Geochemical aspects of aqueous iron, phosphorus and dissolved organic carbon in the humic Lake Tjeukemeer, The Netherlands

SUMMARY. 1. The main source of P, Fe and DOC in the humic Lake Tjeukemeer is superfluous water pumped from surrounding peaty polders. Most particulate P is intracellular but almost all particulate Fe is abiotic.
2. The size and molecular weight of the P, Fe and DOC (mainly fulvic acids (FA)) were determined by ultrafiltration and Sephadex G-100 gel filtration. Throughout the year most dissolved P and Fe was in colloids >35 nm with apparent molecular weight between 30,000 and 150.000. The bulk of FA occurred in particles <35nm.
3. Calculating the atomic ratios of P, Fe and organic C in the different size classes revealed that Fe-FA chelates are a minor species of the Fe pool. Less than 10% of the fulvic acids occurred as Fe-FA chelates.
4. Based on their apparent size and molecular weight, the Fe-FA chelates are colloidal aggregates, probably with the formula Fe_n-1(FA)_n.
5. About 50% of the dissolved P had the same size as the Fe-FA aggregates. These aggregates were only noticeable in winter when humus-rich polder water was flushed through the lake. During the rest of the year the dissolved Fe and P consisted mainly of acid-labile inorganic colloids which might have been organically coated.     

Feeding behaviour of major 0 + fish species in a shallow, eutrophic lake (Tjeukemeer, The Netherlands)

The forage base and the food selectivity of 0+ representatives of six abundant freshwater fish species were studied in a shallow, eutrophic Dutch lake. Most species relied on the zooplankton; the size-selective predation in early summer was directed to the smaller copepods and in late summer to larger cladocerans and copepods than concurrently present in the lake. Daphnia spp. and cyclopoid copepods were the main zooplankton taxa for smelt, perch and pikeperch. Energetically, the large cladoceran, Leptodora kindtii, was especially important for pikeperch. Bream and roach preyed upon smaller zooplankton than the other fish species. The influence of the zooplankton predation by abundant 0+ fish was clear from a small mean Daphnia size in September; this size is to be used as an indicator in fishery management. Neomysis integer, the most important macrofauna species, was consumed by perch, pikeperch and ruffe; pikeperch was most size-selective in this respect. The 0+ ruffe was à typically benthivorous fish. Only the 0+ pikeperch became piscivorous, especially in years when smelt was abundant.     

Formation of year-class strength in the bream population in the shallow eutrophic Lake Tjeukemeer

Formation of year-class strength of bream Abramis brama , the most abundant fish in shallow eutrophic Lake Tjeukemeer, was studied during a 14-year period. Although the size of the spawning stock of bream was rather stable during this period, analysis of length—frequency distributions indicates that it comprised only a few year-classes. The strength of a year-class was determined at a fork length of about 11 cm, at the end of the second summer. Mortality rates of fish larger than 11 cm were low (<50% year⁻¹). As the number of fish at the end of the second summer was not correlated to the number of 0+ in the previous year, mortality rates during the second summer (ranging from 75.6 to 97.7% year⁻¹) determine the final year-class strength. These mortality rates were positively correlated to the year-class strength of 2- to 4-year-old pikeperch. The abundance of this predator, therefore, is likely to be the most important factor controlling the year-class strength of bream.     

Distribution patterns of zooplankton in Tjeukemeer, The Netherlands

The vertical and horizontal distribution of Copepoda and Cladocerain Tjeukemeer were studied. Indices of dispersion are discussed,the sample variance of log-transformed data proved to be anappropriate way to quantify patchiness. Indices of dispersionwere calculated to express species specific differences. Onlyduring short periods of the year the population densities appearedto be influenced by canals and pumping stations that bring waterto the lake. Dispersion values were influenced by windspeed,inhomogenities in the chlorophyll-a concentrations and by thepopulation densities of the populations studied. Sample variancesare relatively low, compared with those of other lakes.     

Dominant diatoms in Tjeukemeer (The Netherlands)

A situation is described where silicon depletion affected a population of diatoms, mainly Diatoma elongatum and Melosira spp., in the shallow lake Tjeukemeer (The Netherlands). After Si-deficiency the numbers of cells dropped quickly, whereas at least for the upper layer (0–1 m) a big increase in the percentage of biprotoplastic D. elongatum cells was observed. Probably the major part of the D. elongatum population arrived alive at the bottom. Heavy winds occurring 2 weeks later stirred up considerable numbers of both diatoms. The numbers of dead D. elongatum cells was then 40–50%, presenting some evidence that a large scale dying of D. elongatum cells occurs rather shortly after arrival at the bottom of Tjeukemeer. In the following weeks, at low Si-concentration, numbers of D. elongatum almost doubled with once again a relatively high percentage of biprotoplastic cells, whereas numbers of Melosira spp. decreased. In the relevant conditions it seems that D. elongatum can take up silicon faster than Melosira spp.     

