Mean Annual Biomass Research Articles

Spatial and Temporal Patterns of Factors Influencing Phytoplankton in a Salt Wedge Estuary, the Swan River, Western Australia

During 1995 the phytoplankton in the Swan River were intensively sampled to assess biomass and species composition. Continuous measurements of fluorescence, salinity, and temperature were made weekly during 40 km sampling trips along the estuary and used to map the seasonal progression of the algal biomass. Weekly measurements of primary production were made and used to model net primary production from the vertical distribution of biomass, irradiance, and phytoplankton species composition. Potential nutrient limitation was assessed with “all but one” nutrient bioassays. The results indicate a complex mixture of potentially limiting factors, which vary in time and space. Although the data sequence is short, it suggests a annual succession pattern of diatoms, chlorophytes, diatoms, and finally dinoflagellates and cryptophytes in late summer-autumn. Peak seasonal biomass was observed during January to April. Mean annual chlorophylla biomass was greatest in upstream stations (5–9), where estimates of net primary production rates averaged 1.55 g C m−2 d−1 and gross primary production was 800–1000 g C m−2 yr−1. Potential nutrient limitation was most severe from November to May, although not during January 1995. Based on bioassay results, during the period of greatest potential for nutrient limitation, nitrogen was 15 to 30 times more limiting to biomass development than phosphate. Runoff due to consistent rainfall during winter eventually breaks down stratification and flushes the estuary with low-salinity, nutrient-rich water, producing, a light-limited, nutrient-rich aquatic ecosystem. Timing and magnitude of physical forcing events, mainly rainfall, appear critical in determining the susceptibility of this ecosystem to summer and autumn algal blooms.

Temporal Variability of Stream Macroinvertebrate Abundance and Biomass Following Pesticide Disturbance

We determined the extent of macroinvertebrate recovery in a former pesticide-treated stream (FTS) relative to a reference stream (RS) by examining macroinvertebrates colonizing red maple (Acer rubrum L.) litter bags between 5 to 10 y following pesticide treatment. Mean abundance and biomass, variability in abundance and biomass (using the coefficient of variation [CV]), and assemblage structure were compared both within and among years to assess recovery. The 5 y of study included 3 drought years followed by 2 wet years. Mean total abundance and biomass of macroinvertebrates, and that of most functional feeding groups (FFG) did not significantly differ between streams during this study, nor did within-year variability of these means, indicating macroinvertebrates in FTS had recovered relative to RS. Some exceptions to the above patterns (shredder abundance and mean annual gatherer biomass) resulted from the dynamics of a single taxon in each group (Leuctra and Paraleptophlebia, respectively). Macroinverte...

The fine sand Abra alba community of the bay of morlaix twenty years after the Amoco Cadiz oil spill

The fine sand Abra alba community from the Bay of Morlaix (western English Channel) was strongly affected by the Amoco Cadiz oil spill of April 1978. The long term changes in the community (1977–1996) show that reconstitution of this community is slow (over 10 yr). A progressive recolonization by amphipod Ampelisca populations constituting the dominant species is observed. The results show that it is necessary to survey the affected communities for a long period of time ( > 10 yr) after an event to identify the real ecological impact of an oil spill. The abiotic and biotic factors existing in the Bay of Morlaix are favorable to the establishment and the persistence of stable dominant Ampelisca populations in time. The ‘climax’ concept seems to be applicable te, this type of stable environment. The soft-bottom Abra alba community of the Bay of Morlaix may only support a maximal carrying capacity of approximately 10–11 g m −2 (mean annual biomass in decalcified dry weight). The deficit of production during 11 annual cycles may have affected the fish biomass feeding on this community.

Long‐term study of the ecology of wild Atlantic salmon smolts in a small Norwegian river

Backcalculated lengths at the end of the first growth season in wild Atlantic salmon Salmo salar differed significantly between parr smolting at age 1, 2 and 3 years over a period of 11 years (i.e. 1983–1993). Mean body lengths of the respective age groups at the end of the first growth period were 11·1, 6·2 and 4·7 cm, respectively. The mean percentage distribution of fish smolting at age 1, 2 and 3 was 14, 78 and 7%, and the mean smolt age was 1·95 years. Mean lengths at smolting of age groups 1, 2 and 3 were 13·6, 15·8 and 17·5 cm, respectively. Females outnumbered males among the downstream migrating smolts with a mean sex ratio (females/ males) estimated at 1·61, with a significant female surplus in 7 of the 11 years sampled. Of the smolts sampled, 14% exhibited enlarged gonads indicative of parr maturation, and all were males (37% of the parr males sampled). Mean annual smolt density from 1975 to 1996 was 13·4 individuals 100 m−2 ranging between 0·3–31 smolts 100 m−2. Mean densities (100 m−2) of the smolts aged 1, 2 and 3 years were 1·5, 9·3 and 0·9 fish, respectively. Mean annual biomass for the 22‐year period (1975–1996) was estimated at 437 g 100 m−2, with a range of variation from 136 to 683 g 100 m−2. Smolt age 2 made up 81% of the mean annual biomass (355 g 100 m−2) and smolt age 1 and 3, 8% (35 g 100 m−2) and 11% (47 g 100 m−2), respectively.

Distribution and Sex Ratio of Ethmolaimus Pratensis De Man, 1880 (Nematoda, Chromadorida) in an Oligotrophic Lake

The distribution of Ethmolaimus pratensis (n = 1627 individuals), a common nematode species of Konigssee, was studied throughout 1986. E. pratensis comprised 3.5% of all nematodes in the littoral, 3.1% in the littoriprofundal and 5.5% in the profundal. The species contributed to a considerable portion of nematode biomass in Konigssee with mean annual biomass being highest at 1 m depth (0.14 mg/100 cm 2 ww; 17% of total nematode biomass) and 2 m depth (0.13 mg/100 cm 2 ww; 22% of total nematode biomass). At 190 m depth it was 40% of total nematode biomass. Population density of E. pratensis peaked three times a year at 1 m and 2 m water depth (spring, summer, winter) and twice in deeper regions. There was considerable water depth and time dependent variation in sex ratios of E. pratensis. In littoral and littoriprofundal zones the sex ratio was about 1 : 1, while in the profundal males were very rare. About 48% of E. pratensis individuals were found in the top sediment layer and the percentage increased with increasing water depth. Juveniles and adults of E. pratensis were partially segregated in the sediment column. Morphometric data revealed negligible sexual dimorphism in size, body shape and weight.

Interannual variations in the population biology and productivity of Thysanoessa spinifera in Barkley Sound, Canada, with special reference to the 1992 and 1993 warm ocean years

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 173:181-195 (1998) - doi:10.3354/meps173181 Interannual variations in the population biology and productivity of Thysanoessa spinifera in Barkley Sound, Canada, with special reference to the 1992 and 1993 warm ocean years R. W. Tanasichuk* Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6, Canada *E-mail: tanasichukr@dfo-mpo.gc.ca ABSTRACT: I monitored the population structure, abundance and growth, as well as the reproductive and production characteristics of Thysanoessa spinifera in Barkley Sound from March 1991 through March 1997. The years 1992 and 1993 were anomalously warm, 1992 due to an El Niño Southern Oscillation (ENSO). Summer upwelling was particularly strong in 1992 and downwelling in the winter of 1994-95. Mean annual larval (<10 mm) abundance was 2.4, 0.6, 0.4, 6.3 and 1.9 times greater in 1992 through 1996 respectively than in 1991. Adult abundance declined steadily since 1991 and in 1996 was about 0.14 of the pre-ENSO levels. Growth conditions, as indicated by the condition factor, varied seasonally but not interannually. There were no apparent interannual differences in length-at-maturity, sex ratio and spawning season length. I found no apparent relationship between parental and larval abundances. Indications are that the number of spawnings declined as of 1993. Mean annual larval biomass fluctuated. Biomass ranged between 0.19 (1994) and 6.9 (1996) times the 1991 value. Total production (growth + moult) was between 0.24 (1994) and 4.9 (1996) the 1991 value. Adult biomass and productivity fell continuously after 1991 and, in 1996, were about 0.13 of the pre-warm ocean years. Population P:B ratios fluctuated between 14.4 and 44.7, with the highest ratio in 1996. KEY WORDS: Thysanoessa spinifera · Euphausiid · Population biology · Productivity · Ocean variability Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 173. Publication date: November 12, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use

