Nitrogen cycling and anthropogenic impact in the tropical interamerican seas

We discuss the mechanisms leading to nutrient limitation in tropical marine systems, with particular emphasis on nitrogen cycling in Caribbean ecosyst ems. We then explore how accelerated nutrient cycling from human activities is affecting these systems. Both nitrogen and phosphorus exert substantial influence on biological prod uctivity and structure of tropical marine ecosystems. Offshore planktonic c ommunities are largely nitrogen limited while nearshore ecosystems are larg ely phosphorus limited. For phosphorus, the ability of sediment to adsorb a nd store phosphorus is probably greater for tropical carbonate sediments th an for most nearshore sediments in temperate coastal systems. However, the ability of tropical carbonate sediments to take up phosphorus can become sa turated as phosphorus loading from human sources increases. The nature of t he sediment, the mixing rate between nutrient-laden runoff waters and nutri ent-poor oceanic waters and the degree of interaction of these water masses with the sediment will probably control the dynamics of this transition. Nearshore tropical marine ecosystems function differently from their temper ate counterparts where coupled nitrification/denitrification serves as an i mportant mechanism for nitrogen depuration. In contrast, nearshore tropical ecosystems are more susceptible to nitrogen loading as depurative capacity of the microbial communities is limited by the fragility of the nitrificat ion link. At the same time, accumulation of organic matter in nearshore car bonate sediments appears to impair their capacity for phosphorus immobiliza tion. In the absence of depurative mechanisms for either phosphorus or nitr ogen, limitation for both these nutrients is alleviated and continued nutri ent loading fuels the proliferation of nuisance algae.