Aquatic Ecosystem Model Research Articles

Recent trends in the development of ecological models applied on aquatic ecosystems.

This paper presents an overview of the application of models on aquatic ecosystems. More than 17% of the models published in the focal journal in the field, Ecological Modelling, are aquatic ecosystem models. An increasing number of papers are dealing with the theoretical aspects of modeling – new modeling approaches and techniques, how models can be used to reveal ecosystem properties, and how models can better reflect the properties of ecosystems. This development implies that today we have more types of models. The characteristics, the advantages, and the disadvantages of these model types are presented briefly. The selection criteria for the presented model types are discussed, and the application of these types to models for aquatic ecosystems is reviewed. A recent improvement in model calibration of particular interest for aquatic ecosystems is presented, and the perspectives resulting from this new calibration procedure and from possible hybrids of the presented model types are discussed.

Chlorodifluoroacetic acid fate and toxicity to the macrophytesLemna gibba, Myriophyllum spicatum, andMyriophyllum sibiricumin aquatic microcosms

AbstractChlorodifluoroacetic acid (CDFA) is a novel haloacetic acid (HAA) and has been recently documented in aquatic systems. It is a suspected degradation product of the refrigerants 1,1,2‐trichloro‐1,1‐difluoroethane (CFC‐113) and 1‐chloro‐1,1‐difluoroethane (HCFC‐142b). Haloacetic acids can be phytotoxic, putatively acting through inhibition of the citric acid cycle. Replicate (n= 3) 12,000‐L model aquatic ecosystems (microcosms) were dosed once at 0.5, 1, 5, and 20 mg/L of neutralized CDFA. Three microcosms served as controls. Each microcosm was stocked with eight individual apical shoots of bothMyriophyllum spicatumandMyriophyllum sibiricumand sampled at regular intervals over a 42‐d exposure period. The plants were assessed for the somatic endpoints of plant length, root growth, node number, and wet and dry mass and the biochemical endpoints of chlorophyll‐a/band carotenoid content as well as citric acid levels. The duckweedLemna gibbawas also introduced into these systems and monitored over a period of 14 d for wet/dry mass, plant/frond number, chlorophyll content, and growth rate. Concentrations of CDFA remained constant in the water column over the course of the fate investigation (241 d), indicating that this compound undergoes little, if any, degradation in aquatic systems. Results showed few statistically significant differences from controls for all three plant species with exposure to CDFA but no biologically relevant impacts. Overall, CDFA does not appear to pose any risk to these aquatic macrophytes at current environmental concentrations.

An inverse relationship between stability and maturity in models of aquatic ecosystems

The relationships among stability, maturity and complexity of ecosystems have been studied for many years. These ecosystem attributes are important in ecological theory and also for the appropriate management of resources. In a previous work, we proposed the U index, which accounts for complexity and recycling of detritus, and measures maturity in ecosystems according to attributes of E. P. Odum's theory of ecosystem development. Here we compare the behaviour of the U index and its ability to measure stability. Using models of ecosystems, we simulated disturbances and measured stability through resistance, persistence, resilience and recovery time. We found that the index is correlated with these attributes, but in an inverse way to that expected; that is, immature ecosystems were more stable and stability decreased when the systems matured.

Strategies for Transforming Fine Scale Knowledge to Management Usability

Simulation tools used for management purposes should fulfill several conditions by being computationally fast, user-friendly, realistic, generic and reliable. These traits are often counteracting since they simultaneously demand for model complexity as well as simplicity. Here we develop a strategy to overcome this general problem of environmental modelling for management use. Major ingredients are model analysis and reduction as new core components of the modelling process. In detail, a set of combined methods is proposed. Within a large class of models the set allows for automatically exploring model behaviour and for aggregating fine scale process knowledge together with spatio temporal resolution. Applications to a huge aquatic European regional seas ecosystem model (ERSEM), a complex photosynthesis model (PGEN) as well as a simple diagenetic model are presented. The analysis and aggregation methods provide first steps towards a new generation of decision support tools able to cope with an increase in scientific knowledge as well as management demands.

Natural control mechanisms in models of aquatic ecosystems

Based on cybernetic categories of natural control mechanisms, four generations of ecosystem models are distinguished: feed-forward, feedback, self-adaptation and self-organization models. The analysis of the natural control mechanisms in aquatic ecosystems suggests that different processes are controlled in different ways, and, although the four mechanisms were identified in historical sequence, they all operate simultaneously. The concept of self-organization of an ecosystem is introduced and specified for a model of an aquatic pelagic ecosystem. The concept of the ecosystem as a multilayer, multigoal and multiechelon hierarchical system with hierarchy of the levels of biological organization is also introduced.

