Showing papers in "Freshwater Biology in 1980"

Relationships between body size and biomass of aquatic insects

Abstract: SUMMARY. Predictive equations were developed to estimate dry weight from body length measurements for forty-three taxa of aquatic insects. The inter-relationships between dry weight, body length and head capsule width of individuals grouped according to the eight major orders of aquatic insects were also examined. Regression analysis indicated that the relationship between biomass and body size was best expressed by a power equation, Y =aXb, rather than by linear or exponential equations. Changes in body length versus head capsule width were best expressed by linear equations, with three distinct relationships being observed. Body length estimated biomass better than head capsule width. Populations often species of insects collected from two different rivers generally did not differ significantly in their dry weight to body length relationships.

Mobilization of sediment phosphorus by submersed freshwater macrophytes

Abstract: SUMMARY. The mobilization of sediment phosphorus (P) by three submersed freshwater macrophyte species was investigated on five different sediments. The study was conducted under controlled environmental conditions in lucite columns that enabled the separation of sediment and plant roots from the overlying P-free ‘complete’ nutrient solution. The species investigated (Egeria densa, Hydrilla verticillata, and Myriophyllum spicatum) had minor root systems (on a biomass basis), but were demonstrated to be fully capable of deriving their P nutrition exclusively from the sediments. Phosphorus absorption and translocation into shoots (i.e., mobilization) was substantial, and in some cases suggested a greater than 1000-fold turnover of interstitial water PO4-P over a 3-month period. Sediment P mobilization, a function of both plant growth and tissue P concentration, differed considerably among plant species and sediments. Phosphorus release from the species investigated appears to be primarily dependent upon tissue decay rather than excretory processes. The mobilization of sediment P by submersed macrophytes represents an important aspect of the P cycle, and may affect the overall metabolism of lacustrine systems.

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

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

The effects of coal industry pollutants on the macroinvertebrate fauna of a small river in the South Wales coalfield

Abstract: SUMMARY. The effects of pollutants from the coal industry on the macroinvertebrate fauna of a small river in the South Wales coalfield were investigated during 1972 and 1973. Three major types of pollution, each spatially separate, affected the river and its tributaries. The upper reaches received acidic drainage from coal stockpiles. The lower reaches received a discharge from a drift mine, bearing high loads of suspended coal particles which caused siltation of the river bed downstream to its confluence with the R. Taff (3.5 km). The largest tributary was subjected, under neutral pH, to siltation by ferric hydroxide, derived from seepage from an abandoned mine. In the upper reaches, a few acid-tolerant invertebrate species survived in low numbers. Downstream recovery was not complete before the discharge of coal particles. In the reaches silted by ferric hydroxide or coal there was a pronounced reduction (80–90%) in faunal abundance. Clear differences in sensitivity of invertebrate groups to these types of siltation were apparent. The data are used to assess the various methods of biological surveillance involving invertebrates.

Acidity and species diversity in freshwater crustacean faunas

Abstract: SUMMARY. On the basis of material from over seventy acidic water bodies in three different areas a relationship is revealed between pH and the species diversity of their crustacean faunas. With increasing acidity species diversity decreases. No such relationship is shown between calcium concentration and species diversity. The very low calcium levels of such waters may, however, play a part in the exclusion of certain species.

Anaerobic metabolism of particulate organic matter in the sediments of a hypereutrophic lake

