A simple model of plankton population dynamics coupled with a LES of the surface mixed layer

The concentration of phytoplankton in the sea is affected by biological processes, such as growth/mortality rates, predatory zooplankton concentrations and nutrient levels. Phytoplankton concentrations are also influenced by physical processes, in particular the mixing properties of the local fluid environment. On planktonic scales (approximately 10-1000 microm) one can assume the local turbulent flow is isotropic, with no distinction between horizontal and vertical mixing. However, agglomerations of phytoplankton into patches are observed on larger scales of up to hundreds of metres, whose formation will be influenced by the anisotropic advection/mixing properties and large-eddy structures prevalent in the surface mixed layer. This paper presents the results of the coupling of a large-eddy simulation (LES) model of the mixed layer with an advection-diffusion system of coupled equations for nitrate-phytoplankton-zooplankton (NPZ) concentration, incorporating sub-grid parameterizations of the biological processes. Typically these include phytoplankton growth due to light levels and ambient nitrate concentration, offset by grazing losses due to the presence of zooplankton. The primary goal of this work is to investigate how the characteristics of the mixed layer turbulence influence the observed distribution of phytoplankton. One novel feature is the incorporation of a 'vortex-force' term in the LES code in order to generate Langmuir circulations. It has been speculated that the enhanced mixing rates associated with 'Langmuir turbulence' play a significant role in regulating planktonic activity. Results derived from the coupled LES-NPZ model, run with and without the presence of Langmuir circulations, are presented in order to investigate these ideas.     

An ecosystem model for the assessment of plankton production in relation to the survival of young fish

A mixed upper layer ecosystem simulation model is used to assessthe effect of changes in the zooplankton stock on young salmonidgrowth and survival. The effect of physical factors such aslight extinction and mixed layer depth cause salmonid productionto optimize at conditions which favour maximum zooplankton standingstock. However, the overwintering stock of ctenophores may decimatethe zooplankton and cause a reduction in young salmonid growthand survival, particularly in unseasonally warm years. The standingstock of salmonids and the time of release have less effecton growth than either physical factors or ctenophores.     

Mathematical model for the ecosystem response of Lake Ladoga to phosphorus loading

The ecosystem response model described in this paper combines an ecosystem model and a three-dimensional circulation model of Lake Ladoga developed earlier by the authors. The ecosystem model describes the process of Lake Ladoga eutrophication, and its biological submodel describes changes in the phyto- and zooplankton. In the earlier model version, lake circulation was determined using a two-dimensional hydrodynamical model which was not completely adequate. The present model allows calculation of the distributions of phyto- and zooplankton and mineral phosphorus and nitrogen. One of its main advantages is that reliable computations of the ecosystem dynamics over an extended period of time are possible. The response of the ecosystem to different levels of phosphorus pollution loading and to weather conditions is studied.     

Seasonal changes in particulate and dissolved lipids in a eutrophic prairie lake

1. Critical periods of lipid energy transfer from phyto- to zooplankton were inferred by comparing seasonal patterns of particulate and dissolved lipid fractions in lake water with temporal changes in lipid energy reserves of the zooplankton in a hypereutrophic lake.
2. The midsummer phytoplankton community was dominated by the bloom-forming cyanobacterium Aphanizomenon flos-aquae. The collapse of the bloom was accompanied by a 2-week period of severe nitrogen deficiency after which there was a marked increase in the concentration of lipid energy reserves in the particulate (algal) fraction.
3. Areal lipid energy reserves of the dominant herbivorous zooplankton responded positively to changes in the tri-and diacylglycerol content of the particulate fraction of lake water in a species-specific manner.
4. Bacterial numbers also peaked in September concomitant with a large increase in free fatty acids in the dissolved lipid fraction probably produced by the decay of the
A. flos-aquae bloom.
5. The association between periods of nitrogen deficiency and increased energy reserve lipids in the particulate fraction supports observations made with laboratory algal cultures that periods of nutrient deficiency may intensify lipid synthesis in some algal species, thereby enhancing the rate of lipid energy transfer from phytoplankton to zooplankton.     

Modeling microzooplankton and macrozooplankton dynamics within a coastal upwelling system

