A hydrodynamic and water quality modeling study of spatial and temporal patterns of phytoplankton growth in a stratified lake with buoyant incoming flow

A three-dimensional hydrodynamic and water quality model was applied to Lake Paldang, a lake in South Korea that is stratified by incoming flows. The spatial and temporal patterns of phytoplankton growth in this lake were determined from the model. The model was calibrated and verified using data measured under different hydrological conditions. The model results were in reasonable agreement with the field measurements, in both the calibration and verification phases. The distributions of water quality and residence time in the lake and phytoplankton response to changes in nutrient loads were examined with the model, and the influence of the hydrodynamics on phytoplankton response was analyzed. The simulation results indicated that Lake Paldang is an essentially phosphorus-limited system, but that phytoplankton growth is limited by low water temperature and short residence time during the winter and the summer monsoon period, respectively. The results of sensitivity analyses also suggested that the hydrodynamics within the lake may have an indirect influence on phytoplankton responses to changes in the limiting nutrient loads, and that reducing phosphorus loading from Kyoungan Stream should be a high priority policy for controlling algal blooms during the pre- and post-monsoon periods. From this study, it was concluded that the three-dimensional water quality model incorporating hydrodynamic processes could successfully simulate phytoplankton response to changes in nutrient loads and that it could become a useful tool for identifying the essential factors determining phytoplankton growth and for developing the best management policy for algal blooms in Lake Paldang.     

A two-dimensional numerical model for eutrophication in Baiyangdian Lake

Hydrodynamic, physical, and biochemical processes in the Baiyangdian Lake water environment were analyzed comprehensively. An eutrophication ecodynamics model including the effects of reed resistance on flow was coupled with the hydrodynamics governing equations. An improvement on the Water Quality Analysis Simulation Program (WASP, a modeling system introduced by the US Environmental Protection Agency) is established, which uses the zooplankton kinetic equation. The model simulates water quality constituents associated with eutrophication in the lake, including phytoplankton, zooplankton, nitrogen, phosphorus, dissolved oxygen, and others. Various kinetic coefficients were calibrated using measured data or information from relevant literature, to study eutrophication in the lake. The values calculated by the calibrated model agree well with field data, including ammonia nitrogen, total nitrogen, total phosphorus and dissolved oxygen. Changes related to nutrition and dissolved oxygen during the processes were simulated. The present model describes the temporal variation of water quality in Baiyangdian Lake with reasonable accuracy. Deviations between model-simulated and observed values are discussed. As an ideal tool for environmental management of the lake, this model can be used to predict its water quality, and be used in research to examine the eutrophication process.     

Structural changes in lake functioning induced from nutrient loading and climate variability

Climate variability is increasingly recognized as an important regulatory factor, capable of influencing the structural properties of aquatic ecosystems. Lakes appear to be particularly sensitive to the ecological impacts of climate variability, and several long time series have shown a close coupling between climate, lake thermal properties and individual organism physiology, population abundance, community structure, and food web dynamics. Thus, understanding the complex interplay among meteorological forcing, hydrological variability, and ecosystem functioning is essential for improving the credibility of model-based water resources/fisheries management. Our objective herein is to examine the relative importance of the ecological mechanisms underlying plankton seasonal variability in Lake Washington, Washington State (USA), over a 35-year period (1964–1998). Our analysis is founded upon an intermediate complexity plankton model that is used to reproduce the limiting nutrient (phosphate)–phytoplankton–zooplankton–detritus (particulate phosphorus) dynamics in the lake. Model parameterization is based on a Bayesian calibration scheme that offers insights into the degree of information the data contain about model inputs and allows obtaining predictions along with uncertainty bounds for modeled output variables. The model accurately reproduces the key seasonal planktonic patterns in Lake Washington and provides realistic estimates of predictive uncertainty for water quality variables of environmental management interest. A principal component analysis of the annual estimates of the underlying ecological processes highlighted the significant role of the phosphorus recycling stemming from the zooplankton excretion on the planktonic food web variability. We also identified a moderately significant signature of the local climatic conditions (air temperature) on phytoplankton growth (r = 0.41), herbivorous grazing (r = 0.38), and detritus mineralization (r = 0.39). Our study seeks linkages with the conceptual food web model proposed by Hampton et al. [Hampton, S.E., Scheuerell, M.D., Schindler, D.E., 2006b. Coalescence in the Lake Washington story: interaction strengths in a planktonic food web. Limnol. Oceanogr. 51, 2042–2051.] to emphasize the “bottom-up” control of the Lake Washington plankton phenology. The posterior predictive distributions of the plankton model are also used to assess the exceedance frequency and confidence of compliance with total phosphorus (15 μg L⁻¹) and chlorophyll a (4 μg L⁻¹) threshold levels during the summer-stratified period in Lake Washington. Finally, we conclude by underscoring the importance of explicitly acknowledging the uncertainty in ecological forecasts to the management of freshwater ecosystems under a changing global environment.     

