Lake restoration with and without dredging of phosphorus-enriched upper sediment layers

Human activity has been the cause of continuing decline of water quality in most Dutch lakes. Development of lake restoration programmes must take into account the lake functions. Major reduction of the nutrient and pollutant loading is the primary step in lake restoration. Still, the recovery of eutrophic lakes is retarded frequently because of internal phosphorus loading by the lakes' sediments. Sediment dredging, as an additional tool for water quality management to accelerate accomplishing the desired water quality, is studied. In this paper we evaluate the preliminary results of eight lake restoration projects in the Netherlands. The lakes are compared in order to estimate the magnitude of the internal phosphorus loading. Dredging as an additional measure was carried out twice in the peatlake Geerplas. In the Nieuwkoop Lakes only the external phosphorus loading was substantially reduced from 0.9 to 0.2 g P m^–2 y^–1. Provisional results of these two shallow peatlake restoration projects focussed on eutrophication abatement with and without dredging, are presented. Both show a decrease in phosphorus concentration in the lakes. The necessity to dredge the lakes is discussed.     

Nutrient control of phytoplankton production in Lake Naivasha,Kenya

Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.     

Sediment removal by the Lake Apopka marsh flow-way

We review the evidence showing that the high turbidity levels of Lake Apopka are due primarily to resuspended sediments rather than phytoplankton, and that this situation is likely to persist unless there is a fundamental change in the lake. We discuss the reasons why reductions in phosphorus inputs, the gizzard shad removal program, and macrophyte plantings would not bring about such a change. Potentially the marsh flow-way could remove the flocculent sediments because of a unique combination of a very large surface area (125 km²), a mean depth of only 1.7 m, a layer of easily resuspended fluid mud, and a marsh flow-way that is designed to filter the lake volume about 2 times a year. Using several different estimates of the rate of sediment formation in the lake, our model calculates that it would take from 275 to 502 years to remove the sediments, so the lake could not attain clear water in a reasonable length of time. The model is mathematically correct but will give nonsense results if one tries to calculate removal times when the lake is accumulating sediments rather than losing them.     

Evaluation of recent limnological changes at Lake Apopka

Recent changes in submersed macrophytes and water quality variables have been offered as the strongest evidence that the current restoration program at Lake Apopka will be effective (Lowe et al., 2000); however, the new beds of submersed plants in Lake Apopka are found only on hard substrates on the fringes of the lake within 40 m of shore and are protected from waves by cattails (Typha spp.). They occupy only 0.02% of the lake area, and there is no indication that they can colonize the flocculent sediments that make up 90% of the lake area. There is no correlation between annual inputs of phosphorus and total phosphorus concentrations in the lake, and patterns of change in chlorophyll and other water quality variables do not follow changes in phosphorus loads. Rather than reflecting decreases in phosphorus loading, the recent changes could be related to the harvest of benthivorous fish or are just the normal fluctuations found in lakes that have not been perturbed. Regardless of the reason the macrophytes were lost in the 1940s, the new analyses confirm our previous findings that the high turbidities in Lake Apopka are due to the resuspension of sediments, and that the fluid mud cannot support the colonization of submersed aquatic macrophytes. Even without the fluid mud, the target phosphorus concentration of 55 mg m^–3 is too high to bring about the restoration of the former macrophyte beds in the lake.     

Field and experimental evidence of the effect of Jenynsia multidentata, a small omnivorous–planktivorous fish, on the size distribution of zooplankton in subtropical lakes

1. Small cladocerans, copepod nauplii and rotifers often dominate the zooplankton community in tropical and subtropical lakes. This is probably because of high predation pressure by small omnivorous–planktivorous fish, but experimental evidence is scarce.
2. This study used two approaches to test the effect of the small omnivorous–planktivorous fish species Jenynsia multidentata , which is frequently abundant in (sub)tropical eutrophic lakes in South America, on the size distribution of zooplankton. In Lake Blanca (Uruguay), which lacks any piscivores, we sampled seasonally for both fish and zooplankton. We also conducted an outdoor mesocosm experiment with treatments containing or lacking J. multidentata .
3. Together, the empirical and experimental data suggest that J. multidentata predation plays an important role in modulating the size structure of the zooplankton community in subtropical lakes. In the absence of J. multidentata , stocked large-sized zooplankters like Daphnia obtusa were abundant in the experiments, while small-sized zooplankton dominated in the presence of fish, as they did in the lake itself from spring to the end of the season.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production, it was found that the periphyte biomass increased with the nutrient concentrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photosynthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors, such as fish farming, storm induced waves and mechanical destruction, and high water levels could be the perturbations during the system shifts, but these are not the key factors. Instead, the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

