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.     

Relationships Between Chemical Characteristics of Sediments and Macrofaunal Communities in the Cabras Lagoon (Western Mediterranean,Italy)

The Cabras lagoon is a large and shallow transitional system which periodically suffers from dystrophic events leading to massive losses of its biological resources. Excessive organic matter (OM) and sulphur compounds in sediments are often claimed to be major environmental co-factors triggering these events. However, still limited information is available on their relationships with benthic macrofaunal communities in coastal lagoons. In order to assess these relationships, we conducted monthly field investigations between 2001 and 2002, at three regularly-spaced stations set between the inlet and the mouth of the Cabras lagoon. Results showed reduced conditions of sediments, worsening during the warm season. This was consistent with very high OM contents, up to >‰20%, with annual means of 14–16%. Organic pollution-tolerant taxa, such as Tubificidae nc, Neanthes succinea and Polydora ciliata, dominated poor macrofaunal communities. Results of multivariate analysis indicated significant differences in both environmental variables and macrofaunal community structure and species composition between stations and through different periods. This study documented for the first time in a coastal lagoon that inorganic reduced sulphur pools (acid volatile sulphide and chromium reduced sulphur) of sediments are leading (‚best-matching’) environmental factors influencing the spatial and temporal distribution of macrofaunal abundance and biomass. We conclude that impoverished macrofaunal communities in the Cabras lagoon may show a slight recovery during winter–spring, but tend to regress to an early stage of faunal succession in late summer. We suggest that the combination of excessive amounts of sedimentary organic matter and high temperatures tends to lead to anoxic conditions especially in relatively deeper and more saline waters. These events may concur to periodic increase of production rates of toxic hydrogen sulphide in sediments, which rapidly diffuses to the water column with deleterious effects both on the benthic and pelagic components of the lagoon ecosystem.     

Macroinvertebrates in the Demmerik ditches (The Netherlands): The role of environmental structure

The relation between the different microhabitats and habitats and the quantitative distribution of macroinvertebrates was investigated as well as the applicability of different sampling devices. Data processing took place by means of multivariate analysis (Flexclus, Canoco). It was concluded that the spatial-physical structure is more important to macroinvertebrates than the plant species. In fact, this spatial physical structure can be indicated as the mesh size of a (micro-)habitat. In order from open vegetation, shore/dense vegetation, artificial substrate, bottom material, and filamentous algae the mesh size decresed. It is advisable to use different sampling devices to sample a macroinvertebrate community.     

应用DGGE法对青海相邻两盐湖中细菌多样性的快速检测

分别对青海相邻两盐湖柯柯盐湖、茶卡盐湖土样、泥样进行富集培养后,从中提取的DNA用两套不同的细菌通用引物进行扩增,分别得到包含V8和V9高变区的16SrDNA片断。经变性梯度凝胶电泳(DGGE)分析,结果显示这两个盐湖富集样品中细菌多样性具有较大的差异,而且同一盐湖不同性质样品中的细菌多样性差异也较大,两湖的泥样富集样品中均表现出了稍丰富的多样性。     

Regional and hierarchical perspectives of thermal regimes in subarctic, Alaskan lakes

