Nutrient Enrichment In Streams Research Articles

Enrichment of nutrients from anaerobically digested centrate minimizing microplastics content using a combination of membrane processes

Centrifuge effluent, or centrate, is a liquid stream generated in the sludge line of wastewater treatment plants in the sludge dewatering process. Nutrients are solubilized in the anaerobic digestion of the sludge, mainly in the form of ammonium-nitrogen and phosphates. Thus, when the dewatering of the digested sludge is performed (usually by centrifugation), a sludge liquor stream enriched in nutrients is generated. However, it also contains microparticles, including microplastics, since most of the microparticles from the wastewater are transferred to the sludge treatment line in wastewater treatment plants. In this work, microplastics in centrate have been analyzed (counted and identified), and membrane technologies (ultrafiltration and forward osmosis) have been applied to concentrate nutrients and to obtain a microplastic-free stream for further nutrient recovery. Two alternative configurations have been compared, changing the application order of these processes. The results showed that obtaining a stream with a concentration higher than 6000 mg/L of ammonium has been possible by using ammonium sulfate (150 g/L) as a draw solution in the forward osmosis process. On the other hand, as a consequence of the ultrafiltration application, microparticles were concentrated in the reject stream up to 800 microparticles/L. At the same time, the permeate presents a lower concentration of microplastics without reducing the concentration of the nutrients. In this way, this pioneering study enables the production of a nutrient-enriched stream with reduced microplastic concentrations, that could be applied to the agricultural soil as biofertilizer.

Macroinvertebrate seedbank survival in pristine and nutrient-enriched intermittent streams and its contribution to flow phase communities

Intermittent stream research focuses mostly on pristine streams. The impacts of nutrient enrichment on invertebrate communities in these systems thus remains underexplored, especially in temperate climates. This study aimed to compare taxa survival during stream drying within pristine and nutrient-enriched sediment streambeds and to evaluate seedbank contribution for community recovery. Ten dry sediment subsamples were each collected from three streams of each nutrient status type. These were rehydrated for 60 days in laboratory with persisting and hatching invertebrates being recorded. Oligochaetes and molluscs were the most abundant; stoneflies and mayflies were common; caddisflies were represented by only a few specimens. The seedbank comprised of around one-third of taxa recorded during flow phases with slightly higher taxa numbers in nutrient-enriched streams. Moisture content positively affected seedbank taxa richness, while dry period duration decreased it. Taxa richness has a unimodal response to nutrient enrichment, with fine sediment proportion revealing no significant effect. Our results show a richer macroinvertebrate seedbank in drying sediments under moist conditions with mild nutrient enrichment, while raised nutrient levels limited macroinvertebrate survival. These results highlight the crucial role of seedbanks in community recovery of intermittent temperate streams, which will become more common with climate change.

Understanding the factors controlling biofilm as an autochthonous resource in shaded oligotrophic neotropical streams

Light and nutrients are bottom-up factors in controlling biofilm in tropical shaded oligotrophic streams. Regardless of the predominant resource source (autochthonous or allochthonous), greater availability of light and nutrients favors the development and, later, grazing (top-down factor) of the biofilm, with reflections on the trophic structure of these ecosystems. The increase in primary productivity in lotic ecosystems may reflect the fractionation of dissolved inorganic carbon (DIC) by the algal cells contained in the biofilm. A global meta-analysis showed that an increase in primary productivity leads to enrichment of the carbon isotope ratios of biofilm. The objective of this study was to evaluate how top-down (grazing) and bottom-up (light and nutrient) factors act on the biofilm and whether algal biomass determines the isotopic fractionation of δ13C in headwaters of Central Brazil. The density of grazing macroinvertebrates was controlled with electric pulses. Electrified (excluding grazers) and non-electrified grids were implemented in open and shaded areas in each stream. The biofilm ash-free dry mass (AFDM) doubled in nutrient-enriched streams. The effect of grazing on the biomass of the biofilm was significant for the nutrient-enriched streams, with a 50% reduction in the AFDM. Additionally, shading influenced chlorophyll-a even in pristine streams, as the concentration increased approximately ten-fold under open canopy conditions. There was a significant negative relationship between chlorophyll-a concentrations and δ13C values. Our study provides evidence that biofilm biomass increases as a result of changes in bottom-up controls. Top-down control significantly reduced biofilm in the food web based on autochthonous organic matter.

