Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Heterotrophic consumption may mask increasing primary production fuelled by anthropogenic nutrient loading in the northern Arabian/Persian Gulf Turki Al-Said a , S. Wajih A. Naqvi a,b, ⁎ , Ayaz Ahmed a , Rakhesh Madhusoodhanan a , Loreta Fernandes a , Raziya Kedila a , Hadeel Almansouri a , Kholood Al-Rifaie a , Faiza Al-Yamani a a Environment and Life Sciences Research Center, Kuwait Institute for Scientific Research, P.O. Box 1638, Salmiya 22017, Kuwait b Council of Scientific & Industrial Research, Rafi Marg, New Delhi 110 001, India ARTICLEINFO Keywords: Nutrients Eutrophication Phytoplankton Respiration Arabian Gulf Kuwait ABSTRACT Monthly measurements of nitrate, nitrite, ammonium and phosphate at three stations of Kuwait during 2002–2015 revealed considerable inter-annual variability, broadly corresponding to fuctuations in the Shatt-al- Arab River discharge, but a lack of secular increasing trend. Nutrient enrichment experiments during two sea- sons revealed nitrate uptake, chlorophyll build-up and growth of micro-phytoplankton, even in the presence of ammonium, provided the availability of phosphate. Primary production was mostly nitrogen limited, but an- thropogenic nitrogen supply may eventually make it phosphorus limited, especially in summer and in the open Gulf. Anthropogenic nutrient inputs appear to have enhanced biological productivity of the northern Gulf, but heterotrophic consumption, indicated by high respiration rates, probably prevented accumulation of phyto- plankton biomass, accounting for the observed lack of chlorophyll increase over the past three decades. Consequently high total organic carbon and emerging hypoxia in the Gulf may lead to expansion/intensifcation of the oxygen minimum zone of the Arabian Sea. 1. Introduction The Arabian (Persian) Gulf is a marginal sea of the Indian Ocean (Fig. 1). Its small size (area 251,000 km 2 , volume 8600 km 3 ), shallow depths (mean ~36 m, maximum ~90 m), geographical setting and highly arid climate make it unique among all marginal seas. The Gulf is particularly distinguished by the extremely large variability in tem- perature (from < 12 °C in winter to > 36 °C in summer) and salinity (from < 30 at the Shatt al-Arab River (SAR) mouth to > 60 in the Gulf of Salwah) (Al-Yamani et al., 2004; Sheppard et al., 2010). Its hydro- graphy is, to a very large extent, driven by the enormous excess of evaporation over precipitation (~2m y −1 ). The river discharge (35–133km 3 y −1 ) is an order of magnitude smaller, and the pre- cipitation is negligible (Sheppard et al., 2010). The resultant highly negative water balance leads to very high salinities that combine with cooling in winter to produce waters with the highest density found in any marine basin (Al-Yamani and Naqvi, 2019). Like other Mediterra- nean-type seas, dense water exits the Gulf into the Sea of Oman through the Hormuz Strait as a deep current with an estimated volume transport of6620km 3 y −1 (Sheppard et al., 2010). The net loss of water through excessive evaporation over precipitation and runof, and by the out- fowing current is made up by the near-surface infow of fresher water fromtheSeaofOman,whichisoftheorderof7250km 3 y −1 (Sheppard et al., 2010). Thus, with reference to the infow or outfow, the fushing time of the Gulf is only 1.2–1.3 y. Nutrient budgets of the Gulf are expected to be largely determined by the water exchange with the Indian Ocean through the Hormuz Strait. Because of the higher nutrient concentrations at depth, export of nutrients by the outfowing current should be more than the import through infow from the Sea of Oman. Therefore, while additional minor sources of nutrients must exist within the Gulf (e.g. river runof), this water body, in its pristine state, would be nutrient-impoverished. Moreover, the short residence time of water would not permit the re- gion to act as a nutrient trap. Historical nutrient data from the Gulf conform to this expectation (Al-Yamani and Naqvi, 2019). The frst signifcant study of nutrient cycling in the Gulf, carried out during a cruise of F.S. Meteor in March 1965, dealt with only phosphate and silicate; it was also geographically restricted to the deeper (Iranian) part of the Gulf. The results of this survey, published by Grasshof (1976), showed the infow of fresher, phosphate-rich water from the https://doi.org/10.1016/j.marpolbul.2019.07.054 Received 26 March 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 ⁎ Corresponding author at: Council of Scientifc & Industrial Research, Raf Marg, New Delhi 110 001, India. E-mail address: snaqvi@csir.res.in (S.W.A. Naqvi). Marine Pollution Bulletin 148 (2019) 30–46 Available online 05 August 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved. T