Food Web Manipulation Research Articles

Silver nanoparticles alter planktonic community structure and promote ecosystem respiration in freshwater mesocosms

The widespread use of silver nanoparticles (AgNPs) has resulted in their release into the aquatic environment, which threatens the health of aquatic ecosystems. Although the ecotoxicological effects of AgNPs have been widely reported at individual and population levels, the impact of long-term exposure to AgNPs on community structure and ecosystem function in aquatic ecosystems remains poorly understood. Herein, the present study investigated the effects of long-term exposure (28 d) to environmentally relevant concentrations (1 μg/L and 10 μg/L) of AgNPs on the community structure and function of freshwater ecosystems by artificially constructed 28 mesocosms freshwater ecosystem in experimental greenhouses, using plastic water tanks and food web manipulation. The results showed that long-term exposure to AgNPs significantly altered the community structure of zooplankton, phytoplankton, and bacterioplankton in the aquatic ecosystem. Exposure to 10 μg/L AgNPs significantly reduced the zooplankton density (70.3%, p < 0.05) and increased the phytoplankton biomass and bacterial richness and diversity via a “top-down effect.” With regards to ecosystem function, AgNPs exposure significantly increased the respiration in freshwater ecosystems but did not have a significant effect on decomposition. The partial least squares path modeling (PLS-PM) further revealed that AgNPs may have a negative impact on ecosystem functions by reducing zooplankton community density and thus increasing phytoplankton biomass. This study is the first to show that long-term exposure to environmentally relevant concentrations of AgNPs leads to alterations in plankton community structure and promotes respiration in freshwater ecosystems. It emphasizes the need for assessing the environmental risk of long-term exposure to AgNPs at the ecosystem level.

Cascading effects of predators on algal size structure

The presence of edible and inedible prey species in a food web can influence the strength that nutrients (bottom-up) or herbivores (top-down) have on primary production. In boreal peatlands, wetter more nutrient-rich conditions associated with ongoing climate change are expanding consumer access to aquatic habitat and promoting sources of primary production (i.e., algae) that are susceptible to trophic regulation. Here, we used an in situ mesocosm experiment to evaluate the consequences of enhanced nutrient availability and food-web manipulation (herbivore and predator exclusion) on algal assemblage structure in an Alaskan fen. Owing to the potential for herbivores to selectively consume edible algae (small cells) in favor of more resistant forms, we predicted that the proportion of less-edible algae (large cells) would determine the strength of top-down or bottom-up effects. Consistent with these expectations, we observed an increase in algal-cell size in the presence of herbivores (2-tiered food web) that was absent in the presence of a trophic cascade (3-tiered food web), suggesting that predators indirectly prevented morphological changes in the algal assemblage by limiting herbivory. Increases in algal-cell size with herbivory were driven by a greater proportion of filamentous green algae and nitrogen-fixing cyanobacteria, whose size and morphological characteristics mechanically minimize consumption. While consumer-driven shifts in algal assemblage structure were significant, they did not prevent top-down regulation of biofilm development by herbivores. Our findings show that increasing wet periods in northern peatlands will provide new avenues for trophic regulation of algal production, including directly through consumption and indirectly via a trophic cascade.

Zooplankton as an alternative method for controlling phytoplankton in catfish pond aquaculture

In pond aquaculture, production of toxins and off-flavor compounds by cyanobacteria can negatively affect fish health and production. Studies have explored chemical or physical methods for controlling algal blooms in aquaculture ponds, which although effective, may be short-lived and can negatively impact non-target organisms, including aquaculture species. Food web manipulations have a long history in lake and fisheries management to improve water quality, but have been rarely considered in aquaculture. This study examined zooplankton and phytoplankton communities, cyanobacterial toxins, and nutrients in nine catfish aquaculture farm-ponds in west Alabama, USA. The goal of this project was to track phytoplankton and zooplankton abundances with respect to each other, with and without efforts to reduce zooplanktivorous fish in some of the ponds. During this project, farm managers reduced zooplanktivorous fish abundance in select ponds to create a large-scale field experiment that addressed the role of zooplankton control of phytoplankton in hypereutrophic catfish aquaculture ponds when zooplanktivorous fish were or were not excluded. There was a strong negative effect of zooplankton on phytoplankton, including cyanobacteria, despite high nutrient concentrations. Although high zooplankton ponds sustained elevated zooplankton biomass during much of this study, including when pond temperatures exceeded 30 °C, the effect of zooplankton on phytoplankton was most pronounced during the non-growing season (November–April). In addition, total ammonia nitrogen was significantly higher in high zooplankton ponds, which could lead to ammonia toxicity in fish at elevated temperature and pH. Our findings suggest that zooplankton biomanipulation may be an efficient method to control algal blooms in farm-pond catfish aquaculture.