The development of a multi-species algal ecodynamic model for urban surface water systems and its application

An ecodynamic model that can simulate four phytoplankton species has been developed to deal with the unique characteristics of urban river systems which has manmade river profile, flow controlled by gates, severe eutrophication status, and fragile aquatic ecosystem. The ecodynamic model was developed referencing two typical models: the water quality simulation model WASP and ecological model CAEDYM. The model can simulate 11 state variables: dissolved oxygen, carbonaceous biochemical oxygen demand, ammonia nitrogen, nitrate nitrogen, organic nitrogen, inorganic phosphorus, organic phosphorus and four phytoplankton species with zooplankton as a boundary condition. The ecodynamic model was applied to Sihai section of the Beijing urban river system, where serious algal blooms broke out in recent years. The dominant phytoplankton species are Cyanophyta, Chlorophyta, Bacillariophyta, and Cryptophyta. Site-specific data on geometry, meteorology, pollution sources, and existing ecosystem parameters were collected and used for model calibration and verification The model results mimic observed trends of water quality and phytoplankton species succession and can be used for forecasting algal blooms as well as assessment of river management measures.     

Microbial activity and carbon, nitrogen, and phosphorus content in a subtropical seagrass estuary (Florida Bay): evidence for limited bacterial use of seagrass production

Bacterial abundance, production, and extracellular enzyme activity were determined in the shallow water column, in the epiphytic community of Thalassia testudinum, and at the sediment surface along with total carbon, nitrogen, and phosphorus in Florida Bay, a subtropical seagrass estuary. Data were statistically reduced by principle components analysis (PCA) and multidimensional scaling and related to T. testudinum leaf total phosphorus content and phytoplankton biomass. Each zone (i.e., pelagic, epiphytic, and surface sediment community) was significantly dissimilar to each other (Global R = 0.65). Pelagic aminopeptidase and sum of carbon hydrolytic enzyme (esterase, peptidase, and α- and β-glucosidase) activities ranged from 8 to 284 mg N m⁻² day⁻¹ and 113–1,671 mg C m⁻² day⁻¹, respectively, and were 1–3 orders of magnitude higher than epiphytic and sediment surface activities. Due to the phosphorus-limited nature of Florida Bay, alkaline phosphatase activity was similar between pelagic (51–710 mg P m⁻² day⁻¹) and sediment (77–224 mg P m⁻² day⁻¹) zones but lower in the epiphytes (1.1–5.2 mg P m⁻² day⁻¹). Total (and/or organic) C (111–311 g C m⁻²), N (9.4–27.2 g N m⁻²), and P (212–1,623 mg P m⁻²) content were the highest in the sediment surface and typically the lowest in the seagrass epiphytes, ranging from 0.6 to 8.7 g C m⁻², 0.02–0.99 g N m⁻², and 0.5–43.5 mg P m⁻². Unlike nutrient content and enzyme activities, bacterial production was highest in the epiphytes (8.0–235.1 mg C m⁻² day⁻¹) and sediment surface (11.5–233.2 mg C m⁻² day⁻¹) and low in the water column (1.6–85.6 mg C m⁻² day⁻¹). At an assumed 50% bacterial growth efficiency, for example, extracellular enzyme hydrolysis could supply 1.8 and 69% of epiphytic and sediment bacteria carbon demand, respectively, while pelagic bacteria could fulfill their carbon demand completely by enzyme-hydrolyzable organic matter. Similarly, previously measured T. testudinum extracellular photosynthetic carbon exudation rates could not satisfy epiphytic and sediment surface bacterial carbon demand, suggesting that epiphytic algae and microphytobenthos might provide usable substrates to support high benthic bacterial production rates. PCA revealed that T. testudinum nutrient content was related positively to epiphytic nutrient content and carbon hydrolase activity in the sediment, but unrelated to pelagic variables. Phytoplankton biomass correlated positively with all pelagic components and sediment aminopeptidase activity but negatively with epiphytic alkaline phosphatase activity. In conclusion, seagrass production and nutrient content was unrelated to pelagic bacteria activity, but did influence extracellular enzyme hydrolysis at the sediment surface and in the epiphytes. This study suggests that seagrass-derived organic matter is of secondary importance in Florida Bay and that bacteria rely primarily on algal/cyanobacteria production. Pelagic bacteria seem coupled to phytoplankton, while the benthic community appears supported by epiphytic and/or microphytobenthos production.     