Seasonal Variation of Virioplankton in a Eutrophic Shallow Lake

Lake Donghu is a typical eutrophic freshwater lake in which high abundance of planktonic viruses was recently revealed. In this study, seasonal variation of planktonic viruses were observed at three different trophic sites, hypertrophic, eutrophic, and mesotrophic regions, and the correlation between their abundances and other aquatic environmental components, such as bacterioplankton, chlorophyll a, burst size, pH, dissolved oxygen, and temperature, was analyzed for the period of an year. Virioplankton abundance detected by transmission electron microscope (TEM) ranged from 5.48 × 10⁸ to 2.04 × 10⁹ ml⁻¹ in all the sites throughout the study, and the high abundances and seasonal variations of planktonic viruses were related to the trophic status at the sampled sites in Lake Donghu. Their annual mean abundances were, the highest at the hypertrophic site (1.23×10⁹ ml⁻¹), medium at the eutrophic site (1.19×10⁹ ml⁻¹), and the lowest at the mesotrophic site (1.02×10⁹ ml⁻¹). The VBR (virus-to-bacteria ratio) values were high, ranging from 49 to 56 on average at the three sampled sites. The data suggested that the high viral abundance and high VBR values might be associated with high density of phytoplankton including algae and cyanobacteria in this eutrophic shallow lake, and that planktonic viruses are important members of freshwater ecosystems.     

Weedbeds and big bugs: the importance of scale in detecting the influence of nutrients and predation on macroinvertebrates in plant‐dominated shallow lakes

1. The scale of investigations influences the interpretation of results. Here, we investigate the influence of fish and nutrients on biotic communities in shallow lakes, using studies at two different scales: (i) within‐lake experimental manipulation and (ii) comparative, among‐lake relationships. 2. At both scales, fish predation had an overriding influence on macroinvertebrates; fish reduced macroinvertebrate biomass and altered community composition. Prey selection appeared to be size based. Fish influenced zooplankton abundance and light penetration through the water column also, but there was no indication that fish caused increased resuspension of sediment. 3. There were effects of nutrients at both scales, but these effects differed with the scale of the investigation. Nutrients increased phytoplankton and periphyton at the within‐lake scale, and were associated with increased periphyton at the among‐lake scale. No significant effect of nutrients on macroinvertebrates was observed at the within‐lake scale. However, at the among‐lake scale, nutrients positively influenced the biomass and density of macroinvertebrates, and ameliorated the effect of fish on macroinvertebrates. 4. Increased prey availability at higher nutrient concentrations would be expected to cause changes in the fish community. However, at the among‐lake scale, differences were not apparent in fish biomass among lakes with different nutrient conditions, suggesting that stochastic events influence the fish community in these small and relatively isolated shallow lakes. 5. The intensity of predation by fish significantly influences macroinvertebrate community structure of shallow lakes, but nutrients also play a role. The scale of investigation influences the ability to detect the influence of nutrients on the different components of shallow lake communities, particularly for longer lived organisms such as macroinvertebrates, where the response takes longer to manifest.     

Comparision of historical and recent data on hydrochemistry and phytoplankton in the Rijnland area (The Netherlands)

In two canals and two lakes in the western part of the Netherlands a comparision is made between data on water chemistry and phytoplankton from 1941/1942 with recent data. Orthophosphate in particular, but also inorganic nitrogen, has increased tremendously, especially in the Gouwe canal, where Rhinewater enters the area. The inorganic N/P mass ratio decreased in the last 45 years, indicating that the limiting nutrient has changed from phosphate in 1941 to nitrogen in 1987.The average seston volume, measured by filtering 1001 water through a plankton net (50 µm), has doubled. In the early 1940s the blue-green alga Microcystis aeruginosa regularly formed waterblooms, as it does now. The plankton composition seems to have become impoverished in the last decennia, since several taxa have disappeared. Others are strongly reduced in number. The saprobic index has not changed.Based on the relations between chlorophyll-a and biological oxygen demand (BOD) and between transparency and seston volume, BOD, dry weight and ash free dry weight in 1987 chlorophyll-a and transparency in 1941 are estimated. The average chlorophyll-a concentration in the lakes has doubled or tripled in the last 45 years and the mean transparency in the Gouwe canal declined from 75 to 50 cm. Submerged higher plants at some sites have disappeared in the last decennia. The results can be used to develop ecological objectives for combatting eutrophication in canals and lakes and possibly in the river Rhine.     

Interactions of temperature and nutrient changes: effects on phytoplankton in the Piburger See (Tyrol,Austria)

1. Contemporary limnological and palaeolimnological data from Piburger See (Eastern Alps, Austria) allowed the reconstruction of its trophic state since the late 19th century and the assessment of changes in phytoplankton biomass and species composition in relation to selected environmental parameters. 2. A radiometrically dated sediment core from Piburger See was analysed for geochemical parameters, spheroidal carbonaceous particles (SCPs), bacterial and algal pigments, and diatoms. The low SCP sediment inventory assigns Piburger See to the ‘cleaner’ sites in Europe with respect to fossil‐fuel related air pollution. The sedimentary pigment and diatom record reveals moderate eutrophication during the 20th century, followed by a slow re‐oligotrophication since the mid‐1980s because of lake restoration starting in 1970. 3. Epilimnetic temperature for Piburger See was reconstructed using air temperature records. A pronounced temperature increase has been recorded during the mid‐1940s and since the late‐20th century, both promoting algal growth and changes in species composition (e.g. increase in centric diatoms and recent bloom of Asterionella formosa). 4. Climate scenarios project additional substantial warming for this mountain lake by the end of the 21st century which will be most pronounced during the growing season. The predicted change in lake water temperature and thermal dynamics represents a key driver for the trophic and ecological status of Piburger See in the future.     

