the coast of Oman showed three major components featuring seasonal and interannual scales; the seasonal cycle, the interannual fluctuations and the interannual rising tendency.
Seasonal cycle has consisted of two extended time periods of maximal values tackling the time of winter and summer monsoons. The development of these periods is mediated by the wind fields, which in turn stimulates the intrusion of coastal upwelling during the SWM, while convective mixing and Iranian coastal upwelling mediate algal blooms during the NEM. Final ecological footprints of the wind activity might be exemplified by the relationships between the annual frequency of algal blooms and zonal or meridional components of the wind field (Figure 7).
In this figure, the meridional component of the wind has a positive value when the wind is blowing from south to north. A south wind has a positive meridional component, while a north wind has a negative meridional component. In case of the zonal component, values are positive when the wind is blowing from west to east. Thus, a west wind has a positive zonal component while an east wind is characterized by negative values.
Wind activity might have a bilateral ecological effect. On one hand, wind stress results in the nutrient enrichment along the coastal areas and increase in phytoplankton biomass (Devassy et al., 1978; Goes et al., 1992; De Souza et al., 1996). On the other hand, the periods of calm weather with low wind speed should lead to a pronounced thermal stratification of the water column. This should increase the sea surface temperature which might be accompanied by a decrease in oxygen concentrations, a warning sign of approaching fish kill incidents. The nutrients data collected in this study was short and very sporadic and therefore could not establish a clear relationship of nutrient changes and algal blooms increase of the bigger scale. Nevertheless the highest algal blooms occurrences are matching with highest nitrates and phosphates concentrations during 2004–2007 years.
As far as the interannual dynamics is concerned, the annual time series of algal blooms occurrence (Figure 2) has a visual periodicity of about 8 years. Indeed, one could notice maxima in 1994, 2002, and 2010. The mechanism underlying this periodicity is yet to be understood. Unfortunately, the length of the algal blooms time series is still far from being adequate for that kind of analysis. One might speculate however, that the above periodicity might be associated with oscillations reported for the basin scale climatic events- El-Nino and Indian Ocean Dipole- both affecting the Arabian Sea (Ali Khan et al., 2008; Saji et al., 1999).
The third component of describing changes is the long-term rising tendency in the number of registered algal blooms. Indeed, one could notice a pronounced tendency of the algal blooms annual occurrence to increase over years (Figure 2). This increase is associated with significant increase in harmful algal blooms species, including toxic and non-toxic over the past decade in association with changes in environmental condition. For instance, it was noticed that Noctiluca scintillans occurred on an annual basis (Figure 8), while mention of Cochlodinium polykrikoides which caused massive blooms in 2008, was the first recorded occurrence of this species blooming in the coastal waters of Oman. Previous studies (Thangaraja, 1990 and 1998) have reported some species associated with fish kill such as Gonyaulax sp., Karenia sp., Ceratium sp., Prorocentrum sp., Dinophysis sp.,Trichodesmium sp., and Noctiluca scintillans in the Sea of Oman. Our study reveals even more diversity (Figure 4) and abundance in algal blooms species with two main types of phenomena. For instance, species such as Noctiluca scintillans, Trichodesmium sp., Cochlodinium sp., exhibit intense accumulation in the surface water causing water discoloration or the formation of unpleasant foam, while the rest of algal blooms species tend to occur in high number without water discoloration (such as Chaetoceros sp., Pseudonitzschia sp.) or occur in low number but could produce potent toxins such as Karenia sp., Gonyaulax sp., Dinophysis sp., and Gymnodinium sp.
The coastal waters of Oman have been exposed to a variety of environmental pressures such as the inter-annual variability in monsoon winds forcing and their role in anomalously strong and persistent upwelling in the south of Oman. In addition, manmade effects such as, coastal developments, industry, desalination and fertilizer plants, represent additional sources of nutrient enrichment to coastal waters. Given the increase in algal blooms frequency and dispersal in the Sea of Oman and Arabian Sea and their impacts on the marine food chain, this clearly illustrated the urgent need for more dedicated time-series programs to monitor algal bloom occurrences and understand the physiological response to changes in the surrounding environments.
The results in the present study show that the occurrence of algal blooms along the coast of Oman is on the rise. This increase is associated with significant increase of phytoplankton species and more diversity including toxic and non-toxic species with the dominance of Noctiluca scintillans. The intrusion of coastal upwelling during the SWM in the Arabian Sea and the convective mixing and Iranian coastal upwelling in the Sea of Oman are considered as the forces triggering algal blooms in the Sea of Oman and Arabian Sea, respectively. The variety of environmental pressures and the physiological response of algal blooms in the coastal waters of Oman still remain as open questions that warrant further investigation.
This work was supported by research grants from Sultan Qaboos University, Agricultural and Fisheries Development Fund, and the Research Council (IG/AGR/FISH/07/01, IG/AGR/FISH/09/01, IG/AGR/FISH/10/01, and ORG/EBR/09/004).We are grateful to technical support from the department of Marine Sciences and Fisheries and the crew of R/V “Al-Jamiah.”