Note Future aquatic nutrient limitations R. Eugene Turner a, * , Nancy N. Rabalais b , Dubravko JusticÕ a , Quay Dortch b a Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803, USA b Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USA Abstract Nutrient limitation of phytoplankton growth in aquatic systems is moving towards a higher incidence of P and Si limitation as a result of increased nitrogen loading, a N:P fertilizer use of 26:1 (molar basis), population growth, and relatively stable silicate loading. This result will likely alter phytoplankton community composition, and may compromise diatom ! zooplankton ! fish food webs. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: N; P; Si; Element ratios; Eutrophication; Nutrient loading; Global change 1. Introduction The concentration and elemental ratios of N, P and Si are known to strongly influence phytoplankton com- munities (Harris, 1986). Empirical expression of these limitations are Redfield ratios (atomic ratios of N:P:Si::16:1:16), which are the stoichiometric require- ments for balanced growth (Redfield, 1958; JusticÕ et al., 1995; Elser et al., 1996). Strong deviations from these ratios indicate that the nutrient in lesser supply becomes limiting for phytoplankton growth if a minimal amount is available. These ratios in rivers are changing as the loading of N, P and Si is influenced by human activities. Human activities have such a strong influence, in fact, that the global variations in the loading of both P and N from watersheds to oceans can be described satisfacto- rily using simple land use categories, population densi- ties and fertilizer use (Caraco, 1993; Howarth et al., 1996). The present (circa 2000) annual agricultural ap- plications of N and P are equivalent to 242% and 83%, respectively, of the annual global riverine fluxes (Schlessinger, 1991) and are expected to double in the next 50 years (Table 1). The present consumption of N fertilizer is 26 times greater than for P, far exceeding the 16:1 ratio required for balanced phytoplankton growth. The applied N leaks from land to water mostly as the highly mobile nitrate ion. Some P may accumulate in soils (Bennett et al., 2001). As a result, the concentration of various forms of N and P in fresh waters are increasing throughout the world (e.g., Cloern, 2001). In contrast to N and P, however, the global variations in silicate loading from rivers are predominantly controlled by its geochemistry, latitude and runoff, although sili- cate concentrations may be reduced by as much as 50% due to hydrologic changes in the watershed (Correll et al., 2000; Humborg et al., 2000). The observed and predicted global increases in N and P loading, and the relatively stable or lower Si loadings have, therefore, a potential to strongly influence dissolved N:P:Si ratios in rivers, hence aquatic food webs. We reviewed data for dissolved inorganic nitrogen, dissolved inorganic phosphate and dissolved silicate (DSi) in the worldÕs largest rivers and plot these in a way that illustrates the present patterns and implies future nutrient limitations, assuming the predicted increases in N and P fertilizer use occurs. 2. Methods The data are described in Turner et al. (in press). These data are part of a Global Environmental Moni- toring System (2000) monitoring program for the worldÕs largest rivers that includes 44% of the EarthÕs land mass (exclusive of the Antarctic) and half the EarthÕs population. The cumulative average annual * Corresponding author. Tel.: +1-225-578-6454; fax: +1-225-578- 6326. E-mail address: euturne@su.edu (R.E. Turner). 0025-326X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0025-326X(03)00049-3 www.elsevier.com/locate/marpolbul Marine Pollution Bulletin 46 (2003) 1032–1034