Section 5.2 (Biological Environment)
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5 Description of the fishery/affected environment 5.1 Physical environment The U.S. Caribbean is located in the eastern extreme of the Caribbean archipelago, about 1,100 mi east-southeast of Miami, Florida (Figure 1) (Olcott 1999). It comprises the Commonwealth of Puerto Rico in the Greater Antilles and the Territory of the USVI in the Lesser Antilles island chain, both of which separate the Caribbean Sea from the western central Atlantic Ocean. The rectangular-shaped island of Puerto Rico is the smallest and the most eastern island of the Greater Antilles (CFMC 2002c), and is located between the North Atlantic Ocean and the Caribbean Sea. The island measures about 110 mi from east to west; 40 mi from north to south. The overall area of Puerto Rico, including its principal offshore islands of Vieques, Culebra, and Mona, is estimated at 3,471 mi2 (Olcott 1999); the combined length of its coasts, 700 mi (CFMC 2002c). The USVI are part of the Virgin Islands chain, which lies about 50 mi east of Puerto Rico and consist of about 80 islands and cays (Olcott 1999). The USVI include the largest and most important islands of the Virgin Islands chain: St. Croix, St. Thomas, and St. John. Together, their coastlines extend about 175 mi. St. Croix is located about 40 nm (74 km) south of St. Thomas and St. John (CFMC 2002c). Covering about 84 mi2, that island is entirely surrounded by the Caribbean Sea. The islands of St. Thomas and St. John are bordered by the Atlantic Ocean to the north and the Caribbean Sea to the south. Their respective areas are about 32 and 19 mi2 (Olcott 1999). More detailed information on the physical environment can be found in Section 3.1 of the EFH FSEIS (CFMC 2004). 5.1.1 Geology The nearshore waters of Puerto Rico range from 0-20 m in depth and outer shelf waters range from 20-30 m in depth at the depth of the shelf break. The north coast of Puerto Rico is marked by a narrow insular shelf that is only 2-3 km wide. Depths extend to over 1,200 ft (400 m) beyond the shelf break (CFMC 2002c); the deepest point in the Atlantic Ocean, the Milwaukee Depth, lies at a depth of 27,493 feet (8,380 m) in the western end of the Puerto Rico Trench, about 100 miles (160 km) northwest of the island. Mona Passage, measuring about 75 mi (120 km) wide and more than 3,300 ft (1,000 m) deep, separates Puerto Rico from Hispaniola to the west. The southeast coast has a narrow shelf approximately 8 km wide (CFMC 2002c), after which the sea bottom descends to the 16,400 ft (5,000 m) deep Venezuelan Basin of the Caribbean Sea. The east coast lies on the same geological platform as the USVI of St. Thomas and St. John. Waters in that area extend to depths of less than 240 ft (73 m) throughout (CFMC 2002c). 92 The shelf shared by the islands of St. Thomas and St. John is about 12.9 km wide on the south and 32.2 km wide on the north (Goenaga and Boulon 1992). St. Croix, which lies on a different geological platform, is separated from the other islands by a 4,000 m-deep trench (CFMC 2002c). The St. Croix shelf is much narrower and shallower than that of the northern islands (Goenaga and Boulon 1992), extending only 4 km wide in the south, less than 0.2 km wide on the northwest, though up to several km wide in the northeast and out on Lang Bank (CFMC 2002c). 5.1.2 Oceanography and climate The North Equatorial Current is the predominant hydrological driving force in the Caribbean region. It flows from east to west along the northern boundary of the Caribbean plateau and splits at the Lesser Antilles. To the north, the current flows westward along the north coasts of the U.S. Caribbean islands, splitting north of the Mona Channel. The north branch flows north of Silver and Navidad Banks, past the Turks and Caicos, to form the Bahama Current. The south branch parallels the north coast of Hispaniola about 30 km offshore. A small gyre has been documented off the northwest corner of Puerto Rico resulting in an easterly flow nearshore in this area (CFMC 2002c). To the south, the current enters the Caribbean Sea through the passages between the Lesser Antilles (Chakalall et al. 1998). The water then continues northwestward as the Caribbean Current, the main surface circulation in the Caribbean Sea. The Caribbean Current flows about 100 km south of the U.S. Caribbean islands at an average speed of 0.5 to 1 knots (CFMC 2002c). The current is characterized by large cyclonic and anticyclonic gyres. Its flow exits the Caribbean through the Yucatan Strait into the Gulf of Mexico and, to the northwest, into the North Atlantic (Kjerfve 1998). Its strength is influenced by changes in the