Showing papers in "Freshwater Biology in 1978"

A review of the biology of British Naididae (Oligochaeta) with emphasis on the lotic environment

Abstract: SUMMARY. Naidids live in a wide range of aquatic habitats but are particularly important numerically as part of the benthic fauna of rivers with stony substrates. In general they graze on bacteria and algae although some, particularly Chaetogaster spp., are mainly predaceous, and C. limnaei vaghini is a parasite of molluscs, chiefly Gastropoda. Food selection seems to be based largely on particle size although the food quality of the particles within the appropriate size-range influences rates of growth and reproduction. Major factors determining the distribution and abundance of naidid species are the nature of the substratum and the presence and kind of vegetation. Plants with a highly dissected form, a thick growth habit, and which permit the greatest periphyton development generally support the most abundant naidid populations. The oligochaete fauna of coarse substrates (stones and gravels) is often dominated by the Naididae but both species-richness and abundance of naidid populations are generally reduced where fine substrates (silts and muds) occur. The occurrence of worms within the substratum is also determined by its nature; naidids penetrate to depths of 20–70 cm in coarse substrates but rarely penetrate below 6 cm in mud. The principal factor limiting both depth penetration and the dominance of naidids in fine substrates is probably oxygen availability. Naidids reproduce both asexually and sexually, the former method predominating for most of the year. Asexual reproduction usually involves the budding-off of zooids but a few species fragment. Sexual reproduction is often infrequent; populations of some species produce few or no sexually mature individuals. In mature worms asexual reproduction virtually ceases. In populations that produce mature individuals there is apparently one sexual generation a year, usually occurring during the summer and autumn. Adults die soon after laying their cocoons. In general, naidids are most abundant during the summer months when rates of growth and asexual reproduction are stimulated by higher temperatures and a plentiful supply of food. A few species, however, e.g. Nais elinguis and Paranais litoralis, are more abundant in the spring. The response of naidid species to different kinds of pollution is varied but generally organic enrichment of rivers which have stony substrates results in a considerable (ten- to twenty-fold) increase in naidid abundance. Nais elinguis, N. barbata, N. communis, N. variabilis, and Chaetogaster diaphanus are often abundant in such rivers, the foremost species reaching densities of 200 000 m−2. Nais alpina, N. bretscheri, and N. pardalis appear to be relatively intolerant of organic enrichment. A substantial increase in naidid abundance is also promoted by the deposition of biologically inert particles (coal dust, china clay, sand) on the river bed. The importance of Naididae in relation to pollution surveillance of fresh waters is discussed.

The food and feeding habits of young perch, Perca fluviatilis, in Windermere

Abstract: SUMMARY. The food of 0+ perch, Perca fluviatilis L. has been investigated in Windermere, the largest lake in the English Lake District. Whole guts and stomachs from 800 fish were investigated during four 24-h periods between June and September in 1975 and in 1976. Ciliates, algae, rotifers and cyclopoid nauplii constituted the first food of larval perch in Windermere. Vertical samples were taken from the zooplankton, at the same time and in the same location where the fish were sampled, and the concentration of the different food organisms studied. Electivity of the different food organisms was studied from lvlev's index, which showed positive electivity, or selection for some organisms, e.g. Bosmina obtusirostris (Sars.) and copepods (both cyclopoid and calanoid), and negative for others such as Daphnia hyalina var. galeata (Sars.), Leptodora kindti (Focke) and Bythotrephes longimanus (Leydig). In studying predatorprey relationships, Daphnia hyalina was chosen as the ‘standard prey’, being the most abundant of all food organisms in the diet of young perch, also because its maximum body-depth (carapace width (mm)) was among the largest of the cladocera and copepods in Windermere. Comparison of the maximum carapace width of Daphnia eaten with that of the plankton revealed that smaller specimens were taken by juvenile perch in July, but they tended to select larger organisms during August and September.

