Preliminary estimation of the export of omega-3 polyunsaturated fatty acids from aquatic to terrestrial ecosystems in biomes via emergent insects

Some terrestrial consumers may be limited by food quality, namely by contents of essential polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (20:5n−3, EPA) and docosahexaenoic acid (22:6n−3, DHA) in their food. Since EPA and DHA are mainly produced in aquatic ecosystems, for future estimating of the potential limitation by food quality in global scale, the water-land fluxes of these PUFA with the biomass of emergent aquatic insects in several biomes were calculated. The water /land area ratios for each biome were calculated by dividing the water area of each biome by its terrestrial area. Data on insect emergence from water bodies (g of dry mass m⁻² year⁻¹), were summarized and averaged for each biome. From available data, EPA and DHA contents (mg g⁻¹dry mass), in the biomass of emergent aquatic insects were calculated first so that annual fluxes of PUFA to land area via aquatic insect emergence could be estimated for each biome. PUFA fluxes occurred between the biomes, ranging from 0.04 to 4.39 mg m⁻² year⁻¹. In this study, the aquatic PUFA supply to land area appeared to be significantly lower than estimated earlier. This suggests that terrestrial consumers may experience food quality limitations mediated by shortage of PUFA compounds.     

Fluxes of terrestrial and aquatic carbon by emergent mosquitoes: a test of controls and implications for cross-ecosystem linkages

Adult aquatic insects are a common resource for many terrestrial predators, often considered to subsidize terrestrial food webs. However, larval aquatic insects themselves consume both aquatic primary producers and allochthonous terrestrial detritus, suggesting that adults could provide aquatic subsidy and/or recycled terrestrial energy to terrestrial consumers. Understanding the source of carbon (aquatic vs. terrestrial) driving aquatic insect emergence is important for predicting magnitude of emergence and effects on recipient food web dynamics; yet direct experimental tests of factors determining source are lacking. Here, we use Culex mosquitoes in experimental pools as an exemplar to test how variation in general factors common to aquatic systems (terrestrial plant inputs and light) may alter the source and amount of energy exported to terrestrial ecosystems in adult aquatic insects that rely on terrestrial resources as larvae. We found strong sequential effects of terrestrial plant inputs and light on aquatic insect oviposition, diet, and emergence of Culex mosquitoes. Ovipositing mosquitoes laid ~3 times more egg masses in high terrestrial input pools under low light conditions. This behavior increased adult emergence from pools under low light conditions; however, high input pools (which had the highest mosquito densities) showed low emergence rates due to density-dependent mortality. Mosquito diets consisted mainly of terrestrial resources (~70–90 %). As a result, the amount of aquatic carbon exported from pools by mosquitoes during the experiment was ~18 times higher from low versus high light pools, while exports of terrestrial carbon peaked from pools receiving intermediate levels of inputs (3–6 times higher) and low light (~6 times higher). Our results suggest that understanding the interplay among terrestrial plant inputs, light availability and biotic responses of aquatic insects may be key in predicting source and magnitude of emergence, and thus the strength and effects of aquatic–terrestrial linkages in freshwater systems.     

Seasonal bat activity related to insect emergence at three temperate lakes

Knowledge of aquatic food resources entering terrestrial systems is important for food web studies and conservation planning. Bats, among other terrestrial consumers, often profit from aquatic insect emergence and their activity might be closely related to such events. However, there is a lack of studies which monitor bat activity simultaneously with aquatic insect emergence, especially from lakes. Thus, our aim was to understand the relationship between insect emergence and bat activity, and investigate whether there is a general spatial or seasonal pattern at lakeshores. We assessed whole‐night bat activity using acoustic monitoring and caught emerging and aerial flying insects at three different lakes through three seasons. We predicted that insect availability and seasonality explain the variation in bat activity, independent of the lake size and characteristics. Spatial (between lakes) differences of bat activity were stronger than temporal (seasonal) differences. Bat activity did not always correlate to insect emergence, probably because other factors, such as habitat characteristics, or bats’ energy requirements, play an important role as well. Aerial flying insects explained bat activity better than the emerged aquatic insects in the lake with lowest insect emergence. Bats were active throughout the night with some activity peaks, and the pattern of their activity also differed among lakes and seasons. Lakes are important habitats for bats, as they support diverse bat communities and activity throughout the night and the year when bats are active. Our study highlights that there are spatial and temporal differences in bat activity and its hourly nocturnal pattern, that should be considered when investigating aquatic–terrestrial interactions or designing conservation and monitoring plans.     