A simple nutrient–phytoplankton–zooplankton (NPZ)pelagic ecosystem model coupled to a two-dimensional primitiveequation circulation model with explicit mixed-layer physicsis configured in a coastal setting to study the biological responseto idealized wind-driven upwelling conditions. Conventionalecosystem model parameterization, which assumes macrozooplanktonas the target grazers, leads to upwelling-induced phytoplanktonblooms that exhaust available nutrient supply and whose zonalscale increases with wind duration. Offshore zooplankton maximaresult from upwelled water with greater total nitrogen concentrationsthan initial ambient surface water. Substantial vertical mixingin the surface boundary layer sets the vertical scale of theproductivity. Phytoplankton sinking contributes to a nearshoreaccumulation of total nitrogen, and enhances the magnitude andduration of the phytoplankton bloom. The system responds differentlywhen the zooplankton are parameterized to represent microzooplankton.The phytoplankton and zooplankton maxima have more limited zonalextent, are more independent of the duration of wind forcing,and near-surface nutrient levels remain high over most of thedomain. When winds are relaxed, the diminished offshore transportreveals the underlying biological oscillations in the microzooplankton-parameterizedecosystem, and reduced vertical mixing decouples surface fromsubsurface dynamics. In contrast, the macrozooplankton systemrelaxes to a steady state supporting limited phytoplankton andlarge zooplankton levels in the upwelling region.     

Adding Detritus to a Nutrient-Phytoplankton-Zooplankton Model:A Dynamical-Systems Approach

The dynamics of two plankton population models are investigatedto examine sensitivities to model complexity and to parametervalues. The models simulate concentrations of nutrients, phytoplankton,zooplankton and detritus in the oceanic mixed layer. In Model1, zooplankton can graze only upon phytoplankton, whereas inModel 2 they can graze upon phytoplankton and detritus. Bothfeeding strategies are employed by zooplankton in the ocean,and both are features of models in the literature. Each modelhere consists of four coupled ordinary differential equations,and can exhibit unforced oscillations (limit cycles) of thefour concentrations. By constructing diagrams that show howsteady states and oscillations persist as each parameter isvaried, a general picture of the dynamics of each model is builtup. The addition of the detritus pool to an earlier nutrient–phytoplankton–zooplanktonmodel appears to have little influence on the dynamics whenthe zooplankton cannot graze upon the detritus (Model 1), butif the zooplankton can graze upon the detritus (Model 2), thenthe dynamics are affected in a significant way. These results,obtained using the theory of dynamical systems, enhance ourknowledge of the factors governing the dynamics of planktonpopulation models.     

Summer dynamics of the deep chlorophyll maximum in Lake Tahoe

Vertical profiles of chlorophyll and phytoplankton biomass weremeasured in Lake Tahoe from July 1976 through April 1977. Adeep chlorophyll maximum (DCM) persisted during summer and earlyautumn (July—October) near 100 m, well below the mixedlayer and at the upper surface of the nitracline. The DCM coincidedwith the phytoplankton biomass maximum as determined from cellcounts. In addition, the composition of the phytoplankton assemblagewas highly differentiated with respect to depth. Cyclotellastelligera was the predominant species in the mixed layer whilethe major species in the DCM layer included C. ocellata andseveral green ultraplanktonic species. In situ cell growth playsa substantial role in maintaining the DCM, but sinking of cellsfrom shallower depths and zooplankton grazing above the DCMmay contribute to the maintenance of the DCM. Calculations supportthe interpretation that the summer DCM persists at the boundarybetween an upper, nutrient-limited phytoplankton assemblageand a deeper, light-limited assemblage.     

Vertical stratification of physical,chemical and biological components in two saline lakes Shira and Shunet (South Siberia,Russia)

A feature of meromictic lakes is that several physicochemical and biological gradients affect the vertical distribution of different organisms. The vertical stratification of physical, chemical and biological components in saline, fishless meromictic lakes Shira and Shunet (Siberia, Russia) is quite different mainly because both mean depth and maximum depth of lakes differ as well as their salinity levels differ. The chemocline of the Lake Shira, as in many meromictic lakes, is inhabited by bacterial community consisting of purple sulphur and heterotrophic bacteria. As the depth of the chemocline is variable, the bacterial community does not attain high densities. The mixolimnion in Lake Shira, which is thermally stratified in summer, also creates different habitat for various species. The distribution of phytoplankton is non-uniform with its biomass peak in the metalimnion. The distribution of zooplankton is also heterogeneous with rotifers and juvenile copepods inhabiting the warmer epilimnion and older copepods found in the cold but oxic hypolimnion. The amphipod Gammarus lacustris which can be assigned to the higher trophic link in the fishless lake’s ecosystem, such as Lake Shira, is also distributed non-uniformly, with its peak density generally observed in the thermocline region. The chemocline in Lake Shunet is located at the depth of 5 m, and unlike in Lake Shira, due to a sharp salinity gradient between the mixolimnion and monimolimnion, this depth is very stable. The mixolimnion in Lake Shunet is relatively shallow and the chemocline is inhabited by (1) an extremely dense bacterial community; (2) a population of Cryptomonas sp.; and (3) ciliate community comprising several species. As the mixolimnion of Lake Shunet is not thermally stratified for long period, the phytoplankton and zooplankton populations are not vertically stratified. The gammarids, however, tend to concentrate in a narrow layer located 1–2 m above the chemocline. We believe that in addition to vertical inhomogeneities of both physicochemical parameters, biological and physical factors also play a role in maintaining these inhomogeneities. We conclude that the stratified distributions of the major food web components will have several implications for ecosystem structure and dynamics. Trophic interactions as well as mass and energy flows can be significantly impacted by such heterogeneous distributions. Species spatially separated even by relatively short distances, say a few centimetres will not directly compete. Importantly, we demonstrate that not only bacteria, phytoflagellates and ciliate tend to concentrate in thin layers but also larger-sized species such Gammarus (amphipods) can also under certain environmental conditions have stratified distribution with maxima in relatively thin layer. As the vertical structure of the lake ecosystem is rather complex in such stratified lakes as ours, the strategy of research, including sampling techniques, should consider potentially variable and non-homogeneous distributions.     