Abstract: SUMMARY. The anaerobic decomposition of particulate organic matter (POM) was examined in the anoxic pelagic sediments of hypereutrophic Wintergreen Lake. Degradation of sedimented POM occurred rapidly as shown by increased production and release of ammonia, hydrogen sulphide, volatile fatty acids and methane from the sediments 2–3 weeks after large inputs of organic matter. Maximum concentrations of most metabolites were found at the sediment-water interface, indicating that the initial anaerobic degradation of freshly deposited POM occurred at this site. The absence of the inorganic electron acceptors, nitrate and sulphate, suggested that fermentation and methanogenesis were the major anaerobic processes involved in the dissimilation of organic matter in these sediments during stratified periods. The amount of carbon input converted to methane in the sediments was determined from May to early November 1976 and 1977. Carbon output as methane was measured by quantifying methane lost from the sediments by ebullition and by estimating soluble methane lost to the water column by diffusion. Total methane release during summer stratification accounted for 34% of the particulate organic carbon input to the sediments in 1976 and 44% in 1977. Methane release was directly related to the rate of sedimentation of POM. However, methane production was temporarily inhibited following high rates of sedimentation in 1976, suggesting that the rate of organic loading may be an important factor controlling anaerobic decomposition in these sediments.

Cadmium and calcium interactions in the freshwater amphipod Gammarus pulex

Abstract: SUMMARY. The accumulation of cadmium from an experimental medium by the freshwater amphipod Gammarus pulex is described. Much of the uptake is internal as opposed to adsorption on the body surface, and after an apparent saturation of the exoskeleton the hepatopancreas becomes an increasingly important site of cadmium storage. The haemolymph cadmium concentration reaches a high level compared with marine crustaceans, achieving a concentration factor (ratio of internal Cd/ external Cd) of 100 after about 60 h uptake. The cadmium uptake process is severely inhibited after exposure of experimental animals to 0.5 mM 2:4 Dinitrophenol, indicating the mediation of an active process. This fact together with the negative relationship between cadmium uptake rate and the calcium concentration of the animal suggests that cadmium accumulation by this species may be at least partially accounted for by a process of ‘accidental’ active cadmium uptake, with cadmium substituting for calcium on a calcium regulatory mechanism. As yet it has not been possible to establish a true stoichiometric relationship between the two metals. Although calcium influx and cadmium uptake (influx) rates are similar over a wide range of external concentrations, calcium influx is clearly inhibited by a low external ratio of cadmium to calcium. This indicates that the relationship between the two metals is far from an equimolar one and the possibility of non-competitive inhibition of calcium influx by cadmium cannot be eliminated.

Further observations on developmental polymorphism and its evolution in the rotifer Brachionus calyciflorus

Abstract: SUMMARY. In laboratory experiments, long, Asplanchna-induced posterolateral spines of Brachionus calyciflorus were very effective in preventing their capture and subsequent ingestion by the predator Asplanchna sieboldi but provided no protection against predation by Mesocyclops edax. Young, short-spined B. calyciflorus were always captured after attack by adult A. sieboldi and were ingested in about 12 seconds. Adult, short-spined forms were captured on c. 35% of occasions when attacked by this predator and were ingested in about 50 s. Young and adult long-spined forms were captured by this predator on c. 60% of occasions when attacked, but they both almost invariably escaped or were rejected 20–35 s after capture. Short- and long-spined B. calyciflorus adults were always captured when attacked by adult, female M. edax and were completely ingested in about 20 s and 30 s, respectively. Life-table experiments conducted with B. calyciflorus at several levels of the food organism, Aerobacter aerogenes, showed that neither the possession of long posterolateral spines nor the production of offspring with long posterolateral spines interfered with survivorship, fecundity, or reproductive potential. In the laboratory, the volumes of the amictic parthenogenetic eggs of B. calyciflorus cultured on Euglena or Aerobacter were significantly greater in individuals from populations maintaining long posterolateral spines than in comparable-sized individuals from populations maintaining short spines. Egg volume was generally independent of adult body length, but it was significantly greater in Brachionus fed on Euglena compared with Aerobacter. Possible reasons why B. calyciflorus does not produce long posterolateral spines in the absence of Asplanchna are discussed. Few organisms other than B. calyciflorus are known to develop novel defensive phenotypes in direct response to the presence of a predator. It is suggested that such developmental responses evolve only when two conditions apply: (1) the defensive structure is primarily effective against a single type of predator, and (2) the prey organism exhibiting the response has a short generation time.