Soil microarthropods were sampled every 3 months for 1 y at 10 sites in the northern Netherlands, varying in soil type and land use. Microarthropods were divided into seven functional groups and biomass-C ha −1 was calculated for the top 10 cm soil layer. The four quantitatively principal functional groups were: cryptostigmatic mites; non-crytostigmatic mites; predatory mites; and omnivorous collembola. Possible relationships between the mean annual biomass of these groups and soil type, land use or soil organic matter were studied. Microarthropod biomass was larger in sandy soil than in loamy and generally larger in meadows than in wheat fields; the mineral layer of forest soils being intermediate. Non-cryptostigmatic mite, omnivorous collembola and predatory mite biomass showed strong positive correlations. Cryptostigmatic mite biomass correlated with lower organic matter input quality, while omnivorous collembola and non-cryptostigmatic mites showed a positive correlation with the amount of input. Omnivorous collembola were negatively affected by a discontinuous input of organic matter to the soil. We found relationships between functional group biomass and either soil organic matter density fractions or soil pore size distribution only when the grassland sites were analyzed separately. Both analyses showed correlation patterns for cryptostigmatic mites to deviate from those of the other three main functional groups. Cryptostigmatic mites showed a positive correlation with the lightest organic matter density fraction, while the non-cryptostigmatic mites and omnivorous collembola were correlated to the heavier fractions. The cryptostigmatic mites correlated with the 6–90 μm pore size class, while the other three groups showed strong correlation to the 1.2–6 μm, as well as the largest (> 90 μm) pore size class. Both observations lead to the conclusion that omnivorous collembola and non-cryptostigmatic mites are related to fungal growth (in the largest pores and on the heavy organic matter fraction), while the cryptostigmatic mites show a more saprovorous feeding mode.

Annual Changes of Abundance and Biomass of Planktonic Ciliates in the Gdańsk Basin, Southern Baltic

AbstractSeasonal changes in the species composition, abundance and biomass of planktonic ciliates were determined every 2–3 weeks at two sites of 30 m depth and one location of 105 m depth in the southwestern Gdańsk Basin between January 1987 and January 1988. A total of 40 ciliate taxa were observed during this period. Autotrophic Mesodinium rubrum dominated ciliate abundance and biomass: maximal values of 50 · 10−1 ind. 1‐1 and 65 μg C 1−1 were recorded. The annual mean biomass of M. rubrum comprised 6 to 9% of the annual mean phytoplankton biomass. The highest abundances and biomasses of heterotrophic ciliates were noted at all stations in the spring and summer in the euphotic zone with maximum values of 28 · 103 ind. 1−1 and 23 μg C 1−1. Three ciliates assemblages were distinguished in the epipelagic layer: large and medium‐size non‐predatory ciliates, achieving peak abundance in spring and autumn; small‐size microphagous ciliates and epibiotic ciliates which were abundant in summer, and large‐size predacious ciliates dominating in spring. Below 60 m, a separate deep‐water ciliate community composed of Prorodon‐like ciliates and Metacystis spp. was found. The ciliate biomass in the 60–105 m layer was similar to the ciliate biomass in the euphotic zone. The heterotrophic ciliate community contributed 10 to 13% to the annual mean zooplankton biomass. The potential annual production of M. rubrum comprised 6 to 9% of the total primary production. Carbon demand of non‐predatory ciliates, calculated on the basis of their potential production, was estimated to be equivalent to 12–15% of the gross primary production.

Consumption of benthic fauna by carnivorous birds in the Wadden Sea

Consumption by carnivorous birds was estimated for the Sylt-Romo tidal inlet in the northern part of the Wadden Sea, as well as the subarea Konigshafen, a small, tidal bay. The bird community of the Sylt-Romo Wadden Sea was dominated by Dunlin (35% of all birds counted), Eider (9%), Oystercatcher (8%), Knot (8%), and Shelduck (7%). The community in the Konigshafen was dominated by Eider (20%), Knot (17%), Bar-tailed Godwit (17%), Dunlin (13%), and Oystercatcher (8%). Annual consumption was estimated at 3.4 g AFDW · m−2 · year−1 for the entire Sylt-Romo Wadden Sea and 19.2 g AFDW · m−2 · year−1 for the Konigshafen. Restricting the calculations to the intertidal area resulted in a consumption of 8.7 g AFDW · m−2 · year−1 for the Sylt-Romo Wadden Sea and 17.6 g AFDW · m−2 · year−1 for the Konigshafen. In the two areas, consumption was dominated by the Eider with 37% and 60% of the total consumption, respectively. In comparison to the western parts of the Wadden Sea the seasonal pattern of consumption as well as species composition differed, most probably as an effect of different climatic conditions, whereas annual consumption on intertidal flats seems to be in the same order of magnitude. On average, 15–25% of the mean annual macrozoobenthic biomass seems to be taken by carnivorous birds in the Wadden Sea, which is in the same order of magnitude as in other northern temperate estuarine areas.

