North Carolina's Pamlico Sound (Tcnore 1972), 

 proving toxic for most benthic organisms and 

 often causing extensive fish kills as well. Anoxia 

 never develops in the water column and in the 

 surface sediments of intertidal flats because of 

 the surface mixing along the shorelines. 



1.4 THE ESTUARY AS A NUTRIENT TRAP 



Coastal lagoons and estuaries ordinarily possess 

 far higher concentrations of nutrients than the sea 

 itself or than the freshwaters running off from 

 land. The high nutrient levels stimulate plant 

 growth resulting in the high organic productivity 

 that characterizes estuarine environments. In par- 

 ticular, nitrogen and phosphorus, the two most 

 important nutrients required for plant growth, 

 can be found in abundance in estuarine waters. 



Estuaries act as nutrient sinks for at least 

 three major reasons. First, and probably most im- 

 portant, is the effect of the sediments. Clay-sized 

 sediment particles tend to adsorb nutrients and 

 other chemicals (Rae and Bader 1960). When con- 

 centrations in the water column decline, the sedi- 

 ments give up their adsorbed nutrients in a classi- 

 cal chemical equilibrium reaction (Pomeroy et al. 

 1965). The sediments thus serve as a buffer for 

 phosphates, nitrates, and other chemical species. 

 When biological reactions occur to utilize availa- 

 ble dissolved nitrates and phosphates, the nutri- 

 ents deposited in the "sediment bank" are released 

 into the water column, helping to maintain nutri- 

 ent concentrations at high levels in estuarine 

 waters. 



A second cause of high nutrient levels in estu- 



arine systems is related to the basic circulation 

 pattern of estuaries and lagoons. Lunar tides pro- 

 duce an ebb and flood of coastal waters with 

 minimal net movement of particles and even 

 water molecules. Physical oceanographers charac- 

 terize estuaries and dissolved species of chemicals 

 within them by their residence times in the sys- 

 tem. Some chemicals have long residence times 

 while others pass through more readily. In gen- 

 eral, coastal waters are characterized by relatively 

 long residence times as a consequence of the ebb 

 and flood patterns of the estuary or lagoon. In 

 well-stratified estuaries, it is easy to see how par- 

 ticles (including adsorbed nutrients) can be car- 

 ried out toward the sea by the freshwater flow at 

 the surface and then returned upstream by the 

 tidal wedge below, which flows as a counter- 

 current when the tide is rising. 



The process of biodeposition also helps ac- 

 count for the nutrient-sink properties of estuarine 

 systems. Large numbers of suspension-feeding 

 molluscs and crustaceans in estuaries and lagoons 

 remove suspended material from the water col- 

 umn and package it into feces and pseudofeces. 

 These act as large particles (which they are, even 

 though a conglomerate of smaller units) and sink 

 to the bottom. There they become buried, but by 

 no means are the nutrients bound within them 

 lost. Subsequent erosion, sediment reworking by 

 animals, and the uptake of deeper nutrients by 

 macrophytic benthic plants maintain these nutri- 

 ents in the estuarine system. Further discussions 

 of the basic structure and biology of estuarine 

 systems appear in a number of texts, including 

 Barnes (1974) and Reid and Wood (1976). 