Laboratory models of diel vertical migration in the dinoflagellate Ceratium hirundinella

Abstract: SUMMARY. Diel vertical migrations of a dinoflagellate, Ceratium hirundinella, were induced in a laboratory tube (1.63 × 0.15 m) under a light-dark cycle. The timing of vertical migrations differed between cultures in the exponential and stationary phases of growth; the latter showed a greater coincidence with the light regime. Migration of cells into the surface layers occurred at low values of surface irradiance ( 1300 μE m−2 s−1) there was a marked avoidance of surface waters, and population maxima were found at depths associated with a relative irradiance level of 10% or c. 150 μE m−2 s−1). Thermal stratification restricted downward movement of cells into the cooler layers. The combination of high surface irradiance and thermal stratification resulted in large, stable, sub-surface maxima of Ceratium, similar to those observed in natural waters under comparable environmental conditions.

Some ecological consequences of fish school volumes

Abstract: SUMMARY. Two consequences of a recently published method of calculating fish school volumes are examined in the light of ecological population data. First, minnow and roach schools occupy only about 0.01% of the volume of water in which they live, graphically illustrating the vagaries of field sampling, and providing reference values for foraging studies. Secondly, I show that roach schools in one river are constantly within attack range of pike predators. If this situation is general, theories which assume that schools function strategically merely to reduce the probability of encountering a predator, break down, whereas my predictions suggest that there is strong selection for direct predator evasion tactics in schools.

Growth in the crayfish Austropotamobius pallipes (Crustacea: Astacidae)

Abstract: SUMMARY. The growth of Austropotamobius pallipes was studied in the River Ouse during 1976–78. Growth of mature crayfish (>2.5 cm carapace length) was followed by determining the relationship between the growth increment at moult and premoult carapace length, together with the frequency of moulting of different categories of crayfish. These data are supplemented by the recapture of marked individuals and the measurement of crayfish held in corves in the river. Growth was limited to the period May–October when water temperatures exceeded 10°C. Growth rates of male and female crayfish are similar until maturity is reached, thereafter males moult twice per year and the majority of females moult once. No crayfish in excess of 3.7 cm carapace length has been observed to moult more than once per year. Growth of juveniles (<2.5 cm carapace length) was estimated from size frequency distributions constructed from regularly taken samples. Growth rates of juveniles showed great variation both between and within year classes. In the hot dry summer of 1976, juveniles exhibited faster growth rates (instantaneous growth rates (G) for 0+ and 1 + crayfish were 0.029 and 0.013 mg mg−1 day−1. respectively) than in other years. Laboratory experiments on the effect of temperature on the growth rate of 0 + crayfish were undertaken; for crayfish at 15°C. G= 0.0138 (0–53 days) and at 10°C, G= 0.0003 (0–90 days). Crayfish held at 10°C failed to undergo a single successful moult. At 20°C crayfish exhibited exponential growth over the first 40 days, with G= 0.0189. declining thereafter to G= 0.012 (40–90 days).

Toxicity of γ-hexachlorocyclohexane (Lindane) to Gammarus pulex

Abstract: SUMMARY. The acute toxicity of the pesticide γ-hexachlorocyclohexane (Lindane) to Gammarus pulex was determined by a standard procedure and by a modification of the conventional toxicity test protocol. Animals were exposed to a range of concentrations for various periods of time from 1 to 1000 min and transferred to clean water. Animals continued to die for up to 3 weeks after the initial exposure to the poison. Toxicity curves were obtained expressing the relationships between concentration of poison and median survival time (LT50) and between concentration of poison and median lethal exposure time, i.e., the duration of exposure required to cause the eventual death of half the animals. For a given concentration of pesticide, the duration of exposure which will cause 50% mortality is far less than is indicated by a conventional toxicity test in which animals are continuously exposed to the poison. It is suggested that the results of conventional acute lethal toxicity tests may have limited predictive value even when applied to field situations involving the discharge of lethal levels of pollutants to receiving waters.