The relation of biotic and abiotic interactions to eutrophication in Tjeukemeer,The Netherlands

During the summer months of 1974–1985 chlorophyll-a and total P concentration, biomass of Daphnia hyalina, smelt Osmerus eperlanus, bream Abramis brema and pikeperch Stizostedion lucioperca, water temperature and water intake from lake IJsselmeer were monitored in Tjeukemeer. During this period there were manipulations with the bream and pikeperch stocks as a consequence of the termination of a gill-net fishery in 1977, and larval smelt immigrated each year from the large lake IJsselmeer and contributed largely to the yearly smelt recruitment.The correlation matrix of the nine variables mentioned above showed a positive correlation between bream and chlorophyll-a, but surprisingly a negative one between smelt and chlorophyll-a. The latter can only be explained when smelt is the dependent variable. In a multi-linear regression there was a negative effect of temperature, chlorophyll a and pikeperch on smelt and a positive effect of water intake. Daphnia hyalina was negatively influenced by the biomass of smelt and the water intake of lake IJsselmeer. The positive relation of Daphnia hyalina and chlorophyll-a was probably related to better survival chances of D. hyalina in an Oscillatoria-rich environment when smelt is the most important predator. An increasing biomass of bream coincided with higher total-P levels and probably contributed to higher chlorophyll-a levels.     

Seasonal variation in sulfate reduction and methanogenesis in peaty sediments of eutrophic Lake Loosdrecht,The Netherlands

During one year, concentration profiles of sulfate and methane were measured in sediment cores of eutrophic Lake Loosdrecht. Sulfate concentrations decreased exponentially with depth towards a constant threshold value of 7.6 ± 6.1 μM. Concentration profiles were used to calculate fluxes of sulfate and methane and to estimate the anaerobic mineralization rate. Anaerobic mineralization was highest in autumn which was probably due to an increased sedimentation of easily degradable organic carbon. At high rates (>600 μ mol organic carbon .m⁻².h⁻¹), sulfate reduction appeared to be limited by sulfate and methanogenesis accounted for over 80% of the anaerobic mineralization. At low anaerobic mineralization rates, measured in winter and spring, sulfate reduction was predominant. There was little methanogenesis below 5 cm depth in the sediment which indicated a rapid decrease of degradable organic matter with depth. There was a remarkable difference, especially in winter, between methane fluxes which were measured in batch experiments and those calculated from the concentration profiles in the sediment. These differences may be due to methane diffusing upward from deep layers.     

Coupling of phytoplankton and detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

An oscillating steady state is described of phytoplankton, dominated by Prochlorothrix hollandica and Oscillatoria limnetica, and sestonic detritus in shallow, eutrophic Lake Loosdrecht (The Netherlands). A steady-state model for the coupling of the phytoplankton and detritus is discussed in relation to field and experimental data on phytoplankton growth and decomposition. According to model predictions, the phytoplankton to detritus ratio decreases hyperbolically at increasing phytoplankton growth rate and is independent of a lake's trophic state. The seston in L. Loosdrecht contains more detritus than phytoplankton as will apply to many other lakes. The model provides a basis for estimating the loss rate of the detritus, including decomposition, sedimentation and hydraulic loss. In a shallow lake like L. Loosdrecht detritus will continue to influence the water quality for years.     

Dynamics of phytoplankton detritus in a shallow,eutrophic lake (Lake Loosdrecht,The Netherlands)

A study was made of the mortality and aerobic decomposition of light- and phosphorus-limited cultures of Oscillatoria limnetica, a dominant phytoplankton species in shallow, eutrophic Lake Loosdrecht (The Netherlands). When placed in the dark at 20 °C, most cells died and lysed within twelve days. The labile organic matter was completely decomposed within three weeks. Absorbance spectra indicated that blue green algae may contributed significantly to the refractory dissolved substances in the lake. Refractory particulate matter constituted from 7 to 24% of the biomass of O. limnetica, depending on the growth rate before incubation in the dark. The decomposition rate of this fraction was 0.005 d^–1. On a basis of a steady-state model of the dynamics of phytoplankton detritus, the areal organic dry weight concentration of the detritus in the lake is ca. 60 g m^–2. This means the quantities of detritus in the seston and epipelon are about equal.     