Can top-down effects of cypriniform fish be used to mitigate eutrophication effects in medium-sized European rivers?

Eutrophication seriously threatens the ecological quality and biodiversity of running waters. In nutrient-enriched streams and shallow rivers, eutrophication leads to excessive periphyton growth and, in turn, biological clogging, oxygen depletion in the hyporheic zone and finally a reduction in the hyporheic habitat quality. Top-down control of the food-web by manipulating fish stocks, similar to the biomanipulation successfully applied in lakes, offers a promising approach to mitigating the effects of eutrophication in shallow rivers, especially those in which major reductions in nutrient input are not feasible. We conducted a reach-scale experiment over 4 years in a medium-sized eutrophic river to assess whether the top-down effects of two important large European cypriniform fish species, herbivorous common nase (Chondrostoma nasus) and omnivorous European chub (Squalius cephalus), would mitigate the effects of eutrophication. The enhancement of fish stocks was expected to reduce biological clogging, via the top-down control of periphyton by benthic grazing and enhanced bioturbation, thus increasing oxygen availability in the hyporheic zone as well as water exchange between the surface water and the hyporheic zone. As expected, enhancing the stocks of nase and chub increased both oxygen availability and vertical exchange flux of water in the upper layer of the hyporheic zone. However, periphyton biomass (chlorophyll a) was significantly reduced only in deeper pool habitat. Thus, while experimental biomanipulation in a shallow river significantly mitigated the effects of eutrophication in the hyporheic zone, top-down effects on periphyton biomass were rather small. Overall, to our knowledge, our results provide first evidence that the biomanipulation achieved by enhancing herbivorous and omnivorous fish stocks has the potential to mitigate the effects of eutrophication in medium-sized European rivers.

Excess of nitrogen reduces temporal variability of stream diatom assemblages

Nutrient enrichment degrades water quality and threatens aquatic biota. However, our knowledge on (dis)similarities in temporal patterns of biota among sites of varying level of nutrient stress is limited. We addressed this gap by assessing temporal (among seasons) variation in algal biomass, species diversity and composition of diatom assemblages in three streams that differ in nutrient stress, but are otherwise similar and share the same regional species pool. We monitored three riffle sections in each stream bi-weekly from May to October in 2014. Temporal variation in water chemistry and other environmental variables was mainly synchronous among riffles within streams and often also among streams, indicating shared environmental forcing through time. We found significant differences in diatom assemblage composition among streams and, albeit less so, also among riffles within streams. Diatom assemblages in the two nutrient-enriched streams were more similar to each other than to those in the nutrient-poor stream. Taxa richness did not differ consistently among the streams, and did not vary synchronously at any spatial scale. Temporal variation in diatom assemblage composition decreased with increasing DIN:TotP ratio, likely via a negative effect on sensitive taxa while maintaining favorable conditions for certain tolerant taxa, irrespective of season. This relationship weakened but remained significant even after controlling for stochastic effects, suggesting deterministic mechanisms between nutrient levels and diatom assemblage stability. After controlling for stochastic effects temporal variability was best explained by DIN suggesting that excess of nitrogen reduces temporal variability(intra-annual beta diversity) of diatom assemblages. The high temporal variation, and especially the lack of temporal synchrony at the within streams scale, suggests that single sampling at a single site may be insufficient to reliably assess and monitor a complete stream water body. Our results also showed that measures including species identity outperform traditional diversity metrics in detecting nutrient stress in streams.

Evaluating transient storage and associated nutrient retention in a nutrient-rich headwater stream: a case study in Lake Chaohu Basin, China.