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate+nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T.japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN):TPis 10 and TN/10 when TN:TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.     

Eutrophication control strategies for three shallow Vecht lakes in the province of North Holland

Like many shallow surface waters in the Netherlands the North Holland Vecht lakes, formerly known for their rich variety of flora and fauna, now face a serious eutrophication problem. Nutrient enrichment has been mainly in the form of (treated) wastewater discharges, and the continuing ingress of nutrient-loaded water from the river Vecht. Yet, this water has to be supplied in order to compensate for water shortages resulting from (i) changes in the groundwater flow pattern due to reclamation of the deep polder Horstermeer, (ii) extensive groundwater extraction in the Gooi hills, and (iii) extensive drainage for agricultural purposes.The present policy of eutrophication abatement and restoration of the Ankeveen and Kortenhoef lakes ecosystems is focused on eliminating wastewater discharges and Vecht water supply. It also allows for additional dredging measures. Because of the un-suitable major ion composition of the Vecht, the aim is to compensate for this water supply by (i) partial restoration of the original groundwater flow from the Gooi hills and (ii) periphere additional supply with fresh seepage water from the skirts of the Horstermeer polder. However, uncertainty exists about the amounts of water needed.Water balances and phosphorus budgets have been established to ascertain the water demands of the lakes and to gain a detailed insight into the nutrient fluxes through the lakes. A groundwater flow model is used to assess the beneficial effects of the proposed measures.The results obtained, question the current unilateral restoration objectives. Calculations reveal that, both in the present situation and after (total) reduction of groundwater extractions in the future, the available quantity of fresh seepage water from the skirts of the polder Horstermeer is not sufficient to replace the inlet from the river Vecht into the Kortenhoef lakes. Additional supply options are available but the ones favoured from an ecological viewpoint are either the most expensive or less favoured from a social point of view. Although the sediments of the lakes appear to be a major source of eutrophication, the possibility of dredging the lakes will be considered only after reviewing results of a pilot-dredging project in the Hollands Ankeveen lakes in 1991.     

The phosphorus status of sediments in a hypertrophic impoundment (Hartbeespoort Dam): implications for eutrophication management

The phosphorus status and distribution of sediments in a hypertrophic water supply reservoir (Hartbeespoort Dam) were investigated, with a view to assessing the role of sediments in counteracting the effects of reduced external phosphorus loading as a restoration measure. In comparison with similar water bodies in South Africa, the sediments in Hartbeespoort Dam contained high levels of both total and potentially mobile phosphorus. The potentially mobile fraction constituted about 60% of the total phosphorus content of the sediments, compared with about 11% in other reservoirs. The excessive eutrophication of Hartbeespoort Dam is clearly reflected in the phosphorus status of the sediments. Sediment distribution in the impoundment was found to be extremely heterogeneous, due to the combined influences of morphometry, hydrology and an imbalance in the nutrient loads entering via rivers at remote points in the water body. It is concluded that sufficient mobile phosphorus has accumulated in the sediments to prolong the response time of the impoundment to phosphorus load reductions. Since phosphorus release from sediments is dependent on dynamic processes not addressed in this study, the extent of the delays in trophic response to load reduction cannot be estimated.     

A two-part model linking multidimensional environmental gradients and seasonal succession of phytoplankton assemblages

Phytoplankton dynamics in Lake Müggelsee, a eutrophic and polymictic lake in Berlin, and in the inflowing lowland River Spree have been comprehensively investigated during the last two decades. Zooplankton dynamics, nutrient supply, light climate, duration of ice cover and of summer stratification have also been regularly measured to help to explain phytoplankton development. The first period (1978–1990) was characterised by high nutrient loads and dominance of cyanobacteria from spring to autumn. Since then, loads of phosphorus and nitrogen have been lowered by 40–50%. Oscillatoria-like cyanobacteria (Limnothrix redekei, Planktothrix agardhii) were favoured under hypertrophic conditions in both the polymictic lake and the river, but they have disappeared nearly completely after nutrient reduction. Development of these species depended on meteorological conditions and nutrient supply in spring rather than on seasonal averages of nutrient concentrations. Diatoms have became dominant and chlorophytes have increased their share of the biomass since the nutrient load was reduced. Species com- position changed even within the algal groups. Retention time of water and duration of thermal stratification of the water column modified phytoplankton structure. Mobile algae like Microcystisor Ceratium occurred in the lake during stratification periods. Otherwise, species composition in the shallow, polymictic lake was very similar to that in the inflowing lowland river. Species with high starting biomass, fed by high riverine import, resting stages or perennation were selected in this flushed system.     

Harnessing the potential of the multi‐indicator palaeoecological approach: an assessment of the nature and causes of ecological change in a eutrophic shallow lake

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