290 research outputs found
Dysfunctionality in ecosystem models: an underrated pitfall?
No full textIncluding causal mechanisms in model formulations is the key to predictive success. Yet it would appear that much of our latest (specifically ecophysiological) understanding accruing from phenomological evidence and experimental work is often not being included within model structures. Dysfunctional equations (which fail to capture mechanistic understanding, and which lead to incorrect model behaviour) are often used instead, the use of fixed Monod formulations to simulate multiple-nutrient interactions in phytoplankton, or Holling type II descriptions of resource-limited predatory activity, being typical examples. In some instances, dysfunctional equations may be adopted through sheer ignorance, a worrying prospect given our incomplete knowledge of many processes occurring in marine ecosystems. One wonders, for example, to what extent the parameterisations used in the current generation of complex ecosystem models being developed for climate studies, and the predictions thereof, can be relied upon. Here, we investigate the underlying problems leading to the use of dysfunctional models within marine food web systems, our perusal of the subject suggesting that ignorance is by no means the only factor, other reasons being indifference, inertia, and subservience to simplicity.<br/
Sensitivity of secondary production and export flux to choice of trophic transfer formulation in marine ecosystem models
No full textThe performance of four contemporary formulations describing trophic transfer, which have strongly contrasting assumptions as regards the way that consumer growth is calculated as a function of food C:N ratio and in the fate of non-limiting substrates, was compared in two settings: a simple steady-state ecosystem model and a 3D biogeochemical general circulation model. Considerable variation was seen in predictions for primary production, transfer to higher trophic levels and export to the ocean interior. The physiological basis of the various assumptions underpinning the chosen formulations is open to question. Assumptions include Liebig-style limitation of growth, strict homeostasis in zooplankton biomass, and whether excess C and N are released by voiding in faecal pellets or via respiration/excretion post-absorption by the gut. Deciding upon the most appropriate means of formulating trophic transfer is not straightforward because, despite advances in ecological stoichiometry, the physiological mechanisms underlying these phenomena remain incompletely understood. Nevertheless, worrying inconsistencies are evident in the way in which fundamental transfer processes are justified and parameterised in the current generation of marine ecosystem models, manifested in the resulting simulations of ocean biogeochemistry. Our work highlights the need for modellers to revisit and appraise the equations and parameter values used to describe trophic transfer in marine ecosystem models
A multi-nutrient model for the description of stoichiometric modulation of predation in micro- and mesozooplankton
No full textChanges in predator behaviour when confronted with prey of disadvantageous composition have been termed stoichiometric modulation of predation (SMP; Mitra and Flynn, 2005; J. Plankton Res. 27, 393–399). Through SMP, a predator may compensate for (positive SMP) or compound (negative SMP) dietary deficiencies. While these responses are documented in experiments, albeit typically with poor parameterization, previous zooplankton models contain no explicit description of these events. A new multi-nutrient biomass-based generic zooplankton model is described, capable of handling SMP at the levels of ingestion and assimilation, for the exploration of zooplankton growth dynamics in situations where prey quality and quantity changes over time. SMP is enabled by configuring ingestion rate and assimilation efficiency descriptors as functions of food quality (indexed here to prey N:C). Sensitivity analysis of the new model shows the structure to be robust against variation in parameter (constant) values. The form of the model enables its use in population dynamic studies of different zooplankton groups; here, the model has been configured to represent micro- and mesozooplankton. It is shown that in the absence of inclusion of SMP, fits of the model to experimental data can be poor with potential for significant misrepresentation of trophic dynamics
Seasonal Distribution Of Non-Constitutive Mixoplankton Across Arctic, Temperate and Mediterranean Coastal Waters
No full textThis report considers the geographic and taxonomic spread of non-constitutive mixoplankton (NCM). NCM are mixoplankton (protists that are both phototrophic and phagotrophic) by virtue of acquired phototrophy; they acquire their potential for photosynthesis from their prey.