Effects of a filter-feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto- and zooplankton in a mesotrophic reservoir: results from an enclosure experiment

SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto- and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m⁻³) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir.
2. Phytoplankton biomass <30 μm and particulate organic carbon (POC) <30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish-free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton.
3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.     

模拟浮游生物的季节变化

本文讨论了海洋中光合作用过程和计算光合作用的速率方程。运用垂直平均值的浮游生物模型研究了在一个固定站和扩展湾流体系中,浮游植物、浮游动物和营养盐的季节变化。结果表明,夹卷过程将含营养盐丰富的中层水带入了混合层,从而导致浮游植物的生长;摸式较好地给出了扩展湾流体系中的水华季,基本上得到了浮游生物的生活周期。使用不同的参数值进行了若干实验,并对结果进行了讨论。     

Habitat structure determines resource use by zooplankton in temperate lakes

While the importance of terrestrial linkages to aquatic ecosystems is well appreciated, the degree of terrestrial support of aquatic consumers remains debated. Estimates of terrestrial contributions to lake zooplankton have omitted a key food source, phytoplankton produced below the mixed layer. We used carbon and nitrogen stable isotope data from 25 Pacific Northwest lakes to assess the relative importance of particulate organic matter (POM) from the mixed layer, below the mixed layer and terrestrial detritus to zooplankton. Zooplankton and deep POM were depleted in 13C relative to mixed layer POM in lakes that can support deep primary production. A Bayesian stable isotope mixing model estimated that terrestrial detritus contributed <5% to zooplankton production, and confirms the role of lake optical and thermal properties; deep POM accounted for up to 80% of zooplankton production in the clearest lakes. These results suggest terrestrial support of lake zooplankton production is trivial.     

A study of time-dependent primary productivity in a natural upper-ocean mixed layer using a biophysical model

A one-dimensional biophysical model of time-dependent photosynthesisin the upper-ocean mixed layer was applied near 36.5°N,74.5°W. The photosynthesis submodel was formulated and parameterizedbased on daylong, light-manipulation experiments performed ontwo phytoplankton communities collected prior to dawn on October13, 1992: one from 5 m depth in the upper mixed layer and theother from 28 m depth below the pycnocline. Time course biologicalmeasurements included chlorophyll a concentration, primary productivityand DCMU fluorescence ratio. The biophysical model was thenused to predict the physical and biological response of thewater column on October 14, 1992. A pre-dawn conductivity-temperature-depth-fluorescence-transmission-rosette(CTDFT-rosette) cast provided data to initialize the water columnstratification and the distribution of phytoplankton biomass.The biophysical model was forced using meteorological and oceanographicmeasurements collected continuously throughout the subsequentdaylight period. CTDFT-rosette casts and measurements of chlorophylla concentration, primary productivity and DCMU fluorescenceratio throughout that daylight period provided the data forcomparison with the model predictions. In general, the biophysicalmodel predicted the physical and biological evolution of thesampled water column. Although the inclusion of vertical mixinginitially improved the accuracy of prediction, agreement decreasedwith time, especially in the lower part of the water column.The one-dimensional model suffered from the effects of excludedhorizontal gradients.     

基于稳定同位素技术的保安湖食物网结构特征研究

文章利用碳、氮稳定同位素技术对江湖阻隔典型湖泊-保安湖的食物网结构进行了研究。结果表明保安湖中鱼类消费者的主要营养级范围为2.1—3.3, 在调查到的16种鱼类中, 顶级肉食性鱼类种类很少, 杂食性鱼类的种类最多。保安湖食物网主要由两条营养传递途径构成, 即由POM、浮游植物为主要食物源的浮游牧食链与沉积物为主要食物源的底栖食物链。POM、浮游植物、浮游动物和底栖动物是保安湖水域食物网中鱼类的主要食物来源, 其次是沉积物中的碎屑和水生植物等。此外, 从基于理论食性数据的食物网与BIMM模型预测的食物网结构可以看出, 从POM、浮游植物、浮游动物到杂食性鱼类的浮游牧食链在整个食物网中具有主导性, 而从水生植物、沉积物和底栖动物到杂食性鱼类的底栖食物链相对重要性较低。     