Reciprocal fluxes of stream and riparian invertebrates in a coastal California basin with Mediterranean climate

Stream and riparian food webs are connected by reciprocal fluxes of invertebrates, and a growing number of studies demonstrate strong effects of these subsidies on consumers and food webs in both habitats. However, despite its importance in understanding energy flow between these habitats, seasonality of reciprocal subsidies has been examined only in a single temperate system in Japan. We measured input of terrestrial invertebrates and emergence of adult aquatic insects for 14?months in two adjacent streams in a coastal Mediterranean basin in California to assess seasonal patterns, annual fluxes, and local variation. Fluxes of terrestrial and aquatic invertebrates fluctuated seasonally and were relatively synchronous, although in the fall of 2004, terrestrial inputs peaked 1?C2?months earlier than emergence. Terrestrial inputs were similar in the two streams with annual flux of 7.9?C8.6?g dry mass?m^?2?year^?1. Emergence differed between the streams: annual emergence was 7.8?g?m^?2?year^?1 (similar to terrestrial flux) in one reach but 5.3?g?m^?2?year^?1 from the other. The presence of streambed travertine in the reach with lower emergence was the primary difference in habitat between the streams, suggesting that travertine may reduce emergence and alter net reciprocal flux. Comparison of our results with those from Japan suggests that seasonality and net annual flux of reciprocal stream-riparian subsidies vary among biomes due to differences in climate, vegetation, and geography. Our results also indicate that local factors, such as travertine, may cause reciprocal fluxes to vary at finer spatial scales.     

Leaf litter quality affects aquatic insect emergence: contrasting patterns from two foundation trees

Reciprocal subsidies between rivers and terrestrial habitats are common where terrestrial leaf litter provides energy to aquatic invertebrates while emerging aquatic insects provide energy to terrestrial predators (e.g., birds, lizards, spiders). We examined how aquatic insect emergence changed seasonally with litter from two foundation riparian trees, whose litter often dominates riparian streams of the southwestern United States: Fremont (Populus fremontii) and narrowleaf (Populus angustifolia) cottonwood. P. fremontii litter is fast-decomposing and lower in defensive phytochemicals (i.e., condensed tannins, lignin) relative to P. angustifolia. We experimentally manipulated leaf litter from these two species by placing them in leaf enclosures with emergence traps attached in order to determine how leaf type influenced insect emergence. Contrary to our initial predictions, we found that packs with slow-decomposing leaves tended to support more emergent insects relative to packs with fast-decomposing leaves. Three findings emerged. Firstly, abundance (number of emerging insects m^?2 day^?1) was 25 % higher on narrowleaf compared to Fremont leaves for the spring but did not differ in the fall, demonstrating that leaf quality from two dominant trees of the same genus yielded different emergence patterns and that these patterns changed seasonally. Secondly, functional feeding groups of emerging insects differed between treatments and seasons. Specifically, in the spring collector-gatherer abundance and biomass were higher on narrowleaf leaves, whereas collector-filterer abundance and biomass were higher on Fremont leaves. Shredder abundance and biomass were higher on narrowleaf leaves in the fall. Thirdly, diversity (Shannon’s H′) was higher on Fremont leaves in the spring, but no differences were found in the fall, showing that fast-decomposing leaves can support a more diverse, complex emergent insect assemblage during certain times of the year. Collectively, these results challenge the notion that leaf quality is a simple function of decomposition, suggesting instead that aquatic insects benefit differentially from different leaf types, such that some use slow-decomposing litter for habitat and its temporal longevity and others utilize fast-decomposing litter with more immediate nutrient release.     

