Freshwater Biology

Published by Wiley
Online ISSN: 1365-2427
Print ISSN: 0046-5070
Publications
Life cycle parameters of Cephalodella acidophila in response to pH, temperature and food levels. (a, e, j). Average life span of females; (b, f, j) cumulative egg production per female; (c, g, k) net reproductive rate R0; (d, h, l) population growth rate r. Experimental temperature is indicated on top of each column. Bars denote standard errors.
3D mesh plots of the net reproductive rate (juveniles female−1) of Cephalodella acidophila in response to pH, food level and temperature.
The planktonic food web in extremely acidic mining lakes is restricted to a few species that are either acidophilic or acidotolerant. Common metazoans inhabiting acidic mining lakes with a pH below 3 include rotifers in the genera Cephalodella and Elosa. The life history response of Cephalodella acidophila to three environmental key factors, pH (2, 3.5, 5.0 and 7.0), temperature (10, 17.5 and 25 °C) and food concentration (10 000, 35 000 and 50 000 algal cells per mL), was investigated in a full factorial design using life-table experiments. The effect of each of the three environmental variables investigated on the rotifer life cycle parameters (life span, fecundity and population growth rate) differed. C. acidophila is a stenoecious species with a pH optimum in the range 3–4 and a comparably high food threshold. Combining the laboratory results with field data, we conclude that C. acidophila is severely growth limited in its natural habitat. However, low pH alone is not harmful as long as temperatures are moderate to warm and food is abundant. The population of C. acidophila in the field is maintained mainly due to release from competitors and predators.
 
In October 2004, plankton samples were collected from six permanent lakes located between 4960 and 5440 m a.s.l. in the Mount Everest region (Nepal) to assess how spatial and local environmental factors affect natural bacterial community composition. Fingerprinting analysis of the bacterial 16S rRNA gene fragment was done by denaturing gradient gel electrophoresis (DGGE).The number of DGGE bands (range: 12-23) was not correlated with lake area or remoteness, but there was a strong negative correlation with the ratio of catchment to lake area (r = -0.826, P < 0.05), suggesting that hydraulic retention time affects the establishment of the bacterial community in these seepage lakes.Most dominant sequences belonged to Betaproteobacteria except in two lakes where members of Bacteroidetes made the largest relative contribution. Up to 81% of the phylotypes had high similarity (>98 to 100%) in partial 16S rRNA gene sequence to those reported from other alpine lakes and glaciers around the world, suggesting the presence of 'cosmopolitan' bacteria.An analysis based on dissimilarity matrices and the Mantel test revealed the existence of dissimilarities in bacterial community composition related to geographical distance over a small spatial scale (<6 km), but determined by local environmental constraints.Our results suggest that several bacterial phylotypes are ubiquitous in the freshwater aquatic realm, but taxon sorting by local environmental constraints is important.
 
1. Variation in detritus quality and quantity can have significant effects on aquatic invertebrate food webs. Allochthonous inputs of detritus are the principal energy source for organisms in aquatic tree hole microsystems. We compared the effects of two major detritus types found in tree holes, senescent leaves (Sugar Maple and White Oak) and invertebrate carcasses (dead adult fruit flies and crickets), on several water quality characteristics of laboratory microcosms as well as on mass, survival and population performance of the dominant tree hole consumer, Ochlerotatus triseriatus (Diptera: Culicidae). To date, no study has documented the effects of animal detritus in tree hole microsystems or on resident consumers.
 
1. Many aquatic species usually considered to be ‘cosmopolitan’ have been identified as cryptic species complexes, based on deep genetic differentiation. However, reproductive isolation among sibling cryptic species has rarely been studied, and interspecific hybridisation is common in some taxa. 2. We investigated isolation mechanisms and possible introgression among three cyclical parthenogenetic rotifer species in the Epiphanes senta complex that are found in very different freshwater habitats: temperate floodplains, subtropical desert rock pools and a tropical alpine lake. Whereas Epiphanes ukera is reproductively isolated from E. chihuahuaensis and E. hawaiiensis, the latter hybridise under laboratory conditions. 3. While reproductive isolation is incomplete, RAPD profiles indicated unique genetic signatures and showed no evidence for introgression, indicating that these three species are diverging and have independent evolutionary trajectories. 4. Testing cues for sexual reproduction in these cyclic parthenogens demonstrated that mixis in E. chihuahuaensis and E. ukera is influenced by population density, whereas E. hawaiiensis females rarely produce mictic offspring regardless of density. Different mixis cues are likely to separate sexual periods and effectively cause reproductive isolation between the species. Epiphanes ukera and E. chihuahuaensis males display mate-guarding behaviour, and E. ukera males distinguish between conspecific and heterospecific females in mate-choice experiments. Geographic isolation, along with different cues for mixis induction and mate recognition, acts as reproductive barrier among these sibling species.
 
1. A flow tank was constructed which generates homogeneous and stable flow of water up lo velocities of 1.2 m s−1. 2 A sensitive, two-component balance with its platform inserted in the bottom of the flume allowed the measurement of drag and lift forces in the mN range. 3 The drag forces experienced by several benthic arthropods, which cling to the roughened surface of the platform, are proportional to the square of the flow velocity. 4 Large biological variations in lift forces, leading even to negative lifts in some cases, are indications of special adaptations of the animals to counter the danger of being swept away from their support.
 
1. Viruses have become widely recognized as the most abundant biological entities and important players in aquatic environments, and this realization has profoundly changed our conceptual understanding of the functioning and regulation of aquatic ecosystems in the last two decades. However, most of this research has focussed on marine viruses, especially in pelagic environments. 2. Here we introduce a special issue of Freshwater Biology dealing with viruses in freshwater ecosystems. It represents the first attempt to summarize progress in freshwater viral ecology made by diverse research groups and to direct attention of viral ecologists towards fresh waters. 3. Six review-type articles and ten original research papers cover a wide range of aspects of freshwater viral ecology. This includes reports on the distribution of freshwater viral communities in contrasting habitats (e.g. sediments, wetlands, littoral zone, open waters), on different roles of viruses in freshwater ecosystems (e.g. mortality rates of bacteria and phytoplankton, transduction, influence on bacterial diversity and organic matter), and on different types of viruses (bacteriophages, cyanophages, algal viruses, and a fish-pathogenic virus). 4. Collectively the series of papers presented in this special issue indicates that freshwater environments cover great habitat diversity and that the significance of some of the mechanisms controlling viral dynamics and impacts may differ between freshwater and marine habitats.
 
