Effects of deforestation on fish community structure in Ecuadorian Amazon streams

SUMMARY 1. There is little information on the impacts of deforestation on the fish fauna in neotropical streams, and on parameters influencing species diversity and community structure of fish. We analysed these aspects in 12 stream sites in the Ecuadorian Amazon. The stream sites represented a large gradient in canopy cover and were located in an area of fragmented forest. While some streams had been deforested, they had not suffered gross degradation of the habitat.
2. The species richness of stream fish was not related to deforestation. Local fish diversity (Fisher's Alpha) was positively related to the surface area of stream pools (m²). Beta diversity was higher among forested than deforested sites, indicating greater heterogeneity in species composition among forested than deforested sites. The percentage of rare species was positively correlated with canopy cover.
3. Total fish density increased with deforestation, and the fish community changed from dominance by omnivorous and insectivorous Characiformes at forested sites to dominance of periphyton-feeding loricariids at deforested sites.
4. Multidimensional statistical analysis of fish community structure showed that six environmental variables (the area of stream bottom covered by leaves, relative pool area, particulate organic matter, mean depth, conductivity and suspended solids) were related to the ordination axes. The presence of leaves, which was strongly correlated to canopy cover, was the variable most closely related to fish community structure, while relative pool area was the second strongest variable. Thus, fish community structure was strongly affected by deforestation.     

Genetic structure of the stonefly, Yoraperla brevis, populations: the extent of gene flow among adjacent montane streams

1. Gene flow in populations of stream insects is expected to depend on the distance between and the connectedness of sites in stream networks, and on dispersal ability (i.e. larval drift and adult flight).
2. Yoraperla brevis (Banks) is an abundant and characteristic stonefly of smaller streams in the northern Rocky Mountains. The present authors analysed genetic structure at 27 sites in sevenz streams flowing into the Bitterroot River in western Montana, USA. Cellulose acetate electrophoresis identified five variable loci with 16 alleles.
3. Genotype frequencies conformed to Hardy–Weinberg expectations. Within-stream differentiation was low and among-stream variation ( F _st) was an order of magnitude higher.
4. UPGMA grouped sites within streams and also grouped adjacent streams. The tree produced by the Neighbour Joining Method was similar although not quite so clear cut.
5. This orderly pattern (i.e. Hardy–Weinberg proportions, homogeneity within streams and geographical structure) contrasts strongly with patterns observed in invertebrates from subtropical streams in Australia. Yoraperla brevis maintains large populations in predictable environments, has a long life-cycle with a likelihood of cohort mixing, emerges synchronously in large breeding populations and occupies streams separated by areas of high relief; the Australian situation is the opposite in most respects.
6. Further analysis of a range of species is required to determine whether the different genetic structure in Y. brevis compared to the Australian species occurs more generally in North American stream insects.     

Interactions among nutrients, algae and invertebrates in a New Zealand mountain stream

SUMMARY. 1. Colonization of nutrient-diffusing substrata by periphyton and invertebrates was investigated at forested and open sites in a small, mountain stream and a spring in the South Island of New Zealand.
2. Substrata had colonization surfaces made from 100μm mesh plankton netting that enabled algal assemblages to be removed intact for scanning electron microscopy. They also allowed small volumes of solvent to be used for the extraction of photosynthetic pigments.
3. At all sites, periphyton assemblages were dominated by species of Achnanthes, Cocconeis and Gomphonema , and except in the forest in winter, periphyton biomass was always greater on enriched (N + P added) than control substrata.
4. Invertebrates colonizing diffusion substrata were principally larval Chironomidae (Orthocladiinac). No larvae were present in winter, but in three spring and summer trials mean larval densities were higher on nutrient-enriched than control substrata at all sites.
5. The inclusion of an insecticide Malathion in diffusion substrata, reduced insect colonization at open and forested sites. After 28 days, no concurrent increases in algal pigment concentration were observed on nutrienl-enriched or control substrata at the forested site. However, pigment concentrations were higher on substrata incorporating Malathion at the open site suggesting that algal standing crop was depressed by the activities of grazers.     