Shoreline urbanization reduces terrestrial insect subsidies to fishes in North American lakes

Despite growing recognition of the energetic connections between aquatic and riparian habitats of streams and lakes, there have been few efforts to quantify the importance of terrestrial insect subsidies to fish in lakes. Further, it is unclear whether lakeshore urbanization alters the magnitude of these fluxes. Because lakeshore development has been found to be negatively correlated with riparian vegetation that serves as habitat for terrestrial invertebrates, we expected that shoreline urbanization would reduce the prevalence of terrestrial invertebrates in fish diets. We quantified the effects of lakeshore urbanization on terrestrial insect subsidies to fish at three scales: a focused comparison of annual patterns in four lakes in the Pacific Northwest, a one‐time field survey of 28 Pacific Northwest lakes, and a literature survey of 24 North American lakes. At all geographical scales, terrestrial invertebrate subsidies to fish were negatively correlated with shoreline development. Terrestrial insects comprised up to 100% of fish diet mass in undeveloped lakes, versus an average of 2% of fish diet mass in developed lakes. Trout, Oncorhynchus spp., in undeveloped lakes had an average of 50% greater daily energy intake, up to 50% of which was represented by terrestrial prey. Temporal variability of the terrestrial subsidy suggests that these inputs are distinctly pulsed, and this subsidy is absent or temporally rare in undeveloped lakes.     

Linking stream habitats and spider distribution: spatial variations in trophic transfer across a forest–stream boundary

In headwater streams, many aquatic insects rely on terrestrial detritus, while their emergence from streams often subsidizes riparian generalist predators. However, spatial variations in such reciprocal trophic linkages remain poorly understood. The present study, conducted in a northern Japanese stream and the surrounding forest, showed that pool–riffle structure brought about heterogeneous distributions of detritus deposits and benthic aquatic insects. The resulting variations in aquatic insect emergence influenced the distributions of riparian web-building spiders. Pools with slow current stored greater amounts of detritus than riffles, allowing more benthic aquatic insects to develop in pools. The greater larval biomass in pools and greater tendency for riffle insects to drift into pools at metamorphosis resulted in an emergence rate of aquatic insects from pools that was some four to five times greater than from riffles. In the riparian forest, web-building spiders (Tetragnathidae and Linyphiidae) were distributed in accordance with the emergence rates of aquatic insects, upon which both spider groups heavily depended. Consequently, the riparian strips bordering pools had a density of tetragnathid spiders that was twice as high as that of the riparian strips adjacent to riffles. Moreover, although limitations of vegetation structure prevented the aggregation of linyphiid spiders around pools, linyphiid density normalized by shrub density was higher in habitats adjacent to pools than those adjacent to riffles. The results indicated that stream geomorphology, which affects the storage of terrestrial organic material and the export of such material to riparian forests via aquatic insect emergence, plays a role in determining the strength of terrestrial–aquatic linkages in headwater ecosystems.     

Land‐use effects on terrestrial consumers through changed size structure of aquatic insects

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Low Levels of Allochthony in Consumers Across Three High-Elevation Lake Types