1. The interactions between calanoid and cyclopoid copepods were examined in an 11-year field study of a eutrophic lake (Heiligensee, Berlin, Germany). 2. A diminishing ratio of calanoids to cyclopoids was observed. The responses, however, were species specific. While two cyclopoid species responded with increased populations (Cyclops vicinus, Thermocyclops oithonoides), one species exhibited marked declines (C. kolensis). Other species extended the duration of their pelagic phase (Mesocyclops leuckarti, Diacyclops bicuspidatus, T. oithonoides, C. vicinus), leading to higher population overlaps. Eudiaptomus graciloides and E. gracilis, which used to be present throughout the year, were more frequently absent. 3. These changes coincided with a series of mild winters during which prey availability (algae and rotifers) was high. 4. Enhanced prey availability, in conjunction with a flexible diapausing strategy, are hypothesized to underly the increased success of C. vicinus. Predatory losses due to early occurrence and enhanced abundances of C. vicinus, rather than competition for the same food resources, are hypothesized to be responsible for the declines of C. kolensis and Eudiaptomus. 5. Summer species tended to develop higher abundances early in the season, which led to enhanced seasonal means during spring. These changes cannot be explained by the interaction between calanoids and cyclopoids alone, but were probably a result of changes in fish predation.
 
1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ13C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km2. Sites fell into two geomorphic categories (i) high-gradient, rock bed (‘rock’) or (ii) low-gradient, sand bed (‘sand’) sites. 3. Variation in δ13C was more strongly related to reach geomorphology than longitudinal position. δ13C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ13C did not vary with bed type. There was significant δ13C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ13C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ13C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ13C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ13C enrichment of biofilm at rock sites. Thus, differences in consumer δ13C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ13C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ13C.
 
Biovolume (mean ± SD) of (a) F. crotonensis and (b) N. palea in control (C), single-grazer with P. antipodarum (P) and A. aquaticus (A), and mixed-grazer treatments (PA) on day 1 and day 2 of incubation.
13C and δ15N of faecal pellets (single measurements) in single-grazer (P and A) and mixed-grazer treatments (PA) on the first (day 1) and the second day (day 2) of incubation.
d 13 C and d 15 N of faecal pellets (single measurements) in single-grazer (P and A) and mixed-grazer treatments (PA) on the first (day 1) and the second day (day 2) of incubation. 
Selectivity index Q [(Dd 13 C/Dd 15 N) · 1000] of P. antipodarum and A. aquaticus in single-grazer (P and A) and mixedgrazer treatments (PA) on the first (day 1) and the second day (day 2) of incubation (mean ± SD). 
1. Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), were conducted to investigate feeding selectivity and interspecific competition between the grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Conventional methods, such as cell counts and estimated biovolume, were used first to detect feeding preferences within the different grazer treatments. 2. The results revealed a significant decline in algal biovolume in all grazer treatments and no indications of active selectivity were observed. In contrast to conventional methods, measurements based on isotope signatures showed strong differences in tracer uptake, thus indicating different degrees of assimilation and digestion by the two grazers. 3. The selectivity index Q, which provides information on the uptake ratio of 13C to 15N, showed a significant time effect for both grazer species and a significant difference between single- and mixed-grazer treatments for P. antipodarum. Thus, this technique enabled the direct quantification of the uptake by grazers and, therefore, served as an ideal tool for the detection of passive selectivity. 4. Our results indicate a shift in feeding preferences related to between-species competition and a potential divergence of trophic niches when species coexist.
 
1. Benthic algae fractionate carbon isotopes less at low water velocities because of reduced boundary layer exchange, and this effect on δ13C is passed on to consumers via trophic transfer. This study examines the relationships between δ13C signatures of consumers (invertebrates and salmonid fishes) and water velocity in the Sainte Marguerite River, QC, Canada, and compares them to patterns for periphyton, both along the river main-stem and in a small tributary. 2. Relationships of δ13C signatures of herbivore/grazers and collector/gatherers with water velocity were strong and similar to those of periphyton, but relationships for filter-feeders were weak, probably reflecting the effect of spatial averaging of their food supply as a result of downstream transport. 3. Velocity effects on salmonid signatures were much weaker than those of lower trophic levels, being barely significant except in the small tributary where the fish were resident and isolated from the main river. In the river main-stem, even when reach standardised (reach mean subtracted from each data point), fish signatures were only weakly related to water velocity. 4. The fidelity with which velocity effects are transmitted to consumers from benthic algae is highly variable, and depends on a combination of consumer and resource movements, in addition to the trophic position of the consumer.
 
Time series of zoobenthos δ13C for four taxon-depth combinations: (a) Chironomidae, 1.5 m; (b) Chironomidae, 3.5 m; (c) Chironomidae, 10 m; (d) Odonata (Libellulidae and Corduliidae), 1.5 m. In each case the circles show observed zoobenthos δ13C (pooled sample of multiple individuals) and the solid line shows predictions from the fitted model of zoobenthos current autochthony. The other lines show daily δ13C values of potential C sources used as drivers in these models: the dashed line represents periphyton and the dotted line represents the ‘detrital’ (allochthonous + old autochthonous) end member. Arrows show the beginning and end of the 13C manipulation.
Proportion of zoobenthos production derived from current autochthonous primary production, as estimated by model fits shown in . Error bars show ±1 bootstrapped SD.
1. Secondary production of benthic invertebrates in lakes is supported by current autochthonous primary production, and by detritus derived from a combination of terrestrial inputs and old autochthonous production from prior seasons. We quantified the importance of these two resources for the dominant benthic insects in Crampton Lake, a 26 ha, clear-water system. 2. Daily additions of NaH13CO3 to the lake caused an increase in the stable carbon isotope ratios (δ13C) of the current primary production of phytoplankton and periphyton. We measured the response of four insect groups (taxon-depth combinations) to this manipulation, quantifying their current autochthony (% reliance on current autochthonous primary production) by fitting dynamic mixing models to time series of insect δ13C. 3. The δ13C of all four groups increased in response to the manipulation, although the magnitude of response differed by taxon and by depth, indicating differences in current autochthony. Odonate larvae (Libellulidae and Corduliidae) collected at 1.5 m depth derived 75% of their C from current autochthonous primary production. Chironomid larvae collected at 1.5, 3.5 and 10 m depths derived, respectively, 43%, 39% and 17% of their C from current autochthonous primary production. 4. Both taxon-specific diet preferences and depth-specific differences in resource availability may contribute to differences in current autochthony. Our results demonstrate significant but incomplete support of insect production by current autochthony, and indicate that allochthonous inputs and old autochthonous detritus support a substantial fraction (25–83%) of insect production.
 
1. Although marine research has indicated that metabolic fractionations of 13C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.2. The δ13C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ15N) or lipid content (C/N ratios).3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ13C and δ15N values, thereby masking any possibility of observing 13C trophic enrichment.4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ13C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.
 
1. The stable carbon isotope ratio δ13C is a useful tracer of energy flow in lake food webs, and the zooplankton signature is commonly used to establish a baseline for the pelagic habitat. However, sources of temporal variability in the δ13C of different zooplankton taxa are rarely considered. 2. Here, we investigate to what extent temporal variation in the δ13C of particulate organic matter (POM) (<41 μm) and the C : N of zooplankton can explain the temporal variability in δ13C of freshwater zooplankton. We compare temporal patterns of δ13C and C : N for Daphnia, Hesperodiaptomus franciscanus and Leptodiaptomus tyrelli over a 6-month period at four sites in two oligotrophic lakes. 3. In all three taxa, seasonal variation in zooplankton C : N explained more of the variation in zooplankton δ13C than did the δ13C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of δ13C in zooplankton. 4. Using these data, we evaluate procedures that estimate the δ13C of only the non-lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or ‘normalising’δ13C based on C : N will exclude a major dietary source, and therefore may be inappropriate. 5. We conclude that temporal variation in body composition (C : N) of zooplankton can significantly influence the temporal variation of zooplankton δ13C signatures.
 
