Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.     

Downstream drift of the larvae of Chironomidae (Diptera) in the River Chew,S.W. England

The drift of larval Chironomidae (Diptera) was investigated at two sites on the River Chew, using a pump filtration system with 50 µm mesh-aperture aerial nets, situated on the river bank.Chironomid larvae were found to drift in far greater numbers than previously reported, even in slow-flowing water. First and second instar larvae dominated the drift.Variation in drifting behaviour between taxa was observed, where Orthocladiinae drifted in all instars and Chironominae predominantly as first and second instars. Proportionally different rates of drift were observed between these taxa from the benthos at the two sites.It is suggested that the majority of chironomid drift represents an active dispersal and colonisation mechanism by which population redistribution and habitat selection occurs.     

Progress in understanding the chemical stratigraphy of metals in lake sediments in relation to acidic precipitation

At the beginning of the reproductive season corixids were excluded from several small-sized rock-pools by netting carried out repeatedly at 5 d intervals. In control pools corixid populations were allowed to develop normally. Changes in the bottom fauna, consisting of chironomid larvae were followed by repeated sampling.When corixids were excluded, the density of chironomid larvae remained at a relatively high level, the total biomass (as organic carbon) was 1.5–4.5 g m^–2 (the midsummer median 2.83 g m^–2). When corixids were present, the chironomid biomass fell to 0.1–1.0 g m^–2 (median at the time of peak corixid biomass was 0.52 g m^–2). The largest chironomid larvae tended to disappear first and ultimately only small larvae were present. When corixids were removed, the density of chironomid larvae returned to the level of the experimental pools in about two weeks.     

Alteration of biogenic structure and physical properties by tube-building chironomid larvae in cohesive sediments

From earlier research it has been suggested that chironomid larvae may significantly affect the physical properties of cohesive sediments, leading to chemical alteration in lake ecosystems. The present study was aimed to evaluate the effect of Tanytarsini larvae on the physical properties of cohesive sediments, both in controlled environments and in natural environments. For this the sediment surface was examined by low temperature scanning electron microscopy (LTSEM) and direct measurement of the shear strength. The larvae are able to modify the surface sediment significantly, e.g., by secretion of silk, which forms a matrix at the sediment surface in the vicinity of the larval tubes. The shear strength increased with greater sediment depth in all the experimental treatments, except where the densities were high. Considerably higher shear strength was observed in the natural sediment than in the experimental set up. A very significant density-dependent effect was observed within the top 1-cm by tube building Tanytarsini larvae. The relationship between larval densities and shear strength was not significant for 2 deeper sections (1–3 cm). The tube length was significantly greater in experimental containers with the low density (4960 ind. m^–2) than in containers with high larval densities (19840 and 39680 ind. m^–2).     

Studies on the small invertebrate plankton of the Sargasso Sea

During the German Eel Expedition in Spring 1979, the horizontal and vertical distribution of the invertebrate plankton was studied in the epipelagic zone of the western central Sargasso Sea, based on 55 µm and 100 µm mesh net samples. In the isothermal waters north of the thermal front, plankton biomass was on average 2–3 times higher than in the warmer stratified waters south of the front. With regard to the fraction of small invertebrates (nauplii and microcopepods) the differences in numerical abundance between the two areas were similar to those reported in the literature for other size ranges of organisms. No divergency was obvious in the plankton composition in terms of major taxonomic groups and size classes. In both parts of the area, organisms smaller than 400 µm, which form a fraction not quantitatively sampled by the conventional 200 µm or 300 µm mesh nets, accounted for 71–92 % of the total number of organisms in the 55 µm net samples and for more than 50 % in the 100 µm net samples. Average concentrations of the potential food supply for early larval fish stages in the upper 100 m appear to be comparable with values reported in the literature for areas well known for larval fish development, such as the California Current.     

