Factors influencing lake recovery from eutrophication--the case of basin 1 of Lake Balaton

Lake Balaton is a large, shallow, and calcareous lake that was subject to a rapid eutrophication during the 1970s. Management measures taken from the mid-1980s decreased the phosphorus load to the lake from 0.5 to 0.3 g P m-2 yr-1. Using long-term load and water quality data, we analyse the response of the formerly hypertrophic Basin 1 of the lake by the means of simple empirical models. Several factors that are commonly neglected during studies of lake recovery modified the apparent settling velocity of total P and consequently, the biomass of the phytoplankton. These factors included the loads of calcium and suspended solids, the loading ratio of the dissolved to particulate phosphorus, and blooming of the dominant cyanobacterium, Cylindrospermopsis raciborskii. Due to the rapid immobilisation of the mobile phosphorus in the surface sediments, moderate reduction (45-50%) in the external load resulted in a surprisingly fast and significant improvement of the water quality in the hypertrophic southwestern basins of the lake.     

Environmental context of Cylindrospermopsis raciborskii (Cyanobacteria) blooms in a shallow pond in France

Cylindrospermopsis raciborskii, a potentially toxic blooming cyanobacterium (blue-green alga), responsible for public health problems in Australia, was identified in France in 1994 in a shallow pond south of Paris. A program monitoring the occurrence of C. raciborskii in this pond was conducted from July 1998 to October 1999. The phytoplankton assemblages were studied, and limnological parameters (water temperature, dissolved oxygen, pH, conductivity, and dissolved inorganic nutrients) were measured. By multivariate analysis (principal component analysis), we showed that sufficiently high temperatures to allow the germination of akinetes, relatively low nutrient concentrations (soluble reactive phosphorus with a mean concentration of 1 microM and nitrate between 0 and 5 microM, except in February 1999 (21 microM)) and a characteristic high and constant sulfate concentration (8981+/-471 microM) seemed to be the main factors involved in the proliferation of C. raciborskii in the "Francs-Pêcheurs" (FP) pond. In the light of these findings and of bibliographic data, C. raciborskii would seem to be characterized by good adaptability, but also by low competitiveness with other phytoplanktonic species in the temperate study area.     

Which offers more scope to suppress river phytoplankton blooms: Reducing nutrient pollution or riparian shading?

River flow and quality data, including chlorophyll-a as a surrogate for river phytoplankton biomass, were collated for the River Ouse catchment in NE England, which according to established criteria is a largely unpolluted network. Against these data, a daily river quality model (QUESTOR) was setup and successfully tested. Following a review, a river quality classification scheme based on phytoplankton biomass was proposed. Based on climate change predictions the model indicated that a shift from present day oligotrophic/mesotrophic conditions to a mesotrophic/eutrophic system could occur by 2080. Management options were evaluated to mitigate against this predicted decline in quality. Reducing nutrient pollution was found to be less effective at suppressing phytoplankton growth than the less costly option of establishing riparian shading. In the Swale tributary, ongoing efforts to reduce phosphorus loads in sewage treatment works will only reduce peak (95th percentile) phytoplankton by 11%, whereas a reduction of 44% is possible if riparian tree cover is also implemented. Likewise, in the Ure, whilst reducing nitrate loads by curtailing agriculture in the headwaters may bring about a 10% reduction, riparian shading would instead reduce levels by 47%. Such modelling studies are somewhat limited by insufficient field data but offer a potentially very valuable tool to assess the most cost-effective methods of tackling effects of eutrophication.     

Seasonal changes of arsenic speciation in lake waters in relation to eutrophication