Primary production and phytoplankton community structure during a winter shelf-scale phytoplankton bloom off Western Australia

Regions of high primary production along the oligotrophic west coast of Australia between 34 and 22°S in May–June 2007 (midway through the annual phytoplankton bloom) were found around mesoscale features of the Leeuwin Current. At 31°S, an anticyclonic eddy-forming meander of the Leeuwin Current had a mixed layer depth of >160 m, a depth-integrated chlorophyll a (Chl a)-normalised primary production of 24 mg C mg Chl a ^?1 day^?1 compared to the surrounding values of <18 mg C mg Chl a ^?1 day^?1. In the north between 27 and 24°S, there were several stations in >1,000 m of water with a shallow (<100 m) and relatively thin layer of high nitrate below the mixed layer but within the euphotic zone. These stations had high primary production at depths of ~100 m (up to 7.5 mg C m^?3 day^?1) with very high rates of production per unit Chl a (up to 150 mg C mg Chl a ^?1 day^?1). At 27–24°S, the majority of the phytoplankton community was the ubiquitous tropical picoplankters, Synechococcus and Prochlorococcus. There was a decline in the dominance of the picoplankters and a shift towards a more diverse community with more diatoms, chlorophytes, prasinophytes and cryptophytes at stations with elevated production. Photosynthetic dinoflagellates were negligible, but heterotrophic dinoflagellate taxa were common. Haptophytes and pelagophytes were also common, but seemed to contribute little to the geographical variation in primary production. The mesoscale features in the Leeuwin Current may have enhanced horizontal exchange and vertical mixing, which introduced nitrate into the euphotic zone, increasing primary production and causing a shift in phytoplankton community composition in association with the annual winter bloom.     

Integrated model projections of climate change impacts on a North American lake

Climate change is likely to impact terrestrial and aquatic ecosystems via numerous physical and biological mechanisms. This study outlines a framework for projecting potential impacts of climate change on lakes using linked environmental models. Impacts of climate drivers on catchment hydrology and thermal balance in Onondaga Lake (New York State) are simulated using mechanistic models HSPF and UFILS4. Outputs from these models are fed into a lake ecosystem model, developed in AQUATOX. Watershed simulations project increases in the magnitude of peak flows and consequent increases in catchment nutrient export as the magnitude of extreme precipitation events increases. This occurs concurrently with a decrease in annual stream discharge as a result of increased evapotranspiration. Simulated lake water temperatures increase by as much as 5 °C during the 2040-2069 time period, accompanied by a prolonging of the duration of summer stratification. Projected changes include shifts in the timing of nutrient cycling between lake sediments and water column. Plankton taxa projected to thrive under climate change include green algae and Bosmina longirostris. Responses for species at higher trophic levels are mixed. Benthic macroinvertebrates may either prosper (zebra mussels) or decline (chironomids), while fish (e.g., gizzard shad) exhibit high seasonal variability without any clear trend.     