Long-term effects of fish biomanipulation and macrophyte management on zooplankton functional diversity and production in a temperate shallow lake

Eutrophication still is one of the greatest anthropogenic impacts affecting aquatic ecosystems. Food web manipulations to reverse eutrophic conditions have been applied, mainly by reducing fish stocks. However, the long-term results of biomanipulation are still ambiguous. The objective of this study was to evaluate biomanipulation effects on zooplankton functional diversity and production during 24 years of monitoring a shallow temperate lake. Temporal variation in crustacean biomass (CB), zooplankton resource use efficiency (RUE), limnological variables, functional diversity indices and functional traits were assessed using the Mann–Kendall test. The effects of the limnological and functional variables on zooplankton production (CB and RUE) were analyzed by generalized least squares models (GLSs). We found decreasing trends for almost all the limnological variables, indicating a reduction in lake eutrophic conditions. We also observed a decreasing trend in functional traits related to pelagic habitat, filter-feeding mode, herbivore feeding preference and body size. The GLS results showed that functional dispersion and chlorophyll a are important determinants of zooplankton production. The effects of fish biomanipulation produced no significant temporal trends in zooplankton biomass. In fact, the increase in submerged macrophytes promoted littoral food webs, which was likely the main determining factor of the changes in zooplankton functional traits and production.

The assessment and remediation of mercury contaminated sites: A review of current approaches

Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production—such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.

Temperature sensitivity patterns of carbon and nitrogen processes in decomposition of boreal organic soils - Quantification in different compounds and molecule sizes based on a multifactorial experiment.

Climate warming and organic matter decomposition are connected in a recursive manner; this recursion can be described by temperature sensitivity. We conducted a multifactorial laboratory experiment to quantify the temperature sensitivity of organic carbon (C) and nitrogen (N) decomposition processes of common boreal organic soils. We incubated 36 mor and 36 slightly decomposed Carex-Sphagnum peat samples in a constant moisture and ambient temperature for 6 months. The experiment included three temperature and two moisture levels and two food web manipulations (samples with and without fungivore enchytraeid worms). We determined the release of carbon dioxide (CO2) and dissolved organic carbon (DOC) in seven molecular size classes together with ammonium N and dissolved organic N in low molecular weight and high molecular weight fractions. The temperature sensitivity function Q10 was fit to the data. The C and N release rate was almost an order of magnitude higher in mor than in peat. Soil fauna increased the temperature sensitivity of C release. Soil fauna played a key role in N release; when fauna was absent in peat, the N release was ceased. The wide range of the studied C and N compounds and treatments (68 Q10 datasets) allowed us to recognize five different temperature sensitivity patterns. The most common pattern (37 out of 68) was a positive upwards temperature response, which was observed for CO2 and DOC release. A negative downward pattern was observed for extractable organic nitrogen and microbial C. Sixteen temperature sensitivity patterns represented a mixed type, where the Q10function was not applicable, as this does not allow changing the sign storage change rate with increasing or decreasing temperature. The mixed pattern was typically connected to intermediate decomposition products, where input and output fluxes with different temperature sensitivities may simultaneously change the storage. Mixed type was typical for N processes. Our results provide useful parameterization for ecosystem models that describe the feedback loop between climate warming, organic matter decomposition, and productivity of N-limited vegetation.

