Nitrogen-Pulsed Systems on the Coast of Northwest Spain

Abstract

Simulation models of two ecosystems—an estuary used for aquaculture and an offshore pelagic community, were used to examine the effects of upwelled pulses of nutrients versus constant inputs at the same annual rate. The estuary, the Ria de Arosa in northwest Spain (Galicia), showed little difference, in either productivity or cyclie behavior, between simulations of pulsed inputs versus constant input. This was partly because of its function as a shortterm sediment and nutrient trap and, more importantly, because of the presence in the estuarine pelagic zone of a large biomass of efficient filter-feeding mussels, unresponsive to advective transport. Most (81%) of the inwelled nutrient was transformed into biomass and harvested as seafood. This was unchanged by replacing the periodic infusions of nutrient with a constant daily rate of input. However, in the pelagic model, nutrient pulsing produced a rich dynamical behavior of the trophic components, which retained 61% of the upwelled nutrients at the end of 60 d. Replacing upwelling and relaxation with a constant current and constant low input of nutrients largely obliterated the dynamical behavior and resulted in a nutritionally impoverished system that lost from 73% to 102% of the upwelled nutrient within 60 d. Mech anistic models are put forward as useful tools for both guiding research on pulsed behavior and predicting its effects.