Comparative estimations of the energy content of fish tissue from bomb calorimetry, wet oxidation and proximate analysis

Abstract: SUMMARY Four methods were used to determine the energy content of somatic tissues of Perca fluviatilis Two forms of direct calorimetry (both adiabatic and non-adiabatic) and wet (dichromate) oxidation gave similar results When energy contents were calculated from proximate analysis using accepted conversion factors (945 cal mg−1 for lipid, 565 cal mg−1 for protein) results were higher than those from the other methods The discrepancy was eliminated when a lower, directly determined energy content for the extracted lipid fraction was used Some historic and technical aspects of the comparison are discussed

A study of the food and feeding of perch, Perca fluviatilis L., in Windermere

Abstract: SUMMARY. Perch were sampled for their stomach contents at regular intervals throughout 24 h from June until October in 1973 and from February 1975 until January 1976. They were found to feed on benthic organisms from November until April, on benthos and plankton during May and June and on perch fry and zooplankton from July until October. Perch over a wide size range feed on similar prey. Fish showed great variability in the weight of their stomach contents. A method based on a points system was developed to estimate the weight of food in the stomach for a given weight of fish at a known time. A diel feeding pattern which varied with the season was apparent from these data. Rates of gastric evacuation were assumed to be exponential and were calculated from the drop in night-time stomach content weights when food intake was assumed to be zero. The rates ranged from 0.18 mg h−1 at a mean water temperature of 11°C in May to 0.35 mg h−1 at a mean water temperature of 17°C in July. Assuming that food consumption followed a linear rate of intake, the standard Bajkov method was considered an adequate model to calculate daily food consumption. Daily food consumption (mg dry weight) was calculated for 150 g perch for all months of the year (November to April and September and October being combined). A second series of values was calculated making corrections for the time spent in the fishing gears when food intake was zero but gastric evacuation continued. Daily food consumption figures for 150g male perch based on Winberg's hypotheses (1956) and growth data showed no significant differences from this second series of values, when both were expressed in energy terms. When certain assumptions have been tested, growth data and Winberg's equations together may be a suitable method for calculating an energy budget for the Windermere perch population.

A technique for predicting in‐stream flow requirements of benthic macroinvertebrates

Abstract: SUMMARY. A total of 225 benthic samples was taken at various riffles along the Tongue River in Montana during three separate weekly intervals in 1975. For each sample, depth, current velocity, substrate microprofile, turbulence (Froude number), and the number and kind of species present were considered. The conditions of highest faunal diversity were 75–125 cm/s current velocity at 20–40cm depth. The optimum condition appeared to be 76 cm/s at a depth of 28 cm over medium cobble substrates. The centroids for optimum conditions of depth and current velocity for thirty-six common macroinvertebrates are also presented. Representative diagrams of the tolerance ranges of some common invertebrate species are presented. Rhithrogena hageni Eaton was chosen as the indicator species since its tolerances closely matched those of the highest community diversity. The presence of Rhithrogena in future samples would indicate maintenance of adequate flow conditions for the present benthic community. A method for applying these data to produce a minimum flow recommendation is discussed. If the optimum conditions can be plotted on composite hydrologic maps of depth and velocity in riffle areas at various discharges, one can predict the discharge at which the optimum conditions are eliminated and, thus, the recommended minimum discharge.

Feeding and growth of Asellus aquaticus (Isopoda) on food items from the littoral of Windermere, including green leaves of Elodea canadensis

Abstract: SUMMARY. In the laboratory, Asellus aquaticus devoured intact green leaves from growing shoots of the aquatic macrophyte Elodea canadensis. In four collections of A. aquaticus on Elodea in a lake (Windermere), c. 20% of the specimens contained in their guts fragments of green Elodea leaves; this material and pieces of oak (Quercus) were identified from characteristic leaf hairs. Some specimens had also eaten the filamentous alga Oedogonium. Fluorescence microscopy is a useful aid for screening invertebrates that may have eaten living plant tissues. Immature A. aquaticus, with an initial mean body length of c. 3 mm, wet weight c. 1 mg, were grown through sexual maturity over a 49-day period at 15°C in a series of twenty-two experiments (six to twelve isolated specimens in each experiment) comparing growth rates on different foods, including instances where no food was given. Animals were fed on a variety of items collected from the littoral of Windermere, plus some laboratory cultures of algae and bacteria. The highest mean specific growth rate (5.8% day−1) was obtained on young Elodea leaves mechanically shaken to remove epiphytes. Other diets yielding fast growth rates (3.7–5.3% day−1) were young growing leaves of Elodea with few epiphytes and older green and brown living leaves covered with a thick growth of epiphytic algae, epiphytic algae removed from Elodea, plastic imitation Elodea immersed in the lake until covered with attached algae, epilithic algae on stones, Oedogonium, and decaying oak leaves. Slower growth (1.3–2.2% day−1) and poorer survival was obtained on the following: a pure culture of the bacterium Sphaerotilus natans; cultured bacteria from lakewater; the filamentous algae Cladophora and Stigeoclonium both with and without epiphytes; faecal matter from Asellus; freshly killed Asellus; lake sediment. Some growth (mean = 0.7% day−1) and 50% survival for 21 days occurred in ‘starved’ animals kept in filtered, sterilized lakewater. Better survival and slightly faster growth (1.0–1.5% day−1) occurred in ‘starved’ animals kept in filtered and unfiltered lakewater. Growth of A. aquaticus was also experimentally determined from birth in animals fed on young green Elodea leaves and on decaying oak leaves. On both diets, growth was curvilinear and approximately exponential from birth to sexual maturity reached at c. 2mg wet weight in 46–60 days at 15°C. In older specimens the relative growth rate gradually fell over a period of 50 days, representing a more linear phase of growth during sexual maturity.

