Leaf Litter Processing Rates Research Articles

Generalization of a density‐dependent ecosystem function in dominant aquatic macroinvertebrates

The processing of organic matter is a central ecosystem function in freshwater ecosystems that allows the integration of terrestrial plant material into aquatic heterotrophic food webs. From an energetic perspective, many temperate aquatic systems fundamentally depend on such allochthonous inputs. Current research established that this central ecosystem function may be linked to population density of few yet dominant invertebrate detritivores, showing a negative density‐dependent relationship. Here, we extended the current knowledge and experimentally assessed the processing of leaf litter by freshwater amphipods, using broad density gradients, interspecific competition, and laboratory and field experiments. We used two species of dominant amphipods, namely the native Gammarus fossarum and the non‐native Gammarus roeselii, varying their density by two orders of magnitude, both in single‐species and two‐species treatments. Results from 252 mesocosms in the lab and 97 mesocosms in a natural stream show that per capita leaf litter processing rates are strongly negatively dependant on population density in monocultures. Interspecific competition in two‐species treatments corroborated the negative density‐dependent ecosystem function and highlighted the functional redundancy of the two detritivore species. We identified a flattening in the processing rates at previously unreported but well‐defined breakpoints. These breakpoints may reflect minimal metabolic requirements at which survival takes precedence over any other process, such as interference competition. The breakpoints were consistent across both species, indicating that they must process leaf litter equalling one fifth to one fourth of their dry body weight per day as a minimal nutritional threshold. Our results suggest the need for integrating nonlinear density‐dependencies and breakpoint population densities in key ecosystem functions in aquatic ecosystem modelling alongside classical biodiversity–ecosystem functioning relationships. Depending on the population density the corresponding ecosystem functioning could differ significantly. This corroborates the need to better understand biodiversity–abundance relationships when protecting aquatic ecosystems.

Procesado de hojarasca en arroyos del Parque Nacional de la Sierra de Guadarrama (Madrid)

Le af litter breakdown in streams of Sierra de Guadarrama National Park (Madrid) The main goal of this study was to identify the main drivers of natural variability of leaf litter processing in six well preserved Mediterranean mountain streams in the Sierra de Guadarrama, through the analysis of the breakdown rates of alder litter, content and dynamics of leaf nutrients, and associated fungal and invertebrates communities. In general, breakdown rates of alder leaves, both exponential (k = 0.013± 0.005 dd –1 , k ′ = 0.041± 0.011 d –1 ) and lineal (b = 0.470± 0.121% PSLC dd –1 , b ′ = 1.522± 0.153% PSLC d –1 ) were very high. Variability in breakdown rates among streams was not correlated to environmental characteristics (discharge, slope or water temperature) neither to stream water characteristics (nutrient concentration, pH, or conductivity). This may be due to the low environmental heterogeneity observed among these streams and also because the physical factors that could regulate this process were outweighed by biological processes that mask them. The highest breakdown rates were recorded in the coldest waters where the activity of decomposers aquatic fungi was very low (measured by sporulation rates, 98.36±148.84 conidia mg –1 PSLC d –1 ), suggesting that consumer invertebrates played a key role in the faster leaf litter processing rates in these sites. Family richness (13.67±3.44 fam. bag –1 ) and macroinvertebrate density (25.33±10.42 ind bag –1 ), as well as shredder density (16.17±7.44 ind bag –1 ) did not show any relationship with the breakdown rates (p> 0.100, neither linear nor exponential rates). A positive relationship was only found between percentage of leaf litter lost and abundance of Limnephilidae (p< 0.010), that pointed out a higher relevance of shredder traits than their total abundance. Limnephilidae are caddisfly shredders adapted to low thermal regimes, they have large body and are able to feed on unconditioned litter debris and poor colonized by aquatic hyphomycetes, which seem to exert a decisive control over leaf litter processing rate in this type of aquatic ecosystems. Therefore, we can suppose that in colder headwater mountain streams consumer macroinvertebrates (shredders) might have a major relevance in the processing of litter debris whereas decomposers (aquatic hyphomycetes) would play an increasing role in warmer streams.

Leaf litter processing and invertebrate assemblages along a pollution gradient in a Maine (USA) headwater stream

During the autumn of 1997 and 1998, leaf litter processing rates and leaf pack invertebrate assemblages were examined at eight stations along a pollution gradient in Goosefare Brook, a first-order coastal plain stream in southern Maine (USA). There was no significant effect on litter softening rate in 1997, and only the most polluted station showed a decrease in 1998. However, litter loss rates showed decreases in both years. The structure of invertebrate assemblages changed in response to the stresses, showing a decline in EPT richness and an increase in the proportion of collecting taxa. However, total shredder biomass was only weakly affected. Shredder biomass at all stations was dominated by Tipula, and biomass of other shredder taxa showed a serial replacement along the gradient of stress related to their pollution tolerance. Rather than the expected relationship with shredder biomass, litter processing rates were directly related to water and sediment quality. Goosefare Brook demonstrates how variable pollution tolerance of community members enables stress resistance and a consequent preservation of ecosystem function.

