Restoration of Botshol (The Netherlands) by reduction of external nutrient load: The effects on physico-chemical conditions,plankton and sessile diatoms

The nature reserve Botshol (Utrecht, The Netherlands), consisting of two shallow lakes, ditches and reedland, originated from excavation of peat by man in the 17th century. Up to 1960 Botshol was a clear-water Charophyte lake system. Since the sixties water quality deteriorated and phytoplankton concentrations increased, while the number and dispersion of Chara species decreased. Several restoration measures were attempted to restablish a Charophyte-dominated ecosystem. This paper reports the promising results of this restoration experiment and mentions some complications that arose in restoring the reserve to a less fertile state. The restoration measures have resulted in a sixfold reduction of the external phosphorus load, from 0.6 to 0.1 g m^–2.y^–1, and in a significant reduction of phosphorus levels at all locations. Moreover, the light climate improved and the phyto- and zooplankton compositions changed considerably Unexpectedly, a bloom ofPrymnesium parvum and a fish kill were observed during the last three months of 1990. Despite this fish kill the restoration of the lake is successful so far.     

Restoration of Botshol (The Netherlands) by reduction of external load: Problem analyses and restoration methods

Since 1960 the water quality of the nature reserve Botshol has been deteriorated. An increase in the nutrient load caused an escalation in phytoplankton biomass and turbidity problems. This may have been caused by nutrient-rich water flowing into the reserve from the agricultural areas and from the polder of Nellestein. From 1980 to 1985 much research has been undertaken and participation of different disciplines gave the opportunity to analyse the cause of the changes. An integrated restoration plan has been drawn up to reduce the eutrophication. The plan was implemented in 1989 and in order to reverse the external nutrient load the following measures were taken. 1. Execution of a water management plan with the intention to isolate the agricultural areas from the reserve area. 2. Dephosphorization of the inlet water. These restoration measures resulted in a reduction of the external load from 0.6 to 0.1 g P m^–2 year^–1.     

Restoration of Botshol (The Netherlands) by reduction of external nutrient load: Recovery of the Characean community

The formerly rich characean community in Botshol with six species of which the rareNitellopsis obtusa andChara hispida dominated at many sites, decreased to only two species,Chara globularis andC. connivens, in the period 1980–1988. The macrophyteNajas marina also remained at some sites, and the aquatic mossFontinalis antipyretica and the filamentous algaVaucheria dichotoma predominated at many sites. These phenomena may have been due to eutrophication by the inlet of polluted water. This process of eutrophication was stopped by restoration measures in 1989, resulting in a lower phosphorus concentration (ca. 0.024 mg l^–1) and a higher water transparency. Immediately after these measures the Characeae community increased strongly in abundance and number of species. During the summer of 1990, and especially of 1991, a spectacular growth occurred ofChara connivens. Chara connivens was often accompanied byChara hispida. Other species with scattered occurrence wereChara aculeolata, C. aspera, C. contraria andC. Globularis. The reasons for the shift in dominance fromNitellopsis obtusa toChara connivens are discussed. One of the reasons may be the recent higher chloride content which is one of the consequences of the restoration measures.     

Restoration of Botshol (The Netherlands) by reduction of external nutrient load: recovery of a characean community,dominated by Chara connivens

Till about 1965 a rich characean community occurred in the shallow peat lake Botshol with six species of which the rare Nitellopsis obtusa and Chara major dominated at many sites. In the period 1980–1988 characean biomass strongly decreased and only two species, Chara globularis and C. connivens, remained in small populations at a few localities. Of the macrophyte Najas marina also some small populations remained, while the aquatic moss Fontinalis antipyretica and the filamentous alga Vaucheria dichotoma predominated at many sites. These phenomena may have been due to eutrophication by the input of polluted water. This process of impoverishment was stopped by restoration measures in 1989, resulting in a lower phosphorus concentration (ca 25 µg l^-1) and a higher water transparency. Immediately after these measures the Characeae community increased in abundance and number of species. During the summer of 1990, and especially 1991, a spectacular growth occurred of Chara connivens. C. connivens was often accompanied by C. major. Other species with scattered occurrence were C. aculeolata, C. aspera, C. contraria and C. globularis. The reasons for the shift in dominance from Nitellopsis obtusa to Chara connivens are discussed. From growth experiments evidence was obtained that neither the recent higher chloride level, nor the lowered phosphate concentration were the main factors for the domination of Chara connivens.     