Aspects of the ecology of Prymnesium parvum (Haptophyta) and water chemistry in the Norfolk Broads, England

Abstract: SUMMARY. Prymnesium parvum, an ichthyotoxic phytoplankter, has been recorded, at times abundantly, in the River Thurne, Norfolk, and its associated Broads. Its occurrence has been apparently more frequent and its population sizes probably larger since the late 1960s than previously and fish mortalities due to it now occur almost annually. The Thurne system is brackish and may have become more so, due to exploitation by drainage pumps, of a saline water table, in recent years. Evidence for this is conflicting, but in any case an increase in salinity is unlikely to have made increased Prymnesium growth more likely. Eutrophication of some Broads in the system is most likely to have increased the populations of Prymnesium since the late 1960s and data are presented on the present water chemistry of the system for comparison with previous records, and on current phytoplankton and Prymnesium crops in different parts of it. P. parvum has been isolated from the system as a unialgal culture and compared in morphology, salinity tolerance and ichthyotoxicity with a strain of P. parvum from Israel. The Broads strain differs slightly in size and pigmentation, but not in salinity tolerance. In culture it produces more ichthyotoxin than the Israeli strain.

Some observations on the use of the in vivo fluorescence technique to determine chlorophyll-a in natural populations and cultures of freshwater phytoplankton

Abstract: SUMMARY. The variation in the ratio (R) between cellular fluorescence of chlorophyll-a and unit extractable chlorophyll-a was examined with natural populations of phytoplankton and unialgal cultures. Photo-inhibition of in vivo fluorescence was observed al high values of irradiance with natural populations and in algal cultures, except with blue-green algae. If samples were dark-adapted (< 6 W m−2) before measurements were made, changes of fluorescence due to photo-inhibition could be minimized. Measurements of in vivo fluorescence with dark-adapted samples of phytoplankton, dominated by species showing large values of R, gave significant correlations (P< 0.001) with determinations of chlorophyll-a concentration over periods of several weeks. The values for R for blue-green algae were appreciably lower than for other algal groups. This, and their lack of photo-inhibition effects at high irradiance values, are attributed to the association of most of chlorophyll-a with photosystem I which fluoresces weakly compared to chlorophyll-a in photosystem II. The significance of these findings for ecological studies is discussed.

The natural prey of larvae of the damselfly, Ischnura elegans (Odonata: Zygoptera)

Abstract: SUMMARY. A method for determining the prey of a damselfly larva under natural conditions is described, with its shortcomings. Within the limits of the method, it would appear that Ischnura elegans consumes different prey items in roughly the same proportions as their occurrence in the field. Anomalous results are explained in terms of differences in encounter rates between predator and prey and/or ease with which prey can be handled.

Community production and respiration by ciliated protozoa in the benthos of a small eutrophic loch

Abstract: SUMMARY. The results of laboratory experiments on ciliate reproductive and respiratory rates have been combined with field data for ciliate numbers, to produce estimates of production and respiratory energy losses by communities of benthic ciliates. Annual production decreased with increasing depth, and it was estimated to be 345, 149 and 40 J cm−2 year−1 at three sites in Airthrey Loch, Scotland. A similar trend with increasing depth was recorded for annual respiratory energy losses, being 16, 11 and 4 J cm−2 year−1 at the three sites. The corresponding annual net production efficiencies (%) were 95.2, 93.1 and 91.5. When net production efficiencies were calculated for each period (the time between successive sampling occasions) as 100 (daily production/daily assimilation), the range extended from 63 to 98. The energetic efficiencies of the ciliate populations are discussed with reference to data in the literature for higher organisms.