Optimization of exergy and implications of body sizes of phytoplankton and zooplankton in an aquatic ecosystem model

Size appears to be an important parameter in ecological processes. All physiological processes vary with body size ranging from small microorganisms to higher mammals. In this model, five state variables — phosphorus, detritus, phytoplankton, zooplankton and fish are considered. We study the implications of body sizes of phytoplankton and zooplankton for total system dynamics by optimizing exergy as a goal function for system performance indicator. The rates of different sub-processes of phytoplankton and zooplankton are calculated, by means of allometric relationships of their body sizes. We run the model with different combinations of body sizes of phytoplankton and zooplankton and observe the overall biomass of phytoplankton, zooplankton and fish. The highest exergy values in different combinations of phytoplankton and zooplankton size indicate the maximum biomass of fish with relative proportions of phytoplankton and zooplankton. We also test the effect of phosphorus input conditions corresponding to oligotrophic, mesotrophic, eutrophic system on its dynamics. The average exergy to be maximized over phytoplankton and zooplankton size was computed when the system reached a steady state. Since this state is often a limit cycle, and the exergy copies this behaviour, we averaged the exergy computed for 365 days (duration of 1 year) in the stable period of the run. In mesotrophic condition, maximum fish biomass with relative proportional ratio of phytoplankton, zooplankton is recorded for phytoplankton size class 3.12 (log V μm 3 volume) and zooplankton size 4 (log V μm 3 volume). In oligotrophic condition the highest average exergy is obtained in between phytoplankton size 1.48 (log V μm 3 volume) and zooplankton size 4 (log V μm 3 volume), whereas in eutrophic condition the result shows the highest exergy in the combination of phytoplankton size 5.25 (log V μm 3 volume) and zooplankton size 4 (log V μm 3 volume).

Expansion of the model ‘Rostherne’ for fish and zooplankton: role of top-down effects in modifying the prevailing pattern of ecosystem functioning

This paper presents a new version of the well-established aquatic ecosystem model ‘Rostherne’, incorporating lake fish and zooplankton. The model currently comprises differential and algebraic equations describing processes and forcing functions most important for a freshwater ecosystem. These include seasonal changes of solar radiation and water temperature, processes of algal and cyanobacterial population dynamics and nutrient uptake, water and chemical budgeting, stratification of the water column and sedimentation of suspended particles, and dynamics of detritus and its chemical constituents. Although it was shown previously for some years that a reasonable simulation of changes in most state variables could be achieved without accounting for fish and zooplankton, in other years, consideration of these compartments improved the fit between observations and model simulations dramatically. This was particularly true for the year 1998, when the high zooplankton numbers in spring appeared to prevent any significant development of the diatom population. The latter, however, bloomed during the first part of the summer (following a decrease in zooplankton counts) causing an unusually delayed major drop in ambient Si levels. As biogoechemical cycles of Si, P and N are interconnected through the dynamics of the primary producers, such a delay has serious implications for the functioning of an aquatic ecosystem model. Since changes in zooplankton dynamics are heavily dependent on the variability in fish recruitment, simultaneous consideration of the two components is helpful both for modelling overall dynamics of the lake ecosystem and for implementation of the biogeochemical regulation proposed previously.

Chlorodifluoroacetic acid fate and toxicity to the macrophytes Lemna gibba, Myriophyllum spicatum, and Myriophyllum sibiricum in aquatic microcosms.

