The nitrogen to phosphorus ratio for lakes: A cause or a consequence of aquatic biology?

External gradients of matter and energy drive the behavior of dissipative structures. Therefore, it can be believed that external gradients also drive, ultimately, the behavior of self-organizing, complex, dissipative systems, like ecosystems. Nutrient gradients are ordinarily expressed between a lake and its environment and also between the lake biota and its chemical-physical environment. For the latter, some energy has to be dissipated by the biota in order to concentrate nutrients, at both high and low TN:TP ratios. Lakes exhibit a huge range of total nutrient concentrations and a great variability in relationships between total nitrogen (TN) and total phosphorus (TP). P originates primarily from soil minerals and can accumulate to a substantial degree at sediments of lakes and oceans. On the other hand, N is unique among lake nutrients, it originates from atmosphere as an inert gas, is closely tied to organic matter, exceptionally accumulates to a significant degree in lake sediments, and has a cycle either more complex than P. Most lakes distributed worldwide were P restricted before cultural eutrophication, independent of its original trophic state. Moreover, a TN:TP decrease is usually displayed during lake eutrophication processes. Therefore, the differences in biogeochemical cycles for phosphorus and nitrogen would be expected to be reflected in TN:TP ratio for lakes. The main purpose of this study is to explore the empirical relation between the TN:TP ratio and the trophic state in lakes ranging from deep stratified to very shallow polymictic lakes. A secondary purpose is to examine the hypothesis that nonlinear patterns in TN-TP relationships for lakes are mainly associated with differences in biogeochemical cycles for N and P, and some inherent properties of living systems. A variety of published data was used comprising measurements of total nitrogen, total phosphorus, chlorophyll concentration, water transparency, lake surface area, and mean depth for more than 1500 water bodies distributed worldwide. The relationship between lake N and P concentrations is not linear. It has a complex parabolic shape, when sets of lakes with a very wide range of trophic states are examined. Therefore, the relationship between TN:TP and TP has a decreasing hyperbolic shape. The TN:TP nutrient ratio decreases abruptly with TP from 1 to 8-10 mg.m, declines more gradually between 10 and 25 mg.m, and practically has a tendency to be stabilized at TN:TP values ranging between 10 and 5 (weight basis) for higher TP concentrations. Therefore, a consistent pattern emerged from the studied data set, despite a large amount of variability among individual lakes. TN:TP ratios are consistently low for eutrophic and hypertrophic lakes. Moreover, similar hyperbolic patterns are displayed for individual lakes during its seasonal cycles. For mesotrophic stratified lakes, TN:TP decreases from the mixed layer to the hypolimnia, and is a minimum at the water-sediment interface. Since human induced nutrient loads to lakes usually have a low TN:TP ratio, would be direct to conclude that lake TN:TP mimics the TN:TP of nutrient loads to lakes. Very often N concentration increases lesser than P concentration during eutrophication.