Stream Water Dissolved Organic Carbon Research Articles

Dissolved organic carbon dynamics in the peat–streamwater interface

A series of experiments were conducted to address the fate of dissolved organic carbon (DOC) in the peat - stream interface zone linking a minerotrophic poor fen and an ombrotrophic mire with surrounding stream water in the drainage area of Lake Ortrasket in northern Sweden. Transport and mineralisation of DOC were quantified in peat - stream interface cores in response to variations in pore water velocity, DOC concentration and the molecular size and source of DOC. Mineralisation and CH4 production were positively correlated with pore water velocity at rates between 0.08 and 0.20 cm h(-1) and negatively correlated at rates between 0.20 and 0.40 cm h(-1). The DOC concentration of the effluent from the peat cores was independent of the pore water velocity but proportional to the DOC concentration of the source water. Higher concentrations of DOC were exported from than imported to the peat cores, and the cores exported DOC molecules of smaller average molecular size than received. Carbon mineralisation in the peat, assessed in a static system, was independent of the concentration of DOC. DOC with a nominal cutoff at 100 Da was mineralised faster by streamwater bacteria than DOC dialysed with a cutoff at 3500 Da, and their mineralisation rate was positively correlated with the DOC concentration. Streamwater bacteria mineralised streamwater DOC at a lower rate than the peat - stream interface zone pore water DOC. The pattern of velocity dependence of mineralisation was the same for both sources of peat DOC but the mineralisation rates, average molecular size, and bioavailability of DOC were different, emphasising the importance of the compositional heterogeneity of the peat - stream interface zone for the DOC budget of streamwater.

Stream and interstitial water DOC of a gravel bar (Sitka stream, Czech Republic): characteristics, dynamics and presumable origin

Temporal changes in dissolved organic carbon (DOC) concentration, biodegradability, molecular weight and optical properties within both the stream and interstitial waters of a gravel bar were assayed in a small lowland stream. The mechanisms responsible for the observed quantitative and qualitative changes are discussed with reference to stream DOC (SDOC) and interstitial DOC (IDOC) sources. Both stream and interstitial waters showed similar temporal patterns of DOC concentrations, biodegradability and molecular weight of humic substances, but different temporal structural patterns of DOC molecules during the year. As was noticeable from annual UV-absorbance characteristics and from biodegradable DOC (BDOC) concentrations, SDOC contained more saturated chemical structures, was less bioreactive and was characterized by a lower molecular weight of humic substances, than IDOC. The character of SDOC suggests that it was composed of diagenetically old organic material which is resistant to microbial uptake and DOC from autochthonous production. The IDOC seemed to be diagenetically younger, and to originate more from allochthonous sources, particularly in the upwelling zone. Besides the difference between SDOC and IDOC quality, we have also found differences in IDOC quality between the downwelling and the upwelling zone, suggesting that decomposition of buried particulate organic matter (POM) was the main source of IDOC.

Biogeochemistry of a Treeline Watershed, Northwestern Alaska

Since 1950, mean annual temperatures in northwestern Alaska have increased. Change in forest floor and soil temperature or moisture could alter N mineralization rates, production of dissolved organic carbon (DOC) and organic nitrogen (DON), and their export to the aquatic ecosystem. In 1990, we began study of nutrient cycles in the 800-ha Asik watershed, located at treeline in the Noatak National Preserve, northwestern Alaska. This paper summarizes relationships between topographic aspect, soil temperature and moisture, inorganic and organic N pools, C pools, CO2 efflux, growing season net N mineralization rates, and stream water chemistry. Forest floor (O2) C/N ratios, C pools, temperature, and moisture were greater on south aspects. More rapid melt of the soil active layer (zone of annual freeze-thaw) and permafrost accounted for the higher moisture. The O2 C and N content were correlated with moisture, inorganic N pools, CO2 efflux, and inversely with temperature. Inorganic N pools were correlated with temperature and CO2 efflux. Net N mineralization rates were positive in early summer, and correlated with O2 moisture, temperature, and C and N pools. Net nitrification rates were inversely correlated with moisture, total C and N. The CO2 efflux increased with temperature and moisture, and was greater on south aspects. Stream ion concentrations declined and DOC increased with discharge. Stream inorganic nitrogen (DIN) output exceeded input by 70%. Alpine stream water nitrate (NO3-) and DOC concentrations indicated substantial contributions to the watershed DIN and DOC budgets.