Chlorodifluoroacetic acid (CDFA) is a novel haloacetic acid (HAA) and has been recently documented in aquatic systems. It is a suspected degradation product of the refrigerants 1,1,2-trichloro-1,1-difluoroethane (CFC-113) and 1-chloro-1,1-difluoroethane (HCFC-142b). Haloacetic acids can be phytotoxic, putatively acting through inhibition of the citric acid cycle. Replicate (n = 3) 12,000-L model aquatic ecosystems (microcosms) were dosed once at 0.5, 1, 5, and 20 mg/L of neutralized CDFA. Three microcosms served as controls. Each microcosm was stocked with eight individual apical shoots of both Myriophyllum spicatum and Myriophyllum sibiricum and sampled at regular intervals over a 42-d exposure period. The plants were assessed for the somatic endpoints of plant length, root growth, node number, and wet and dry mass and the biochemical endpoints of chlorophyll-a/b and carotenoid content as well as citric acid levels. The duckweed Lemna gibba was also introduced into these systems and monitored over a period of 14 d for wet/dry mass, plant/frond number, chlorophyll content, and growth rate. Concentrations of CDFA remained constant in the water column over the course of the fate investigation (241 d), indicating that this compound undergoes little, if any, degradation in aquatic systems. Results showed few statistically significant differences from controls for all three plant species with exposure to CDFA but no biologically relevant impacts. Overall, CDFA does not appear to pose any risk to these aquatic macrophytes at current environmental concentrations.

Response of zooplankton communities to liquid creosote in freshwater microcosms.

In this study, the response of zooplankton communities to single applications of liquid creosote in model aquatic ecosystems (microcosms) was evaluated. Liquid creosote was applied to 14 microcosms at concentrations ranging from 0.06 to 109 mg/L. Two microcosms served as controls. Zooplankton samples were collected from each microcosm on days 7 and 1 before treatment and on days 2, 5, 7, 14, 21, 28, 43, 55, and 83 following treatment. Temporal changes (response-recovery) in composition of the zooplankton community were assessed using principal response curves (PRC). Creosote induced a rapid, concentration-dependent reduction in zooplankton abundance and number of taxa, with maximum response (50-100% reduction in population densities) occurring between 5 and 7 d after treatment. Taxa that dominated at the time of treatment experienced the greatest impact, as indicated by large, positive species weight values (> 1) from the PRC analysis. Many of these taxa recovered to pretreatment or control levels during the posttreatment period, with the degree and duration of recovery being strongly dependent on concentration. Creosote had little effect on species composition at less than 1.1 mg/L, because changes in the types and relative proportion of species contributed from Cladocera, Rotifera, and Copepoda were comparable to those observed in control microcosms. However, a significant shift in species composition was observed at concentrations greater than 1.1 mg/L; these microcosms were generally dominated by low numbers of rotifers, some of which had not been collected before treatment. Community-level effect concentrations (EC50s) were 44.6 and 46.6 micrograms/L at 5 and 7 d, respectively, based on nominal creosote. Corresponding no-effect concentrations were 13.9 and 5.6 micrograms/L. The results of this field study indicate that creosote may pose a significant risk to zooplankton communities at environmental concentrations potentially encountered during spills and/or leaching events.

Metabolically active bacteria in Lake Kinneret

Three staining methods were used to identify metabolically active bacteria in Lake Kin- neret, northern Israel: CTC, DAPI staining followed by a propanol wash, and the Molecular Probes Live/Dead stain. Positive results from these methods purport to show, respectively, actively respiring bacteria (CTC+), cells with intact nucleoids (NuCC), and cells with intact membranes (MEM+). Con- comitantly, bacterial metabolic activity was measured as electron transport system (ETS) flux, O2 uptake, activities of peptidase, β-glucosidase and lipase, and rate of leucine incorporation in monthly samples taken for 2.5 yr at a pelagic lake station. Laboratory experiments followed changes during 22 or 40 h in the percentages of 'active' bacteria in GF/C-filtered lake water with or without substrate enrichment or antibiotic inhibitors of cell division, or with bacterivorous protists. In lake samples, each of the staining methods detected different aspects of cellular state or metabolic activity but all 3 indicated low percentages of 'active' bacteria relative to total bacterial abundance. CTC+ ranged from 1.0 to 27.3% (average 5.1%), NuCC from 1.4 to 42.9% (average 8.3%) and MEM+ from 1.0 to 29.9% (average 8.8%), with no clear seasonal or spatial patterns. No significant correlations were found between the proportions of 'active' bacteria in lake water as determined by these methods, although such correlations were observed in the laboratory experiments. Significant correlations were obtained between ETS and O2 uptake, peptidase and β-glucosidase, and between leucine incorporation and peptidase. ETS was significantly correlated with CTC+ and NuCC cell abundance, but not with total bacteria (DAPI counts). In contrast, peptidase activity correlated with total bacter- ial counts. Results of time course experiments indicated that some bacteria which initially appear to be inactive can become active when stimulated by substrate addition, even though cell division is inhibited. Grazing by protists increased the percentage of active bacteria, at least during the active predator-prey phase. Our data support the hypothesis that in natural waters usually only a small frac- tion (probably <20%) of the entire bacterial assemblage is strongly active metabolically at any given time. This proportion may increase dramatically with localized substrate inputs. The concept of bac- terial assemblages, heterogeneous not only in terms of phylotype, but also in terms of levels of meta- bolic activity will need to be considered in future aquatic ecosystem models.

