12 CARmU. MAR. STUD., SPECIAL EDITION, 2007 Box 2.1: (Concluclecl) Water Diversion and Mortality of Mangrove Ecosystems consequent die-off of almost 270 km 2 of mangrove rorcsts during Magdalena lagoon/dcltn complex and associated coastal zones, the mangrove urcu has been reduced !rom 62,000 ha in 1991 to 52,478 ha in I 996, a loss of ulmost2.000 ha per year. In addition, fresh-water input from the Magdulena River lo the lagoon was also diverted tor irrigation purposes. and interrupted by dikes built along the delta distributaries to prevent tlooding of farm lands. The changes in the hydrological regime have nlso caused the pasl 39 years. 13elwcen 1956 and 1995, 66% of the original mangrove forest died. Recent estimates indicate that li1r the whole dissolved oxygen concentration, lish kills, and eutrophication. \Vater diven;ion due to the construction of a highway in the Magdalena dcllallagoon complex, the Cit!naga Grande de Santa Marta, has resulted in hypersalinizmion of mangrove soils, and the water quality changes in the lagoons ond canals. resulting in low 2.7 Major Coastal and Marine Habitnts ecosystem services. Unregulated sand mining, lor example, causes loss of sand and prevents the natural replenishment of other beaches as material is carried around the coast by tides and cutTcnts. 2.7.1 Beaches Beaches are deposits of sand between the high- and low-tide marks, transported to shore and moulded by waves. The sand can be calcareous (dcriwd from the broken skeletons of corals, calcareous algae, molluscs, and echinodenns) or siliceous (derived from eroded rocks). Beaches arc dynamic, the sand being constantly subjected to deposition (accretion) or loss (erosion). Storms, olTshore reels, sand shoals, currents, and onshore dunes play important roles in controlling deposition and erosion on a beach. The stability of a beach, whether eroding or accreting, depends on a balance, over time, between the supply of sand and the rate at which it is transported away. Another key threat is being brought about largely by the tourism industry itself, despite its reliance on beaches to attract visitors. Many poorly planned developments arc simply too close to the edge of the sea. They frequently lack adequate wuste-disposal facilities, which leads to contamination with sewage and other elllucnts, causing a health hazard and badly diminishing the aesthetic value of beaches. Failure to set buildings back 50 m or more !rom the shore also exposes them to stonns and damages dunes which arc part of the dynamic system stabilizing beaches. In addition, construction on beaches may alter pattems of water cun-cnts which, in tum, could increase erosion. It has recently been estimated that 70% of beaches on the islands of the Caribbean arc eroding at rates of between 0.25 m and 9 m per year (Cambers 1997). It is possible that the decline in Caribbean coral reels (a source of much of the calcareous sand) has reduced both protection from wave action and the supply of sand, thereby increasing erosion. The cost of artilicial replacement of sand, tt process known as beuch nourishment, can run into millions of U.S. dollars (U.S.$). Beach and dune sands serve as one of the world's major sources of construction aggregate. Noncalcareous sand is also used to produce minerals and orcs for various industries, including electronics. However, beaches in the Caribbean arc best known for their importance to tourism: the quality of the beach is cited by most tourists as the main feature of a successful holiday (Uyarra et a!. 2005). They also provide areas of recreation and enjoyment for local people throughout the region and, thcrelorc, have great cultural value in addition to their economic importance in attracting overseas visitors. 1. i.2 Seagmss Beds Seagrasscs arc !lowering plants that nourish in shallow, sheltered, marine environments, such as lagoons ncar mangroves or coral reels, or just ollshorc li·om beaches. The Caribbean region has six species of scagrass, the most common of which is turtle grass (Tiw/assia testudimuu). Beaches arc important habitats lor sen turtles, which nest in the zone above the high-tide mark. This can create a conl1ict between the usc of a beach for recreation and its contribution to the biodiversity of the Wider Caribbean Sea ecosystem, but it can also provide income, community employment, and educational opportunities through wellmanaged ceo-tourism. A number of threats linked to human activity arc causing beach erosion and polluting coastal waters, compromising the ability of Caribbean beaches to continue providing The beds lonned by seagrass perlonn a number of important roles in the Caribbean Sea ecosystem, including the stabilization of' sediments, reducing the energy of waves as they approach the shore, and the provision of a nursery habitat for organisms that as adults live in other systems. -33-