Effect of riparian land use on contributions of terrestrial invertebrates to streams

Since terrestrial invertebrates are often consumed by stream fishes, land-use practices that influence the input of terrestrial invertebrates to streams are predicted to have consequences for fish production. We studied the effect of riparian land-use regime on terrestrial invertebrate inputs by estimating the biomass, abundance and taxonomic richness of terrestrial invertebrate drift from 15 streams draining catchments with three different riparian land-use regimes and vegetation types: intensive grazing — exotic pasture grasses (4 streams), extensive grazing — native tussock grasses (6 streams), reserve — native forest (5 streams). Terrestrial invertebrate drift was sampled from replicated stream reaches enclosed by two 1 mm mesh drift nets that spanned the entire channel. The mean biomass of terrestrial invertebrates that entered tussock grassland (12 mg ash-free dry mass m^–2 d^–1) and forest streams (6 mg AFDM m^–2 d^–1) was not significantly different (p > 0.05). However, biomass estimated for tussock grassland and forest streams was significantly higher than biomass that entered pasture streams (1 mg AFDM m^–2 d^–1). Mean abundance and richness of drifting terrestrial invertebrates was not significantly different among land-use types. Winged insects contributed more biomass than wingless invertebrates to both pasture and tussock grassland streams. Winged and wingless invertebrates contributed equally to biomass entering forest streams. Land use was a useful variable explaining landscape-level patterns of terrestrial invertebrate input for New Zealand streams. Evidence from this study suggests that riparian land-use regime will have important influences on the availability of terrestrial invertebrates to stream fishes.     

Variation in horizontal zooplankton abundance in mountain lakes: shore avoidance or fish predation?

Hypothesizing that fish predation, active shore avoidance and outlet stream avoidance may be separately affecting horizontal zooplankton distribution, the effects of fish presence, sampling location (midlake, outlet and non-outlet shore) and time (day or night) on zooplankton abundance and body size were tested. Statistically significant horizontal zooplankton abundance gradients occurred in both fish-present and fish-absent lakes. Fish may strengthen zooplankton spatial patterns common to both fish-present and fish-absent lakes, as abundance differences among locations were often greater in fish-present systems compared to fish-absent systems. Horizontal zooplankton abundance gradients differed through a diel cycle, but were species specific with some species exhibiting gradients only during the day, while others exhibit gradients only during the night. Avoidance of the outlet over and above active shore avoidance appeared to take place in Daphnia sp. Other taxa provided equivocal support of active outlet avoidance with most showing no significant difference between shore and outlet abundance (seven of nine), one taxa showing a significant decrease and one a significant increase in outlet compared to shore abundance. No gradients in zooplankton body size were found.      

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and a Pteronarcys stonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.     

Selective foraging on terrestrial invertebrates by rainbow trout in a forested headwater stream in northern Japan

The important contribution of terrestrial invertebrates to the energy budget of drift-foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m^–2 day^–1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m^–2 day^–1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.     

Changes in benthic community composition in response to reservoir aging

The effects of reservoir aging on the benthic macroinvertebrate community in Pawnee Reservoir were documented by comparing species composition and biomass of samples collected from October 1991 through September 1992, to a similar survey conducted in 1968–70 by Hergenrader & Lessig (1980). Filling of the basin with sediment and associated material and the subsequent change in the benthic environment, has resulted in a relatively homogenous bottom substrate at each of the three sampling transects (dam, middle, and inflow). Sediment enrichment has limited the benthic fauna to species tolerant of brief periods of bottom anoxia and increased levels of organic matter, which has resulted in the disappearance of many taxa and a decrease in the abundance of remaining invertebrates. Significant differences in total biomass were found at each transect, as well as for the whole lake, between study periods. The dam, middle, inflow and total biomasses for the 1968–70 study period were 2.4, 1.5, 2.3 and 2.0 g m^–2, respectively, compared to 0.2, 0.2, 0.3, and 0.2 g m^–2 for the 1991–92 study period. The total disappearance of eight invertebrate taxa, in particular two sphaerid clam species, and significant declines in other dominant taxa such as Chaoborus punctipennis and Chironomus sp. accounted for these major differences in biomass between study periods. Reductions in the number of taxa present has resulted in an increase in benthic faunal similarity at each transect, with tubificid oligochaetes, Coelotanypus sp., C. punctipennis, and Chironomus sp., comprising 90% of both the total density and biomass of benthic invertebrates in Pawnee Reservoir.     

