The most important physical aspects of the Gulf with respect to effects of marine pollution are its semi-enclosed, shallow nature and its arid setting. The Gulf is principally a sedimentary basin whose substrate is mainly biogenic, with outcrops of older limestone, and with a few actively growing reefs. It is relatively shallow, so that almost all parts of it lie within the photic zone. Its enclosed nature means that it has a low rate of water exchange (up to 5 years), and large parts of it experience extremes of salinity and temperature which have considerable effect on the marine communities. Pelagic productivity is typical for waters of this latitude, though the high productivity values commonly attributed to the Gulf refer to benthic components rather than to the main water mass. It suffered complete drying out in the late Pleistocene, so that its present relatively low biotic diversity is perhaps due as much to its relatively short existence in Recent time as much as to the extremes of environmental conditions which it experiences.