Publications of Krivtsov, V.

Ecological study of Stephanodiscus rotula during a spring diatom bloom: dynamics of intracellular elemental concentrations and correlations in relation to water chemistry, and implications for overall geochemical cycling in a temperate lake

The population of Stephanodiscus rotula in Rostherne Mere (Cheshire, UK) was studied during the spring of 1996 using light microscopy, hydrochemical analyses and scanning electron microscopy X-ray microanalysis (SEM XRMA). Data obtained were statistically analysed to reveal relationships between the elements within the cells and in the lake water. The results confirmed that algal biomass production was mainly controlled by availability of Si. The depletion of ambient Si coincided with changes in the intracellular elemental composition, particularly a considerable reduction of intracellular Si whose depletion was also reflected in changes of intracellular elemental correlations and ratios. A simple structural model of elemental relationships in S. rotula cells is proposed to explain the pattern of intracellular elemental associations. This model can be used as a reference for species response to changes in environmental parameters. S. rotula provided a significant contribution to overall biogeochemical cycling due to the removal of nutrients from the water column and transporting them to bottom sediments following the culmination of the bloom. The role of S. rotula was particularly prominent in the biogeochemical cycle of silicon, as the diatom's spring growth may account for more than 20% of its total annual loss to bottom sediments. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Water and nutrient budgeting of Rostherne Mere, Cheshire, UK

This paper presents a simple model of the Rostherne Mere water catchment. The model allows calculation of water and elemental budgets on meteorological data using regressions derived from a monitoring data set. Estimates of yearly inputs and outputs of Ca, Mg, K and N suggest that the lake is currently acting as a sink for these elements. Comparison with earlier estimates shows an increase in N loading. High inputs of particulate P appear to be balanced by outputs of inorganic P, suggesting that the two may be coupled by certain physical and biogeochemical processes within the lake ecosystem, including adsorption to/desorption from particles, coprecipitation with various chemical compounds, uptake and excretion by biota, mineralisation and release from sediments.

Elemental concentrations and correlations in winter micropopulations of Stephanodiscus rotula: an autecological study over a period of cell size reduction and restoration

The population of Stephanodiscus rotula in a temperate eutrophic lake was studied over a 2 month period (January to early March). Although during the study period nutrient concentrations in the lake water remained far in excess of phytoplankton requirements, no notable increase in chlorophyll levels was recorded. This suggests that algae were limited by shortage of light and low temperatures. Frequency distribution analysis of cell diameters showed a dramatic size reduction at the end of winter, followed by restoration of higher values by early spring. Electron probe X-ray microanalysis spectra from Stephanodiscus cells routinely showed peaks of Ca, K, Si, P, S, Fe. Al, Mn, Mg, Na and Cl, with substantial variation in elemental concentrations both between and within samples. End-of-winter reduction in the cell size coincided with a considerable depletion of intracellular chemical levels of Si, P, Cl and K and could be related to the concurrent decrease in dissolved organic C and increase in intracellular Al. Correlation and factor analysis of intracellular elemental concentrations showed that statistical elemental associations within Stephanodiscus cells were mainly determined by three factors, with P, Cl, Si and K showing higher loadings on the first, Ca, Mn and S on the second, and Fe, Ca and Al on the third factor. Significant correlations among the elements of the first association may indicate the importance of P (ATP), K (through involvement in P metabolism) and Cl (possibly charge balance) in the active Si uptake during the study period.

Determination of P Release from Rostherne Mere Sediment Cores

Sediment cores taken in November 1998 were subsequently incubated under a range of experimental conditions simulating the most important situations likely to occur in Rostherne Mere. Sediment P release into the apparently aerobic water under undisturbed conditions was estimated at 7...11 mg/(m2 d). Disturbance of the boundary layer by mixing, and/or an increase in nitrate concentration resulted in either absence of any conclusive release or even loss of the already released P back to the sediments. Estimates of P release obtained in this study are only slightly lower than the estimate obtained more than 10 years ago before sewage diversion, suggesting that rapid changes in the lakes trophic status are at present unlikely

A one-year study of the Rostherne Mere ecosystem: seasonal dynamics of water chemistry, plankton, internal nutrient release, and implications for long-term trophic status and overall functioning of the lake