The integration of lakes into watershed-scale energy budgets remains a major goal of aquatic ecosystem ecology. However, this enterprise has focused almost exclusively on temperate and boreal systems and on zooplankton as representatives of system-wide energy flow. We investigated the proportion of consumer biomass derived from terrestrial sources, allochthony, in three classes of high-elevation lakes—alpine, large and small montane—of varying geometry and watershed ecosystem development, and across five taxa, including macrobenthic invertebrates and fish. We used stable isotopes of carbon (¹³C:¹²C), nitrogen (¹⁵N:¹⁴N), and hydrogen (²H:¹H) to fit a modular Bayesian mixing model, which estimated proportional assimilation of phytoplankton, algal periphyton, and terrestrial organic matter by each consumer. The stable isotope analysis was supplemented with a comparison of fatty acid profiles between consumers and producers, and with a Daphnia magna rearing study involving aquatic and terrestrial nutrient sources. The most probable estimate of allochthony across consumer taxa was 41% in small montane lakes (< 0.1 ha) with high terrestrial C loading. For large montane (3–11 ha) and alpine lakes (0.8–3 ha), with substantially less terrestrial influence, allochthony was just 4 and 3%, respectively. Allochthony was also lower on average for benthic grazers than for pelagic consumers. Our results corroborate previous findings that lake size, depth, and light penetration are dominant physical controls on allochthony, but add that it sharply declines at high elevation due to changes in terrestrial primary production near or above tree line.     

Ecosystem Linkages Between Lakes and the Surrounding Terrestrial Landscape in Northeast Iceland

Despite a recent emphasis on understanding cross-habitat interactions, few studies have examined the ecological linkages between lakes and surrounding terrestrial habitats. The current paradigm of land–lake interactions is typically unidirectional: the view is that nutrients and matter are transported downslope from the surrounding watershed to their ultimate lacustrine destination. Emergent aquatic insects, which spend their larval stages in lake sediments and emerge as adults to mate over land, can act as vectors of material, energy and nutrients from aquatic to terrestrial habitats. In this study, we document a gradient of midge (Diptera: Chironomidae) infall rates into terrestrial habitats (measured as g dw midges m^?2 d^?1) surrounding eight lakes in Northern Iceland (≈66°N latitude). Lakes ranged from having virtually no midge infall (for example, Helluvaðstjörn, 0.03 g m^?2 d^?1) to extreme levels (for example, Mývatn, 19 g m^?2 d^?1) with abundances of midges decreasing logarithmically with distance from shore. Annual midge input rates are estimated as high as 1200–2500 kg midges ha^?1 y^?1. As midges are approximately 9.2% total N, this can result in a significant fertilization effect of terrestrial habitats with consequences for plant quality and community structure. In addition, we used naturally-occurring δ¹³C and δ¹⁵N isotopes to examine food web structure and diet sources of terrestrial arthropod consumers surrounding lakes with differing amounts of midge input. Terrestrial arthropods showed increased utilization of aquatic-derived (that is, midge) C relative to terrestrial sources as midge infall increased. This pattern was particularly pronounced for predators, such as spiders and opiliones, and some detritivores (Collembola). These findings suggest that, despite being largely ignored, aquatic-to-terrestrial linkages can be large and midges can fuel terrestrial communities by directly serving as resources for predators and decomposers.     

Boomerang ecosystem fluxes: organic carbon inputs from land to lakes are returned to terrestrial food webs via aquatic insects

Many ecosystems are linked to their adjacent ecosystems by movements of organisms. For instance, aquatic and terrestrial ecosystems are linked via emerging aquatic insects that serve as prey for terrestrial consumers. However, the role of these organisms in returning recycled carbon to the ecosystem from which it originated is not well known. This is due to the fact that values of carbon isotope signatures from terrestrial leaves and aquatic resources are usually similar and hence results of isotope mixing models need to be considered with caution. We overcame this problem by adding isotopically distinct terrestrial particulate organic carbon (tPOC) as a tracer to the experimental sides of two lakes that were divided in two equal halves with plastic curtains. We focused on aquatic insect larvae (Chironomidae) that fed on maize Zea mays leaves experimentally added to the lakes, and subsequently became prey for terrestrial predators (spiders) after emergence. The carbon isotope values of Chironomidae and spiders were significantly elevated in the lake treatment sides as compared to reference sides, whereas the values of all autochthonous resources were not affected by maize additions. Estimates from stable isotope mixing models indicated a low but demonstrable contribution of maize leaves to the diet of Chironomidae. Overlap between the isotope values of alder leaves, the major natural tPOC source, and autochthonous resources prevented a reliable quantification of allochthony of Chironomidae. However, we qualitatively demonstrated the flow of terrestrial particulate organic carbon to lakes, as leaf fall, and back to terrestrial surroundings via emerging insects. This ‘boomerang’ carbon flux between land and lakes blurs the distinction between autochthonous and allochthonous carbon sources.     

