摘要:

SUMMARY1 Picoplankton community production (0.2–2μm) was investigated over 3 months, June‐September 1991, in Llyn Padarn, a mesotrophic upland lake in north Wales.2 The picoplankton was differentiated into autotrophic algae (<1–3μm) and heterotrophic bacteria (<0.2–1 μm) using differential filtration through a 1 μm pore size Nuclepore filter.3 Efficient separation of these distinct metabolic constituents of picoplankton was obtained. A good correlation (r= 0.81,P3μm) 68.3%.7 Bacteria accounted for about half the picopfankton community production via uptake of EOC. Thus although autotrophic picoplankton were ubiquitous, it is likely that their contribution via primary production to the carbon balance of planktonic environments has be

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00834.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 The changes in abundance ofAsterionella formosain the South Basin of Windermere between 1946 and 1990 are described and analysed. The average seasonal cycle for the 45‐year period shows an overwintering population of about 10 cell ml−1which increases with an exponential rate of 0.09 logeday−1to an annual maximum of 4000 cell ml−1by about Day 124. There is then a rapid decline at an exponential rate of loss of 0.29 logeday−1to values which typically are less than 0.01 cell ml−1in mid‐summer. By about Day 240 a second period of rapid increase occurs with an exponential rate of increase of 0.18 loge. day−1to a plateau of about 7 cell ml−1in late autumn and early winter.2 This average pattern is subject to considerable year‐to‐year variation. The timing and extent of the increase in the autumn was particularly variable. The rate of increase in the spring was strongly positively correlated, and that in the autumn strongly negatively correlated, with the day at which the exponential phase started. Rates for these two phases of increase were not statistically different when expressed in terms of time from mid‐summer, which reinforces earlier conclusions that light availability is the main factor governing the rate of spring increase and suggests that this is also the case for the autumn increase.3 Eight descriptors of seasonal development showed statistically significant changes over the 45 years. Early winter populations declined from 27 to 4 cell ml−1, and linked to this the day at which cell concentrations exceeded 50 cell ml−1occurred later by 24 days from Day 54 in 1946 to 78 in 1990. The lower early winter population appears to be linked to a lower end of year population as this decreased between 1946 and 1968 from 46 to 2 cell ml−1, and then increased slightly to 7 cell ml−1in 1990. The start of the spring exponential increase occurred on Day 57 in 1946 and started earlier by 19 days in 1968 but then occurred later, at Day 76, in 1990. The duration of the spring increase got shorter by 23 days, from 67 days in 1946 to 44 days in 1990. The maximum rate of increase rose from 0.065 logeday−1in 1946 to 0.112 logeday−1in 1990. The annual maximum declined from 9863 cell ml−1in 1946 to 2278 cell ml−1in 1968 and then increased to 6159 cell ml−1in 1990. The annual geometric mean decreased from 61 cell ml−1in 1946 to 5 cell ml−1in 1968 and remained nearly constant subsequently.4 In many cases, the precise underlying causes of these changes were not apparent. However, the increase with time of rate of increase in the spring appeared to be linked to a later start and hence growth under higher light. There was no signific

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00835.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 The interaction of water depth and velocity with size, sex and morphotype of nymphs belonging to the leptophlebiid mayfly genusDeleatidiumwas investigated in a New Zealand river.2 Velocity had a significant effect on distribution such that larger nymphs tended to be found more commonly in faster water. Depth also influenced size distribution, but in different ways at different sites.3 Water velocity or depth alone did not significantly influence the distribution of sexes (1.0 mm head width), but the combined effect of these two factors was significant. Final instar males appeared to be relatively more abundant in slower water than final instar females.4 Nymphs (1.0 mm head width) belonging to the informallilliigroup (apically pointed gills) were present over a wide range of depth and velocity conditions, but those belonging to themyzobranchiagroup (round‐tipped gills) were largely restricted to fast water (>0.9ms−1).5 Several mechanisms thought to cause variations in microdistribution are discussed. These include differential effects of oxygen availability, food requirements and drag forces, nymphal behaviour, and selection of oviposition si