Intact Myriophyllum spicatum plants were grown in compartmentalized containers in a growth room so that the roots were separated from the shoots by a watertight partition. Nitrogen 15N was added to the water or sediment to trace the uptake of inorganic N by the plant shoots or roots. Myriophyllum spicatum is capable of taking up inorganic N through both roots and shoots. Plant N requirements can apparently be met by root uptake alone. However, when about 0·1 mg/l of NH4-N were present in the water, foliar uptake supplied more N to the plants than did root uptake. Foliar uptake of NH4-N was found to be several times faster than that of NO3-N when both forms of N were present in the water. Only about 1% of the N taken up by the roots was subsequently released to the water through the foliage.
 
1. Several dozen summer meltwater streams are located in the McMurdo Sound region (c. 78°S 165°E) of southern Victoria Land. They are characterized by a highly variable flow regime at diel, seasonal and annual times caleis; wide fluctuations in temperature and nutrient content; and a very simple epilithic community of cyanophytes (Nostoc spp., Oscillatoriaceae), bacteria, fungi and microherbivores.2. The epilithon survives the dark Antarctic winter as dry, frozen mats which provide a large inoculum for growth the following summer. This overwintering assemblage retains a high metabolic capacity and responds rapidly to rehydration.3. In a series of artificial substrate experiments, biomass accumulation rates were generally less than 0.1 In units d−1. Colonization and growth on the substrates was inversely related to the suspended sediment load of the stream. There was also a visual correspondence between per cent algal cover of the natural streambed and the clarity of the streamwater. Sloughing losses may limit community biomass, particularly in the turbid flowing waters.4. During running water conditions the mature communities had very low gross photosynthetic rates per unit chlorophyll (<0,1 μg C (μg chl a.h)−1 and per unit carbon (<0,2 μg C (mg biomass C.h)−1). Respiration was generally a high percentage (up to 92%) of gross photosynthesis, which probably reflected the high population densities of microheterotrophs in the community.5. The floristically simple epilithic mats slowly accumulate to extreme biomass levels (>20 μg chl a cm −2, <20 mg C cm−2). Production rates per unit biomass are low, probably in response to the cold temperatures of the Antarctic stream environment, and the accumulated biomass represents several seasons of growth.
 
1. We developed equations which describe relationships between the relative abundance of diatom assemblages in the surface sediments of twenty-seven Sierra Nevada lakes and the present pH and acid neutralizing capacity in these lakes. 2. These equations were employed to reconstruct pH and acid neutralizing capacity at twenty-four depth intervals in a 20 cm long sediment core from Emerald Lake, a dilute, high elevation lake, located in Sequoia National Park, California, U.S.A. 3. The diatom-inferred pH and acid neutralizing capacity values indicated that neither pH nor acid neutralizing capacity exhibited significant trends since about A.D. 1825. We conclude from our diatom analysis that Emerald Lake has not been affected by acidic deposition.
 
1. Age, growth and reproduction in an Atlantic population of the small fish, Aphanius iberus (Cuv. & Val., 1846), from the estuary of the Guadalquivir river were studied. 2. The growth period was from April to September and the annulus on the scales was formed in April. 3. The life-span was short with a winter age-structure of 95.9% 0+ fish, 3.9% 1+ and 0.2% 2+. 4. The fecundity of this stock, before the beginning of reproduction, is represented by the formula: Fec=0.907 TL (cm)4.099. During the reproductive period 1+ females contained more, smaller eggs g−1 of gonad than 0+ fish. 5. Males matured before females, but females were larger than males. The overall sex ratio did not differ from 1:1 but males dominated significantly during most of the spawning period. 6. Aphanius iberus spawned intermittently during the reproductive period. From April to July, 1+ group specimens (7–12 months old) reproduced. Their offspring (0+ group; 3–4 months old) reproduced from July to September. Very few of the 1+ group specimens survived to spawn the following year. 7. Mean gonadosomatic indices were much higher in the 1+ group than in the 0+ group and in females compared to males. 8. The loss of condition and the disappearance of the 1 + group during spawning, may indicate the cost of a prolonged high level of reproductive effort. 9. In this productive environment, the life history of Aphanius iberus is similar to that of Gambusia affinis (Baird & Girard, 1853) and Fundulus heteroclitus (Linnaeus) found in the same area.
 
1. This is the first study on the life cycle, growth and production of Sigara selecta, a Palearctic corixid species typical of brackish and saline waters, at the warmest limit of its European distributional range. The study combines field and laboratory approaches. 2. The S. selecta population studied was multivoltine, producing four asynchronous cohorts from early spring to December and overwintering in the adult state. Development time from egg to first adult ranged from 2 to 3 months. A minimum temperature threshold of 10 °C and diel amplitude of ≥10 °C were observed for reproduction and oviposition. 3. Maximum density and biomass were reached in mid spring and early autumn. The sex ratio was unbalanced, females dominating during most of the year, except in spring, when the sex ratio was balanced or dominated by males during the first adult emergence. 4. Laboratory rearing experiments at constant temperatures (18, 22 and 26 °C) pointed to a significant effect of temperature on egg development and nymphal growth. In the range of temperatures tested, both egg and nymphal instar duration decreased with increasing temperature. Mean nymphal development time varied from 43 days at 26 °C to 71 days at 18 °C, with a mean of 57 days. Survivorship was independent of temperature. 5. A reduction in nymphal and adult length was observed with increasing temperature. 6. Growth rates decreased with increasing body mass and increased as temperature increased. The first nymphal instar had the highest length increments and growth rates in all temperature treatments. 7. Satisfactory agreement was found between the field and laboratory degree-days required for complete development from egg to first adult. At constant and variable thermal regimes, degree-days decreased with increasing temperature. 8. Rate of growth in the field could be predicted with reasonable accuracy from a simple model obtained as a function of body mass. The model explained 67% of the variability in growth rates. 9. Annual production and production/biomass ratio (P/B) of S. selecta estimated by the Instantaneous Growth method were 1.28 g m−2 year−1 and 13.71, respectively. Spring and autumn cohorts contributed 32% and 54%, respectively, of total annual production. Maximum production corresponded to intermediate temperature periods, although summer production may have been underestimated because of the longer sampling interval relative to cohort interval production. The Size Frequency method underestimated production by at least 18% with respect to the Instantaneous Growth method.
 