position of the inter-tropical convergence zone (ITCZ). It increases in strength during the winter when the thermal equator is farthest south. It decreases in strength during the summer when the thermal equator shifts north, and surface waters in the Caribbean are influenced by increasing precipitation. This is the time of year when the North Equatorial Counter Current is established and surface waters of the equatorial Atlantic are displaced to the east (Kjerfve 1998). Fluctuations in the water mass transport of the Gulf Stream are influenced by seasonal changes in Caribbean surface salinity transport and to wind speed changes in the tropical-subtropical trade wind zone (Kjerfve 1998). Westerly trade wind circulations to the north are responsible for the major wind and wave patterns. High winds occur in the winter; hurricanes in the autumn (CFMC 2002c). The zonal shift of the ITCZ is also responsible for the seasonal change in precipitation in the Caribbean. The dry season occurs when the ITCZ is near the equator (Kjerfve 1998), generally in the late winter to spring (Kjerfve 1998; Olcott 1999). The wet season occurs when the ITCZ is at its most northerly position in the Caribbean, generally in the late summer into late fall (Kjerfve 1998; Olcott 1999); about 50 % of the annual rainfall occurs during this wet season. Average 93 annual precipitation in Puerto Rico ranges from less than 40 in (101.6 cm) on the southern coastal plain, to greater than 200 in (512 cm) in the mountains. Along the coasts, average annual precipitation ranges from about 30 in (76.2 cm) on the lee side of the island along the southwestern coast to about 75 in (190.5 cm) on the windward north coast. Average annual precipitation ranges from less than 30 in (76.2 cm) to greater than 55 in (139.7 cm) in the USVI. Most of the precipitation in this region is returned to the atmosphere by evapotranspiration – evaporation from the land and water surfaces and transpiration by plants (Olcott 1999). Surface water salinity changes along with the seasonal change in precipitation. But precipitation affects salinity only indirectly. The discharge from the Amazon, Orinoco, and Magdalena rivers is the main contribution to buoyancy in the Caribbean, increasing silica concentrations, decreasing salinity and chlorophyll pigments, and increasing the input of terrestrial materials (Kjerfve 1998). The plume of the Orinoco River, as tracked by satellite imagery, seasonally penetrates across the Caribbean Basin, potentially exerting a region-wide influence (Kjerfve 1998). It could be responsible for events of high turbidity and algal blooms that often occur in the Caribbean Basin in October (CFMC 2002c). Locally, Puerto Rico's rivers influence the nearshore environment by discharging silt, nutrients, various chemicals and, of course, freshwater. The USVI has no permanent streams, and outflows only occur during periods of heavy rainfall. But these are sometimes sufficient to muddy coastal surface waters up to one half mile (0.8 km) from shore (CFMC 1985). Sea surface temperature ranges from a minimum of 25 degrees Celsius in February-March to a maximum of about 28.5 degrees Celsius in August-September. Inshore temperatures may be higher (e.g., 30 degrees Celsius) due to shallower depths or, in some cases, to thermal plumes from generator plants (CFMC 2002c). Tidal regimes differ between the north and south coasts. The fluctuations range from a diurnal tide of about 10 cm in the south coast to a semi-diurnal regime of between 60-100 cm along the north coast, where waves are larger (CFMC 2002c). But the astronomical tidal range is slight (20-30 cm) (Kjerfve 1998). 5.1.3 Major habitat types The coastal-marine environment of Puerto Rico and the USVI is characterized by a wide variety of habitat types. NOAA’s National Ocean Service has mapped 21 distinct benthic nearshore habitat types using aerial photographs acquired in 1999. Those maps display 49 km2 of unconsolidated sediment, 721 km2 of submerged vegetation, 73 km2 of mangroves, and 756 km2 of coral reef and colonized hard bottom over an area of 1600 km2 in Puerto Rico. They document 24 km2 of unconsolidated sediment, 161 km2 of submerged vegetation, 2 km2 of mangroves, and 300 km2 of coral reef and hard bottom over an area of 490 km2 in the USVI. Coral reefs, seagrass beds, and mangrove wetlands are the most productive marine habitat areas (CFMC 2002c). CFMC (2002c) provides an in-depth description of the distribution of these 94 habitats, along with information on their ecological functions and condition. A summary of the habitat-life history associations of Caribbean Council-managed species is provided in Section 5.2 (Biological Environment).