The presence of rainbow trout reduces drift of aquatic insects in Yungas streams (Tucumán, Argentina)

The drift or transport of individuals downstream is an important phenomenon of rivers and streams, since most aquatic organisms participate in it at some period of their life cycle. This work describes the assemblage of macroinvertebrate drift from ten sites in eight mountain streams of Tucumán, and evaluates the effect of the presence of rainbow trout (Oncorhynchus mykiss Walbaum), an exotic fish and visual predator, on this fraction of the community. Thirty-one (31) families of drifting aquatic invertebrates were identified, mainly immature insects and to a lesser extent water mites (Acari: Hydrachnidia), oligochaetes (Annelida: Oligochaeta) and nematodes (Nematoda). The richness and diversity of the sites with trout did not differ from the others, except when comparing lower and upper reaches of a same stream. Samples from rivers without trout or with low density of trout did not show significant changes in drift density of macroinvertebrates. In contrast, in rivers with high trout densities, a notable decrease in insect taxa was observed (those with large sized individuals): Baetidae (Ephemeroptera), Gripopterygidae (Plecoptera) and Leptoceridae (Trichoptera). On the contrary, the density of small Diptera, water mites, oligochets and nematodes increased in trout-streams

STUDIES ON THE DRIFTING BEHAVIORAL PATTERNS OF MACROZOOBENTHOS IN KYUNJA GAD, A MOUNTAIN STREAM FROM GARHWAL HIMALAYA, INDIA

Macrozoobenthos displayed several interesting trends with regards to their occurrence and movement in the streams in response to various abiotic and biotic factors constituting the aquatic ecosystem. These are considered of great ecological significance in understanding the quality of the particular ecosystem. The present communication focused on understanding the spatial and temporal diel drifting patterns displayed by macrozoobenthos in different levels of stream orders at different duration round the clock. It was observed that there was a general preference of nocturnal and early evening hours of drifting. The drift by benthic species was performed mainly due to presence of predators, lack of proper food and breeding sites. The study revealed that highest diel drift density and diversity of macrozoobenthos was found in 3rd order in comparison to 2nd and 1st order of Kyunja Gad stream. Variations were also noted in the physico-chemical parameters of streams Kyunja Gad at different orders.

Meiofauna constitute a considerable portion of invertebrate drift among moss-rich patches within a karst hydrosystem

Aiming to establish the most frequent invertebrate taxa in drift at the small spatial scale within a moss-rich karst tufa-precipitating hydrosystem, we sampled drift among microhabitats differing in substratum type and flow conditions along a tufa barrier-cascading lotic reach. Additionally, we addressed the question of the contribution and the potential significance of meiofauna within the overall invertebrate drift at the small spatial scale. During the study period, a total of 60 invertebrate taxa were recorded in the drift. Six of these taxa belonged to the annelid/arthropod meiofauna and they represented 35% of total drift density. Macroinvertebrates found in drift were represented mainly by larval insects. The composition of the most abundant taxa in total drift was as follows: Alona spp. (Cladocera 26.7%), Riolus spp. (Coleoptera: Elmidae 13.2%), Simulium spp. (Diptera: Simuliidae 12.2%), Enchytraeidae (Oligochaeta 10.4%), Hydrachnidia (6.3%), Orthocladinae (Diptera: Chironomidae 3.9%) and Naididae (Oligochaeta 3.6%). Faunal drift densities and amounts of transported particulate matter (PM) were highest at the fast-flowing sites located at the barriers and lowest at the slow-flowing sites within pools. Similarly to the seasonal amounts of transported PM, faunal drift was lowest in winter, and peaked in autumn and in late spring/early summer. Correlation between flow velocity and PM-faunal drift densities suggested a significant effect of the dislodged PM, though a minor influence of discharge and flow velocity on faunal drift. We suggest that the small-scale habitat heterogeneity and the respective feeding and refugial strategies of the fauna, as well as faunal passive dislodgement initiated by the shear forces of the flow were the most important drivers of observed drift patterns.