Characteristics and classification of the lakes of Signy Island, South Orkney Islands, Antarctica

Abstract: SUMMARY. The lakes of Signy Island (60° 43′S, 45° 38′W), South Orkney Islands, Antarctica, provide a unique laboratory for the study of many processes including natural eutrophication. Current relationships between the lakes have been examined by principal components analysis of environmental data from a long-term monitoring programme. The results from the analysis have been combined with some biological data to provide a classification scheme for the lakes. The study has produced a valuable baseline for use in future research.

Fossil diatoms and scales of Chrysophyceae in the recent history of Hall Lake, Washington

Abstract: SUMMARY. Scales of several members of the Chrysophyceae (family Synuraceae) were examined from a core of the recent sediments of Hall Lake, Washington, in conjunction with a study of fossil diatoms. Both types of microfossils revealed communities changing with the historical development of the lake. Increases in planktonic diatoms and in two species of Mallomonas occurred about the time of European settlement. Several acidophilous species among the Chrysophyta disappeared with the establishment of a sawmill and meromictic conditions in the lake. When extensive road construction around the lake ended the period of meromixis, the ratio of Synuraceae, in general, to diatoms, declined. Besides the change in ratio of Synuraceae to diatoms there was a change in the composition of the community of Synuraceae at that time. Species of Synura that had been rare before became the most abundant, and Mallomonas became relatively rare. Throughout the recent history of the lake the dominant Synuraceae seemed to respond more strongly to disturbance around the lake than the most abundant diatoms; thus, scales of Mallomonas, Synura, and other Synuraceae are valuable microfossils for paleolimnological studies.

Seasonal distribution and life histories of Ephemeroptera in a Northumbrian River

Abstract: SUMMARY. The life histories of twelve abundant species in the River Coquet are given together with notes on certain scarce species. The life histories are compared with those recorded for other river systems and major differences in findings between studies are related to geographical location and temperature. Catastrophic mortalities noted for some populations coincided with the presence of slush-ice in the river bed and subsequent winter spates caused by meltwaters. Extended hatching periods are apparently adaptive and compensate for such losses. Life histories are also compared along the length of the R. Coquet. Hatching periods, growth rates and emergence periods show regional differences which are apparently dependent on altitudinal variations in temperature regime.

Temporal and vertical distribution of ciliophoran communities in the benthos of a small eutrophic loch with particular reference to the redox profile

Abstract: SUMMARY. The ciliate communities occurring at three benthic sites in a small eutrophic loch have heen investigated over a 2-year period. Two characteristics of the community were studied in detail, the pattern of vertical distribution within the sediment and the temporal distribution of ciliates occurring in the surface sediment. The relationships between ciliate distributions and the environmental factors recorded were analysed by multiple regression. Significant relationships were revealed between vertical distribution of ciliates and the sediment redox (Eh) profile, the larger ciliate communities being associated with regions of higher potential. Other factors such as sediment density, organic matter, temperature and daylength, combined with other indicators of benthic metabolism (sulphide ion activity (Es2−), pH, oxygen flux) were selected in the regression analysis as accounting for much of the variation in the depth distribution of ciliates. In analysing the temporal distribution of ciliates in the surface sediment, numbers were inversely related to Eh, Es2− and oxygen flux, a result of the upwards migration of reducing conditions and greater microbial activity in the sediment surface during the summer. Daylength, temperature, organic carbon and benthic chlorophyll-a were also selected as accounting for much of the variation in ciliate number. It is proposed that the large increases in number and biomass of surface-sediment cilates in the summer months resulted from an intolerance of reducing conditions developing immediately beneath the surface and the increased productivity of the benthos as a whole during this period. Methods are also described for the construction, calibration and operation of electrodes used in measuring Eh, Es2− and oxygen flux in the freshwater benthos. Data recorded for these three variables revealed similar seasonal patterns at each site in each of two consecutive years.

