The energy bound in producer biomass is utilized in many ways. Some is 

 utilized by the plants themselves for growth and respiration. The other 

 pathways result in energy transfers to higher trophic levels, either directly 

 through grazing on plants or indirectly through the detrital chain. Many 

 plant cells are grazed directly by zooplankton, filter-feeding benthic 

 organisms (e.g., clams), and pelagic fish. Some plant cells die and enter the 

 pool of suspended organic material and detritus. The detrital pool also 

 receives organic matter through decay, resuspension of peat and sediments, and 

 death of other organisms. 



The zooplankton feed on living phytoplankton and suspended organic matter and 

 its associated bacteria and, in turn, are devoured by certain pelagic fish and 

 filter-feeding benthic invertebrates. Excretion by and death of consumers 

 recycle nutrients and contribute to the pools of nonliving organic material. 

 Detritivores (e.g., polychaetes and crustaceans) consume primary production as 

 detritus. This is the major pathway in which producer energy enters the 

 estuarine system. 



The top consumers of the estuarine system are the fish and waterbirds. These 

 mobile carnivores import or export energy to or from the system by their 

 migratory habits, and some of their biomass is removed from the system by 

 fishing and hunting. 



BIOGEOCHEMICAL CYCLES 



The maintenance of the estuarine system is dependent upon nutrients supplied 

 by chemical, geological, and biological processes. Several such processes or 

 cycles have been described in the literature; for example, the cycling of 

 nitrogen, organic matter, silica, phosphorus, and sulfur. One or more of 

 these cycles could involve a rate-limiting step, which affects primary 

 production within a given system or subsystem. 



The nutrient and organic matter cycles, the seasonal cycling of nutrients in 

 the water column, and processes that may affect nutrient levels in the 

 estuarine environment are discussed below. 



Plant Nutrients 



Photosynthesis is the fundamental process by which energy and essential 

 nutrients enter the estuarine food chain. This process requires light (solar 

 energy) and nutrients. In the surface layers of most natural waters 

 sufficient light exists to varying depths to support plant growth (primary 

 production), and the process continues until some other requirement for growth 

 is exhausted. 



Nitrogen generally is thought to limit primary production in estuarine and 

 marine waters (Ryther and Dunstan 1971; and Goldman 1976), while phosphorus 

 may fulfill the same role in riverine and lacustrine systems (Schindler 1971). 

 A meager supply of either of these nutrients limits productivity, whereas a 

 plentiful supply usually produces large quantities of plant material. 

 Although the concept of a limiting nutrient is not new (Liebig 1840) it is 

 difficult to determine which element is potentially limiting and, under 

 natural conditions, the extent to which a limitation exists. 



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