Organic Matter Budgets for Streams: A Synthesis

Suspended Particulate Organic Matter Concentration and Export in Streams

Transport of coarse particulate organic matter (CPOM) derived from forest litterfall has been hardly studied in rivers, unlike fine particulate organic matter (FPOM) or dissolved organic matter (DOM). Yet, many rivers are dammed or run into lakes, and there is growing evidence that CPOM accumulation in river delta participates substantially in ecological processes such as greenhouse gas emissions of lakes and reservoirs. We investigated the transport of CPOM and FPOM by the Leysse River (discharge from 0.2 to 106 m3 s−1) to Lake Bourget (France) in relation to aerial litter deposition, river network length, and discharge. Over a 19-month study period, the volume-weighted mean CPOM and FPOM concentrations were 1.3 and 7.7 g m−3, respectively. Most CPOM and FPOM transport occurred during major flood events, and there were power relationships between maximum discharge and particulate organic matter (POM) transport during these events. The annual export of CPOM (190 t AFDM) was 85% of the litter accumulation in autumn on permanent sections of the riverbed (224 t AFDM), which suggests that export is a major process compared to breakdown. Export of CPOM was 1.25 t yr−1 km−2 of the forested catchment area. This study highlights the need to account for long-range CPOM transport to describe the fate of litter inputs to streams and to quantify the organic matter input and processing in lakes and reservoirs.

Large-Scale Trends for Stream Benthic Respiration

Export of coarse particulate organic matter (CPOM) from three headwater streams was studied continuously over a period of 8 (one stream) to 9.5 years (two streams) at the Coweeta Hydrologic Laboratory, North Carolina. Annual CPOM export among years varied by factors of 9.2 to > 16 ×, much greater than the 3.5 to 5.0 × variation in annual stream discharge. Annual export was poorly correlated with annual discharge and was more a factor of storm frequency and magnitude. Over 63 to >77% of all CPOM export occurred during the largest 20 storms. Export during individual sampling intervals was strongly related to maximum discharge during the interval (r² 0.43-0.54, p<0.001) for all streams throughout the study. Total leaf export and leaf export per unit maximum discharge were greatest during the autumn and lowest during the spring and summer months in all streams. Export of woody debris was more seasonally variable than that of leaves in two of the three streams. Although allochthonous CPOM is the greatest source of organic matter inputs to these streams (>86%), CPOM represented only 1.8-3.8% of total organic matter export, indicating that CPOM retention is high in these small, high-gradient streams. Most export occurs in the form of fine particulate organic matter (FPOM) and dissolved organic matter (DOM), or respired CO₂. Stream retentiveness was further illustrated by movement of marked artificial "leaves" and "sticks", which displayed maximum downstream movement of ∼42 m/yr for "leaves" and 10 m/yr for "sticks". Surprisingly, export as a percent of inputs was greatest for woody debris. This is probably attributable to both underestimates of woody litter inputs and slower decomposition rates for woody debris, resulting in greater standing crops and availability of woody debris for transport during storms. For three of the eight years of study, one stream (C 54) received seasonal treatments with an insecticide that drastically reduced invertebrate populations. Large increases in leaf litter standing crop were observed following three years of treatment compared with untreated streams. However, CPOM export from the treated stream did not increase significantly compared with untreated streams. During treatment the CPOM:FPOM ratio of exported particulate organic matter was over twice that of untreated streams, or C 54 during periods when the stream was not treated. This increase was primarily a result of massive reduction in FPOM export, rather than large increases in CPOM export. Although storms removed large amounts of CPOM from the wetted perimeter of all streams during 1989 and 1990, most removed material was retained within the narrow riparian zone, and not exported to downstream reaches. Movement and exchange of CPOM between the wetted perimeter and adjacent riparian zones in these high-gradient streams demonstrates similarities with streams draining lowland regions with extensive floodplains.

Organic material transport in the New River, Virginia, was investigated over a 12 month period. Collections were made using drift nets and grab water samples from bridges at two sites about 210 km apart. About midway between the two sampling sites is a 1920 ha impoundment used for flood control and power generation. Dissolved organic matter (DOM) ranged 1–50 mg l−1 at Site 1, upstream from the impoundment, and 11–19 mg l−1 at Site 2 and was the most abundant form of organic matter at both sites during most periods of the year. Fine particulate organic matter (FPOM) ranged 1–45 mg l−1 at Site 1 and 1–9 mg l−1 at Site 2. Concentration of coarse particulate organic matter (CPOM) ranged 0.1–0.7 mg l−1 at Site 1 and 0.1–0.2 mg l−1 at Site 2. On an annual basis, the organic matter loads at Site 1 and Site 2 were computed to be 67 000 and 76 800 T y−1, respectively, suggesting that the impoundment trapped and processed POM, and that municipal and industrial treatment facilities between the study sites supplemented DOM in the river.