Summary1. Historical archives, published data sets and lake sediments from the North and South Basins of Windermere were analysed to reconstruct changes in the lake and catchment since 1850 and determine the drivers of limnological variability.2. Pastoral sheep farming has remained the dominant form of agriculture since 1850, but there was a decline in horse numbers (because of agricultural mechanisation), an increase in nitrogen fertiliser use and a doubling in sheep numbers since the 1950s. The human population in the Windermere catchment almost quadrupled between 1801 and 1921 coincident with the arrival of the railway in 1847, which led to urban expansion, sewer and piped water installation (since ∼1860s) and the development of sewage treatment systems (after 1886).3. C : N ratios and stable isotopes of carbon in 210Pb-dated sediment cores suggested that land disturbance increased the transport of terrestrial carbon to sediments of both basins after 1870, but algae increasingly contributed to the sediment matrix after ∼1890. Nitrogen-stable isotope ratios generally declined in both basins consistent with an increase in an isotopically light source such as atmospheric N deposition or synthetic fertilisers.4. Chlorophyll and carotenoid pigments in sediment cores showed that algal production increased gradually in the North Basin from ∼1890 to 1940 to be maintained at higher abundance thereafter. South Basin pigments changed most markedly ∼1860 (increase in cyanobacterial pigments), ∼1945 (increase in siliceous algal and chlorophyte pigments) and ∼1987 (increase in cyanobacterial pigments).5. Redundancy and variance partitioning analyses suggested that sewage influx and the modernisation of agriculture were strongly and positively correlated with algal abundance. However, climate variables were a secondary driver of algal change in both lake basins.6. Enrichment of the lake was caused by a combination of increases in population (after 1847), more efficient sewage disposal, intensification of agriculture and atmospheric deposition of N, which led to a greater than fivefold increase in primary production in the North Basin and marked shifts in algal communities of the South Basin.7. After enrichment, lake phytoplankton responded markedly to fluctuations in climate. High precipitation in January–March led to lower abundance of siliceous algae (mostly Asterionella formosa) in both lake basins, possibly because A. formosa inocula were diluted, thus limiting the spring diatom bloom.8. This combined palaeolimnological and archival study has shown that eutrophication of Windermere occurred before the start of the lake monitoring programme in 1945, but that nutrient enrichment enhanced the lake response to meteorological change. Therefore, future management of Windermere should aim for baseline conditions similar to those existing c. 1850 and consider that the control of eutrophication is essential in building resilience to future climate change.
 
Laboratory observations on the egg production, development and growth in the Cladoceran Moina micrura show that this species has a life span of 13 days during which it produces 6M 8 eggs. Two preadult and eleven adult instars were recorded at a temperature range of 28°–30°C. The rate of egg production calculated on a cumulative basis was found to be lower (a = 1.0368) than that of other tropical species. The embryonic stages in Moina micrura closely resembled other cladoceran species, although with a short development period of 24 h. The total life span, growth rate and the total number of eggs produced in Moina micrura have been compared with those of allied tropical, temperate and arctic daphnids. The significance of this relationship to secondary productivity in tropical ecosystems has been discussed.
 
(1) The occurrence of haemoglobin in Phaenocora typhlops (Vejdovsky) is reported. (2) Rhabdocoels were examined spectrophotometrically. The distribution of the pigment was studied by a sodium nitroprusside-benzidine technique, and the distribution of ferric iron with Perl's technique. (3) Characteristic bands of oxyhaemoglobin were observed at 540 and 580 nm. The oxyhaemoglobin was reduced to haemoglobin by the addition of sodium dithionite and an absorption band appeared at about 560 nm. (4) Haematin was found principally around the pharynx, though a little was found in front of the latter and along the length of the gut. Ferric iron was distributed throughout the worm. (5) The occurrence of haemoglobin in P. typhlops and P. unipunctata is compared. It is suggested that less occurs in the former species. Whether or not this is related to the presence of zoochlorellae is considered. Certainly both species burrow actively in mud where reduced oxygen conditions can occur.
 
1. Ecological stoichiometry deals with the mass balance of multiple key elements [e.g. carbon (C), nitrogen (N), phosphorus (P)] in ecological systems. This conceptual framework, largely developed in the pelagic zone of lakes, has been successfully applied to topics ranging from population dynamics to biogeochemical cycling. More recently, an explicit stoichiometric approach has also been used in many other environments, including freshwater benthic ecosystems. 2. Description of elemental patterns among benthic resources and consumers provides a useful starting point for understanding causes of variation and stoichiometric imbalance in feeding interactions. Although there is considerable overlap among categories, terrestrially-derived resources, such as wood, leaf litter and green leaves have substantially higher C : nutrient ratios than other resources of both terrestrial and aquatic origin, such as periphyton and fine particulate organic matter. The elemental composition of these resources for benthic consumers is modulated by a range of factors and processes, including nutrient availability and ratios, particle size and microbial colonisation. 3. Among consumers in benthic systems, bacteria are the most nutrient-rich, followed (in descending order) by fishes, invertebrate predators, invertebrate primary consumers, and fungi. Differences in consumer C : nutrient ratios appear to be related to broad-scale phylogenetic differences which determine body size, growth rate and resource allocation to structural body constituents (e.g. P-rich bone). 4. Benthic consumers can influence the stoichiometry of dissolved nutrients and basal resources in multiple ways. Direct consumption alters the stoichiometry of food resources by increasing nutrient availability (e.g. reduced boundary layer thickness on substrata) or through removal of nutrient-rich patches (e.g. selective feeding on fungal patches within leaf litter). In addition, consumers alter the stoichiometry of resources and dissolved nutrient pools through the return of egested or excreted nutrients. In some cases, consumer excretion supplies a large proportion of the nutrients required by algae and heterotrophic microbes and alters elemental ratios of dissolved nutrient pools. 5. Organic matter decomposition in benthic systems is accompanied by significant changes in the elemental composition of organic matter. Microbial colonisation of leaf litter influences C : nutrient ratios, and patterns of microbial succession (e.g. fungi followed by bacteria) may be under some degree of stoichiometric control. Large elemental imbalances exist between particulate organic matter and detritivores, which is likely to constrain growth rates and invertebrate secondary production. Such imbalances may therefore select for behavioural and other strategies for dealing with them. Comminution of large particles by benthic consumers alters detrital C : nutrient ratios and can influence the stoichiometry of elemental export from whole catchments. 6. A stoichiometric framework is likely to advance understanding of biogeochemical cycling in benthic ecosystems. A set of scenarios is developed that explores the influence of microbial elemental composition on nutrient spiralling parameters in streams, such as uptake length and uptake rate ratios. The presented hypothetical examples identify when the elemental composition of benthic stream organisms is likely to predict nutrient uptake ratios and conditions that would cause benthic stoichiometry and nutrient uptake from the water column to become uncoupled.
 
Changes in macrophyte richness, relative abundances, depth distributions and dominant species from 1937 to 1975 were assessed in Chautauqua, a eutrophic lake. Change was greatest in dominant species, with several formerly abundant native Potamogeton species being replaced by the exotics, Potamogeton crispus and Myriophyllum spicatum. Two extensively practiced management techniques, herbicidal poisoning and mechanized cutting, were considered to be primarily responsible for the observed macrophyte changes.
 