Water quality research in the Loosdrecht lakes: proposals for the follow-up restoration measures

Reduction of the external phosphorus load in 1984 did not lead to recovery of Loosdrecht lakes. Therefore the Provincial Water Authorities of the Province of Utrecht proposed, after consulting the research group Water Quality Research Loosdrecht lakes (WQL), measures to reduce the present external phosphorus loading from 0.35 to 0.1 g Pm^–2y^–1. Mathematical modelling suggested that such a reduction would benefit the water quality of the lakes. The results of the WQL investigation did not lead to additional measures (dredging, biomanipulation, chemomanipulation).     

Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

The shallow, brackish (11–18% salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m^–3. Nutrient loadings in 1985 were 11.3–62.4 g N m^–2 yr^–1 and 0.4–7.3 g P m^–2 yr^–1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from –32.2 to 17.9 g P m^–2 yr^–1 and from –3.3 to 66.8 g N m^–2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary.Gross sedimentation rates measured by sediment traps were between 7 and 52 g p m^–2 yr^–1 and 50 and 426 g N M^–2 yr^–1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m^–2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m–1. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d^–1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks.The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m^–2 d–1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250 µmol N m^–2 h^–2 achieved at loading rates of about 25–125 g N m^–2 yr^–1.     

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.     

Development and practical application of limiting values for the phosphate concentration in surface waters in the Netherlands

Summary To counteract eutrophication it is important to possess sufficient information (1) on the relationship between the content of nutrients and the algal biomass, and (2) on the cycling of nutrients in lakes.A comparative study of a large number of surface waters in the Netherlands has demonstrated that it is possible to derive a relationship between the nitrogen and phosphorus concentrations and the upper limit of the chlorophyll concentration, averaged over the summer season.For the authority in charge of water quality it is essential to know how far the phosphorus loading must be reduced to attain the desired phosphate concentration. The results of an extensive study of the nutrient budgets of Lake Wolderwijd-Nuldernauw over the period 1976–1979 demonstrate that in certain cases a relative high phosphate concentration can occur (approx. 0.30 mg/l) in spite of a low external phosphorus loading (approx. 0.7 g P/m² year). Such high concentrations can be explained by the continuous presence of a bloom of blue-green algae and the release of phosphate from the aquatic sediments during the summer. In such lakes it is necessary to take additional measures, such as flushing with water poor in algae and phosphates, or, where necessary, the removal of aquatic sediment rich in phosphate.     

Patterns of nutrient release from nutrient diffusing substrates in flowing water

As nutrient diffusing substrates age, the availability of nutrients to periphyton may decline with time either because of diffusion or dilution of nutrients into the water column or because of the effects of grazing by herbivores. Typically, large amounts of nutrients are added to nutrient diffusing substrates (NDS) to insure continuous enrichment throughout experimental periods of 2 to 8 weeks. This study examined the release of phosphates and nitrates from NDS exposed to three different current velocities (0.07 m s^–1, 0.11 m s^–1, 0.20 m s^–1) in recirculating laboratory flumes. Replicated agar samples from four treatments (control, nitrate (N), phosphate (P), and N+P) were sampled throughout 32 days (day 1, 2, 3, 6, 12, 18, 24, 32). Increasing concentrations of agar were required to solidify the P and N+P treatments.Nutrient release rates from NDS were independent of agar concentrations (with the exception of [PO₄] in the medium velocity flume). Nutrient concentrations in the agar of spiked samples declined substantially within a week when exposed to flowing water. Nitrates were retained in agar to a greater extent than phosphates particularly when NDS were exposed to low or medium flows. Although floods physically remove or abrade periphyton in natural streams, findings from this laboratory study suggest that ambient flows deplete the availability of nutrient concentrations to potential periphyton colonizers within the first week of incubation. Because of the rapid decline of nutrients from NDS, short incubation periods in natural running waters seem warranted.     