Transient storage has been studied intensively in small streams, but some processes and mechanisms are not yet entirely understood regarding this issue, especially in chronically nutrient-enriched streams. The exploration of transient storage dynamics in nutrient-rich headwater streams has great significance for stream nutrient management in China and other developing countries, which are suffering from eutrophication. In the present study, we conducted five instantaneous slug additions composed of a conservative tracer dissolved with two nonconservative nutrients injections in a suburban small stream (Guanzhen Creek), Lake Chaohu Basin, China. Transient storage metrics were estimated using the model-fitted hydrologic parameters from the one-dimensional transport with inflow and storage (OTIS) model. Regression analyses were performed to examine the relationship between hydraulic parameters and transient storage metrics. Moreover, nitrogen and phosphorus retention efficiency was qualitatively evaluated based on the OTIS model-fitted nutrient parameters. Our results showed that the OTIS model-fitted hydrologic parameters in Guanzhen Creek were within the range of previously published literature. The transient storage metrics of Guanzhen Creek were generally comparable to those in streams with low-to-moderate nutrient levels in other catchments. Moreover, most of the transient storage metrics showed a strong relationship with stream discharge, while only hydrological retention factor showed a markedly negative correlation with flow rate. Given the negative uptake rates for NH4-N and SRP in half cases, we reasonably concluded that Guanzhen Creek was hardly incapable of retaining nitrogen and phosphorus.

Relationships Between Land Use and Stream Nutrient Concentrations in a Highly Urbanized Tropical Region of Brazil: Thresholds and Riparian Zones.

Nutrient enrichment in streams due to land use is increasing globally, reducing water quality and causing eutrophication of downstream fresh and coastal waters. In temperate developed countries, the intensive use of fertilizers in agriculture is a main driver of increasing nutrient concentrations, but high levels and fast rates of urbanization can be a predominant issue in some areas of the developing world. We investigated land use in the highly urbanized tropical State of Rio de Janeiro, Brazil. We collected total nitrogen, total phosphorus, and inorganic nutrient data from 35 independent watersheds distributed across the State and characterized land use at a riparian and entire watershed scales upstream from each sample station, using ArcGIS. We used regression models to explain land use influences on nutrient concentrations and to assess riparian protection relationships to water quality. We found that urban land use was the primary driver of nutrient concentration increases, independent of the scale of analyses and that urban land use was more concentrated in the riparian buffer of streams than in the entire watersheds. We also found significant thresholds that indicated strong increases in nutrient concentrations with modest increases in urbanization reaching maximum nutrient concentrations between 10 and 46% urban cover. These thresholds influenced calculation of reference nutrient concentrations, and ignoring them led to higher estimates of these concentrations. Lack of sewage treatment in concert with urban development in riparian zones apparently leads to the observation that modest increases in urban land use can cause large increases in nutrient concentrations.

Nutrient enrichment alters the magnitude and timing of fungal, bacterial, and detritivore contributions to litter breakdown

Microbial decomposers and metazoan detritivores rely on and affect processing rates of detrital C in streams. Nutrient enrichment can affect the biomass of both groups, but we lack a quantitative understanding of how these groups affect breakdown rates of C under elevated nutrient conditions. We quantified the relative contribution of decomposers (fungi, bacteria) and detritivores (shredders) to breakdown rates in a reference vs a nutrient-enriched stream. We measured breakdown rates and litter-associated shredder, fungal, and bacterial biomass on Rhododendron maximum L. and Acer rubrum L. and modeled the contribution of these groups over the decay sequence for R. maximum and for a single time period for A. rubrum. We used published metabolic parameters for fungi and bacteria under high- and low-nutrient conditions and shredder feeding rates based on organisms and leaf litter from our study area. Breakdown rates of both litter types were higher (∼4–5×) in the treatment than reference stream and were associated with large changes in biomass of fungi, bacteria, and shredders. Changes in microbial metabolic parameters (growth rate, growth efficiency) affected estimates of mass loss. Fungal and shredder contributions were roughly equivalent and greater than bacterial contributions under reference and treatment conditions, but nutrient enrichment accelerated the colonization sequence of fungi and shredders, and shredders had relatively greater effects on mass loss in later stages of breakdown. Our results suggest facilitation of detritivores by decomposers and large combined effects on breakdown with nutrient enrichment. Decomposer contributions could exceed those of detritivores with changes in microbial metabolic rates. Large increases in breakdown rates could be attributed to biotic response to nutrient enrichment, are sensitive to microbial metabolic rates, and have the potential to alter residence times of litter in streams.