Spatial and temporal variability of phytoplankton biomass,primary production and community structure in the Pontevedra Ria (NW Iberian Peninsula): oceanographic periods and possible response to environmental changes

The spatial distribution of phytoplankton assemblages, chlorophyll, primary production and physical and chemical parameters were studied in the Pontevedra Ria in Galicia (NW Iberian Peninsula) from October 1997 to October 1998. In addition to the usual oceanographic periods described for the Galician Coast, two other periods were observed: a Prebloom or winter bloom, occurring during calm, sunny days in winter and a Continental period, related to the allochthonous intrusion of low salinity water from the Miño River in late spring. The phytoplankton biomass and production in both periods reached values of up to 145 mgChl-a m^?2 and 3.6 gC m^?2 day^?1, respectively, which were similar to those found in summer upwelling blooms. Throughout the year, the phytoplankton biomass and primary production gradients along the ria’s axis were highly dependent on the balance between upwelling and runoff. When the latter prevailed, increased values were measured toward the inner ria, while the opposite pattern was observed during summer upwelling blooms. According to projections derived from climate models and the analysis of wind patterns, temperature and precipitation trends in the area, a drop in the productivity of the ria would be expected as a result of reduced upwelling intensity in summer and decreasing rainfall in spring. In any case the estuarine part of the ria would be the most seriously affected.     

Development of Lake Ladoga ecosystem models: modeling of the phytoplankton succession in the eutrophication process. I

New models of Lake Ladoga ecosystem and the results of modeling are presented. In the first part the model of phytoplankton succession in the process of anthropogenic eutrophication of the lake is considered under the evolution of the phosphorus loading. The still continued anthropogenic eutrophication of the lake started in 1962 when the phosphorus load began to increase. Since 1962 during the evolution of the lake’s state from oligotrophic to developed mezotrophic one, the structure of phytoplankton community dominating species was significantly changed as well as its total productivity. The system state in the model is described by 14 parameters: nine phytoplankton complexes, zooplankton, dissolved organic matter, detritus, dissolved mineral phosphorus and dissolved oxygen. The number of parameters of this model is noticeably larger than that of previous models created by the authors. The relative dynamics of phytoplankton complexes in the lake’s ecosystem evolution was simulated by the new model. It is shown that the modeling results are adequately corresponding to the observation data. The results of phytoplankton structure modeling allow to estimate the impact of phytoplankton on the water quality as well as give the prediction of the lake’s ecosystem evolution with the changes of the phosphorus loading.     

Evaluation and application of a three-dimensional water quality model in a shallow lake with complex morphometry