Further studies on the palaeolimnology and changes in the phosphorus budget of Barton Broad, Norfolk

Abstract: SUMMARY. Patterns of sedimentation have been studied in Barton Broad, Norfolk, a man-made lake created by peat excavation prior to the fifteenth century. Sedimentation rates since before 1800 have increased by as much as 50-fold owing to the effects of hypereutrophication and sediment movement downriver into the lake. A striking horizontal pattern has been found; a long black tongue of sulphide-rich sediment extends into the broad from the inflow river. This dates (210Pb method) back to 1920–30 and is associated with enrichment from sewage effluent and a change from a macrophyte-dominated to a plankton-dominated lake. Past phosphorus concentrations (1800, 1900, 1920, 1940) and budgets have been predicted from sediment core analyses by use of a model previously validated by contemporary measurements on the lake. The analyses and calculations provide a perspective for plans to restore the lake to a less fertile state capable of supporting aquatic macrophytes.

A survey of the macro‐invertebrate riffle fauna of the River Wye

Abstract: SUMMARY. Surveys of the Wye catchment in March and September 1975, and July and September 1976 and 1977, yielded 227 macro-invertebrate taxa, 77 of which were Chironomidae. Total macro-invertebrate density ranged from 520 to 22020 m−2. Total density estimates in July 1976. during a period of severe drought, were significantly higher than in July 1977 but no differences were established between estimates on other comparable dates in 1975, 1976 and 1977. The total number of taxa and total macro-invertebrates collected from sites with total dissolved solids concentrations less than 50 mg I−1 were significantly lower than at sites with higher concentrations. In general, upstream sites were dominated numerically by Plecoptera in March; Ephemeroptera, Trichoptera and Diptera were relatively more abundant during the summer months. Sites lower in the catchment were more variable in composition with Oligochaeta, Ephemeroptera, Trichoptera and Diptera all abundant at certain sites and times. The densities of Ecdyonuridae (Ephemeroptera). Hydropsychidae (Trichoptera) and Elminthidae (Coleoptera) in the impounded R. Elan, a tributary of the R. Wye, were significantly lower than at nearby sites on the Wye although no differences in total macro-invertebrate density were established. Average linkage clustering of Spearman rank correlation coefficients between selected taxa allowed the comparison of taxa groups with contemporary site groups by nodal analysis. Six major taxa groups were established and these characterized five site groupings.

Quantification and characterization of sedimenting particulate organic matter in a shallow hypereutrophic lake

Abstract: SUMMARY. Sedimenting seston was collected in May-November 1976 and 1977 in sedimentation traps anchored 0.5 m above the pelagic sediments of hypereutrophic Wintergreen Lake. Sedimentation rates differed greatly in the two years studied, ranging from 2.7 to 19.3 g m−2 day−1 in 1976 and from 2.6 to 9.5 g m−2 day−1 in 1977. Because of the shallowness of the lake (maximum depth 6.5 m), the quantity and quality of sedimenting particulate organic matter (POM) was closely linked to the production dynamics of the phytoplankton. Chemical analyses indicated that the sedimenting POM was planktonic in origin and, in contrast to that of many deeper lakes, was dominated by protein. Short sedimentation distances coupled with the close proximity of the anaerobic hypolimnion to the photic zone ensured that the majority of sedimenting POM reached the sediments in a relatively undegraded form.

Methods for the isolation, sterilization and cultivation of Lemnaceae

Abstract: SUMMARY. A method is described for isolating and sterilizing fronds of Lemna gibba, L. minor, L. trisulca, L. polyrrhiza and Wolffia arrhiza, employing enrichment followed by sodium hypochlorite treatment. The method enables about 4% of the total number of treated fronds to be used as initiators of clonal cultures in which bacteria responding to organic enrichment and algae are excluded. Appropriate growth media, culture vessels and culture conditions are detailed.