In this study, the influence of eutrophication on arsenic speciation in lake waters was investigated. Surface water samples (n = 1-10) were collected from 18 lakes in Japan during July 2007 and February 2008. The lakes were classified into mesotrophic (7 lakes) and eutrophic (11 lakes) based on the total phosphate (T-P) and chlorophyll-a (Chl-a) concentrations in water column. Inorganic, methylated and ultraviolet-labile fractions of arsenic species were determined by combining hydride generation atomic absorption spectrometry with ultraviolet irradiation. Organoarsenicals (mainly methylated and ultraviolet-labile fractions) comprised 30-60% of the total arsenic in most lakes during summer. On the other hand, inorganic arsenic species (As(III + V)) dominates (about 60-85%) during winter. The occurrence of ultraviolet-labile fractions of arsenic was higher in eutrophic lakes than those in mesotrophic lakes in both seasons. The concentration of dimethyl arsenic (DMAA) was high in eutrophic lakes during winter; and in mesotrophic lakes during summer. The results suggest that the conversion of As(III + V) to more complicated organoarsenicals occurred frequently in eutrophic lakes compared to that in mesotrophic lakes, which is thought to be the influence of biological activity in the water column. The distribution of arsenic species were well correlated with phosphate concentrations than those of Chl-a. This might be due to the competitive uptake of As(V) and phosphate by phytoplankton. The organoarsenicals (OrgAs)/As(V) ratio was higher at low phosphate concentration indicating that conversion of As(V) to OrgAs species was more active in phosphate-exhausted lakes with high phytoplankton density.     

Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): the need for a dual nutrient (N & P) management strategy

Harmful cyanobacterial blooms, reflecting advanced eutrophication, are spreading globally and threaten the sustainability of freshwater ecosystems. Increasingly, non-nitrogen (N₂)-fixing cyanobacteria (e.g., Microcystis) dominate such blooms, indicating that both excessive nitrogen (N) and phosphorus (P) loads may be responsible for their proliferation. Traditionally, watershed nutrient management efforts to control these blooms have focused on reducing P inputs. However, N loading has increased dramatically in many watersheds, promoting blooms of non-N₂ fixers, and altering lake nutrient budgets and cycling characteristics. We examined this proliferating water quality problem in Lake Taihu, China’s 3rd largest freshwater lake. This shallow, hyper-eutrophic lake has changed from bloom-free to bloom-plagued conditions over the past 3 decades. Toxic Microcystis spp. blooms threaten the use of the lake for drinking water, fisheries and recreational purposes. Nutrient addition bioassays indicated that the lake shifts from P limitation in winter-spring to N limitation in cyanobacteria-dominated summer and fall months. Combined N and P additions led to maximum stimulation of growth. Despite summer N limitation and P availability, non-N₂ fixing blooms prevailed. Nitrogen cycling studies, combined with N input estimates, indicate that Microcystis thrives on both newly supplied and previously-loaded N sources to maintain its dominance. Denitrification did not relieve the lake of excessive N inputs. Results point to the need to reduce both N and P inputs for long-term eutrophication and cyanobacterial bloom control in this hyper-eutrophic system.     

Cyanobacterial blooms: statistical models describing risk factors for national-scale lake assessment and lake management

Cyanobacterial toxins constitute one of the most high risk categories of waterborne toxic biological substances. For this reason there is a clear need to know which freshwater environments are most susceptible to the development of large populations of cyanobacteria. Phytoplankton data from 134 UK lakes were used to develop a series of Generalised Additive Models and Generalised Additive Mixed Models to describe which kinds of lakes may be susceptible to cyanobacterial blooms using widely available explanatory variables. Models were developed for log cyanobacterial biovolume. Water colour and alkalinity are significant explanatory variables and retention time and TP borderline significant (R²-adj = 21.9%). Surprisingly, the models developed reveal that nutrient concentrations are not the primary explanatory variable; water colour and alkalinity were more important. However, given suitable environments (low colour, neutral-alkaline waters), cyanobacteria do increase with both increasing retention time and increasing TP concentrations, supporting the observations that cyanobacteria are one of the most visible symptoms of eutrophication, particularly in warm, dry summers. The models can contribute to the assessment of risks to public health, at a regional- to national level, helping target lake monitoring and management more cost-effectively at those lakes at the highest risk of breaching World Health Organisation guideline levels for cyanobacteria in recreational waters. The models also inform restoration options available for reducing cyanobacterial blooms, indicating that, in the highest risk lakes (alkaline, low colour lakes), risks can generally be lessened through management aimed at reducing nutrient loads and increasing flushing during summer.     

