CARmUEAN SEA EcosYsTEM major inllucncc on the dynamics of plankton and, hence, on the lishcries of the Caribbean. These river plumes extend to very shallow depths in the southcm Caribbean (less than I 0 m) but, under the inlluencc of wind and current, slowly mix into the underlying saltier water. They can achieve depths ofbetween40 m and 60 min the northcast Caribbean. Eddies crossing the sea further complicate the transport of lh:sh water, resulting in a complex mosaic of diJTerent levels of salinity on the surlltce. 10 9 AssESSMEN'I Recent studies have shown that the impact of these river plumes can be very great, even hundreds of kilometres li·mn the deltas where they originate. Dissolved organic matter from the Orinoco, for example, has been found to stimulate the growth ofplankton far out into the Caribbean, through a process known as photomineralization which releases nutrients into the marine environment (Corredor ct a!. 2003). Box 2.1: The Magdalemt River and its Impacts on Coastal Ecosystems 11 The t..-lugdalena River is one of the most important rivers in the world in terms of its impact on the wider environment. For cxumple: [t discharges more sediment for each square kilometre ot'ils catchment area than any of the other large rivers ulong and Atlantic coasts of South America (the rate is 560 t km 'year 1). Caribbean The total mmmnt of setlimcnt transporh:d into the Caribbcun by the Mugdalcna is oft he same magnitudc as the three larger rivers of the continent, Amazon. Orinoco, and Parami (Plata). which all drain into the Atlantic. ll has a large drainage busin (257,438 km) covering 24'% of the territory of Colombia. The Magdalena River extends tbr l ,6 l 2 km and drains the West em and Central Curdillems of the Andes. The basin is characterized by high h!ctunic activity, hmdslidcs. steeply sloping tributary (71 of the catchment area cmTesponds to c]c,·ations above 1,000 m). and motlerute precipitation, with an average raint\•11 of 2.500 mm per year. 600 lE§i!J 01\re.t '!., 400,_ Ylold 1- 1- 1;- = IOilE!Ih SouJtcr,: Restrepo and Kjerl\•c 2000b. 2002, 2004. During the past 50 ycm-s, the Magdalena River has com\! umh:r increasing cnvironml!ntal stress accompanying economic development in Colombia between the l 970s and l 990s, with major implications for the Caribbean coustul areas into which the river dmins. Ongoing trends include (I) escalating population densities along the basin and at the river mouth. Eighty per cent of the Colombian population, including the cities of 13ogota. Medellin, Cali. and 13mrJnquilla ure located in the Magdalena watershed. This gives rise to a density of 120 inhabitantsikm 2 lbr the basin us a whole, compared to just 0.24 inhahitants/km2 in the Amazon basin; (2.) accelerating upland erosion rates duf! to increasing Uef(m:station. mining. and poor agricultural pructices; and (3) increasing levels of wnlcr pollution. also linked to increased population, deforestation, and poor limning practices. The ovcrull result has been a distortion of the nutural dynamics of the river system. in turn leading to the loss of critical habitat, biodiversity, and altered patterns of the transport of sediments and other material. 10 Dramatic saldlitc·clcrivcd imagery of these intcmctions can be \'icwt.!d on lhl'! website of the US Nuvul Research Lubumtury nt http://www7320.nrlssc. n:.lvv.mil!IASNFS \VW\V I infonnutiun in this box is largdy sourced from Rcstrcpn mul Kjcrfvc 2000a, 2000b. 2002, - 30- Rcslrcpo und Syvilski 2006; Rcslrcpo c1 al. 2005.