1. We examined 60 clear, stained and glacial lakes in Alaska to quantify the relative importance of climate setting, morphometry, transparency, and lake typology influences on various thermal characteristics including duration of growing season, water temperature, mixing depth (MD) and heat content. We used analysis of variance (ANOVA ) to test for differences in thermal characteristics in association with lake type and employed simple and multiple regression techniques to determine functional relationships between variables. 2. Latitude accounted for 60% of the total variance in length of growing season. Although the date of maximum heat content was consistent among lake types, stained lakes had longer growing seasons compared with clear and glacially turbid lakes. 3. Maximum water temperatures were approximately 3 °C higher in stained lakes and 3 °C lower in glacial lakes compared with clear lakes. Mean water column temperature was significantly lower in glacial lakes (5.9 °C) compared with clear lakes (7.4 °C), but there was no statistical difference between clear and stained lakes (7.2 °C) or between stained and glacial lakes. Maximum surface temperatures were positively related (r²=0.51) to colour (humic stain), but negatively related (r²=0.40) to inorganic turbidity (glacial silt). 4. Only about half of the lakes in our data set underwent summer stratification. None of the glacial lakes developed a distinct thermocline, but stained lakes had shallower MDs (mean 8 m) than clear lakes (mean 12 m). Thus, the MD to total depth ratio for glacial lakes was unity compared with mean values of 0.66 for clear lakes and 0.34 for stained lakes. Fetch explained a significant fraction (51%) of the total variance in MD. Considering all lakes, MD was inversely related to transparency (Secchi depth). In contrast, considering only stratified clear and stained lakes, MD was positively related to Secchi depth (SD), the fraction of the total variance explained was 23%. The sign of the slope was dependent on the mixture of lake types. 5. Despite significant (ANOVA ) differences in water temperatures, growing season, and MDs among the three lake types, there were no statistical differences in the summer heat budget associated with lake type. In addition, heat budgets were poorly correlated with lake area, depth and volume. In contrast, mean water column temperature was strongly and inversely related (r²=0.77) to mean depth. 6. Potential explanations for the similarity in summer heat budget among lake types and weak correlation with morphometry were attributed to different patterns in vertical heat distribution associated with lake typology (colour and turbidity) differences. 7. Multiple linear regression including climatic (latitude and altitude), morphometric, and lake typology (colour and turbidity) factors demonstrated a hierarchical (climate–morphometry–typology) regulation of growing season characteristics, water temperatures, stratification and heat retention. A regional and hierarchical framework for lake thermal characteristics adds to our understanding of potential responses to climatic change and may be important for regional management objectives for fisheries.     

An assessment of the importance of emergent and floating-leaved macrophytes to trophic status in the Loosdrecht lakes (The Netherlands)

The potential importance of the six major emergent and floating-leaved macrophyte species in recycling of sediment phosphorus in the Loosdrecht lakes was studied. Representative plant samples were collected at the time of maximum biomass, and analysed for biomass and carbon, nitrogen and phosphorus contents. Species cover was determined by aerial photography.Total cover in the seven lakes studied ranged between 2 and 26 percent. For the four main species, biomass per unit area increased with lake trophic status. Consistent differences in C, N and P contents per unit biomass were not observed. Although cover values were small, significant amounts of C, N and P were contained in the macrophytes when compared with maximum sestonic content.Potential P loads from macrophyte decay were calculated. In Lake Loosdrecht, the P load represented 15 percent of current external P inputs. The potential importance of macrophyte decay to P recycling in the other lakes is greater.Decay of macrophyte species at the end of the growing season appears to affect autumnal nutrient and chlorophyll a levels in the water column of some lakes. The re-establishment of submerged species following lake restoration may increase the importance of this pathway in the lakes.     

Hydrology and substrates: determinants of oligochaete distribution in lowland streams (The Netherlands)

In most soft-bottomed, lowland streams in the Netherlands discharge regimes largely follow the precipitation pattern. Winter discharges are higher and much more dynamic then summer discharges, although rain storms throughout the year cause unexpected peak flows. Minimal precipitation, reduced stream flow and droughts can occur during the summer months. Lowland stream habitat, particularly in The Netherlands, is hydrologically dynamic, with substrates frequently moved or disturbed. Differences in discharge patterns in Dutch soft-bottomed lowland streams are expected to affect distribution patterns of macroinvertebrates and thus oligochaetes. Ten small to medium sized lowland streams, differing from one another in hydrological regime, were studied. Five major habitats in each stream were assessed on three occasions over a 15-month period. Each habitat sampled with a micro-macrofauna shovel; during each sampling period, several environmental parameters, especially hydrological and substrate parameters, were measured. Ordination (CANOCO) (Ter Braak, 1989) and statistical tests (chi-squared test) (Lindgren & McElrath, 1970) were used to determine the major oligochaete distribution patterns between streams as well as between habitats within each stream. Each stream was characterized by its oligochaete assemblage; general distribution patterns and individual oligochaete–substratum relationships were documented. Hydro-morphological parameters together explained the differences in major distribution patterns. Preferences of oligochaetes for specific structural habitats are discussed.     

