1,721,065 research outputs found
Impact of including dissolved organic matter in a global ocean box model on simulated distributions and fluxes of carbon and nitrogen
No full textTwo ecosystem models, one with dissolved organic matter and the other without, were run to equilibrium in a global ocean box model. Predicted distributions of nitrate and dissolved inorganic carbon were similar between the two runs. Export fluxes to the deep ocean were dominated by sinking particles in both instances, and so showed little sensitivity to the choice of whether or not an explicit representation of DOM is chosen. Significant differences did however occur in predicted primary production and f-ratio, giving rise to alkalinity changes (up to 22 mmol m-3) which could be important when calculating pCO2 and atmospheric exchange of CO2 in studies addressing the global carbon cycle
Coupling the PLANKTOM5.0 marine ecosystem model to the OCCAM 1º ocean general circulation model for investigation of the sensitivity of global biogeochemical cycles to variations in ecosystem complexity and physical environment
Full text linkThe earliest marine ecosystem models consisted of a simple representation of the main features of marine ecosystems, including, typically, variables for phytoplankton, zooplankton, nutrient and detritus (NPZD models). These have been incorporated into ocean general circulation models to give a basic representation of ecosystem function, providing predictions of bulk quantities such as global primary production, export and biomass which can be compared with available observations. A recent trend has been to increase the number of phytoplankton and zooplankton groups modelled, as analogues of different plankton groups observed to exist in the ocean, for example diatoms and cocolithophores (the so-called plankton functional type or PFT approach). It is usually assumed that the increase in complexity of the model will result in simulated ecosystems which more faithfully reproduce observations than NPZD models, but this has not been demonstrated systematically. The robustness of the PFT models to changes in model parameters and to changes to the physical environment in which it is embedded, have not been investigated. As a first step towards these goals, we incorporate a state-of-the-art PFT model, PLANKTOM5.0 into the OCCAM ocean general circulation model. A 6 year simulation is performed, covering the years 1989-1994 with identical parameter choices to an existing run of PLANKTOM5.0 coupled to the OPA general circulation model. This document describes the development of the coupled model and the 6 year simulation. Comparison with the OPA model and sensitivity of the solution to parameter choices will be described in a forthcoming journal paper
Deserts on the sea floor: Edward Forbes and his azoic hypothesis for a lifeless deep ocean
No full textWhile dredging in the Ægean Sea during the mid-19th century, Manxman Edward Forbes noticed that plants and animals became progressively more impoverished the greater the depth they were from the surface of the water. By extrapolation Forbes proposed his now infamous azoic hypothesis, namely that life would be extinguished altogether in the murky depths of the deep ocean. The whole idea seemed so entirely logical given the enormous pressure, cold and eternal darkness of this apparently uninhabitable environment. Yet we now know that the sea floor is teeming with life. Curiously, it took 25 years for the azoic hypothesis to fall from grace. This was despite the presence of ample contrary evidence, including starfishes, worms and other organisms that seemingly originated from the deep seabed. This is a tale of scientists ignoring observations that ran counter to their deep-seated, yet entirely erroneous, beliefs
Threshold elemental ratios for carbon versus phosphorus limitation in Daphnia
No full text1. The transition from carbon (C) to phosphorus (P) limited growth in Daphnia depends not only on the C : P ratio in seston, i.e. food quality, but also on food quantity. Carbon is commonly believed to be limiting at low food because of the energetic demands of basal metabolism. The critical C : P ratio in seston (otherwise known as the threshold elemental ratio, TER) above which P is limiting would then be high when food is scarce.2. A new model that differentiates between the C : P requirements for growth and maintenance is presented that includes terms for both C and P in basal metabolism. At low food the calculated TERs for Daphnia of around 230 are only slightly higher than values of 200 or so at high intake. Seston C : P often exceeds 230, particularly in oligotrophic lakes where phytoplankton concentration is low and detritus dominates the diet, indicating the potential for limitation by P.3. The analysis highlights the importance of P, as well as C, in maintenance metabolism and the overall metabolic budget, such that food quality is of importance even when intake is low. Further measurements of C and P metabolism at low food, in particular basal respiration and excretion rates, are needed in order to improve our understanding of the interacting roles of food quantity and quality in zooplankton nutrition
Stoichiometric theory extended to micronutrients: comparison of the roles of essential fatty acids, carbon, and nitrogen in the nutrition of marine copepods