Dissolved organic matter of a low‐coloured stream

Abstract: SUMMARY. The dissolved organic matter (DOM) of White Clay Creek (Pennsylvania, U.S.A.), a low-coloured stream, was characterized by gel permeation chromatography, humic acid determination, compound classification analysis and gas chromatography. Large polymers (approximate mol.wt above 5000) were virtually absent. The majority of the DOM consisted of fulvic acid-like material, of probable mol.wt less than 3000. Highly coloured material represented only a small portion of the total. Substances classifiable as phenols, carbohydrates, lipids, amino acids or proteins made up only a small fraction of the DOM. Sixteen low mol. wt compounds (carboxylic acids, amino acids, carbohydrates and an amide) were tentatively identified. DOM concentration had a mean value of 6.4 mgC/l and showed annual fluctuations, with maxima in autumn and in late winter.

Water quality and the occurrence of Chironomus riparius Meigen (Diptera: Chironomidae) in a stream receiving sewage effluent

Abstract: SUMMARY. The effects of a sewage effluent in Moat Brook, Staffordshire, were determined from regular collections of water and biological samples from directly comparable stations above and below the sewage outfall. The eutrophic conditions below the outfall result in the replacement of Polypedilum laetum by Chironomus riparius (except in the spring) as the dominant member of the Chironominae. Continuous recruitment of first instar larvae to the population of C. riparius in summer and autumn results in much overlap of generations, but emergence trap captures suggest at least five generations in the year. The usefulness of C. riparius as an indicator of enriched conditions in lotic ecosystems should be considered in relation to oviposition behaviour and the occurrence and seasonal variation in density of the adults and aquatic stages (including the more conspicuous third and fourth instar larvae).

The effects of temperature on the development and mortality of eggs of perch, Perca fluviatilis

Abstract: SUMMARY. The rate of development and mortality of perch Perca fluviatilis was studied at ten different constant temperatures. The rate of development was inversely related to the incubation temperature, whereas the rate of mortality was directly related to the incubation temperature. The sum of heat (ΣH, degree-days) required for 10, 50 and 90% of the eggs to hatch was found to be constant, regardless of the incubation temperature, with mean values (with 95% confidence limits) of 91.4 (83.3–102.0) degree-days above 4.6°C for 10% hatched, 97.0 (90.9–104.2) degree-days above 4.9°C for 50% hatched and 101.0 (94.3–108.7) degree-days above 5.0°C for 90% hatched. Mortality among the different embryological stages was highest for the pre-hatching stage (i.e. when eye-pigment has been formed) at all temperatures. High mortality among the early stages occurred at temperatures below 8°C and above 12°C.

Comparative limnology of a deep-discharge reservoir and a surface-discharge lake on the Madison River, Montana

Abstract: SUMMARY. Phytoplankton standing crop, primary production, light penetration, temperature, and various chemical concentrations were measured in a man-made, deep-discharge, reservoir and in a natural, surface-discharge, lake in order to relate limnological conditions in the two bodies of water to their depth of outflow. The quantity and depth distribution of heat stored during the summer varied markedly. The reservoir functioned as a heat trap, whereas heat was readily dissipated from the lake. Salinity increased more in the lower layer of the lake. Throughout the summer, nutrient-rich water was discharged from the reservoir whereas nutrient-poor water was discharged from the lake. Phytoplankton standing crops were greater in the lake and were dominated by flagellates and diatoms. In the reservoir, blue-green algae were predominant. Rates of primary production and respiration were higher in the lake, but estimated algal turnover times were faster in the reservoir. It was concluded that depth of outflow has a direct and predictable effect on certain physical and chemical conditions within these two bodies of water; but effects of discharge depth on phytoplankton were secondary and thus difficult to ascertain.

The influence of turbidity on net phytoplankton photosynthesis in some Irish lakes

Abstract: SUMMARY. An important influence on gross photosynthetic rates per unit area is the success with which phytoplankton competes for the available light with non-algal suspended material and dissolved organic compounds. Using a range of Irish lakes, with euphotic zones varying between 0.7 and 20 m and with chlorophyll-α values between 1 and 860 mg m−3, the effect on gross rates of photosynthesis is analysed for changes in the balance between the factors contributing to light attenuation. Net values per unit area are also likely to be modified in well-mixed systems as the ratio of light to dark regions in the water column are altered with changes in light penetration. Depth gradients in dark respiration are reported for L. Neagh which vary according to previous light history and nutrient stress. Possible cases of restraint on phytoplankton growth are discussed for optically deep situations.

