consumption by grazers, much macroalgal production is at least partially 

 broken down by bacteria to form conglomerates of algal detritus and bacteria. 

 Some algal detritus (called wrack) is temporarily washed up on shore, while 

 most remains in subtidal marine waters. This material then forms a major food 

 pathway to higher trophic levels. These conglomerates are consumed by 

 omnivores and deposit-feeding (detrital) invertebrates. These invertebrates, 

 as well as grazers, are consumed by other invertebrates, demersal fish, and 

 waterbirds . 



A similar type of energy dynamics applies to eelgrass beds. As with 

 macroalgae, little of the net productivity of Zostera marina is consumed 

 directly by grazers, although epiphytes may be heavily grazed. Much of the 

 net production goes into formation of detritus. Areas populated by Z. marina 

 are frequented by waterfowl species, including Canada geese and black ducks. 

 These and many other birds graze directly on Z. marina , or the associated 

 invertebrate community that Z. marina supports. 



People remove energy from the benthic marine system by harvesting demersal 

 fish, filter-feeding molluscs, and crustaceans, which support major commercial 

 fisheries. Waterbirds, which are highly mobile, also remove energy from the 

 system. 



In the coastal zone, mixing of the water column is the major physical factor 

 determining water quality and water chemistry. Mixing transports various 

 nonmotile components, such as nutrients and organic materials, which allows 

 their effective utilization and recycling. The principal forcing functions of 

 the mixing process are ocean currents caused by tides, density gradients, and 

 wind (see "Hydrography" and "Climate" above). 



Biogeochemical Cycles 



Many essential substances are cycled through the chemical, geological, and 

 biological components of the marine system. Several such cycles have been 

 described in the scientific literature, for example, those of nitrogen, 

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

 may involve a rate-limiting step of primary production within a given system 

 or subsystem. 



Two cycles discussed below will be referred to throughout the chapter: the 

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

 water column and processes that may affect nutrient levels will be described 

 in the discussion of the nutrient cycle. The discussion of the organic matter 

 cycle will address the different categories of organic matter in the marine 

 environment and their cycling through biotic and abiotic segments of the 

 marine system. 



Nutrient cycle . Many nutrients are required for growth of phytoplankton 

 and macroalgae; however, in the marine system, nitrogen in various forms is 

 generally considered to be the limiting element. The same is also true in 

 estuarine waters. There is, however, a major difference between coastal 

 marine and estuarine waters with respect to the mechanisms of the supply of 

 nutrients . 



4-48 