Leaf litter processing rates in a Kenyan highland stream, the Njoro River

Mass loss of leaf litter from five common riparian trees in Kenya was determined using mesh bags over a 70-day period. The study was carried out in a river which previous research has shown to be apparently devoid of invertebrate shredders. There was a five fold difference between the most rapid and slowest decay rates, but all were within the fast range identified from temperate zone trees, demonstrating high rates of microbial activity. We conclude, therefore, that the low biomass of invertebrate shredders in Kenyan highland streams cannot be explained by poor quality of the leaf litter that enters them.

Field and laboratory investigations on the effects of road salt (NaCl) on stream macroinvertebrate communities

Field and laboratory experiments were conducted to examine the effects of road salt (NaCl) on stream macroinvertebrates. Field studies investigated leaf litter processing rates and functional feeding group composition at locations upstream and downstream from point source salt inputs in two Michigan, USA streams. Laboratory studies determined the effects of increasing NaCl concentrations on aquatic invertebrate drift, behavior, and survival.Field studies revealed that leaves were processed faster at upstream reference sites than at locations downstream from road salt point source inputs. However, it was sediment loading that resulted in partial or complete burial of leaf packs, that affected invertebrate activity and confounded normal leaf pack colonization. There were no significant differences that could be attributed to road salt between upstream and downstream locations in the diversity and composition of invertebrate functional feeding groups. Laboratory drift and acute exposure studies demonstrated that drift of Gammarus (Amphipoda) may be affected by NaCl at concentrations greater than 5000 mg/l for a 24-h period. This amphipod and two species of limnephilid caddisflies exhibited a dose response to salt treatments with 96-h LC50 values of 7700 and 3526 mg NaCl/l, respectively. Most other invertebrate species and individuals were unaffected by NaCl concentrations up to 10,000 mg/l for 24 and 96 h, respectively.

Restoration of Biogeochemical Function in Mangrove Forests

AbstractForest structure of mangrove restoration sites (6 and 14 years old) at two locations (Henderson Creek [HC] and Windstar [WS]) in southwest Florida differed from that of mixed‐basin forests (&gt;50 years old) with which they were once contiguous. However, the younger site (HC) was typical of natural, developing forests, whereas the older site (WS) was less well developed with low structural complexity. More stressful physicochemical conditions resulting from incomplete tidal flushing (elevated salinity) and variable topography (waterlogging) apparently affected plant survival and growth at the WS restoration site. Lower leaf fall and root production rates at the WS restoration site, compared with that at HC were partly attributable to differences in hydroedaphic conditions and structural development. However, leaf and root inputs at each restoration site were not significantly different from that in reference forests within the same physiographic setting. Macrofaunal consumption of tethered leaves also did not differ with site history, but was dramatically higher at HC compared with WS, reflecting local variation in leaf litter processing rates, primarily by snails (Melampus coffeus). Degradation of leaves and roots in mesh bags was slow overall at restoration sites, however, particularly at WS where aerobic decomposition may have been more limited. These findings indicate that local or regional factors such as salinity regime act together with site history to control primary production and turnover rates of organic matter in restoration sites. Species differences in senescent leaf nitrogen content and degradation rates further suggest that restoration sites dominated by Laguncularia racemosa and Rhizophora mangle should exhibit slower recycling of nutrients compared with natural basin forests where Avicennia germinans is more abundant. Structural development and biogeochemical functioning of restored mangrove forests thus depend on a number of factors, but site‐specific as well as regional or local differences in hydrology and concomitant factors such as salinity and soil waterlogging will have a strong influence over the outcome of restoration projects.