A hybrid neural-genetic algorithm for reservoir water quality management

A combined neural network and genetic algorithm (GA) was developed for water quality management of Feitsui Reservoir in Taiwan. First, an artificial neural network (ANN) model was employed to simulate the behavior of nutrient loads into the reservoir. The data from watershed loads, precipitation in the watershed, and outflow were used in the ANN model to forecast the total phosphorus concentration in the reservoir. A 6-year (1992-97) record of water quality data was used for network training, and additional data collected in 1998-2000 were used for model verification. Further, a GA was used with this ANN model to optimize the control of nutrient loads from the watershed. The GA was used as a search strategy to determine the proper reduction rates of nutrient loads from the watershed so that the objective function could be as close to the optimal value as possible. The study results indicate that the ANN model can effectively simulate the dynamics of reservoir water quality. The GA is able to identify control schemes that reduce the in-reservoir total phosphorus concentration by as much as 60%, and water quality in the reservoir can be expected to achieve an oligotrophic (most of the time) or mesotrophic level if the watershed nutrient loads are reduced by 10-80%.     

Phytoplankton growth control and risk of cyanobacterial blooms in the lower Senegal River delta region

Eutrophication, and the resulting proliferation of phytoplankton can affect water quality. Although environmental and ecological processes that lead to phytoplankton proliferations in temperate areas have received considerable attention, scientific background regarding this topic in tropical areas, especially West Africa, are scarce. In this study, bioassays in batch cultures were carried out in order to identify factors that may be stimulating or limiting the biomass of phytoplankton at various sites in the lower Senegal River delta region. Complete factorial designs were used to test several factors both alone and combined (nitrogen, phosphorus, sulfate, EDTA and trace elements, stirring). The effects of these factors on the growth of the whole phytoplankton communities were followed by the daily measurement of in vivo fluorescence. At all but two of the sites, enrichment with certain nutrients significantly stimulated phytoplankton biomass growth, with nitrogen and/or phosphorus appearing to be the primary limiting factor(s). Nitrogen was the primary limiting factor in coastal ocean waters and in the Senegal River estuary. In the freshwater ecosystems studied, nitrogen and phosphorus acted as alternating or concomitant limiting factors. We assessed the changes in the phytoplankton composition induced by nutrient enrichments in Lake Guiers, a drinking water reservoir for the population of Dakar. Our findings lead us to define the conditions of a possible proliferation of the potentially toxic cyanobacterium Cylindrospermopsis raciborskii mainly in response to phosphorus inputs.     

Seasonal variation of phosphorus release from the sediments of Shallow Lake Balaton (Hungary)

Phosphorus release was low from intact sediment cores of the mesotrophic area of the lake throughout the year, and amounted to 0.3 mg P m⁻² day⁻¹ during autumn in short-term incubations. In the hypertrophic area maximum release (2.8 mg Pm⁻² day⁻¹) was measured during summer.

Phosphorus release showed a rapid increase from long-term incubated intact sediment cores with the increasing pH of the overlying water. At the ecologically real maximum pH the release may amount to 0.8 and 4.0 mg P m⁻² day⁻¹ in the mesotrophic and hypertrophic areas, respectively. A release of 2.0–3.9 mg P m⁻² day⁻¹ was estimated from sediment suspensions of the hypertrophic area within a pH range of 8–9. These values are similar to the external phosphorus loadings of the respective areas.

The most important phosphorus mobilizing factors are pH and the decomposition of the organic matter in the sediments. Redox conditions may play a significant indirect role in the regulation of the internal loading.

A positive feedback is hypothesized between the internal phosphorus loading and primary production both processes being affected by the external loading in different ways.     

A shallow lake remediation regime with Phragmites australis: Incorporating nutrient removal and water evapotranspiration

