Stream Ecosystem Function Research Articles

Effects of riparian vegetation restoration and environmental context on ecosystem functioning in tropical streams of southeastern Brazil

Tropical stream ecosystems are under increasing human pressure, making the development of effective restoration approaches and expanding knowledge in this field urgent. This study evaluated the impact of riparian vegetation restoration and environmental context on stream ecosystem functioning by measuring key ecosystem functions - gross primary production (GPP), ecosystem respiration (ER), and nutrient uptake of ammonium and soluble reactive phosphorus - across ten tropical streams in southeastern Brazil. The streams represented a gradient from clearcut areas (impacted reaches) to relatively pristine conditions (reference reaches), including intermediate stages of vegetation recovery (restored reaches). In the short-term (~15–20 years after restoration), restoration led to reduced GPP akin to reference reaches. Yet, ER did not show the anticipated increase, suggesting a longer timeframe is necessary for restored streams to emulate the functional characteristics of reference reaches. Additionally, the restored reaches did not achieve the nutrient uptake efficiencies observed in both impacted and reference reaches, pointing to a partial recovery of ecosystem function. This study suggests that while riparian vegetation restoration contributes positively to certain aspects of stream function, environmental variables less related to this type of restoration, such as discharge and hydromorphology, significantly influence stream ecosystem functioning, highlighting the importance of considering environmental context in restoration efforts. A more holistic approach, possibly encompassing broader hydromorphological and habitat enhancements, is needed to fully restore ecological processes in these vital ecosystems. These insights are critical for informing future tropical stream restoration projects, advocating the use of ecosystem function metrics as comprehensive indicators of ecological recovery and restoration success.

Salamander loss alters montane stream ecosystem functioning and structure through top‐down effects

AbstractAmphibians are among the most endangered taxa worldwide, but little is known about how their disappearance can alter the functioning and structure of freshwater ecosystems, where they live as larval stages. This is particularly true for urodeles, which often are key predators in these ecosystems. The fire salamander (Salamandra salamandra) is a common predator in European fresh waters, but the species is declining due to habitat loss and the infection by fungal pathogens. We studied the consequences of fire salamander loss from three montane streams, by comparing two key ecosystem processes (periphyton accrual and leaf litter decomposition) and the structure of three communities (periphytic algae, aquatic hyphomycetes and invertebrates) using stream enclosures with and without salamander larvae. Salamander loss did not cause changes in invertebrate abundance or community structure, except for one stream where abundance increased in the absence of salamander larvae. However, salamander loss led to lower periphyton accrual, changes in algal community structure and slower leaf litter decomposition, with no associated changes in fungal communities or microbial decomposition. The changes observed may have been caused by release of salamander predatory pressure on invertebrates, which could have promoted their grazing on periphyton, in contrast to their preference for leaf shredding in the presence of salamander. Our study demonstrates an important role of salamander larvae in montane streams through top‐down control of lower trophic levels and thus in regulating key stream ecosystem processes. Our results highlight the need for improving protection measures for amphibians to prevent these alterations on ecosystem structure and function.

The Effects of Urbanization and Economic Growth on The Igarapes of the Legal Amazon

This study shows the primary mechanisms causing alteration and degradation of water bodies. Data analysis on the condition of urban streams in the Eastern Amazon (53,8%) shows that Pará state is the most affected (n = 67,8%). The Almas and Mata Fome creeks in the Bengui neighborhood have undergone the most significant and irreversible environmental changes. Similarly, the Paragominas stream is experiencing ongoing harm due to urban and economic expansion. In the Western Amazon (46,1%) and Amazonas state (9.5±4.9), research on 16 streams says a housing shortage and an increase in peripheral development along these water bodies. Urbanization and economic growth are altering the ecosystem functions of urban streams, highlighting the urgent need for water management authorities in the legal Amazon region to focus on this issue.