Summary1. We describe the changes in trophic dynamics in Lake Maggiore from c. 1943 to 2002 using subfossil cladoceran data from a high resolution sediment record, long-term contemporary data series and historical information. During this period the lake went through a eutrophication phase until 1980 followed by oligotrophication.2. During the eutrophication period a major increase occurred in the abundance of Chydorus sphaericus, the proportion of planktonic cladocerans and total abundance of cladocerans in the sediment. Since 1980 the abundance declined again and subfossil Eubosmina mucro length and contemporary Daphnia body length increased, most probably as a result of higher abundance of invertebrate predators.3. Changes in the fish stock composition caused by the introduction of exotic fish during the pre-eutrophication period and a complete ban on fishing because of Dichloro-diphenil-ethanes (DDTs) pollution of the lake (during oligotrophication) could also be detected in the community assemblage and size structure of the sediment zooplankton.4. We found good correspondence between trophic changes inferred from cladoceran subfossils (community composition, size and predation pressure) and contemporary data, suggesting that sediment samples can be used to infer past development in trophic dynamics, including predation by fish and pelagic invertebrates in lakes with scarce neolimnological data.5. Furthermore, by combining palaeolimnological cladoceran data rarely obtained from contemporary samples (e.g. benthic and plant-associated cladocerans, mucro length of bosminids) with contemporary data of organisms poorly represented in the sediment record (e.g. remains of Bythotrephes and fishes) a more complete understanding of changes in trophic dynamics was obtained.6. The detection in the sediments of meteorological events whose effects on zooplankton had been recorded in the long-term studies also provided evidence that eutrophication tends to override climate signals.7. We conclude that a combined palaeo-neolimnological approach can be a powerful tool for elucidating past changes in the trophic dynamics of lakes and the interaction with climate induced changes, not least when high resolution sediment records are available.
 
Statistical analysis has been carried out on 11 years (1965–75) weekly data of nitrate concentrations and discharge values from the R. Frome (Dorset). The results have shown a trend over the 11-year period with nitrate concentrations increasing by 0.11 mg NO3-N 1−1 per year. A predictive model for nitrate concentrations for 1976 was made and even though discharge conditions were extreme, the mean predicted nitrate concentration of 3.37 ± 0.136 mgNO3-N1−1 was very close to the observed mean nitrate concentration of 3.44mgN03-N1−1.
 
This study covers a 7-year period from 1965 to 1972, and shows the variation in flow, water chemistry and throughput of nutrients. The annual discharge of the River Frome varied considerably from year to year, ranging from a mean value of 4.48 m3 sec−1 in 1971 to one of 8.38 m3 sec−1 in 1966. The mean annual nutrient losses calculated over all 7 years of study from the River Frome catchment were: Nitrate 11.4 kg ha−1, phosphorus 0.49 kg ha−1 and potassium 8.4 kg ha−1. There has been an increase in annual throughput (in years of similar flow) of 21% in phosphate and 41% in nitrate from 1965 to 1972.
 
The seasonal and depth distributions of dominant phytoplankters, in situ primary productivity, organic carbon, and major chemical variables are described for Gull Lake, Michigan for the years 1971–74. Despite efforts by residents surrounding this moderately large dimictic lake to reduce phosphorus loading, all indications are of progressive eutrophication over the period. Previously dominant Cyclotella michiganiana has almost completely disappeared while araphidinean diatoms have increased greatly; certain blue-green algae that were absent previously are becoming characteristic. The ratio of araphidinean to centric diatoms in recent sediments has also increased. Primary productivity and particulate and dissolved organic carbon concentrations have also increased. Voluntary attempts to reduce nutrient loading have apparently been inadequate to arrest increasing rates of eutrophication.
 
In this study of the Lough Neagh catchment a relationship has been sought between the loads of dissolved and total silica and the flow of each of the six inflowing rivers. Two of these rivers-the Main and Six Mile Water-carry a higher ratio of dissolved to total silica than do the remaining four rivers due to the geology of the catchment. The multiple regressions of loads in tonnes against river flows in cubic metres show a linear relationship for dissolved but not for total silica. The explanation for this non-linearity has been sought in the river proaies, where the fall for the last 30 m varies from 5 to 25 km depending on the river. The rivers Main and Six Mile Water are the steepest, and linear relationships were obtained in their regressions while the shallowest profile was found in the Blackwater where non-linearity in the multiple regression was obtained. In Lough Neagh the dissolved silica content was gradually reduced from early winter through to springtime by diatom growth and a balance has been drawn up for this utilization. This reduction also compares with the quantity in the diatom crop at maximum based on cell volume measurements. The summer build up of the dissolved silica content of the lough has been shown to be greater than the quantity contributed by the inflowing rivers from late spring to autumn and the data show that the recycling of silica from some 65% of the frustules from the spring crop after its collapse would account for the summer increase.
 
Significant Spearman rank correlation coefficients (P<0.01) for positive (+) and negative (-) correlations Macrophytes on a shaded section of river 13 Suspended solids Si a Na K Conductivity Temperature Discharge
The growth and recession of macrophytes on a shaded section of the R. Lambourn were documented by a mapping procedure. With the exception of Ranunculus spp., the changes in total cover did not indicate directly the pattern of growth and recession of the macrophyles. Analysis of gross changes, expressed as gains and losses on cover, indicated that colonization of gravel and silt by the dominant macrophyte, Berula erecta, did not vary seasonally. Colonization was at a constant rate of about 8% of the site each month throughout the year and this accounted for 50% of the total number of gains by Berula. Gains of Berula from Ranunculus showed an annual cycle with a maximum during the summer when Ranunculus was in recession. Gains of Berula from Callitriche spp. also varied annually but the maximum was during the autumn. Total losses of Berula were at a constant rate throughout the year but were to gravel and silt during the winter, to Ranunculus in spring and early summer and to Callitriche in late summer and autumn. Analysis of loss of Berula with time indicated that the position of the Berula carpet was constantly changing. The growth and recession of Berula could not be linked in a meaningful way to environmental variables. Callitriche and Ranunculus both showed an annual pattern of growth and recession. There was temporal separation of the two macrophytes with Ranunculus growing mainly in spring and early summer and Callitriche showing maximum growth in late summer and autumn, and some evidence of spatial separation. The observed differences between years in the growth of Callitriche could not be attributed to any of the environmental variables measured.
 
The growth and recession of the macrophytes on an unshaded section of the R. Lambourn was documented by a mapping procedure from March 1971 to December 1976. The seasonal patterns were determined mainly by the dominant macrophytes. Ranunculus penicillatus var. calcareus and Ranunculus peltatus. Ranunculus grew rapidly in spring and early summer, mainly colonizing areas of gravel and reached a summer maximum. The greatest loss of plants was in August and September but losses continued through the winter to give minimum cover in March. Berula and Callitriche did not show regular seasonal growth patterns but their growth was largely confined to times when there was little growth of Ranunculus. Increase in area of Ranunculus in the spring was positively correlated with the mean discharge at that time. The growth of Ranunculus appeared to be restricted when shading was caused by turbid water due to dredging or by accumulation of epiphytic algae on the surface of the plants in years when discharge was low. Discharge also influenced the substrata underlying Ranunculus and the area of silt under Ranunculus beds was negatively correlated with discharge. The factors thought to influence growth of Ranunculus were used to develop a conceptual model of growth.
 
