In-stream Processes Research Articles

Experimental Investigation of Potassium and Nitrate Dynamics in A Headwater Stream in Mid-Wales

Abstract Potassium and nitrate were added experimentally to a small moorland stream in the headwaters of the River Wye, mid-Wales, during summer and winter low-flow conditions. Nutrient losses at three downstream sampling locations were calculated using concentrations of an added bromide tracer to correct for dilution effects. During the summer experiment, approximately 18% of the added nitrate and 58% of the potassium were removed from the stream water between the point of addition and the catchment outlet. During the winter experiment, nitrate depletion was not observed and the added nitrate travelled along the stream at the same rate as the bromide tracer, while approximately 93% of the added potassium passed through the stream but, at a slower rate than the bromide and nitrate. the results show that in-stream processes, probably related to biological activity of macrophytes and microflora, can regulate stream water concentrations of nitrate and potassium in the summer under stable flow conditions. Dur...

Identification of processes regulating sulphate content in upland catchment waters: the role of empirical models

A conceptual hydrological model has been extended to incorporate empirical relationships permitting the prediction of storm event sulphate response in a small headwater catchment in Mid-Wales. Results from simulation of transient events have highlighted inadequacies in hydrological conceptualisation and a need for incorporation of preferential pathway and in-stream processes. This has led, for example, to an investigation into the effects of Sphagnum spp. which has enhanced process understanding and yielded a plausible explanation of performance mismatch. Specific additional data requirements permitting the further verification of empirical functions have been pinpointed. The modelling has thus been successfully used to refine field investigations as a basis for improved process understanding.

Identification of processes regulating sulphate content in upland catchment waters: The role of empirical models

A conceptual hydrological model has been extended to incorporate empirical relationships permitting the prediction of storm event sulphate response in a small headwater catchment in Mid-Wales. Results from simulation of transient events have highlighted inadequacies in hydrological conceptualisation and a need for incorporation of preferential pathway and in-stream processes. This has led, for example, to an investigation into the effects of Sphagnum spp. which has enhanced process understanding and, yielded a plausible explanation of performance mismatch. Specific additional data requirements permitting the further verification of empirical functions have been pinpointed. The modelling has thus been successfully used to refine field investigations as a basis for improved process, understanding.

Dynamics of phosphorus compounds in a lowland river system: Importance of retention and non‐point sources

AbstractIntensive measurements of the fluxes of phosphorus (P) and of P retention were carried out in a Danish lowland watershed (Gjern Å) during two hydrological years (June 1987 to May 1989). Seasonal and short‐term variations of P concentrations were measured by intensive automatic sampling covering P fluxes from the entire Gjern Å watershed and two subcatchments: the Gelbæk and Lake Søbygård. Moreover, infrequent sampling was performed in all major tributaries of the river system. Transport of P compounds (dissolved and particulate P) exposed a seasonal pattern which was highly affected by flow conditions, instream processes, as well as point and non‐point sources. Stormflow P transport during the two study years (1987‐8 and 1988‐9) constituted 56 and 61% in the Gelbæk, respectively, compared with 25 and 23% in the Gjern Å, respectively. Particulate P transport in the Gelbæk constituted 56 and 75% of annual total P transport during the two years compared with 77 and 75% in the Gjern Å. The magnitude of monthly baseflow and stormflow total P loads was significantly correlated with the mean monthly discharge during baseflow and stormflow in the Gelbæk, whereas only stormflow loads were correlated to discharge in the Gjern Å. In situ measurements of retention showed that the maximum amount of P stored in the Gelbæk during summer (June to August) was 22gPm−2 (1988) compared with 27gPm−2 (1987) in the Gjern Å. Lake Søbygård was a P source during summer but a P sink during winter and on an annual basis the net P released was 810 and 1420 kg, which corresponded to 7 and 20% of the annual P export from the watershed, respectively. Retention constituted about 30% of gross P transport in the Gelbæk during summer compared with 20% in the Gjern Å. Resuspension of retained P during stormflows in September 1987 constituted 94% of the stormflow P transport and 54% of the total P transport. Monthly total P and dissolved reactive P (DRP) mass balances for the main channel of the Gjern Å revealed a significant DRP retention over 17 months (p < 0.05) caused by P adsorption on sediments and biological uptake. On an annual basis the main channel was a sink of DRP (1220 and 1660 kg P) but a source of total P (3440 and 1000 kg P). implying that the channel is an important source of particulate P. Bank erosion proved to be a significant P source in the main channel of Gjern Å, whereas P delivery from soil erosion was possibly of minor importance. The annual non‐point P export from two intensively sampled catchments was significantly higher (0.89 and 0.45 kg Pha−1 yr−1) than from the five extensively sampled subbasins (0.43 and 0.17 kg Pha−1 yr−1). No significant relationship could be established between non‐point P export and the proportion of arable land and soil type. Only for the wet year of 1987‐8 was a significant relationship established between non‐point P export and the P load from scattered dwellings outside sewage areas in five subbasins.