Biotic Indices and Stream Ecosystem Processes: Results from an Experimental Study

We investigated the ability of the North Carolina Biotic Index (NCBI) and the Ephemeroptera + Plecoptera + Trichoptera (EPT) index to track an experimental manipulation of the invertebrate community and resultant alteration of several ecosystem‐level processes in a headwater stream at the Coweeta Hydrologic Laboratory in western North Carolina. Indices were calculated from quantitative monthly or bimonthly benthic samples of moss‐covered rockface and mixed substrate habitats, as well as habitat‐weighted values based on the proportion of each habitat in the two streams. One stream (C 55) served as a reference stream over the 6‐yr period of late 1984 through 1990, whereas the other (C 54) received seasonal treatments with an insecticide for 3 yr (1986‐1988). Throughout pretreatment, treatment, and recovery, both the NCBI and EPT indices tracked the disturbance regime of the treatment stream. Indices for the reference stream varied little during the 6‐yr period. Both the NCBI and EPT suggested strong changes in the treatment stream during treatment relative to both pretreatment and the reference stream. Following cessation of insecticide treatments, both indices reflected improved biotic conditions during first and second years of recovery in C 54. Compared with fauna of mixed substrates, rockface fauna had lower (better) NCBI values during pretreatment, and exhibited a greater proportional increase in tolerant taxa during treatment than mixed substrates, emphasizing the importance of including rockface communities in environmental monitoring programs. Changes in both the EPT and NCBI indices closely corresponded to changes in ecosystem‐level processes observed in C 54 from pretreatment to treatment, and recovery periods. These processes include: leaf litter processing rates, organic matter storage, fine particulate organic matter generation and export, and secondary production. With the exception of organic matter storage, all of these processes declined during treatment of C 54, and subsequently increased during recovery. Our results demonstrate the potential of such indices to detect and monitor stream ecosystem changes during and following disturbance. The EPT index was by far the easiest to use from both the standpoint of time required for sample processing and ease of application. Compared with the labor‐intensive sample processing, specimen identification and measurement, and data entry required for secondary production calculations, the EPT index was relatively simple and displayed a remarkable ability to track secondary production of invertebrates in the treatment stream. Our data strongly support the inclusion of the EPT and NCBI indices in these southern Appalachian headwater streams as indicators of both degradation and recovery of stream ecosystem processes from chemical‐induced disturbance.

Fungal biomass and leaf litter processing in streams of different water chemistry

Leaf litter processing rates and fungal biomass on leaf detritus were compared in four streams of different water chemistry. The streams drained catchments underlain by different bedrock types and varied in mean pH from 4.3 to 7.5 and in mean alkalinity from 0.0 to 35.8 mg CaCO3 l−1. Processing rates were fastest in WS3 and WS4, which had a pH of 6.0; slowest in SFR, which had a pH of 4.3; and intermediate in HSR which had a pH of 7.5. Fungal biomass as measured by the fungal sterol, ergosterol, was similar in WS3, WS4, and HSR but was much lower in SFR. These results suggest that reduced processing rates in SFR were associated in part with reduced fungal biomass on the leaves, whereas reduced processing rates in HSR were not related to differences in fungal biomass on the leaves.

The Influence of Lepidostoma (Trichoptera: Lepidostomatidae) on Recovery of Leaf-Litter Processing in Disturbed Headwater Streams

-Two headwater streams draining catchments 53 and 54 (C53 and C54, respectively) at the Coweeta Hydrologic Laboratory in western North Carolina were treated with insecticide in 1980 (C53) and 1986-1988 (C54). During recovery periods in both streams, Lepidostoma spp. were abundant, early colonizers. Densities of Lepidostoma in litterbags and benthic samples collected from recovering streams were substantially higher than in untreated streams (pretreatment and reference streams). During treatment years, leaf-litter processing rates were severely reduced relative to pretreatment and reference streams. In contrast, litter processing rates in C53 and C54 during recovery were faster than those in untreated streams (pretreatment and reference). Rhododendron is one of the most refractory leaves commonly found in Coweeta streams; however, percent increase of rhododendron processing rates from treatment periods to recovery was greater than that of more labile red maple. Laboratory feeding experiments were performed in order to examine and quantify use of rhododendron and red maple litter by Lepidostoma larvae. In the laboratory, Lepidostoma consumed significantly more rhododendron (1.062 mg AFDM/mg AFDM body wt/day) than red maple (0.479 mg AFDM/mg AFDM body wt/day) (P = 0.001). Results suggest that changes in the taxonomic composition of stream macroinvertebrate communities due to disturbance may have significant effects on ecosystem processes for at least 2 yr after termination of disturbance. Lepidostoma colonize rapidly following disturbance, enhancing the restoration of vital ecosystem processes such as litter decomposition in Coweeta streams.

Colonization and processing of leaf litter by macroinvertebrate shredders in streams of contrasting pH

SUMMARY Leaf litter processing rates and macroinvertebrate shredder assemblages in leaf packs were compared in four streams on the Allegheny plateau in the central Appalachian Mountains, U.S.A.; these streams were characterized by different bedrock geology and streamwater pH. Leaf litter processing rates were fastest in the neutral streams, slowest in the acidic stream, and intermediate in the most alkaline stream. Slower processing rates in the acidic stream were associated with lower total shredder biomass, made up predominantly by small leuctrid and nemourid stoneflies. The differences in processing rates between the more alkaline stream and the neutral streams were not associated with differences in shredder biomass, but appeared to be related to taxonomic differences in the shredder assembiages. Insects were dominant in the neutral streams, and amphipods were dominant in the more alkaline stream.

