Recycling of nutrients is dependent 

 on the structure of the food web. Fig- 

 ure 2 illustrates a food web which would 

 be appropriate for almost any coastal 

 zone with some type of macroscopic 

 plants. Grazers consume perhaps 10% of 

 the macroscopic plants, and 90% of them 

 are degraded by bacteria and fungi. The 

 elements in the tissues of the plants, 

 such as phosphorus and nitrogen, are re- 

 cycled primarily through death and de- 

 generation. It may not be generally ap- 

 preciated that the obvious grazer does 

 not consume most of the grass. The graz- 

 ers in forest and grassland rarely get 

 10% of the crop. Most of it really fol- 

 lows the detritus route, which is much 

 less obvious. We all see large animals 

 and small ones; we do not see the bacte- 

 ria. Nevertheless, they are the degrad- 

 ers of much of the total material, and 

 move it through particulate and dis- 

 solved pathways. The end result is the 

 recycling of such elements as nitrogen 

 and phosphorus into inorganic forms 

 which are available to plants. 



Although it may not be wholly de- 

 served, phosphorus has received much 

 attention in recent years. The classical 

 view of the phosphorus cycle still has 

 good circulation in textbooks and in the 

 scientific community. This view is one 

 of seasonal changes in the abundance of 

 phosphorus in the water. The concentra- 

 tion of phosphorus in the water increas- 

 es during the winter in the temperate 

 zone. It is utilized by aquatic plants 

 in the spring when the weather gets warm 

 and days longer. Then, there is a summer 

 crash when the supply of nutrients be- 

 comes depleted. Production presumably 

 slows down for that reason, staying at a 

 relatively lower level through fall and 

 winter. 



This classical view of the annual 

 cycle of abundance of phosphorus was 

 developed 50 yr ago when it was first 

 possible to measure phosphorus chemical- 

 ly by colorimetric methods. In the 

 1950 ' s , people began to use 32P in 

 aquatic research. They quickly found 

 that phosphorus was more mobile than 

 they had realized. While observers had 

 been thinking in terms of seasonal 

 changes in abundance, in fact, the 

 standing stock of phosphorus in any 

 water body was usually replaced every 

 few days. In some lakes it was replaced 



every few minutes (Rigler 1956). The 

 seasonal cycle that people had been see- 

 ing and measuring chemically was really 

 a shifting equilibrium point, superim- 

 posed on a rapid recycling seen only by 

 labeling the pool with a radioactive 

 tracer. Now that we are aware of this 

 and use tracer methods, we knew that 

 there is a great deal of recycling of 

 phosphorus. 



The high productivity of coastal 

 systems, in many cases, depends heavily 

 on recycling, rather than on a continued 

 supply of new phosphorus into the sys- 

 tem. This has been examined in many eco- 

 systems including the coastal upwel lings 

 off Peru. Dugdale and Goering (1967) 

 have estimated that 50% of the nitrogen 

 used is recycled and 50% is newly up- 

 welled. For the coastal waters off 

 Georgia, Haines (1975) estimated that 

 95% of the nitrogen was recycled. I am 

 sure the same is true of phosphorus. 

 Recycling is a major factor in continu- 

 ing productivity of many coastal ecosys- 

 tems. 



Figure 3 is a simplified version of 

 how I view the phosphorus cycle in a 

 shallow coastal system where sediments 

 are present. In the classical view, bac- 

 teria are generators or remineralizers 

 of phosphorus, but nobody has yet suc- 

 ceeded in finding these bacteria in nat- 

 ural waters. We now think that the major 

 role of bacteria really is scavenging 

 phosphorus. Bacteria take phosphorus 

 wherever they can get it. They may take 

 it from food material or they may take 

 it from the pool of phosphorus in the 

 water. In fact, the bacteria are compet- 

 ing with the phytoplankton for a common 

 source of dissolved phosphate, and the 

 bacteria compete very well and will get 

 some of the phosphate away from the phy- 

 toplankton. 



But the life cycles of bacteria are 

 very short. They die o-r they are con- 

 sumed by other organisms. The turnover 

 time of bacteria is probably a matter of 

 one day in most systems. So the result 

 is that phosphate moves through both 

 phytoplankton and the bacteria to fil- 

 ter-feeding consumers and benthic- 

 deposit feeding consumers. These con- 

 sumers excrete the phosphate since most 

 of what they consume must be utilized to 

 supply energy. Most of the phosphorus 

 in the organic matter that they consume 



142 