Water environment change over the period 2007-2016 and the strategy of fishery improve the water quality of Lake Taihu

太湖水环境是国内外关注的焦点，其生态环境质量影响到流域经济社会发展.2007年太湖水危机事件，催生了对太湖的综合整治.本文基于2007-2016十年对太湖水质与蓝藻水华面积等的监测，分析了太湖水环境的演变趋势.太湖十年水质变化阶段性明显，2008-2012年各项水质指标下降明显，而后趋于平缓，近3年个别指标如总磷、叶绿素a浓度等呈现快速上升的反弹趋势；另外，水质指标在空间上的差异性逐步缩小，原来污染严重的西北部水域水质改善效果较为显著，其正从污水湖向自然湖状态过渡，而原来水质相对较好的东南部水域水质却逐步下降.本文也综述分析了太湖鱼类群落结构变化与水质环境变化的相关性，基于太湖局部水域的鱼类调控实践，提出了太湖现阶段以渔改水的鲢鳙控藻非经典生物操纵与鱼类结构调控的经典生物操纵结合的渔业途径.湖泊富营养化治理需要充分关注到鱼类对湖泊浮游生物和水质变化的重要驱动效应，需要充分考虑到鱼类群落结构优化和食物网调控对环境的改善作用.;Currently,water environment in the Lake Taihu has become one of the focuses due to its importance to the rapid growth of economy and the process of society in the Lake Taihu Basin. Water Crisis of the Lake Taihu in 2007 has resulted in a series of comprehensive treatment program for its eco-environmental improvement. Based on the water quality data from 2007 to 2016, the characteristics and dynamic variation of water environment in the last decade were analyzed. The results showed that the periodic change of water quality in Lake Taihu was obvious. All water quality indexes continued to decline and then leveled off over the period of 2008 to 2012, while certain indexes, such as total phosphorus and chlorophyll-a concentration, showed a rapidly increasing trend in nearly three years. Additionally, the spatial heterogeneity of water quality in the Lake Taihu decreased. As for the heavily polluted northwestern waters, changing from contaminated to natural status, had acquired a massive improvement in the ecological environment, while the water quality of the southeastern waters had gradually deteriorated. Based on the correlation analysis between the fish community structure and water quality variation in Lake Taihu, and combined with the practice of certain fish bio-manipulation projects in local lake regions, a bio-manipulation approach Fishery improve the water quality combined using traditional and non-traditional bio-manipulations was proposed. Eutrophication control in lakes requires sufficiently concern the irreplaceable role of fish in driving plankton community and water quality modification, as well as considering the advantageous effects of structural optimization of fish community and food web manipulation.

No evidence for trade-offs in plant responses to consumer food web manipulations.

Plants face a range of trade-offs as they attempt to maximize their fitness within a complex web composed of competitors, mutualists, and herbivores. In addition to growth-defense and competition-defense trade-offs, plants must balance their response to a wide range of potential enemies including pathogens and vertebrate and invertebrate herbivores. We tested for trade-offs in plant species' responses to different types of consumers using a foodweb manipulation experiment in which we selectively excluded large vertebrate herbivores and removed foliar fungi, soil fungi, and insects from natural and experimentally planted grassland communities. We found no evidence for trade-offs in the ability of plants to defend themselves against different sets of consumers, although plants varied widely in their responses to removal of different consumer groups. In addition, the species-level responses to consumer removal in monoculture were uncorrelated with each species' response in more diverse communities, highlighting the important role of local context (e.g., competition and apparent competition) in determining the effects of consumers. Plants must balance their allocation of energy among a wide variety of tasks including growing, competing for limited resources, and defending themselves against an array of potential enemies. We found that while plant species differed greatly in their response to the removal of consumers, species that were susceptible to the effects of one consumer group (e.g., insect herbivores) also were susceptible to other consumer groups (e.g., fungal pathogens). This suggests that plants differ in their overall allocation to defense, but defense investment can proffer protection against a wide array of natural enemies. We also found that plant responses to consumers depended on the diversity of the surrounding plant community, suggesting that among-plant interactions can alter their susceptibility to the impacts of consumers.

