is excreted as phosphate. By the time 

 food has gone through the first level of 

 carnivores, 90% or more of the phosphate 

 has been excreted. The rapid recycling 

 that we find with tracers is not the 

 production of phosphate by bacteria, but 

 rather the rapid consumption and excre- 

 tion of most of it back as phosphate. 



There is a substantial experimental 

 basis for this view of the recycling of 

 phosphate (Pomeroy 1970). Microscopic 

 organisms with short life spans tend to 

 be the important organisms in terms of 

 recycling nutrients as well as in expen- 

 diture of energy. The larger organisms, 

 although they have other valuable attri- 

 butes, are not major recycling organisms 

 in terms of moving phosphorus or other 

 elements around in the ecosystem. Fall- 

 out of organic matter carrying phospho- 

 rus to the bottom is a potential sink 

 taking phosphorus out of circulation. 

 If the fallout is not very far, phospho- 

 rus does not get out of the system. What 

 reaches bottom will go into the mouths 

 of hungry organisms or into the bodies 

 of bacteria and get back into the system 

 again. The recycling of elements in 

 shallow water tends to be more complete 

 than in the deep ocean. 



Also, there is a physicochemical 

 equilibrium between phosphate in the 

 water and the sediments, especially clay 

 sediments which adsorb phosphate on the 

 surfaces of the platelets of clay. There 

 will be many times more phosphate on the 

 clay than in the water when there is an 

 equilibrium. The equilibrium establishes 

 itself in a matter of minutes and is a 

 continuing process, going on all the 

 time. To some extent, this tends to be 

 a stabilizing influence on the amount of 

 phosphate in the water (Pomeroy et al. 

 1965). In the real world, the clay is 

 not fully suspended, so the equilibrium 

 is only realized to the extent that 

 there is interaction between water and 

 clay. 



The clay is probably more important 

 in another way. Plants are growing in 

 it, and they are getting phosphate out 

 of the interstitial water. There is a 

 continual pumping of phosphate out of 

 the sediments by the grass; then, when 

 the grass dies, it is degraded and phos- 

 phate goes into the water. So, recycling 

 between water and sediments is driven by 

 the growth of marine grass. As it grows, 



the grass also leaks, and Reimold (1972) 

 studied the rate at which it loses phos- 

 phate. The washing of the grass by the 

 tide removes about as much phosphate as 

 is actually incorporated in the annual 

 growth of grass, so the amount of phos- 

 phate pumped from the sediment may be 

 twice as much as that incorporated by 

 growth. 



There is also inflow of phosphorus 

 from rivers to coastal waters, which 

 supplies a portion of the annual re- 

 quirement of plant populations in the 

 coastal zone. This is a very indirect 

 contribution and its importance is not 

 clear. Phosphorus coming down a river 

 is not going directly to plants. For 

 phytoplankton or even kelps the effect 

 of phosphorus carried by rivers may be 

 more immediate than in a system with 

 intertidal plants. In a salt marsh, for 

 example, phosphorus recycling is much 

 more important in the short term than 

 input from rivers. In terms of geologi- 

 cal time, rivers are important. 



We often overlook the fact that 

 phosphorus comes from the ocean as well 

 as the rivers. In fact, probably more 

 of the phosphorus that cycles through 

 the coastal zone comes from the ocean. 

 There are a number of mechanisms that 

 bring phosphorus in from the ocean, such 

 as the upwel lings off Peru and South 

 Africa, that are well known and very 

 dramatic. There is a less known type of 

 upwel ling along the edge of the conti- 

 nental shelf of the U.S. Atlantic coast. 

 Periodically, the Gulf Stream washes up 

 on the shelf. The idea that nutrients 

 move toward shore across the continental 

 shelf was originally proposed in a math- 

 ematical model by Riley (1967). His mod- 

 el showed that nutrients had to go in- 

 ward across the shelf with the inner 

 part of the shelf being supplied with 

 nutrients by the ocean. What the physi- 

 cal oceanographers are beginning to tell 

 us now would verify this, that indeed 

 the ocean is of major importance in sup- 

 plying nutrients to the coastal zone. 



In any case, phosphorus is not an 

 element that is limiting in coastal zone 

 water. There is plenty of it around, 

 except in the cleanest tropical situa- 

 tions, such as the Florida Keys or Ha- 

 waii; those are the only situations in 

 which phosphorus might be a limiting 

 factor. 



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