Life histories, secondary production and microdistribution of hydropsychid caddisflies (Trichoptera) in a tropical forest stream

The ecology of seven species of hydropsychid caddisflies was investigated in Tai Po Kan Forest Stream (TPKFS), New Territories, Hong Kong, over a two-year period (1977–79). Quantitative benthic samples were taken from different microhabitats in the stream on 47 occasions and, in combination with light-trap catches of adult caddisflies, were used to investigate life-history patterns and estimate secondary production. Cheumatopsyche spinosa (141.9 individuals 0.5 m-2; 63% of total hydropsychid densities) and C. ventricosa (54.7 individuals 0.5 m-2; 24%) were the most abundant hydropsychids in TPKFS, followed by Macrostemum fastosum (15.2 individuals 0.5 m-2; 7%). Hydatopsyche melli, Herbertorossia quadrata, Polymorphanisus astictus and Hydropsyche chekiangana were present at relatively low densities (each < 2% of total hydropsychid abundance). Macrostemum fastosum made up 42% (mean value 1977–79: 40.0 mg AFDW 0.5 m-2) of total hydropsychid biomass. Hydatopsyche melli was ranked second (13.3 mg AFDW 0.5 m-2; 14%), followed by C. spinosa and C. ventricosa. There was little inter-year variation in total hydropsychid standing stocks: densities (mean of all species combined) were 7% higher in 1978–79 than in 1977–78 (mean value 1977–79: 224.8 individuals 0.5 m-2), while total biomass was only 3% greater (mean value 1977–79: 96.1 mg AFDW 0.5 m-2). However, mean densities and biomass of C. ventricosa and P. astictus were higher in 1977–78, while Herbertorossia quadrata, Hydatopsyche melli and Hydropsyche chekiangana exhibited the converse pattern. Standing stocks of M. fastosum and C. spinosa were relatively stable between years. Total hydropsychid abundance in TPKFS followed an annual pattern of wet-season decrease and dry-season increase, but this tendency was more apparent in some species (e.g. C. spinosa) than others. Macrostemum fastosum was most numerous following recruitment during the wet season. Cheumatopsyche spp. and Herbertorossia quadrata had bivoltine life histories while P. astictus and M. fastosum were univoltine; the latter grew rapidly to the final instar and then spent several months increasing in weight before emergence. Hydatopsyche melli and Hydropsyche chekiangana were probably univoltine. Total hydropsychid annual production over the period 1977–79 was 537.0 mg AFDW 0.5 m-2, and was 11% higher during 1978–79 than in 1977–78. Inter-year variations in production reflected fluctuations in annual mean biomass: thus production of P. astictus and C. ventricosa was higher in 1977–78, and production of Herbertorossia quadrata, Hydatopsyche melli and Hydropsyche chekiangana was greater in 1978–79. Production of M. fastosum, which constituted 35% of the total, was relatively stable between years. Species-specific annual production during 1977–79 was in the order M. fastosum > C. spinosa > C. ventricosa > Hydatopsyche melli > Herbertorossia quadrata > P. astictus > Hydropsyche chekiangana. Production:biomass (P:B) ratios varied from 3.59 (Hydatopsyche melli, 1977–78) to 25.46 (Herbertorossia quadrata, 1977–78), and P:B ratios (1977–79) decreased in the order C. ventricosa > C. spinosa > Herbertorossia quadrata > M. fastosum > Hydropsyche chekiangana > P. astictus > Hydatopsyche melli (Table II). Bivoltine species had the highest P.B ratios. All TPKFS hydropsychids (except P. astictus) were significantly more abundant in midstream microhabitats than close to the stream banks. This pattern was most marked in Hydatopsyche melli and Hydropsyche chekiangana. Multivariate statistics revealed that hydropsychid microdistribution was influenced significantly by sediment grain-size characteristics, although algae or detritus also had an influence on all species except Herbertorossia quadrata. Despite the statistical significance of best-fit regression models, the proportion of variation in the abundance of each species explained by the independent variables did not exceed 23% (in C. spinosa), and was typically much less.