Macrophyte-mediated fluctuations in stream ecosystem function

Macrophyte-mediated fluctuations in stream ecosystem function

Wildfires alter stream ecosystem functioning through effects on leaf litter

BackgroundWildfires have strong impacts on terrestrial and aquatic ecosystems, whose frequency, severity, and intensity are increasing with climate change. Moreover, the expansion of exotic monoculture plantations, such as those of eucalypts, increases this risk. When wildfires do not cause the disappearance of riparian vegetation, they still imply the fall of leaf litter exposed to the fire (i.e., crown scorch), which consequences for ecosystems are unknown.Experimental designTo explore how these leaf litter inputs may affect stream ecosystem functioning, we conducted a microcosm experiment where we quantified the decomposition of leaf litter from three tree species (alder, oak, and eucalypt) under two conditions (control litter simulating natural entries and litter subjected to 150 °C for 3 h mimicking exposure to fire). We also examined the interaction between this factor and a temperature rise (which is often associated to the loss of riparian vegetation caused by the wildfire) by manipulating water temperature (10, 12.5, and 15 °C). Finally, we explored the effects of these variables on the growth of a common detritivore, the caddisfly Sericostoma pyrenaicum.ResultsControl alder presented the highest decomposition rates, which were notably reduced due to fire exposure. On the contrary, eucalypt litter decomposition was even slower than that of oak and hardly showed any effect derived from fire exposure. The different leaf litter types determined detritivore growth, to a greater extent than variation related to warming, which generally had negligible effects.ConclusionsOur study shows the negative effects of wildfires on stream ecosystem functioning even when they only involve brief exposure of leaf litter to the fire. Effects are greater on the most palatable native species, which represents the highest quality input in streams of the study area. Our results highlight the importance of protecting riparian forests, especially those composed of native species, against wildfires.

No effects of macroconsumer exclusion on macroinvertebrate functional composition in Brazilian karst streams along a gradient of riparian forest loss

AbstractDegradation and loss of riparian vegetation are some of the main drivers of change in the trophic interactions among aquatic macroconsumers. These changes can often have major consequences for biodiversity and stream ecosystem functioning. Yet, many aspects of food‐web dynamics are still not well understood, including how food webs among tropical streams respond to riparian vegetation loss. We examined the role of macroconsumers on community structure of functional feeding groups of benthic macroinvertebrates along a forest cover gradient within the Bodoquena Plateau, Brazil. We compared the effect of excluding macroconsumers (using electric fences) on functional feeding groups to those in controls within seven karst streams along a gradient of riparian vegetation cover. Aquatic insect and gastropod communities were dominated by groups of collector‐gatherers, scrapers, and predators across all sites. The exclusion of macroconsumers did not lead to an increase in the abundance of macroinvertebrates and was not affected by the forest cover during the experiment. These findings demonstrated that the experimental exclusion of macroconsumers resulted in a lack of or negligible effect on functional feeding groups along the gradient of forest cover. It is important to consider the specificity of trophic relationships to fully understand the effects of vegetation loss on streams where complex groundwater hydrology can also affect food webs such as in karst terrain.Abstract in Portuguese is available with online material.

Joint effects of warming and salinization on instream leaf litter decomposition assessed through a microcosm experiment

Stream ecosystem functioning is often impaired by warming and salinization, but the joint effect of both environmental stressors on key processes such as leaf litter decomposition is virtually unknown, particularly in the tropics. We experimentally explored how increased water temperature (26, 29 and 32°C) and salinity (no salt addition, 0.1, 1.0 and 10 g l−1 NaCl added) affected the rates of total, microbial and detritivore-mediated decomposition, in stream microcosms containing leaf litter of Ficus insipida and larvae of Chironomus sp. collected from tropical streams. Effects of temperature were strong and consistent with previous findings: it promoted microbial decomposition and reduced decomposition mediated by detritivores, which showed greater feeding activity at 26°C. Salinity was negatively correlated with microbial decomposition at 32°C; it also had a negative influence on detritivore-mediated decomposition, which was nevertheless non-significant due to the high detritivore mortality at higher salinities. Notably, total decomposition was reduced with the joint presence of both factors (32°C and salt addition treatments, compared to 26°C and no salt addition), indicating the existence of additive effects and highlighting the relevance of multiple-stressor contexts when assessing the consequences of global change on stream ecosystems.