Epiphyte load and seagrass performance are decoupled in an estuary with low eutrophication risk

Seagrass can decline due to blooms of algae following either loss of top-down control or increase in bottom-up nutrient delivery. Macroalgae negatively affect seagrass especially at high ratios of algal biomass to seagrass size. In contrast, for microalgae, epiphyte loads that affect seagrass performance are not well synthesized. Competition has been demonstrated primarily following nutrient or food web manipulations, which increase epiphyte load from background levels, rather than through experimental removal of epiphytes. Although the Willapa Bay (Washington, USA) is an estuary with low eutrophication risk, microalgae reach large biomass in summer, up to 60% of the mass of their intertidal seagrass host (eelgrass, Zostera marina). Based on a novel field technique, experimental reduction of epiphytes did not accelerate growth (g g−1 d−1) of Z. marina. Neither did two other manipulations that were expected to improve resource access: removing nearby shoots or older, epiphytized leaves. Instead, removing older leaves slowed growth from 2.3%d−1 to 1.8%d−1, and removing lightly-epiphytized younger leaves slowed growth to 1.7%d−1. To evaluate top-down and bottom-up causes of epiphyte load, two widespread methods were used. Observationally over time, epiphyte load was positively correlated with eelgrass leaf emergence rate and unrelated to mesograzer density; however, a crossed experiment reducing crustacean mesograzers and adding nutrients indicated top-down control, since chlorophyll-a in epiphytes increased with grazer reduction and no added nutrients (deterrent ×nutrient interaction). At current epiphyte loads in the Willapa Bay, intertidal Z. marina tolerates seasonally-abundant epiphytes coinciding with rapid leaf emergence, thus producing fresh leaves where microalgal competition cannot be detected.

Enrichment of Omnivorous Cercozoan Nanoflagellates from Coastal Baltic Sea Waters

Free-living nano-sized flagellates are important bacterivores in aquatic habitats. However, some slightly larger forms can also be omnivorous, i.e., forage upon both bacterial and eukaryotic resources. This hitherto largely ignored feeding mode may have pronounced implications for the interpretation of experiments about protistan bacterivory. We followed the response of an uncultured group of omnivorous cercozoan nanoflagellates from the Novel Clade 2 (Cerc_BAL02) to experimental food web manipulation in samples from the Gulf of Gdańsk (Southern Baltic Sea). Seawater was either prefiltered through 5 µm filters to exclude larger predators of nanoflagellates (F-treatment), or prefiltered and subsequently 1∶10 diluted with sterile seawater (F+D-treatment) to stimulate the growth of both, flagellates and bacteria. Initially, Cerc_BAL02 were rapidly enriched under both conditions. They foraged on both, eukaryotic prey and bacteria, and were highly competitive at low concentrations of food. However, these omnivores were later only successful in the F+D treatment, where they eventually represented almost one fifth of all aplastidic nanoflagellates. By contrast, their numbers stagnated in the F-treatment, possibly due to top-down control by a concomitant bloom of other, unidentified flagellates. In analogy with observations about the enrichment of opportunistically growing bacteria in comparable experimental setups we suggest that the low numbers of omnivorous Cerc_Bal02 flagellates in waters of the Gulf of Gdańsk might also be related to their vulnerability to grazing pressure.

Coupling stable isotope studies with food web manipulations to predict the effects of exotic fish: lessons from Cuatro Ciénegas, Mexico

1. Exotic species threaten native species worldwide, but their impacts are difficult to predict. 2. Stable isotope analysis was combined with field competition experiments to predict how an invasive African cichlid fish, Hemichromis guttatus, might affect native fish in the desert springs of Cuatro Ciénegas, Mexico. 3. Stable isotope analysis suggested diet overlap between the invader and juvenile endemic cichlids, and field experiments verified that the invader reduces growth rates of the juvenile endemics through competition, but has smaller effects on adults. 4. Competition between juvenile endemic cichlids and the invader was asymmetric, with the exotic out-competing the native, suggesting the potential for competitive exclusion if the invasion is not stopped. 5. These results suggest that exotic removal programmes in Cuatro Ciénegas should focus on removing/reducing populations of the exotic cichlid in habitats where juvenile native cichlids are concentrated. 6. This approach could help focus efforts to manage exotic species before populations of native species have crashed, when it is too late to intervene. Copyright © 2011 John Wiley & Sons, Ltd.