The role of leaf litter macroinvertebrates in stream-floodplain dynamics

The floodplain has been viewed as a transitional system or ecotone between aquatic and terrestrial habitats. This research has evaluated the role of floodplain macroinvertebrates in the degradation of leaf litter in a Michigan woodland floodplain, and examined the interrelationships among floodplain-stream macroinvertebrates, microbial colonization and selected environmental factors. Although taxonomically different decomposer groups operate in both systems, their functional roles as leaf litter detritivores are basically the same. Leaf litter processing rates were compared and contrasted between the stream and adjacent floodplain using leaf packs and litter bags, respectively. In the stream, the major time period for detritus processing by invertebrates is in the fall and winter, while in the floodplain it is during the spring. Therefore, the sequence of events involved in processing are similar, yet the timing and rate of these events are strikingly different. An understanding of the significant ecological processes linking the stream and floodplain as complementary systems is discussed within the context of watershed management.

Experimental evidence quantifying the role of benthic invertebrates in organic matter dynamics of headwater streams

SUMMARY. 1. The insecticide methoxychlor was applied seasonally to one of three small headwater streams in the southern Appalachian Mountains in North Carolina, U.S.A. The initial application caused massive invertebrate drift (&gt;1,000,000 organisms/week) and resulted in a community with few shredders and reduced abundances of most insect taxa.2. Bacterial densities and microbial respiration rates were not affected by treatment.3. Disruption of the invertebrate community resulted in significant reductions in leaf litter processing rates (50–74% reduction depending on leaf species) and in the amount of leaf litter processed annually (reduction of 25–28%).4. Reductions in leaf litter processing rates resulted in significant reductions in fine particulate organic matter (FPOM) export. Declines in both concentration and total export were detectable within 1 week of treatment. Annual FPOM export was reduced to 33% of pretreatment levels. Alteration to the invertebrate community had a much greater effect on FPOM export than a severe (50–200 year) drought.6. Course particulate organic matter (CPOM) export was not significantly influenced by treatment but was influenced by hydrologic differences among years.

Effects of Dimilin on stream leaf-litter processing rates

(1988). Effects of Dimilin on stream leaf-litter processing rates. SIL Proceedings, 1922-2010: Vol. 23, No. 2, pp. 1255-1260.

Recovery of a Headwater Stream from an Insecticide-Induced Community Disturbance

Faunal assemblages, leaf litter processing rates, and seston export were followed in two streams two years after one was altered with an insecticide. Two years after treatment ended, the macroinvertebrate trophic structure of the treated stream (TS) community had recovered and was similar to that of the reference stream (RS); however, major differences in taxonomic composition persisted. Some taxa, e.g., Diplectrona metaqui and Pseudostenophylax, which were rare or absent in TS before treatment became locally abundant during recolonization. Other taxa (e.g., Peltoperla, Pycnopsyche, Diplectrona modesta, and Fattigia), which were known to be abundant in TS before treatment, remained absent or rare in TS and common in RS. Recovery of shredder functional group biomass in TS was accompanied by restoration of leaf litter processing rates, seston concentrations, and export. These had all decreased significantly in TS, relative to RS, during the treatment period. Despite the recovery of the shredder functional group biomass, the taxa dominating this group remained strikingly different from RS and pretreatment conditions in TS. Biomasses of some shredders (e.g., Lepidostoma spp. and Tipula) were twice as high in TS as in RS, while others (e.g., Peltoperla, Fattigia, and Pycnopsyche) were low or absent in TS. Although taxonomic differences still existed between TS and RS, trophic structure was similar between streams. Restoration of shredder biomass, leaf litter processing, and organic seston concentrations in TS supports the concept that stream ecosystems are resilient to disturbance. Thus recovery of trophic function may occur in a fashion not recognizable solely by taxonomic criteria.

Leaf Litter Processing Rates in Four Texas Streams

Abstract Rates of leaf litter processing were investigated in four Texas streams during fall-winter. Assuming an exponential decay model, processing coefficients (-k) of some leaf species were among the highest yet reported (e.g. Celtis laevigata = 0.0619). Values for % loss day−1 ranged from 0.54–6.00. However, when compensated for the higher water temperatures found in these streams compared to more northern streams, processing rates were equivalent or even slightly lower. It is hypothesized that (on a per degree-day basis) this slower processing rate may result from the depauperate shredder fauna in such Texas streams.