A study of Rostherne Mere (Cheshire, UK) was undertaken to reveal relationships among various ecosystem components and assess the progress in lake recovery following sewage diversion. An intensive monitoring programme included measurements of dissolved oxygen. T, pH, electric conductivity, K, Mg, Ca, Si, N and P species, Secchi depth, suspended solids, chlorophyll-a, phyto- and zooplankton counts. Recorded changes and the results of correlation analysis broadly confirmed to the classic limnoecological theory and allowed detailed interpretation of the dynamics observed. The Si level was used to estimate the amount of diatom detritus produced in spring. Chemical profiles were used to estimate the amount of nutrients ac:cumulated in the hypolimnion during the stratified period. These estimates were compared with simulations made using the model of exponential decay, which showed a good performance in the case of Si but considerable underestimation in the case of P. The differences between the values simulated by the model and estimates based on field observations resulted from the additional P release from the sediments. This release could have been stimulated by a combination of factors, including the development of anoxic layers on the sediment-water interface and Si-induced desorption from Fe, Al and Mn oxides. Internal P loading during the stratified period was thus estimated at about 4-9 g/m(2), suggesting that rapid changes in the lake's trophic status are at present unlikely. Certain aspects of this work (including interpretation of interrelationships between ecosystem components, estimation of the decomposition constant. analysis of factors controlling nutrient accumulation in the hypolimnion, and a low-cost method to estimate intern; l P release) may be useful for studies of other aquatic systems and have, therefore, general limnological applicability. Copyright (C) 2001 John Wiley & Sons, Ltd.

Interrelations between Si and P biogeochemical cycles – a new approach to the solution of the eutrophication problem

Because the biogeochemical cycles of P and Si in temperate lakes are strongly connected by the dynamics of primary producers, it should be possible to influence the former cycle by causing changes in the latter. It is shown using the mathematical model 'Rostherne' that winter levels of ambient Si have a major influence both on spring levels of ambient P and on the summer cyanobacterial maxima. Additions of Si to the lakes could be used for the fine regulation of the biogeochemical balance and may prescribe a recipe for improvement of water quality, as well as a new solution to the problem of eutrophication. Copyright (C) 2000 John Wiley & Sons, Ltd.

Incorporation of the intracellular elemental correlation pattern into simulation models of phytoplankton uptake and population dynamics

Compelling evidence of complex statistical relationships among various elements contained within phytoplankton cells has traditionally been ignored in models of algal nutrient uptake and population dynamics. Here we present a new approach, incorporating a phytoplankton intracellular elemental correlation pattern into the existing dynamic simulation model of a freshwater lake. Within this approach, uptake and cycling of elements that are likely to become limiting during the simulation period are described by ordinary differential equations. Dynamics of nutrients that are unlikely to become limiting are described either by differential equations or, when more practicable, by multiple regressions on environmental variables and cell quotas of other elements. This allows an easy simultaneous consideration of a wide range of elements. The model adopting the described approach was tested on a data set for Rostherne Mere, Cheshire, UK. It showed a good fit between observations and simulations for all considered variables, including the population dynamics of Ceratium hirundinella and Microcystis aeruginosa, the outcome of interspecific competition and changes in concentrations within algal cells and in the surrounding lake water. The approach could easily be implemented in models of bioreactors, chemostat experiments and aquatic ecosystems.

Modelling of elemental associations in Anabaena

Changes in the elemental composition of the blue-green alga Anabaena at Rostherne Mere (Cheshire, U.K.) were studied over an extended bloom period in 1996. Elemental analysis was carried out in relation to intracellular concentrations, intracellular elemental correlations, time series correlations and total quantities of elements in lake water. Most of the intracellular elements showed considerable variation both between and within samples. Correlations between these elements also varied considerably, though certain pairs – K/Cl, K/Mg, P/K and P/Mg were significantly correlated in at least 50% of the samples. Analysis of the time series demonstrated clear correlations between lake water P and intracellular P and Mg. The growth of Anabaena coincided with a considerable increase in the overall amount of chemicals contained in its population. The total amounts of chemicals in the lake water, however, always substantially exceeded those present in the population of Anabaena cells. Elemental relationships within cells are considered in relation to an elemental correlation pattern, consisting of three main groupings – Mg-K-P, Na-S and Ca-Cl. The structural model of intracellular elemental associations presented here may be helpful in the investigation of algal response to changes in environmental parameters.The incorporation of this model into a dynamic model simulating phytoplankton–environmental interactions is discussed.

X-ray microanalytival study of the Protozoan Ceratium hirundinella from Rostherne Mere (Cheshire, UK) : dynamics of intracellular elemental concentrations, correlations and implications for overall ecosystem functioning

The population of Ceratium hirundinella in a freshwater lake was studied both by traditional limnological methods and X-ray microanalysis during the final phase of stratification. Correlation and Factor analysis of intracellular elemental concentrations revealed a clear pattern of statistical relationships mainly determined by three factors, with K, P and Mg demonstrating higher loadings on the first, Ca and Si on the second, and S, Cl and Na on the third factor. S also showed significant correlations with K and P, elements belonging to the first association. The results of the statistical analysis are summarised in a structural model of intracellular elemental associations. Consideration of the latter in the mathematical description of nutrient uptake and plankton population dynamics should prove useful in aquatic modelling. Development of the population resulted in a dramatic decrease in transparency and nutrient concentrations in the epilimnion water. The recorded peak of the population coincided with minimum levels of ambient PO4.P and DIN and maximum intracellular levels of a number of important chemicals (Mg, K, P, S). The decrease in the epilimnion concentrations of orthophosphate did not match the simultaneous increase in the total amount of P contained within the population of Ceratium. Ambient nutrient scarcity, therefore, could have been alleviated by Ceratium's capacity for vertical movements, heterotrophic consumption of organic materials and/or partial entrainment of the hypolimnion water during periods of colder weather. Following the demise of the population, chemicals accumulated inside the cells were removed from the epilimnion with easily sedimented biomass. The results clearly showed the importance of Ceratium in transforming nutrients between different levels of the Rostherne ecosystem. Considering the high lake water retention time, the geochemical consequences of Ceratium's ecochemistry and population dynamics may considerably delay changes in the lake's trophic status expected after sewage diversion.

