Coarse and Fine Particulate Organic Matter Transport by a Fourth-Order Mountain Stream to Lake Bourget (France)

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.

Abundance- and biomass-based metrics of functional composition of macroinvertebrate as surrogates of ecosystem attributes in Afrotropical streams

The composition of macroinvertebrate functional feeding groups (FFGs) has been used as surrogates of ecosystem attributes in aquatic ecosystems but studies that utilize such knowledge are still limited in the tropics. This study investigated the suitability of abundance- vs. biomass-based metrics of macroinvertebrate FFGs as surrogates of ecosystems attributes of the Sosiani-Kipkaren River in western Kenya. Macroinvertebrates were sampled in wet and dry seasons, classified into five FFGs and used to derive five metrics that are surrogates of ecosystem attributes; 1) a balance between autotrophy and heterotrophy, 2) linkage between riparian inputs of coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM), 3) top-down predator control, 4) geomorphic channel stability, and 5) relative dominance of fine particulate organic matter (FPOM) in transport compared to FPOM deposited in the sediments. Taxon richness, abundance and biomass of shredders were higher in forested sites, scrapers were numerically dominant in mid-order streams, whereas collectors dominated agricultural and urban sites. Abundance-based metrics were better predictors of ecosystem attributes and displayed a greater response to changes in stream size than biomass-based metrics. Moreover, there was incongruence between abundance- and biomass-based indicators for P/R and CPOM/ FPOM. Catchment land use did not influence metric performance, suggesting that reach scale influences played a predominant role in structuring communities and determining ecosystem functioning. Although the use of FFGs as indicators of ecosystem integrity and functioning in this river show promise, the lack of agreement between abundance- and biomass-based measures suggests that more studies are needed to refine the metrics used.

Spatially Varying Trophic Effects of Reservoir-Derived Plankton on Stream Macroinvertebrates Among Heterogeneous Habitats Within Reaches

SummaryDam reservoirs often supply high amounts of plankton to downstream reaches, leading to a critical shift of trophic origins of stream ecosystems from natural sources (e.g. attached algae and terrestrial inputs) to reservoir-oriented plankton. Although this is a widely observed phenomenon, previous studies focused only on lotic habitats (e.g. riffles) rather than lentic habitats such as backwaters and isolated ponds (IP).Using a stable isotope three-source mixing model, we evaluated trophic contributions of reservoir-derived plankton, epilithon and terrestrial leaves to stream macroinvertebrates at four dam outlet reaches and two reference reaches in the Natori River catchment, Japan. We compared four different habitat types co-occurring within the reaches: lotic habitat (riffle and pool), bar-head (BH) lentic habitat, bar-tail (BT) lentic habitat (backwater) and isolated pond (IP) on sandy bars.The trophic contributions of reservoir-derived plankton were significantly lower in lentic habitats (BH, 15.4%; BT, 10.4%; IP, 9.1%) than in lotic habitats (mean, 27.7%). This was especially notable for filter feeders that feed on suspended fine particulate organic matter (SFPOM). The three-source model analysis indicated a lower biomass proportion of dam plankton in lentic SFPOM (mean, 21.2%) than in lotic SFPOM (mean, 35.6%). This difference in SFPOM composition was reflected in the lower trophic contribution of dam plankton to lentic filter feeders.The abundance ratio of filter feeders in the community was decreased in lentic habitats, while the abundance ratios of collector-gatherers, scrapers and shredders were increased. Macroinvertebrates in lentic habitats fed on sources less mixed with reservoir-derived plankton (e.g. benthic coarse particulate organic matter [BCPOM], benthic fine particulate organic matter [BFPOM] and epilithon); therefore, the trophic impact of reservoirs was indistinctive at the community level, indicating that lentic habitats can function as trophic refugia to mitigate the trophic impact of reservoirs.Because lentic habitats were decreased in area (accounting for 5.7% of average total area) in the downstream reaches of dams due to riverbed degradation, lentic habitats must be created in order to restore the trophic impact of reservoirs in river ecosystems.

Metabarcoding-based fungal diversity on coarse and fine particulate organic matter in a first-order stream in Nova Scotia, Canada

Most streams receive substantial inputs of allochthonous organic material in the form of leaves and twigs (CPOM, coarse particulate organic matter). Mechanical and biological processing converts this into fine particulate organic matter (FPOM). Other sources of particles include flocculated dissolved matter and soil particles. Fungi are known to play a role in the CPOM conversion process, but the taxonomic affiliations of these fungi remain poorly studied. The present study seeks to shed light on the composition of fungal communities on FPOM and CPOM as assessed in a natural stream in Nova Scotia, Canada. Maple leaves were exposed in a stream for four weeks and their fungal community evaluated through pyrosequencing. Over the same period, four FPOM size fractions were collected by filtration and assessed. Particles had much lower ergosterol contents than leaves, suggesting major differences in the extent of fungal colonization. Pyrosequencing documented a total of 821 fungal operational taxonomic units (OTU), of which 726 were exclusive to particles and 47 to leaf samples. Most fungal phyla were represented, including yeast lineages (e.g., Taphrinaceae and Saccharomycotina), Basidiomycota, Chytridiomycota and Cryptomycota, but several classes of Pezizomycontina (Ascomycota) dominated. Cluster dendrograms clearly separated fungal communities from leaves and from particles. Characterizing fungal communities may shed some light on the processing pathways of fine particles in streams and broadens our view of the phylogenetic composition of fungi in freshwater ecosystems.