A combination of Broenkow's nutrient oxygen model and Bowden's residence time evaluation of upwelled waters enabled us to calculate the "net community production" (NCP) in coastal upwelling areas (NW Africa, SW Africa, Peru) and in an open upwelling (Costa Rica Dome). Since NCP represents mainly new production in terms of phosphorus or nitrogen, and since it derives from integrated nutrient consumption over the main production area, a good picture of the average fertility of these zones is obtained. In terms of carbon, NCP averaged 0.6 g C m-2 d-l off Peru, 1.1 off SW Africa, and 2.3 off NW Africa. However, production budget per kilometer of coastline is greater for both Peru and SW Africa than it is for NW Africa. The lower activities off Peru and off SW Africa correspond to a relatively high nutrient-low chlorophyll (HNLC) situation, characterized by a slowly growing phytoplankton standing stock. Such HNLC situations are greatly pronounced in the Costa Rica Dome, with NCP values as low as 0.14 g C m-2 d-l. Herbivorous grazing in the freshly upwelled source water appears to be the most plausible explanation for the low chlorophyll. This mechanism for cropping phytoplankton and retarding nutrient uptake provides a way to spread nutrient input to the open ocean. Italso explains the prevalence of oxygen undersaturation (and CO, oversaturation) in the surface waters of the equatorial upwelling. By using the NW African data and NCP and ammonium excretion rates from the literature we could calculate an Eppley ffactor (pN0, -: pNO,-f pNH,+) of 0.64. *
Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ‐S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.
Nutrients, phosphorus in particular, were studied at five locations in the brackish lake Etang de Berre's waters in order to examine vertical and horizontal distributions, seasonal and annual evolutions, and circulation.The phosphorus cycle was surveyed taking into account the extensive freshwater input from the river Durance which was recently deviated into the Etang de Berre.Of primary importance is the phenomenon of trapped nutrients in the deepest layer of thelake, corresponding to the occurrence of a pycnocline (halocline). This layer is in a steady state characterized by anoxic conditions and shows denitrification processes.
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