Larval habitat preference of the endemic Hawaiian midge,Telmatogeton torrenticola Terry (Telmatogetoninae)

Telmatogeton torrenticola Terry is a large endemic chironomid (lastinstar >20 mm) commonly found in high gradient Hawaiian streams on smoothrock surfaces with torrential, shallow flow and in the splash zones ofwaterfalls. We have quantified benthic water flow in larval habitat in a 50m segment of Kinihapai Stream, Maui using a thermistor-based microcurrentmeter. Under base flow conditions at sites suitable for larval attachment,depth was measured and bottom water velocity measurements were made 2 mmabove populations. Larval densities ranged from 386.9–1178m^–2, habitat bottom water velocities from 13.4–64.2 cms^–1, and water depths from 1.5–50 cm. Bottom velocitiesof sites with zero larvae ranged from 20.8–21.8 cm s^–1with depths from 50 to >160 cm. Larval densities were greatest inareas with high bottom water velocities and shallow depths. Stepwisemultiple regression analyses showed that density could be confidentlypredicted best by Froude number (r=0.81; p=0.008). In the absence of Froudenumber as a regression term, the best variable to predict larval density wasbottom velocity ratio: relative depth ratio (r=0.75; p=0.019). In addition,the torrential habitat of the larvae was always characterized by aperiphyton community that appeared to be the primary food resource for thelarvae. These data suggest that torrential flows over appropriate substratesare important factors regulating habitat availability for T. torrenticolaand that reduced discharge (e.g. affected by water diversions) couldsignificantly reduce the amount of available habitat for this organism andother flow sensitive stream fauna.     

Vertical microdistribution of benthic chironomid larvae within a section of the littoral zone of a lake

A study of the vertical distribution of all instars of chironomid larvae was carried out in the littoral zone of a lake in southwest England over a period of 14 months. Samples were collected every month by using a 44 mm (i.d.) piston core sampler. Each core sample was sectioned at 1 cm intervals and preserved immediately after collection. The density of larvae in the study area varied from a low of 16,368 larvae m^–2 in August 1989 to maximum values of 106,249 and 129,954 larvae m^–2 in June 1989 and May 1990, respectively. During spring and early summer, first instar larvae comprised approximately half of the larval population. No significant differences were found in seasonal vertical distribution of the chironomid larvae. Over 85% of the larval population inhabited the top 6 cm of the sediment, with the first and second instar larvae of all taxa most abundant in the top 1 cm section.     

The drift of zooplankton and microzoobenthos in the river Strandaelva,western Norway

The drift of zooplankton (rotifers, cladocerans, cyclopoid copepods) and microscopical zoobenthos (mainly bdelloid rotifers and small chironomid larvae) was investigated by filtering samples of river water. The number of drifting benthic rotifers varied between 1 000 and 6 000 ind. m^–3 in the lake inlet, and between 30 and 500 ind. m^–3 in the lake outlet, without any seasonal trend. The number of drifting insect larvae was approx. equal in the lake inlet and outlet, with a maximum in summer (250–300 ind. m^–3) and minimum in winter (ca. 10 ind. m^–3). Increasing water flow resulted in an increasing number of drifting zoobenthos. Downstream from the lake, the number of drifting benthic rotifers was increasing from approx. 300 ind. m^–3 in the outlet to 6 500 ind. m^–3 3.4 km downstream, while the number of insect larvae was ca. 100 ind. m^–3 in the outlet and leveled off at approx. 300 ind. m^–3 after 200 m. The number of drifting zooplankton in the lake outlet varied between 20 and 2 000 ind. m^–3 for crustaceans, and between 300 and 20 000 ind. m^–3 for rotifers, both with a maximum in late summer/autumn and a minimum in winter. The number of drifting zooplankton decreased by some 45% in the first 200 m from the lake outlet, but some zooplankton was still found in the drift 3.4 km downstream. The largest species was removed first from the drift. The diurnal variation in the number of drifting zooplankton in lake outlets appear to be related to the vertical migration in the lake, i.e. the largest number drifting when most animals are in the upper water layers.Contribution from the Voss Project, University of OsloContribution from the Voss Project, University of Oslo     

Diel drift of Chironomidae in a large lowland river (Central Poland)

The diel drift patterns of Chironomidae larvae were investigated in a seventh order section of the Warta River (Central Poland) over two diel cycles during May 1989. Three nets (mesh size 400 m) were installed in a cross section of the Warta River.The estimated drift density was low, but was comparable to that calculated for other large rivers. Spatio-temporal fluctuations in abundance and composition of macroinvertebrate drift, including Chironomidae, were observed with the highest density of drifting macrobenthos recorded near the depositional bank of this river. The ratio benthosdrift indicated differing propensities for of the older instars of a given chironomid taxon to drift. Orthocladiinae larvae were the most abundant subfamily of Chironomidae in drift but not in benthos, reaching up to 73% of the total drifting chironomid larvae. More taxa but fewer individuals (about 20% of the chironomid larvae collected) belonged to the tribe Chironomini, the dominant group in benthos.A major part of chironomid drift collection may represent behavioural drift because the net mesh size used in the Warta River was insufficient to catch the earliest instars (distributional drift). Both at the family and subfamily level chironomid larvae exhibited a distinct nocturnal drift periodicity. Nocturnal periodicity was documented for the dominant species, but due to the low density of many chironomid species, it was impossible to determine their diel drift pattern. Some Chironomidae appeared to be aperiodic.     