Organic matter chemistry and dynamics in clear-cut and unmanaged hardwood forest ecosystems

Forest harvesting alters the organic matter cycle by changing litter inputs and the decomposition regime. We hypothesized that these changes would result in differences in organic matter chemistry between clear-cut and uncut watershed ecosystems. We studied the chemistry of soil organic matter (SOM), and dissolved organic carbon (DOC) in soil solutions and stream samples in clear-cut and uncut sites at the Hubbard Brook Experimental Forest in New Hampshire using DOC fractionation techniques and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Alkyl-C (aliphatic compounds) and O-alkyl-C (carbohydrates) were the largest C fractions in soil and dissolved organic matter at Hubbard Brook. Alkyl-C ranged from 29–48% of soil C, 25–42% of soil solution C, and 22–42% of streamwater DOC. Carbohydrates comprised 32–49%, 36–43%, and 29–60% of C in soils, solutions, and streamwater, respectively. In both soils and soil solutions, the carbohydrate fraction decreased with increasing soil depth, while the aromaticity of organic matter increased with depth. There were no significant differences in the structural chemistry of SOM between clear-cut and uncut watersheds. The aromatic-C fractions in soil solutions at the clear-cut site ranged from 12–16%, approximately 40% greater than at the uncut site (8.5–11%). Thus, clear-cutting has resulted in the leaching of more highly decomposed organic matter, and depletion of more aliphatic compounds in the soluble organic pool. Because DOC fluxes are small compared to the SOM pool, large differences in soil solution chemistry do not substantially alter the overall composition of SOM. While the organic chemistry of stream DOC varied greatly among 3 sampling dates, there were no obvious clear-cutting effects. Thus, temporal variations in flowpaths and/or in-stream processes appear to be more important than disturbance in regulating the organic carbon chemistry of these streams.

The Trophic Significance of Bacteria in a Detritus-Based Stream Food Web

We compared relative use of streamwater dissolved organic carbon (DOC) by bacteria and the trophic significance of bacteria to invertebrates in two headwater streams at Coweeta Hydrologic Laboratory in North Carolina: a stream with all leaf litter inputs excluded for 1 yr, and a reference stream. Leaf litter standing crop in the treatment stream was <1% that of the reference stream, and fine benthic organic matter (FBOM) was 50% lower than the reference. We used a whole-stream tracer addition of 13C-1 sodium acetate for 3 wk to label bacteria and hence their consumers during both July and December. Bacterial δ13C was measured by collecting respired bacterial carbon. We estimated the contribution of bacterial carbon to consumers using a mixing model for invertebrates and bacteria. The acetate label declined exponentially downstream with a 10-m uptake length in each stream and season. FBOM and biofilm were the only detrital samples to show a strong label; both were more labeled in the litter-excluded stream. Bacteria in the litter-excluded stream had 7–10 times more label than those in the reference stream during both seasons, showing their higher relative use of streamwater DOC. The percentage of invertebrate carbon derived from bacteria was >20% for many taxa. This was significantly related to the percentage of amorphous detritus in invertebrate guts, suggesting that bacterial carbon supporting higher trophic levels was associated with amorphous detrital particles. Predatory invertebrates were labeled, showing that bacterial carbon was important for higher trophic levels. Some invertebrates were more highly labeled than the bacteria. Stenonema in the treatment stream contained eight times more label than measured bacteria. This suggests that they were using an unmeasured bacterial source such as bacteria in exposed epilithic biofilms, which had higher δ13C than all other detrital components. Invertebrates in the treatment stream did not appear to use more bacterial carbon than in the reference stream despite a lower standing crop of detritus. Tallaperla, a shredding stonefly, derived 20–40% of its carbon from bacteria in both streams, even though it was more labeled in the treatment stream. Our estimates of the percentage of invertebrate carbon derived from bacteria were higher than those found in laboratory-based studies. To investigate reasons for this difference, we examined the possibility that bacterial carbon was principally found in exopolymers, as our labeling method would have labeled exopolymers. We found 6 g/m2 of colloidal carbohydrates in the reference stream, which was five times greater than bacterial biomass; thus the high use of bacterial carbon by invertebrates may be a consequence of the availability of these polymers.