An East-West perspective on riverine loads of nutrients in the Vistula and Rhine basins

Measures proposed to reduce the load of nutrients to the Baltic Sea focus on the removal of point emissions through introduction of environmental technology in the eastern and southern parts of the drainage basin. This article draws attention to the mixed success of Western European river basin management by comparing water quality trends and turnover of nutrients in the Vistula and Rhine basins. Removal of point sources in the Rhine basin has caused a marked drop in the concentrations of phosphorus and ammonium nitrogen, whereas the long-term upward trend in the concentration of nitrate was broken only recently. As compared to the Vistula river, the Rhine is still in some respects more polluted. Expressed as area-specific export, the riverine load of nitrate from the Vistula basin is only about one sixth of that of the Rhine basin. The area-specific export of phosphorus is comparatively higher in the Vistula, but still only half of that of the Rhine. In addition to the lower input of nutrients in the Eastern European agriculture, the much lower runoff in the Vistula basin is an important explanation to the observed differences. Furthermore, in-stream processes, such as sedimentation of phosphorus, play an important role.

A Leaf-Toughness Method for Directly Measuring the Processing of Naturally Entrained Leaf Detritus in Streams

The leaf pack method has been used extensively as the standard protocol for tracking the in-stream processing of leaf detritus. However, a major deficiency of the method has been that it does not directly measure the processed condition of natural, multispecies accumulations of leaves. The method reported here allows mass loss of leaf detritus to be related to leaf toughness. Studies of dry-mass loss and leaf toughness of black cherry (Prunus serotina) leaves were conducted in summer 1992 in Linesville Creek and Powdermill Run (Pennsylvania, USA), and during fall/winter 1992-1993 in Powdermill Run. Leaf toughness was measured with a digital force-gauge penetrometer. Penetrometer measurements determined the peak force (nearest 0.001 Newton) necessary to drive a steel rod through the leaf tissue. In all studies, dry mass loss was best explained by a linear regression model and leaf toughness loss by a negative exponential regression model. The natural logarithm of mean toughness per leaf pack predicted perc...

Regulation of nutrient concentrations in a temperate forest stream: Roles of upland, riparian, and instream processes

The roles of terrestrial and instream processes in controlling stream‐water N and P concentrations were studied over a 2‐yr period in a deciduous forest stream in eastern Tennessee. Upper soil horizons were highly effective sinks for inorganic N and P in throughfall, and weathering of the parent dolomite was the dominant source of inorganic P to the stream. The riparian zone was a potential source of NH4+ and P to the stream when dissolved oxygen concentrations in riparian groundwater were low, but a sink for P when dissolved oxygen concentrations were high. High rates of instream immobilization of inorganic N and P were observed from late autumn to spring, primarily as a result of uptake by microbes on decomposing leaves and secondarily by algae. Immobilization of inorganic N and P resulted in longitudinal declines in concentrations with distance downstream from groundwater inputs (springs), thereby increasing the importance of organic forms of these nutrients in stream water in downstream reaches. The seasonal pattern of winter minima and summer maxima in stream‐water N and P concentrations observed here is opposite to the pattern observed in many northern streams, suggesting the importance of winter nutrient cycling processes in soils and streams in warmer climates.

