CE and RW acknowledge support

from WWF-Canada for compiling ship traffic and source level data. EA and RW acknowledge ongoing support from Marisla Foundation for incorporating noise into conservation and management EPZ5676 plans for killer whales and their critical habitat. The funders had no role in study design, data collection and analysis, decision to publish, or conclusions of the manuscript. “
“The Arabian Gulf is of paramount economic importance in the world. The tremendous oil resources and their maritime transportations in the area have constantly drawn compelling attention. The Arabian Gulf is a shallow, semi-enclosed marginal sea, which is connected to the Gulf of Oman through the Strait of Hormuz in the east (Fig. 1). Its mean depth is ∼35 m, its length is ∼990 km, and its maximal width is 370 km. The Arabian

Gulf has asymmetric bathymetrical features along Pirfenidone mouse the main axis with a deeper zone off the Iranian coast and broad shallow shelf along the southern and western coasts from Kuwait to the United Arab Emirates (UAE). Additionally, the surrounding arid climate, in which evaporation surpasses the combination of precipitation and river runoff, results in hypersaline water mass production (Nezlin et al., 2010). These extreme conditions lead to an inverse estuarine circulation of cyclonic nature (Reynolds, 1993). The basin-scale circulation consists of two components. One current flows northwesterly from the Strait of Hormuz along the southern Iranian coast. The other one is a southeastern-flowing current in the southern Arabian Gulf (Reynolds, 1993). It flows out of the Arabian Gulf and spreads into the Gulf of Oman and the Arabian Sea at 200–300 m depth through the Strait of Hormuz (Prasad et al., 2001). The major rivers that empty into the Arabian Gulf are located in the north and from northwest with an average discharge rate of 36–1000 km3 year−1 (Reynolds, 1993). The maximum river discharge was recorded in late spring-early summer (Nezlin et al., 2010). The precipitation over the Arabian Gulf area is very low at a rate of 0.07–0.1 m year−1 (Marcella and Eltahir, 2008). Evaporation exceeds combined rainfall and

freshwater discharge, which results in high salinity up to 44.3‰ (Jacob and Al-Muzaini, 1990). In addition to the high salinity, the Arabian Gulf is one of the warmest water bodies on earth with water temperature reaching 32 °C during the summer (ROPME, 1999). Other special characteristic of the Arabian Gulf region is its high aerosol concentration. Dust storms occur frequently in the gulf area, mainly in May–July, when dust deposition can amount to over 30 g m−2 (Subba Rao and Al-Yamani, 1999). Red tide, also known as harmful algal bloom, is caused by proliferation of a toxic or nuisance algae species and has been a pre-eminent topic of world-wide research communities for several decades (Cullen et al., 1997, Kahru et al., 2000, Stumpf et al.