The effect of vegetation patterns on oviposition habitat preference: A driving mechanism in terrestrial chironomid (Diptera: Chironomidae) succession?

Relationships between changes of two species of terrestrial chironomids(Smittia cf. aterrima andBryophaenocladius cf. virgo) and changes of vegetation cover during succession were studied with respect to plant — chironomid interaction in particular stages of chironomid life cycle. Several vegetation types forming vegetation mosaics in suburban fallow were distinguished according to their spatial architecture (height; density etc.). Abundance of chironomid larvae in particular type of vegetation and an area covered by these vegetation types were studied for three years. During one year the effect of vegetation types on density of aerial drift, oviposition habitat preference and adult emergence was studied in detail. Aerial drift was studied using pan traps, oviposition using soil sampling and adult emergence using emergence trap. Main results were similar for both species. Chironomid larvae were abundant in soil in initial years of succession. They were concentrated in patches with open and low height vegetation. Later their abundance decreased corresponding to a reduction of open and low vegetation areas. Greater density of aerial drift and oviposition preference was observed in areas of open and low vegetation. The success of larval development agreed with oviposition preference for winter larval generation only. The less preferred habitats seemed to be more suitable for summer generation. The results indicated that successional changes of terrestrial chironomids can be explained by the effect of vegetation cover on oviposition habitat preference.     

Effect of drift sampler exposure time and net mesh size on invertebrate drift density in the Njoro River,Kenya

Although invertebrate drift is an important ecological process in lotic ecosystems, very little is known about it in Kenyan rivers. The primary aim of this study was to investigate the effect of driftnet mesh size and exposure duration on drift density in 2017. Drift samples were dominated by Chironomidae, Baetidae, Simuliidae, Caenidae and Culicidae. The 100 µm mesh driftnet had the highest mean invertebrate density, followed by the 250 µm and 500 µm nets. Invertebrate drift densities decreased with increased exposure time. This study demonstrates that sampler mesh size and exposure time should be taken into account when characterising invertebrate drift in streams. Future studies should consider sampling different biotopes and during different seasons.     

Dried-up zone as a temporal stock of chironomid larvae: survival periods and density in a reservoir bank

The survival period, community composition and density of chironomid larvae were investigated at the reservoir bank in Yachiyoko Lake (Haji Dam), Hiroshima, Japan to verify their survivability in a dried-up zone. We define “dried-up zone” as the one that had been exposed to drought due to drawdown by evaporation and/or drainage. The larvae were collected from sediments at different altitudes along a transectional line. After larval rearing, 189 indiv. m⁻² of adults emerged even from the sediment of a 7-day drought (17.1% sedimental water content). The proportion of Tanytarsus oyamai Sasa in the community tended to be larger with dry periods, indicating that this species has a higher drought tolerance than other species. The mean density in the dried-up zone was estimated as 886.4 indiv. m⁻². Biomass is generally estimated by using the samples of only underwater sediment. These results show that some species have the potential to survive for at least one week in a dried-up zone and suggest that the biomass is likely to be highly underestimated for the entire ecosystem. A pathway of matter flow from aquatic to terrestrial ecosystems via the larvae in a dried-up zone should be taken into consideration.     

