Sergey A. Piontkovski

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

Field sampling, remote sensing and modelling were employed to understand the seasonal and interannual changes of chlorophyll-a concentrations in the Gulf of Oman in comparison to open sea regions. In these regions, maximal chlorophyll concentrations were reported during the summer monsoon (with peaks in June and August), while in the Gulf of Oman, the chlorophyll maximum was observed during the winter monsoon (February–March). From 1997 through to 2008, the interannual variability in chlorophyll-a concentrations in the Gulf of Oman has not exhibited pronounced trends and neither have the other two (oceanic) regions in the western Arabian Sea. However, an increase of the annual variation in chlorophyll concentrations over the years was noticed. The diatom biomass decreased two-fold from 1997 to 2007. Nitrate concentration and mixed-layer depth also declined. In comparison to the seasonal blooms driven in the Gulf of Oman by the dinoflagellate Noctiluca scintillans, the year 2008 was markedly different. The summer bloom was shifted to September; it was gradually extended in time and formed by the other species. An applicability of the concept of ecosystem regime shift is discussed.

Interannual Changes in the Sea of Oman Ecosystem

Historical data were used to estimate interannual tendencies for the Sea of Oman over the past 50 years. Declining trends were shown for the atmospheric surface level pressure in the region affected by the Siberian High atmospheric anomaly, the zonal component of wind speed, concentration of nitrates, biomass of diatoms and sardine landings. Changes in Zooplankton biomass showed no clear trend. These trends were associated with and accompanied by rising atmospheric temperature, sea surface temperature, annual variability of the kinetic energy of mesoscale eddies, frequency of fish kills (along with the death of other animals- dolphins, turtles, and sea birds) and harmful algal bloom incidents. In terms of interannual coupling between physical and biological processes, the evaluated trends imply that the weakening of the Siberian High atmospheric anomaly results in the decline of the zonal wind speed and a regional increase of air temperature. This in turn increases the temperature in the mixed layer which strengthens thermo-haline stratification of the water column. Increasing stratification prevents the penetration of nutrients into the mixed layer and does not favor the interannual increase of biological productivity, although annual variation of biological productivity has increased, from 1997 to 2008.

Interannual changes of the Arabian Sea productivity

Abstract Inter-annual changes in temperature and chlorophyll a across the Arabian Sea (subdivided into 61 2-degree regions) were analysed. For each 2-degree region, from appropriate databases, remotely sensed chlorophyll a, sea surface temperature, and wind speed time series were retrieved. Spatial and temporal trend analysis showed physical–biological oscillations with dominant periods of 12 and 6 months (reflecting the seasonality of monsoonal winds) with a globally warming trend, but no overall increase in chlorophyll during the period 1997–2009. Variation coefficients of the inter-annual time series of chlorophyll a implied high variability in western regions of the sea in comparison to eastern regions. The basin-wide maps of chlorophyll distribution did not show the enlargement of the productive area over time and overall, not only did the Arabian Sea not get more productive, but several regions in its eastern basin showed a decline in chlorophyll a concentration.

The occurrence of algal blooms in Omani coastal waters

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.

Mesoscale eddies and variability of chlorophyll-a in the Sea of Oman

Satellite-derived (TOPEX/Poseidon, Jason-1, Jason-2, SeaWIFS (Sea-Viewing Wide Field-of-View Sensor) and MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua) weekly and monthly products for sea surface height, sea surface temperature (SST) and chlorophyll-a (CHL) concentration were used to analyse the trends of physical–biological coupling. In the variability of chlorophyll, no interannual trends were found for the Sea of Oman (1997–2008). However, the variation of chlorophyll within the annual cycle has increased. A similar tendency was evaluated for the variability of the energy of mesoscale eddies. The median level of kinetic energy and the coefficient of variation of this energy within the annual cycle both increased from 1997 through 2008.