1. We measured temporal variability of the fish assemblage in the Wabash River using a 25-year electrofishing dataset through a combination of a time-lag analysis and multivariate analysis. 2. The fish assemblage had substantial year-to-year variation, but the overall abundance pattern did not change. Based on time-lag analyses and ordinations, there was only weak predictable directional change at individual sites. At the scale of the entire reach, however, the overall combined data had a statistically significant directionality with the time-lag analysis. 3. These results support the hypothesis that the middle Wabash River stream fish assemblage has undergone change over a time period of 25 years, although at the scale of individual sites this change was not detectable.
 
An account is given of the distribution of macrophytes in the River Tees and its tributaries, north-east England, together with a critical discussion of the problems involved in recording data in a form which can be used for monitoring long-term changes of vegetation in rivers. Among the conventions and methods adopted in the present study are an attempt to define the vertical limits of the river sensu strictu as opposed to the bank, the extension of the term macrophyte to include as many visually obvious photosynthetic organisms as possible, the use of a check-list prepared in advance which contains a wider range of species than are actually expected for the river, and the collection of two different types of standard record for each 0.5 km length of river. The Tees has been the subject of intermittent observations on its vegetation for some 45 years. The most obvious change has taken place since 1965, with a substantial upstream spread of four submerged angiosperm species. Potamogeton crispus, Zannichellia palustris and Myriophyllum spicatum were previously present only in the lower reaches of the Tees, and have spread 25, 9.5 and 4.5 km respectively upstream from their previously known most upstream localities. Ranunculus penicillatus var. calcareus is an apparent invader to the river. It seems probable that these changes have taken place since 1971 as a result of regulation of the Tees by Cow Green Reservoir. Further changes are predicted with the advent of water transfer from the Tyne to Tees around 1980, because at least twenty-six macrophytes are known to be present in the former river that are either absent or confined to the lowest stretches of the latter.
 
Spatial and temporal distributions of inorganic carbon and related ionic species (Ca2+, H+) were monitored weekly for 1 year in Cazenovia Lake, a small mesotrophic marl lake. Attendant [H2CO*3]([H2CO3] + [CO2(aq)]) and CaCO3 equilibrium conditions were determined through application of equilibrium equations adjusted for appropriate temperatures and ionic strength. A seasonal cycle was demonstrated for the inorganic carbon species which was temporally correlated to the lake's thermal and productivity cycles. Vertical homogeneity in carbonate chemistry was evident during turnover periods while dramatic differences developed between the epilimnion and hypolimnion during winter and summer stratification. The lake was supersaturated throughout the year with H2CO*3, though saturation was approached during the productive summer months in the epilimnion. The epilimnion was supersaturated with respect to CaCO3, from the start of spring turnover to the end of autumn turnover. Calcium (Ca2+) levels within the epilimnion decreased through much of the same period. Dissolution of CaCO3 in the hypolimnion is indicated by increases in Ca2+ and alkalinity as stratification periods progress. Analysis of potentially influencing factors indicates that the seasonal trends were mostly a result of CO2 metabolism. This was further supported by concurrent algal biomass and 14C uptake.
 
1. Adult Trichoptera were caught during 3 years (1980–82) in the ‘Ritrodat’ research area of the Upper Lunzer Seebach. a mountain brook near Lunz. Lower Austria, using a set of thirty pyramid type emergence traps. During the three collecting periods (112 collecting days) a total of 1810 specimens were caught. The most abundant species were Micrasema minimum McL. (Brachycentridae). Rhyacophila vulgaris Pictet (Rhyacophilidae), and the three Iimnephilid species Potamophylax cingulatus Steph., Allogamus auricollis Pictet and Ecclisopteryx guttulata Pictet. 2. In six species (Micrasema minimum, Rhyacophila vulgaris, Atlogamus auricollis, Ecclisopteryx guttulata, Drusus biguttatus Pictet, Chaetopteryx fusca Brauer) the sex ratio was significantly different from 1:1. In Micrasema minimum 422 females were caught but only three males. 3. In each year there were peaks in total emergence in early summer and early autumn. 4. The length of the emergence period of the most abundant species ranged from 38 days in Micrasema minimum to 210 days in Rhyacophila vulgaris (mean values 1980–82); there was also a difference in the intensity of emergence in these species: the percentage of time required for 50% of the animals to emerge (0%=onset of emergence, 100%= emergence completed) was 16% in Allogamus auricollis but 71% in Rhyacophila vulgaris (mean percentages 1980–82). 5. The distribution pattern of individuals per trap was tested against a Poisson distribution; in all three years the distribution was contagious. 6. The dry weight of specimens of Drusus biguttatus, Micrasema minimum. Ecclisopteryx guttulata and Wormaldia copiosa McL. was measured and. together with published dry weight values of other species (Malicky, 1976), these data were used to estimate the biomass emerging annually of twenty-three species of Trichoptera (99.2% of emergence, 1980–82). This was equivalent to 35.3 kJ m−2 in 1981 and 32.5 kJ m−2 in 1982.
 
1. Adult Trichoptera were caught for 1 year (May 1986 to June 1987) on the banks of the Danube at Altenwörth, Lower Austria, using a set of three Jermy-type light traps. From a total of 62,693 specimens caught, the most abundant species were Brachycentrus subnubilus Curtis (Brachycentridae), the two hydropsychids Hydropsyche bulgaromanorum Mal. and Hydropsyche contubernalis McL., Tinodes waeneri L. (Psychomyidae) and the leptocerid Ceraclea dissimilis Steph.2. The total catch included sixty-five of the 278 Austrian species from thirteen families. Hydroptila pulchricornis Pictet and Oecetis notata Ramb. were new to the Austrian fauna.3.Of the meteorological parameters tested, only air temperature seemed to influence flight activity.4. Twenty-four species were studied in detail; Halesus tesselatus Ramb. was the only autumn species, the remainder being summer species. In Tinodes waeneri, the flight period was divided into two well-defined peaks.5. Of the total, 49.9% were caught during the first third of the night, 39.1% in the second and 11.0% in the last third.6. In seventeen species the sex ratio was significantly different from 1:1, and it changed during the night.7. The indication of water quality by light trap catches of caddisflies is discussed.
 