Indirect regulation rule for consecutive stages of ecological succession

The lake ecosystem model 'Rostherne' allowed a theoretical insight into delayed causal relationships in aquatic ecosystems. Model simulations were used to demonstrate the possibility of influencing a species dominant at a later stage of ecological succession, by alleviating growth limitation of a different species, dominant at an earlier stage. Such delayed relationships are characteristic of various types of systems (including ecological), and can be illustrated by using a simple Stella model presented here. The stated indirect regulation rule for consecutive stages of ecological succession provides an important theoretical basis both for certain ecological manipulations and for the better understanding of various environmental relationships. It should, therefore, prove useful for theoretical analysis of system dynamics, studies of terrestrial and aquatic ecosystems, management of natural resources, Environmental Assessment and Auditing. (C) 2000 Elsevier Science B.V. All rights reserved.

Examination of the phytoplankton of Rostherne Mere using a simulation mathematical model

Changes of phytoplankton populations in Rostherne Mere in 1996 were examined by means of simulation mathematical models. Simple models, solely based on Monod or Michaelis Menten equations, failed to give a reasonable simulation of the phytoplankton succession. A more complex model Rostherne (version 1.1a) calibrated on an extensive set of XRMA and conventional data, however, proved to be useful both for prediction of the outcome of the spring and summer competition and for the estimation of values of certain non-measured variables. It also helped to identify the limiting factors for different times of the year. Alteration of the simulated magnitude of the spring diatom bloom had a major influence on summer cyanobacterial maxima, demonstrating fine regulation of the biogeochemical balance within the modelled system.

Elemental concentrations, correlations and ratios in micropopulations of Ceratium hirundinella (Pyrrhophyta): an X-ray microanalytical study

The elemental composition of <e1>Ceratium hirundinella</e1> was determined in mixed phytoplankton samples collected over a 2-month period (late June to early September 1995). Electron probe X-ray microanalysis spectra of single cells routinely showed clear peaks of monovalent (Na, K) and divalent (Mg, Ca) cations, plus Si, P, S and Cl. Considerable variation in elemental concentrations occurred both within and between samples (20 cells). The overall ratio of monovalent to divalent cations was relatively constant at about 1.3. Intracellular concentrations of anions/cations and electronegative/electropositive elements were significantly correlated in most samples, suggesting a controlled internal balance of these ionic groups. Correlation analysis of elemental concentrations revealed clear patterns of statistical association within individual samples, with significant positive correlations between particular pairs of elements (Mg–P, K–Cl and K–P). Some pairs of highly correlated elements (particularly Mg–P) occurred in defined ratios throughout the samples, while others (e.g. K–Cl, Na–Mg) were variable. Factor analysis showed that elemental associations were determined by two Principal Factors in most samples, possibly related to insoluble and soluble components of cells. Decrease in the level of available P in lake water was associated with a sharp decrease in the intracellular P concentration and the concentration of other correlated elements. The estimated C/P ratio markedly differed from the classical (Redfield) value at times of low P availability. The concentration of P in cells of <e1>Ceratium</e1> was approximately 10⁴ times greater than in the surrounding water medium (total P) compared with values of 10²–10³ for K.

Application of SEM XRMA data to lake ecosystem modelling

The model Rostherne represents the first attempt to apply SEM XRMA (scanning electron microscopy X-ray microanalysis) data to lake ecosystem modelling. It considers subsystems proved to be most important for Rostherne mere (Cheshire, UK) with incorporation of uptake dependency of one nutrient upon internal deficiency in another. The model showed a reasonable fit (R-2 = 0.87, P < 0.001) between measured data and simulation curves for most of the considered variables (i.e. P, Si, chlorophyll-a and algal concentrations in the lake water, nutrient mass fractions of algal cells, etc.) and could, therefore, have been used to estimate some parameters and variables which were not measured otherwise (e.g. sedimentation and growth rates, etc.). The possibility of incorporating alternative expressions for the processes considered is discussed and tasks for future research in relation to coupling of Various submodels with the proposed submodel of nutrient uptake are envisaged. (C) 1998 Elsevier Science B.V. All rights reserved.