Shallow lake eutrophication has been an important issue of global water environment. Based on the simulation and field sampling experiments in Baiyangdian Lake, the largest shallow lake in North China, this study proposed a shallow lake remediation regime with Phragmites australis (reed) incorporating its opposite effects of nutrient removal and water evapotranspiration on water quality. The results of simulation experiments showed that both total nitrogen (TN) and phosphorus (TP) removal efficiencies increased with the increasing reed coverage. The TN removal efficiencies by reed aboveground uptake and rhizosphere denitrification were 11.2%, 13.8%, 22.6%, 28.4%, and 29.6% for the reed coverage of 20%, 40%, 60%, 80%, and 100%, respectively. Correspondingly, TP removal efficiencies by aboveground reed uptake were 1.4%, 2.5%, 4.4%, 7.4% and 7.9%, respectively. However, the water quality was best when the reed coverage was 60% (72 plants m⁻²). This was due to the fact that the concentration effect of reed evapotranspiration on nutrient increased with reed coverage. When the reed coverage was 100% (120 plants m⁻²), the evapotranspiration was approximately twice that without reeds. The field sampling results showed that the highest aboveground nutrient storages occurred in September. Thus, the proposed remediation regime for Baiyangdian Lake was that the reed coverage should be adjusted to 60%, and the aboveground biomass of reeds should be harvested in each September. With this remediation regime, TN and TP removal in Baiyangdian Lake were 117.8 and 4.0 g m⁻², respectively, and the corresponding removal efficiencies were estimated to be 49% and 8.5% after six years. This study suggests that reed is an effective plant for the remediation of shallow lake eutrophication, and its contrasting effects of nutrient removal and evapotranspiration on water quality should be considered for establishing the remediation regime in the future.     

Determining reference conditions for nutrients,chlorophyll a and Secchi depth in Yungui Plateau ecoregion lakes,China

Establishing nutrient reference condition (baseline environmental condition) of lakes in ecoregion is a critical consideration in the development of scientifically defensible aquatic nutrient criteria. Three methods were applied to determine reference conditions in Yungui Plateau ecoregion lakes with respect to total phosphorus, total nitrogen, planktonic chlorophyll a (chl a) and Secchi depth. First, minimally developed lake/catchment units were identified based on existing geographical database and visual basin survey. Lakes in these catchments were considered minimally disturbed ‘reference lakes’. Second, the frequency distribution of all of the lake data presently available by each variable was plotted, and the lower 25th percentile was selected as the reference condition for each value. Third, median nutrient, chl a and Secchi depth values were determined for the best one‐third of lakes and applied as indicators of reference condition. The result of model prediction and data from historical record were used to test three techniques for determining reference condition. Reference conditions, as determined by these three methods, broadly agreed for all parameters. Overall, the data suggest that multiple methods can be used to determine reference condition, and that in Yungui lakes, reference condition corresponds to oligotrophic–mesotrophic status.     

Distributions of rare earths and heavy metals in field-grown maize after application of rare earth-containing fertilizer

Macroalgal blooms have been considered to be an indicator of eutrophication. A new and rapid method is described for the assessment of macroalgal cover in the intertidal zone of estuaries. Macroalgal cover in the intertidal of the nutrient-enriched River Deben estuary was found to reach a maximum of 50% coverage, but this varied seasonally with the highest percentage cover during June and July. Macro-algae mats were particularly associated with areas of hard substrata providing suitable attachment points, rather than with the nutrient concentrations along the estuary. The occurrence of macroalgae may be more related to the substrate than to the nutrient status of the estuary.     

Nutrient and grazing factors in relation to phytoplankton level in a eutrophic shallow lake: the effect of low macrophyte abundance

The ability of submerged macrophytes to moderate the structure of food webs with respect to lake eutrophication management has been intensively studied in recent years. Many lake managers have adopted the option of increasing macrophyte abundance in order to restore eutrophic waters, with a view to improve water quality, increase water transparency and reduce phytoplankton biomass. These studies are mostly based upon the comparison of macrophyte-rich and macrophyte-free waters. There is less quantitative information on how different levels of macrophyte abundance, in particular very low levels, influence ecosystem structure, or vice versa, within the same ecosystem. This paper takes a multivariate statistical approach to examine ecosystem function with low macrophyte abundance (ca. 5%) in the context of nutrient-phytoplankton-zooplankton interaction in a eutrophic shallow lake. It shows that with low levels of macrophyte coverage, phytoplankton biomass was only limited by nutrient (TP and Si) availability, whilst nutrient (Si) as well as grazing (zooplankton and Daphnia) factors were both limiting phytoplankton levels in macrophyte-free waters. With respect to interactions between Daphnia and chlorophyll-a, the results suggest that low macrophyte cover does not result in a reduction of phytoplankton biomass due to the presence of Daphnia. Rather, low macrophyte cover provides a refuge that maintains constant Daphnia predation, so reducing chlorophyll-a fluctuation. These results add credence to the causal interpretation of different interactions amongst nutrients, phytoplankton and zooplankton at very low macrophyte densities.     