Nutrient enrichment, habitat variability and trophic transfer efficiency in simple models of pelagic ecosystems

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 223:73-87 (2001) - doi:10.3354/meps223073 Nutrient enrichment, habitat variability and trophic transfer efficiency in simple models of pelagic ecosystems W. M. Kemp*, M. T. Brooks, R. R. Hood University of Maryland, Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA *E-mail: kemp@hpl.umces.edu ABSTRACT: We developed 4 simple numerical models of plankton dynamics to explore how nutrient enrichment and habitat variability might influence the efficiency by which phytoplankton (P) production is transferred to growth of zooplankton (Z) consumers in coastal ecosystems. The 4 models range in complexity from 2 (P and Z) to 5 state variables (including detritus, nutrients, and 2 algal size-groups). The models employ generic equation formulations, which are generally well supported by empirical studies and are widely used in coastal ecosystem modeling. Simulation experiments revealed that trophic transfer efficiency (TTE = zooplankton growth per unit phytoplankton production) tends to be enhanced with increased variability of resources, particularly at low nutrient levels. Numerical and analytical studies also showed that, regardless of resource variability, these model formulations produce a trend of initial enhancement of trophic efficiency with increasing nutrient levels, followed by a marked reduction in efficiency beginning at moderately eutrophic conditions. This precipitous drop in trophic efficiency is attributable to a saturation of the ability of zooplankton to utilize the increased primary production associated with nutrient enrichment. Under these conditions, an increasing fraction of the primary production is shunted to microbial food chains and associated respiratory losses. The steepness of this reduction in trophic efficiency with nutrient enrichment is related to the strength of predation (or disease) control at upper trophic levels. Model formulations simulating more intense top-down control (i.e. increasing mortality rates with increasing Z abundance) resulted in sharper declines in TTE with increasing nutrients. We speculate that these model results may help to explain how observed reductions in relative fish yield (per unit primary production) in many shallow nutrient-enriched estuaries and lakes are related to interacting effects of cultural eutrophication and intense fisheries exploitation. Furthermore, we surmise that these relationships are robust characteristics of most existing aquatic ecosystem models. KEY WORDS: Trophic efficiency · Eutrophication · Simulation models · Pelagic ecosystems · Topdown control Full text in pdf format PreviousNextCite this article as: Kemp WM, Brooks MT, Hood RR (2001) Nutrient enrichment, habitat variability and trophic transfer efficiency in simple models of pelagic ecosystems. Mar Ecol Prog Ser 223:73-87. https://doi.org/10.3354/meps223073Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 223. Online publication date: November 28, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

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

Compelling evidence of complex statisticalrelationships among various elements contained withinphytoplankton cells has traditionally been ignored inmodels of algal nutrient uptake and populationdynamics. Here we present a new approach,incorporating a phytoplankton intracellular elementalcorrelation pattern into the existing dynamicsimulation model of a freshwater lake. Within thisapproach, uptake and cycling of elements that arelikely to become limiting during the simulation periodare described by ordinary differential equations.Dynamics of nutrients that are unlikely to becomelimiting are described either by differentialequations or, when more practicable, by multipleregressions on environmental variables and cell quotasof other elements. This allows an easy simultaneousconsideration of a wide range of elements.The model adopting the described approach wastested on a data set for Rostherne Mere, Cheshire, UK.It showed a good fit between observations andsimulations for all considered variables, includingthe population dynamics of Ceratium hirundinellaand Microcystis aeruginosa, the outcome ofinterspecific competition and changes inconcentrations within algal cells and in thesurrounding lake water. The approach could easily beimplemented in models of bioreactors, chemostatexperiments and aquatic ecosystems.