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00836.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Invertebrate herbivory on the submerged macrophytePotamogeton perfoliatuswas studied in a Danish lowland stream during the main growth seasons of summer 1991 and 1992. Young apical leaves escaped consumption probably because of their location and dense packing, but herbivory loss increased linearly with age and exposure time of older leaves. Nitrogen content was relatively high in both young (4.61% DW) and old leaves (3.65% DW) but a food preference experiment showed that young leaves were preferred by the main herbivore, the trichopteranAnabolia nervosa.2 The percentage of standing plant biomass lost to herbivory (the apparent loss) between sampling periods increased from early May to a mid‐June maximum (24.8% in 1991, 4.2% in 1992), and subsequently declined to zero within a month. The averages for the growth seasons were 10.5% in 1991 and 2.0% in 1992. Yet, the proportions of annual plant production harvested by herbivores were low and almost the same (1.3 and 1.8%) because consumption was low when plant production peaked in late summer.Consumption was almost the same early in the two years, but plant growth dynamics differed markedly and was the main factor determining apparent herbivory loss.3 Despite heavy damage early in the summer,P. Perfoliatuscontributed a minor fraction (1–5%) of the trichopteran diet. This fraction appeared to be restricted by the low macrophyte biomass during early summer.4 The results emphasize that apparent herbivory loss does not estimate the harvested proportion of plant production, and that plant growth dynamics should be analysed to attain precise estimates of herb

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00837.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Phosphorus enrichments of 5 weeks' duration were performed in two woodland streams in eastern Tennessee, U.S.A. to determine their effect on the dominant stream bryophyte,Porella pinnata.2 In a second‐order reach of Walker Branch, which had an N:P ratio (by atoms) of 3.5:1 in the stream water, the P:C ratio ofPorellawas not significantly affected by enrichment, but the P:N ratio did increase significantly. In Sludge Creek, which had an N:P ratio of 21.6:1 in the stream water, both P:C and P:N ratios ofPorellaincreased significantly following phosphorus addition. Increased phosphorus ratios may have resulted from either assimilation or adsorption.3 Absolute phosphorus concentrations inPorellatissue were significantly greater prior to enrichment in the control reach of Walker Branch, confounding the effect of phosphorus enrichment. In Sludge Creek, absolute phosphorus content inPorellatissue increased significantly in the treated reach and showed no significant difference in the control reach.4 Although mean primary production increased by approximately 15% following enrichment, the increase was not significantly different from that prior to enrichment.5 Epiphyte structure and abundance were not significantly influenced by enrichment in either stream. It is suggested that grazing pressure by snails may have masked any potential epiphyte response to

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00838.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Decomposition of the leaves ofNymphaea albaL. andBetula pubescensEhrh. was studied in two ponds of contrasting pH, buffering capacity and trophic level. Rates of carbon loss, concentrations of nitrogen, protein and phenolics and colonization of the leaf material by macroinvertebrates were studied.2 Decomposition ofB. pubescensleaves was slower than that of N.albain both ponds. Protein and phenolic concentrations were higher in the decomposing N.albamaterial, whereas no significant difference was found for nitrogen concentrations.3 Decomposition of both species was slower in the acid water. The effect of acid water on decomposition rate was similar for both species. Both nitrogen and protein concentrations of N.albaremained higher under acid conditions, whereas no effect of study site was observed on the protein and nitrogen concentrations inB. pubescenslitter. The concentration of phenolic compounds was higher inN. albalitter from the acid pond; no such difference was observed forB. pubescenslitter. The high concentrations of phenolic compounds in N.albalitter from the acid pond probably induced chemical immobilization (tanning reaction), yielding high concentrations of nitrogen and protein in the litter.4 Diversity of the detritivorous macroinvertebrate fauna on the litter of both species was extremely low in the acid pond, while a relatively high diversity occurred in the alkaline water. Exclusion of large macrofauna inhibited decomposition in the alkaline water, while there was no comparable influence in the acid water.5 The effects of low pH on the decomposition of both terrestrial and aquatic macrophyte litter appeared to be caused by interactive effects of low pH on the microbial decomposition, on the detritivorous macroinvertebrates and, in the case ofN. alba, on the resource quality of the