Temporal changes in secondary production of a population of the subtidal sand snail Umbonium costatum in Hakodate Bay, northern Japan: importance of annual change in age structure

Year-to-year changes in age structure, biomass ( B), annual secondary production ( P) and P B ratio are described of a population of the subtidal snail Umbonium costatum in Hakodate Bay, northern Japan, during a 6-y period (1982–1988). Population structure and values of biomass and production were highly variable from year to year; the ranges of the annual mean biomass, annual production and P B ratio were 3.71–9.22 g dry tissue m −2, 1.01–4.92 g dry tissue m −2 y −1 and 0.13–1.33 y −1 respectively. Change in the age structure was the most important single factor affecting temporal changes in annual production in this population. The annual production of the population was high when young individuals, which have a small body size and high growth rate, dominated the population. While annual P B ratios in 1983 and 1984 fell within the range of values reported for various other gastropods, those in 1985, 1986, 1987 and 1988 were markedly lower, if the relation between the population P B ratio and life span is taken into account. This demonstrates that production estimates from annual biomass and life-span values may lead to incorrect results in a recruitment-limited population.

Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 13:275-284 (1997) - doi:10.3354/ame013275 Comparative effects of benthic microalgae and phytoplankton on dissolved silica fluxes Sigmon DE, Cahoon LB Microalgal effects on dissolved silica fluxes in an estuarine sandflat were investigated over an annual cycle. Benthic microalgal and phytoplankton chlorophyll a were measured at 17 times in Masonboro Sound, NC, USA, between June 1994 and May 1995. Using light and dark benthic chambers, dissolved silica (DSi) concentrations were measured in situ and used to calculate flux rates. Mean annual biomass was 64.6 and 10.0 mg chl a m-2 for benthic microalgae and phytoplankton, respectively. Benthic microalgae and phytoplankton (both containing diatoms), and cultured benthic diatoms were used to calculate Si:chl a weight ratios. Ratios of Si:chl a showed that benthic microalgae contain more silica per unit chlorophyll than phytoplankton, with mean Si:chl a ratios of 14.3, 10.9, and 2.8:1, for benthic microalgae, cultured benthic diatoms, and phytoplankton, respectively. DSi flux rates were always higher in the dark, while some negative flux rates (into the sediment) were seen in the light. DSi flux rates averaged 81.0 μmol m-2 h-1 in the dark and 37.0 μmol m-2 h-1 in the light. Lab and field experiments showed that benthic microalgae can take up water column DSi, while typical phytoplankton concentrations exert little control of water column DSi. Temperature was found to be the major regulator of DSi fluxes, with flux rates decreasing with decreasing temperature. Benthic microalgae were significant regulators of light chamber DSi flux at temperatures <20°C. Due to benthic microalgal uptake and low water column DSi concentrations observed under lower temperatures, nutrient limitation experiments were performed to determine if phytoplankton were silicate-limited. Of the 3 major nutrients, nitrogen, phosphorus, and silicon, only silicon was limiting for phytoplankton growth in every experiment. Benthic microalgae are important regulators of DSi flux, and can regulate water column DSi concentrations for at least half of the year, possibly limiting phytoplankton growth and species composition in shallow, coastal environments. Diatoms · Benthic microalgae · Phytoplankton · Dissolved silica · Sediments · Flux rates Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 13, No. 3. Publication date: September 11, 1997 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1997 Inter-Research.

