C ARIBUEAN SEA EcosYSTEM AssESSMEN"I Box 2.1: (Contiuuecl) M.l!]dnfcnu N N 111:!. N 10 11 A Delta Grande dr! SilnraMart. ················-·--··' Bauanqu;lfa'.,.._\. Caribbean Seil' / Sediment load trom the Magdalena River has had a strong impact on coastal ecosystems. Since 1954, the government of Colombia has dredged the El Dique Canal, a 114-km mun-made channel from the Magdalena River at C'ulamar to Cartagena Bay. Because of increased sedimentation in the bay during the 1970s, new cunals were constructed tiom El Diquc to Burbacoas Bay. und since then, the suspended sediment lout! in Burbacous has reached ant! impacted the El Rosario Islands, a 68-km' coral-reef ecosystem in the Caribbean Sea. Sediment load is responsible lor most of the observed reef mortality. with dead corul-reef cover reaching 58 per cent. Also, the suspended sediment load from the Sinu River is probably responsible lor impacL' on the largest coral reef on the Colombian Caribbean coast. a system covering I 35 km' between the San Bemardo and Fuerte Islands, north und south lfom the Morrosquillo Gulf. Livl! coral cover had. in some areas, decreased 25% between 1995 and 2000. Cariogena-.._-.. Rosano ls.l;mds--:-, s: Bay··· ..r Sau Morra!Jquilfo GI.JI/ tsh1nd--' Nutrient fluxes of phosphate (PO, ·Jl and nitrate (N03 ) in nonpristine fluvial systems of the Caribbean basins of Colombia. Nutrient values are b:.L"icd on avcmucs culculated from monthlv samples covering three-year period J998-2000 • Total phosphate Water discharge (km3 yr 1) Total nitrate (X Magdalena 228 186 47 Diquc 9.4 12 3.0 Sinu 11.8 1.5 0.07 Leon (Urabu Gult) 2.1 1.5 0.7 River Sediment Load and its Impact on Coastal Ecosystems ro,' 10-'t yr 1) Caribhean Turbo (Urabn Gult) 12 0.1 0.003 Atruto (Umba Gull) 81 58 2.4 Human-induced Nutrient Fluxes and their· Impact on Coastal Systems Since the 1950s. the usc of nitrogen (N) and phosphorous ( P) in agricultuml fertilizers, and in a mnge of products such us detergents, has resulted in a mpid increase in the flow of these clements through rivers. now exceeding pristine levels by u fhetor of 10 in some world rivers. This can dmstically ulter the ecoloc.'Y of aquutic systems. ovcrstimuluting the growth of algae and leading to oxygen depletion. toxic blooms, fish kills, and coml-rcef dcgmdution. In Colombia, non-pristine fluvial systems like those draining the Caribbean basins have much more phosphate .1) und nitrate (NOJ ) loads when compared to pristine rivers (e.g., Pacific rivers). The Magdalcnu and Atmto rivers are the Colombian systems which contribute by tar the highest P and N fluxes to the Caribbean Sea, with totul phosphate and nitrnte flows up to 186,000 t and 47,000 t per year, respectively. Many causes arc responsible tor these high nutrient loads, including massive };e\vage discharges in cities and towns, mainly in the Magdalena basin, and also the use of chemical fertilizers on banana plantations in the lower course of the Atmto River. SoURCE: Restrepo and Kjcrfvc 2004. Urban, agricultural, mining, und industrial waste inputs from the Magdalena basin have aggmvated the condition of the CiCn:Jga Grande h1goon and coastal ecosystems. Biodiversity has been reported to be considerably lower in the area atl'ected by mangrove mortality, us well as in the coastal zone. A decline in fisheries catches by a factor of eight, from 63,700 tin 1978 to 7,850 t in 1998, is another indication of the environmental degmdution caused by reduced water quality. - 32- 11