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00839.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Life history characters (body size of adults, egg diameter, egg sac length and breadth) of nineteen species of central European cyclopoid copepods were measured and sexual size dimorphism (adult female length x adult male length−1), relative egg size (egg weight X body weight−1), weight of adult females and of eggs, egg sac shape (egg sac length x egg sac breadth−1), and reproductive effort (clutch weight produced per female weight per day) were calculated to detect trends in life history strategies.2 Typical planktonic species exhibited the lowest reproductive effort. Among planktonic species, the value for egg sac shape increased with clutch size.3 Large species and small species exhibited different trends in life history characters. Large species had larger clutches, larger eggs, and a greater sex size dimorphism than small species. However, small species had a greater relative egg size.4 Large species live in cold water and reproduce during the spring bloom of phytoplankton where the production of large clutches with relatively small eggs is advantageous. Reserves are unnecessary for juveniles because food is abundant. Small species generally are most abundant during the warm season, when conditions are less predictable, and relatively large eggs, possibly provided with reserves, are

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00840.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Holopedium gibberum, from twenty lakes in Rhode Island and Maine, were examined for allozyme variation at five loci to determine the pattern and degree of generic variation among sites and the genetic structure within individual lakes.2 There were significant differences in allele frequencies among sites. Most populations were fixed for a particular allele at each locus. Only five lakes had polymorphic populations.3 Polymorphic populations showed significant deviation from expected Hardy‐Weinberg genotype frequencies. In each case, there was an excess of homozygotes.4 Two lakes were examined for intra‐lake allele frequency differences. In one lake there were no differences. The other lake exhibited significant allele frequency differences between stations at the north and south ends of the lake.5 Populations were examined for the frequency distribution of composite genotypes over three loci. Most populations were dominated by one or two genotypes.6 The results suggest sporadic sexual recruitment and a high degree of genetic isolation among these populations ofH. gibberum. In these respects they resemble the permanent pond populations ofDaphnia magnaexamined by

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00841.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Dry weight, body length and spine length were measured for the exotic cladoceranBythotrephes cederstroemiicollected from offshore and inshore stations in southeastern Lake Michigan. Average dry weight of each developmental stage exhibited seasonal variation by a factor of more than 5.2 Mean dry weight ofBythotrepheswas closely correlated with water temperature. Contrary to the inverse relationship between water temperature and body size frequently observed for other invertebrates, the dry weight ofBythotrephesincreased at higher ambient temperatures.3 No significant correlation was observed between abundances of major zooplankton taxa and the dry weight ofBythotrephes. An indirect effect of temperature on prey consumption may cause seasonal variation in dry weight ofBythotrephesin Lake Michigan.4 Distances between adjacent pairs of barbs, added to the caudal spine with each moult, are significantly shorter inBythotrepheswhich produce resting eggs. Less material investment in the exoskeleton of sexually reproducing females was observed in favour of growth and repro

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00842.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SUMMARY1 Naupliar and copepodid development times (DnandDc, respectively) of two African freshwater calanoids (Metadiaptomus meridianusandTropodiaptomus spectabilis) were measured on mono‐specific diets of comparably sizedChlamydomonas reinhardii, Scenedesmus acutus, Cryptomonassp.,Rhodomonas minuta, Cyclotella meneghiniana, andSelenastrum capricornutum, to test the nutritional adequacy of these algae. Comparisons were made at a standard temperature (17°C) and food supply level (1 mgCl−1).2 All diets other thanScenedesmusandSelenastrumsupported complete naupliar development at broadly comparable times within and between calanoids, apart from greatly protractedDnvalues forM. meridianusonCyclotella. Dcdurations were more variable between diet types, and bothChlamydomonasandCyclotellawere inferior or inadequate for copepodid development.3 Both naupliar and copepodid stages ingested radiolabelledScenedesmusandSelenastrumreadily. Comparative incorporation rate measures ofSelenastrumandCryptomonasrespectively exceeded estimated metabolic maintenance needs of stage 3/4 nauplii ofT. spectabilisby some 56% and 790%. Scope for growth (‘surplus’ energy) was accordingly fourteen‐fold greater onCryptomonasthan onSelenastrum /Scenedesmus. The dietary inadequacy of these two green algae is thus attributed largely to low digestibility, and perhaps some bioche

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1994.tb00843.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY