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In tropical lakes relatively little is known about the general relationship between nutrient concentration and phytoplankton biomass. Using data from 192 lakes from tropical and subtropical regions we examine the relationship between total P (TP) and chlorophyll (Chl). The lakes are all located between 30° S to 31° N include systems in Asia, Africa, and North and South America but are dominated by Brazilian (=79) and subtropical N. American (=67) systems. The systems vary in morphometry (mean depth and lake area), trophic state as well total N (TN) to total P (TP) ratios and light extinction. Despite a nearly 500-fold range in TP concentrations (2–970 µg P l), there was a poorer relationship between log TP and log Chl (=0.42) than is generally observed for temperate systems from either narrow or broad geographic regions. N limitation is not a likely explanation for the relatively weak TP-Chl relationship in the tropical-subtropical systems. Systems had high average TN:TP ratios and neither a multiple regression with log TP and log TN nor separating systems with high TN:TP (>17 by weight) improved the predictive power of the log TP-log Chl relationship.

Here we present the within-site, seasonal, and interannual variations of the carbon (δC) and nitrogen (δN) isotope ratios of leaves, wood, bark and litter from four sites in the Amazon region, Brazil. Samples were collected in Manaus (3° 06′07″ S; 60°01′30″ W), Ji-Paraná (10°53′07″ S; 61°57′06″ W), and Santarém (2°26′35″ S; 54°42′30″ W) with mean annual precipitation of 2207, 2040 and 1909 mm respectively. The overall average for all leaf samples was−32.3 ± 2.5‰ for δC and +5.8 ± 1.6‰ for δN ( = 756). The leaf δ values at these sites were often but not always statistically distinct from each other. The dC values varied from−37.8‰ to−25.9‰ Pronounced differences in δC values occurred with height associated with differences in forest structure. The δC of leaf dry matter showed seasonal variations associated with the length of the dry season, despite the fact that total annual precipitation was similar among the studied sites. Leaf δN values ranged from+0.9‰ to a maximum value of+10.9‰, and the Santarém sites showed more enriched values than Manaus and Ji-Paraná sites. No seasonal variation was detected in the δN of leaves, but significant differences were observed among sites and with changes in canopy height. The isotope ratio data are consistent with our current understanding of the roles of light, water availability, and recycling of soil-respired CO influences on δC and consistent with our understanding that an open nitrogen cycle can lead to high δN values despite a significant number of legumes in the vegetation.