The importance of aquatic vegetation in beaver diets and the seasonal and habitat specificity of aquatic‐terrestrial ecosystem linkages in a subarctic environment

The extent and ecological significance of trophic linkages across ecosystem boundaries have been the subject of considerable recent research attention. North American beavers Castor canadensis engineer terrestrial influences in aquatic ecosystems by constructing terrestrial food caches near their lodges and aquatic influences in terrestrial ecosystems by building dams and flooding low lying areas. However, it is poorly resolved to what extent beavers rely on aquatic food sources and whether this reliance is greater during winter when ice cover physically confines beavers to aquatic habitats or during summer when warm, ice free water promotes the growth and accessibility of aquatic vegetation. Working in a subarctic region, we surveyed the abundance of aquatic and terrestrial food sources in and around lotic and lentic environments and estimated their contributions to beaver diets during open water and ice covered periods using carbon and nitrogen stable isotope analysis of hair samples. Ponds had four times more aquatic vegetation than streams, but terrestrial habitats around ponds had less than half as much shrub cover as habitats adjacent to streams. Beaver diets in this subarctic environment are estimated to be comprised of 60 to 80% aquatic vegetation, with beavers occupying ponds consuming more aquatic vegetation in winter than beavers occupying streams, which rely more on terrestrial shrubs cached near their lodge. Collectively, these results show how the influence of physical barriers on ecosystem linkages can be modified by habitat‐ and season‐specific abundances of preferred resources and the potential for animals to consume food in ecosystems and seasons different from where and when the food was harvested.     

Increasing donor ecosystem productivity decreases terrestrial consumer reliance on a stream resource subsidy

Because nutrient enrichment can increase ecosystem productivity, it may enhance resource flows to adjacent ecosystems as organisms cross ecosystem boundaries and subsidize predators in recipient ecosystems. Here, we quantified the biomass and abundance of aquatic emergence and terrestrial spiders in a reference and treatment stream that had been continuously enriched with nitrogen and phosphorus for 5 years. Because we previously showed that enrichment increased secondary production of stream consumers, we predicted that aquatic emergence flux would be higher in the treatment stream, subsequently increasing the biomass and abundance of terrestrial spiders. Those increases were predicted to be greatest for spiders specializing on aquatic emergence subsidies (e.g., Tetragnathidae). By adding a ¹⁵N stable isotope tracer to both streams, we also quantified nitrogen flow from the stream into the riparian community. Emergence biomass, but not abundance, was higher in the treatment stream. The average body size of emerging adult insects and the relative dominance of Trichoptera adults were also greater in the treatment stream. However, spider biomass did not differ between streams. Spiders also exhibited substantially lower reliance on aquatic emergence nitrogen in the treatment stream. This reduced reliance likely resulted from shifts in the body size distributions and community composition of insect emergence that may have altered predator consumption efficiency in the treatment stream. Despite nutrient enrichment approximately doubling stream productivity and associated cross-ecosystem resource flows, the response of terrestrial predators depended more on the resource subsidy’s characteristics that affected the predator’s ability to capitalize on such increases.     