Body mass–abundance relationships are robust to cascading effects in marine food webs

Body mass has been shown to scale negatively with abundance in a wide range of habitats and ecosystems. It is believed that this relationship has important consequences for the distribution and maintenance of energy in natural communities. Some studies have shown that the relationship between body mass and abundance may be robust to major food web perturbations, fuelling the belief that natural processes may preserve the slope of this relationship and the associated cycling of energy and nutrients. Here, we use data from a long‐term experimental food web manipulation to examine this issue in a semi‐natural environment. Similar communities were developed in large experimental mesocosms over a six month period. Some of the mesocosms were then subjected to species removals, based on the mean strength of their trophic interactions in the communities. In treatments where the strongest interactors were removed, a community‐level trophic cascade occurred. The biomass density of invertebrates increased dramatically in these communities, which led to a suppression of primary production. In spite of these widespread changes in ecosystem functioning, the slope of the relationship between body mass and abundance remained unchanged. This was the case whether average species body mass and abundance or individual organism size spectra were considered. An examination of changes in species composition before and after the experimental manipulations revealed an important mechanism for maintaining the body mass–abundance relationship. The manipulated communities all had a higher species turnover than the intact communities, with the highest turnover in communities that experienced cascading effects. As some species increased in body mass and abundance, new species filled the available size–abundance niches that were created. This maintained the overall body mass–abundance relationship and provided a stabilising structure to these experimental communities.

Nitrogen fixation may not balance the nitrogen pool in lakes over timescales relevant to eutrophication management

We explored multiyear linear trends in nutrient concentrations, nitrogen (N) : phosphorus (P) ratio, and phytoplankton biomass within the 37‐yr, whole‐ecosystem nutrient enrichment experiment in Lake 227 of the Experimental Lakes Area, Canada. Based on experimental conditions, data were divided into subsets, which included (1) the period from 1969 to 1989 when the lake was fertilized with both N and P; (2) the period from 1990 to 2005 when the lake was fertilized with P alone; and (3) the period from 1997 to 2005 when the lake was fertilized with P alone and which also postdated a food web manipulation experiment, which left the lake without fish. After N fertilization was halted in 1990, total N concentrations decreased, which resulted in a decrease in the ratio of total N to total P and suggested increasing N deficiency. Chlorophyll ά concentration decreased over this same period. Phytoplankton biomass (mg m−3) was highly variable during the food web manipulation experiment but exhibited a clear decrease from 1997 to 2005, which was the longest period of monotonic change in phytoplankton biomass over the entire 37‐yr study. Collectively, these results suggest that Lake 227 has become increasingly N‐limited since N fertilization was halted and indicate that N fixation by cyanobacteria was not sufficient to offset the decrease in external N inputs to Lake 227. Furthermore, phytoplankton biomass decreased in response to decreased N availability, suggesting that the degree of eutrophication can be controlled by managing N inputs concurrently with P.

Theoretical Analysis of the Potential of Silver Carp Hypophthalmichthys Molitrix in the Control of Water Blooming by Different Species of Cyanobacteria

Searching the way to prevent reservoirs from blooming by cyanobacteria has been one of the principal objectives of the modern hydrobiology during the last decades. A great number of different physical, chemical and biological methods to prevent blooming have been developed since, with the method of foodweb manipulation occupying a special place. This approach is the most ecologically sound management strategy (Datta, Jana, 1998). In most cases biomanipulation led to an improvement in the water quality (e.g. Annadotter et al., 1999; Gulati, van Donk, 2002). The ‘classical’ approach in biomanipulation is the increase of carnivorous fish biomass and/ or decrease of planktivorous fish biomass (e.g. Lammens, 1999; Drenner, Hambright, 2002), which leads to the increase of zooplankton biomass and reduction of cyanobacteria concentration. Journal of Siberian Federal University. Biology 4 (2009 2) 403-417 ~ ~ ~

