1,721,015 research outputs found
Split-domain calibration of an ecosystem model using satellite ocean colour data
Full text linkThe application of satellite ocean colour data to the calibration of plankton
ecosystem models for large geographic domains, over which their ideal parameters cannot be assumed to be invariant, is investigated. A method is presented for seeking the number and geographic scope of parameter sets which allows the best fit to validation data to be achieved. These are independent data not used in the parameter estimation process. The goodness-of-fit of the optimally calibrated model to the validation data is an objective measure of merit for the model, together with its external forcing data. Importantly, this is a statistic which can be used for comparative evaluation of different models. The method makes use of observations from multiple locations, referred to as stations, distributed across the geographic domain. It relies on a technique for finding groups of stations which can be aggregated for parameter estimation purposes with minimal increase in the resulting misfit between model and observations.The results of testing this split-domain calibration method for a simple zero dimensional model, using observations from 30 stations in the North Atlantic, are presented. The stations are divided into separate calibration and validation sets.
One year of ocean colour data from each station were used in conjunction with a
climatological estimate of the station’s annual nitrate maximum. The results
demonstrate the practical utility of the method and imply that an optimal fit of the model to the validation data would be given by two parameter sets. The corresponding division of the North Atlantic domain into two provinces allows a misfit-based cost to be achieved which is 25% lower than that for the single parameter set obtained using all of the calibration stations. In general, parameters are poorly constrained, contributing to a high degree of uncertainty in model output for unobserved variables. This suggests that limited progress towards a definitive model calibration can be made without including other types of observations
RRS "Challenger" Cruise 79/5, 10 April - 23 April 1979. Physical and biological sampling on the Nymphe Bank, Celtic Sea and in the vicinity of the Goban Spur using a batfish and continuous surface water sampling
Full text linkRRS Discovery Cruise 182, 8 May - 8 June 1989. BOFS Cruise Leg 1: Investigations in the region of 47N, 20W and 60N, 20W
Full text linkRRS Discovery Cruise 120, 9 May - 1 June 1981. Biological and physical investigations of the oceanic front to the south-west of the Azores
Full text link
A modelling exploration of vertical migration by phytoplankton
No full textThe behaviour of phytoplankton having different abilities to assimilate N in darkness was considered in simulations of vertical migrations. Such behaviour is especially important for the competitive advantage of flagellates, including harmful algal species. Three phases of biomass development were apparent. (1) Cells remained at a subsurface location with migration down to avoid photoinhibitory light at midday; as the attenuation of light increased with biomass growth, the mean depth became shallower. (2) On exhaustion of nutrients in surface waters, cells migrated down through the nutricline in the latter half of the daylight period, with a subsurface maximum in the photic zone as long as light penetration matched requirements. When that condition was no longer met (3), cells migrated between the very surface (forming dense aggregations) and the nutricline. While the ability to perform dark N-assimilation is not critical when N-sources are available at low concentrations, it is important when (as encountered following migration down to a nutricline), nutrients are available at higher concentration in darkness. The most advantageous configuration tested, where nitrate assimilation (as well as that of ammonium) continued at a high rate in darkness as long as C-reserves remained, is not actually used in migratory species but in non-migratory diatoms. The use of the outwardly inferior configurations typical of migratory species, in which dark nitrate-assimilation is notably poorer than assimilation in the light, reflects a deficient metabolism or indicates that N-sources other than nitrate are more important. It is unfortunate then that most attention has been paid to nitrate nutrition in experiments on migrating species. While an ability to continue N-assimilation in darkness as well as during daylight is advantageous, there is no evidence for phytoplankton to be able to grow at high growth rates when decoupling photosynthesis at the surface and N-assimilation at depth. <br/
Parameterizing the microbial loop: an experiment in reducing model complexity
Full text linkThe structure of the plankton food web in the upper mixed layer has important implications for the export of biogenic material from the euphotic zone. While the action of the microbial loop causes material to be recycled near the surface, activity of the larger zooplankton leads to a significant downward flux of material. The balance between these pathways must be properly represented in climate models to predict carbon export. However, the number of biogeochemical compartments available to represent the food web is limited by the need to couple biogeochemical models with general circulation models. A structurally simple model is therefore sought, with a number of free parameters, which can be constrained by available observations to produce reliable estimates of export.A step towards addressing this aim is described: an attempt is made to emulate the behavior of an 11 compartment model with an explicit microbial loop, using a 4 compartment model. The latter, incorporating a basic microbial loop parameterization, is derived directly from the 'true' model. The results are compared with equivalent results for a 4 compartment model with no representation of the microbial loop. These non-identical twin experiments suggest that export estimates from 4 compartment models are prone to serious biases in regions where the action of the microbial loop is significant. The basic parameterization shows some promise in addressing the problem but a more sophisticated parameterization would be needed to produce reliable estimates. Some recommendations are made for future research
- …