Selective feeding by three native North American freshwater mussels implies food competition with zebra mussels

The response of a benthic macroinvertebrate assemblage to whole-lake biomanipulation was studied in a small Finnish mesotrophic lake. From 1993 to 1997, over 200 kg ha^–1 of fish, mainly roach (Rutilus rutilus (L.)) and bream (Abramis brama (L.)) were caught and the fish biomass was reduced by nearly 80%. The biomass and density of benthic invertebrates were investigated during the years of fish removal and for the following three years. The decrease in benthivorous fish stock led to a higher biomass and density of all major groups of benthic invertebrates during the early years of fish removal. Non-biting midges (Chironomidae), water mites (Hydrachnellae), mayfly nymphs (Ephemeroptera), sphaeriid clams (Bivalvia: Sphaeridae) and biting midges (Ceratopogonidae) seemed to respond most profoundly to changes in fish biomass. The biomass of most invertebrate groups correlated negatively with the catch-per-unit-effort (CPUE). The total biomass and density of invertebrates had strong negative correlations with the CPUE (r= -0.85, p = 0.016, r = -0.84, p = 0.019, respectively), but they did not correlate significantly with total phosphorus, chlorophyll a, or temperature. However, the variation in total biomass that was not explained by the CPUE, was significantly associated with total phosphorus.The fish stock recovered to almost its initial level within three years after fish removal had been discontinued. As an apparent response to increased predation pressure, the biomasses of many invertebrate groups decreased again in the years 1999–2000. The strong relationship between macrozoobenthos and fish populations in the studied lake is likely to be a consequence of the open and sparsely vegetated bottom, which offers minimal shelter to invertebrate prey. An additional factor behind the recent low biomass levels may be changes in primary production. Phosphorus and chlorophyll a concentrations started to decrease markedly after three years of fishing and they have remained at a low level.     

Allochthonous input and retention in a small mountain stream,South Africa

Allochthonous input and benthic coarse particulate organic matter (CPOM) standing stocks were investigated in a first-order stream in South Africa between May 1984 and April 1985. Monthly falls into the stream of all litter types (total) ranged from 11 (September) to 79 g m^–2 (March). Total annual litter fall was 426 g dry weight, which corresponds to 1.2 g m^–2 d^–1. Flowers, fruits and seeds contributed 37 g m^–2, woody debris, 122 g m^–2, and leaves 267 g m^–2 to this total. Leaf fall from native trees, which accounted for approximately 57% of total litter input (244 g m^–2 a^–1), was significantly higher in summer than in winter. The summer peak in leaf fall recorded is far smaller and more protracted than the autumnal peak recorded for many Northern Hemisphere streams.Monthly total standing stocks of CPOM ranged from 14 g dry weight m^–2 in January to 69 g m^–2 in August, and a mean total CPOM standing stock of 41 g m^–2 mth^–1 was estimated. This comprised 18 g m^–2 mth^–1 soft litter, and 23 g m^–2 mth^–1 hard litter. CPOM standing stocks showed no seasonal trends, and with the exception of two species, standing stocks of endemic leaf species reflected their contributions to the total litter fall. Contrary to earlier reports for streams in the Fynbos Biome, Window Stream has CPOM standing stocks well within the ranges reported for low-order streams worldwide.     

Diurnal bottom feeding of predator fish strengthens trophic cascades to benthic algae in experimental flow-through pools

Mechanisms that determine the strength of trophic cascades from fish to benthic algae via algivorous invertebrates in stream communities have not been clarified. Using seven fish species, we tested the hypothesis that the interspecific variation of predatory behavior of fishes affects the strength of trophic cascades in experimental streams. One or two species of fish were introduced into flow-through pools of 2.5 m² and the abundances of benthic invertebrates and algae were monitored. Pike gudgeon, a diurnal benthic feeder, triggered a strong trophic cascade but masu salmon, a diurnal drift feeder, did not have a cascading effect. Japanese dace, which is both a diurnal benthic and drift feeder, increased the algal biomass, but the nocturnal benthic feeder cut-tailed bullhead had little cascading effect. The diurnal benthic feeder silver crucian carp also had a cascading effect, but no trophic cascade was triggered either by Asian pond loach or by Japanese common catfish, both of which are nocturnal benthic feeders. Thus, diurnal benthic fish exerted a stronger cascading effect than diurnal drift feeders or nocturnal fish. The combination of two fish species enhanced the per-capita strength of trophic cascades, probably because one of the two species, the benthic feeder, preyed on more invertebrates than in the single-species pools.     

