Primary production in the Saudi coastal waters of the Arabian Gulf

Highlights

• This Ms presents data, for the first time, on nutrients and primary production in a large sea area of the Arabian Gulf.

• The waters are oligotrophic, with low rates of primary production.

• Nutrients play a major role in controlling primary production.

• Sea surface temperature and incident light may negatively influence primary production between spring and summer.

• Increased anthropogenic inputs may render eutrophic specific locations of this pristine sea area.

Abstract

The Arabian Gulf is a warm (summer SST > 30 °C) and hypersaline (salinity > 40 psu at any time) marginal sea of the Indian Ocean. This paper reports on a 3-year study of seasonal and spatial changes of primary production and associated physico-chemical and biological parameters in the coastal waters of Saudi Arabia in the western Arabian Gulf. The primary production rates were low and yet showed a seasonality, with a major spring peak and a minor autumn peak, and a possible significant role for heterotrophs. While the strong relationships between the net changes of carbon uptake and nutrients between seasons showed a control of primary production by the availability of nutrients, the decrease in primary production between spring and summer when nutrients continued to increase suggests that the primary production at this time could have been controlled by higher ambient temperatures and intensities of incident light.

Introduction

The Arabian Gulf is one of the two marginal seas of the Indian Ocean, with several distinctions of its own. It is a shallow basin, with the maximum depth not >100 m. It is also a low volume (6 × 10¹² m³) basin, with a water residence time between 2 and 5 years (Koske, 1972; Reynolds, 1993). The freshwater inflow into the basin is only of the order of 45 × 10⁶ m³ a year (Reynolds, 1993) and the annual precipitation over the Arabian Gulf is <100 mm year⁻¹ (Almazroui et al., 2012). The combined effect of all these, coupled with the fact that the Arabian Gulf borders arid countries, is an enormous loss of water, unmatched by precipitation and freshwater advection, leading to sea surface temperatures rising seasonally to >30 °C and salinities to >40 psu.

It is a common knowledge that marine phytoplankton production is regulated by light, temperature, availability of nutrients and salinity and that an extreme in any one of these sets the limit to primary, and hence biological, productivity of a water body (Parsons and Takahashi, 1973). Nonetheless, primary production in the Arabian Gulf still remains least studied, in part owing to logistic issues and in part owing to partitioning of the Gulf area between many countries. Besides the economic interest of determining the biological productivity of this marginal sea, an additional impetus for measuring primary production in these waters comes from the context of global changes: if higher sea surface temperature and salinity, besides surface incident light, along with resultant environmental changes, are expected to affect primary production of the world ocean, then what happens in a partially enclosed sea already with extremes of these would be a harbinger of changes yet to come elsewhere.

The paucity of information on primary production and restriction of most of them to a small area off Kuwait is evident in the review of phytoplankton ecology of the Arabian Gulf by Subba Rao and Al-Yamani (1998). Almost all of the studies reviewed also come from areas not typical of the Arabian Gulf – estuary of Shatt Al Arab (Hadi et al., 1989), sewage canals (Al-Saadi and Antoine, 1981), algal blooms (Schiewer et al., 1982; Subba Rao et al., 1999) – or temporally restricted (Huq et al., 1978). Only in the study of Al-Yamani et al. (2006), which also happens to be the most recent, had there been sampling in waters un- or less-impacted and primary production measured at regular intervals over an annual cycle. Similarly, measurements of nutrients in the Arabian Gulf are also very limited, with most of them confined to UAE waters (Hassan et al., 1995a, Hassan et al., 1995b; Shriadah, 1997, Shriadah, 2001, Shriadah, 2006; Shriadah and Al-Ghais, 1999) and one from Qatar waters (Emara et al., 1989)

The present study had two objectives. The first was to generate a better spatial and temporal coverage of primary production in the coastal waters of the Arabian Gulf (~300 km of the Saudi Arabian coast and 3 years), by carrying out measurements in six locations ranging from least- to most-affected by human impacts in Saudi waters. The second was to evaluate the role of controlling variables on primary production so that the usefulness of the data as reference for detecting effects of global changes and anthropogenic impacts can be elucidated.