Photochemical and microbial transformation of particulate organic matter depending on its source and size

Coarse particulate organic matter (CPOM), especially leaves, plays a key role in food webs of many streams and can be converted to fine particulate organic matter (FPOM) by specific macroinvertebrate functional groups (i.e., shredders). In this study, we examined crayfish [Orconectes obscurus (Hagen)] effects on FPOM generation from two leaf types, red maple and white oak, that differ in recalcitrance and decomposition rates. Further, we examined potential quality differences between FPOM generated by crayfish via fragmentation and defecation. Crayfish were fed stream-conditioned maple or oak leaves in hanging 1-mm mesh-bottom baskets in aquaria. After 12 h, C:N ratios and dry/ash-free dry weights of remaining CPOM, FPOM fragments that fell through the mesh, and crayfish feces (collected using finger cots that encased the crayfish abdomens) were determined. Loss of CPOM attributable to crayfish feeding was higher for maple than oak; fragment FPOM and crayfish feces generation were also higher for maple. Maple CPOM percent organic matter was lower than oak CPOM but feces and fragment FPOM percent organic matter did not differ among leaf species suggesting that crayfish actions homogenize the properties of particulate organic matter. Both C:N ratios and bacterial abundance were also altered by crayfish processing and digestion underscoring potential crayfish effects on FPOM bioavailability. Overall, crayfish altered the ontogeny of the detritus, which may, in turn, affect stream FPOM dynamics.

The McMurdo Dry Valleys of South Victoria Land, Antarctica, form the largest of the desert oases found along the coast of Antarctica. Although this region is an extreme cold desert, glacial meltwater streams, which flow for 6-10 wk during the austral summer, are a prominent feature of the landscape. Canada Stream is in Taylor Valley and is one of 3 major streams draining Canada Glacier. The surrounding landscape is characterized by an absence of plant life, an underlying permafrost at a depth of 0.5 m, and prominent "patterned ground" features, which are large (3-5 m across) polygonal patterns formed through freeze/thaw cycles. In contrast to the barren landscape, many of the streams contain perennial algal mats and mosses, illustrating that the presence of liquid water allows for a photosynthetically based ecosystem even under the harshest conditions. Canada Stream is one of 10 streams that flow into Lake

To characterize organic matter dynamics in headwater streams of southwestern British Columbia, Canada, we monitored epilithic biomass and fluvial transport of organic matter. Fluvial organic matter was collected once to twice monthly from 7 streams: samples were processed as coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and dissolved organic matter (DOM). Epilithon was collected monthly from unglazed, ceramic tiles in 12 stream reaches and measured for ash-free dry mass (AFDM) and chlorophyll-a. CPOM and FPOM concentrations were higher during the wet season (1 October-30 April) than the dry season (1 May-30 September). Averaged across sites, CPOM and FPOM concentrations were positively related to mean daily discharge averaged over the 7 days before sample collection. In contrast, discharge and DOM concentration were inversely related. DOM concentration was positively correlated to chlorophyll-a accrual. Seasonal variation in epilithic chlorophyll-a and AFDM accumulation was high. AFDM accumulation rates ranged from a low in January 1997 of 0.31 to a high of 3.6 μg cm -2 d -1 in May 1997. As with organic matter in stream water, discharge influenced the temporal dynamics of epilithic biomass accumulation; there was a negative correlation between discharge and epilithic AFDM.

Nutrient enrichment affects bacteria and fungi associated with detritus, but little is known about how biota associated with different size fractions of organic matter respond to nutrients. Bacteria dominate on fine (<1 mm) and fungi dominate on coarse (>1 mm) fractions, which are used by different groups of detritivores. We measured the effect of experimental nutrient enrichment on fungal and bacterial biomass, microbial respiration, and detrital nutrient content on benthic fine particulate organic matter (FPOM) and coarse particulate organic matter (CPOM). We collected FPOM and CPOM from 1 reference and 1 enriched stream. CPOM substrates consisted of 2 litter types with differing initial C:nutrient ratios (Acer rubrum L. and Rhododendron maximum L.). Fungal and bacterial biomass, respiration, and detrital nutrient content changed with nutrient enrichment, and effects were greater on CPOM than on FPOM. Fungal biomass dominated on CPOM (∼99% total microbial biomass), whereas bacterial biomass dominated on FPOM (∼95% total microbial biomass). These contributions were unchanged by nutrient enrichment. Bacterial and fungal biomass increased more on CPOM than FPOM. Respiration increased more on CPOM (up to 300% increase) than FPOM (∼50% increase), indicating important C-loss pathways from these resources. Microbial biomass and detrital nutrient content were positively related. Greater changes in nutrient content were observed on CPOM than on FPOM, and changes in detrital C:P were greater than changes in detrital C:N. Threshold elemental ratios analyses indicated that enrichment may reduce P limitation for shredders and exacerbate C limitation for collector-gatherers. Changes in CPOM-dominated pathways are critical in predicting shifts in detrital resource quality and C flow that may result from nutrient enrichment of detritus-based systems.