Dieback and Replacement of Riparian Trees May Impact Stream Ecosystem Functioning

Alders are nitrogen (N)-fixing riparian trees that promote leaf litter decomposition in streams through their high-nutrient leaf litter inputs. While alders are widespread across Europe, their populations are at risk due to infection by the oomycete Phytophthora ×alni, which causes alder dieback. Moreover, alder death opens a space for the establishment of an aggressive N-fixing invasive species, the black locust (Robinia pseudoacacia). Shifts from riparian vegetation containing healthy to infected alder and, eventually, alder loss and replacement with black locust may alter the key process of leaf litter decomposition and associated microbial decomposer assemblages. We examined this question in a microcosm experiment comparing three types of leaf litter mixtures: one representing an original riparian forest composed of healthy alder (Alnus lusitanica), ash (Fraxinus angustifolia), and poplar (Populus nigra); one with the same species composition where alder had been infected by P. ×alni; and one where alder had been replaced with black locust. The experiment lasted six weeks, and every two weeks, microbially driven decomposition, fungal biomass, reproduction, and assemblage structure were measured. Decomposition was highest in mixtures with infected alder and lowest in mixtures with black locust, reflecting differences in leaf nutrient concentrations. Mixtures with alder showed distinct fungal assemblages and higher sporulation rates than mixtures with black locust. Our results indicate that alder loss and its replacement with black locust may alter key stream ecosystem processes and assemblages, with important changes already occurring during alder infection. This highlights the importance of maintaining heathy riparian forests to preserve proper stream ecosystem functioning.

Land use composition, configuration and nutrient: Key drivers of benthic metabolism in streams

Benthic metabolism is an essential process which plays a critical role in the carbon and nutrient cycles and in the transfer of material and energy in stream ecosystems. Benthic algae are one of principal primary producers in stream food webs that converts dissolved inorganic carbon to organic matter, and algae are more palatable basal resources for aquatic consumers due to its higher unsaturated fatty acid content than terrestrial carbon. Despite the importance of algae, few studies have investigated how the benthic metabolism is influenced by distal factors, such as upland land use and landscape configuration, and proximal factors, such as stream temperature, nutrient availability, and light availability at various spatial scales. Here, we measured the benthic metabolism and proximal factors at the outlet of 14 selected sub-catchments in a subtropical river. Land use and landscape metrics at the catchment, riparian, and reach scales were processed as distal factors. Using stepwise regression models, we found that the proximal factors, such as water temperature (WT) and total phosphorus (TP), significantly increased primary production, and that nitrate-nitrogen (NO3--N), pH, and electrical conductivity (EC) decreased gross primary production (GPP). Also, TP was positively correlated with respiration (R) while NO3--N negatively impacted R. Structural equation modeling demonstrated that the patch cohesion index negatively affected GPP by restraining WT and TP at the catchment scale while the increased shape index elevated the WT and TP to promote GPP. Our results suggested that stream metabolism is vulnerable to anthropogenic disturbances, such as upland land use and landscape patterns. Thus, land use planning and administration, particularly at catchment scale, should consider how to minimize the influence of human activities on ecosystem function in streams.

Basin‐scale variables drive macroinvertebrate biomass in low‐order streams across different mountain ecoregions