Biomanipulation additional to nutrient control for restoration of shallow lakes in The Netherlands

Eutrophication control is one of the major issues in the environmental policy in The Netherlands. As a result of international action programmes the average phosphorus loading of freshwater systems should decrease by 50% between 1985 and 1995. However, in many cases the restoration of water quality requires additional measures. Recovery is hampered by the structure and functioning of the present food-chain.

The feeding behaviour of the dominant fish species in Dutch lakes, bream and roach, tend to impose a homeostasis on the system, resisting restoration of water quality. In shallow lakes, biomanipulation, including drastic reduction of fish-stocks, may induce a shift from a stable ‘turbid-water state’ to a stable ‘clear-water state’.

To assess the possibilities of biomanipulation for the restoration of a particular lake, three questions are relevant: (1) is a drastic reduction of fish-stocks feasible?, (2) will a shift occur from ‘turbid to clear’ after the fish reduction? and (3) will the new situation of clear water be stable? This paper focuses attention on the last two questions. The increase in water clarity, following fish reduction, largely depends on the increase in the density of the Daphnia-population and the contribution of benthivorous fish to the resuspension of sediments. A ‘turbid to clear’ shift may be expected if the total biomass of planktivorous and benthivorous fish is reduced to levels<50 kg ha^?1. The stability of the achieved clear-water state largely depends on the development of submerged macrophytes in the lake and on the level of nutrient loading. It is tentatively concluded that a stable clear-water state may be expected at initial total-P concentrations<0.10 mg l^?1.

Because the water managers in The Netherlands have no fishing rights, they have to.co-operate with anglers and commercial fishermen to apply biomanipulation as a tool for water management.

     

Microbial community structure in moraine lakes and glacial meltwaters, Mount Everest

The bacterial diversity and abundance in two moraine lakes and two glacial meltwaters (5140, 5152, 5800 and 6350 m above sea level, respectively) in the remote Mount Everest region were examined through 16S rRNA gene clone library and flow cytometry approaches. In total, 247 clones were screened by RFLP and 60 16S rRNA gene sequences were obtained, belonging to the following groups: Proteobacteria (8% alpha subdivision, 21% beta subdivision, and 1% gamma subdivision), Cytophaga-Flavobacteria-Bacteroides (CFB) (54%), Actinobacteria (4%), Planctomycetes (2%), Verrucomicrobia (2%), Fibrobacteres (1%) and Eukaryotic chroloplast (3%), respectively. The high dominance of CFB distinguished the Mount Everest waters from other mountain lakes. The highest bacterial abundance and diversity occurred in the open moraine lake at 5152 m, and the lowest in the glacial meltwater at 6350 m. Low temperature at high altitude is considered to be critical for component dominancy. At the same altitude, nutrient availability plays a role in regulating population structure. Our results also show that the bacteria in Mount Everest may be derived from different sources.     

The distribution of aquatic oligochaetes in brackish inland waters in the SW Netherlands

Macrobenthic fauna was sampled in inland waters with unstable environmental parameters in the SW Netherlands. The distribution of 23 species of aquatic oligochaetes was related to chlorinity. Oligochaetes and other macrofaunal taxa were grouped by cluster analysis. Brackish-marine oligochaetes appeared to have a similarly large tolerance for chlorinity variations, as is known for the better researched euryhaline macrofauna.Communication no. 221.     

Macroinvertebrates in Dutch ditches: A typological characterization and the status of the Demmerik ditches

Different aspects of using a new multivariate analysis technique namely detrended canonical correspondence analysis (DCCA), for the elaboration of macrofaunal data from Dutch ditches are shown. In this technique a graphic representation is given of combined main biotic and abiotic parameters. Especially for typological purposes, this technique is promising. A typological scheme resulted from typifying small, stagnant, line-shaped waters' or ditches in the province of Overijssel. All ditches were separated into eight major clusters. The most important related variables were acidity, drought, and stream velocity within which nutrient content/organic load diversified. The Demmerik ditches (see Hpigler and Verdonschot, 1989) represent larger sized ditches on a peaty substrate. They belong to the real ditches. Their taxon composition is most similar to that of clear, eutrophic, permanent ditches with a densely developed vegetation.     

