2,540 research outputs found
Biological activity in the wake of an island close to a coastal upwelling
Hydrodynamic forcing plays an important role in shaping the dynamics of
marine organisms, in particular of plankton. In this work we study the
planktonic biological activity in the wake of an island which is close to an
upwelling region. Our research is based on numerical analysis of a kinematic
flow mimicking the hydrodynamics in the wake, coupled to a three-component
plankton model. Depending on model parameters different phenomena are
described: a) The lack of transport of nutrients and plankton across the wake,
so that the influence of upwelling on primary production on the other side of
the wake is blocked. b) For sufficiently high vorticity, the role of the wake
in facilitating this transport and leading to an enhancement of primary
production. Finally c) we show that under certain conditions the interplay
between wake structures and biological growth leads to plankton blooms inside
mesoscale hydrodynamic vortices that act as incubators of primary production.Comment: 42 pages, 9 figure
Population growth and persistence in a heterogeneous environment: the role of diffusion and advection
The spatio-temporal dynamics of a population present one of the most
fascinating aspects and challenges for ecological modelling. In this article we
review some simple mathematical models, based on one dimensional
reaction-diffusion-advection equations, for the growth of a population on a
heterogeneous habitat. Considering a number of models of increasing complexity
we investigate the often contrary roles of advection and diffusion for the
persistence of the population. When it is possible we demonstrate basic
mathematical techniques and give the critical conditions providing the survival
of a population, in simple systems and in more complex resource-consumer models
which describe the dynamics of phytoplankton in a water column.Comment: Introductory review of simple conceptual models. 45 pages, 15 figures
v2: minor change
Modeling Nutrient and Plankton Processes in the California Coastal Transition Zone: 1. A Time- and Depth-Dependent Model
A time- and depth-dependent, physical-bio-optical model was developed for the California coastal transition zone (CTZ) with the overall objective of understanding and quantifying the processes that contribute to the vertical and temporal development of nutrient and plankton distributions in the CTZ. The model food web components included silicate, nitrate, ammonium, two phytoplankton size fractions, copepods, doliolids, euphausiids, and a detritus pool. The wavelength-dependent subsurface irradiance field was attenuated by sea water and phytoplankton pigments. The one-dimensional (1-D) model adequately simulated the development and maintenance of a subsurface chlorophyll maximum in different regions within the CTZ. An analysis of the individual terms in the model governing equations revealed that phytoplankton in situ growth was primarily responsible for the creation and maintenance of the subsurface chlorophyll maximum at both coastal and oceanic regions in the CTZ. The depth to which the maximum in situ growth occurred was controlled by the combined effect of light and nutrient limitation. Also, the simulated bio-optical fields demonstrated the effect of nonlinear couplings between food web components and the subsurface irradiance field on vertical biological distributions. In particular, the epsilon-folding scale of the subsurface photosynthetically available radiation (PAR) was influenced by the level of zooplankton grazing
Modeling the role of constant and time varying recycling delay on an ecological food chain
summary:We consider a mathematical model of nutrient-autotroph-herbivore interaction with nutrient recycling from both autotroph and herbivore. Local and global stability criteria of the model are studied in terms of system parameters. Next we incorporate the time required for recycling of nutrient from herbivore as a constant discrete time delay. The resulting DDE model is analyzed regarding stability and bifurcation aspects. Finally, we assume the recycling delay in the oscillatory form to model the daily variation in nutrient recycling and deduce the stability criteria of the variable delay model. A comparison of the variable delay model with the constant delay one is performed to unearth the biological relevance of oscillating delay in some real world ecological situations. Numerical simulations are done in support of analytical results
The reduction of plankton biomass induced by mesoscale stirring: a modeling study in the Benguela upwelling
Recent studies, both based on remote sensed data and coupled models, showed a
reduction of biological productivity due to vigorous horizontal stirring in
upwelling areas. In order to better understand this phenomenon, we consider a
system of oceanic flow from the Benguela area coupled with a simple
biogeochemical model of Nutrient-Phyto-Zooplankton (NPZ) type. For the flow
three different surface velocity fields are considered: one derived from
satellite altimetry data, and the other two from a regional numerical model at
two different spatial resolutions. We compute horizontal particle dispersion in
terms of Lyapunov Exponents, and analyzed their correlations with phytoplankton
concentrations. Our modelling approach confirms that in the south Benguela
there is a reduction of biological activity when stirring is increased.