Predation and drift of lotic macroinvertebrates during colonization

Summary A field experiment was carried out to determine the effect of an invertebrate predator on the colonization and drift of benthic macroinvertebrates in experimental stream channels. Lotic invertebrates colonized four replicate channels: two controls with no predators, and two channels with low densities (2.8 m^–2) of predatory stonefly nymphs, Doroneuria baumanni (Perlidae). Immigration rates were measured at the inflow of two other channels. Drift rates of invertebrates immigrating to and emigrating from channels were measured daily, and benthic samples were collected every five days. Over a 25-day colonization period, benthic densities of Baetis nymphs and larval Chironomidae were reduced by D. baumanni. Colonization curves were fit with a power function and significantly different colonization rates were indicated for both Baetis and chironomids in predation and control channels. A predator-induced drift response was exhibited by Baetis only and this response was size-dependent. In the presence of D. baumanni, large Baetis drifted more frequently than small nymphs and, correspondingly, small nymphs were more frequent in the benthos. Net predator impacts on invertebrate densities in channel substrates were partitioned into predator-induced drift and prey consumption. These estimates suggest that predator avoidance by Baetis is a prominent mechanism causing density reductions in the presence of predators. Reductions in the density of Chironomidae, however, were attributed to prey consumption only. A rainstorm during the experiment demonstrated that stream flow disruptions can override the influence of predators on benthic invertebrates, at least temporarily, and re-set benthic densities.     

Trophic‐level dependent effects on CO2 emissions from experimental stream ecosystems

Concern over accelerating rates of species invasions and losses have initiated investigations into how local and global changes to predator abundance mediate trophic cascades that influence CO₂ fluxes of aquatic ecosystems. However, to date, no studies have investigated how species additions or losses at other consumer trophic levels influence the CO₂ flux of aquatic ecosystems. In this study, we added a large predatory stonefly, detritivorous stonefly, or grazer tadpole to experimental stream food webs and over a 70‐day period quantified their effects on community composition, leaf litter decomposition, chlorophyll‐a concentrations, and stream CO₂ emissions. In general, streams where the large grazer or large detritivore were added showed no change in total invertebrate biomass, leaf litter loss, chlorophyll‐a concentrations, or stream CO₂ emissions compared with controls; although we did observe a spike in CO₂ emissions in the large grazer treatment following a substantial reduction in chlorophyll‐a concentrations on day 28. However, the large grazer and large detritivore altered the community composition of streams by reducing the densities of other grazer and detritivore taxa, respectively, compared with controls. Conversely, the addition of the large predator created trophic cascades that reduced total invertebrate biomass and increased primary producer biomass. The cascading effects of the predator additions on the food web ultimately led to decreased CO₂ emissions from stream channels by up to 95%. Our results suggest that stream ecosystem processes were more influenced by changes in large predator abundance than large grazer or detritivore abundance, because of a lack of functionally similar large predators. Our study demonstrates that the presence/absence of species with unique functional roles may have consequences for the exchange of CO₂ between the ecosystem and the atmosphere.     