Land-use influences macroinvertebrate community response following a pulse disturbance

1. We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press‐pulse disturbance) and native forest (pulse disturbance) catchments before and after a one‐in‐28‐year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways. 2. Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance. 3. Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2‐year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies. 4. Taxa numbers and densities recovered to preflood levels within 5–7 months at both sites, but a range of taxa‐specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood. 5. Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site. 6. Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non‐Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera. 7. This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land‐use impacts.     

Use of both benthic and drift sampling techniques to assess tropical stream invertebrate communities along an altitudinal gradient, Costa Rica

1. Two sampling techniques were used to characterize invertebrate communities in eight, low-order streams along an altitudinal gradient in Costa Rica that represents the last continuous tract of primary forest spanning such extremes in elevation (i.e. near sea level to 2900 m a.s.l.) along the Caribbean Slope of Central America. A standard Surber sampler was used to sample invertebrates on the stream bottom, and drift sampling nets were used to sample invertebrates drifting in the stream flow. 2. Sites were established at 30, 50, 700 1800 and 2700 m a.s.l. In one to two streams per site, six Surber samples were collected, and drift was sampled every 3 h over one 24-h period between April and August 1994. All sites were in primary forest, with the exception of the lowest elevation site (30 m) which was located in banana plantations. 3. Both sampling techniques indicated that Diptera (Chironomidae) and Ephemeroptera were the dominant insect groups at all sites. Disturbed streams draining banana plantations were dominated by Chironomidae and had lower taxon richness and diversity than other sites. 4. While data from benthic samples indicated that insects were the major faunal component (> 90%) at all sites, drift samples were dominated by larval shrimps (> 50%) at the 30 m and 50 m sites. 5. Drift periodicity of invertebrates was observed at those sites characterized by predaceous fishes: nocturnal drift densities were higher than diurnal densities at 30, 50 and 700 m a.s.l., however, no periodicity was observed at 1800 and 2700 m a.s.l. where fish were absent. 6. This study shows the importance of measuring invertebrate drift, in addition to directly sampling the benthos. Drift sampling provided data on a major community component (shrimps) of lowland tropical streams, that would have been overlooked using traditional benthic sampling techniques, and in some cases provided additional information on taxon richness. 7. Based on results of the present study, it is recommended that drift sampling be included as a standard complementary tool to benthic sampling in biological assessments (e.g. bioassessment protocols) of tropical streams, which are often characterized by migratory invertebrate species such as shrimps. Drift samples provide critical information on the presence or absence of shrimps and also on the timing and magnitude of their migration which is an important link between many tropical rivers and their estuaries.     

Seasonal and diel patterns of invertebrate drift in different alpine stream types

1. We examined the seasonal and diel patterns of invertebrate drift in relation to seston and various habitat characteristics in two each of four different kinds of alpine streams [rhithral (snow‐fed) lake outlets, rhithral streams, kryal (glacial‐fed) lake outlets and kryal streams]. Samples were collected at four times of the day (dawn, midday, dusk and midnight) during three seasons (spring, summer and autumn). 2. Habitat characteristics differed mainly between rhithral and kryal sites, with the latter having higher discharge and turbidity, lower water temperature, and higher concentrations of ammonium, and particulate and soluble reactive phosphorus. Seasonality in habitat characteristics was most pronounced for kryal streams with autumn samples being more similar to rhithral sites. 3. The concentration of seston was lowest in the glacial‐influenced lake outlets and slightly higher in the stream sites; no seasonal or diel patterns were evident. 4. The density of drifting invertebrates averaged less than 100 m^?3 and was lowest (<10 m^?3) at three of the four kryal sites. Taxon richness and diversity were lowest at rhithral lake outlets. Chironomidae dominated the drift as well as benthic communities and <30% of benthic taxa identified were found in the drift. 5. Drifting invertebrates showed no consistent seasonal pattern. However, density tended to be highest in spring at rhithral sites and in autumn at kryal sites. No diel periodicity in drift density was found at any site and the lack of diel pattern may be a general feature of high altitude streams. 6. Glacially influenced habitat parameters were a major factor affecting drift in these alpine streams, whereas no clear differences were observed between streams and lake outlets. Our findings indicate that invertebrate drift in alpine streams is primarily influenced by abiotic factors, and therefore, substantially differs from patterns observed at lower altitude.     