The origin, composition and downstream transport of plant material in a small chalk stream

Abstract: SUMMARY. This study was a large-scale comparative in situ experiment on the fragmentation and breakdown of typical aquatic and terrestrial inputs of a chalk stream. Two similar lengths of stream were separated by screens of 5-mm mesh across the entire stream width at their upper and lower limits. The material transported downstream was collected continuously for 3 years. The order of breakdown was: <2 weeks for the emergent herbaceous macrophyte Rorippa nasturtium-aquaticum; 2–3 months for the dominant submerged macrophyte Ranunculus calcareus and 4–6 months for the terrestrial material, mainly leaves of Salix viminalis and Fraxinus excelsior. The degree of fragmentation of material varied with its structure, composition and time of availability relative to the seasonal changes of discharge. In general, fragmentation of material increased with its time of retention at or near its sites of origin, but this could be greatly modified by the date of the major autumnal increase in discharge. Thus, in a year of early rain, terrestrial leaf material was mainly whole when transported downstream but in a year with late rain, fragments predominated. The breakdown order coincided with an increasing ratio of carbon/nitrogen in the materials. The total annual movement of plant material was between 231 and 426 kg dry weight for both sites but there was considerable variation in the quantities of the individual species present. On an areal basis, with equal inputs of aquatic and terrestrial material, aquatic material was fragmented more than terrestrial material and therefore more of the latter was transported downstream. The aquatic macrophytes in the open section of the stream retarded the progress of terrestrial allochthonous material and resulted in a substantial increase in fragmentation of this material which would have been lost to this part of the stream.

A fish‐kill in Heart Lake, Ontario, associated with the collapse of a massive population of Ceratium hirundinella (Dinophyceae)

Abstract: SUMMARY. A massive population of the common dinoflagellate Ceratium hirundinella developed in Heart Lake. Ontario, Canada during the summer of 1976 and its sudden collapse and subsequent decomposition depleted dissolved oxygen and resulted in a fish-kill in the lake. The lake was being artificially mixed at the time by supplying compressed air to the bottom waters and the limnological events contributing to the development of the Ceratium population and its collapse appear to be closely related to the artificial destratification process. Artificial destratification during 1976 precluded the development of blue-green algue. The process also led to an increase in the density of herbivorous zooplankters which controlled the development of smaller planktonic algae. Ceratium flourished in Heart Lake because there was little competition for nutrients from other algae and because Ceratium cells are too large to be grazed by the zooplankton. The maximum size of the Ceratium population (53 mm3 1−1) is apparently the highest biomass reported in the literature and its collapse may have been related to a depletion of inorganic nitrogen. There is apparently no previously published record of a Ceratium-induced fish-kill in a freshwater lake.

The temperature-determined growing season of a submerged hydrophyte

Abstract: SUMMARY. Temperature determines the growing season of Utricularia purpurea, a common rootless, carnivorous plant at depths of 0–6.5 m in a dimictic oligotrophic lake (Mirror Lake, New Hampshire, U.S.A.). The dynamics of thermal stratification limit the growing season to 8 weeks at a depth of 6 m compared to 17 weeks at 2 m. Light may limit growth at 6 m during the short growing season, but is not limiting in midsummer at 2 m or 4 m. Plants of equivalent size produce 60–75% less biomass annually at 6 m than at 2 or 4 m. The seasonally maximal biomass (in mid-August) is equivalent to the annual net production as calculated from periodic inventories of new, old and senescent tissue (thus P/B= 1.0). Uptake of nutrient elements (N, P, Ca, Mg, Na, K) is synchronous with biomass production. Nutrients do not become enriched in the overwintering tissue (52% of the maximum biomass). This species is unusually rich in Zn (1000 μg g dry wt−1), relatively rich in N (2.9% dry wt). but quite poor in P (0.084%).