Relationship between Distance Sampling and Carbon Dioxide Emission under Oil Palm Plantation

Relationship between Distance Sampling and Carbon Dioxide Emission under Oil Palm Plantation

Global pressures, specific responses: effects of nutrient enrichment in streams from different biomes

We assessed the effects of nutrient enrichment on three stream ecosystems running through distinct biomes (Mediterranean, Pampean and Andean). We increased the concentrations of N and P in the stream water 1.6–4-fold following a before–after control–impact paired series (BACIPS) design in each stream, and evaluated changes in the biomass of bacteria, primary producers, invertebrates and fish in the enriched (E) versus control (C) reaches after nutrient addition through a predictive-BACIPS approach. The treatment produced variable biomass responses (2–77% of explained variance) among biological communities and streams. The greatest biomass response was observed for algae in the Andean stream (77% of the variance), although fish also showed important biomass responses (about 9–48%). The strongest biomass response to enrichment (77% in all biological compartments) was found in the Andean stream. The magnitude and seasonality of biomass responses to enrichment were highly site specific, often depending on the basal nutrient concentration and on windows of ecological opportunity (periods when environmental constraints other than nutrients do not limit biomass growth). The Pampean stream, with high basal nutrient concentrations, showed a weak response to enrichment (except for invertebrates), whereas the greater responses of Andean stream communities were presumably favored by wider windows of ecological opportunity in comparison to those from the Mediterranean stream. Despite variation among sites, enrichment globally stimulated the algal-based food webs (algae and invertebrate grazers) but not the detritus-based food webs (bacteria and invertebrate shredders). This study shows that nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but that its magnitude and extent within the food web are complex and are strongly determined by environmental factors and ecosystem structure.

The ecology of freshwater bivalves in the Lake Sapanca basin, Turkey

Despite the ecological importance and globally threatened status of freshwater bivalves, the freshwater mussel fauna of Turkey has hitherto received little attention. The aim of this study was to describe the ecology of freshwater mussels of the Lake Sapanca basin (northwest Turkey) to provide baseline data against which to measure future trends. Five native mussel species belonging to 3 genera were encountered in surveys: Anodonta cygnea, A. anatina, Unio pictorum, U. crassus, and Dreissena polymorpha. In addition, an invasive species from Asia, A. woodiana, was recorded in the region for the first time. A. cygnea and U. pictorum were the most abundant species in the lake, while U. crassus was most abundant in the associated stream. Age and growth parameters were estimated for U. pictorum and U. crassus, and indicated that the latter species had a younger median age and faster growth rate than the former, possibly a function of exclusively occupying a nutrient-enriched stream. The bioecological features of the bivalve fauna of the Lake Sapanca basin, as well as the implications of the introduction of A. woodiana, are discussed.

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom-up (e.g. nutrients and light, which determine growth rates) or from the top-down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient-enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient-enriched stream habitats of the Owennagearagh River, south-west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high-voltage localised electric pulses. Replicates of treatment (grazer-excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2-week Cladophora cover, periphytic chlorophyll a and biofilm ash-free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer-excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient-enriched site. 4. Our results demonstrate that in eutrophic, high-light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.

