NUTRIENT CYCLING IN COASTAL ECOSYSTEMS 



L. R. Pomeroy 



Institute of Ecology 



University of Georgia 



Athens, Georgia 30602 



The coastal zone is an active 

 place: biologically, geologically, and 

 in terms of human activity. The coastal 

 zone really extends out to the edge of 

 the continental shelf, and it extends 

 inland on the coastal plain, but in the 

 estuaries and nearshore ocean we see the 

 real meeting ground of the oceanic and 

 continental regimes. The meeting of the 

 continent with the ocean produces a num- 

 ber of conditions that are important in 

 making the coastal zone one that is bio- 

 logically active and productive. 



One important feature is the shal- 

 lowness of the water so that the bottom 

 influences the system. In the open 

 ocean, the bottom is so far away from 

 the surface that material falls out, and 

 although materials do come back, they do 

 not come back very swiftly. The time 

 necessary for water to return from the 

 bottom of the ocean to the surface is on 

 the order of thousands of years. In 

 shallow water, however, it takes only 

 weeks or months to bring materials back 

 from the bottom into the water column. 

 Materials need not be lost permanently, 

 in biological terms, in the sediments of 

 the coastal zone. 



The coastal zone is an area of vig- 

 orous water movement and so is the mid- 

 dle of the ocean. But in the middle of 

 the ocean, most of the vigorous water 

 movement is near the surface. In much 

 of the coastal zone, vigorous water 

 movement extends much of the way or all 

 of the way to the bottom. This enhances 

 further interaction with the bottom and 

 results in the exchange of materials be- 

 tween the water and the bottom of the 

 system. These factors work together with 

 the penetration of light, because of the 

 shallowness of the water in the coastal 

 zone, to produce a system that is highly 

 productive. 



Coastal ecosystems are highly pro- 

 ductive and contain a variety of plant 

 communities. Figure 1 is modified from 



Mann (1972) showing the relative rates 

 of productivity of a number of the plant 

 types in the coastal zone and comparing 

 them with some terrestrial systems and 

 the open sea. Highly productive systems 

 include Zostera , the eelgrass, Thalas- 

 sia , the turtlegrass, and the giant 

 kelps, particularly in Canada, according 

 to Mann. Johannes et al. (1972) and 

 others show that coral reefs do in fact 

 have productive macroalgae. I have add- 

 ed the coral reef values we have for 

 Pacific atolls. I do not know if the 

 Caribbean reefs are in this same range 

 or not. This indicates that most estua- 

 rine situations or other coastal areas, 

 especially those with macroscopic 

 plants, are regions of very high primary 

 productivity. 



That leads into the question of 

 nutrient supply because, to maintain 

 high productivity, plant nutrients must 

 be sufficient. These systems will not 

 function without a continuing supply of 

 nutrients. Coastal ecosystems may have 

 clear water or turbid water. That ap- 

 pears to be of fundamental importance in 

 determining the way such plant nutrients 

 as phosphorus and nitrogen are supplied 

 (Pomeroy et al. 1972). Both clear and 

 turbid systems must have a nutrient sup- 

 ply. The clear water system must obtain 

 its nutrient supply from the water. Tur- 

 bid systems appear to have the advantage 

 of a stable nutrient reserve in the sed- 

 iments, particularly if there are clays 

 or fine organic sediments. 



In clear water systems around coral 

 reefs, most of the nutrient elements are 

 tied up in living material. Turnover of 

 the living material, plus the flow of 

 water, maintains a continuing supply of 

 nutrients. Obviously, there are systems 

 which are not at either extreme (clear 

 or turbid). In fact, a relatively small 

 supply of fine sediments conveys stabil- 

 ity of nutrient availability (Pomeroy 

 et al. 1972). 



140 