Effect of temperature on the hatching time of eggs of Ephemerella ignita (Poda) (Ephemeroptera:Ephemerellidae)

Abstract: SUMMARY. Eggs of Ephemerella ignita (Poda) were kept at eight constant temperatures (range 5.9–19.8°C) in the laboratory. Over 85% of the eggs hatched in the temperature range 10.0–14.2°C but the percentage decreased markedly to 39% at 5.9°C and 42% at 19.8°C. Hatching time (days after oviposition) decreased with increasing water temperature over the range 5.9–14.2°C and the relationship between the two variables was well described by a hyperbola. Therefore, the time taken for development was expressed in units of degree-days above a threshold temperature. Mean values (with 95%CL) were 552 (534–573) degree-days above 4.25°C for 10% of the eggs hatched, 862 (725–1064) degree-days above 3.57°C for 50% hatched and 1383 (1294–1486) degree-days above 3.14°C for 90% hatched. These values can be used to predict hatching times at temperatures below 14.68°C for 10% hatched, 14.54°C for 50% hatched and 14.45°C for 90% hatched. At higher temperatures, the hatching time and the number of degree-days required for development both increased with increasing temperature. Equations were developed to estimate the number of degree-days required for development at these higher temperatures. Eggs were also placed in the Wilfin Beck, a small stony stream in the English Lake District. Maximum and minimum water temperatures were recorded in each week and the summation of degree-days was used to predict the dates on which 10%, 50% and 90% of the eggs should have hatched. There was good agreement between these estimates and the actual hatching times. Only 10–15% of the eggs hatched between October and late February with most of the eggs hatching in March, April and May. Nymphs hatching in October and November probably did not survive the winter.

Effects of temperature on gut‐loading and gut‐clearing times of the burrowing mayfly, Hexagenia limbata

Abstract: SUMMARY. The effect of temperature on gut-loading times, gut-clearing times, and the calculated ingestion rates, egestion rates, and consumption indices of the deposit-feeding burrowing mayfly, Hexagenia limbata, were investigated in laboratory experiments. Rates of movement of two natural sediments of differing colour through the digestive tract were monitored to quantify feeding intensity when ambient water temperatures approached 5, 10, 15, 20, and 25°C. At each temperature, gut-loading times (GLT) and gut-clearing times (GCT) increased as nymph length increased. Mean GLT and GCT values decreased as temperature increased from 5 to 20°C but were longer at 25°C than at 20°C. Relationships between GLT, GCT, and length of nymphs and temperature were best described by multiple regression equations. No diel variation in gutclearing times was observed. Low water temperatures resulted in lower ingestion and egestion rates and consumption indices. At most temperatures nymphs ingested over 100% of their dry body weights per day.

The adaptive significance of cyclomorphosis in Daphnia: more possibilities

Abstract: SUMMARY. Morphological variability in Daphnia populations has often been uncritically ascribed to phenotypic plasticity. For instance, detailed study revealed that the ‘cyclomorphic species’D. carinata s. l. was a complex of nine species. Several of these species often cohabit and seasonal change in their relative frequencies causes phenotypic cycles which mimic true cyclomorphosis. Intraspecific genetic variation in head shape also seems widespread and is likely to be important in explaining phenotypic changes in many single species populations. The hypothesis that helmet formation in Daphnia is primarily related to predator avoidance is not supported by work on the D. carinata group. Seasonal trends in species composition can be explained without reference to differential predation. Natality differences exist between species with disparate head size suggesting that helmet formation may have direct effects on fitness. Two possibilities are considered. The length of the anterior adductor muscle is directly correlated with helmet size and such variation may affect swimming efficiency. In addition the laminar design of helmets suggests a role in gas exchange.

Spatial variation in epilithic algae in a stony stream (Wilfin Beck) with particular reference to Cocconeis placentula

Abstract: SUMMARY. Epilithic algae in a small softwater stream in the English Lake District were studied using a direct counting epifluorescence technique. A small trickling filter sewage treatment plant discharged into the stream and cattle grazed in fields adjoining the upper reaches. Both increased the inorganic nitrogen and phosphorus concentrations in the stream but the effect of the cattle was of a more intermittent nature. Cocconeis placentula was the dominant diatom but large numbers of encrusting green, blue-green and red algae also occurred in samples from the middle reach, which was heavily shaded by trees in summer. Although it was possible to demonstrate the effect of the sewage effluent on the rate of colonization of clean surfaces placed in the stream, it was only during periods of stable water flow that this was reflected in the standing crop of algae on the stones. The Cocconeis counts below the sewage outfall tended to be highest in the autumn whereas those of the other diatoms were highest in the spring. The results were, however, extremely variable and apart from the effect of the nutrient inputs, the Cocconeis standing crops were largely related to the rate of water flow. Counts decreased and the variability between and within stones increased as higher rainfall and flow caused instability and scouring of the stones. A similar decrease in standing crop and high variability occurred in conditions of very low water flow but this could not be explained.