1. The current extent and distribution of acidified surface waters in the U.K. is assessed using data collected between 1988 and 1993 as part of the U.K. Acid Waters Monitoring Network (AWMN). The data have been collected against a background of constant S deposition across the U.K. throughout the monitoring period. 2. Surface waters with mean pH < 5.0 occur in south-west Scotland, south-east Northern Ireland, the Lake District and south-east England. In addition, sites with mean pH < 5.5 are found in central and north-east Scotland, the Pennines and north Wales. These areas receive, and have historically received, high levels of S deposition and xSO4 is identified as the main contribution to non-marine acid anion concentrations. 3. Mean total oxidized nitrogen (TON) concentrations are > 10 μeq l–1 at ten of the twenty-two sites which are found in areas broadly corresponding to areas of high N deposition. The mean contribution of TON to total non-marine acid anion concentration is > 10% at sixteen sites and > 20% at six sites. Most sites, except for those with TON < 10 μeq l–1 and mainly located in north-west Scotland and Northern Ireland, demonstrate strong seasonality in TON concentrations and during autumn and winter instantaneous contributions of TON to total non-marine acid anions is up to 60%. This confirms the importance of N with respect to surface water acidification and strengthens the case for implementation of controls on future N emissions. 4. The data period is currently too short for rigorous trend analysis given the monthly/quarterly sampling frequency in streams/lakes, respectively. Nevertheless, increased SO4 and xSO4 can be observed in the time series at sites in south-west Scotland and north Wales despite constant deposition to these areas. Similar trends are evident in longer-term more frequent data at two sites and may reflect hydrometeorological influences, especially the duration and intensity of hot and dry weather in summer which promotes mineralization of organic S.
 
SUMMARY1Studies carried out in Lake Baikal in late spring (late May-early June) 1991 showed marked differences in the species composition and abundance of phytoplankton in different regions of the lake. The south and north basins were characterized by small forms of algae. The middle basin, Maloe More and the shallow waters of the Selenga had species with large cells including Aulacoseira islandica subsp. helvetica, Dinobryon cylindricum and D. divergens.2Areas of high biomass were correlated with shallow waters and river inputs. This was especially apparent in the region of the Selenga delta. The vertical distribution of phytoplankton indicated the non-synchronous start of the spring homothermy throughout the lake. Phytoplankton were concentrated in the upper 100 m layer with subsurface maxima resulting from the sinking of large algae.3The concentration of phytoplankton biomass in general at this time characterized the lake as moderately productive.
 
SUMMARY1 Near-bed flow equations and definitions have been reviewed by Carling (1992). The main errors and ambiguities in this contribution are identified.2 The hypotheses required to derive the governing equations are usually violated in complex flows of natural streams.
 
SUMMARY1 This response to the paper of Frutiger & Schib (1993) focuses on substantial errors that are relevant to the interpretation of results and deficiencies in their study design. I include some reanalysis of the original data of Schib (1991), which formed the basis of Frutiger & Schib's paper.2 The paper of Frutiger & Schib (1993) is flawed because of their sampling strategy (sequence of measurements, depth stratification, and the long duration of the study in relation to temporal changes in the insect populations), the selection of study sites (one lake outlet, one river disturbed by bathers), the mixing of single-species and multiple-species units in the analysis, the pooling of data and its analysis, the omission of essential information given by Schib (1991), and the generation of doubtful arguments. Their approach and interpretation places FST hemispheres at an unreasonable disadvantage in a debate about the relative utility of hemispheres and propeller-type current meters and consistently discriminates against the general role hydraulics play in the micro-distribution of the eight insect taxa that were studied.3 Despite the deficiencies of their study, my reanalysis of the original data of Schib (1991) provides significant models of insect abundance in relation to hemisphere density for six of the eight insect taxa, with r2 values between 0.62 and 0.86 for five of them.
 
1. Harris (Freshwater Biology, 32, 143–160, 1994) asserts that while empirical modelling of lake ecosystem properties has yielded general predictions and useful explanations, research into the effects of food-web interactions has not. I provide an explicit example of a useful, quantitative prediction gleaned from experiments on food-web effects—the relationship between total phosphorus and the magnitude of response of algal biomass.2. I further argue that Harris errs in asserting that food-web effects will be ineffective in eutrophic lakes. This error stems from a misconception about the relevance of ecosystem behaviour in unmanipulated systems for predictions of how an ecosystem will respond to manipulation. I cite empirical evidence demonstrating that, contrary to Harris’s contention, the response of algal biomass to a change in food-web structure increases as lakes are enriched.
 
1. Population characteristics (density, size, reproductive patterns) of the predatory cladoceran Bythotrephes cederstroemii in south-east Lake Michigan were monitored at an offshore station (110 m) in 1995–98 and at a nearshore station (45 m) in 1997–98. 2. The mean density of B. cederstroemii at the offshore station was generally highest in July–September (145–914 m−2) and at the nearshore station in October–November (168–1625 m−2). In 1995 and 1998, density was also high at the offshore station in November (211–284 m−2). Fish predation may limit B. cederstroemii in nearshore regions in the summer. The maximum annual densities of B. cederstroemii for 1995–98 were generally similar to those reported from the late 1980s, when the species arrived in Lake Michigan. 3. Body size increased rapidly each year to a maximum in August. Thereafter, body size declined and converged for stage-2 and 3 individuals, suggesting food scarcity or size-selective fish predation was affecting large individuals. 4. Most reproduction occurred asexually (90%), and by stage 2 or 3 females (99%). Asexual brood size was highest when B. cederstroemii first appeared each year, and decreased in August, when larger neonates were produced. There appeared to be differences in reproductive mode for stage 2 and 3 females, with a higher percentage of stage 2 females reproducing sexually.
 
1. The unicellular green alga Chlamydomonas acidophila accumulates in a thin phytoplankton layer in the hypolimnion (deep chlorophyll maximum, DCM) of an extremely acidic lake (Lake 111, pH 2.6, Lusatia, Germany), in which the underwater light spectrum is distorted and red-shifted. 2. Chlamydomonas acidophila exhibited a significantly higher absorption efficiency and a higher cellular chlorophyll b content when incubated in the red shifted underwater light of Lake 111 than in a typical, blue-green dominated, light spectrum. 3. Chlamydomonas acidophila has excellent low light acclimation properties (increased chlorophyll b content, increased oxygen yield and a low light saturation point for photosynthesis) that support survival of the species in the low light climate of the DCM. 4. In situ acclimation to the DCM under low light and temperature decreased maximum photosynthetic rate in autotrophic C. acidophila cultures, whereas the presence of glucose under these conditions enhanced photosynthetic efficiency and capacity. 5. The adaptive abilities of C. acidophila to light and temperature shown in this study, in combination with the absence of potent competitors because of low lake pH, most probably enable the unusual dominance of the green alga in the DCM of Lake 111.
 