Fundamental hydrodynamic and ecological processes of a lake or reservoir could be adequately depicted by one-dimensional (1D) numerical simulation models. Whereas, lakes with significant horizontal water quality and hydrodynamic gradients due to their complex morphometry, inflow or water level fluctuations require a three-dimensional (3D) hydrodynamics and ecological analyses to accurately simulate their temporal and spatial dynamics. In this study, we applied a 3D hydrodynamic model (ELCOM) coupled with an ecological model (CAEDYM) to simulate water quality parameters in three bays of the morphologically complex Lake Minnetonka. A considerable effort was made in setting up the model and a systematic parameterization approach was adopted to estimate the value of parameters based on their published values. Model calibration covered the entire length of the simulation periods from March 29 to October 20, 2000. Sensitivity analysis identified the top parameters with the largest contributions to the sensitivity of model results. The model was next verified with the same setup and parameter values for the period of April 25 to October 10, 2005 against field data. Spatial and temporal dynamics were well simulated and model output results of water temperature (T), dissolved oxygen (DO), total phosphorus (TP) and one group of algae (Cyanobacteria) represented as chlorophyll a (Chla) compared well with an extensive field data in the bays. The results show that the use of the model along with an accurate bathymetry, a systematic calibration and corroboration (verification) process will help to analyze the hydrodynamics and geochemical processes of the morphologically complex Lake Minnetonka. An example of an ecological application of the model for Lake Minnetonka is presented by examining the effect of spatial heterogeneity on coolwater fish habitat analysis in 3D and under a scenario where horizontal spatial heterogeneity was eliminated (1D). Both analyses captured seasonal fish habitat changes and the total seasonal averages differed moderately. However, the 1D analysis did not capture local and short duration variabilities and missed suitable fish habitat variations of as much as 20%. The experiment highlighted the need for a 3D analysis in depicting ecological hot spots such as unsuitable fish habitats in Lake Minnetonka.     

Aspects of a zooplankton,phytoplankton, phosphorus system

In this paper a three-dimensional dynamical system which models the three-species system made up of phytoplankton, zooplankton and organic phosphorus nutrient in a lake environment is studied. The system is part of a more general limnological model for eutrophic lakes and impoundments which has been developed by Battelle Northwest Laboratories. It is shown that this system, henceforth referred to as Z-P-P, has a phase portrait comprised of a plane portrait embedded in the three-dimensional space R³ as an “attractor”.¹ Under a small perturbation of the nutrient equation it is shown that the system is essentially a classical Volterra-Lotka system embedded in a three-dimensional phase space R³.The system derived from Z-P-P by the addition of a term to the nutrient equation which represents the organic phosphorus contribution of dying phytoplankton is also considered. The equilibria of this system are studied and what can be deduced of the phase portrait is compared with that of the above systems. It is found that these phase portraits are qualitatively indifferent to the form of the growth rate functions for zooplankton and phytoplankton provided they are monotone increasing. Some discussion about the stability of these systems is included. Throughout this paper results are interpreted in limnological terms.     

Food web structure and trophic interactions of the tropical highland lake Hayq, Ethiopia

We generated a mass-balance model to figure out the food web structure and trophic interactions of the major functional groups of the Ethiopian highland Lake Hayq. Moreover, the study lay down a baseline data for future ecosystem-based investigations and management activities. Extensive data collection has been taken place between October 2007 and May 2009. Ecotrophic efficiency (EE) of several functional groups including phytoplankton (0.8) and detritus (0.85) was high indicating the utilization of the groups within the system. However, the EE of Mesocyclops (0.03) and Thermocyclops (0.30) was very low implying these resources were rather a ‘sink’ in the trophic hierarchy. Flows based on aggregated trophic level sensu Lindeman revealed the importance of both phytoplankton and detritus to higher trophic levels. The computed average transfer efficiency of 11.5% for the first four trophic levels was within the range for highly efficient African lakes. The primary production to respiration (P/R) ratio (1.05) of Lake Hayq indicates the maturity of the ecosystem. We also modeled the food-web by excluding Tilapia and reduced phytoplankton biomass to get insight into the mass balance before Tilapia was introduced. The analysis resulted in a lower system omnivory index (SOI = 0.016) and a reduced P/R ratio (0.13) that described the lake as immature ecosystem, suggesting the introduction of Tilapia might have contributed to the maturity of the lake. Tilapia in Lake Hayq filled an ecological empty niche of pelagic planktivores, and contributed for the better transfer efficiency observed from primary production to fish yield.     

Resistance and re-organization of an ecosystem in response to biological invasion: Some hypotheses

Using a dynamic model of Lake Chozas developed by Marchi et al. (2011), we tested three hypotheses about recovery of the indigenous community and water quality after radical changes caused by introduction of an invasive allochthonous crayfish, Procambarus clarkii:

1.