AbstractAimBiomass is an important descriptor of macroinvertebrate communities, providing insights into stream productivity and ecosystem functioning. Nevertheless, the drivers of biomass are still poorly assessed, especially in multiscale studies. Here, we aimed at filling this gap by assessing the relationship between environmental variables measured at patch, reach and basin scale and macroinvertebrate biomass, as well as the transferability of such relationships across different mountain regions.LocationMaritime‐Cottian Alps and the Tuscan‐Emilian Apennine (Northern Italy).TaxaFreshwater macroinvertebrates.MethodsPatch‐, reach‐ and basin‐scale variables, either measured in the field or computed with kriging techniques, were summarised with sparse principal component axes and later used in mixed modelling and variance partitioning to assess the relative effect of environmental variables on macroinvertebrate biomass. An independent model was run for each region, and results were compared to assess the transferability of drivers.ResultsBasin‐scale variables explained most of the biomass variability in both regions, supporting the transferability of results. More specifically, mean cumulated daily precipitation was the main driver of macroinvertebrate biomass. Patch‐ and reach‐scale variables also significantly affected macroinvertebrate biomass, but their relative role varied depending on the region considered, without any clear pattern.Main ConclusionsOverall, we highlighted a dominance of basin‐scale drivers—and specifically of the mean cumulated precipitation—and a consistency of results across different mountain regions. This implies that climatic variables regulate energy fluxes to higher trophic levels and subsidies to both downstream river sections and terrestrial ecosystems. These findings offer insights into ecosystem‐level vulnerability to climate change.

Macrophyte removal affects nutrient uptake and metabolism in lowland streams

Macrophytes provide essential ecosystem services in lowland streams, including nutrient uptake that can reduce downstream transport to vulnerable coastal areas. Despite that, to ensure water conveyance and effective run off from agricultural fields, aquatic plant biomass is removed regularly in many European streams (i.e. weed cutting practices). However, the impacts of weed cutting on stream ecosystem processes are not yet well documented. Here, we studied the effect of weed cutting on nutrient retention and ecosystem metabolism in three lowland streams with contrasting dominant vegetation communities (submergent and emergent plants) during summer in Denmark. Our results showed a decrease in nutrient retention; uptake velocity of ammonium decreased 34–77 % and of phosphate decreased 50–77 %. Ecosystem metabolic rates also decreased after weed cutting, both in gross primary production (9 %, 60 % and 85 %) and respiration (47 %, 69 % and 76 %). The effects of weed cutting on these ecosystem processes prevailed three weeks after the cutting occurred. Understanding the effects of weed cutting on stream ecosystem functioning can improve nature-based management strategies to control eutrophication of downstream coastal areas.

Downstream impacts of peatland drainage on headwater stream biodiversity and ecosystem functioning

Intensive peatland drainage alters the physico-chemical status of the recipient streams, potentially leading to the loss of biodiversity and impaired ecosystem functioning. However, the extent of these changes, and particularly their impacts on downstream ecosystems, remain poorly understood. We studied the downstream effects of peatland drainage on stream biodiversity (aquatic bryophytes and macroinvertebrates) and key ecosystem processes (primary productivity, organic matter (OM) decomposition and OM standing stock). Our survey design comprised upstream (directly below drainage network) and downstream (∼300 m downstream) locations in 18 boreal headwater streams encompassing an extensive gradient of peatland drainage intensity (0–48 %). Drainage modified environmental conditions, with nutrient (TP, TN) and dissolved organic carbon (DOC) concentrations and inorganic sediment cover increasing with intensifying drainage, in both upstream and downstream sites. Species richness of both benthic macroinvertebrates and bryophytes was strongly reduced with increasing drainage intensity. Upstream and downstream sites responded in a similar manner, suggesting wide-ranging impacts of drainage on stream biodiversity. Regardless of site location, ecosystem processes were unrelated to drainage intensity. Our results confirm that intense peatland drainage not only modifies environmental conditions and biodiversity in adjoining streams, but these impacts propagate further downstream in the stream network. To prevent further degradation of drainage-impacted freshwater ecosystems, large-scale peatland restoration is needed, with prioritization of sites with the greatest potential (least drainage-induced damage) for biodiversity recovery.

