INTRODUCTION 



The Ria de Arosa is an estuary on the northwest Atlantic coast of Spain 

 which has been intensively exploited by raft culture of mussels ( Mytilus edulis ) 

 since the late 1940's (Tenore et al . in press). The Ria Arosa has an area of 

 250 km^ with an average depth of 19 m. The yield of mussels averages 86,000 Tm 

 (wet weight) yr - * from approximately 2,000 rafts (Perez and Roman 1979). Each 

 of the rafts is about 19 m square and supports 700-800 ropes, some 8-9 m long. 

 The mussels and associated organisms attach and grow on these ropes. An 

 additional 200 oyster rafts produce currently about 2,500 Tm yr - * (wet weight) 

 of Ostrea edulis . 



For the past several years the Ria de Arosa has been the site of an 

 intensive ecological study, performed by a joint team of Spanish and American 

 scientists. The objectives were: 1 ) to document the major processes occurring 

 in the estuary, 2) to ascertain the factors responsible for the high productivity, 

 3) to compare the effect of the mussel rafts in Ria de Arosa with a relatively 

 unmodified estuary immediately to the north (Ria de Muros ) , and 4) to obtain 

 the data necessary to construct a simulation model capable of predicting the 

 optimal number of mussel rafts. 



Two years ago we began building a multicompartment ecosystem simulation 

 model of the Ria de Arosa, one which employed realistic (= definable) parameters 

 and functional feedback relationships which included provision for threshold 

 effects (see Wiegert et al. 1975; Wiegert et al. 1981). Thus the model is 

 'explanatory' in the sense of postulating mechanistic causes for the observed 

 sets of field data. The model is a set of hypotheses, arrived at by induction 

 and utilizing wherever possible ecological, physiological and physical data. 

 The predictions by these (model) hypotheses can be tested by the independent 

 seasonal standing crop measurements from the estuary. 



This kind of model, although required if any advance in theory is desired, 

 is far too demanding of data and time to be preferred when only simple prediction 

 is required. Unfortunately, in the present instance, no other course was 

 possible because the data requisite for construction of a multiple-regression 

 type "predictive" model were not available. There is only one estuary being 

 heavily exploited. Furthermore, experimental manipulation of the Ria de Arosa 

 in the sense of increasing or decreasing numbers of rafts and observing 

 the results is not possible because the rafts are individual family enterprises. 



The conserved or bookkeeping unit in this first-generation model is 

 elemental N, because this is a major factor limiting primary production in these 

 Spanish estuaries. The overall high productivity of the Ria de Arosa Is maint- 

 ained by periodic upwelling which results in a large volume of N-rich water 

 being moved into the estuary (Tenore et al . in press). The structure of this 

 model plus associated submodels and details of many simulation experiments 

 performed with them will be published elsewhere (Penas and Wiegert, in prepara- 

 tion). Here we present only a brief report of the salient features of this 

 first-generation model and the effect of varying the number of rafts in the 

 estuary. 



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