Changes in water clarity during fish manipulation and post-manipulation periods in a shallow eutrophic lake

A food web manipulation experiment was conducted in the small, shallow, eutrophic Lake Major, part of the Kis-Balaton Water Protection System (KBWPS) wetland area. Several factors affected the development of turbid water during the fish manipulation period (FMP) in 2000-2001 versus clear water during the post manipulation period (PMP) in 2002 and 2005-2006. A > 40 % percentage macrophyte cover was the most important factor to differentiate between the years with diverse water quality. However, the concentrations of soluble reactive phosphorus (SRP) did not change significantly between the periods. Lake Major exhibited maximum fish species richness (12 species) during PMP, when richness was twice as high as during FMP. There was a clear succession of macrophyte taxa, mostly related to the natural variability in fish. Initially, the macrophyte community was dominated by Najadosum marini, but after fish removal and restocking of piscivores, Ceratophyllum demersi became dominant. This study showed that there is a high potential for restoration of species-rich submerged macrophyte stands even in a hypertrophic lake having been for more than two decades in a turbid state. Water of Lake Major, as the whole KBWPS wetland area, had not previously been in such a good ecological condition since its establishment in 1986. However, it cannot be concluded from this study whether the new trophic state created by fish manipulation is sustainable for longer period.

Forecasting and explanation of algal dynamics in two shallow lakes by recurrent artificial neural network and hybrid evolutionary algorithm

Long-term time-series of the two eutrophic Dutch lakes Veluwemeer and Wolderwijd were subject to predictive modelling by recurrent supervised ANN (RANN) and hybrid evolutionary algorithms (HEA). A combination of bottom-up and top-down eutrophication control measures has been implemented in both lakes since 1979. Dividing the time-series data into training and validation datasets based on three distinctive management periods has facilitated a comparative analysis of the two lakes regarding the long-term dynamics in response to eutrophication control. Results of the study have demonstrated that RANN and HEA can be applied for (1) 5-day-ahead prediction of Oscillatoria spp. and (2) 5-day-ahead prediction of Scenedesmus spp. Firstly RANN achieved reasonably accurate results for 5-day-ahead forecasting of abundances of blue-green algae Oscillatoria and green algae Scenedesmus in both lakes. Secondly HEA achieved similar good forecasting results and also provided model representations for both algae species in the form of rule sets. The limitation with single lake models is that the rule sets discovered are lake specific. Merging of the two lake datasets using merged lake models for both training and testing have produced simpler and generic rule sets that explain the dynamics of Oscillatoria and Scenedesmus for both lakes. The results from this study have shown that both external nutrient control combined with food web manipulation have turned the lakes from hypereutrophic conditions and from Oscillatoria to Scenedesmus dominance. These complex dynamics that are associated with the shift from periods of turbid with Oscillatoria dominance to clear-water conditions with increased Scenedesmus in both lakes are predictable and can be explained by the key-driving variables in the generic rule sets discovered.

Food web (bio-)manipulation of South African reservoirs – viable eutrophication management prospect or illusory pipe dream? A reflective commentary and position paper

An overview of prospects and limitations for the application of ‘classical' top-down biomanipulation as a management tool to ameliorate the consequences of eutrophication under the conditions applicable to reservoirs in South Africa is presented. This is structured by considering successive stages in reservoir food-web structure and function as far as can be generalised for South African biophysical conditions. Features and conditions that influence the potential vulnerability of local reservoirs to the effects of eutrophication, and prospects for its amelioration by biomanipulation intervention are examined. Physical factors linked to latitude (irradiation pattern and water-column stability) enhance the potential severity of eutrophication consequences in local reservoirs, although conversely, these are offset by suspended-clay turbidity. The predominance of Microcystis in local eutrophic waters is perceived as a primary major constraint in implementing ‘classical' food-web manipulation. Intrinsic limitations on the ability of zooplankton grazers to control this cyanobacterium, and subsequent food-web linkages widely applicable in local reservoirs are discussed and evaluated accordingly. On balance, available evidence indicates a range of limitations that are likely to apply in respect of familiar contemporary approaches to biomanipulation in use today, although the uniqueness of each reservoir ecosystem is recognised. Some novel alternative management approaches are suggested, and a range of associated research requirements necessary to advance locally relevant scientific comprehension of ‘biomanipulation' and its application are itemised. National revitalisation of reservoir limnology remains paramount. Water SA Vol.32 (4) 2006: pp.567-575