Invertebrate responses to short-term water abstraction in small New Zealand streams

1. Small permanent streams are coming under increasing pressure for water abstraction. Although these abstractions might only be required on a short‐term basis (e.g. summer time irrigation), the highest demand for water often coincides with seasonal low flows. 2. We constructed weirs and diversions that reduced discharge in three small streams (<4 m width) to test the hypotheses that short‐term water abstractions would decrease habitat availability and suitability for invertebrates, resulting in increased invertebrate drift, reduced taxonomic richness and decreased benthic invertebrate densities. 3. We sampled benthic invertebrates, invertebrate drift and periphyton at control (upstream) and impact (downstream) sites on each stream before and during 1 month of discharge reduction. 4. Discharge decreased by an average of 89–98% at impact sites and wetted width decreased by 24–30%. Water depth decreased by 28–64% while velocity decreased by 50–62%. Water conductivity, temperature and dissolved oxygen showed varying responses to flow reduction among the three streams, whereas algal biomass and pH were unaffected in all streams. 5. The densities of invertebrate taxa tended to increase in the impact reaches of these streams, even though invertebrate drift increased at impact sites in the first few days following discharge reduction. There were a higher proportion of mayflies, stoneflies and caddisflies at the impact site on one stream after flow reduction. There were no changes to the number of taxa or species evenness at impact sites. 6. Our results suggest that for these small streams, the response of invertebrates to short‐term discharge reduction was to accumulate in the decreased available area, increasing local invertebrate density.     

Effects of environmental factors on the phytoplankton community in the Gulf of Finland – unattended high frequency measurements and multivariate analyses

We studied life history and secondary production of Cheumatopsyche spp. (Trichoptera: Hydropsychidae) in StroublesCreek, a 3rd order stream near Blacksburg, Virginia to compare lifehistories and production dynamics in two reaches with differentland-uses. The upstream reach was influenced by agriculturalactivities and more annual degree-days accumulated in this sectionof the stream. The downstream reach was surrounded by forest,received less thermal input than the upstream reach, and showedlittle evidence of human disturbance. Differences between the twosites included a shift in dominant taxa and changes in voltinismand production of hydropsychids. Cheumatopsyche pettiti(Banks) was dominant at the upstream site, while C. pettitiand Cheumatopsyche oxa Ross were both abundant downstream.Cheumatopsyche larvae and other aquatic insects had higherdensities at the forested downstream site. C. pettiti wasbivoltine at the upstream site, but C. pettiti and C. oxa were partially bivoltine downstream, with a small summergeneration overlapping with continuous emergence of overwinteringlarvae. Cheumatopsyche production at the two sites inStroubles Creek was higher than in forested, low-order streams ofNorth America, but similar to some values obtained at streams inagricultural areas and large rivers. Production was higher at thedownstream site, but the superior bivoltinism at the upstream sitecounterbalanced, to some extent, the increase in larval densitythat caused higher production downstream. The adjustment in lifehistory and production dynamics, in the two reaches, showed theecological plasticity of Cheumatopsyche and the importance ofhydropsychids in the energy flow of moderately disturbedstreams.     

Filters and templates: stonefly (Plecoptera) richness in Ouachita Mountains streams, U.S.A.

1.  We collected adult stoneflies periodically over a 1-year period at 38 sites in two headwater catchments in the Ouachita Mountains, Arkansas, U.S.A. The 43 species collected were a subset of the Ozark-Ouachita fauna and the much larger fauna of the eastern U.S.A. We estimated 78–91% species coverage in the two catchments using jackknife extrapolation of species richness from our survey.
2.  Many streams, especially small ones, lacked surface water for months, but others, both small and large, flowed permanently.
3.  Using published regional presence–absence and coarse ecological data in a discriminant function analysis (DFA), we identified stream size (negative) and regional frequency of occurrence (positive) as predictors of presence in these headwater catchments. For the combined catchments, the extrapolated richness (51 spp.) was similar to an estimate (48 spp.) based on predicted absences from DFA and the Ouachita provincial total of known stonefly species (57 spp.).
4.  Local species richness (1–27 spp. per site) was correlated strongly with stream size (catchment area) but was independent of stream drying. Generic richness was correlated negatively with stream drying and positively, but less strongly, with stream size.
5.  Regionally endemic stoneflies dominated in drying streams, and widely distributed species dominated in more permanent streams. The composition of stonefly assemblages was associated with regional factors (species pools, regional abundance, evolution of tolerant endemic species, regional climate) and local factors (drying, stream size).     