Elemental Dynamics in Streams

We discuss elemental dynamics in streams and seek to identify areas where there are critical gaps in our understanding. Both landscape-level processes (e.g., geology, land-use practices, vegetation) and heterogeneous in-stream processes influence the supply and availability of elements to the stream biota. Stream ecologists need to consider the relative availability of different compounds or groups of compounds to the biota rather than lumping all forms of an element into operationally-defined units such as dissolved organic nitrogen or carbon. The impact of short-term events like storms on the elemental dynamics in streams needs to be assessed and compared with other controls. The relative importance of longitudinal (upstream), lateral (riparian zone, flood-plains), and in-stream controls of supply and availability of elements needs to be compared in a variety of streams. Availability of essential elements is a key factor controlling rates of primary productivity and decomposition in streams. Whole syste...

Diel Variations in Iron Chemistry in an Acidic Stream in the Colorado Rocky Mountains, U.S.A.

In the Snake River, an acidic mountain stream in the Colorado Rocky Mountains, the concentration of dissolved iron is apparently unrelated to seasonal changes in hydrologic regime, which strongly influence the concentrations of most other dissolved constituents. Hourly sampling indicated that short-term fluctuations in iron chemistry occur, whereas the concentrations of most other dissolved constituents, including other trace metals, remain stable. During the day, greater concentrations of dissolved total and ferrous iron generally occurred during periods of full sunlight. Photoreduction of hydrous iron oxides, which are abundant in the fine sediment and as coatings on the rocks, may be responsible for these observations. Iron chemistry also varied at night, decreasing in dissolved total and ferrous iron until about midnight and increasing until dawn. Oxidation of ferrous iron, and several microbial processes, may contribute to these nighttime changes. In an on-site batch experiment using rocks and streamwater, an increase in dissolved total and ferrous iron occurred on exposure to sunlight, and ferrous oxidation occurred on return to darkness. Short-term fluctuations in iron chemistry are consistent with the lack of correlation between iron and other constituents in the long-term data, and illustrate the potential importance of complex in-stream processes in such stream systems.

Characteristics of Sediment and Organic Carbon Export from Pristine Boreal Forest Watersheds

Estimates of the amount of material moving annually from terrestrial ecosystems to the ocean are largely based on an incomplete understanding of events occurring throughout the hydrologic year, and only a vague comprehension of in-stream processes controlling that export. Discharge, suspended sediment, particulate organic matter (POM; > 0.5 μm), dissolved organic carbon (DOC; <0.5 μm diameter), and the percentage of organic matter were measured from 1979 to 1981 in five pristine Quebec streams: First Choice Creek (1st order; watershed area: 0.25 km2), Beaver Creek (2nd order; 1.83 km2), Muskrat River (5th order; 204 km2), Matamek River (6th order; 673 km2), and the Moisie River (9th order; 19 871 km2). All streams, with the exception of First Choice Creek, have a strong spring freshet when 43–55% of the annual discharge occurs. By describing sediment and organic carbon export throughout the annual hydrologic cycle, 1 showed that during the 2-mo spring freshet 71–92% of the annual sediment load is exported but only 59–65% of the annual POM load, and only 47–51% of the annual DOC load. Sediment yield is relatively constant between watersheds (1.5–7.6 g∙m−2∙yr−1), as is POM export (1.0–6.7 g ash-free dry-weight [AFDW]∙m−2∙yr−1); however, export DOC varies from 3.1 g C∙m−2∙yr−1 in First Choice Creek to 48.4 g C∙m−2∙yr−1 in Beaver Creek. There appears to be rapid loading of carbon between 1st- and 2nd-order streams in boreal forests, followed by biological and physical processing as watershed area increases. Thus, for the Moisie River watershed, export of total organic carbon (TOC) is reduced to only 4.7 g C∙m−2∙yr−1. Export of coarse particulate organic matter (> 1 mm) is negligible (normally < 0.1 mg∙L−1), as is oxidation of the suspended load (< 0.5%∙d−1). Effects of summer storms, natural diel variations, and depth of sample from the water column are shown to have a minimal influence on concentrations. Rating curves (kg∙d−1 vs. discharge) are developed to estimate the annual yield of sediment, POM, and DOC, and to evaluate long-term variations. From the results I suggest that in-stream processing and retention devices exert considerable control over the quantity and nature of suspended organic material. Physical processes such as the discharge regime and stream power are relatively less important in determining organic concentrations, but more important in determining sediment concentrations.Key words: seston, carbon, sediment, boreal forest, watershed, river, stream, export