Sedimentation rates in a Swiss-Italian lake measured with sediment traps

Tripton sedimentation was investigated in the eutrophic Lake Lugano (Ponte Tresa basin) from October 1979 to October 1980. The annual amount of tripton collected was 748 g · m^–2 · y^–1. Phosphorus, nitrogen and organic carbon fluxes into the hypolimnion were estimated to be 1.9, 16.2 and 121 g · m^–2y^–1 respectively. Mineralization rates into the trophogenic layer varied from 11% to 19% per day during summer stratification. The regeneration processes contribute about 60% of the calculated P deficit in the epilimnion. The tripton is decomposed mostly in the metalimnion, out of the euphotic zone; from here the phosphorus is carried back to the overlying waters by diffusion processes.     

Nutrients in pore waters from Dead Sea sediments

Pore waters were separated from 50 cm-long cores of Dead Sea sediments raised from waters depths of 25, 30 and 318 m. The salinity of the pore water is close to that of the overlying water at 225–230 g l^–1 chloride. The titration alkalinity of the pore water is about 60 % of the overlying water, and sulfate is also depleted. Ammonia and phosphate concentrations are higher than those of the water column with up to 50 mg l^–1 N-NH₃ (ten times increase) and 350 µg l^–1 P-PO _inf4 ^sup3– (four to eight times increase). Early diagenetic reactions are a result of decomposition of organic matter and of water-sediment interactions, resulting in aragonite precipitation, phosphate removal to the sediments, probably by absorption on iron-oxyhydroxides followed by remobilization, reduction of sulfate and formation of iron sulfides and accumulation of ammonia. Mass balance calculations show that pore water contribute about 80% of the ammonia and 30% of the phosphate input into the Dead Sea water column. On the other hand, the sediments act as a sink for carbonate and sulfate.     

Yearly variation in primary productivity of marine phytoplankton from Cabo Frio (RJ,Brazil) region

Light saturation curves of natural assemblages of phytoplankton at 4 stations in Cabo Frio coastal waters were examined and related to changes in environmental conditions, based on 263 experiments carried out weekly during two years. No differences in specific productivity at saturating light (Pm ^B ) between stations were detected. Global mean value of Pm ^B was 4.7 mgC mgChla ^–1 h^–1 with a range from 0.50 to 15.29. Significant seasonal variations were not observed. Correlation and regression analysis attributed most of Pm ^B variation to temperature, salinity, nutrients and the phaeopigment: chlorophyll-a ratio.     

The Shatt al-Arab River: A nutrient salt and organic matter source to the Arabian Gulf

The distribution of dissolved reactive phosphate, nitrate and nitrite in the waters as well as total organic carbon, total phosphorus and Kjeldahl nitrogen in the sediments of the Shatt al-Arab Estuary and the NW Arabian Gulf were studied from November 1979 to April 1980. The Shatt al-Arab waters contain 0.18 to 0.70 µg-at P-PO _inf4 ^sup3– l^–1, 26.12 to 52.39 µg-at N-N0 _inf3 ^sup– l^–1 and 0.53 to 0.70 µg-at N-NO _inf2 ^sup– l^–1, indicating that this river should be considered a source of nutrients to the Arabian Gulf. It is concluded that most of the nitrate is supplied in dissolved form, while an appreciable amount of phosphate is absorbed to fine suspended particles and released at higher salinities. Total organic carbon in surficial sediments was found to vary between 0.14% and 0.96%. These rather low values are attributed to dilution by dust fallout, which is a major cource of sediments in this area.     

