A A SEWAGE 



RUNOFF 



-'^"...v 



WETLANDS 







Figure 2. Conceptual model illustrating five common hypotheses concerning 

 the factors responsible for bringing about high levels of estuarine primary 

 production: 1) The estuarine nutrient pool (B) is simply fertilized by fresh- 

 water input (A) which may or may not include anthropogenic inputs; 2) Nutrient- 

 rich deep water from offshore (D) is brought into the estuary (E) by the estu- 

 arine circulation pattern where it is rapidly mixed up into the euphotic zone 

 (B) by strong tidal and wind effects; 3) The estuarine nutrient pool is ferti- 

 lized by inputs to rivers (A) and the estuarine tidal waters (B) by outwelling 

 of nutrients from fresh and salt marshes and other wetlands; 4) Nutrients in 

 the estuary (B) are taken up by the estuatine plankton (B) which fall to the 

 deeper water (E) and are carried back upstream by the landward flowing bottom 

 water where they are entrained in the seaward flowing fresher surface water 

 (A) and can be taken up once again by the estuarine plankton (B) and; 5) In- 

 puts to (B) from (A) and (D) are much less important than the recycling of 

 nutrients within the system (B,E,F). 



36 