Dissolved Carbohydrate Concentration, Composition, and Bioavailability to Microbial Heterotrophs in Stream Water

Dissolved total carbohydrates (DTCH), dissolved free monosaccharides (DFMS), dissolved organic carbon (DOC), biodegradable DOC (BDOC), and humic substances (HS) were measured in White Clay Creek (WCC), a stream in southeastern Pennsylvania Piedmont, USA. Samples were collected over different seasons and under baseflow and stormflow conditions. DOC concentrations ranged from 1.0 to 12.8 mg/L C with the highest concentrations associated with stormflows. Carbohydrates ranged from 0.42 to 12.4 μM and accounted for 2.9 to 12.7 % of the DOC. Humic substances represented the major DOC fraction, accounting for 55 to 72 % of the DOC pool under all flow conditions. The humic fraction had a lower carbohydrate content (4.4%) than the non-humic fraction of DOC (7.2 %). Stormflow DOC was enriched in carbohydrates relative to baseflow DOC, but the percentage of humic-C changed little. Carbohydrates were primarily present as dissolved polysaccharides (55 %), but a significant fraction was bound to humic substances (40%), while a small proportion was present as monosaccharides (5%). The major monosaccharides in stream water, listed in order of decreasing concentration, included glucose, galactose, rhamnose, xylose, arabinose, mannose, and fucose. On average (30.6 ± 7.4)% (n = 44) of the stream water DOC was biodegradable, and carbohydrates accounted for 9.9 to 17.7% of the BDOC.

The chemistry of streams in southwestern and central Nova Scotia, with particular reference to catchment vegetation and the influence of dissolved organic carbon primarily from wetlands

The percentage of wetlands in a catchment accounted for about half of the variance in transformed data for concentrations of dissolved organic carbon (DOC) in 42 Nova Scotian streams draining catchments with 11 different kinds of vegetation. Color increased with DOC, as did total dissolved nitrogen (TDN). Thecolor/DOC andDOC/TDN quotients also rose with increasing DOC, indicating a change in the quality of dissolved organic matter with increasing wetland influence. Dissolved Fe, and to a much lesser extent dissolved Al, showed a strong positive correlation with DOC. Stream pH showed a strong negative correlation with DOC, largely from wetlands, and a strong positive correlation with non- marine Ca2+ weathered from mineral soils. Non-marine SO4 2− from acid deposition had no apparent influence on stream pH and decreased with increasing streamwater DOC in summer, presumably owing to reduction processes in wetlands that mitigated the effects of acid deposition. Apparently, these reduction processes also produced small amounts of dissolved, non-ionic organic sulfur. Non-marine Ca2+ was related strongly to the percentage of upland hardwood forests in the catchments. Wetlands exert a profound influence on the chemistry of streams, principally through their export of DOC but also because of reduction reactions in their anerobic peats.

Modification of Snowmelt Chemistry by Forest Floor and Mineral Soil, Northern Michigan

AbstractPrecipitation, snowpack, snowmelt, forest floor percolate, soil water, and streamwater chemistry were studied throughout winter for 7 yr in a small (176 ha) Northern Michigan watershed vegetated by northern hardwoods. The objective was to see how precipitation and snowmelt chemistry is modified before it reaches the stream. Intermittent snowmelt occurred throughout winter, and 65 to 78% of total snowpack loss of H+, NH+4, NO−3, and SO2−4 occurred before peak snowpack and snowmelt. From 70 to 95% of snowmelt H+, NH6+4, and NO−3 was retained in the forest floor and shallow mineral soil by exchange, adsorption, or biological uptake. Snowmelt SO2−4 concentration was enriched in forest floor percolate and shallow soil water probably by organic mineralization, low adsorption, and perhaps desorption. The significant relationship between dissolved organic carbon (DOC) and SO2−4 concentration in forest floor percolate and shallow soil water indicates organics may contribute to reduced SO2−4 adsorption. Sulfate and organic anions were major contributors to base cation (CB) flux in the forest floor while SO2−4 was related to CB flux in shallow soils. A decline in mineral soil and streamwater DOC concentration and increase in streamwater HCO−3 relative to concentrations in forest floor percolate suggests the relative importance of organic decomposition decreased and soil mineral weathering increased with soil depth. High streamwater Ca2+, Mg2+, and HCO−3 concentrations indicated that soil mineral weathering was a major process altering snowmelt chemistry at the watershed level.