Observations on the biology of the perch, Perca fluviatilis, in the River Stour, Dorset

Abstract: SUMMARY. Opercular bones from 323 perch from the River Stour were used for age and back-calculated growth determinations. Annuli were formed during May at the beginning of the growth period. Growth was minimal from October to April. Female perch grew faster than males, and the growth rates of both sexes were higher than those observed in most other European waters. Spawning occurred during late April and early May; male gonads began development in August and had attained their maximum weight in September, but the ovaries developed gradually from August until April. Immature perch had an annual cycle of condition with a maximum in June-July and a minimum in December-January. The condition of mature males and females was affected by the gonad cycle. The fecundity of Stour perch is expressed by the formula: log egg number = 2.40 log length (mm) - 1.34. Approximately 25% of males were mature at age I and all were mature at age II, whereas most females did not spawn until age III. Ephemeroptera nymphs and minnow fry constituted the bulk of the diet of 0-group perch; Ephemeroptera nymphs, minnow fry and Corixidae were the most numerous items in I group perch, whereas older perch contained Corixidae and a wider range of fish prey species, although minnows were the most numerous of these.

The fauna of the muddy sediments of Lough Neagh, with particular reference to eutrophication

Abstract: SUMMARY. The main groups of animals found in the muddy sediments of Lough Neagh were Chironomidae (Diptera) and Oligochaeta (mainly Tubificidae). At 8 m depth, the average biomass of the main species in these groups was: Chironomus anthracinus, 2.64gm−2; Procladius, 342.8 mgm−2; tubificids (Tubifex, Potamothrix, Aulodrilus, Limnodrilus), 418.7mgm−2. At 25m depth C. anthracinus and Procladius were less abundant, C. plumosus was present and among the tubificids Limnodrilus had a higher density (up to 50000m−2) often an order of magnitude greater than at 8m. Kinnego Bay, a semi-enclosed, polluted area of the lough, had fewer chironomids than the open lough, with chiefly C. plumosus and Procladius. Limnodrilus was more abundant than at 8m. Dero obtusa (Naididae) was also present, sometimes in large numbers (31000m−2). The response of this fauna to eutrophication in L. Neagh appears to be a diminution in the number of species present, with C. plumosus gradually replacing C. anthracinus as the dominant Chironomus, and tubificids, particularly Limnodrilus, becoming more abundant, A further stage, shown in Kinnego Bay, has Procladius as the most numerous' chironomid. Average total biomass at the three sites was 3.9gm−2 at 8m, 13.2gm−2 at 25 m in the main lake, 2.8gm−2 in Kinnego Bay. At 8m depth, C. anthracinus contributed 76% of the total biomass, at 25m Limnodrilus contributed 74% and C. anthracinus 15%, and in Kinnego Bay, Limnodrilus contributed 40%, C. plumosus 28% and Procladius 20%.

Sexual swarms in Daphnia magna, a cyclic parthenogen

Abstract: SUMMARY. Dense swarms were observed in a population of Daphnia magna (Crustacea: Cladocera). The swarms appeared during a sexual period in which females were abundant but males were rare. Each swarm contained several males and large numbers of ‘imminently sexual’ females, a class of females that was rare except in the swarms. This implies that both sexes respond to attractant stimuli, and that the female response occurs only during a brief critical phase in the ovarian cycle.

The biology of an Antarctic aquatic moss community

Abstract: SUMMARY. Two species of aquatic moss (Calliergon sarmentosum and Drepanocladus sp.) occur in deep water in Moss Lake, Signy Island (60° 43′S, 45° 38′W). Their elongate stems support a structurally complex community of epiphytic algae and associated invertebrates which has a greater diversity than the surrounding benthic algal communities. Qualitative differences in the distribution and abundance of the epiphytic algae are described. Two groups of algae present on both mosses are distinguished–‘habitual’ and ‘casual’ epiphytes. On Calliergun, algae are most abundant in the leaf axil. The succession of epiphytes down the stem and the progressive deterioration of the moss plant are used to characterize six stem zones. The invertebrates are benthic and most move actively among the moss. Six species of rotifers are more or less permanently attached and show a preference for the middle stem zones where epiphyte cover is highest. Four species favour the leaf axil, the other two colonize the bare underside of the leaf. A settlement experiment has shown the importance of wind-induced mixing in summer for the transport of some epiphytic species from shallow parts of the lake. The settlement of sessile rotifers is effected by their larvae. Photosynthesis declines from the stem apex to the dead old stem. Much of the primary production of the community is algal. The respiration maximum occurs where the highest concentration of epiphytic algae and invertebrates is found. Interrelationships between the invertebrates and the epiphytic algae are suggested and a comparison is made between these data and other studies.