No full textConventional stoichiometric theory, which is used to study the limitation of zooplankton production by C, N, and other elements, is extended to include the essential polyunsaturated fatty acids (PUFAs) 20:5(n-3) and 22:6(n-3). Using typical biochemical compositions of consumer (Calanus helgolandicus) and algal food (hypothetical diatom-dinoflagellate mixtures), the analysis shows that PUFAs or macronutrients can be important in limiting zooplankton production, depending on the biochemical compositions of consumer and food and the efficiencies with which dietary components are used. Predicted limitation by fatty acids is strongest when zooplankton use a monospecific diet, indicating that such limitation may be of particular significance in laboratory zooplankton, which are often fed phytoplankton monocultures. The analysis illustrates several factors that operate to minimize the limiting potential of fatty acids to the extent that limitation by C or N could occur: selective grazing to obtain a nutritionally balanced diet, plasticity in consumer biochemical composition, and high C requirements for respiration. The possibility of macronutrient limitation is increased further if zooplankton are able to actively synthesize essential PUFAs, although this is not thought to be the case in most aquatic systems. The work highlights the need for complete data sets incorporating fatty acids and bulk elemental properties of consumers and prey, as well as an improved understanding of the roles and cycling of essential fatty acids, if we are to be able to provide a unified view of zooplankton nutrition
Use of spherical and spheroidal models to calculate zooplankton biovolume from particle equivalent spherical diameter as measured by an optical plankton counter
No full textThree methods of calculating the biovolume of particles from their shadows as recorded by and optical plankton counter (OPC), based on optical geometry, are presented. In the first method (Vsphere), particles are assumed to be opaque spheres. In the other two methods, particles are represented as opaque spheroids, oriented with their major axes either parallel to the flow thus presenting maximum shadow area (Vmax), or randomly orientated relative to the flow (Vran). The models were tested by comparing with net biovolume, measured from samples of a zooplankton assemblage dominated by Calanus finmarchicus collected during a cruise to the northeast Atlantic during 2001. The randomly orientated spheroidal model (Vran) provided the best fit with the net data: on average the ratio of OPC biovolume to net biovolume was 1.02, compared to ratios of 0.84 when calculating OPC biovolume as Vmax and 1.50 when calculating as Vsphere. The Vran and Vmax methods gave reasonable estimates of net biovolume from OPC measurements without recourse to the use of empirical tuning parameters that are otherwise required. This success was enhanced by the fact that the community chosen for validation purposes was dominated by a single species, C. finmarchicus, which could be approximated by spheroids of known dimension. The calibration methods are less likely to be effective when applied to zooplankton communities incorporating a diverse range of organisms
Low bacterial growth efficiency in the oligotrophic eastern Mediterranean Sea: a modelling analysis
No full textEffect of food quality on carbon and nitrogen growth efficiency in the copepod Acartia tonsa
No full textPopulations of the copepod Acartia tonsa were fed a mixture of algal prey (diatom Thalassiosira weissflogii, prymnesiophyte Emiliania huxleyi, dinoflagellate Aureodinium [Gymnodium] pigmentosum) supplied at saturating concentrations, grown under either nitrogen-sufficient or nitrogen-deplete conditions, in order to study the impact of food quality on production and development throughout the life cycle of the copepod. Changes in predator population structure and biomass were recorded, along with consumption of each of the algal groups, permitting C and N growth efficiencies to be estimated. There was a clear difference in the Acartia tonsa population structure when fed N-sufficient or N-deplete prey, with those fed N-deplete prey slower to develop and reproduce and laying fewer eggs. Algal nutrient status affected selectivity between the diatom and dinoflagellate, the latter being favoured under nutrient-deplete conditions, perhaps in part because their C:N ratio was less susceptible to altered nutrient status. There was no clear difference in the N growth efficiency (N-GE, typically 5%) between N-sufficient and N-deplete prey, but certainly efficiency did not increase with N-deplete prey. C growth efficiency (C-GE) declined from 5 to 2% with N-deplete prey. However, while the ratio of N-GE:C-GE was clearly different between N-sufficient (1) and N-deplete (2.5) treatments, actual growth efficiencies increased with time during the progression to later life history stages, culminating in highest efficiencies during active egg production. Caution should be exercised in assigning GE and predation rates in models incorporating zooplankton feeding on prey of variable nutrient status; these parameters are not constants and GE estimates from egg production experiments are likely to significantly overestimate efficiencies over the whole copepod life cycle
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