Trophic structure and productivity of the lagoonal communities of Tikehau atoll (Tuamotu Archipelago,French Polynesia)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988.The average POC concentration (0.7–2000 µm) was 203 mg C m^–3. The suspended living carbon (31.6 mg C m^–3) was made up of bacteria (53%), phytoplankton < 5 µm (14.2%), phytoplankton > 5 µm (14.2%), nanozooplankton 5–35 µm (5.7%), microzooplankton 35–200 µm (4.7%) and mesozooplankton 200–2000 µm (7.9%). The microphytobenthos biomass was 480 mg C m^–2.Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production.It was estimated that 50% of bacterial biomass was adsorbed on particles. the bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44–175 mg C m^–2 day ^–1) and zooplankton (50 mg Cm^–2 day^–1)2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus.50% of the phytoplankton biomass belongs to the < 5 µm fraction. This production (440 mg C m^–2 day^–1) exceeded phytobenthos production (250 mg C m^–2 day^–1) when the whole lagoon was considered.The zooplankton > 35 µm ingested 315 mg C m^–2 day^–1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m^–2 day^–1.     

The Ladoga seal (Phoca hispida ladogensis Nordq.)

Oligochaetes are the dominant group of macrofauna in a river reach (1–450 m) below the Dobczyce dam on the River Raba. The Oligochaeta made up from 60.2% to 78.7% of the community. The next most abundant group were the Chironomidae comprising 20.5%–38.8% of the fauna. Upstream-downstream movements of the macrofauna were studied at one station, with an oligochaete density of 50 000 ind. m^–2, and a chironomid density about 20 000 ind. m^–2. In the drift the proportions of these two groups were reversed—Chironomidae represented 59% of the drift fauna, and the oligochaetes — about 40%. On the river bottom and in the drift a similar number of species (12–13) of Naididae were identified.In order to compare the intensity of the movement of the various taxonomic groups, the percentage of animals moving upstream and downstream over 24 h was calculated. The highest percentage of the population migrating were dipterans (7.3%). The Chironomidae and Ephemeroptera had similar proportions migrating (3.2% and 2.6% respectively). The Oligochaeta had the lowest value — only 0.6% of animals were displaced in a 24 h period.The Naididae appear to be strongly attached to the substratum and, under normal environmental conditions, their reproduction sufficiently compensated for the decrease in their number associated with the drift or predation.     

Analysis of sibling cannibalism among pike,Esox lucius,juveniles reared under semi-natural conditions

Synopsis Sibling cannibalism in pike, Esox lucius, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae m^–2, equivalent to 12 and 36 larvae m^–3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae m^–2 (36 larvae m^–3), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60 mm) onwards. At a mean total length of 100 mm and for initial stocking densities of 6 and 18 larvae m^–2, (12 and 36 larvae m^–3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.     

Larviculture of the fairy shrimp,Streptocephalus proboscideus (Crustacea: Anostraca): effect of food concentration and physical and chemical properties of the culture medium

Food concentration (0.5 × 10³ – 5 × 10⁵ Scenedesmus cells m1^–1) significant influenced the somatic growth, maturation and survivorship ofS. proboscideus larvae. A density of 5 × 10⁴ cells m1^–1 was optimal for early larval stages. Of four temperature tested (20–35 °C), 30 °C resulted in the best growth and survival. Maturation time was inversely related to temperature, and was size- rather that age-dependent. Larvae were tolerant of a wide conductivity range, but optimal growth and survival were observed at 260 µS cm^–1. Nitrate-Nitrogen (NO₂-N) caused a larval mortality of 50% after 24 h at 0.58 mg1^–1.     

Occurrence and abundance of three larval codlet species (Bregmacerotidae, Teleostei) in the Southwest Atlantic Ocean (12–22°S)

There are still only a few studies related to Bregmacerotidae larvae in Brazil, although these larvae are frequently collected in oceanic ichthyoplankton samples. Thus this study analyzes the occurrence and abundance of Bregmaceros larvae along the Brazilian central coast and describes larval characteristics. During three oceanographic cruises samples were collected between the Real River (12°S) and the São Tomé Cape (22°S). Oblique hauls were conducted from a maximum depth of 200 m, using bongo nets of 330‐ and 500‐μm mesh size. Among the 951 bregmacerotids collected, three species were identified corresponding to 89%Bregmaceros atlanticus, 8%Bregmaceros cantori and 3%Bregmaceros n. sp. B. atlanticus was widely distributed along the study area and was more frequent in stations situated beyond the 200‐m isobath; this species was more abundant during the winter, showing a peak of density between Vitória and the São Tomé Cape (355 larvae per 10 m²). Bregmaceros cantori densities were very similar during the three cruises; the highest value (25.6 larvae per 10 m²) was recorded during the winter and higher concentrations were observed in neritic stations along the 200‐m isobath. Bregmaceros n. sp. was rare in collections made during the three cruises; it was concentrated along the Bahia coast (12–18°S), and was more frequent in stations situated in the oceanic region. Bregmaceros atlanticus varying from 2.0 to 6.0 mm and Bregmaceros n. sp. larvae varying from 3.0 to 6.0 mm were present in all cruises, indicating that they spawn year‐round. Bregmaceros cantori did not show a distribution pattern among the different length classes, probably associated with the fact that this species has a coastal distribution.     