The effect of sewage effluent on Grasmere (English Lake District) with particular reference to inorganic nitrogen transformations

Abstract: SUMMARY. In the summer of 1971 the village of Grasmere was converted from septic tank to mains drainage with sewage treatment at an activated sludge plant. The effluent was discharged into the River Rothay, the main inflow of a nearby small lake (Grasmere). This paper describes some of the effects on the lake. The mean areal hypolimnetic oxygen deficit increased from 274 to 434 mg O2 m−2 day−1 with an accompanying marked increase in the degree of deoxygenation in the hypolimnion. Plate counts of bacteria in the surface water increased for 2 years but this increase was not sustained. The mean summer soluble reactive phosphorus concentration did, however, increase significantly (P= 0.05), but the same was not true of nitrate levels. This paper is particularly concerned with inorganic nitrogen transformations and analyses of the main inflow have shown that 50–98% of the ammonia and 10–40% of the nilrate entering in this river was derived from the sewage effluent. The concentrations in the main body of the lake were usually lower, possibly due to assimilation and denitrification in the shallower reaches of the lake. Seasonal changes in the inorganic nitrogen species in the hypolimnion showed three distinct phases of activity, ammonification, nitrification and denitrification. Nitrification accounted for approximately a quarter of the oxygen uptake in the hypolimnion.

The distribution of some freshwater planktonic bacteria in two stratified eutrophic lakes

Abstract: SUMMARY. Changes in bacterial populations and certain physical and chemical variables in Esthwaite Water between June and September 1975 were studied and compared with results obtained from 1972 to 1974 in the hypolimnia of Blelham Tarn and the Lund tubes. The counts of total bacteria ranged between 1 and 7 × 106ml−1 and were highest in the anoxic hypolimnion. The bacterial genera examined in more detail constituted only a small percentage of this count and included Ochrobium (104ml−1), Naumanniella (103ml−1), Leptothrix (102ml−1), Planctomyces (103ml−1), and Metallogenium (102ml−1). The iron bacteria appear to grow best in the oxycline where there was not only sufficient oxygen for aerobic growth but also a plentiful supply of reduced iron. Planctomyces numbers increased as the thermocline became depressed in September. The results from Blelham Tarn might be interpreted as further evidence of growth by iron bacteria in the absence of dissolved oxygen, but other explanations are possible. Examination of the results by multiple regression analysis showed that it was possible to explain a significant proportion of the bacterial variation (with the notable exception of the Planctomyces counts) in spite of considerable intercorrelation of the regressor variables.

Limnological studies of Lake Wisdom, a large New Guinea caldera lake with a simple fauna

Abstract: SUMMARY. Lake Wisdom is of interest because it is large and deep, has oxygen throughout, and yet, apparently as a result of its recent formation, its biota has some surprising lacunae. It is nearly circular and fills the central caldera of Long Island, Papua New Guinea (5° 20′ S, 147° 6′ E). Its maximum length is 13.4 km, and area approximately 95 km2. A bathymetric map of the lake to depths of 300 m has been constructed from fathometer transects. Repeated soundings in the deeper parts gave depths of about 360 m. The surface of the lake is approximately 190 m above sea-level and water samples from the deepest part of the lake are fresh, so the basin is apparently sealed. The lake level shows annual fluctuations of c. 1.0 m. Limnological information has been collected during seven visits to Lake Wisdom over the period 1969–76. The surface temperature of the lake was constant at 28°C throughout this period and the temperature falls very gradually to 26–27°C at 60 m, except for a relatively rapid drop of approximately 1°C from 10 to 20 m. One of the most unusual features is the relatively high oxygen concentration in the deepest parts of the lake. Living chironomid larvae and molluscs were collected from the bottom in depths of 360 m. Light penetration in the lake varies greatly depending on the amount of rainfall and the volcanic activity of Motmot, a secondary cone within the lake. The biota of Lake Wisdom is rather simple. There is a low standing crop of phytoplankton. Benthic algae are abundant and diverse although they cover only a limited area because of the depth of the lake. There are no vascular aquatic plants. The pelagic fauna consists of two species of Cladocera and one species of notonectid. There is one species of sponge, four species of molluscs, and a small number of species of aquatic insects including Hemiptera. Odonata, and the larvae of chironomids, mayflies, a caddisfly, and a pyralid moth. Water birds, including ducks, grebes and waders, are fairly numerous. The lake also contains one or more crocodiles.