Forests and the temperature of upland streams in Wales: a modelling exploration of the biological effects

SUMMARY. 1. Daily temperature data from six streams in upland Wales were used to explore the thermal effects of afforestation on stream ecology. The data were linked to published biological models to simulate fish and invertebrate development.
2. Mean daily temperatures in forest streams were lower than those of moorland streams in spring and summer, and higher in winter. These spatial comparisons were supported by the results of experimental bank-side clearance at a forest site, where there was evidence that stream temperatures fell in winter and rose in spring following treatment.
3. Simulations indicated that brown trout (Salmo trutta) could weigh over 30% more by the end of their second growing season in a moorland compared with a forest stream. Several species of insects showed slower simulated egg development at forest sites. For two ephemeropteran species simulated nymphal growth was also retarded, suggesting significant alterations to the life cycle. Two plecopteran species were affected only slightly by the different temperature regimes.
4. Overall, the simulations suggested that afforestation, by reducing summer temperatures, could lead to marked reductions in rates of development of some invertebrates and fish.     

Allochthonous coarse particulate organic material in forest and pasture reaches of two south-eastern Australian streams

1. Terrestrial plant litter entering two forest and two pasture sites on upland streams in south-eastern Australia was measured over a 2-year period using traps suspended above the streams. Laterally transported terrestrial plant litter was also determined in one stream, but found to be only 10% of the total. 2. Litter accession to traps suspended above the stream was significantly lower at both forested sites than was litter fall in the adjacent riparian vegetation. 3. Litter input was high (600–700g DW m^?2yr^?1) and seasonal, with 30–50% of the annual total entering the stream during December—February (summer). However, seasonality was not as great as that recorded in Northern Hemisphere deciduous forest streams. 4. Leaves comprised the largest litter component, but represented a relatively smaller proportion of the litter than is usually the case in Northern Hemisphere deciduous forest streams; in contrast bark was a relatively large proportion of the litter. 5. At the sites flowing through pasture, litter accession was about two orders of magnitude lower than at the forested sites.     

Experimental increases and reductions of light to streams: effects on periphyton and macroinvertebrate assemblages in a coniferous forest landscape

Increased light reaching streams as a result of riparian vegetation management is often thought to be responsible for enhanced algal productivity. However, concomitant changes in nutrients and other physical processes confound that interpretation. We manipulated light in two separate experiments to test the role of light as a controlling factor for periphyton productivity and biomass, and to observe invertebrate responses in small streams in central British Columbia, Canada. We did this by adding artificial light to reaches of three forested streams, and in a second experiment we used shadecloth to cover reaches of two streams flowing through clearcuts. Periphyton growth, productivity and composition, and macroinvertebrate benthic densities were contrasted with control reaches within the same streams. Gross primary production (GPP) was increased at least 31% by light addition to forested streams. Periphyton biomass was higher under light additions, but only significantly so in one of the streams. In one stream grazers increased along with the periphyton response, whilst in the other two lit streams invertebrates, including grazers, decreased with increased light. The shading significantly reduced GPP to about 11% of that in clearcut sections, but failed to produce any significant responses in either periphyton standing stock or invertebrates in the clearcut streams. Measures of algal production and biomass responded as predicted; however, invertebrate responses to increased and decreased light were idiosyncratic amongst streams, perhaps indicating lagged responses and limitation by other resources.     