Trout Production Dynamics and Water Quality in Minnesota Streams

We sampled fish assemblages and quantified production dynamics of brook trout Salvelinus fontinalis, brown trout Salmo trutta, and rainbow trout Oncorhynchus mykiss in 13 southeastern Minnesota streams during 1988–1990 to examine the influence of water quality on fish populations in fertile trout streams. Fish assemblages in 15 stream reaches were abundant, but low in diversity; 13 species were collected. Parameter means (ranges) over the reaches were species richness, 4.1 (1–8); density, 29,490 (1,247–110,602) fish/ha; and biomass, 253.5 (49.6–568.6) kg/ha. Means (ranges) for salmonids were annual mean density, 2,279 (343–8,096) fish/ha; annual mean biomass, 162.0 (32.5–355.5) kg/ha; and annual production, 155.6 (36.7–279.6) kg/ha. Salmonid production and mean biomass were greater during the spring-fall interval than during fall-spring; young cohorts (ages 0–1) contributed the greatest proportion to population biomass and production. Salmonid annual production-to-mean-biomass ratio (P/B) averaged 1.06 (0.64–1.42), and means were significantly different among species (1.03 for brown trout, 1.54 for brook trout, and 1.92 for rainbow trout). A significant linear model was developed that describes P/B as an inverse function of population age structure and may be used to improve accuracy in approximations of annual production from mean biomass. Fish density, biomass, or production were not correlated with eight water quality variables describing ionic and nutrient content in these streams, but when data from other United States streams with a wide range in alkalinity were incorporated, salmonid production was strongly, positively correlated with alkalinity. The wide range in fish population and production statistics and their lack of correlation with water quality suggest that no uniform fish carrying capacity exists among these streams and that factors other than water fertility limit fish density, biomass, and productivity at this spatial scale, but the overall maximum production rate in the region may be governed by water quality.

Production Dynamics of Hyalella azteca (Amphipoda) among Different Habitats in a Small Wetland in the Southeastern USA

The life history, population dynamics, and secondary production of Hyalella azteca (Amphipoda) were studied in a small southeastern wetland pond. Habitat-specific sampling was used to estimate density, biomass, and production in each of 3 habitats (benthic, Nymphaea odorata leaves, and submerged wood). Laboratory experiments at different temperatures (10, 15, 20, 25, and 30°C) provided predictive equations for estimating growth rate and egg development times that could be applied to field populations. Size-frequency data from field sampling suggested that H. azteca had 2 major cohorts per year, but reproduction was continuous from spring through fall. Mean annual density and biomass were 904/m² and 85 mg dry mass (DM)/m². Daily growth rate was a function of both individual mass and temperature. Annual production of somatic tissues as estimated by the instantaneous growth method (675 mg DM· m^-2· y^-1) agreed well with that using the size-frequency method (714 mg DM· m^-2· y^-1). Egg production (252 mg DM· m^-2· y^-1) represented 27% of total production (927 mg DM· m^-2· y^-1). Annual production:biomass (P/B) was 7.8 without eggs and 9.8 with eggs. Habitat-specific estimates of density, biomass, and production were highest in the benthic habitat. The contribution of somatic production from Nymphaea leaves to total amphipod production was low (6.2%), but temporal patterns of production followed trends in leaf abundance. Production on leaves reached 29% in September when leaves were most abundant. Contributions of animals from submerged wood were consistently low (2.9%). Monthly densities and production peaked in May at the start of the summer cohort, decreased throughout summer, and showed a small increase in October at the start of the winter cohort. Although egg production and birth rates were highest in summer months, high death rates were responsible for the late-summer declines in density and production. These temporal trends in production suggest heavy predation or environmental stress, or both, during summer months.

Mesozooplankton biomass distribution in the upper 100 m layer of the Atlantic sector of the Southern Ocean

The distribution of annual mean biomass of mesozooplankton (animals in the size range 0.2–20 mm) in the upper layer (0–100 m) of the Atlantic sector of the Southern Ocean was mapped. The map was constructed from 4216 measurements at 2184 oceanographic stations, from the Russian and Brazilian archive records, from published sources, and from the British Oceanographic Data Centre. The annual mean mesozooplankton biomass value for the South Atlantic is estimated in this study to be 93 mg m−3 wet weight.

