Fine Particulate Organic Matter Production Research Articles

Macroinvertebrate identity mediates the effects of litter quality and microbial conditioning on leaf litter recycling in temperate streams.

Plant litter decomposition is an essential ecosystem function that contributes to carbon and nutrient cycling in streams. Aquatic shredders, mainly macroinvertebrates, can affect this process in various ways; they consume leaf litter, breaking it down into fragments and creating suitable habitats or resources for other organisms through the production of fine particulate organic matter (FPOM). However, measures of litter‐feeding traits across a wide range of aquatic macroinvertebrates are still rare. Here, we assessed the contributions of 11 species of freshwater macroinvertebrates to litter decomposition, by measuring consumption rate, FPOM production, and assimilation rate of highly decomposable (Alnus glutinosa) or poorly decomposable (Quercus robur) leaf litter types. In general, an increase in the quality of litter improved the litter consumption rate, and fungal conditioning of the leaf litter increased both the litter consumption rate and FPOM production. Macroinvertebrates specializing in leaf litter consumption also appeared to be the most sensitive to shifts in litter quality and the conditioning process. Contrary to expectations, the conditioning process did not increase the assimilation of low‐quality litter. There was a strong correlation between the relative consumption rate (RCR) of the two litter types, and the relative FPOM production (RFP) was strongly correlated to the RCR. These findings suggest a consistent relationship between RCR and macroinvertebrate identity that is not affected by litter quality, and that the RFP could be inferred from the RCR. The varying responses of the macroinvertebrate feeding traits to litter quality and the conditioning process suggest that the replacement of a shredder invertebrate species by another species could have major consequences for the decomposition process and the detritus‐based food web in streams. Further studies onto the importance of invertebrate identity and the effects of litter quality in a variety of freshwater ecosystems are needed to understand the whole ecosystem functioning and to predict its response to environmental changes.

Exotic Eucalyptus leaves are preferred over tougher native species but affect the growth and survival of shredders in an Atlantic Forest stream (Brazil).

We evaluated the effect of leaves of native and exotic tree species on the feeding activity and performance of the larvae of Triplectides gracilis, a typical caddisfly shredder in Atlantic Forest streams. Leaves of four native species that differ in chemistry and toughness (Hoffmannia dusenii, Miconia chartacea, Myrcia lineata and Styrax pohlii) and the exotic Eucalyptus globulus were used to determine food preferences and rates of consumption, production of fine particulate organic matter (FPOM), growth and survival of shredders. We hypothesized that the consumption rates of leaves of Eucalyptus and their effects on the growth and survival of shredders could be predicted by leaf chemistry and toughness. The larvae preferred to feed on soft leaves (H. dusenii and M. chartacea) independently of the content of nutrients (N and P) and secondary compounds (total phenolics). When such leaves were absent, they preferred E. globulus and did not consume the tough leaves (M. lineata and S. pohlii). In monodietary experiments, leaf consumption and FPOM production differed among the studied leaves, and the values observed for the E. globulus treatments were intermediate between the soft and tough leaves. The larvae that fed on H. dusenii and M. chartacea grew constantly over five weeks, while those that fed on E. globulus lost biomass. Larval survival was higher on leaves of H. dusenii, M. chartacea and S. pohlii than on E. globulus and M. lineata leaves. Although E. globulus was preferred over tougher leaves, long-term consumption of leaves of the exotic species may affect the abundance of T. gracilis in the studied stream. Additionally, our results suggest that leaf toughness can be a determining factor for the behavior of shredders where low-quality leaves are abundant, as in several tropical streams.

The role of aquatic fungi in transformations of organic matter mediated by nutrients

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 >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.