Two-dimensional offshore advection and latitudinal difference in Primary
Production, also mediated by the flow, seem to be the dominant processes
involved. We estimate that mesoscale processes are responsible for 30 to 50% of
the offshore fluxes of biological tracers. In the northern area, other factors
not taken into account in our simulation are influencing the ecosystem. We
suggest explanations for these results in the context of studies performed in
other eastern boundary upwelling areas
Analytical solution of the nitracline with the evolution of subsurface chlorophyll maximum in stratified water columns
In a stratified water column, the nitracline is a layer where the nitrate concentration increases below the nutrient-depleted upper layer, exhibiting a strong vertical gradient in the euphotic zone. The subsurface chlorophyll maximum layer (SCML) forms near the bottom of the euphotic zone, acting as a trap to diminish the upward nutrient supply. Depth and steepness of the nitracline are important measurable parameters related to the vertical transport of nitrate into the euphotic zone. The correlation between the SCML and the nitracline has been widely reported in the literature, but the analytic solution for the relationship between them is not well established. By incorporating a piecewise function for the approximate Gaussian vertical profile of chlorophyll, we derive analytical solutions of a specified nutrient-phytoplankton model. The model is well suited to explain basic dependencies between a nitracline and an SCML. The analytical solution shows that the nitracline depth is deeper than the depth of the SCML, shoaling with an increase in the light attenuation coefficient and with a decrease in surface light intensity. The inverse proportional relationship between the light level at the nitracline depth and the maximum rate of new primary production is derived. Analytic solutions also show that a thinner SCML corresponds to a steeper nitracline. The nitracline steepness is positively related to the light attenuation coefficient but independent of surface light intensity. The derived equations of the nitracline in relation to the SCML provide further insight into the important role of the nitracline in marine pelagic ecosystems
Vertical distribution and composition of phytoplankton under the influence of an upper mixed layer
The vertical distribution of phytoplankton is of fundamental importance for
the dynamics and structure of aquatic communities. Here, using an
advection-reaction-diffusion model, we investigate the distribution and
competition of phytoplankton species in a water column, in which inverse
resource gradients of light and a nutrient can limit growth of the biomass.
This problem poses a challenge for ecologists, as the location of a production
layer is not fixed, but rather depends on many internal parameters and
environmental factors. In particular, we study the influence of an upper mixed
layer (UML) in this system and show that it leads to a variety of dynamic
effects: (i) Our model predicts alternative density profiles with a maximum of
biomass either within or below the UML, thereby the system may be bistable or
the relaxation from an unstable state may require a long-lasting transition.
(ii) Reduced mixing in the deep layer can induce oscillations of the biomass;
we show that a UML can sustain these oscillations even if the diffusivity is
less than the critical mixing for a sinking phytoplankton population. (iii) A
UML can strongly modify the outcome of competition between different
phytoplankton species, yielding bistability both in the spatial distribution
and in the species composition. (iv) A light limited species can obtain a
competitive advantage if the diffusivity in the deep layers is reduced below a
critical value. This yields a subtle competitive exclusion effect, where the
oscillatory states in the deep layers are displaced by steady solutions in the
UML. Finally, we present a novel graphical approach for deducing the
competition outcome and for the analysis of the role of a UML in aquatic
systems.Comment: 20 pages, 8 figure
Integrating functional diversity, food web processes, and biogeochemical carbon fluxes into a conceptual approach for modeling the upper ocean in a high-CO2 world
Marine food webs influence climate by channeling carbon below the permanent pycnocline, where it can be sequestered. Because most of the organic matter exported from the euphotic zone is remineralized within the "upper ocean" (i.e., the water column above the depth of sequestration), the resulting CO2 would potentially return to the atmosphere on decadal timescales. Thus ocean-climate models must consider the cycling of carbon within and from the upper ocean down to the depth of sequestration, instead of only to the base of the euphotic zone. Climate-related changes in the upper ocean will influence the diversity and functioning of plankton functional types. In order to predict the interactions between the changing climate and the ocean's biology, relevant models must take into account the roles of functional biodiversity and pelagic ecosystem functioning in determining the biogeochemical fluxes of carbon. We propose the development of a class of models that consider the interactions, in the upper ocean, of functional types of plankton organisms (e.g., phytoplankton, heterotrophic bacteria, microzooplankton, large zooplankton, and microphagous macrozooplankton), food web processes that affect organic matter (e.g., synthesis, transformation, and remineralization), and biogeochemical carbon fluxes (e.g., photosynthesis, calcification, respiration, and deep transfer). Herein we develop a framework for this class of models, and we use it to make preliminary predictions for the upper ocean in a high-CO2 world, without and with iron fertilization. Finally, we suggest a general approach for implementing our proposed class of models
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