Life histories, secondary production, and microdistribution of heptageniid mayflies (Ephemeroptera) in a tropical forest stream

The ecology of five species of heptageniid mayflies was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two‐year period (1977‐79). Quantitative benthic samples were taken from different microhabitats in the stream on 47 occasions and, in combination with light‐trap catches of adult mayflies, were used as a basis for investigating life‐history patterns and estimating secondary production. Electrogena sp. (49.8 individuals 0.5 m−2; 60% of total heptageniid densities) and Cinygmina sp. (15.7 individuals 0.5 m−2) were the most abundant heptageniids in TPKFS, followed by Epeorus sp. (9.1 individuals 0.5 m−2). Iron sp. and Paegniodes cupulatus, which were present in relatively low densities, made up &lt; 10% of total heptageniid abundance. In terms of mean biomass (1977‐79), Electrogena sp. and Cinygmina sp. were equally important (10.3 vs. 10.5 mg ash‐free dry weight (AFDW 0.5 m−2), while Epeorus sp. was ranked third (6.8 mg AFDW 0.5 m−2). There was, however, considerable inter‐year variation in heptageniid standing stocks: mean densities and biomass of Cinygmina sp. in 1977‐78 were (respectively) eight and nine times greater than in 1978‐79, while Iron sp. and P. cupulatus exhibited the converse trend, with densities more than twice as high and biomass more than three times greater in 1978‐79. Heptageniid abundance in TPKFS seemed to follow an annual pattern of wet‐season decrease and dry‐season increase, but this tendency was more apparent in some species (e.g. Iron sp.) than others (e.g. Cinygmina sp.). The life histories of TPKFS heptageniids were unclear. All species showed rather stable larval size‐frequency distributions throughout the study period, which was indicative of asynchronous growth and continuous recruitment. Adult flight periods did not exhibit strong seasonality. Calculations of secondary production were based upon the conservative assumption that each species was univoltine, but multivoltine life histories may have been possible for these tropical mayflies.Total heptageniid annual production over the period 1977‐79 was 212.0 mg AFDW 0.5 m−2, and was slightly higher during 1977‐78 than in 1978‐79 (225.0 vs. 202.1 mg AFDW 0.5 m−2). Inter‐year variations in production reflected variations in annual mean biomass: production of Cinygmina sp. was 7.9 times greater in 1977‐78 than in 1978‐79, but production of Iron sp. was 2.7 times higher during 1978‐79, and that of P. cupulatus was 4.6 times greater. As a consequence of these inter‐year variations, species‐specific production decreased in the order Cinygmina sp. &gt; Electrogena sp. &gt; Epeorus sp. &gt; Iron sp. &gt; P. cupulatus in 1977‐78, but the sequence was Electrogena &gt; Iron sp. &gt; Epeorus sp. &gt; P. cupulatus &gt; Cinygmina sp. in 1978‐79. For the study period as a whole (1977‐79), the order was Electrogena sp. &gt; Cinygmina sp. &gt; Epeorus sp. &gt; Iron sp. &gt; P. cupulatus. Production:biomass (P:B) ratios varied from 4.8 (Cinygmina sp., 1977‐78) to 11.1 (Iron sp., 1977‐78), and P:B ratios (1977‐79) decreased in the order Iron sp. &gt; Epeorus sp. &gt; Electrogena sp. &gt; P. cupulatus &gt; Cinygmina sp. Production calculations and P:B ratios are highly sensitive to the conservative assumptions made about heptageniid voltinism in TPKFS, and therefore may be significant underestimates. Epeorus sp., Iron sp., and P. cupulatus were most abundant in midstream microhabitats but Cinygmina sp. was more numerous close to the stream banks. The microdistribution pattern of Electrogena sp. was less distinct, but larvae seemed to favour midstream microhabitats. Multivariate statistics revealed that the microdistribution of all heptageniids (except Electrogena sp.) was influenced significantly by sediment grain‐size characteristics rather than by algae or detritus, but the proportion of variation (4‐14%) in the abundance of each heptageniid species accounted for by sediment characteristics was low.

The Effects of Nitrogen, Potassium and Phosphorus Addition on the Vegetation of a Somerset Peat Moor Under Cutting Management