Prevalence of indirect toxicity effects of aluminium flakes on a shredder‐fungal‐leaf decomposition system

Summary Polyaluminium chloride (PAC) addition has been proposed as a technique to precipitate dissolved phosphorus (P) and reduce the internal loading of P in eutrophic hardwater lakes. However, the direct and indirect detrimental effects of PAC on aquatic communities and ecosystem processes are virtually unknown. We determined how PAC may affect a leaf litter–fungus–shredder decomposition system in the littoral habitat of eutrophic hardwater lakes.We hypothesised that PAC will exert little direct toxicity on aquatic organisms at high pH but trigger indirect bottom‐up effects by affecting fungal biomass, leaf litter food quality (Al, N and P contents), trophic transfer and the behaviour and performance of a leaf‐shredding consumer. We tested these hypotheses in a microcosm experiment involving a leaf‐shredding amphipod, Gammarus pulex, offered conditioned aspen leaves as food. We measured concentrations of Al, N, P and organic C in water, leaves and gammarids; fungal biomass associated with leaf litter; gammarid survival, body mass, feeding rate (FR) and N assimilation efficiency; and fine‐particulate organic matter (FPOM) generation in microcosms receiving 0, 2 and 20 g Al flakes per m2. Survival rates of G. pulex were similar in all microcosms, suggesting that even the high dose of Al flakes was below lethal levels. However, exposure to Al flakes decreased fungal biomass in leaves, the FR of gammarids, FPOM production and N assimilation efficiency. The C/N and C/P ratios of both leaves and gammarids also decreased in microcosms receiving Al flakes, supporting the hypothesis that stoichiometric imbalances between food and consumers were altered by exposure to Al flakes. These results point to the importance of indirect effects of PAC on lake communities and ecosystems beyond direct toxicity on invertebrates. Although not all those effects are necessarily undesirable, their numbers, interdependencies and potential to act in concert suggest they need to be considered in ecotoxicological assessments.

Plant litter diversity affects invertebrate shredder activity and the quality of fine particulate organic matter in streams

There is evidence that loss of riparian plant diversity alters the availability and quality of resources in streams, but little is known about how such effects change with time after loss of diversity. We used a microcosm approach with leaves of alder, oak and eucalypt previously colonised by microbes in a mixed forest stream to test how loss of litter diversity and time (2 and 6 months after loss of diversity) affect leaf consumption by invertebrate shredders, the elemental composition of shredder tissues, and the quality of fine particulate organic matter (FPOM). The number and identity of leaf species affected leaf consumption and FPOM production by shredders. Effects of leaf species diversity were positive and became more frequent with time after loss of diversity. Carbon (C) and nitrogen (N) composition of invertebrate tissues changed with the leaf identity. FPOM quality (C:N ratio) was positively correlated with leaf quality. Leaf consumption by the animals decreased linearly with the increase in C:N imbalance between leaf litter and invertebrate tissues. Our results suggest that changes in plant litter diversity affect the activity of shredders (leaf consumption and FPOM production), and the quality of food resources (FPOM and shredders) to higher trophic levels in streams; such effects are likely to become stronger with time after loss of plant diversity.

Food type and temperature constraints on the fitness of a dominant freshwater shredder

Freshwaters receive a large amount of materials and biologically available energy from their surrounding catchments. In particular, small forested streams are dependent on allochthonous organic matter inputs. However, since the middle 20th century large areas of forests in the Iberian Peninsula have been converted to eucalypt plantations, impairing invertebrate communities and stream energy fluxes. In addition, the dependence of organismal metabolic rates on temperature may influence invertebrate life cycles and development, and ultimately the riparian-stream system. We tested the effects of two food resources (alder and eucalypt) under two different temperatures (9 vs. 14°C) on the life history parameters and elemental composition of Brillia bifida (Chironomidae), a numerically dominant shredder in forested streams of NW Spain. Our results showed: (i) warmer temperature accelerated the development of B. bifida larvae (i.e., three times faster at warm than at cold temperature), (ii) Brillia's growth rate when fed on eucalypt leaves was only 30% of what it was on alder leaves, and (iii) both factors together modified larvae stoichiometry and nutrient fluxes (i.e., through fine particulate organic matter production). The observed elemental imbalances suggest, to some extent, a deviation from the “strict homeostasis” assumption for heterotrophs. Differential effects observed on larval development, growth and elemental composition point out that food type and temperature are influencing B. bifida performance, and this fact may reach other trophic compartments through detritivore-mediated nutrient cycling.