Effect of plant richness on the dynamics of coarse particulate organic matter in a Brazilian Savannah stream

Loam and sandy soils, and the earthworm casts produced with 14C-labelled plant material in both soils, were incubated in airtight glass vessels with and without enchytraeids to evaluate the effects of soil fauna on the distribution and fragmentation of organic matter. After 1, 3, and 6 weeks, the amount of C mineralised was determined in soils and earthworm casts, and the soil was fractionated into particulate organic matter (POM), the most active pool of soil organic matter, after complete physical dispersion in water. The percentage weight of fine fractions (0–50 µm) was 67.4% in the loam soil. Sand (coarse, i.e. 150–2,000 µm and fine 50–150 µm) represented 87.2% of total weight in sandy soil, while the percentages of C (PC) were 23.2% in coarse POM (2,000–150 µm) and 11.9% in fine POM (150–50 µm). These percentages were higher than those in loam soil, i.e. 3.4% (coarse POM) and 5.4% (fine POM). The PC in coarse POM (9.50%) and fine POM (16.4%) remained higher in casts from sandy soil than in casts from loam soil (4.7% in coarse and 14.3% in fine POM). The highest percentages of 14C-labelled leaves were found in fine fractions, 55.9% in casts from loam soil and 48.8% in casts from sandy soil. The C mineralisation of the added plant material was higher in casts from the sandy soil (20.3%) than from the loam soil (13.5%). Enchytraeids enhanced C mineralisation in the bulk sandy soil, but did not affect the mineralisation of added plant material in either soil. The main enchytraeid effect was enhancement of the humification process in the bulk sandy soil, the casts from this soil, and the bulk loam soil.

Summary We assessed the key role of aquatic fungi in modifying coarse particulate organic matter (CPOM) by affecting its breakdown rate, nutrient concentration and conversion to fine particulate organic matter (FPOM). Overall, we hypothesised that fungal‐mediated conditioning and breakdown of CPOM would be accelerated when nutrient concentrations are increased and tested the degree to which fungi were critical to CPOM processing and to FPOM production by an invertebrate consumer. We manipulated the presence and absence of fungi, exogenous nutrients [nitrogen (N) and phosphorus (P)] and an invertebrate consumer in a full‐factorial laboratory experiment and quantified their effects on CPOM mass loss and nutrient concentration, and the quantity and nutrient concentration of FPOM produced during leaf breakdown. Mass of CPOM lost and the quantity of FPOM produced were highest in nutrient‐enriched treatments containing fungal decomposers. Across all treatments, FPOM produced was a constant proportion of CPOM lost (67%). Loss of CPOM due to shredders was highest at fungal biomass values &gt;16 mg g dry mass−1, which occurred with nutrient enrichment. Nitrogen concentration in CPOM increased in treatments with nutrients and fungi, while CPOM P concentration was primarily affected by nutrients. In contrast, FPOM P concentration declined in treatments with fungi and nutrients, suggesting either sequestration via CPOM‐associated fungi or preferential assimilation by shredders. Nitrogen in FPOM increased with nutrients, but was unaffected by fungi. Our results indicate that aquatic fungi play a critical role in facilitating energy and nutrient flow through detrital pathways and that their ability to mediate organic matter transformations is significantly influenced by nutrient enrichment.

Introduction: Coarse particulate organic matter originated in riparian vegetation is the main source of energy in many headwater streams. However, the transfer of such material is altered by the destruction of forests. Objective: To assess flow differences of this organic matter in streams with forests and grasslands. Methods: We compared input, storage and export of this organic matter in the riparian belts of streams with forests, and streams with grasslands, in the central Andean region of Colombia. For each stream, we measured vertical and lateral litter with baskets; stream bed litter with a quadrant, and matter export with drift nets, in a 100 m reach. Results: The streams with riparian forest received an average of 915 g m-2 of coarse particulate organic matter annually, exported a total of 334 g m-3 and stored 732 g m-2, values that were significantly higher than in grassland streams, where the corresponding values were: 125.4 g m-2; 128 g m-3 and 205.5 g m-2. Conclusions: The removal of tree cover from the riparian zone reduced the organic matter in these headwater streams of Colombia.

In response to water quality standard violations linked to excessive organic matter (OM) and a lack of sampling data informing the Total Maximum Daily Load (TMDL), an organic matter budget was created to quantify and identify sources of OM in the lower Jordan River (Salt Lake City, UT). By sampling dissolved, fine, and coarse particulate OM, as well as measuring ecosystem metabolism at seven different sites, the researchers aimed to identify the origin of excess OM, and understand pathways by which different size classes of the OM pool are generated. The dissolved fraction (DOM; 94 %) was found to be the dominant form of OM transported within the river with fine particulate organic matter (FPOM; 6 %) the second most abundant, and coarse particulate organic matter (CPOM; 1 %) transport relatively insignificant in the overall OM budget. Primary production exceeded respiratory losses in the upper river, and this, along with OM inputs from two tributaries (where water reclamation facilities discharge into the river) delivered excess OM to the impaired lower reaches. Increasing stream metabolism index (SMI) with distance downstream (>1 in the lower river) further demonstrated that transport of excessive organic matter into the lower river was from upstream sources and not due to lateral inputs. This simple approach to characterizing the organic matter budget as it relates to water quality in the Jordan River was effective and could serve as a model for future studies attempting to quantify and identify sources of OM in urban ecosystems.