Influence of Hydrological Situations on Benthic Organisms in a Small River in Saxony (Germany)

This research was focused on the relationship between river discharge and organism drift. It was carried out for three years in a small heavily modified river in Saxony (Germany). The amount and species composition of drifting invertebrates were observed, depending on discharge and flow velocity. A station was installed where the flow velocity was continually measured and drifting organisms were caught with nets. An inventory of the aquatic community (benthic invertebrates) was taken to determine the species living in the river at the research station. The highest drift density measured was 578 organisms per m3 at a flow velocity of 0.90 m s-1, the mainly drifting organisms were Chironomidae. Different organisms groups started drifting at different flow velocities. Heavy impacts, such as dredging the river and flood waves, affected the aquatic ecosystems and severely changed the aquatic community regarding the number and the diversity. Some of the aquatic invertebrates such as the Anthothecata completely disappeared after dredging. It was found that many different terrestrial organisms were part of the drift. The typical family of soil biota Collembola represented the largest share.

Spatial and temporal variations of Cocconeis placentula var. euglypta (Ehrenb.) 1854 Grunow, 1884 in drift and periphyton

Spatial and temporal variations of Cocconeis placentula var. euglypta in drift and periphyton were studied in mountain streams of the Córdoba Province (Argentina). The sampling program was conducted in study sites located on a confluence between different order streams during an annual cycle. Samples were also taken every two hours during the daylight period in high and low water conditions. The relationship between drift and cellular reproduction was evaluated by valve length biometrics analysis. C. placentula var. euglypta drift was continuous; its density was not always dependent on periphyton density in each locality. C. placentula var. euglypta drift could be related to abiotic factors such as temperature and flow during the annual cycle. There were significant differences between periphyton and drift valve lengths. Moreover, drift can be associated with cellular reproduction because density was higher when valve lengths were shorter at different hours of the day. C. placentula var. euglypta epiphytims on Cladophora glomerata also influenced drift density and size distribution, modifying the relationship between periphyton and drift during the late spring when C. placentula var. euglypta was detached from senescent mats.

The effects of riparian forestry on invertebrate drift and brown trout in upland streams of contrasting acidity

Variations in macroinvertebrate drift and benthic invertebrate abundance were assessed in 30 upland Welsh streams of varying acidity (pH < 5.7 or pH.> 6.0) and riparian land-use (conifer, moorland or native broadleaf). The consequences for the diet and condition of wild brown trout Salmo trutta were also assessed. As expected from previous studies, there were significant reductions in benthic invertebrate abundance, aquatic drift density (by >60%), aquatic drift biomass (by >35%), total drift density (by >35%) and total drift biomass (by >20%) at acid sites by comparison with circumneutral sites due largely to the scarcity of mayflies. Absolute drift from terrestrial sources was unrelated to stream pH but formed a significantly greater proportion of total drift at acid sites (30-65% of density) than at circumneutral sites (20-40%) as aquatic contributions declined. Most of this apparent land use effect reflected significantly increased terrestrial drift under broadleaves. There was no significant reduction in terrestrial or aquatic drift at conifer forest sites per se after accounting for low pH. Trout diet varied substantially between locations partly reflecting variations in drift: significantly fewer mayflies and stoneflies were eaten at acid sites, and significantly more terrestrial prey were eaten under broadleaves. However, acidity did not reduce trout condition or gut-fullness. Unexpectedly, trout condition was significantly enhanced at conifer sites, irrespective of their pH. Hence, acidity has greater effects on the benthic abundance and drift density of invertebrates in upland streams than does riparian land use. However, trout forage flexibly enough to offset any possible food deficit, for example by switching to chironomids and terrestrial invertebrates. Enhanced terrestrial contributions to invertebrate drift from riparian broadleaf trees may be important in supplementing foraging opportunities for trout where aquatic prey are scarce. These data illustrate the value of native tree species in riparian locations in upland Britain and the energy subsidy they provide might well be disproportionately important for otherwise impoverished acid streams Keywords: brown trout, land-use, acidification, drift, forestry, streams