Fish Species Richness in Lowland Streams with a Geothermal Input Phenomenon, Sarapiquí de Heredia, Costa Rica

Lowland streams of the Caribbean Costa Rican slope are naturally enriched by minerals and solutes such as phosphorus, a phenomenon known as geothermal input. The resulting stream phosphorus gradient affects the food web by altering primary production, leading to complex biotic and abiotic interactions. The objective of this study was to describe fish species richness patterns in relation to a naturally occurring phosphorus gradient in three lowland streams. We hypothesized that phosphorus input in La Selva Biological Station streams is correlated with species richness. 354 individuals were identified at 4 sites: Surá, El Salto, Sábalo and Jaguar, using a cast net, a total of 150 throws were made, distributed in 10 samplings, at each site. An estimation of species richness along with alpha and beta diversity indices were calculated in order to describe fish community structure. Species richness was estimated to be higher at Jaguar but presents a lower phosphorus enrichment. Jaguar and Sábalo, differed the most while Salto and Surá were the most similar as indicated by species turnover among streams. There is a non significant relationship between phosphorus levels and diversity; however, this interesting trend encourages further studies on species richness patterns and natural nutrient enrichment of streams.

Synergistic effects of water temperature and dissolved nutrients on litter decomposition and associated fungi

In woodland streams, the decomposition of allochthonous organic matter constitutes a fundamental ecosystem process, where aquatic hyphomycetes play a pivotal role. It is therefore greatly affected by water temperature and nutrient concentrations. The individual effects of these factors on the decomposition of litter have been studied previously. However, in the climate warming scenario predicted for this century, water temperature and nutrient concentrations are expected to increase simultaneously, and their combined effects on litter decomposition and associated biological activity remains unevaluated. In this study, we addressed the individual and combined effects of water temperature (three levels) and nutrient concentrations (two levels) on the decomposition of alder leaves and associated aquatic hyphomycetes in microcosms. Decomposition rates across treatments varied between 0.0041 day−1 at 5 °C and low nutrient level and 0.0100 day−1 at 15 °C and high nutrient level. The stimulation of biological variables at high nutrients and temperatures indicates that nutrient enrichment of streams might have a higher stimulatory effect on fungal performance and decomposition rates under a warming scenario than at present. The stimulation of fungal biomass and sporulation with increasing temperature at both nutrient levels shows that increases in water temperature might enhance fungal growth and reproduction in both oligotrophic and eutrophic streams. The stimulation of fungal respiration and litter decomposition with increasing temperature at high nutrients indicates that stimulation of carbon mineralization will probably occur at eutrophied streams, while oligotrophic conditions seem to be ‘protected’ from warming. All biological variables were stimulated when both factors increased, as a result of synergistic interactions between factors. Increased water temperature and nutrient level also affected the structure of aquatic hyphomycete assemblages. It is plausible that if water quality of presently eutrophied streams is improved, the potential stimulatory effects of future increases in water temperature on aquatic biota and processes might be mitigated.

Algal foraging by a rheophilic chironomid (Eukiefferiella claripennis Lundbeck) extensively encountered in high nutrient enriched streams

Water enrichment has been described to have major impact on the freshwater food-webs structure. We investigated the structure of the epilithic algal assemblage and the gut contents of a rheophilic orthoclad in situ in a nutrient enriched stream. 4-th instar larvae of Eukiefferiella claripennis, a widespread orthoclad in the Palearctic ecozone and abundant in nutrient-enriched streams, were chosen to be studied due to their great densities in such environment. The taxonomic composition of the epilithic algal assemblage, the gut contents and the ingested algal-cell biovolumes were used to establish the foraging behavior of this chironomid, and to explain its success under such environmental conditions. A Colnertia analysis, and a Two Way INdicator SPecies ANalysis were uses to compare the co-structure of algae assemblages in the epilithon and the guts. Then Strauss's linear food selection index was used to assess the electivity of each algae taxon. Cell biovolumes and distribution were also compared and tested for significant differences between the epilithic algal assemblage and the gut contents. Biovolumes of cells in the epilithon were tested for significant differences from the mean biovolume value of the ingested cells. Eukiefferiella claripennis larvae seem to feed preferentially on Chlorophyceae and Cyanophyceae; Diatomophyceae were randomly grazed without preference while Rhodophyceae were avoided. Cell biovolumes were significantly different between the epilithon and the gut contents, and the larvae preferentially consumed a narrow range of cell biovolumes compared to the wide range of cell biovolumes encountered in the environment. Stigeoclonium and Oscillatoria were the key taxa of the algal assemblage foraged by the larvae. Two complementary approaches seem to drive the foraging behavior of this orthoclad, 1) taxa-type preference and 2) cell-bivolume preference. In nutrient enriched streams, an algal assemblage consisting of cells of Chlorophyceae and Cyanophyceae averaging 1702 μm 3 offers the best diet to this chironomid.