Can the lake resist the pressure of an invasive species, like P. clarkii, by adaptation?

2.

Can the ecosystem recover when all the crayfish are removed and low phosphorus concentrations persist in inflow water?

3.

Does the simulated recovery of submerged vegetation occur at a total phosphorus concentration below 100 mg TP m⁻³, as estimated by Scheffer et al. (1993), Scheffer (1997), Jeppesen et al. (1998) and Zhang et al. (2003)?

We obtained the following answers:

1.

Lake Chozas can at least partly resist by adaptation. A combination of possible parameter changes could lead to a significant increase in eco-exergy.

2.

Removal of the phosphorus represented by crayfish (by harvesting) implies complete recovery of the lake and its eco-exergy, albeit not necessarily with the same organisms having the same properties.

3.

The expected hysteresis created by introduction and harvesting of crayfish is observed under the following conditions: phytoplankton dominance at total phosphorus ≥ about 200-250 mg TP m⁻³ and submerged vegetation returns at total phosphorus < 100 mg TP m⁻³.

     

Estimation of winter respiration rates and prediction of oxygen regime in a lake using Bayesian inference

In this paper, we estimate the winter respiration (oxygen depletion per unit area of hypolimnetic surface) in a hyper-eutrophic shallow lake (Tuusulanjärvi) in the northern hemisphere (Finland, northern Europe, latitude 60^∘26′, longitude 25^∘03′) under ice-cover periods in the years 1970–2003. We present a dynamic nonlinear model that can be used for predicting of the oxygen regime in following years and to dimensioning of needed artificial oxygenation efficiency that will prevent fish kills in the lake. We use Bayesian estimation of respiration using Markov chain Monte Carlo (MCMC) method (Adaptive Metropolis–Hastings algorithm). This allows for analysis and predictions that take into account all the uncertainties in the model and the data, pool information from different sources (laboratory experiments and lake data), and to quantify the uncertainties using a full statistical approach. The mean estimated respiration in the study period was $301\pm 105$ mg m⁻² d⁻¹, which is on the upper limit of winter respiration of eutrophic Canadian lakes on the same latitude. The reference rate of the respiration $k$ (d⁻¹) at 4 ^∘C indicated cyclic behavior of about 9-year amplitude and had a statistically significant negative trend through out the study period. The temperature coefficient and respiration rate of the model prove to be highly correlated and unidentifiable with the given data. The future winters can be predicted using the posterior information coming from the past observations. As new observations arrive, they are added to the analysis. Methods are shown to be applicable to the dimensioning of artificial oxygenation devices and to the anticipation of the need for oxygenation during the winter.     

Extensive cross-disciplinary analysis of biological and chemical control of Calanus finmarchicus reproduction during an aldehyde forming diatom bloom in mesocosms

Egg and faecal pellet production and egg hatching success of the calanoid copepod Calanus finmarchicus were monitored over a period of 14?days (14?C28 April, 2008) while fed water from 4 differently treated mesocosms and ambient water. Two of the mesocosms used were inoculated with the polyunsaturated aldehyde (PUA)-producing diatom Skeletonema marinoi, while 2 received only nutrient additions with or without silica. The mesocosms developed blooms of S. marinoi, mixed diatoms or the haptophyte Phaeocystis pouchetii, respectively. Faecal pellet production of C. finmarchicus increased with increasing food availability. Egg production increased with time in all mesocosms to a maximum single female production of 232 eggs female^?1?day^?1 (average of 90 eggs female^?1?day^?1) and followed the development of ciliates and P. pouchetii, but was not affected by the observed high (up to 15?nmol?L^?1) PUA production potential of the phytoplankton. The hatching success of the eggs produced on the mesocosm diets was high (78?C96%) and was not affected by either aldehydes in the maternal diet or exposure to the dissolved aldehydes in the water.     