Patternising phytoplankton dynamics of two shallow lakes in response to restoration measures by applying non-supervised artificial neural networks

Long-term time-series data sets of two shallow Dutch lakes, Lake Veluwemeer and Lake Wolderwijd were subjected to ordination and clustering by means of non-supervised artificial neural networks (ANN). Splitting of the data sets into sub-series corresponding with three different management periods have allowed a comparative analysis of both the short-term seasonal and long-term phytoplankton dynamics in relation to the restoration measures. The lakes were considered as hyper-eutrophic and have been managed both with bottom-up and top-down management approaches. Results of the study have demonstrated that non-supervised ANN allow to elucidate causal relationships of complex ecological processes (1) within the specific genus, Oscillatoria and Scenedesmus and (2) the combination of external nutrient control and in-lake food web manipulation of the two lakes achieved to control eutrophication.

Phytoplankton community dynamics of two adjacent Dutch lakes in response to seasons and eutrophication control unravelled by non-supervised artificial neural networks

Long-term time-series of the eutrophic Dutch lakes Veluwemeer and Wolderwijd were subject to ordination and clustering by means of non-supervised artificial neural networks (ANN). A combination of bottom-up and top-down eutrophication control measures has been implemented in both lakes since 1979. Dividing time-series data from 1976 to 1993 into three distinctive management periods has facilitated a comparative analysis of the two lakes regarding both the seasonal and long-term dynamics in response to eutrophication control. Results of the study have demonstrated that non-supervised ANN are an alternative technique: (1) to elucidate causal relationships of complex ecological processes, and (2) to reveal long-term behaviours of ecosystems in response to different management approaches. It has been shown that external nutrient control combined with food web manipulation have turned both lakes from nitrogen to phosphorus limitation, and from blue-green algae to diatom and green algae dominance.

The roles of natural temporal and spatial variation versus biotic influences in shaping the physicochemical environment of intermittent ponds: a case study

The importance of top-down and bottom-up mechanisms in lentic systems has largely been explored in stable systems; however, their influence is likely to change along a habitat duration gradient. Using a case study of four intermittent ponds, we test the generality of these paradigms in systems nearing the extreme short end of this gradient. Intermittent ponds fill up with water on a seasonally predictable basis, and are considered more regulated by physicochemical constraints than by biological factors. We found that natural spatial (i.e., pond-to-pond) and temporal variations (i.e., seasonal and annual) were more important than food-web manipulations (i. e., exclusion of aerial colonization, addition of top-predators [beetle larvae and odonate nymphs], and addition versus removal of allochthonous resources [leaf litter] in field enclosures) in shaping the physicochemical environment. Spatial variation included mainly depth-related variables (e. g., conductivity and pH), whereas annual variation included variables associated with hydroperiod length and productivity (e. g., weeks after flooding, pH, nutrients, and chlorophyll-a). Seasonal variation was pronounced but lower in ponds with longer hydroperiod and in mid-season. Enhanced invertebrate predation pressure increased levels of chlorophyll-a. Exclusion of aerial colonizers increased levels of chlorophyll-a and phosphorus, and affected levels of dissolved oxygen (increasing in one pond and decreasing in another). Addition of leaf litter resulted in lower concentrations of dissolved oxygen, higher phosphorus, and lower pH. Leaf litter removal led to increased levels of dissolved oxygen, changed chlorophyll-a (increasing in one pond and decreasing in another), and higher pH (in one pond). We conclude that although pond characteristics, seasonal development, and annual differences were of greater importance than biological factors for shaping the physicochemical characteristics of these intermittent ponds, bottom-up and top-down effects were also influential.