Limnology of Crater Lakes in Los Tuxtlas,Mexico

Investigations on the abundance, biomass and position of heterotrophic flagellates (HF) in the benthic microbial food web of a melt water stream on King George Island, Antarctic Peninsula, were undertaken during the Antarctic summer from 23rd December 1997 until 13th March 1998. Abundance and biomass of potential HF resources (picophotoautotrophic and non-photoautotrophic bacteria) as well as potential predators on HF (ciliates and meiofauna) were also investigated. HF abundance ranged from approximately 9 × 10³ to 81 × 10³ cells cm^–3, values which fall into the same range as those found in lower latitudes. Numerically important benthic HF were euglenids, kinetoplastids, thaumatomastigids and especially chrysomonads. Most species identified have been shown to have a worldwide distribution. Abundance of the benthic ciliates ranged from 27 to 950 cells cm^–3. Mean bacterial abundance was 1.9 × 10⁷ and 5.2 × 10⁸ cells cm^–3 for picophotoautotrophic and non-photoautotrophic benthos, respectively. The well-developed microbial community was able to support the large number of nematods, gastotrichs, tardigrads and rotifers with abundances reaching more than 1000 individuals cm^–3. The largest portion of heterotrophic biomass was formed by the meiofauna with a mean of 63 g C cm^–3, followed by that of the heterotrophic bacteria with 4.80 g C cm^–3. Picophotoautotrophic bacteria contributed a mean of 1.37 g C cm^–3. HF and ciliates mean biomass was 0.61 and 1.99 g C cm^–3, respectively, with the HF biomass comprising between <10 and 70% of the total protozoan biomass. The data obtained in this study identify the melt water stream as a hot-spot of heterotrophic microbial and meiofaunal activity during the austral summer. The HF in the melt water stream formed a diverse group in terms of taxa and potential feeding types. Chrysomonads, kinetoplastids, euglenids and thaumatomastigida were the most abundant taxa. A classification into feeding types identified an average of 34% of the total HF as bacterivorous while all others were able to utilise other, larger organisms as resources. Potential trophic interactions between HF and bacteria and higher trophic levels are discussed.     

Small stonefly predators affect microbenthic and meiobenthic communities in stream leaf packs

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Nutrient additions to wetlands in the Interlake region of Manitoba,Canada: effects of a single pulse addition in spring

This study examined the responses of algae and invertebrates to a single application of N and P in a series of experimental wetland enclosures in the Interlake region of Manitoba during 1989 and 1990. N and P levels in the water, sediment and vegetation were also monitored. The 3 fertilization treatments were: dissolved inorganic (6200 µg 1^–1 N, 420 µg 1^–1 P), dissolved inorganic (3200 µg 1^–1 N, 210 µg 1^–1 P) and organic (ground alfalfa meal: 6200 µg 1^–1 420 µg 1^–1 P).Dissolved nutrients in the inorganic treatments were quickly depleted from the water column, but dissolved N increased in the water column of the alfalfa treatment as the alfalfa decomposed. No changes in N or P concentrations in the sediments or vegetation were detected. Phytoplankton biomass increased in all fertilized enclosures while epiphytic periphyton exhibited only minor responses. Epipelon biomass increased in the alfalfa treatment and metaphyton standing crops were highest in the high inorganic treatments.In the alfalfa treatment, high microbial respiration rapidly reduced dissolved oxygen concentrations which negatively affected invertebrates. This trend reversed as oxygen levels increased later in the experiment. Dominant nektonic and benthic invertebrates increased in the high inorganic and alfalfa treatments. Orthocladiinae emergence increased in the high inorganic and alfalfa treatments, while Chironominae and Tanypodinae increased in the alfalfa treatment. Second year responses by algae and invertebrate communities to the fertilization treatments were minimal. Annual single pulse fertilization has the potential to increase the productivity of Interlake wetlands when nutrients are applied in the spring, however it should be noted that at the levels used in this study the effects did not extend to the second year.     

The influence of invertebrates on the breakdown ofPotamogeton pectinatus L. in a coastal marina (Zandvlei,South Africa)