Dietary influences on production, stoichiometry and decomposition of particulate wastes from shredders

Summary Aquatic shredders produce large quantities of fine particulate organic matter (FPOM) as fragments and egesta, but the significance of shredder FPOM as a pathway of consumer‐mediated nutrient transformation remains understudied. We fed the stream shredders Pycnopsyche lepida, Lepidostoma sp. and Tipula abdominalis oak or maple litter conditioned under contrasting phosphorus concentrations to produce gradients in dietary carbon:phosphorus (C:P) and carbon:nitrogen (C:N) content (range = 850–4480 and 30–49 by moles, respectively). We measured total production and stoichiometry of FPOM to estimate particulate N and P release rates, compared resultant rates to those of P excretion and measured microbial decomposition of FPOM. FPOM production was lowest for Lepidostoma and higher for Tipula and Pycnopsyche; FPOM production by Tipula increased on higher‐nutrient diets. The C:P, C:N and N:P of FPOM from Pycnopsyche and Tipula often diverged from diet stoichiometry depending on litter type, and rates of particulate N and P release by shredders were greater with increasing nutrient content of the diet. Shredders fed high‐nutrient diets produced FPOM with greater microbial decomposition rates, although these trends differed between litter types. Bottom‐up changes in litter type and nutrient content may modify production, stoichiometry and decomposition of FPOM from shredders, and shredder‐mediated nutrient transformations may differ across shredder species.

The effect of shredder community composition on the production and quality of fine particulate organic matter

AbstractDecomposition of senesced primary production starts processing chains in aquatic systems. Shredding macroinvertebrates convert coarse particulate organic matter (CPOM) to fine particulate organic matter (FPOM) that supports 2 other feeding groups, collecting and filtering macroinvertebrates. This linkage is often invoked by aquatic ecologists, but the effect of detritivore assemblage composition on production of FPOM is relatively understudied. I manipulated detritivore assemblage composition (Limnephilus sp., Caecidotea sp., and Hyalella azteca) in aquatic mesocosms stocked with green speckled alder leaves (Alnus incana rugosa). I measured production rate, size distribution, and stoichiometry of FPOM produced through time. Detritivore species richness had a positive effect on FPOM production resulting from inclusion of the functionally dominant shredder, Limnephilus sp., in mixed-species treatments (e.g., sampling effect). Mixed-species treatments had significantly faster particle production than...

Red maple dominance enhances fungal and shredder growth and litter processing in temporary ponds

We used in situ and laboratory mesocosm experiments to test the effects of red maple (Acer rubrum), white oak (Quercus alba), and northern red oak (Q. rubra) leaf litter on microbial biomass and detritivore (Limnephilus indivisus) growth, survival, and organic matter processing in temporary forest ponds. Fungal biomass was highest in red maple leaf litter but was reduced in both maple and oak litter by detritivore presence, indicating a foraging preference based on fungal biomass. Organic matter processing and fine particulate organic matter (FPOM) generation rates were highest in red maple treatments containing detritivores, and red maple diets allowed for rapid larval growth and development to adulthood, while oak diets (Q. alba or Q. rubra) resulted in slower development and increased mortality of larvae. Red maple dominance around pond margins may have positive effects on detritivore growth and generation of FPOM in temporary ponds. Because many of the organisms that feed on FPOM also serve as prey for larval amphibians, the effects of increased FPOM production resulting from red maple dominance may travel through the food web to affect higher trophic levels.

Grazer–collector facilitation hypothesis supported by laboratory but not field experiments

Grazing invertebrates in streams feed by harvesting algal cells from surfaces, and in doing so release fine particulate organic matter (FPOM). The "grazer–collector facilitation hypothesis" holds that FPOM production by grazers facilitates growth and (or) survival of FPOM-collecting invertebrates. We tested for grazer–collector facilitation in laboratory and field experiments. In recirculating flumes in the laboratory, we tested for facilitation of the collector Hydropsyche slossonae by the grazers Physa gyrina, Glossosoma intermedium, and Baetis tricaudatus. All three grazers increased FPOM levels in flume water, but only Physa facilitated Hydropsyche growth. In the field, we manipulated Physa and Glossosoma densities to test for facilitation (at a local scale) of natural collector assemblages in an eastern Iowa stream. We did not detect facilitation of any collector by either grazer in the field, despite high power to detect such interactions. We suspect that grazer–collector facilitation was not observed in the field because (unlike in our laboratory flumes) field FPOM levels are often high and extremely variable in time and space and because organic particles can arise from sources other than grazer activity (= grazer-independent processing). Therefore, at local scales, collectors may not be significantly limited by the supply of grazer-derived FPOM.