Bdelloid rotifers and nematodes in a small Italian stream

Abstract: SUMMARY. The bdelloid rotifers and nematodes of a small clean watercourse in the area surrounding the Po river near Parma have been studied. The samples were collected at two sites 400 m apart. The species found in twelve collections of benthos between May 1977 and June 1978 were eighteen belonging to Bdelloidea and forty-seven to Nematoda (Tylenchida have not been considered). Nearly all the bdelloid species were oligosaprobic or beta-mesosaprobic; the Shannon index of diversity was 3.0 at the first site and 2.4 at the second. Among the Nematoda the dominant species were Paraplectonema pedunculatum and Paraphanolaimus anisitsi which were not previously recorded for Italy. Nearly all nematodes are microphagous; predators represent only 10% of collected specimens. Polluted water indicator species (Secernentia group) were only 3.5% of the total nematodes recorded. Most rotifers and nematodes did not show seasonal fluctuations in numbers. The two sampling sites do not seem to have substantial differences in the taxocoenosis of Rotifera and Nematoda; their percentage of similarity is 69% and 79% for the two classes respectively.

Colonization of artificial substrata by macro‐invertebrates in a stream and variations according to stone size

Abstract: SUMMARY. Plastic cages containing artificial substrata were placed on the stony bottom of a stream in a section where the environmental conditions appeared to be homogeneous. The section was divided into eight equal sub-sections (A-H) and four cages were placed in each sub-section. The stone-size of the artificial substrata was similar within each cage but differed between the four cages with average diameters of 14, 24, 48 and 96 mm, respectively. The baskets were removed one month later and the catches of macro-invertebrates were recorded. Analysis of the catches (density and number of taxa in each cage) revealed no significant differences in connection with the position of the cages in the section of stream. Cages with 48-mm stones contained the least abundant fauna. The taxa which colonized cages with 14- or 24-mrn stones were more numerous than those collected from cages with 48- or 96-mm stones. Statistical analysis by laxon confirmed the preceding results. Variability in connection with the position of the cages along the section of stream was never significant. A little more than half the taxa were distributed independently of stone size but always showed a contagious type distribution. The density of the other taxa was higher, either in cages with 14- or 24-mm stones (particularly Gammarus) or only in cages with 96-mm stones (especially Hydropsyche). Only one taxon was more abundant in cages with 48-mm stones, the type of substratum in which the overall density was lowest. Catches in the cages were not the same as those taken with a Surber sampler because the two samplers did not take samples from the same habitats and also because the baskets offered a more specialized habitat than the surrounding bottom. The basket with 24-mm stones gave the least distorted picture of the natural population.

Diurnal fluctuations within a littoral plankton community in oligotrophic Lake Pääjärvi, southern Finland

Abstract: SUMMARY. The plankton community within an Equisetum fluviatile stand in oligotrophic Lake Paajarvi had distinct diurnal fluctuations in the total cell volume and species composition of algae and in the abundance of microcrustaceans (Bosmina coregoni). Diurnal fluctuations in pH, oxygen saturation and temperature of the water were also recorded. Within the littoral region, daytime pH values > 9 were recorded, whereas in the pelagial region of the lake, values remained near pH 7. Diatoms and green algae dominated the littoral phytoplankton especially in the innermost parts of the macrophyte stand, with cryptophytes (dominant in the pelagial area) occurring only in small numbers. At the outer parts of the macrophyte stand, water movements between the pelagial and littoral areas might rapidly increase the contribution of cryptophytes in the phytoplankton. The fall in algal cell volume during the night may have resulted from settling out of cells in the absence of wind-induced water movements, perhaps together with increased grazing pressure from dense swarms of Bosmina (up to 3000 individuals per litre) which appeared during the night.

Abnormalities in the tracheal gills of aquatic insects collected from streams receiving chlorinated or crude oil wastes

Abstract: SUMMARY. Altered tracheal gills were found on some specimens of Phasganophora capitata (Plecoptera: Perlidae) and net-spinning caddisflies (Trichoptera: Hydropsychidae) collected from streams in New York State. At one location only the basal portion remained of the normally highly-branched structures. At another location the gills were impregnated with a tar-like substance. Chlorine and crude oil are the suspected causal agents.