The influence of invertebrates upon the decomposition ofPotamogeton pectinatus L. in a coastal Marina system was examined over 112 days using litter bags. Invertebrate inclusion bags (2 mm mesh, 5 mm holes) registered a dry mass loss of 1% d^–1, while exclusion litter bags (80 µm mesh) produced a 0.4% mass loss d^–1 (a 2.5 fold difference). Losses of ash and N from inclusion bags were greater than those from exclusion bags (p < 0.05). There was a three fold difference between the two treatments in the time taken for litter to breakdown to half the initial stock: T_1/2 for inclusion bags = 43 d, exclusion bags = 130 d. In both treatments, minerals showed an expected rapid loss, due to leaching, with a subsequent slow increase relative to the dry material remaining. A total of nine invertebrate taxa was recorded from inclusion bags, with a peak biomass of 64 mg g^–1 dry massPotamogeton bag^–1 reached at 64 days after immersion. Grazing amphipods,Melita zeylanica Stebbing andAustrochiltonia subtenuis (Barnard), numerically dominated the litter bag fauna, whileM. zeylanica and nymphs of the zygopteran predatorIschnura senegalensis (Rambur) formed most of the biomass. Scanning Electron Microscopy indicated heavy grazing of micro-organisms by invertebrates, with major qualitative differences occurring 112 days after immersion. Invertebrates significantly accelerated the rate of litter breakdown through their feeding activities, assisting fragmentation and thus contributing to plant losses and also by increasing the surface area for microbial colonisation and attack.     

Biomass of planktonic crustaceans and the food of young cyprinids in the littoral zone of Lake Balaton

The littoral zone of Lake Balaton and its periphyton-zooplankton-fish communities have been investigated intensively in recent years. Total average number of crustacean plankton varied from 36 to 126 ind l^–1, their biomass from 0.49 to 1.86 mg ww l^–1 month^–1 at different areas of the littoral zone. In general, these values for the above parameters were higher in hypertrophic areas. 23 fish species occurred in the littoral zone with cyprinids dominating. The seasonal food spectra of Y-O-Y roach (Rutilus rutilus), white bream (Blicca bjoerkna) and bream (Abramis brema) were based mainly on planktonic crustaceans and benthic/periphytic invertebrates. According to the frequency of occurrence of crustaceans and other invertebrates, the food composition of young cyprinids differed significantly in the NE and SW-basins of the lake.     

Population dynamics,production and angling catch of brown trout,Salmo trutta,in a mature upland reservoir in mid-Wales

Synopsis The brown trout in Llyn Frongoch, a mature upland reservoir, and its nursery stream was sampled during 1983. The stream stock consisted largely of the 1983 and 1982 year classes, with fish reaching mean lengths of 7.0 and 11.6 cm at one and two years of age. The size and biomass of the stream stock at the beginning of 1983 and 1984 were estimated to be 120 and 125 (1.20 and 1.25 fish m^–2) and 1.41 and 0.69 kg (14.1 g m^–2 and 6.9 g m^–2) respectively. Annual stream production ranged from an estimated minimum of 2.49 kg (24.9 g m^–2) to an estimated maximum of 4.59 kg (45.9 g m^–2). Both downstream and upstream movements of 0+ juveniles were recorded. The adult spawning stock was estimated at 79 males and 32 females, a sex ratio of 2.5:1, with most spawners belonging to the 1980 yearclass. The average size of the lake stock over the year was estimated to be 1 650 (229 fish ha^–1) or 250.8 kg (34.8 kg ha^–1). The 1980 yearclass was predominant; there were few fish older than five years. Seasonal variations in netting catches suggested movements to and from the littoral region. Growth in the lake was moderately fast, with fish reaching mean lengths of 21.7 and 27.2 cm by three and four years of age. Fish entering the lake after one year appeared to grow faster than fish which remained in the stream for two years. Annual production in the lake was estimated at 136.7 kg (19.0 kg ha^–1). The total angling catch for the season was estimated to be 62.6 kg (8.7 kg ha^–1).     

Asymmetric competition between distant taxa: poecilid fishes and water striders

Summary Competition for water surface prey between fish (Priapichtus annectens: Poeciliidae) and water striders (Potamobates unidentatus: Gerridae) was studied in the laboratory and in pools in a small tropical stream. Laboratory experiments showed that fish depressed activity and foraging success of water striders. Large fish (4–5 cm) had a greater effect than small fish (2–3 cm). The field experiment showed that competition was highly asymmetric. Presence of fish decreased water strider foraging success while the reverse interaction was insignificant. It is suggested that the higher individual foraging success of the fish, harassment of water striders by fish and the use of an exclusive resource, benthic invertebrates, by the fish, contribute to this pattern. Habitat use differed between the two species. Fish used the deeper parts of stream pools and water striders used the shallower parts of the pools. Asymmetric interference and exploitation competition may force water striders to use shallow edge habitats.     