1. A range of nitrogen (N), phosphorus (P) and potassium (K) fertilizer treatments was applied for 4 years in a randomized block experiment to a species-rich hay meadow on peat soil, within the Somerset Moors and Levels Environmentally Sensitive Area. 2. The percentage composition of each species present was recorded in May and October each year on plots cut for hay, followed by further cuts of aftermath growth. The effects on species richness, diversity and dominance were analysed, and ordination techniques were used to investigate the effects of fertilizers on plant community composition. Data for mean annual biomass production are also presented. 3. Botanical results were compared with those of a concurrent experiment where aftermath growth was grazed. 4. P was more important than N in determining both biomass production and botanical change. In both cases, the effects were small when substantial amounts of N and K were applied without P, but when high rates of P were included biomass increased very significantly and species diversity was severely reduced, with Holcus lanatus, Rumex acetosa and Lolium perenne dominating. 5. Lolium perenne was not increased by N and modest rates of P in the absence of aftermath grazing, but dominated all fertilized plots when aftermath grazing was maintained. Agrostis canina came to dominate plots receiving 200 kg ha -1 of N with modest rates of P and K in the absence of aftermath grazing, but was negatively associated with N where the aftermath was grazed. 6. Trifolium pratense became very abundant where P and K were applied with nil or 25 kg ha -1 of N each year, both with and without aftermath grazing, but all legumes were suppressed at high rates of N, particularly in conjunction with high P. 7. Substantial botanical change occurred on unfertilized plots as a result of the cessation of aftermath grazing. These plots became dominated by Plantago lanceolata, with significant increases in Leontodon hispidus and L. autumnalis.

Light limitation of phytoplankton development in an oligotrophic lake ‐ Loch Ness, Scotland

SUMMARY The underwater light climate in Loch Ness is described in terms of mixing depth (Zm) and depth of the euphoric zone (Zeu). During periods of complete mixing, Zm equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient Zm/Zeu is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth. Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m‐3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch. The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light‐saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm‐3) and maximum photosynthetic rate (1–4 mg C mg Chla‐1 h‐1) are within the range reported from other lakes. Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited. Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m‐3) and the probable annual specific areal loading of total phosphorus (0.4–1.7 g TP m‐2 yr‐1) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate. These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability.

Effects of El Ni�o/Southern Oscillation on the population dynamics of a Gari solida population (Bivalvia: Psammobiidae) from Bah�a Independencia, Peru

Population dynamics of a Gari solida (Gray, 1828) population from Bahia Independencia, Peru (14°S) was studied between November 1989 and February 1995 (except for 1991) in order to investigate the effects of the climate phenomenon El Nino/Southern Oscillation (ENSO) which occurred in 1992/93. Results indicate that ENSO had negative effects on gonad production in 1992, when temperatures were very high, and positive effects in 1993, when temperatures were lower, but the warm period lasted longer than in 1992. Higher reproductive output in 1993 was achieved by several spawning events in addition to the normal pattern. Growth rate in 1990 was higher (K=0.495) than in 1993/94 (K=0.346). A possible explanation could be that more energy was allocated to reproduction, and thus less surplus energy remained for growth. The negative effect of ENSO on production in 1992 is indicated by the low P/B ratio (0.298); in all other years P/B was consistently higher (1990: 0.570; 1993: 0.597; 1994: 0.597). From 1990 to 1994 a reduction of mean annual biomass from 189.3 to 14.4 g ash-free dry weight m-2 was also observed which may be related to fishery activities. The consistency of P/B ratio is best explained by the higher somatic production of the smaller individuals remaining after the larger individuals had been removed selectively by fishermen. Based on the results presented in this study and from the literature, a model is proposed depicting how ENSO may negatively affect bivalve populations. Two main factors are mentioned, increased temperature and reduction of assimilated energy.

The effects of shoot age on colonization of an emergent macrophyte (Typha latifolia) by macroinvertebrates

SUMMARY1. The colonization and dynamics of epiphytic aquatic macroinvertebrates are described on first‐year and second‐year shoots of an emergent macrophyte (Typha latifolia) in a Swiss pond. Effects of shoot senescence on composition, richness, density and biomass of the macroinvertebrates are quantified.2. There were two phases of colonization: a short‐term colonization process which corresponded with that usually observed on inert substrates and a longer term colonization process related to the attractiveness of the shoots for the colonizers. In this second process, the older shoots showed a higher attractiveness for most invertebrate taxa.3. Taxa colonized the shoots at different rates. Rapid colonizers included the limpet Ferrissia wautieri and the mayfly Cloeon dipterum. Conversely, Oligochaeta, Chironomidae and Ceratopogonidae were particularly slow colonizers.4. The older shoots supported a higher mean annual richness, abundance and biomass of invertebrates. Compared with younger shoots, the older shoots demonstrated a higher carrying capacity for most invertebrate taxa, as is the case for many other freshwater macrophytes.