Tolerance of estuarine benthic diatoms to high concentrations of ammonia,nitrite ion,nitrate ion and orthophosphate

A carbon-14 assimilation method was used to determine action spectra and photosynthesis versus irradiance (P versus I) curves of natural populations of phytoplankton and zooxanthellae from a coral reef fringing Lizard Island in the Australian Barrier Reef. The action spectra were related to the phytoplankton species composition. The curves showed shade adaptation in phytoplankton from deeper waters and in the zooxanthellae. Rates of photosynthesis of zooxanthellae were shown to be highly but variably dependent on their host organisms. Photosynthetic production by zooxanthellae was about 0.9 gC m^-2 day^-1, which is about three times higher than phytoplankton production in the waters close to the reef.     

Release of dissolved organic matter from natural populations of marine phytoplankton

A new method is presented for measuring the rate of release of dissolved organic matter (DOM) produced by natural populations of marine phytoplankton. The method has been field-tested using natural populations of estuarine, coastal, and oceanic phytoplankton. Problems associated with the necessity for long incubation times, high initial activity of the inorganic ¹⁴C added to the sample, and self-absorption are overcome. Improved sensitivity is obtained by utilizing a large portion of the sample filtrate, Results from sampling at different times of day and for different lengths of incubation suggest that these problems should be reexamined in terms of both the percent and rate of relase of DOM from natural populations of phytoplankton. The rate of release of DOM from natural populations of marine phytoplankton decreases seaward. Phytoplankton in Georgia (USA) estuaries release <1 to 40 mg C m^-3 day^-1, in southeastern USA coastal waters from 0 to 19 mg C m^-3 day^-1, and in the westernmost Sargasso Sea from 0 to 2 mg C m^-3 day^-1. The percent of photoassimilated carbon released as DOM increases seaward. The percents are less than 7% in the Georgia estuaries, generally less than 13% in southeastern USA surface coastal waters, generally less than 21% for these coastal waters below the surface, and less than 44% for the westernmost Sargasso Sea. The results of this study and of other studies suggest that the measurement of primary productivity in estuarine and nearshore coastal waters is not seriously in error because of the absence of measurements on the rate of release of DOM from such phytoplankton. However, the estimation of primary-productivity values for oceanic waters is underestimated in some cases by about 50%.Contribution No. 595 from the Department of Oceanography, University of Washington, Seattle, Washington, USA and Contribution No. 224 from the University of Georgia Marine Institute, Sapelo Island, Georgia, USA.Extracted in part from a dissertation submitted to the Graduate Faculty of the University of Georgia in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

Dealing with microtopography of an ombrotrophic bog for simulating ecosystem-level CO2 exchanges

Peatlands contain approximately 25% of the global soil carbon (C), despite covering only 3% of the earth's land surface. In order to evaluate the role of peatlands in global C cycling, models of ecosystem biogeochemistry are required, but peatland ecosystems present a number of unique challenges, particularly how to deal with the large variability that occurs at scales of one to several metres. In models, spatial variability is considered either explicitly for each individual unit and the outputs averaged, referred to as flux upscaling, or implicitly by weighting model parameters by the fractional occurrence of the individual units, referred to as parameter upscaling. The advantage of parameter upscaling is that it is much more computationally efficient: a requirement for hemispheric scale simulations. In this study we determined the differences between modelling a raised bog peatland with hummock-hollow microtopography using flux and parameter upscaling. We used the McGill Wetland Model (MWM), a process-based ecosystem C model for peatlands, configured for hummocks and hollows separately and then a weighted mixture of both. The simulated output based on flux and parameter upscaling was compared with eddy-covariance tower measurements. We found that net ecosystem production (NEP) for hollows was much larger than that for hummocks because total ecosystem respiration (TER) for hummocks was greater while gross primary production (GPP) did not differ significantly between the two topographic features. However, despite differences in components of NEP between hummocks and hollows, there was no statistically significant difference between the NEP based on flux and parameter upscaling using the MWM. Both flux and parameter upscaling show equivalent capability to capture the magnitude, direction, seasonality and inter-annual variability. The root-mean-square-errors (RMSE) are 0.66, 0.45, and 0.49 g C m⁻² day⁻¹, respectively for GPP, TER and NEP based on the flux upscaling, while 0.67, 0.44, and 0.48 g C m⁻² day⁻¹, respectively based on the parameter upscaling. The degree of agreement (d*) is 0.96, 0.97, and 0.88, respectively for GPP, TER and NEP based on the flux upscaling, while 0.96, 0.97, and 0.89, respectively based on the parameter upscaling. This result suggests that differences in processes caused by peatland microtopography scale linearly, which means an ecosystem-level model set-up (i.e. parameter upscaling scheme), is sufficient to simulate the C cycling.     