The Omani shelf hypoxia and the warming Arabian Sea

Interdecadal changes in oxygen depletion, with a special reference to artisanal landings of large pelagic fishes, were analysed. Data from 53 expeditions incorporating 29,043 vertical profiles of temperature and 2114 of dissolved oxygen implied an increase in temperature of 1.2 °C over the past 50 years in the upper 30 m layer of sea water during the south-west (summer) monsoon. The thermal stratification of the water column increased and the oxycline shoaled from 153 m in the 1960s to 80 m in the 2000s. Concentration of dissolved oxygen <3.5 mL L−1 is known to induce symptoms of stress for many tropical pelagic fishes, compressing them within upper layers and exposing them to fishery. The habitat compression by the Oman shelf hypoxia has two components: a seasonal oxycline shoaling and an interdecadal trend.

Influence of a Tropical Cyclone Gonu on Phytoplankton Biomass (Chlorophyll a) in the Arabian Sea

Tropical cyclones could gradually affect the physical, chemical, and biological processes in the upper layer of the ocean. In terms of biological consequences, the cyclone wind field causes local mixing which results in the injection of nutrients into the upper layer of the ocean and triggering phytoplankton bloom (Subrahmanyam et al. 2002). In some cases, the magnitude of the hurricane-induced bloom could reach a gradual (30-fold) increase in the surface chlorophyll a concentration, as well as an increase in the primary production (Lin et al. 2003; Smitha et al. 2006). In the regions where cyclones often occur, their propagation could chiefly influence the annual productivity of the ocean. For example, an average of 14 cyclones pass over the South China Sea annually, which suggests the contribution of cyclones to annual production to be as much as 20-30% (Lin et al. 2003).

Understanding how physical-biological coupling influences harmful algal blooms, low oxygen and fish kills in the Sea of Oman and the Western Arabian Sea

In the last decade, green Noctiluca scintillans with its symbiont and other dinoflagellates such as Cochlodinium polykrikoides, Prorocentrum micans and Scrippsiella trochoidea have become the dominant HABs, partially replacing the previously dominant diatoms and red Noctiluca scintillans, especially during the northeast monsoon. Fish kills in the Sea of Oman are linked to a slow seasonal decline in oxygen concentration from January to November, probably due to the decomposition of a series of algal blooms and the deep, low oxygen waters periodically impinging the Omani shelf. In the western Arabian Sea, cyclonic eddies upwell low oxygen, nutrient-rich water and the subsequent algal bloom decays and lowers the oxygen further and leads to fish kills. Warming of the surface waters by 1.2°C over the last 5 decades has increased stratification and resulted in a shoaling of the oxycline. This has increased the probability and frequency of upwelling low oxygen water and subsequent fish kills.

Seasonal and interannual changes of Indian oil sardine, Sardinella longiceps landings in the governorate of Muscat (the Sea of Oman)