Invertebrate drift along a longitudinal gradient in a Neotropical stream in Serra do Cipó National Park,Brazil

The diversity and composition of drift invertebrate assemblages were evaluated along a longitudinal gradient of an altitudinal stream in southeastern Brazil. The main goal of this study was to evaluate the influence of seasonality, stream order, and some abiotic factors on invertebrate drift and the use of drifting invertebrate assemblages to assess aquatic invertebrate diversity. Drift samples were collected over a 24 h period using nets (open area of 0.08 m²; mesh 0.250 mm), partially submerged (60%) in the water column. Taxonomic richness, Pielou evenness (J), Shannon–Wiener diversity (H), and total density of drift invertebrate assemblages were used in unpaired t-tests, Kruskal–Wallis and stepwise multiple regression analysis. The results showed a high taxonomic richness of aquatic invertebrates, with 91 taxa found. Chironomidae and Ephemeroptera represented together c. 80% of the total density of drift organisms. The drift approach allowed the collection of new and rare taxa, besides the knowledge of pupae stage of several chironomid genera. Significant differences in the taxonomic richness and diversity of drift invertebrate assemblages were found between the rainy and dry periods, indicating a significant influence of seasonality. An increase in water flow and electrical conductivity were associated with the increase in the taxonomic richness and diversity in the rainy period. No significant differences were found among the other abiotic variables among the stream orders.     

Emergence of Chironomidae from the shallow eutrophic Lake Kasumigaura,Japan

Seasonal chironomid emergence was monitored in the shallow eutrophic Lake Kasumigaura and 18 species were collected with a battery-operated light trap fixed on a floating stage and with surface emergence traps. During October–December, samples in the light trap comprised exclusively Tokunagayusurika akamusi (Tokunaga) and small numbers of one or two other species. T. akamusi, Procladuis (Holotanypus) culiciformis (L.), and Chironomus plumosus (L.) constituted 91.6% of the annual catch of chironomid adults. The predominance of T. akamusi (75.3 % of chironomid catch) and the high ratio (13) of T. akamusi to C. plumosus was more marked in this lake than other Japanese eutrophic lakes. Glyptotendipes tokunagai Sasa and Dicrotendipes pelochloris (Kieffer) were also caught abundantly with the light trap, but not with surface traps, indicating these were littoral species. The dry weight of emerging adults during May–December 1982 was 2.87 g m^–2, of which 1.92 gm^–2 (67%) was T. akamusi and 0.67 gm^–2 (23%) C. plumosus and 0.23 g m^–2 (8%) Clinotanypus sugiyamai Tokunaga and 0.03 gm^–2 (1%) P. (H.) culiciformis. The weight of emerging Tanypodinae was much higher than the annual mean larval biomass or estimated larval production, which have been due to underestimating the population density using an Ekman-Birge dredge. High numbers of individuals and species of chironomids were caught during April–July, presumably as a result of the high food supply for chironomid communities.     

Feeding and growth of larval herring,Clupea harengus,in relation to density of copepod nauplii

Synopsis Feeding and growth rates of 1–3 wk old herring larvae from four different stocks were compared in laboratory experiments (8°C). For most of the larval groups, feeding rate was saturated at nauplii (Acartia tonsa, nauplii stages 3–5) densities over 301^–1 (5 g d.w. 1^–1). Specific growth rate increased asymptotically with nauplii density, and reached about 6% d^–1 at densities over 120 nauplii 1^–1. The growth rates attained in the laboratory were similar to field measured growth rates of similarly aged herring larvae at comparable food densities. Since food particles were homogenously distributed in the laboratory tanks, patches of dense plankton concentrations are, thus, apparently not necessary for larval growth and survival in the sea. Growth efficiency differed between larval groups, with large sized larvae being the most efficient in transforming ingested matter into growth. The difference probably relates to different sizes rather than to the different geographical origins of the larvae.