Warming overrides eutrophication effects on leaf litter decomposition in stream microcosms

Several human activities often result in increased nitrogen (N) and phosphorus (P) inputs to running waters through runoff. Although headwater streams are less frequently affected by these inputs than downstream reaches, the joint effects of moderate eutrophication and global warming can affect the functioning of these ecosystems, which represent two thirds of total river length and thus are of major global relevance. In a microcosm study representing streams from a temperate area (northern Spain), we assessed the combined effects of increased water temperature (10.0, 12.5, and 15.0 °C) and nutrient enrichment (control, high N, high P, and high N + P concentrations) on the key process of leaf litter decomposition (mediated by microorganisms and detritivores) and associated changes in different biological compartments (leaf litter, aquatic hyphomycetes and detritivores). While warming consistently enhanced decomposition rates and associated variables (leaf litter microbial conditioning, aquatic hyphomycete sporulation rate and taxon richness, and detritivore growth and nutrient contents), effects of eutrophication were weaker and more variable: P addition inhibited decomposition, addition of N + P promoted leaf litter conditioning, and detritivore stoichiometry was affected by the addition of both nutrients separately or together. In only a few cases (variables related to detritivore performance, but not microbial performance or leaf litter decomposition) we found interactions between warming and eutrophication, which contrasts with other experiments reporting synergistic effects. Our results suggest that both stressors can importantly alter the functioning of stream ecosystems even when occurring in isolation, although non-additive effects should not be neglected and might require exploring an array of ecosystem processes (not just leaf litter decomposition) in order to be detected.

Land use differentially alters microbial interactions and detritivore feeding during leaf decomposition in headwater streams

Abstract Human‐driven land conversion from forest to agriculture introduces nutrients, like nitrogen (N) and phosphorus (P), which upon entering nearby streams can stimulate growth and metabolism of microbial autotrophs (algae) and heterotrophs (fungi and bacteria). Recent evidence indicates that algae also can stimulate microbial heterotrophs associated with decaying plant litter by exuding labile carbon (C), defined as the priming effect on decomposition. These autotroph–heterotroph interactions may further alter detritivore feeding ecology, suggesting many complex mechanisms of land‐use change on stream ecosystem functions. In order to investigate the effects of land use on autotroph–heterotroph interactions, we used a full‐factorial design manipulating labile C (glucose and acetate) and nutrients (N and P) to test their effects during red maple litter decomposition within four forested and four pasture agriculture streams in the Point Remove catchment, Arkansas. We deployed nutrient‐diffusing substrates (NDS) containing litter and either agar, agar + labile C, agar + NP, or agar + CNP for 20 days and measured microbial biomass and metabolism, and detrital mass loss. Additionally, we fed litter to Tipula sp. to assess shifts in consumption and egestion rates. Both microbial heterotrophs and autotrophs responded to NDS manipulations, but not land use. Heterotrophic microbes showed evidence of nutrient limitation and negative priming, while algal biomass was negatively associated with heterotrophs. Tipula consumption and egestion rates were both elevated when feeding on litter incubated in agriculture streams, but only egestion responded to NDS manipulations, suggesting a change in assimilation efficiencies. Cumulatively, we suggest that altered stream conditions, such as those resulting from land‐use change, can alter food webs and detritivore feeding more heavily than microbial interactions during litter decomposition in stream ecosystems. These results add to the literature demonstrating shifts in autotroph–heterotroph interactions with anthropogenic nutrient pollution, and the implications for detritivore feeding and overall C cycling in streams.

Canopy development influences early successional stream ecosystem function but not biotic assemblages

Determining how streams develop naturally, particularly the ecological role of newly developed riparian canopy cover, is essential to understanding the factors that structure new stream communities and provides valuable information for restoring highly disturbed ecosystems. However, attempts to understand primary succession in riverine ecosystems have been hindered by a lack of data owing to the infrequent formation of new rivers on the landscape. In the present study, we used five streams formed following the 1980 eruption of Mount St. Helens (WA, USA) to examine the influence of canopy cover development on algal and benthic macroinvertebrate assemblages, biomass, and organic matter processing. Newly established closed canopy reaches had less available light, but no significant differences in algal biomass or macroinvertebrate assemblages compared to open canopy reaches. Instead, algal and macroinvertebrate communities were structured mainly by hydrologic differences among watersheds. In contrast, organic matter processing rates were sensitive to canopy cover development, and rates were faster under closed canopies, especially in late summer or after terrestrial preconditioning. After 40 years of stream and riparian primary successional development, canopy cover strongly influences ecosystem function, but aquatic organism assembly was more influenced by physio-chemical and hydrologic variation. Our findings provide insight into the development of in-stream assemblages and ecosystem functions, which is also relevant to efforts to address major disturbances to stream channels, such as volcanic eruptions, floods, forest fires, and clear-cut logging.