NCM are quantitatively important members of the protistan communities in Arctic, temperate and Subtropical waters. Generally, ciliates were the quantitatively dominant NCM across climate zones, with NCM dinoflagellates and amoebic were of less importance
Defining the “to” in end-to-end models
No full textRobust models relating climate change to fish production require an adequate description of planktonic intermediaries between phytoplankton and fish in end-to-end models. In turn this requires and justifies a proper testing of zooplankton models. Fundamental issues regarding inclusion of zooplankton in these end-to-end models are discussed. It is argued that the complexity of the zooplankton component requires careful consideration and should not be simplified arbitrarily relative to higher and lower trophic levels. Future modelling studies are needed to rigorously examine the effects of increasing complexity within the zooplankton component on ecosystem dynamics. Acquisition of data from targeted field and laboratory studies, including mesocosms, is needed for testing mechanistic end-to-end models and optimizing the balance between fidelity and simplicity in the zooplankton component
Seasonal Distribution Of Non-Constitutive Mixoplankton Across Arctic, Temperate And Mediterranean Coastal Waters
No full textThis report considers the geographic and taxonomic spread of non-constitutive mixoplankton (NCM). NCM are mixoplankton (protists that are both phototrophic and phagotrophic) by virtue of acquired phototrophy; they acquire their potential for photosynthesis from their prey.
NCM are quantitatively important members of the protistan communities in Arctic, temperate and Subtropical waters. Generally, ciliates were the quantitatively dominant NCM across climate zones, with NCM dinoflagellates and amoebic were of less importance
A Guide For Field Studies And Environmental Monitoring Of Mixoplankton Populations
No full textExecutive Summary
• A guide for the sampling and analysis of mixoplankton in natural environments is provided that offers guidelines to assist students and research scientists initiating studies of mixoplankton in natural waters.
• The guide contains methods on how to sample, preserve and analyse mixoplankton abundance and diversity directly from natural environments. As mixoplankton are fully integrated constituents of the protist plankton community, many of the sampling strategies and techniques described in this guide are applicable also to phytoplankton and protozooplankton. Accordingly, methods
cover traditional methods but also evolving new molecular techniques developed for applications to field and discrete studies of plankton diversity.
• The guide contains specific information on various topics including:
o Sampling and sample preservation for optical microscopy analyses.
o Protist microplankton and mixoplankton community sampling shipboard.
o Size-fractionated eukaryotic protist plankton sampling for molecular
purposes.
• Topics not considered in this work include continuous and autonomous methods of sampling (though these are commented upon) and identification of environmental parameters required to contextualise drivers of changes in diversity
Mixoplankton interferences in dilution grazing experiments
No full textIt remains unclear as to how mixoplankton (coupled phototrophy and phagotrophy in one cell) affects the estimation of grazing rates obtained from the widely used dilution grazing technique. To address this issue, we prepared laboratory-controlled dilution experiments with known mixtures of phyto-, protozoo-, and mixoplankton, operated under different light regimes and species combinations. Our results evidenced that chlorophyll is an inadequate proxy for phytoplankton when mixoplankton are present. Conversely, species-specific cellular counts could assist (although not fully solve) in the integration of mixoplanktonic activity in a dilution experiment. Moreover, cell counts can expose prey selectivity patterns and intraguild interactions among grazers. Our results also demonstrated that whole community approaches mimic reality better than single-species laboratory experiments. We also confirmed that light is required for protozoo- and mixoplankton to correctly express their feeding activity, and that overall diurnal grazing is higher than nocturnal. Thus, we recommend that a detailed examination of initial and final plankton communities should become routine in dilution experiments, and that incubations should preferably be started at the beginning of both day and night periods. Finally, we hypothesize that in silico approaches may help disentangle the contribution of mixoplankton to the community grazing of a given system
Are closure terms appropriate or necessary descriptors of zooplankton loss in nutrient–phytoplankton–zooplankton type models?
No full textOur current knowledge of plankton ecology ascribes a large proportion of zooplankton losses to zooplankton cannibalism and carnivory, rather than via the activity of higher trophic levels beyond the plankton. However, planktonic ecosystem models, such as the widely used nutrient–phytoplankton–zooplankton (NPZ) type models, typically represent all zooplankton losses by mathematically (rather than biologically) justified closure functions. Even where it is assumed that these closure functions include zooplanktonic cannibalism and carnivory, these processes are not explicitly implemented within the grazing function of the zooplankton. Here it is argued that this representation of zooplankton losses through “closure” terms within planktonic food web models is neither appropriate nor necessary. The general consequences of implementing a simple function incorporating zooplankton cannibalism and carnivory (intra-guild predation) within a planktonic food web model are compared against models implementing different types of traditional closure functions. While the modelled biomass outputs may appear similar, the fate of annual primary production and f-ratios vary widely. There appears no justification for the continued use of traditional closure term to depict zooplankton loss processes on biological or modelling arguments. To do so can seriously misrepresent the fate of primary production and thence trophic dynamics
- …
