Publications of Krivtsov, V.

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.

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.

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.

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.