An ecosystem model for assessing ecological risks in Québec rivers, lakes, and reservoirs

The comprehensive aquatic systems model (CASM) was adapted for estimating ecological risks posed by toxic chemicals in rivers, lakes, and reservoirs in Québec, Canada. Populations of aquatic plants, invertebrates, and fish characteristic of these aquatic ecosystems were identified and generic food webs were constructed. Bioenergetics parameters that determine the growth dynamics of these populations were derived from published values for these same or similar species. Input values of light, water temperature, concentrations of dissolved nitrogen (N), phosphorus (P), and silica (Si) were constructed from available regional data or data from similar Canadian systems at similar latitudes. The model provides the capability to estimate the probability of changes in the biomass of multiple populations of primary producers and consumers as a function of the concentration of dissolved chemical contaminant. The CASM permits the evaluation of direct toxic effects, as well as indirect toxic effects that result from changes in competitive or predator–prey relations in complex aquatic food webs. Hypothetical risk assessments were constructed for pentachlorophenol, copper, mercury, and diquat dibromide in generalized rivers, lakes, and reservoirs in Québec. Numerical sensitivity and uncertainty analyses were used to describe the relative contributions of direct and indirect toxic effects on overall ecological risks estimated for functional guilds of producers and consumers in these ecosystems. This aquatic ecosystem model may become one component in a decision support system for assessing ecological risks.

Degradation of pyrethroids in an aquatic ecosystem model

A study was conducted to evaluate the distribution of bioresmethrin, cypermethrin, deltamethin, permethrin, fenvalerate and Nurelle DX 550 EC (cypermethrin and chlorpyriphos) in an aquatic ecosystem model. The investigations were carried out at a temperature of 15–19°C in two different concentrations of each of the tested toxic substances in two parallel series. Cumulative effects and the influence of adaptation capabilities of microflora on their distribution were also studied. The degradation processes developed in accordance with the first kinetic order and depended on a group of determined compounds and initial concentrations. The pyrethroid half-live times at a temperature of 15–19°C fell within the range of 0.5–30.8 days. Chlorpyriphos, a component of Nurelle D 550EC, decomposed faster than most pyrethroids, its relevant half-live times were 5.0–7.1 days. In the case of sediments and organisms of the ecosystem, no accumulation effect of the pyrethroids was observed. However, a negative influence on the aquatic biocenosis was noted.

Environmental fate of amitrole: Influence of dissolved organic matter

In this study the environmental fate of amitrole in terrestrial and aquatic model ecosystems was investigated. Under aerobic conditions mineralization of amitrole is the main degradation pathway. The experiments revealed that the leaching behaviour is low in the presence or the absence of dissolved organic matter (DOM) despite the high water solubility due to a strong binding of amitrole to soil constituents. Under anaerobic conditions the addition of DOM increases the transport of amitrole in soil columns. The tests with water/sediment model ecosystems showed that the mineralization of amitrole is lower in comparison to aerobic soil experiments. Up to 80.6% of the applied 14C-labelled amitrole transfer into the sediment and about 1 3 of this amount formed bound residues, which are not extractable.

Interaction of aquatic substances with anilazine and its derivatives: The nature of the bound residues

This study is focused on the binding behaviour of the triazine fungicide anilazine and its main metabolites such as dihydroxyanilazine in aquatic ecosystems. For this purpose anilazine and its derivatives were incubated with ultrafiltrated dissolved organic matter (DOM) and isolated fulvic and humic acids, respectively and the adsorption isotherms according to Freundlich were determined. More than 90% bound residues were formed and up to 60% of anilazine metabolites were released with a recently developed silylation method. The study of the reactivity of metabolites and model compounds as well as our NMR investigations showing ether bondings led to a suggested binding scheme of anilazine to humic substances. To have more realistic conditions the behaviour of anilazine and dihydroxyanilazine in aquatic model ecosystems with water and sediment from a river was also part of this study. Furthermore the release of active ingredients or derivatives from sediment by different extraction methods was examined.