Curie point pyrolysis mass-spectrometry of fulvic acids from Tjeukemeer, The Netherlands

SUMMARY. Curie point pyrolysis mass-spectrometry of freshwater fulvic acids from Tjeukemeer indicates the presence of polysaccharides, amino sugars, proteinaceous material and phenols. These results resemble those of the pyrolysis of soil fulvic acids. However, freshwater fulvic acids may be richer in aliphatic components than those from soil.     

Population structure, life histories and abundance of copepods in Tjeukemeer, the Netherlands

SUMMARY. The copepod zooplankton of Tjeukemeer was sampled at weekly intervals for 3 years (1969–1971), and ten species were recorded. Five species contributed more than 99% of all individuals collected. Acanthocyclops robustus is the most abundant species (66%), followed by Mesocyclops leuckarti (13%), Cyclops vicinus (11%), Eurytemora affinis (9%) and Diacyclops bicuspidatus (1%).
The ecology of the A. robustus population in Tjeukemeer differs from other A. robustus populations in that it dominates the open water zone. This is an additional argument for a taxonomic revision of the A. vernalis-robustus complex.
Seasonal periodicity in abundance of each species was very similar in 3 successive years.
All cyclopoid species went into diapause, A. robustus and M. leuckarti during winter, D. bicuspidatus and C. v. vicinus during summer. Most species diapause in a particular copepodite stage.
The seasonal variation in the population structure, the fecundity and the sex ratio are described. Both population structure and sex ratio were influenced by diapause and size-dependent mortality. The fecundity was more influenced by the water temperature and the age structure of the female population than by food situation or size of the females.     

Variation in abundance and survival of fish larvae in shallow eutrophic lake Tjeukemeer

Synopsis Variations in abundance and survival of larvae of the major fish species, bream, roach, pikeperch, perch and smelt were studied in shallow eutrophic lake Tjeukemeer (1986–1988). Except for smelt, the size of the spawning stock and the abundance of larvae were not related. Both the timing of appearance of larvae and their growth rate were determined by water temperature as was shown by a growth model developed earlier for Tjeukemeer fishes. In cyprinids and percids the survival of the larvae was hypothesized to be negatively related to the duration of the earliest stages. Except in cyprinids, the abundance of 0+ juvenile fish could be explained by the number of larvae. In cyprinids 0+ juvenile abundance at the end of the year was not related to the number of larvae in May and June. The cyprinid juvenile survival rates are hypothesized to be related to the size of the main cyprinid predator, 0+ pikeperch.     

The population density, growth rate and production of bream, Abramis brama, in Tjeukemeer, the Netherlands

A method of estimating the population density of bream in Tjeukemeer (21.3 km²) using 16 690 introduced fish (fin-clipped and opercular tagged) is described. Gill nets of the winter fishery proved to be a more effective method of sampling the population for marked fish than fyke nets. The population density of bream (⋝25 cm) was estimated to be 180 000. There was no significant difference between the estimates derived from fin-clipped and opercular tagged fish. The growth rate of bream in Tjeukemeer (L_∞=41 cm) is poor compared with that of bream in other waters, due to its high density and the scarcity of zoobenthos available to it.
The production of bream (I–XV) is estimated to be 34 kg ha⁻¹ of which 25% is contributed by two (1959, 1963) strong year classes. Of a total biomass of 37.5 kg ha⁻¹ available to the fishery in 1969 only 2.7 kg ha^−l was removed as yield. The average P/B ratio for the population was low (0.39). It is concluded that a major flow of energy to bream is through zooplankton.     

The population density, growth rate and production of roach Rutilus mtilus (L.) in Tjeukemeer, The Netherlands

A method of estimating the population density of roach in Tjeukemeer (21.3 km²) using 20 529 introduced fin-clipped fish is described. Fyke nets proved to be an effective method of sampling the population for marked fish during the spawning season. A total of 20 277 roach were processed during the recapturing period. The population density of roach (⋝ 14 cm) was estimated to be 1 246 458. The growth rate of roach in the lake although relatively poor (von Bertalanffy's L ∞ for males and females, 22 and 26 cm respectively) for the species was similar to that recorded in three other Friesian lakes. The mortality rate of males was higher than that of females. Logarithmic length-weight regression analyses showed that the value of the coefficient varied both within and between the sexes, that of females being higher (range 3.03–3.375) than that of males (range 2.76–3.254). Seasonal changes in the size of the coefficient were due to a disproportionate loss of gonad weight in larger fish. The fecundity of the population was comparatively high for the species.
The total production of the population was estimated to be 95 hg ha⁻¹ of which 39 kg ha⁻¹ was contributed by fry. In older (⋝ IV) fish the production of females (12 kg ha⁻¹) exceeded that (2 kg ha⁻¹) of males, due to differences in their growth and mortality rates. The relatively poor performance of roach in Tjeukemeer, in terms of biomass and production was due to a scarcity of zoobenthos and competition from other species offish. There is no evidence either from this study or the literature that the productive potential of roach in lakes is high, even though macrophytes and detritus can be consumed in significant amounts.     