Seasonal variation in nutrient limitation of microbial biofilms colonizing organic and inorganic substrata in streams

Humans have increased the availability of nutrients including nitrogen and phosphorus worldwide; therefore, understanding how microbes process nutrients is critical for environmental conservation. We examined nutrient limitation of biofilms colonizing inorganic (fritted glass) and organic (cellulose sponge) substrata in spring, summer, and autumn in three streams in Michigan, USA. Biofilms were enriched with nitrate (NO3 −), phosphate (PO4 3−), ammonium (NH4 +), NO3 − + PO4 3−, NH4 + + PO4 3−, or none (control). We quantified biofilm structure and function as chlorophyll a (i.e., primary producer biomass) and community respiration on all substrata. In one stream, we characterized bacterial and fungal communities on cellulose in autumn using clone library sequencing and denaturing gradient gel electrophoresis to determine if community structure was linked to nutrient limitation status. Despite oligotrophic conditions, primary producer biomass was infrequently nutrient limited. In contrast, respiration on organic substrata was frequently limited by N + P combinations. We found no difference between biofilm response to NH4 + versus NO3 − enrichment, although the response to both N-species was positively related to water column PO4 3− concentrations and temperature. Molecular analysis for fungal community composition suggested no relationship to nutrient limitation, but the dominant members of the bacterial community on cellulose were different on NO3 −, PO 4 3 , and NO3 − + PO4 3− treatments relative to control, NH4 +, and NH4 + + PO4 3− treatments, which matched patterns for biofilm respiration rates from each treatment. Our results show discrete patterns of nutrient limitation dependent upon substratum type and season, and imply changes in bacterial community structure and function may be linked following nutrient enrichment in streams.

The Role of Early Fungal Colonizers in Leaf‐Litter Decomposition in Portuguese Streams Impacted by Agricultural Runoff

AbstractWe conducted a transplant experiment between two streams in NW Portugal impacted by agricultural runoff, mainly differing in phosphate concentration, to determine whether fungi on decomposing leaves would adapt to the new environment or would be replaced by fungi of the recipient stream. The most nutrient enriched stream had lower fungal diversity but faster leaf decomposition. Leaf transplantation did not alter fungal activity or species dominance. Multidimensional scaling ordination of fungal communities, from DNA fingerprint or conidial production, revealed that transplanted communities resembled more those of the original stream than the recipient stream. Results suggest that early fungal colonizers will determine the development and activity of fungal communities on decomposing leaves in streams impacted by agricultural practices. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Modeling nutrient in-stream processes at the watershed scale using Nutrient Spiralling metrics