Impacts of diffuse urban stressors on stream benthic communities and ecosystem functioning: A review

Catchment urbanisation results in urban streams being exposed to a multitude of stressors. Notably, stressors originating from diffuse sources have received less attention than stressors originating from point sources. Here, advances related to diffuse urban stressors and their consequences for stream benthic communities are summarised by reviewing 92 articles. Based on the search criteria, the number of articles dealing with diffuse urban stressors in streams has been increasing, and most of them focused on North America, Europe, and China. Land use was the most common measure used to characterize diffuse stressor sources in urban streams (70.7 % of the articles characterised land use), and chemical stressors (inorganic nutrients, xenobiotics, metals, and water properties, including pH and conductivity) were more frequently reported than physical or biological stressors. A total of 53.3 % of the articles addressed the impact of urban stressors on macroinvertebrates, while 35.9 % focused on bacteria, 9.8 % on fungi, and 8.7 % on algae. Regarding ecosystem functions, almost half of the articles (43.5 %) addressed changes in community dynamics, 40.3 % addressed organic matter decomposition, and 33.9 % addressed nutrient cycling. When comparing urban and non-urban streams, the reviewed studies suggest that urbanisation negatively impacts the diversity of benthic organisms, leading to shifts in community composition. These changes imply functional degradation of streams. The results of the present review summarise the knowledge gained to date and identify its main gaps to help improve our understanding of urban streams.

Alder stands promote N-cycling but not leaf litter mass loss in Mediterranean streams flowing through pine plantations

During the last century, the abandonment of traditional dryland farming and pastures in the Mediterranean basin promoted the development of ambitious afforestation programs causing a drastic transformation of the landscape. Afforestation programs were usually accomplished without considering the potential ecological impacts on the recipient ecosystems. Forest streams rely on terrestrial organic detritus, so their functioning and conservation status can be altered by changes in riparian forest biodiversity and composition. However, the influences of conifer plantations in particular on stream functioning are still unclear, possibly because the presence of multiple species in plantations or the presence of other species in the riparian zones of streams may act as buffers of such effects. Here, by means of a field experiment, we assessed whether the presence of black alder in the riparian zone and/or as leaf litter within the stream may mitigate the impacts exerted by pine plantations on stream ecosystem functioning. We found (i) that streams were functionally similar but differed in water N concentrations; (ii) no differences in litter mas loss between riparian types, but higher total litter mass loss of those mixtures containing alder leaf litter; and (iii) higher N losses (or lower N gains) for all litter types in streams without riparian alder. These results demonstrate that microbial decomposers can use either stream water N or litter N, and that detritivores can feed simultaneously on resources of contrasting quality to balance their diet. Our study underscores the effect of even low-density riparian cover of alder promoting microbial nutrient cycling by moderate increases of water N concentrations. We suggest thinning of pine plantations combined with planting of native deciduous species as alder to alleviate the effects of pine plantations on Mediterranean streams.

Drying, more than warming, alters ecosystem functioning in streams with different energy pathways