Variability in Removal of Dissolved Organic Carbon in Hyporheic Sediments

Dissolved organic carbon (DOC) is consumed by microbial metabolism as streamwater perfuses through a lateral gravel bar of the East Branch of the Wappinger Creek. The rate of DOC removal was estimated from the decline in DOC and travel time through the bar. Variability in DOC removal together with potential regulatory factors was determined for 14 dates spanning more than a 2-y period. DOC removal was not correlated with temperature, availability of oxygen, or residence time within hyporheic sediments. Hyporheic DOC could be predicted (r² = 0.68) from streamwater DOC concentrations, with a surprisingly constant 57% (±9% [1 SD]) removal of DOC. This pattern suggests an initial concentration-dependent adsorption of DOC onto surfaces. This mechanism allows for efficient retention of DOC within hyporheic sediments even under conditions (low temperature, high interstitial velocity) that might be expected to minimize biotic consumption of DOC.

Water quality trends at an upland site in wales, UK, 1983–1993

Ten years of detailed upland stream and bulk deposition water quality data from Plynlimon, mid-Wales, are examined for trend. A robust statistical test (the seasonal Kendall test) is applied and data are presented graphically. Smoothing techniques are used to highlight the patterns of change which underlie high data scatter. The graphs show long-term cycles within the data which violate the assumptions of common statistical tests for trend. These cycles relate to fluctuations in the weather patterns at Plynlimon. Even though the seasonal Kendall test is significant for some determinands, the evidence from the graphs suggests that many of these ‘trends’ are unlikely to continue. For solutes in rainfall, there is no convincing long-term trend. There is a possible increase in ammonium concentrations, which may indicate an increasing atmospheric source generated by farming activities, but this will require a longer data series for confirmation. Several trace metal concentrations increased significantly part way through the study period, but later returned to the original levels. The bulk precipitation sea salt input has been uneven over the 10-year sampling period, with the highest inputs occurring during the wetter winters. For solutes in streamwaters, there are clear trends in dissolved organic carbon (DOC), iodine and bromide, which increase over time and may be attributed to an increase in organic decomposition in the catchment. Previous studies in Wales have shown similar behaviour for colour, which is related to DOC, but the corresponding changes for bromide and iodine are new. For most other streamwater determinands, any changes are masked by the effects of year to year variations in the quality and quantity of rainfall. For example, zinc and chromium variations parallel the corresponding rainfall quantity variations. The effect of rainfall quality variation is marked for marine-derived elements such as chloride. For sulphate, streamwater variations are inverted relative to chloride. This suggests that dry deposition may vary with weather conditions: high when the wind direction is from the land and low when weather systems are predominantly frontal and laden with sea salts. Alternatively, high sea salt rainfall may be affecting absorption/solubility reactions in the soils. There are four main conclusions. Firstly, there is no indication of changing acid deposition inputs or changing acidity within the runoff, despite a decline in UK sulphur dioxide emissions. Secondly, streamwater DOC has shown an increase over time, but there is no clear corresponding decrease in pH as might be expected from acidification theory. Thirdly, there are cyclical variations in bulk precipitation inputs and in streamwater quality, which mean that trends cannot be established even with 10 years of data. Long-term cycles are likely to exist in other environmental data and extreme care is required for the interpretation of trend, especially if data sets are short. This aspect strongly supports the continuation of long-term monitoring programmes over several decades. Finally, the graphical application strongly enhances data analysis and should be considered an essential component of trend investigation.