Ecology of leaf pack macroinvertebrate communities in streams of the Fraser River Basin, British Columbia

1. To characterise geographic and small scale variation in the structure of macroinvertebrate communities in stream leaf packs, we collected one to three natural leaf pack communities from 119 reference streams in the Fraser River Basin and quantified their variability and correlation with aspects of the stream environment at several scales. We also sampled leaf packs in 19 test streams in the same geographic area exposed to stressors (nine logged, seven farmed, three mined catchments) to evaluate the leaf pack community as a tool for bioassessment. 2. There was substantial variation in the composition of invertebrate communities in leaf packs among reference streams of the Fraser River Basin. Capnia and Zapada (stoneflies), Baetis and Ephemerella (mayflies) and Tvetnia (midge) were the most common taxa found in the leaf packs. There were three types of assemblages identified by non‐metric multidimensional scaling; Capnia, Baetis and Ephemerella communities. 3. Leaf pack communities from the 19 test streams were plotted on a non‐metric multidimensional scaling ordination of the reference communities, and 14 of 19 sites fell outside the 80% confidence ellipse of the reference sites, including eight of nine logged, four of seven farmed and one of three mined catchments. Most of these streams plotted on the ordination near the Ephemerella reference communities. Reference stream communities had a similar number of genera per leaf pack (12.0) and genera per site (18.7) as the test streams (12.6 genera per leaf pack and 18.7 genera per site). Among the test sites, the farmed catchments had higher genera per leaf pack (17.8) and genera per site (21.9) than either the logged (11.5 genera per leaf pack; 19.9 genera per site) or mined (3.4 genera per leaf pack; 7.7 genera per site) catchments. 4. Heterogeneity of leaf pack communities within a site decreased as the number of genera found at the site increased. This was determined by allometric regression of the number of genera found at a site on the maximum number of genera possible, given the average number found per leaf pack. 5. There was a significant relationship between the composition of the leaf pack invertebrate community and stream geography (latitude, longitude, altitude, stream order). Canonical correspondence analysis showed differences among ‘big river’, ‘mountain stream’ and ‘southern’ communities. 6. There was no relationship between the composition of the leaf pack invertebrate community and stream channel and flow characteristics (bank dimensions, flow, slope). There was a significant relationship between the composition of the leaf pack invertebrate community and water quality of the stream (oxygen, nitrogen, phosphorus, conductivity, pH, temperature). ‘Cold, oxygen rich water’ communities were distinguishable from communities in streams with warmer, lower oxygen concentration. ‘High nutrient water’ communities were also distinct from communities in low nutrient streams. There was no relationship between the composition of the leaf pack invertebrate community and the nature of the leaf pack itself (i.e. morphology, decomposition, coniferous needle content). 7. Invertebrate communities in leaf packs show substantial, interpretable variation among reference streams. They are sensitive to human stressors at a landscape scale such as forestry and agriculture. Their diversity and composition varies at different spatial scales in a way that is at least partially explained by the environment of the stream and its catchment area.     

Patterns of species richness for blackflies (Diptera: Simuliidae) in the Nearctic and Neotropical regions

Abstract.  1. Patterns of simuliid species richness were examined over a variety of scales at 532 stream sites in the Nearctic (394) and Neotropical (138) regions. In Nearctic streams, species richness of immature blackflies both within and across ecoregions and over two seasons was examined. Stream variables at each site included seston, width, depth, velocity, discharge, conductivity, pH, dissolved oxygen, water temperature, dominant streambed-particle size, canopy cover, and riparian vegetation. These variables were subjected to a principal component analysis and derived principal components were related back to richness, using regression analysis. At the level of the stream reach, richness was not highly correlated with single-point measurements of stream conditions.
2. Using data from both Nearctic and Neotropical sites, the effect of regional richness on local richness was examined. As regional richness increased, local diversity reached an asymptote in which further increases in regional richness were not matched by increases in local richness. Hence, simuliid communities are best described as saturated (type II) communities, consistent with the current view of lotic communities as non-equilibrium systems.
3. The well-established pattern of greater species richness in tropical regions was not observed in this study. To the contrary, blackfly richness is higher in temperate streams than in tropical streams at both local and regional scales.     

