Khalid Al-Hashmi

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.

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.

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.

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).

Abundance of harmful algal blooms in the coastal waters of Oman: 2006–2011

Over the last decade, the ecosystem of the Sea of Oman and Arabian Sea has been showing signs of rapid and profound changes in terms of phytoplankton diversity and harmful algal bloom outbreaks. Frequent blooms have been on the rise in the coastal waters of Oman causing adverse impacts on marine life. The population dynamics of potentially harmful phytoplankton in relation to environmental parameters was investigated from June 2006 to April 2011. Our studies recorded 24 potentially harmful species. Dinoflagellates Prorocentrum minimum, Scrippsiella trochoidea, Cochlodinium polykrikoides and Noctiluca scintillans were the most abundant species. Diatoms Pseudo-nitzschia seriata, Climacodium frauenfeldianum and Guinardia flaccida were the most abundant, but occurred at low concentrations. Scrippsiella trochoidea and Noctiluca scintillans were reported previously as common phytoplankton in Oman coastal waters; however, Prorocentrum minimum and Cochlodinium polykrikoides are reported for the first time. Here we report their occurrence and persistence in relation to changes in environmental parameters. In addition, the potential long-term implications of changes in phytoplankton species and harmful algal blooms outbreaks on ecological, economic, social and human health impacts will be discussed.

Atmospheric cyclones and seasonal cycles of biological productivity of the ocean

Remotely sensed wind speed, translation speed, atmospheric pressure, atmospheric precipitation rate and chlorophyll-a concentration were used to verify the hypothesis that the response of the phytoplankton community to propagating atmospheric cyclones should be associated with the phase of the seasonal cycle of this community and the translation speed of a cyclone. For the 12 cyclones investigated from the Indian Ocean, Pacific Ocean and the Mediterranean Sea the maximal sustained wind speed varied twofold, whereas the translation speed of cyclones and the chlorophyll ratio (characterizing chlorophyll concentration before and after cyclone passage) varied by 10-fold. It was shown that cyclones affecting the phytoplankton community approaching its seasonal maximum either could not stimulate or stimulated a weak response of further increase of chlorophyll-a concentration (due to explicitly available nutrients). Controversially, cyclones affecting the community approaching its seasonal minimum could induce gradual increase of chlorophyll-a concentration. An exponential type of relationship between chlorophyll ratio and translation speed of cyclones was evaluated. In the range of translation speeds from 1 to 10 ms−1, the increase of chlorophyll-a concentration due to cyclone passage was most pronounced with regard to slow moving cyclones.

Influence of Light Availability and Prey Type on the Growth and Photo-Physiological Rates of the Mixotroph Noctiluca scintillans

A strain of the mixotrophic green Noctiluca scintillans (Noctiluca) isolated from the Arabian Sea afforded us an opportunity to investigate the photosynthetic and feeding characteristics of this organism which has recently replaced the once diatom dominated food chain of winter blooms in the Arabian Sea. Here we present the first in a series of experiments undertaken to study the interactive effects of irradiance and grazing response of this mixotroph to four phytoplankton species provided as food. Noctiluca showed a distinct preference for the dinoflagellate Peridinium foliciaeum and the diatom Phaeodactylum tricornutum but not the chlorophyte Pyramimonas nor the chain forming diatom Thalassiosira weissflogii. However, irrespective of food provided, adequate light was required for Noctiluca to grow as evidenced by its maximum growth rates of 1.44 cells day-1 when fed the preferred Peridinium and exposed to optimal irradiance of 250 E m-2 s-1 versus growth rates of 0.18 cells day-1 with the same food but at a low irradiance of 10 E m-2 s-1. Measurements of Noctiluca’s electron transport rates (ETR) per PSII Reaction Center as a function of irradiance also indicated severe light limitation of photosynthesis at 10 E m-2 s-1. The active fluorescence derived ETR versus Irradiance curves revealed an interesting finding in that there was no significant difference in photosynthetic parameters such the maximum photosynthetic capacity (ETRmax) nor α, the rate of increase of photosynthesis with light between fed and unfed cells under optimal light conditions. These results suggest that feeding does not enhance the photosynthetic activity of the endosymbionts when nutrients are not limiting as was the case in these experiments. Measurements of Noctiluca’s intracellular ammonium concentrations under optimal light conditions, the first for this strain, show significant accumulation of NH4+ (0.003-0.012 M NH4+ cell-1) after 14 days for fed and unfed Noctiluca which was undetectable 4 days later. A similar 14-day increase but of significantly higher concentrations (0.005-0.08 M NH4+ cell-1) was obtained under low light conditions. For Phaeodactylum and Thalassiosira fed cultures under light limitation, NH4+ continued to increase past the 14-day period suggesting Noctilucas strong and efficient mechanism for regulation of intracellular nutrients.