Zooplankton community composition of lakes in the Yukon and Northwest Territories (Canada): relationship to physical and chemical limnology

We analysed associations between zooplankton species composition and local abiotic factors in 30 lakes located along a 900 km south-north transect from Whitehorse (Yukon Territory) to Inuvik (Northwest Territories). The lakes were situated in three broadly defined vegetation zones: (i) Boreal forest (between Whitehorse and Dawson City), (ii) alpine tundra (Ogilvie mountains north of Dawson City) and (iii) subarctic forest-tundra (near Inuvik). Lakes in the alpine tundra were characterised by lower conductivity, temperature, chlorophyll a and nutrients than those in the other two zones. Those in the forest-tundra were generally small and shallow, and had higher chlorophyll a concentrations than lakes further south. Lakes in forested catchments spanned a larger latitudinal range and exhibited a greater variety of physical and chemical characteristics. However, they were generally deeper, with higher conductivity, temperature and ionic concentrations. Forty-one zooplankton taxa were identified from the 30 lakes, of which the most frequently occurring were the rotifers Conochilus unicornis, Kellicottia longispina, Keratella cochlearis and Polyarthra vulgaris, the cladocerans Daphnia middendorffiana and Bosmina longirostris, and the copepods Leptodiaptomus pribilofensis, Heterocope septentrionalis and Cyclops spp. The lakes contained between two and fifteen species (mean = 6.9). Alpine tundra lakes contained slightly less species (mean = 5.8) than those at lower elevations; in particular the cladoceran fauna was depauperate or absent. Relationships among the lakes, species and environmental factors were examined using canonical correspondence analysis, with forward selection and associated Monte Carlo permutation tests. Chloride, silica and temperature showed statistically significant relationships with species distribution, and together these abiotic factors explained 25% of the variation in zooplankton communities within Yukon and Northwest Territories lakes.     

The occurrence of internal phosphorus loading in two small,eutrophic, glacial lakes in Northeastern Ohio

Internal loading of phosphorus for the summers of 1972–1974 in the eutrophic Twin Lakes, Ohio, USA was calculated from nutrient budgets, and was found to account for 65–100% of the increase in phosphorus content of the lakes during this period. Recovery of lakes of this type after nutrient diversion may be delayed by internal loading and chemical inactivation of phosphorus may be needed. A discussion of sources of this internal loading is presented.     

The abundance and distribution of Filinia Terminalis in various types of lakes as related to temperature,oxygen, and food

Although morphologically variable, Filinia terminalis constitutes a well defined ecological unit living below 12–15°C, and forming large maxima up to several thousand ind./l at an oxygen content < 2 mg/l. Field investigations and laboratory experiments indicate facultative anaerobiosis and food sources of chemosynthesizing or plankton decomposing bacteria. On the grounds of these physiological requirements a hypothetical schema is presented attempting to predict the occurrence of the two species, F. terminalis and F. longiseta in oligotrophic, eutrophic, meromictic, and polymictic, shallow lakes.     

Vertical distribution of bacterial community structure in the sediments of two eutrophic lakes revealed by denaturing gradient gel electrophoresis (DGGE) and multivariate analysis techniques

Vertical distribution of bacterial community structure was investigated in the sediments of two eutrophic lakes of China, Lake Taihu and Lake Xuanwu. Profiles of bacterial communities were generated using a molecular fingerprinting technique, denaturing gradient gel electrophoresis (DGGE) followed by DNA sequence analysis, and the results were interpreted with multivariate statistical analysis. To assess changes in the genetic diversity of bacterial communities with changing depth, DGGE banding patterns were analysed by cluster analysis. Distinct clusters were recognized in different sampling stations of Lake Taihu. Canonical correspondence analysis (CCA) was carried out to infer the relationship between environmental variables and bacterial community structure. DGGE samples collected at the same sampling site clustered together in both lakes. Total phosphorus, organic matter and pH were considered to be the key factors driving the changes in bacterial community composition.     

Ostracods (Ostracoda,Crustacea) from two freshwater lakes near Maarssen,The Netherlands

Summary In Lake Maarsseveen I (with the lowest trophic level) 15 ostracod species were collected, whereas 9 species were found in Lake Maarsseveen II.Physocypria kliei Schäfer, 1934, andCypridopsis obesa Brady and Robertson, 1869, are new to the Dutch fauna.