The effect of the acanthocephalan parasite (Pomphorhynchus laevis) on the drift of its intermediate host (Gammarus pulex)

SUMMARY 1. The drift of Gammarus pulex in a population in which approximately 20% of adults are infected with the acanthocephalan parasite Pomphorhynchus laevis was monitored at margin and mid-river sites in the River Teme, England over a 24-h period in mid-summer.
2. Drift densities showed a diurnal pattern with a large increase at night, independent of parasite burden.
3. There was no significant effect of site (margin or mid-river) on the proportion of parasitized and unparasitized G. pulex found in the drift or the benthos.
4. The drift of parasitized G. pulex was significantly greater than unparasitized animals. Gammarids harbouring only one parasite were found in significantly higher proportions in the drift than those with two or more parasites.
5. At both sites (margin and mid-river) the proportion of unparasitized adult G. pulex in the drift was significantly lower than that in the benthos. However, there was a significantly higher proportion of parasitized animals in the drift than in the benthos.     

Hydrolytic extracellular enzyme activity in heterotrophic biofilms from two contrasting streams

SUMMARY. 1. Extracellular hydrolytic enzyme activities and cell densities were monitored during undisrupted biofilm formation on pristine surfaces in two contrasting river sites in North Wales: an oligotrophic mountain stream (Nant Waen) and a mildly eutrophic river (River Clywedog).
2. Bacterial densities generally increased at both sites over a 33-day monitoring period. Densities in the eutrophic site were approximately 14 times greater than in the mountain stream.
3. Using fluorescent substrate analogues, biofilms from Nant Waen produced low, variable xylosidase and β-glucosidase activities. Biofilms from the more eutrophic River Clywedog produced higher xylosidase and β-glucosidase activities and detectable endopeptidase, though these activities also fluctuated during the colonization period.
4. Unlike the other activities measured, esterase activities in the River Clywedog were correlated with cell densities ( P <0.05). When extracellular esterase activities per cell were calculated, the oligotrophic biofilm was found to contain about twice as much extracellular esterase activity as the more eutrophic River Clywedog biofilm.     

Effects of land use on water quality and aquatic biota of three North Carolina Piedmont streams

Three streams in the Piedmont ecoregion of North Carolina were studied to evaluate the effect of land use (forested, agricultural, urban) on water quality and aquatic biota. In comparison with the forested stream, there were few changes in water quality at the agricultural and urban streams. Suspended-sediment yield was greatest for the urban catchment and least at the forested catchment. Suspended-sediment concentrations during storm events followed this same pattern, but at low-moderate flows suspended-sediment concentrations were greatest at the agricultural site. Most nutrient concentrations were highest at the agricultural site, and the amount of available dissolved nitrogen was elevated at both the urban and agricultural sites. High concentrations of metals (totals) in the water column were sometimes observed at all sites, but maximum average concentrations were recorded at the urban site (especially Cr, Cu, and Pb). Maximum sediment metal concentrations, however, were not found at the urban site, but were usually recorded at the forested site. Only minor differences were noted between fish communities of the forested and agricultural sites, although both abundance and average size of some species increased at the agricultural site. The fish community at the urban site was characterized by low species richness, low biomass, and the absence of intolerant species.Invertebrate taxa richness, a biotic index, and the number of unique invertebrate species (found at only one site) indicated moderate stress (Fair water quality) at the agricultural site and severe stress (Poor water quality) at the urban site. At the agricultural site, declines in taxa richness within intolerant groups were partially offset by increases within tolerant groups. The agricultural stream had the highest abundance values, indicating enrichment. The urban site, however, was characterized by low species richness for most groups and very low abundance values. Analysis of seasonal patterns suggested detritus was the most important food source for invertebrates in the forested stream, while periphyton was of greater importance in the agricultural stream. Dominant macroinvertebrate groups shifted from Ephemeroptera at the forested site, to Chironomidae at the agricultural site, and Oligochaeta at the urban site. There was little between-site overlap in dominant species (8–7%), indicating that land use strongly influenced the invertebrate community. Chemical and physical parameters measured at the three sites did not seem sufficient to account for all of the observed differences in the invertebrate communities, suggesting some unmeasured toxicity. Biological measurements, especially macroinvertebrates community structure, consistently indicated strong between-site differences in water and habitat quality.     

