Sixth-order Streams Research Articles

Characterization of Stream Benthic Communities Using Substrate-Filled Trays: Colonization, Variability, and Sampling Selectivity

ABSTRACT This research examined colonization rate, variability and sampling selectivity of substrate-filled trays collected from six streams (second-sixth order) in Virginia and West Virginia. The length of time required to obtain equilibrium communities in trays varied among streams. Community composition in a second order stream was highly variable among days. In contrast, benthic communities collected from fourth and sixth order streams after 18–20 d colonization were similar to those collected on day 30. These results suggest that longer colonization periods may be necessary to characterize the benthic communities of small streams. Sampling selectivity of substrate-filled trays was examined by comparing relative abundance of dominant taxa in trays and in a natural substrate sampler (0.1 m2 Hess sampler). Trays were selective for collector-filterers, which comprised 19–73% of total individuals; however, most dominant taxa present in the natural substrate were also present in trays. Sampling variability...

Particulate Allochthonous Inputs: Relationships with Stream Size in an Undisturbed Watershed

Four streams of a pristine boreal watershed in eastern Quebec were examined to assess the composition, seasonal pattern, and ash-free dry mass (AFDM) of particulate allochthonous input. Study sites on streams ranging from first to sixth order exhibited significant differences in the types and magnitude of litter input. Composition of litterfall varied according to vegetation and physical structure of the riparian zone. Although the forest in the watershed is dominated by conifers, deciduous leaves dominated annual budgets and seasonal input patterns. Annual litterfall per unit area of stream surface declined exponentially with increasing stream order (r2 = 0.98) from 307–534 g AFDM∙m−2∙yr−1 in a first-order stream to15–17 g AFDM∙m∙yr−1 in a sixth-order stream. Lateral inputs of 25–70 g AFDM∙m−1∙yr−1 were not related to stream size, but were strongly influenced by riparian structure and entrainment of organic matter during spring flooding. Total input per unit stream length increased logarithmically with increasing order (r2 = 0.87), approaching 1000 g AFDM∙m−1∙yr−1 in the fifth- and sixth-order streams. A watershed budget calculated from these results estimates a total input of 729 t∙yr−1 to streams of this 673-km2 watershed. When particulate allochthonous input is compared with primary production measurements in these streams, 81–95% of the organic carbon supplied to the first- and second-order streams is allochthonous, but 85–95% of the supply to the fifth- and sixth-order streams is autochthonous. While these trends agree with predictions of the synthetic continuum model, the importance of local effects in our results illustrates the need for a more mechanistic approch in allochthonous input research.

Stream Geomorphology and Benthic Habitat Predictability as Determinants of the Population Dynamics and Life History of the SnailJuga pllclfera

ABSTRACT Populations of J. plicifara were examined for a second, third and sixth order stream in western Oregon over all seasons. I hypothesized that traits indicative of r-selection should be more apparent as benthic habitat predictability or stream size decrease. Mean individual biomass and apparent survivorship were directly correlated with stream width and benthic habitat predictability while mean fecundity and population density were inversely related to the above factors. Fecundity and population density were regulated primarily by geomorphological factors through the abundance of breeding sites and the number of trematode parasites causing snail gonadal castration. Mortality and survivorship were directly influenced by the severity of the winter freshets and the predictability of the benthic habitat at that time. Although a trade-off between fecundity and survivorship was apparent, consistent with predictions from r and K-selection theory, this pattern was in fact the result of more complex interactions. I conclude from these results that: (1) benthic habitat certainty may be an important factor determining life history and population dynamics patterns in stream benthic invertebrates and (2) local stream geomorphological and biotic conditions, indirectly correlated with the degree of habitat certainty, can be critical in explaining overall life history patterns in stream benthic invertebrates.

Riverbed-material texture and composition of Bishnumati River, Kathmandu, Nepal; implications in provenance analysis

The Bishnumati River, a major tributary of the Bagmati River in the Kathmandu Basin, is a low-gradient, low-sinuosity river with a short high-gradient head. The tributaries contributing the Bishnumati River supply sediments from granite-gneiss sources located towards the north of the river area, and from sedimentary rocks of the Phulchoki Group from the northwest and the west of the Bishnumati watershed. Four representative segments each from third, fourth, fifth and sixth order stretches were sampled and analysed for riverbed-material size, and gravel shape and composition, to characterise riverbed materials, to understand downstream distribution of shape, size and composition of sediments, and to understand provenance of riverbed materials. The D50 of the segments 1, 2, 3 and 4 are 25.11 mm, 0.871 mm, 3.75 mm and 27.86 mm, respectively. The riverbed materials are classified as Gravel, muddy sandy Gravel and muddy Gravel, which are very poorly sorted indicating textural immaturity. Gravels are compact bladed in form. Oblate prolate index (OPI) shows more oblate nature of gravels from downstream segments (sixth order stream). Gravels have high settling sphericity (0.6-0.8) and slightly higher flatness index (0.51-0.54). They are subrounded to rounded although there exist some angular to well rounded gravels. Downstream changes in shape parameters are not remarkable perhaps because of short distance of transport, insufficient abrasion, or durability of gneissic and granitic clasts, or quick transport of gravels during floods. Shape also seems to be influenced more by inherited properties of parent rocks. The gravels from sedimentary rocks increase from the upstream to downstream sites of the river at the expense of reduction of gneissic and granitic gravels. Existence of siltstone and sandstone gravels in the fifth (Segment 2) and forth order (Segment 3) mainstreams is remarkable as there is no primary parent source upstream of these segments. Existence of sedimentary gravels shows local provenance (fluvio-deltaic terrace deposits) and perhaps suggests existence of paleodrainage system different from the present doi: 10.3126/bdg.v12i0.2250 Bulletin of the Department of Geology, Vol. 12, 2009, pp. 55-62