of their cultivation on the whole ecosystem. 



The standing stocks are expressed in grams of nitrogen m - ^ , as an average 

 for the whole system. 



Besides the state variables listed on the next page, some other nitrogen 

 pools are considered only as sources or sinks. These are oceanic water (source 

 of inorganic nitrogen, sink for dissolved and particulate organic nitrogen), 

 dissolved N2 (source/sink by f ixation/denitrif ication) , and man (sink by 

 fishing). 



The flow diagram (fig. 1) shows the main pathways considered in nitrogen 

 transfer and transformation. Fluxes are expressed in grams nitrogen m - ^ 

 day - * . Most excretion and detritus production (egestion plus nonpredatory 

 mortality) fluxes are omitted to make the diagram clearer. Because of their 

 quantitative importance, fluxes of excretion and detritus production by mussels 

 have been retained in the diagram. 



The main pathways of nitrogen transfer are: the inorganic N rate of supply 

 (by upwelling, mineralization, and mussel excretion), inorganic N uptake by 

 seaweeds and phytoplankton, and secondary production by zooplankton and mussels. 

 This secondary production has been strongly affected by extensive mussel culture. 

 Formerly, phytoplankton production gave rise to a relatively long and complex 

 pelagic food chain. This has been "short circuited" by the mussels. This 

 fact, along with the other natural short circuit, that of the seaweed production 

 (larger than that of phytoplankton and very lightly grazed) produced a shortening 

 of the initial food chain to one with a large production of detritus. From 

 this arise several questions that the model was constructed to answer. 



1) Is the system capable of recycling this much detritus or will it 

 accumulate in the sediments? 



2) Can the system support a much larger mussel and oyster culture taking 

 into account the food available and question 1? The important factors affecting 

 the answers to these two questions are, besides nitrogen itself: 



1) Light limits production in winter; its influence is simulated by seasonal 

 changes in the maximum rates of uptake by the photosynthetic state variables. 



2) Oxygen concentration affects sediment transfers by limiting production 

 by meiofauna and infauna. These bioturbators enhance microbial colonization of 

 organic matter and, thus, organic nitrogen recycling. 



3) The saturation threshold of microbial uptake of organic nitrogen in 

 sediments is low because once a certain level of O2 depletion is achieved (in 

 relation to organic matter supply), the realized rate of uptake of organic 

 nitrogen by microbes cannot increase, substrate availability notwithstanding. 



4) The carrying capacity of mussels determines the number of mussel rafts. 

 Because most of the data available on the ecology of the Ria de Arosa were 



in units of biomass or carbon, conversion factors have had to be used. For 

 animals, Vinogradov's (1953) average of 7.6 percent nitrogen has been used. In 

 the case of phytoplankton, two different C/N ratios were used: 6.0 for blooming 

 phytoplankton, and 10.0 for slow-growing phytoplankton (estimated from Donaghay 

 et al. 1977). 



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