Effectiveness of an artificial substrate for sampling macroinvertebrates in small streams

Abstract: SUMMARY. Factors affecting invertebrate colonization of samplers consisting of twenty-five 5-cm diameter porcelain balls were studied in small northern California streams. Peaks in diversity, in number of individuals and in taxa, occurred after 2—4 weeks in each of four 10-week colonization runs. Most variation in number of organisms was correlated with increases in organic detritus. Time was the second most important variable. The average coefficient of variation in total numbers collected from the artificial sampler was 0.40, slightly less than the average 0.56 for the Surber sampler. Published coefficients of variation for artificial substrates range from 0.109 to 0.849, which are similar to values reported from work with the Surber sampler. We had several practical difficulties with the artificial substrates. Samplers were lost, became clogged or buried, supported vertebrate predators and were costly. The samplers also collected some organisms in greater proportion than their occurrences in the natural substrate. In sampling small streams these disadvantages (compared to a Surber sampler) outweigh the slight reduction in sampling variability.

The peritrophic membrane and faecal pellets of Gammarus lacustris limnaeus Smith

Abstract: SUMMARY. Gammarus lacustris limnaeus Smith was fed decomposed autumnshed leaves of maple (Acer saccharum Marsh.) and poplar (Populus tremuloides Michx.). Faecal pellets were collected at various time intervals after egestion and examined under a light and a scanning electron microscope. Nearly all the faecal pellets collected up to a period of about 7 h after egestion possessed a thin, tightly-fitting peritrophic membrane while those that had been outside the gut of the animal for a longer time lacked a peritrophic membrane. Presumably, after egestion faecal pellets swell because of absorption of water leading to eventual rupture and loss of the membrane. The surface of newly extruded pellets is devoid of microbes and microbes seem to play a very insignificant role in the loss of peritrophic membrane from the pellets.

Variations in the shallow water form of Potamogeton richardsonii induced by some environmental factors

Abstract: SUMMARY. Populations of Potamogeton richardsonii in Sparrow Lake, Ontario, vary greatly in leaf dimensions and internode length. Leaf length/breadth (L/B) ratio is increased by low irradiance, significantly at 4% daylight and, in a contrary fashion, by ontogenetic drift at high irradiances of shallow water. L/B ratio was not correlated with substrate. These findings suggest that this ratio is of doubtful value taxonomically or as an integrator, as specific leaf area is confirmed to be, of factors like photosynthetically active radiation (PAR). Longest internodes belong to plants from deep(1.8 m) water, and, in a summer-grown high-density population, internode length decreases logarithmically as depth lessens; both observations implicate PAR and ageing. Relative to undisturbed, shallow-water shoots, young transplants in full daylight elongate almost twice as fast because of the production of more, and longer, internodes. Young shoots in 12% daylight lengthen even more rapidly than those in full daylight (and four times faster than undisturbed shoots) in the same period because of quicker elongation of the same number of internodes. At ambient summer temperatures, this rate of elongation is inversely related to PAR. Experimental and seasonal field data indicate that maximal internode extension occurs on young shoots in very low irradiances at temperatures of 9–15°C; in nature, effects of increasing water temperatures are depressed by increasing irradiances and ageing until minimal extension takes place in old shoots at mid- to late-summer temperatures and in high irradiances. In tanks (40 m3) in full daylight, nutrients limit growth of plants on sand before they limit growth on clay (with marl intermediate) whereas, at 12% daylight, light limits growth on sand before soil nutrients do. The leaf area index on silty sand on an exposed shore was 0.4 at 0.5 m depth, in contrast to a cultured population where it was 4.0 in silty loam at the same depth. Thus, while it has not been possible to explain some variations in leaf morphology of P. richardsonii in environmental terms, the differential effects of ontogeny, PAR and temperature on shoot growth have been assessed, along with overall effects of light and nutrients on biomass and of shelter on leaf area index.