Lead-210 dating of sediments compared with accumulation rates estimated by natural markers and measured with sediment traps

Methods to provide accurate accumulation rates for lake models are discussed. Cores were taken in 1979 in two basins of Lake Lucerne, Switzerland, and accumulation rates were calculated by using Pb-210 dating and by a natural landslide marker of 1795 in one basin (Weggis). In the other basin (Horw Bay) the sediment accumulation rates based on the lead method were compared with yearly sedimentation rates measured by sediment traps in 1969/70. At the Weggis station, the core dating yielded sediment accumulation rates of about 400 g dry wt. m^–2 y^–1 with the lead method, averaged over a sediment depth of 4–20 cm; accumulation was about 700 g dry wt. m^–2 y^–1 with the marker method, averaged over 0–33 cm. In Horw Bay, the trap method yielded about 1300 g dry wt. M^–2 y^–1 compared with 400–1000 g dry wt. m^–2 y^–1 obtained with the lead method and related to various depth intervals. The characteristic sources of error of the three methods as well as several hypotheses for these discrepancies are discussed.     

A reference system for continental running waters: plant communities as bioindicators of increasing eutrophication in alkaline and acidic waters in north-east France

Two bioindication scales of the degree of eutrophication based on aquatic macrophyte communities were established in two types of running waters free of organic matter, the one in acidic soft waters (pH 5.5–7.0, conductivity 40–110 S.cm^–1), the other in alkaline hard waters (pH 7–8, conductivity 500–900 S.cm^–1). We show that the main determining factor of the macrophyte distribution is the nutrient level (trophy), especially the level of phosphate and ammonia. The acidic scale, with increasing pH, includes four stages ranging from oligotrophic to eutrophic level (traces to 300 g.l^–1 N-NHinf4^p+ and P-PO₄ ^3–), while the alkaline scale at constant pH comprises six stages of a trophic gradient. For the most part, the floristic composition found in the two sequences is different and depends on conductivity and alkalinity variation. However, some species occur in the two scales and may reflect differences in the trophic level, depending on whether the waters are alkaline or acidic. This change of trophic level for these species is discussed.     

Production,Respiration, and Overall Carbon Balance in an Old-growth <Emphasis Type="Italic">Pseudotsuga</Emphasis>-<Emphasis Type="Italic">Tsuga</Emphasis> Forest Ecosystem

Ground-based measurements of stores, growth, mortality, litterfall, respiration, and decomposition were conducted in an old-growth forest at Wind River Experimental Forest, Washington, USA. These measurements were used to estimate gross primary production (GPP) and net primary production (NPP); autotrophic respiration (R_a) and heterotrophic (R_h) respiration; and net ecosystem production (NEP). Monte Carlo methods were used to calculate uncertainty (expressed as ± 2 standard deviations of 200–400 calculations). Live carbon (C) stores were 39,800 g C m^–2 (34,800–44,800 g C m^–2). The store of C in detritus and mineral soil was 22,092 g C m^–2 (20,600–23,600 g C m^–2), and the total C stores were 61,899 g C m^–2 (56,600–67,700 g C m^–2). Total NPP was 597 g C m^–2 y^–1 (453 to 741 g C m^–2 y^–1). R_a was 1309 g C m^–2 y^–1 (845–1773 g C m^–2 y^–1), indicating a GPP of 1906 g C m^–2 y^–1 (1444–2368 g C m^–2 y^–1). R_h, including the respiration of heart rots in tree boles, was 577 g C m^–2 y^–1 (479–675 g C m^–2 y^–1). Long-term NEP was estimated to be +20 g C m^–2 y^–1 (–116 to +156 g C m^–2 y^–1), indicating this stand might be a small sink. These estimates contrast with the larger sink estimated at the same site using eddy-flux methods. Several hypotheses to explain this discrepancy were explored, including (a) undetected biomass increases, (b) underestimates of NPP, (c) unmeasured losses, and (d) a temporal mismatch between the two sets of measurements. The last hypothesis appears the most likely.     

Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Eight years of internal phosphorus loading and changes in the sediment phosphorus profile of Lake Søbygaard,Denmark

During each of the first 8 years following an 80–90% reduction in external phosphorus loading of shallow, hypertrophic Lake Søbygaard, Denmark in 1982, phosphorus retention was found to be negative. Phosphorus release mainly occurred from April to October, net retention being close to zero during winter. Net internal phosphorus loading was 8 g P m^–2 y^–1 in 1983 and slowly decreased to 2 g P m^–2 y^–1 in 1990, mainly because of decreasing sediment phosphorus release during late summer and autumn. The high net release of phosphorus from Lake Søbygaard sediment is attributable to a very high phosphorus concentration and to a high transport rate in the sediment caused by bioturbation and gas ebullition. Sediment phosphorus concentration mainly decreased at a depth of 5 to 20 cm, involving sediment layers down to 23 cm. Maximum sediment phosphorus concentration, which was 11.3 mg P g^–1 dw at a depth of 14–16 cm in 1985, decreased to 8.6 mg P g^–1 dw at a depth of 16–18 cm in 1991. Phosphorus fractionation revealed that phosphorus release was accompanied by a decrease in NH₄Cl-P + NaOH-P and organic phosphorus fractions. HCl-P increased at all sediment depths. The Fe:P ratio in the superficial layer stabilized at approximately 10. Net phosphorus release can be expected to continue for another decade at the present release rate, before an Fe:P ratio of 10 will be reached in the sediment layers from which phosphorus is now being released.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

The dynamics and role of limnetic zooplankton in Loosdrecht lakes (The Netherlands)

The paper summarizes the results of a ten-year (1981–1991) zooplankton research on the Lake Loosdrecht, a highly eutrophic lake. The main cause of the lake's eutrophication and deteriorating water quality was supply up to mid 1984 of water from the River Vecht. This supply was replaced by dephosphorized water from the Amsterdam-Rhine Canal in 1984. The effects of this and other restoration measures on the lake's ecosystem were studied. Despite a reduction in the external P-load from ca. 1.0 g P m^–2 y^–1 to ca. 0.35 g m^–2 y^–1 now, the filamentous prokaryotes, including cyanobacteria and Prochlorothrix, continue to dominate the phytoplankton.Among the crustacean plankton Bosmina spp, Chydorus sp. and three species of cyclopoid copepods and their nauplii are quite common. Though there was no major change in the composition of abundant species, Daphnia cucullata, which is the only daphnid in these lakes, became virtually extinct since 1989. Among about 20 genera and 40 species of rotifers the important ones are: Anuraeopsis fissa, Keratella cochlearis, Filinia longiseta and Polyarthra. The rotifers usually peak in mid-summer following the crustacean peak in spring. The mean annual densities of crustaceans decreased during 1988–1991. Whereas seston (< 150 µm) mean mass in the lake increased since 1983 by 20–60%, zooplankton (> 150 µm) mass decreased by 15–35%.The grazing by crustacean community, which was attributable mainly to Bosmina, had mean rates between 10 and 25% d^–1. Between 42 and 47% of the food ingested was assimilated. In spring and early summer when both rotifers and crustaceans have their maximal densities the clearance rates of the rotifers were much higher. Based on C/P ratios, the zooplankton (> 150 µm) mass contained 2.5 times more phosphorus than seston (< 150 µm) mass so that the zooplankton comprised 12.5 % of the total-P in total particulate matter in the open water, compared with only 4.5% of the total particulate C. The mean excretion rates of P by zooplankton varied narrowly between 1.5 and 1.8 µg P 1^– d^–1, which equalled between 14 and 28% d^–1 of the P needed for phytoplankton production.The lack of response to restoration measures cannot be ascribed to one single factor. Apparently, the external P-loading is still not low enough and internal P-loading, though low, may be still high enough to sustain high seston levels. Intensive predation by bream is perhaps more important than food quality (high concentrations of filamentous cyanobacteria) in depressing the development of large-bodied zooplankton grazers, e.g. Daphnia. This may also contribute to resistance of the lake's ecosystem to respond to rehabilitation measures.