A vertical-compressed three-dimensional ecological model in Lake Taihu, China

A three-dimensional ecological model on the basis of the analyses of environmental characteristics is set up for Lake Taihu, one of the largest shallow lakes in China. The hydrodynamic processes, nutrient cycling, chemical processes and biological processes are integrated in the model. Model state variables include: water current, surface displacement, nutrients of nitrogen and phosphorus, as well as their different forms such as ammonia nitrogen, nitrate nitrogen, phosphate phosphorus, etc., biomasses of macroplankton, phytoplankton, zooplankton and fish, and also the nutrient levels of macroplankton and phytoplankton. A nutrient budget and sediment transformation are also coupled in the model. The data from January 17, 1997 to January 18, 1998 are use to calibrate the model. The model results have shown good agreement with the observations. It implies that the model could be used for the lake environmental management and research for examining the processes and determining the water quality. The reasons of deviations between the modelled results and the observed values are also discussed. There are six factors that explain the deviations of the modelled results from the observed values and they can be grouped into two sets. One set of problems is associated with the standard deviation introduced by sampling and analyses. The second set of problems can be solved by introduction of processes lacking in the present model (resuspension, phytoplankton transportation mode under the wind with low speed, shifts in species composition and varied size of phytoplankton and zooplankton). The latter two processes should be included in the model at a later stage by integration of a structurally dynamic approach into the three-dimensional model.     

Production organique primaire dans un milieu saumâtre eutrophe (Etang de Berre). Effets d'une forte dilution (dérivation des eaux de la Durance)

¹⁴C primary production measurements were made over a period of 5 years (1965–1969, inclusive) in the brackish lake Etang de Berre, near Marseilles (France). The diversion of the River Durance into the Etang de Berre took place during this period (March 1966) and introduced an important modification into the organic production ecosystem, mainly through increased and variable freshening, accompanied by substantial nutrient input. The seasonal distribution of production rates displayed 3 bloom periods: the first (short and slight) in spring, the second (the most important as regards intensity and duration) in summer, and the third in autumn (October). Before the diversion of the river in 1965, the carbon-uptake rates in the lake ranged between 25 mg/m²/day in winter and 800 mg/m²/day in summer-autumn, the mean value for the year being 150 g C/m², which represents 2.5×10⁴ tons of photosynthesized carbon for the whole lake. After the diversion, more than 3000 mg C/m² day were measured; for 1968, the inclusive uptake rate was 384 g C/m², representing 6×10⁴ tons of synthesized carbon for the whole lake. Nevertheless, noticeable variations occurred from one year to another. From the annual nutrient input of phosphate to the Etang de Berre through the inflow of Durance waters, the quantity of potentially synthesizable elements has been calculated, in terms of carbon, according to the normal P:C ratio of organic substances; this quantity is called R. The difference between measured production, P, and R gives a measure of the regenerated production. This portion of production represented about 80% of the total production before 1968 but only 16% in 1969, a year of maximum fresh-water inflow. This phenomenon could be due to modifications of the ecophysiology of the phytoplankton resulting from the considerable freshening. With increasing nutrient load, eutrophication first occurs, then still greater dilution results in inhibition of production.