A comparison of populations of the isopod Asellus aquaticus above and below power stations in organically polluted reaches of the River Trent

Abstract: SUMMARY. Five populations of Asellus aquaticus, subjected to different combinations of temperature and organic pollution in the River Trent, were examined over a period of 18 months. The life-cycle was probably completed in a year or less in all the populations sampled, a larger proportion of the animals completing the life-cycle in less than a year at warmer locations. Young animals also hatched 1 or 2 months earlier at the warmer locations. At the locations subjected to most organic pollution, population densities and the size of animals of both sexes were the largest. The size of ovigerous females, however, appeared to be inversely related to temperature, the smallest occurring at the warmest location. Possible explanations for these differences are given in terms of the amount of food available and the effect of temperature on growth and sexual maturation.

Embryonic duration and post‐embryonic growth rates of the tropical freshwater shrimp Caridina nilotica (Decapoda:Atyidae) under laboratory and experimental field conditions

Abstract: SUMMARY. Embryonic durations and post embryonic growth rates of Caridina nilotica were determined under laboratory conditions at constant temperatures near 18, 24 and 30°C. Embryonic durations and intermoult intervals were negative curvilinear functions of temperature. At a given temperature moulting frequency varied inversely with shrimp size and slight sexual differences were apparent. Moulting frequency of berried females was governed by the temperature-specific embryonic durations. Growth rates were determined from changes in carapace length (CL) of individual shrimps (laboratory) or batches of shrimps (field enclosures) over 1 month and these data were used to calculate temperature-specific life-long growth curves for males and females. Growth in body mass was estimated indirectly from the carapace length-mass relationship of C. nilotica. On average, males grew marginally faster than females during the first 2 months of life, but growth of males larger than CL= 4 mm was considerably depressed relative to that of females. Inflexions in growth rate, apparently related to the onset of sexual maturity, were apparent in both sexes. Under laboratory conditions, the growth rate of males increased with temperature, but temperature-related differences were not as marked in females. Notwithstanding the more rapid moulting rate at 30°C the growth rate of females was slightly slower at 30 than at 24°C as a result of marginally but significantly smaller per moult growth increments observed at 30°C in animals up to CL= 5.5 mm. Possible reasons for this depressed growth are discussed. Growth rates of animals in field enclosures in Lake Sibaya over 1 month in winter (20 ± 3°C) were generally comparable to those estimated for the 18°C laboratory experiments. Growth rates in enclosures containing tripled standing stocks were almost identical to those containing the naturally occurring biomass of animals, suggesting a non-limited environment at least during the time of the experiment.

Calcium balance in premoult and post‐moult Gammarus pulex (Amphipoda)

Abstract: SUMMARY. Adult Gammarus pulex lose about 42% body calcium into solution over a 2–3-day period preceding the moult. A further 54% body calcium is lost with the exuviae, leaving c. 4% in the newly moulted animal. Nearly all of the body calcium in a stage A (post moult) animal is contained in the hepatopancreas and a transient increase in the calcium content of this tissue is seen in some individuals. The haemolymph calcium compartment is heavily depleted at this stage. In stage B animals the hepatopancreas calcium level has returned to, or even fallen below, the intermoult level while the haemolymph calcium concentration remains lower than in intermoult animals. Animals switch from a strongly negative calcium balance to a state of rapid calcium uptake immediately following moult. Uptake at 10°C proceeds at a rate of between 3 and 10 μmol g−1 h−1 depending on the calcium concentration of the external medium. The lower rate is found in starved animals in 0.1 mM calcium and this concentration is probably close to a minimum for the satisfactory restoration of body calcium. Recalcification is completed in 10–14 days in 0.1 mM calcium and is apparently enhanced by the presence of food in the form of a portion of oak leaf together with the cast exuviae. In 1.0 mM calcium the recalcification period is shortened to 3–4 days. This is considerably longer than the time recorded for French populations of this species (Vincent, 1969), although reasons for this are offered.