Watersheds Eight primary, and 17 subsidiary, watersheds replenish the Gulf. These cover some , with 2,240 sq. mi. in Belize, 7,070 sq. mi. in Guatemala, and 11,430 sq. mi. in Honduras. They contain 13 major, and various minor, rivers, with the former discharging some of freshwater per second, on average, and the latter, some 7,060 cubic feet per second, on average. Annually, the Gulf receives some 17.75–18.23 cubic miles (74–76 km3) of water from its watersheds. Sedimentary discharge from Belizean rivers into the Gulf was, on average, 80, 15, and 5 per cent mud, clay, and sand, respectively. Peak freshwater and sedimentary discharge occurs in the wet season, which usually exceeds dry season discharge by a factor of 59.
Currents The Gulf's open sea experiences the Caribbean Current and a quasi-permanent cyclonic eddy generated in the southwest corner of the Cayman Tranch. The latter is centred at about 19°N 86°W, generating a sea surface height anomaly of negative , with peripheral current velocities of 7.9 to 15.8 inches per second (0.2–0.4 m/s). The Caribbean Current flows from east to west in the deep waters off the continental shelf of Honduras. In doing so, every few months, it generates cyclonic, counterclockwise gyres, characterised by a central water level depression of 812 inches (2030 cm), which take 23 months to progress westwards along the Honduran coast towards the Belize Barrier Reef. Along the Gulf's northern (ie Belizean) coast, persistent northeasterly trade winds maintain a constant southerly downwelling, with speeds of 3.9 to 7.9 inches per second (0.1–0.2 m/s). This southern drift, in turn, drives a counterclockwise eddy along the Gulf's mouth, and along its southern coast (ie the Guatemalan and Honduran coasts).
Tides The Gulf experiences a mixed, mainly semidiurnal microtide with a mean sea surface elevation range of some . The semidiurnal and diurnal constituent amplitudes range within 1.182.76 inches (0.030.07 m). The dominant semidiurnal and diurnal tidal constituents propagate westwards along the coast of Honduras, and northwards along the Belize Barrier Reef. Currents induced by the tide may be appreciable in constricted channels along the Belize Barrier Reef, reaching 15.75 inches per second (0.4 m/s) here. Astronomical tides are weak, and at times completely dominated by meteorological tides. For instance, the storm surge associated with
Hurricane Mitch raised the mean water level at Gladden Spit 9.2 feet (2.8 m) on 27 October 1998.
Waves The northeasterly and easterly trade winds give rise to both wind waves and swell, especially during December–May. These are intensified by cold northerlies and north-westerlies blowing in from North America during November–April in bursts lasting one to three days. Waves are typically 3.28–9.84 feet (1–3 m) high, with periods of 3–7 seconds, though hurricanes may increase their height to 32.81 feet (10 m), and their period to 12.7 seconds. Mean wave direction is towards 255° (ie towards the west-southwest). The wet season's thunderstorms are often accompanied by intense gusts from varying directions, followed by periods of calm.
Water Sea-surface temperatures in the Gulf range from in JanuaryFebruary to in AugustSeptember, with the former lower temperatures associated with coastal upwelling driven by intensified trade winds. The Gulf's upper mixed layer, which has uniform temperature and salinity, is about in its northwest section, featuring temperatures of to . A thermocline develops offshore at 49 to 66 feet (15 to 20 m). Sea-surface salinity in the Gulf averages 36,200 parts per million (36.2 g per kg) offshore, but drops off closer towards the coast, and especially near estuaries like the Amatique Bay, where salinity dips to 5,000–10,000 ppm (5–10 g/kg) during the wet season. Visibility in the Gulf ranges from less than near estuaries to a maximum of inside the Belize Barrier Reef. Dissolved oxygen ranges from 0.9 ppm (0.9 mg/L) near coastal areas of wastewater discharge, to 5.2–9.6 ppm (5.2–9.6 mg/L) near the Belize Barrier Reef. pH ranges from 5.8 in areas without marine vegetation (eg seagrass beds), to 7.4 in lagoons and estuaries, to 8.8 near coral reefs. Nitrate and phosphate concentrations range from levels below the detection threshold (near coral reefs), to 7.0 μM and 1.0 μM respectively (near estuaries). Chlorophyll-a concentrations range from undetectable to 0.55 ppm (0.55 mg/L). == Ecology ==