ROLES OF CRAYFISH: CONSEQUENCES OF PREDATION AND BIOTURBATION FOR STREAM INVERTEBRATES

Two field experiments were performed to examine the impacts on stream invertebrates of crayfish (Paranephrops zealandicus) as predators and bioturbators. A den- sity manipulation experiment involved three treatments with different crayfish densities in streamside channels (0, 5, and 10 crayfish/m2) in which accrual of fine sediment, leaf decomposition, and associated invertebrates were measured. During the 50-d experiment, leaf decomposition was not affected by the presence of crayfish, whereas fine sediment was reduced to similar extents in both crayfish treatments. Some invertebrates followed a similar pattern to that of sediment accrual, suggesting that removal of fine sediment by crayfish was a primary mechanism underlying differences in invertebrate densities. Medium (4-6 mm body length) and large (-7 mm) Tanypodinae were rare in crayfish treatments, probably as a result of selective predation. Conversely, crayfish appeared to facilitate Deleatidium mayflies, as predicted by a trophic cascade (large Tanypodinae predator removal) or shred- der-collector facilitation (fine particulate organic matter production). In many cases, the response of dependent variables was equally strong at both crayfish densities. In a sediment flush experiment, sediment removal treatments were established, along with no-crayfish control and crayfish treatments, to assess the relative impacts of predation and sediment removal on each invertebrate taxon. Crayfish had little impact on leaf decomposition, although decomposition rate was significantly faster than in sediment flush treatments, where collector-scraper invertebrates were reduced as a result of sediment removal. Despite the fact that fine sediment was similarly reduced in crayfish and flush treatments, the response of invertebrates to treatments differed according to invertebrate taxon and size class. Overall, large Tanypodinae were selectively preyed upon; Orthocladiinae, Oribatei, and Oligochaeta responded primarily to sediment removal; Deleatidium were indirectly facilitated; and Chi- ronominae, Ostracoda, and Cyclopoida were relatively unaffected. These results are likely to be relevant at various levels of local scale in the natural environment, but not at the catchment scale. Crayfish are therefore considered to play important roles in determining local distribution of invertebrates through both trophic and nontrophic effects.

Is invertebrate shredding critical for collector invertebrates? A test of the shredder–collector facilitation hypothesis

A laboratory experiment was conducted to test the hypothesis that leaf processing by Japanese crayfish (Cambaroides japonicus de Haan) enhances the growth and reduces the mortality of collector amphipods (Sternomoera yezoensis Ueno) through fine particulate organic matter (FPOM) production. Two treatments (crayfish shredding, no shredding control) were established using flow‐through artificial streams and permitting differential FPOM production rates. An amphipod enclosure, associated with juvenile amphipods and preconditioned leaf packs, was placed at the downstream end of each channel. During a 4‐week period, shredding by crayfish greatly increased the amount of drifting FPOM compared to that in the control channels, in which only shredder‐independent FPOM occurred. Despite the substantial contribution of crayfish to FPOM production, amphipods in both treatments grew during the 4‐week period and neither amphipod survival nor growth differed between the two treatments. Furthermore, the residual leaf mass in the amphipod enclosures did not differ between the two treatments. However, residual leaf mass in amphipod enclosures was about 20% less than that expected from the processing rates of the leaf packs in the absence of invertebrate shredding. It is likely that amphipods were directly utilizing leaf packs as their food source regardless of FPOM abundance. These results indicate that juvenile amphipods, despite their classification as collectors, were plastic in their foraging habits, and that the FPOM produced by crayfish was not critical for amphipod growth or survivorship.