Migratory drift of larval freshwater shrimps in two tropical streams, Puerto Rico

1. Migratory shrimps are often major biotic components of tropical stream communities, yet spatial and temporal patterns of their migration have yet to be described. This information is of increasing importance given the continued fragmentation of tropical streams by damming and water abstraction/diversion, which can disrupt migratory life cycles. 2. Larval amphidromous shrimps are released by adult females in freshwater streams. They then drift passively to an estuarine habitat where they metamorphose before migrating back upstream. Drift of larval shrimps was sampled over two to five 24-h periods at each of three sites along two rivers that drain the Luquillo Experimental Forest in Puerto Rico: the Espíritu Santo (10, 135 and 335 m a.s.l.) and the Mameyes (10, 90 and 290 m a.s.l.). A total of seventeen diel samplings were conducted. 3. Shrimp drift increased in the downstream direction in both catchments, and had a significant positive exponential relationship with length of stream channel above each site. There was no significant difference between catchments with respect to mean daily drift rate per km of stream channel. Maximum observed larval shrimp density was 69 102 larvae 100 m^–3 (1.7 g dry mass 100 m^–3), which is high relative to published invertebrate drift studies. 4. The pattern of shrimp drift agreed with the ’risk of predation hypothesis‘. In stream reaches with predatory fish, drift of larval shrimps occurred at night and was slight during the day. A nocturnal peak in drift occurred between 19.00 and 22.00 h. At a high-altitude site, where predatory fish were absent, no diel pattern was discernible. 5. The present study provides information on the timing of migratory drift of larval shrimps, which can minimize the adverse effects of water abstraction from streams draining the Luquillo Experimental Forest. Elimination of water withdrawal during peak larval drift after dark will significantly reduce shrimp mortality.     

Community structure in some southern English streams: the influence of species interactions

SUMMARY. 1. The results of a survey of thirty-four stream sites, differing in pH and invertebrate species richness, indicated that the pool of locally available, suitably adapted species was smaller in the acid streams. This plays a part in determining the general pattern of lower species richness at more acid sites.
2. Diversity of feeding categories increased with species richness, indicating that a greater range of food resources was available in the less acid, more species-rich communities.
3. The pattern of predation varied with pH and species richness. The numbers of large insect predators were lower in the less acid, more species-rich communities and this was correlated with the presence of fish.
4. A detailed study of the guild of detritivorous stoneflies in four streams differing in species richness provided evidence that density compensation occurs, niche width decreases and niche overlap declines as species richness increases.
5. We discuss the roles that competition and predation play in determining the structure and richness of stream invertebrate communities.     

Population dynamics and feeding of mayfly larvae in some acid and alkaline New Zealand streams

SUMMARY. 1. Population dynamics (density, biomass, annual production), gut contents and feeding rates of mayflies ( Deleatidium spp.; Leptophlebiidae) were compared in two naturally acid (mean pH≃4.8). brownwater streams and two alkaline (mean pH 7.5), clearwater streams in South Westland, New Zealand.
2. Mean densities of larvae (range 234–2318 m⁻²) were higher in alkaline streams on most of the six bimonthly sampling dates. Mean biomass (range 0.020–0.376 g larval dry weight (LDW) m⁻²) was always highest at the stable, spring-fed, alkaline site and was lower at the acid sites and another alkaline site where the population was always dominated by small larvae.
3. Annual production was high at the more stable, alkaline site (10.35 gLDW m⁻²) but much lower at the other sites (2.49–3.77 g m⁻²).
4. Gut contents of larvae were dominated by fine (45–75 μm widest diameter) paniculate matter (69–99%), diatoms (up to 21%) and. at one site, filamentous algae (8–13%).
5. Grazing rates of mayflies on epilithon were significantly higher on stones taken from acid than alkaline streams and material grazed from the former contained a higher proportion of inorganic material (87–93% and 61–83% inorganics, respectively).
6. Higher grazing rates may reflect lower quality of epilithic food in acid, brownwater streams, a factor that could contribute to the lower productivity of Deleatidium populations at these sites.