Monthly data on Muscat’s landings of the Indian oil sardine, Sardinella longiceps, along with 23 environmental parameters ( sea surface temperature, temperature of the mixed layer, wind speed, kinetic energy of mesoscale eddies, concentration of nitrates, dissolved oxygen, chlorophyll-a, abundance of phytoplankton, zooplankton, and several others) were analyzed for the period 1994–2011. Seasonal changes were associated with the time of t h e winter (Northeast) monsoon, with maximal landings in February. The multiple regression analysis of the statistically signifi cant variables selected through the Principal Component Analysis has implied that 51% of seasonal variability in sardine landings might be approximated by the seasonal variations of the zonal component of wind speed and chlorophyll-a concentration in the coastal and open-sea regions. In terms of interannual changes, sardine landings exhibited a declining trend from 2001 to 2011 (the time covered by the most complete data set). Rising sea temperature, thermal stratifi cation of the water column, and the trophic pressure imposed on sardine populations by large pelagic predators (talang queenfi sh, Scomberoides commersonnianus; kingfi sh, Scomberomorus commerson; longtail tuna, Thunnus tonggol; and some others) might be the factors mediating this trend. Introduction Oman is one of the most important countries engaged in fi shing in the Middle East. The 3,240 km coastline, with a commercial fi shing area of 350,000 km2, has rich fi shing grounds, the potential of which has yet to be fully evaluated. A 200 nmi exclusive economic zone extending seaward from the baseline from which the territorial waters are determined, has been declared. Omani fi sheries may be divided into two broad categories, traditional (artisanal) and commercial, with the former representing the cornerstone of the national industry and accounting for 96% of landings (Fishery Statistics Book, 2011). Given the high abundance of fi sh in Omani waters and its importance to the livelihood of thousands of people, the fi sheries sector is a signifi cant sector in the Omani economy. There is a strong fi shing tradition in Oman, and a large number of small villages scattered along the coast, from which, in 2011, around 40,161 fi shermen were directly employed in the fi sheries sector operating 18,731 fi shing boats of which 96% were fi berglass, 8–10 m in length (Fishery Statistics Book, 2011). Small pelagic fi sh are an important component of the artisanal fi shery in Oman (Al-Barwani et al., 1989). In some regions, coastal pelagic fi sh contribute directly to human consumption of fresh, dried, canned, smoked, or frozen fi sh, hence a large proportion of protein and nutrition needed for poor communities, as well as providing income for fi shermen. Purse seining is a major fi shing method for these species, including large-scale fi shing for conversion of fi sh into fi sh meal and oil (particularly by Peru and Chile). In Omani waters, small pelagic fi sh, including the Indian oil sardine, Sardinella longiceps, occur in large quantities (Haleem et al., 2011). Small pelagic stocks in Oman waters appear to be sustainably fi shed at the moment, and research surveys suggest that there is room for some further expansion in the Arabian Sea. These species form an important component of the marine food web because they comprise the bulk of the forage for large fi sh and other predators (Al-Barwani et al., 1989). They also contribute signifi cantly to the Omani marine fi shery: in 2011 about 27,931 metric tons (t) of sardines were landed along the coast of Oman valued at 5.8 million RO (~15 million USD). Sardines are exploited primarily with beach seines, cast nets, and gill nets. Small-scale sardine purse seining is already conducted in some locations (in particular in south and north Al-Batinah regions) along the Sea of Oman as a result of a recent modifi cation by adding rings and a drawstring to gillnets, thereby forming encircling gill nets. Because they require little investment in manpower and equipment, and are effi cient in catching pelagic fi sh, encircling gillnets are widely used by traditional fi shermen. Due to sardine schooling behavior (Misund et al., 2003), beach seines, purse seine, and encircling gillnets are an effi cient and effective method of harvesting. Schooling can also be facilitated through fi sh aggregating devices (FAD’s) emitting strong light at night (Fréon and Dagorn, 2000). Fishmeal and oil are essential ingredients in feeds used by the aquaculture industry, which grew at an average rate of 8–20% per year. If this trend continues, the demand and

Recent Outbreaks of Harmful Algal Blooms Along the Coast of Oman: Possible Response to Climate Change?

The Sultanate of Oman has an extensive coastline of 3,615 km, which includes the Gulf of Oman (GOO) in the north and the Arabian Sea (NAS) in the south (Fig. 1). In comparison to the extensive research on phytoplankton productivity processes carried out in the offshore oceanic waters of the Arabian Sea during the International Joint Global Fluxes program (Wiggert et al. 2000; Smith 2001; Barber et al. 2001), phytoplankton variability in the coastal waters of Oman is poorly known. One of the limitations has been the lack of a dedicated program with systematic sampling along the coast at regular time intervals. Consequently, no information is available on plankton variability on an annual cycle. Based on the extensive array of investigations designed to observe the physical and biochemical processes in the Arabian sea, we can infer that physical-biological coupling in the coastal ecosystem of the GOO and NAS is largely driven by meteorological forcing, termed the monsoon, which manifests itself as strong, seasonally reversing winds (Wiggert et al. 2000). The two components of this annual cycle are referred to as the northeast monsoon (NEM) and the southwest monsoon (SWM).