Productivity-diversity relationships in lake plankton communities

One of the most intriguing environmental gradients connected with variation in diversity is ecosystem productivity. The role of diversity in ecosystems is pivotal, because species richness can be both a cause and a consequence of primary production. However, the mechanisms behind the varying productivity-diversity relationships (PDR) remain poorly understood. Moreover, large-scale studies on PDR across taxa are urgently needed. Here, we examined the relationships between resource supply and phyto-, bacterio-, and zooplankton richness in 100 small boreal lakes. We studied the PDR locally within the drainage systems and regionally across the systems. Second, we studied the relationships between resource availability, species richness, biomass and resource ratio (N:P) in phytoplankton communities using Structural Equation Modeling (SEM) for testing the multivariate hypothesis of PDR. At the local scale, the PDR showed variable patterns ranging from positive linear and unimodal to negative linear relationships for all planktonic groups. At the regional scale, PDRs were significantly linear and positive for phyto- and zooplankton. Phytoplankton richness and the amount of chlorophyll a showed a positive linear relationship indicating that communities consisting of higher number of species were able to produce higher levels of biomass. According to the SEM, phytoplankton biomass was largely related to resource availability, yet there was a pathway via community richness. Finally, we found that species richness at all trophic levels was correlated with several environmental factors, and was also related to richness at the other trophic levels. This study showed that the PDRs in freshwaters show scale-dependency. We also documented that the PDR complies with the multivariate model showing that plant biomass is not mirroring merely the resource availability, but is also influenced by richness. This highlights the need for conserving diversity in order to maintain ecosystem processes in freshwaters.     

Determination Analysis as a Tool to Investigate the Contingencies of Various Components of Biocenosis

Specific application of determination analysis to investigate the contingencies of various components of natural biocenosis was illustrated by the example of fish production and biomass of phyto- and zooplankton. Determination analysis confirms the theoretic assumptions on food preferences of herbivorous fish: both silver and bighead carps avoided feeding on cyanobacteria. Being a facultative phytoplankton feeder, silver carp preferred microalgae to zooplankton. Determination analysis allowed us to demonstrate the contingencies of the mean biomass of phyto- and zooplankton during both the whole fish production cycle and its individual periods.__________Translated from Izvestiya Akademii Nauk, Seriya Biologicheskaya, No. 3, 2005, pp. 327–335.Original Russian Text Copyright © 2005 by Bulgakov, Maximov.     

Limnological Aspects of Some Moroccon Atlas Lakes,with Reference to Some Physical and Chemical Variables,the Nature and Distribution of the Phyto- and Zooplankton,Including a Note on Possibilities for the Development of an Inland Fishery

Four mountain lakes were studied. Two are eutrophic, one is mesotrophic and one is oligotrophic. The first three have a well-developed phyto- and zooplankton, with a relatively high diversity index. There are differences between the faunas that may be ascribed to differences in surface, depth, altitude and also to biological phenomena such as interspecies competition. The Ifni Lake is a genuine high-mountain lake, subject to severe environmental conditions. It has a very simple food chain: there is only one species of algae, an unicellular blue-green, one copepod species, one rotifer and one fish. In general some features of a definite interest for fundamental limnology were found: the presence of a pelagic Rhabdocoelid in Dayat Ifrah, some particularities in the vertical distribution of a number of plankton species, the existence of a layer of relatively warm water near the bottom of Lake Ifni. The latter is explained by a strong bottom current which flows through the lake between its phreatic in- and outlets. Finally, stress is laid on the under-exploitment of Moroccon inland waters with respect to a freshwater fishery. Yet, perspectives are good and some suggestions are forwarded.     

Modelling lake cyanobacterial blooms: Disentangling the climate‐driven impacts of changing mixed depth and water temperature

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Zooplankton mortality and the dynamical behaviour of plankton population models

We investigate the dynamical behaviour of a simple plankton population model, which explicitly simulates the concentrations of nutrient, phytoplankton and zooplankton in the oceanic mixed layer. The model consists of three coupled ordinary differential equations. We use analytical and numerical techniques, focusing on the existence and nature of steady states and unforced oscillations (limit cycles) of the system. The oscillations arise from Hopf bifurcations, which are traced as each parameter in the model is varied across a realistic range. The resulting bifurcation diagrams are compared with those from our previouswork, where zooplankton mortality was simulated by a quadratic function—here we use a linear function, to represent alternative ecological assumptions. Oscillations occur across broader ranges of parameters for the linear mortality function than for the quadratic one, although the two sets of bifurcation diagrams show similar qualitative characteristics. The choice of zooplankton mortality function, or closure term, is an area of current interest in the modelling community, and we relate our results to simulations of other models.