Characterization of the light field in laboratory scale enclosures of eutrophic lake water (Lake Loosdrecht,The Netherlands)

Light conditions in laboratory scale enclosures (LSE) of shallow, eutrophic Lake Loosdrecht (The Netherlands), including a method for simulating a natural incident light course, are described. Total PAR (400–700 nm) and spectral irradiance distribution were measured at sestonic chlorophyll a and dry weight concentrations 100 mg m^–3 and 16 g m^–3, respectively. Phytoplankton was dominated by Oscillatoria spp. The euphotic depth (Z _eu) was 0.7–1.0 m. Shortly after filling the LSE with lake water, diffuse attenuation coefficients ranged from 14 m^–1 for blue to 5 m^–1 for red light. Around Z _eu, attenuation in the blue region was markedly lower and irradiance reflectance (R) continued to increase; these anomalies were caused by lateral incident light from the LSE's waterbath. Spectrophotometry indicated that absorption was mainly by particles, but dissolved humic substances (gilvin) were also important. The particles were likely to be dominated by detritus absorbing more blue relative to red light. Subsurface R in lake water in the LSE had a maximum around 705 nm and low values in the blue band, but was lower than that previously reported for measurements in situ. Wash-out of detritus, presumably both dissolved and particulate fractions, by flow-through with synthetic medium, greatly affected the spectral reflectance measured outside the LSE. The maximum value of R decreased from 0.022 to 0.009, and the peak shifted to 550 nm.     

Effect of temperature in laboratory studies on development and growth of Cladocera and Copepoda from Tjeukemeer, The Netherlands

SUMMARY. The effects of temperature on development and growth are described for the common Cladocera and Copepoda in Tjeukemeer. Eight cladoceran species and five copepod species have been investigated: Daphnia hyalina, Daphnia cucullata, Bosmina coregoni, Bosmina longirostris, Chydorus sphaericus, Ceriodaphnia pulchella, Diaphanosoma brachyurum, Leptodora kindtii, Acanthocyclops robustus, Mesocydops leuckarti, Diacyclops bicuspidatus, Cyclops vicinus vicinus and Eurytemora affinis . Growth is expressed as length increment per size class (cladocerans) or instar (copepods) per week and as the relationship between time and length. The curvilinear logarithmic equation: In D = In a + b (In T)² is fitted to the development times of eggs, combined naupliar instars and combined copepodite instars, as well as to the longevity of the adults. The results are compared with those found in the literature and differences are discussed.     

Biomanipulation,new perspectives for restoration of shallow,eutrophic lakes in The Netherlands

Eutrophication of Dutch lakes has led to massive algal growth, disappearance of most of the macrophytes and disturbance of the food chain. The pike population has fallen sharply and bream developed very strongly, in the absence of this predator. Eutrophication problems are primarily being tackled by reducing nutrient loading. Restoration of water quality, however, seems to be impeded by the present structure of the food chain,i.e. the large bream stock. Biomanipulation, especially fish stock control with the aim of reducing the bream stock and increasing that of predatory fish, can accelerate the process of restoration. For the further development of biomanipulation it is very important that water authorities and managers of fish stocks agree on a common strategy.     

The return of marked roach (Rutilus rutilus L.) to spawning grounds in Tjeukemeer, The Netherlands

A reciprocal transplant of marked roach between two spatially separated (3.3 km) spawning grounds is described. The recapture rates on the'home grounds'were higher than those obtained elsewhere in the lake, which suggests that the roach exhibit a degree of fidelity to specific spawning grounds.     

A note on the dispersion pattern of marked bream Abramis brama released into Tjeukemeer, The Netherlands

Following the release of 4619 opercular tagged bream in Tjeukemeer, 101 fish were subsequently recaptured outside the lake (1969–72). The maximum distance travelled was 60 km. Although the initial dispersal of fish out of the lake appeared to be random there was some evidence of homing once fish had gained access to the IJsselmeer.     

Seasonal variations in the composition of fulvic acids in Tjeukemeer,The Netherlands,as studied by Curie-point pyrolysis-mass spectrometry

Three different molecular weight fractions of fulvic acid from Tjeukemeer, sampled monthly between January and September 1978, were studied by Curie-point pyrolysis-mass spectrometry. The winter samples apparently differed markedly in composition from the summer samples. These differences, which were particularly striking in the pyrograms of the high molecular weight fraction, can be explained in terms of fragment molecules attributable to polysaccharides, proteins and/or phenolic polymers.