Trophic relationships between primary producers and fish yields in Lake Balaton

Relationships between chlorophyll a content of the water, the shoreline-length: water area ratio and the annual total fish yield as catch per unit effort (CUE: kg ha^–1 100 h^–1 as annual mean values) have been calculated by multivariable regression. The determination coefficient (r ² = 0.913) showed a significant dependence of fish yield on morphometry of different lake areas. Accordingly, fish carrying capacity of the open water areas, calculated from chlorophyll a content and S/A, ranged from 12 to 34%, but that of the littoral zone between 66 and 88%. These findings have also been supported by echo-sounding records of the horizontal distribution of fish.Bream (Abramis brama L.) contributes the majority (70–80%) of fish stock and yield. Its food mainly consists of zooplankton and benthic invertebrates in ratios that are widely variable with season and depend on the age of fish. Average daily food consumption of individuals (age group 3 + and over) varies between 2 and 5 g. Bream consumes two- to three-times more food in the SW basin than in the NE one. This means that the present stocks inhabiting areas from NE to SW consume annually 13249–20085 t yr^–1 of food. According to estimated calorific values, the annual energy consumption of local populations along the longitudinal axis of the lake varies between 93 and 141 kJ m^–2 yr^–1. The efficiency of energy transfer from primary producers to fish is low and varies from 0.04 to 0.1%.     

Tubifex tubifex as a link in food chain transfer of hexachlorobenzene from contaminated sediment to fish

Sediments contaminated with poorly water-soluble organic chemicals pose a risk to aquatic food chains. Sediment-associated chemicals can be accumulated by endobenthic, sediment-ingesting invertebrates. Some tubificid species – or other benthic annelids – serve as food for benthivorous fish, which thereby ingest the sediment-borne chemicals and may accumulate contaminant concentrations far higher than from water exposure only, and transfer them to organisms of higher trophic levels. For measurement of biomagnification, a sediment based food chain was developed and established in the laboratory. The two-step food chain included the sediment-dwelling freshwater oligochaete Tubifex tubifex (Müller) as a representative species of benthic infauna. The three-spined stickleback (Gasterosteus aculeatus, Linné), a small teleost fish which often feeds primarily on benthic invertebrates, served as a model predator. Spiked artificial sediment and reconstituted water as the overlying medium were used. Experiments were performed using ¹⁴C-labelled hexachlorobenzene, a hydrophobic pollutant as a model compound. To examine the influence of benthic prey on the bioaccumulation of the test substance in the predator, fish were exposed to spiked water, spiked sediment, pre-contaminated prey organisms, or combinations of these exposure routes. The results of these experiments indicate that for hexachlorobenzene, the presence of contaminated Tubifex tubifex as a food source in combined exposure leads to significantly higher accumulation in fish than exposure to single pathways.     

Forest floor biomass,litter fall and nutrient return in Central Himalayan oak forests

The seasonal dynamics of forest floor biomass, pattern of litter fall and nutrient return in Central Himalayan oak forests are described. Fresh and partially decomposed litter layers occur throughout the whole year in addition to herbaceous vegetation. The highest leaf litter value is found in April and May and the minimum in September. Partially and largely decomposed litter tended to increase from January to May with a slight decline in June. The wood litter peaked in March and April. The relative contribution of partially decomposed litter to the forest floor remains greatest the year round. The maximum herbaceous vegetation development was found in September with a total annual net production of 104.3 g m^-2yr^-1. The total calculated input of litter was 480.8 g m^-2yr^-1. About 68% of the forest floor was replaced each year with a subsequent turnover time of 1.47 yr. The total annual input of litter ranged from 664 (Quercus floribunda site) –952 g m^-2 (Q. lanuginosa site), of which tree, shrub and herbaceous litter accounted for respectively 72.0–86.3%, 6.4 – 19.4% and 5.2 – 8.6%. The annual nutrient return through litter fall amounted to (kg ha^-1) 178.0 – 291.0 N, 10.0 – 26.9 P, 176.8 – 301.6 Ca, 43.9 – 64.1 K and 3.98 – 6.45 Na. The tree litter showed an annual replacement of 66.0 – 70.0%, for different nutrients the range was 64 and 84%.