Changes in Models of Aquatic Ecosystems Approaching Carrying Capacity

Changes in Models of Aquatic Ecosystems Approaching Carrying Capacity

The Changes of Ecosystem Goal Functions in Stressed Aquatic Communities

The changes of biomass, exergy and structural exergy under the action of various chemical pollution (addition of nutrient compounds, pesticides, chlorinated phenols, oil, heavy metal ions, acidification etc.) are analysed for 50 experimental works with model aquatic ecosystems -microcosms, mesocosms and experimental ponds.<br>Structural exergy is shown to remain at constant level or to increase when the allochtonous compounds can be metabolised by ecosystem, or when the ecosystem can adapt itself to the input of toxicant by structural changes. When the substance is too conservative, too toxic or/and is coming in too high concentrations, structural exergy is decreasing, demonstrating the inability of ecosystem to adapt to this influence and irreversibility of changes in ecosystem.<br>The possibility to use such parameter as structural exergy for estimation of ecosystem state and its changes under various external influences is demonstrated. It reflects the state of ecosystem and can indicate the degree of ecosystem adaptation, decreasing when important for ecosystem functioning components are eliminated.<br>In experiments with mathematical model of lake Baikal ecosystem exergy content is shown to increase after addition of nutrients and to decrease after addition of phenolic compounds, reflecting the general shifts in ecosystem.;The changes of biomass, exergy and structural exergy under the action of various chemical pollution (addition of nutrient compounds, pesticides, chlorinated phenols, oil, heavy metal ions, acidification etc.) are analysed for 50 experimental works with model aquatic ecosystems -microcosms, mesocosms and experimental ponds.<br>Structural exergy is shown to remain at constant level or to increase when the allochtonous compounds can be metabolised by ecosystem, or when the ecosystem can adapt itself to the input of toxicant by structural changes. When the substance is too conservative, too toxic or/and is coming in too high concentrations, structural exergy is decreasing, demonstrating the inability of ecosystem to adapt to this influence and irreversibility of changes in ecosystem.<br>The possibility to use such parameter as structural exergy for estimation of ecosystem state and its changes under various external influences is demonstrated. It reflects the state of ecosystem and can indicate the degree of ecosystem adaptation, decreasing when important for ecosystem functioning components are eliminated.<br>In experiments with mathematical model of lake Baikal ecosystem exergy content is shown to increase after addition of nutrients and to decrease after addition of phenolic compounds, reflecting the general shifts in ecosystem.

Modelling the effect of atmospheric deposition on shallow heathland lakes in the Netherlands: dry deposition on water — now you see it, now you don't

The most important nitrogen source for isolated heathland lakes in the Netherlands is the atmospheric deposition of nitrogen compounds. Calculations were performed to relate in-lake concentrations with deposition. Using a dynamic retention model, all major components deposited (NH 4 +, NO 3 − and SO 4 2−) appeared to be retained by the lake. Simulations with the aquatic ecosystem model Aqu. Acid showed a big discrepancy between calculated and measured concentrations when using the estimated total deposition. However, using only wet deposition data, a reasonable fit with measured data was obtained. It was concluded that the dry deposition of nitrate and ammonium actually entering the lakes was far lower than the deposition estimated. The reasons for the apparent absence of dry deposition of nitrogen compounds on heathland lakes in general, and the impact on estimates of critical nitrogen deposition, are discussed.

Dynamics of natural rotifer populations

New tools for analyzing ecological time series have permitted the construction of rigorous models from relatively short series. We have applied these techniques to abundance data for nine natural rotifer populations to construct realistic models of their dynamics. Species included are Asplanchna girodi, Filinia pejleri, Keratella tropica, Monostyla bulla, Brachionus rotundiformis, and four other Brachionus species. The overall shapes of the time series were similar with an initial peak followed by oscillations of varying amplitude around a mean of lower population density. Auto correlation functions (ACF) for all populations were positive at small time lags and decayed rapidly to zero. This suggest that these are stationary, exponentially damped time series, fluctuating arround a constant mean with constant variance. The rapid decay of the ACFs indicates that the effect of a perturbation on these populations is quickly removed in one or two days. Phase portrait plots of log current population density vs log lagged density indicate that the time series are stable and non-chaotic. One type of model yielded the highest R2 for four of the nine species and was designated the consensus model. The mean R2 of this model for all nine species was 0.53 with a coefficient of variation of 38%. Lyapanov exponents were strongly negative, indicating that these populations rapidly return to equilibrium after an exogenous perturbation. Rotifer populations appear to be tracking very recent perturbations and their dynamics cannot be predicted from perturbations in the more distant past. We investigated the effects of increasing the level of stochasticity in the consensus model on the length of the growing season and resting egg production. Increasing stochastic variance increased the probability of extremely low population densities, shortening the growing season. In shorter growing seasons, fewer resting eggs were produced, other factors being equal. Counteracting this negative effect, was an increased probability of extremely high populations densities which increased mixis and resting egg production. Constructing models accurately depicting the dynamics of natural zooplankton populations should improve aquatic ecosystem models.