Abstract. One of the fundamental problems of using large-scale biogeochemical models is the uncertainty involved in aggregating the components of fine-scale deterministic models in watershed applications, and in extrapolating the results of field-scale measurements to larger spatial scales. Although spatial or temporal lumping may reduce the problem, information obtained during fine-scale research may not apply to lumped categories. Thus, the use of knowledge gained through fine-scale studies to predict coarse-scale phenomena is not straightforward. In this study, we used the nutrient uptake metrics defined in the Nutrient Spiralling concept to formulate the equations governing total phosphorus in-stream fate in a deterministic, watershed-scale biogeochemical model. Once the model was calibrated, fitted phosphorus retention metrics where put in context of global patterns of phosphorus retention variability. For this purpose, we calculated power regressions between phosphorus retention metrics, streamflow, and phosphorus concentration in water using published data from 66 streams worldwide, including both pristine and nutrient enriched streams. Performance of the calibrated model confirmed that the Nutrient Spiralling formulation is a convenient simplification of the biogeochemical transformations involved in total phosphorus in-stream fate. Thus, this approach may be helpful even for customary deterministic applications working at short time steps. The calibrated phosphorus retention metrics were comparable to field estimates from the study watershed, and showed high coherence with global patterns of retention metrics from streams of the world. In this sense, the fitted phosphorus retention metrics were similar to field values measured in other nutrient enriched streams. Analysis of the bibliographical data supports the view that nutrient enriched streams have lower phosphorus retention efficiency than pristine streams, and that this efficiency loss is maintained in a wide discharge range. This implies that both small and larger streams may be impacted by human activities in terms of nutrient retention capacity, suggesting that larger rivers located in human populated areas can exert considerable influence on phosphorus exports from watersheds. The role of biological activity in this efficiency loss showed by nutrient enriched streams remained uncertain, because the phosphorus mass transfer coefficient did not show consistent relationships with streamflow and phosphorus concentration in water. The heterogeneity of the compiled data and the possible role of additional inorganic processes on phosphorus in-stream dynamics may explain this. We suggest that more research on phosphorus dynamics at the reach scale is needed, specially in large, human impacted watercourses.

Influence of shredder feeding and nutrients on fungal activity and community structure in headwater streams

In stream detrital food webs, interactions occur between aquatic hyphomycetes associated with decomposing leaves and shredders consuming those leaves. However, few studies have examined how the feeding activity of shredders affects aquatic hyphomycetes. We examined the effect of shredder feeding on aquatic hyphomycete communities associated with submerged leaves in two southern Appalachian headwater streams in Coweeta Hydrologic Laboratory, North Carolina, USA. Coarse (allowing shredder access) and fine (preventing shredder access) mesh litter bags containing red maple (Acer rubrum) leaves were placed in the treatment stream (C54) which was enriched with nitrogen (N) and phosphorus (P), and in the reference stream (C53) and were retrieved monthly. Both shredder feeding and nutrient enrichment enhanced breakdown rates. The breakdown rates of leaves in coarse mesh bags in the reference stream (k = 0.0275) and fine mesh bags in the nutrient enriched stream (k = 0.0272) were not significantly different, suggesting that the higher fungal activity stimulated by nutrient enrichment could increase the relative contribution of fungi to leaf breakdown to the level similar to that of shredders in the reference stream. Macroinvertebrate abundance and biomass were higher in the litter bags submerged in the treatment stream. Fungal sporulation rates and biomass were higher in the treatment stream than in the reference stream, but neither fungal biomass nor sporulation rate was affected by shredder feeding in either stream. The enrichment with N and P altered fungal community composition more than shredder feeding. Species richness was higher in the nutrient enriched stream than in the reference stream, and fungal assemblages from fine and coarse mesh bag treatments within a stream were more similar to each other than the fungal assemblages from the same mesh bag treatments but from different streams.

Parasitic mite and trematode abundance are associated with reduced reproductive output and physiological condition of freshwater mussels

Although historically understudied, parasites may play an important role in freshwater invertebrate population ecology and evolution. We quantified abundance of parasitic mites and trematodes in the freshwater mussel Pyganodon grandis, in a southeastern Alabama stream (USA), to assess parasite impact on adult mussel physiological condition and reproductive output. We used stepwise multiple regression analyses to assess the effects of mussel size and parasites on reproduction and condition. Multiple regression analysis found no multivariate models that predicted reproductive output or physiological condition. However, univariate models revealed that increased parasite densities predict reduced mussel reproductive output and physiological condition. These findings suggest that parasites may have important negative consequences for freshwater mussels. We hypothesize that elevated parasite loads may reduce mussel fitness in impounded or nutrient-enriched streams with high densities of intermediate hosts (chironomid midges).