Isolation as a restoration strategy for nutrient reduction in a small hypereutrophic lake

The restoration of a small hypereutrophic urban lake in Baton Rouge, Louisiana was accomplished by isolating the lake from its primary source of nutrient loading: a larger, connecting lake that was heavily contaminated by sewage. Restoration objectives were directed towards reducing phosphorus levels in the system to minimize chronic summer fish kills. Previous studies had identified a critical phosphorus level of 0.4 mg l⁻¹, below which no fish kills had been observed in this system. Above this level the kill frequency was a uniform 3.0%. Water quality data, consisting of 13 parameters, were collected over a four and one-quarter year period, representing conditions before, during and after the isolation. Evaluation of total phosphorus data indicated an approx. 90% reduction in estimated risk of summer fish kills as a result of the isolation. A general reduction in nutrient levels, as well as an increase in system stability, was also observed. Isolation of the lake proved to be a simple and economical means of attaining management objectives.     

Cost assessment and ecological effectiveness of nutrient reduction options for mitigating Phaeocystis colony blooms in the Southern North Sea: an integrated modeling approach

Nutrient reduction measures have been already taken by wealthier countries to decrease nutrient loads to coastal waters, in most cases however, prior to having properly assessed their ecological effectiveness and their economic costs. In this paper we describe an original integrated impact assessment methodology to estimate the direct cost and the ecological performance of realistic nutrient reduction options to be applied in the Southern North Sea watershed to decrease eutrophication, visible as Phaeocystis blooms and foam deposits on the beaches. The mathematical tool couples the idealized biogeochemical GIS-based model of the river system (SENEQUE-RIVERSTRAHLER) implemented in the Eastern Channel/Southern North Sea watershed to the biogeochemical MIRO model describing Phaeocystis blooms in the marine domain. Model simulations explore how nutrient reduction options regarding diffuse and/or point sources in the watershed would affect the Phaeocystis colony spreading in the coastal area. The reference and prospective simulations are performed for the year 2000 characterized by mean meteorological conditions, and nutrient reduction scenarios include and compare upgrading of wastewater treatment plants and changes in agricultural practices including an idealized shift towards organic farming. A direct cost assessment is performed for each realistic nutrient reduction scenario. Further the reduction obtained for Phaeocystis blooms is assessed by comparison with ecological indicators (bloom magnitude and duration) and the cost for reducing foam events on the beaches is estimated. Uncertainty brought by the added effect of meteorological conditions (rainfall) on coastal eutrophication is discussed. It is concluded that the reduction obtained by implementing realistic environmental measures on the short-term is costly and insufficient to restore well-balanced nutrient conditions in the coastal area while the replacement of conventional agriculture by organic farming might be an option to consider in the nearby future.     

Is the future blue-green? A review of the current model predictions of how climate change could affect pelagic freshwater cyanobacteria

There is increasing evidence that recent changes in climate have had an effect on lake phytoplankton communities and it has been suggested that it is likely that Cyanobacteria will increase in relative abundance under the predicted future climate. However, testing such a qualitative prediction is challenging and usually requires some form of numerical computer model. Therefore, the lake modelling literature was reviewed for studies that examined the impact of climate change upon Cyanobacteria. These studies, taken collectively, generally show an increase in relative Cyanobacteria abundance with increasing water temperature, decreased flushing rate and increased nutrient loads. Furthermore, they suggest that whilst the direct effects of climate change on the lakes can change the timing of bloom events and Cyanobacteria abundance, the amount of phytoplankton biomass produced over a year is not enhanced directly by these changes. Also, warmer waters in the spring increased nutrient consumption by the phytoplankton community which in some lakes caused nitrogen limitation later in the year to the advantage of some nitrogen-fixing Cyanobacteria. Finally, it is also possible that an increase in Cyanobacteria dominance of the phytoplankton biomass will lead to poorer energy flow to higher trophic levels due to their relatively poor edibility for zooplankton.     

Nutrient fluxes in the Po basin.

