60 



them out of reach for ordinary plants. Eenitrifiers exist in the 

 sea, and much discussion has centered around their supposed activi- 

 ties there. But, lecking- quantitative inforrriation , we have no clear 

 concept of the scale on which they actually affect such losses in 

 the open ocean, both because of our iernorance of their actual abun- 

 dance in its different parts, and {mor:^ important) because the 

 factors thot govern their denitrifying activities there erf not yet 

 fully understood. It has long been supposed that b^icteria of this 

 group operate more efficiently at high temperatures than at low, . 

 Hence it has often been suggested that the scarcity of nitrates 

 actually recorded in the tropics, as contrasted to cold'^r wat-rs, 

 reflects the greater activity of these bacteria, in Vijsrin s"as, end 

 consequently that the regional differences in the losses of nitro- 

 gen that they affect are responsible for the genral paucity of 

 phytoplankton in the Tropics, as comp--;red with higher latitudes. 



But this theory, like m?ny others that hpve b^en set up in 

 Oceanic Biology, is based on only one factor in the environment, 

 when actually others mny be more import'^nt; and on a numerical abun- 

 dance of the organisms concerned, which, while, easily maintsinid in 

 the laboratory, may never exist in the open sea. Recent experiments, 

 for example, hav; suggested that while temperature controls the rate 

 of activity of the denitrif iers, perhaps they actually attack 

 nitrates only when oxygen is deficient. If true, this points to 

 great possiblv. losses of nitrogen by their activities in the oottoms 

 of certain enclosed basins, in the mud generally and wherever oxygen 

 is relatively scarce in the raid-strata of the oce^ins, but to little 

 or no activity on their part in the surface layers which are nearly 

 saturated with oxygen. If, however, these bacteria are active in 

 the mid-depths, the results of their cumulative work there may lead 

 to a great loss of nitrogen, that must oe made up in some other way, 

 for most of the decomposition of dead carcasses sifting down from 

 above, takes place at this level. What is needed here is not theo- 

 retic discussion of what might happen, so much as empiric determina- 

 tion of what actually does happen. 



How active the denitrifiers are in the mud is equally a live 

 question, because the sediments on the sea floor, in deep water as 

 well as in shoal, contain considerable quantities of organic nitro- 

 genous compounds, which, so long as they continue in chemical conn- 

 binations, are a potential food supply, that may be brought to the 

 plants in the upper waters by vertical currents. In the '7ulf of 

 Maine, for instance, the bottom muds contain on an aver^.;= e about 

 as much nitrogen as good garden soil, much of which is distributed 

 throughout the water at the seasons when vertical circulation is 

 most active. It may be assumed that a scarcity of oxygen everywhere 

 in the mud below the superficial skim sets the stase for the destruc- 

 tive effect of the denitrifiers there, unless the temperature be too 

 low for them. But v;e are still entirely in the dark as to how 

 effectively they act in the mud, i.e. what role they play in pre- 

 venting the accumulation of nitrates in the submarine deposits, for 

 while these salts are extremely soluble in the sea water, organic 

 particles teid to be trapped in thi mud wherever sedimentation is 

 rapid, and thereby protected from the action of the water. A.ay 

 nitrogen locked up in this way would be a dead loss to the oceanic 

 complex until it should in some way be brought again into circula- 

 tion. 



While bacteria of certain sorts may perhaps be acting constant- 

 ly to denude the sea water of its nitrates, the fact there is any 