Abstract Empirical evidence and theory suggest that climate warming and an increase in the frequency and duration of drying events will alter the metabolic balance of freshwater ecosystems. However, the impacts of climate change on ecosystem metabolism may depend on whether energy inputs are of autochthonous or allochthonous origin. To date, few studies have examined how warming and drying may interact to alter stream metabolism, much less how their impacts may depend on the energy‐base of the food web. To address this research gap, we conducted a multi‐factorial experiment using outdoor mesocosms to investigate the individual and synergistic effects of warming and drought on metabolic processes in stream mesocosms with green (algal‐based) vs. mixed (algal‐ and detritus‐based) vs. brown (detritus‐based) energy pathways. We set up 48 mesocosms with one of three different levels of shade and leaf litter input combinations to create mesocosms with different primary energy channels. In addition, we warmed half of the mesocosms by ~2–3°C. We assessed changes in ecosystem respiration (ER), gross primary production (GPP), net ecosystem production (NEP) and organic matter biomass in warmed and ambient temperature mesocosms before a 24 day drying event and after rewetting. Surprisingly, experimental warming had little effect on metabolic processes. Drying, however, led to decreased rates of ER and GPP and led to an overall reduction in NEP. Although the effects of drying were similar across energy channel treatments, reductions in ER and GPP were primarily driven by decreases in biomass of benthic and filamentous algae. Overall, we demonstrate that drying led to lower rates of NEP in mesocosms regardless of energy inputs. While warming showed little effect in our study, our results suggest that an increase in the frequency of stream drying events could greatly alter the metabolic balance of many aquatic ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Effects of incomplete sampling on macroinvertebrate secondary production estimates in a forested headwater stream

Estimates of secondary production depend on the efficiency of sampling methods in capturing abundances and body lengths of the entire macroinvertebrate community. The efficiency of common sampling methods in fulfilling these criteria is poorly understood. We compared the effects of a Surber sampler (250 µm mesh size) and a Freeze corer in capturing abundance, biomass, and secondary production of macroinvertebrates in a forested headwater stream. We then examined how the use of nets with different mesh sizes could affect estimates of secondary production. Macroinvertebrate abundance was three times lower, and biomass was three times higher with the Surber than with the Freeze corer. Neither method captured the entire length distribution, and incomplete sampling of body lengths and abundance resulted in underestimating total secondary production by 48% (Surber) and 49% (Freeze corer). We estimated that reducing the mesh size from 250 to 100 µm would reduce the underestimation of production from ~ 48 to ~ 12% due to the inclusion of smaller individuals. Our results improve the efficiency of common sampling methods, allowing a reliable quantification of the role of macroinvertebrates in stream ecosystem functioning.

Resource modification by ecosystem engineers generates hotspots of stream community assembly and ecosystem function.

Ecosystem engineers can generate hotspots of ecological structure and function by facilitating the aggregation of both resources and consumers. However, nearly all examples of such engineered hotspots come from long-lived foundation species, such as marine and freshwater mussels, intertidal cordgrasses, and alpine cushion plants, with less attention given to small-bodied and short-lived animals. Insects often have rapid life cycles and high population densities and are among the most diverse and ubiquitous animals on earth. Although these taxa have the potential to generate hotspots and heterogeneity comparable to that of foundation species, few studies have examined this possibility. We conducted a mesocosm experiment to examine the degree to which a stream insect ecosystem engineer, the net-spinning caddisfly (Tricoptera:Hydropsychidae), creates hotspots by facilitating invertebrate community assembly. Our experiment used two treatments: (1) stream benthic habitat with patches of caddisfly engineers present and (2) a control treatment with no caddisflies present. We show that compared to controls, caddisflies increased local resource availability measured as particulate organic matter (POM) by 43%, ecosystem respiration (ER) by 70%, and invertebrate density, biomass, and richness by 96%, 244%, and 72%, respectively. These changes resulted in increased spatial variation of POM by 25%, invertebrate density by 76%, and ER by 29% compared to controls, indicating a strong effect of caddisflies on ecological heterogeneity. We found a positive relationship between invertebrate density and ammonium concentration in the caddisfly treatment, but no such relationship in the control, indicating that either caddisflies themselves or the invertebrate aggregations they create increased nutrient availability. When accounting for the amount of POM, caddisfly treatments increased invertebrate density by 48% and richness by 40% compared to controls, suggesting that caddisflies may also enhance the nutritional quality of resources for the invertebrate assemblage. The caddisfly treatment also increased the rate of ecosystem respiration as a function of increasing POM compared to the control. Our study demonstrates that insect ecosystem engineers can generate heterogeneity by concentrating local resources and consumers, with consequences for carbon and nutrient cycling.