The nutrient load of the river Po needs to be reduced in order to bring the eutrophication problems in the Northwest Adriatic Sea under lasting control. In this paper we analyse the contribution of the different pollution sources to the nitrogen and phosphorus loads of the river Po (source apportionment). We also estimate the effects of measures that aim to reduce nitrogen and phosphorus pollution in the Po basin (scenario analyses). Using a model (PolFlow) that was previously applied to the Rhine and Elbe basins, we found that more than half of the nitrogen and phosphorus loads in the river Po originates from diffuse sources. The results of the scenario analyses indicate that the measures imposed by the EU Nitrates Directive and the EU Wastewater Treatment Directive may not be stringent enough to achieve a large reduction in the N and P loads in the river Po.     

Eutrophication and nutrient release in urban areas of sub-Saharan Africa — A review

Eutrophication is an increasing problem in sub-Saharan Africa (SSA), and, as a result, the ecological integrity of surface waters becomes compromised, fish populations become extinct, toxic cyanobacteria blooms are abundant, and oxygen levels reduce. In this review we establish the relationship between eutrophication of fresh inland surface waters in SSA and the release of nutrients in their mega-cities. Monitoring reports indicate that the population of mega-cities in SSA is rapidly increasing, and so is the total amount of wastewater produced. Of the total amounts produced, at present, less than 30% is treated in sewage treatment plants, while the remainder is disposed of via onsite sanitation systems, eventually discharging their wastewater into groundwater. When related to the urban water balance of a number of SSA cities, the total amount of wastewater produced may be as high as 10-50% of the total precipitation entering these urban areas, which is considerable, especially since in most cases, precipitation is the most important, if not only the ‘wastewater diluting agent’ present. The most important knowledge gaps include: (1) the fate and transport mechanisms of nutrients (N and P) in soils and aquifers, or, conversely, the soil aquifer treatment characteristics of the regoliths, which cover a large part of SSA, (2) the effect of the episodic and largely uncontrolled removal of nutrients stored at urban surfaces by runoff from precipitation on nutrient budgets in adjacent lakes and rivers draining the urban areas, and (3) the hydrology and hydrogeology within the urban area, including surface water and groundwater flow patterns, transport velocities, dynamics of nutrient transport, and the presence of recharge and discharge areas. In order to make a start with managing this urban population-related eutrophication, many actions are required. As a first step, we suggest to start systematically researching the key areas identified above.     

Statistical models of river loadings of nitrogen and phosphorus in the Lough Neagh system

To establish nutrient budgets of Lough Neagh in N. Ireland, and provide strategies for the reduction of nitrogen and phosphorus it became necessary to obtain estimates of their inputs to the lake. On the six major rivers entering the lake continuous flow metering was available but chemical concentrations were only available every 8 or 15 days. To associate river chemical concentrations and loadings with flows for these observed dates, and hence predict from their relation the loads at the unobserved dates a computer program using eight possible statistical relations was used. Those methods examined included various regressions with adjustments, higher order (polynomial) fits, logarithmic and other transforms, and different rising and falling flow relations. Results produced by the various methods with discussion of their relative merits are presented. Reasons are given for the final selection of a simple log load on log flow relation for rivers derived from terrestrial catchments.     

Biological and chemical factors influencing shallow lake eutrophication: a long-term study

The focus of eutrophication research has tended to be upon short-term and experimental studies. However, given the range of factors that can influence eutrophication dynamics, and that these matter over a range of time scales, some discrete, some continuous, eutrophication dynamics may only be fully investigated when long-term, time-series data are available. The present study aims to evaluate the interacting effects of abiotic processes and biotic dynamics in explaining variations of phytoplankton biomass in a eutrophic shallow lake, Barton Broad (Norfolk, UK) using a long-term data set. Multivariate statistical analysis shows that the inter-relationships between phytoplankton variability, nutrient and grazing factors were highly sensitive to seasonal periodicity. In spring phytoplankton biomass was related to phosphorus, nitrogen and silicon. In summer phytoplankton biomass was associated with phosphorus, nitrogen and zooplankton. In autumn phytoplankton was related to phosphorus, nitrogen, silicon and zooplankton. In winter, no significant relationship could be established between phytoplankton and environmental variables. This paper improves our understanding of the governing role of nitrogen, phosphorus, silicon and zooplankton upon phytoplankton variability, and hence, improves management methods for eutrophic lakes.