SULFUR, NITROGEN, AND CARBON IN THE BIOSPHERE 183 



illustrate two of my most general, or more speculative, conclusions: (1) the 

 biosphere is being eutrophicated by accelerated recycling of nutrients, and 

 (2) this implies a significant change in the mean redox state of the globe. 



As long as a lake is thermally stratified, it is neatly divided into two quite 

 different environments. Oxidizing conditions prevail in the lighted, warmer 

 upper waters, and, although carbon compounds are reduced there, reduction is 

 confined to a cellular environment in close proximity to molecules of 

 chlorophyll. Below the thermocline, on the other hand, reducing conditions 

 prevail outside cells, but whether they prevail in the water column or only in the 

 mud depends on the oxygen-storage capacity of the lake, a function of its mean 

 depth. As the concentration of dissolved oxygen and the redox potential 

 diminish seasonally toward mg/liter and mV, respectively, the ferrous iron, 

 manganous manganese, and ammonium cations and the nitrate, phosphate, and 

 bicarbonate anions appear and accumulate during stagnation. Even clearer 

 evidence of reduction appears in the form of three (or four or five) volatile 

 substances, which, diffusing upward, may or may not be oxidized before they 

 escape to the atmosphere. The three volatile substances, one for each of the 

 elements of my title, are hydrogenated carbon or methane, hydrogenated 

 nitrogen or ammonia, and hydrogenated sulfur; the fourth reduced gas is carbon 

 monoxide, and nitrous oxide perhaps ought to be added to the list. 



We know very little about the escape of these reduced gases to the 

 atmosphere. At first thought it seems unlikely that any would escape, for they 

 should all be rapidly oxidized and retained in solution. Ammonia, in particular, 

 is so soluble and so avidly used as a plant nutrient that it should never reach 

 supersaturation outside a feedlot. On second thought, gas bubbles initially 

 composed mainly of methane rise regularly from mud, 3 ' 4 against pressures as 

 great as 3 atm, and, when the bubbles burst at the lake's surface, they can and 

 do liberate gases to the atmosphere. Any mechanism that retains these gases 

 within a bubble, such as differential solubility or adsorption on organic films, 

 will assist in the transit of reduced substances through oxidizing water; and 

 natural waters are notoriously rich in dissolved organic compounds- 

 surfactants — that cause foaming. 



The loss to the atmosphere of a few microbars of dissolved gases has not 

 attracted much attention from limnologists, because the loss makes very little 

 difference to the economy of the lake itself. What can make a difference is the 

 tendency of ferrous iron to solubilize phosphorus. The existence of a reducing 

 hypolimnion has long been thought to promote recycling in this restricted sense. 

 It is not easy to quote direct evidence, for eutrophication is a complex process 

 with multiple causation, 5 and the chemistry of "the hypolimnion" obeys 

 complex rules that differ in force from lake to lake. Still, stagnant water has 

 biological properties that are very different from those of flowing water, and 

 two lakes in the same drainage system and containing the same water can differ 

 sharply in respect to productivity and nutrient economy. One of the main 

 sources of this kind of difference is the different amount of nutrient feedback 



