Modelling the temperature and the phytoplankton distributions at the Aveiro near coastal zone,Portugal

The main objective of this paper is to implement a coupled three-dimensional physical and ecological model for the Aveiro coast, and to apply it to study the temperature and the phytoplankton biomass spatial distributions along the coastal ecosystem. The Aveiro coast is located at Portugal within the upwelling system of the Atlantic Iberian coast, characterized by nutrients availability and phytoplankton biomass accumulation, from April to October. In order to implement the ecological model, its validation was assessed by comparing simulations to data relative to the horizontal and vertical distributions of the temperature, nutrients and phytoplankton biomass, obtained during the CICLOS I survey off the Portuguese coast [Moita, M.T., 2001. Estrutura, Variabilidade e Dinâmica do Fitoplâncton na Costa de Portugal Continental. PhD Thesis. Faculdade de Ciências da Universidade de Lisboa, 272 pp.]. A sensitivity analysis of the model has been performed in order to assess the influence of the main ecological model variables. The simulation results show that the model is capable of predicting realistic the temperature, the nutrients and the chlorophyll-a distributions for the study area. The scenarios evidence the setup of a thermal stratification pattern resulting from the upwelling of deep and rich in nutrients water to the surface layer and a chlorophyll-a maxima extending offshore, along the picnocline and the nutricline. The results confirm the crucial role played by the physical processes in the phytoplankton bloom along the Aveiro coast. They also evidence the close link between the surface phytoplankton distribution and the surface temperature distribution.     

A simple plankton model for the Oregon upwelling ecosystem: Sensitivity and validation against time-series ocean data

Simple plankton models serve as useful platforms for testing our understanding of the mechanisms underlying ecosystem dynamics. A simple, one-dimensional plankton model was developed to describe the dynamics of nitrate, ammonium, two phytoplankton size-classes, meso-zooplankton, and detritus in the Oregon upwelling ecosystem. Computational simplicity was maintained by linking the biological model to a one-dimensional, cross-shelf physical model driven by the daily coastal upwelling index. The model sacrificed resolution of regional-scale and along-shore (north to south) processes and assumed that seasonal productivity is primarily driven by local cross-shelf Ekman transport of surface waters and upwelling of nutrient-rich water from depth.Our goals were to see how well a simple plankton model could capture the general temporal and spatial dynamics of the system, test system sensitivity to alternate parameter set values, and observe system response to the effective scale of potential retention mechanisms. Model performance across the central Oregon shelf was evaluated against two years (2000-2001) of chlorophyll and copepod time-series observations. While the modeled meso-zooplankton biomass was close in scale to the observed copepod biomass, phytoplankton was overestimated relative to that inferred from the observed surface chlorophyll concentration. Inshore, the system was most sensitive to the nutrient uptake kinetics of diatom-size phytoplankton and to the functional grazing response of meso-zooplankton. Meso-zooplankton was more sensitive to alternate parameter values than was phytoplankton. Reduction of meso-zooplankton cross-shelf advection rates (crudely representing behavioral retention mechanisms) reduced the scale of model error relative to the observed seasonal mean inshore copepod biomass but had little effect of the modeled meso-zooplankton biomass offshore nor upon phytoplankton biomass across the entire shelf.     

Hydrographic characterization of southeast Arabian Sea during the wane of southwest monsoon and spring intermonsoon

Seasonal variation of the hydrography along the southeast Arabian Sea is described using data collected onboard FORV Sagar Sampada in September–October 2003 (later phase of Southwest monsoon, SWM) and March–April 2004 (Spring inter monsoon, SIM). During the later phase of the SWM, upwelling was in the withdrawal phase and the frontal structure was clearer in the northern sections (13 and 15°N lat) indicating strong upwelling in the area. The driving force of upwelling is identified as the combination of alongshore wind stress and remote forcing with a latitudinal variability. Although a more prominent upwelling was found in the north, a maximum surface Chlorophyll-a was found in the south (10°N). During the SIM, the area was characterized by oligotrophic water with relatively high Sea Surface Temperature (>29°C) and low salinity (33.8 to 35.4). During March, the surface hydrography was found to be controlled mainly by the intrusion of low-saline waters from the south, while during September by the high saline water from the north. The presence of various water masses [Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), Red Sea Water (RSW)] and their seasonal variations in the region is discussed and their decreasing influence towards the south is noted during both periods of observation. During the SWM, the dynamic topography showed the equator-ward flow of the West India Coastal Current (WICC) at the surface and a pole-ward coastal under current at sub-thermocline depth. During the SIM, surface circulation revealed the WICC flowing pole-ward north of 13°N, but equator-ward flow in the south, with a clockwise circulation around the Lakshadweep High.     

Estimates of upwelling rates in the Arabian Sea and the equatorial Indian Ocean based on bomb radiocarbon

Radiocarbon measurements were made in the water column of the Arabian Sea and the equatorial Indian Ocean during 1994, 1995 and 1997 to assess the temporal variations in bomb 14C distribution and its inventory in the region with respect to GEOSECS measurements made during 1977-1978. Four GEOSECS stations were reoccupied (three in the Arabian Sea and one in the equatorial Indian Ocean) during this study, with all of them showing increased penetration of bomb 14C along with decrease in its surface water activity. The upwelling rates derived by model simulation of bomb 14C depth profile using the calculated exchange rates ranged from 3 to 9 m a(-1). The western region of the Arabian Sea experiencing high wind-induced upwelling has higher estimated upwelling rates. However, lower upwelling rates obtained for the stations occupied during this study could be due to reduced 14C gradient compared to that during GEOSECS.