Nitrogen has many similarities to 

 phosphorus and some differences in its 

 cycles. One of the biggest differences 

 is that the main reservoir of nitrogen 

 is the atmosphere rather than planetary 

 rocks. As nitrogen gas, it is available 

 only through nitrogen-fixing organisms. 

 The nitrogen fixers are bacteria and 

 blue-green algae. So a very limited 

 range of organisms is involved in nitro- 

 gen fixation. Other organisms depend 

 upon the continuing fixation of nitrogen 

 by a taxonomically limited group. 



Nitrogen fixers are widespread; 

 many are in the coastal zone. We have 

 always thought of them as being present 

 in soil; however, they are present in 

 much of the ocean as well. There are 

 blue-green algae in the tropical and 

 sub-tropical oceans. We now know that 

 there are abundant blue-green algae on 

 coral reefs, and there is active nitro- 

 gen fixation there. There are blue- 

 green algae in salt marshes, and there 

 is nitrogen fixation there as well. In 

 most coastal systems which we have exam- 

 ined with modern methods, we have found 

 nitrogen fixation. This does not neces- 

 sarily mean that nitrogen is abundantly 

 available in the coastal zone because 

 there is denitrification going on as 

 well. The important point about denitri- 

 fication is it is done by an even more 

 limited group of specialists. These are 

 obligate anaerobic bacteria. Denitrifi- 

 cation occurs only where there is an 

 anaerobic environment, notably in sedi- 

 ments such as those found in marshes. A 

 stagnant estuary, with the bottom water 

 depleted in oxygen, might have some de- 

 nitrification. Much of the denitrifica- 

 tion is probably associated with the 

 bottom of the continental shelf or with 

 the estuaries. 



Nitrogen also goes through the same 

 kind of food web cycle as phosphorus. 

 With a few exceptions, nitrogen is accum- 

 ulated by bacteria and plants. It is 

 consumed and regenerated, and most of 

 the regeneration is in the form of am- 

 monia, so there is a very rapid cycle of 

 ammonia much like the rapid cycle of 

 phosphate. At any one time, there is 

 very little ammonia in the system be- 

 cause ammonia is, for most plants, the 

 preferred form. It is the most reduced 

 form and, therefore, the energetically 

 optimal form. Aquatic plants will take 



the ammonia first and leave the nitrate 

 until last. Ammonia rapidly recycles, 

 being taken up by the plants, passed on 

 to the animals and bacteria, and excret- 

 ed as ammonia again into the water. 

 Nitrogen coming in from the ocean 

 is mainly nitrate. There is a big reser- 

 voir of nitrate in the ocean, and when 

 deep water washes up onto the continen- 

 tal shelf in one way or another, it 

 brings some nitrate into the coastal 

 zone. So there is an input of nitrate 

 from the ocean, and this, of course, 

 will be utilized by plants, will go into 

 the cycle, and will be recycled as 

 ammonia. 



There is a tendency to view micro 

 organisms in two categories: the good 

 guys who are nitrogen fixers and the bad 

 guys who are the denitrifiers. We should 

 remember that in reality what is impor- 

 tant is that the cycle keeps turning 

 over. If we did not have the denitrifi- 

 ers, the nitrogen would become locked up 

 somewhere, and not be in the atmosphere. 

 The atmosphere could be depleted, not 

 overnight, but in a rather short extent 

 of geological time. 



Carbon is rarely a limiting ele- 

 ment, as far as I am aware, in the 

 coastal zone. There is approximately a 

 thousand times as much of it in the wa- 

 ter as the plants could utilize. There 

 is a good supply of it in the atmosphere 

 for the intertidal grasses and man- 

 groves. We look upon the cycle of car- 

 bon as something important to study, but 

 not as something that is limiting the 

 system in any way. 



The cycle of sulfur in the coastal 

 zone may be more significant than we 

 have realized. Sulfate is abundant in 

 the ocean and is not going to be a lim- 

 iting element. We are now interested in 

 the sulfur cycle because there are vola- 

 tile sulfur compounds being produced by 

 organisms in the coastal zone. h^S and 

 volatile organic compounds are going 

 into the atmosphere. Such things as di- 

 methyl sulfide and a number of other low 

 molecular weight sulfur compounds are 

 apparently produced by algae in substan- 

 tial quantities (Lovelock et al . 1972). 

 These are being produced, not just in 

 anerobic sediments, but also by kelp 

 beds or even phytoplankton, and con- 

 tribute to the sulfur supply in the 

 atmosphere. There is an input to the 



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