Summary1. In extremely acid mining lakes, benthic filamentous green algae (Zygnemataceae, Chlorophyta) thrive as effective competitors for limited carbon (C). These algae could supply C for microbial-mediated benthic alkalinity generation. However, biomass, productivity and impact of the acidobiontic filamentous green algae at pH ≤3 have not previously been determined.2. Periphytic filamentous green algae was mapped by harvesting their biomass from 85 1 × 1 m quadrats in mining lake Grünewalder Lauch. Zygogonium ericetorum colonised water depths between 1.6 and 10.5 m covering 88% of total area. Biomass peaked at 5–6 m depth. Total Zygogonium biomass amounted to 72.2 t dry weight for the whole lake (0.94 km2), which corresponds to 16.1 t C and the accumulation of primary production from 2.2 years.3. Growth of Zygogonium is moderately N, C and extremely P deficient, and seriously stressed by high rates of Fe deposition during summer. Consequently, net primary production (NPP) of Zygogonium, calculated from measured photosynthesis versus irradiance characteristics and calculated underwater irradiance (0.13 g C m−2 year−1) and in situ oxygen measurements (7.8 g C m−2 year−1), corresponds to only 0.3% and 18.1% of pelagic NPP.4. Neither pelagic nor benthic Zygogonium primary production can supply enough C for efficient acidity removal. However, at rates of benthic NPP in summer of 21.4 mg C m−2 day−1, Zygogonium contributed 26% of the C equivalents to remove acidity associated with ferric iron, contributing at least seasonally to efficient alkalinity generation.
 
1. Welsh & Hodgson (2008) argued that stream-associated amphibians (Ascaphus truei, Dicamptodon tenebrosus and Rhyacotriton variegatus) are reliable indicators of ecosystem health in the Pacific Northwest (PNW), U.S.A. We demonstrate that their assertions rely on circular reasoning as well as logical and empirical assumptions that have not received rigorous evaluation. 2. Welsh & Hodgson (2008) based their arguments on data collected in northwestern California, U.S.A. However, all three taxa occur across a wide range of environmental gradients in the PNW, and other research results suggest that the taxa respond differentially to environmental perturbations across their geographical distribution. Hence, differences in abundance are not always reliable indicators of the state of an ecosystem, as population numbers can be influenced by factors that may not be associated with habitat quality (e.g. predation and disease). 3. Welsh & Hodgson (2008) present indices (rather than estimates) of abundance, which contain an unquantifiable amount of uncertainty not reliably attributable to either environmental or management factors. Evaluating parameters such as reproductive success and survival are more likely to provide strong inference about factors influencing population size than correlative studies that associate some measure of relative abundance with individual habitat features. 4. We argue that direct measurement of physical features (water temperature, sediment and wood), for which Welsh & Hodgson (2008) suggest using amphibians as surrogates, is more accurate, efficient and cost effective compared with monitoring cryptic species with methods that are unable to quantify spatial and temporal uncertainty in biological responses. 5. Headwater streams are critical components of catchments in the PNW, and all three amphibian taxa show strong associations with low-order streams. However, we think research and monitoring of headwater streams should be guided by hypotheses that evaluate causal mechanisms, rather than relying on descriptive patterns that are limited in geographical scope and which support only weak inference about ecosystem processes. Other studies provide direct links between physical features and responses of target populations (i.e. fish). Adding a second correlative link (i.e. amphibians) provides no added benefit, and increases uncertainty in the association between management actions and changes in population size.
 
Summary1. Two senior ecologists summarised their experience of the scientific publication process (Statzner & Resh, Freshwater Biology, 2010; 55, 2639) to generate discussion, particularly among early career researchers (ECRs). As a group of eight ECRs, we comment on the six trends they described.2. We generally agree with most of the trends identified by Statzner & Resh (2010), but also highlight a number of divergent perspectives and provide recommendations for change. Trends of particular concern are the use of inappropriate metrics to evaluate research quality (e.g. impact factor) and the salami slicing of papers to increase paper count. We advocate a transparent and comprehensive system for evaluating the research.3. We stress the importance of impartiality and independence in the peer review process. We therefore suggest implementation of double-blind review and quality control measures for reviewers and possibly editors. Besides such structural changes, editors should be confident to overrule biased reviewer recommendations, while reviewers should provide helpful reviews but be explicit if a submission does not meet quality standards. Authors should always conduct a thorough literature search and acknowledge historical scientific ideas and methods. Additionally, authors should report low-quality copy editing and reviews to the editors.4. Both early and late career researchers should jointly implement these recommendations to reverse the negative trends identified by Statzner & Resh (2010). However, more senior scientists will always have to take the lead with respect to structural changes in the publication system given that they occupy the majority of decision-making positions.
 
1. The transition from carbon (C) to phosphorus (P) limited growth in Daphnia depends not only on the C : P ratio in seston, i.e. food quality, but also on food quantity. Carbon is commonly believed to be limiting at low food because of the energetic demands of basal metabolism. The critical C : P ratio in seston (otherwise known as the threshold elemental ratio, TER) above which P is limiting would then be high when food is scarce. 2. A new model that differentiates between the C : P requirements for growth and maintenance is presented that includes terms for both C and P in basal metabolism. At low food the calculated TERs for Daphnia of around 230 are only slightly higher than values of 200 or so at high intake. Seston C : P often exceeds 230, particularly in oligotrophic lakes where phytoplankton concentration is low and detritus dominates the diet, indicating the potential for limitation by P. 3. The analysis highlights the importance of P, as well as C, in maintenance metabolism and the overall metabolic budget, such that food quality is of importance even when intake is low. Further measurements of C and P metabolism at low food, in particular basal respiration and excretion rates, are needed in order to improve our understanding of the interacting roles of food quantity and quality in zooplankton nutrition.
 
Summary1. Global warming has increased the mean surface temperature of the Earth by 0.6 °C in the past century, and temperature is probably to increase by an additional 3 °C by 2100. Water temperature has also increased, which in turn can affect metabolic rate in rivers. Such an increase in metabolic rate could alter the role of river networks in the global C cycle, because the fraction of allochthonous organic C that is respired may increase.2. Laboratory-based incubations at increasing water temperature were used to estimate the temperature dependence of benthic respiration in streams. These experiments were performed on stones taken from seven reaches with different thermal conditions (mean temperature ranging 8–19 °C) within the pre-alpine Thur River network in Switzerland, June–October 2007.3. The activation energy of respiration in different reaches along the river network (0.53 ± 0.12 eV, n = 94) was similar, indicating that respiration was constrained by the activation energy of the respiratory complex (E = 0.62 eV). Water temperature and the thickness of the benthic biofilm influence the temperature dependence of respiration and our results suggest that an increase of 2.5 °C will increase river respiration by an average of 20 ± 1.6%.
 
1. Ontogenetic changes during the life cycle of aquatic insects are important not only in life-history studies but also in evaluating food-web structure. They require information on the growth and number of larval instars but such information is lacking for many species, including Plecoptera. Therefore, the chief objectives of the present study were to determine inter- and intra-specific differences in the number of larval instars in British populations of 24 species of stoneflies, to test Dyar’s hypothesis that growth followed a geometric progression, and to synthesise this information with previously published values for four British species.
 
Top-cited authors
Erik Jeppesen
  • Aarhus University
Torben L. Lauridsen
  • Aarhus University
N. LeRoy Poff
  • Colorado State University
Klement Tockner
  • Austrian Science Fund
Martin Søndergaard
  • Aarhus University