Dispersal is linked to habitat use in 59 species of water beetles (Coleoptera: Adephaga) on Madagascar

Lentic habitats (standing water, such as ponds and lakes) differ from lotic habitats (running water; streams and rivers) in their spatiotemporal persistence, with lentic habitats being more ephemeral in evolutionary time. This habitat instability is thought to select for dispersal, and several phylogenetic and macroecological studies have suggested that high rates of dispersal are more characteristic of lentic than lotic species. We tested this hypothesis using a comparative population genetic and phylogeographic approach based on mitochondrial DNA for 59 aquatic beetle species, sampled across Madagascar. Species were classified as lotic (n = 25), lentic (n = 25), or lotolentic (associated with both running and standing water; n = 9). Hierarchical population genetic structure (AMOVA), nucleotide diversity (π), and geographic structure were compared among habitat types. Lotic species had significantly greater population structure (Ф_ST = 0.55, hierarchical AMOVA) than lentic (Ф_ST = 0.13) and lotolentic (Ф_ST = 0.19) species using phylogenetic generalized least squares (PGLS) to correct for phylogeny. Body size was independent of habitat preference, and did not explain any of the intraspecific variation. A greater proportion of lotic species were endemic to Madagascar and lotic species had more pronounced geographic structure in their haplotype networks. The results indicate that dispersal is consistently lower among lotic species, independent of phylogenetic relatedness. This has macroevolutionary and biogeographical consequences for the freshwater fauna of this tropical biodiversity hotspot where remaining natural habitats are becoming increasingly isolated from one another.     

Aquatic and terrestrial invertebrate drift in southern Appalachian Mountain streams: implications for trout food resources

1. We characterised aquatic and terrestrial invertebrate drift in six south‐western North Carolina streams and their implications for trout production. Streams of this region typically have low standing stock and production of trout because of low benthic productivity. However, little is known about the contribution of terrestrial invertebrates entering drift, the factors that affect these inputs (including season, diel period and riparian cover type), or the energetic contribution of drift to trout. 2. Eight sites were sampled in streams with four riparian cover types. Drift samples were collected at sunrise, midday and sunset; and in spring, early summer, late summer and autumn. The importance of drift for trout production was assessed using literature estimates of annual benthic production in the southern Appalachians, ecotrophic coefficients and food conversion efficiencies. 3. Abundance and biomass of terrestrial invertebrate inputs and drifting aquatic larvae were typically highest in spring and early summer. Aquatic larval abundances were greater than terrestrial invertebrates during these seasons and terrestrial invertebrate biomass was greater than aquatic larval biomass in the autumn. Drift rates of aquatic larval abundance and biomass were greatest at sunset. Inputs of terrestrial invertebrate biomass were greater than aquatic larvae at midday. Terrestrial invertebrate abundances were highest in streams with open canopies and streams adjacent to pasture with limited forest canopy. 4. We estimate the combination of benthic invertebrate production and terrestrial invertebrate inputs can support 3.3–18.2 g (wet weight) m⁻² year⁻¹ of trout, which is generally lower than values considered productive [10.0–30.0 g (wet weight) m⁻² year⁻¹]. 5. Our results indicate terrestrial invertebrates can be an important energy source for trout in these streams, but trout production is still low. Any management activities that attempt to increase trout production should assess trout food resources and ensure their availability.     

Linking the evolution of habitat choice to ecosystem functioning: direct and indirect effects of pond-reproducing fire salamanders on aquatic-terrestrial subsidies

Shifts in life history traits and in the behaviour of species can potentially alter ecosystem functioning. The reproduction of the central European fire salamander (Salamandra salamandra), which usually deposits its larvae in first-order streams, in small pool and pond-like habitats, is an example of a recent local adaptation in this species. Here we aimed to quantify the direct and indirect effects of the predatory larvae on the aquatic food webs in the ponds and on the flux of matter between the ponds and adjacent terrestrial habitats. Our estimates are based on biomass data of the present pond fauna as well as on the analysis of stomach content data, growth rates and population dynamics of the salamander larvae in pond habitats. By their deposition of larvae in early spring, female fire salamanders import between 0.07 and 2.86 g dry mass m^?2 larval biomass into the ponds. Due to high mortality rates in the larval phase and the relatively small size at metamorphosis of the pond-adapted salamanders compared to stream-adapted ones, the biomass export of the metamorphosed salamanders clearly falls below the initial biomass import. Catastrophic events such as high water temperatures and low oxygen levels may even occasionally result in mass mortalities of salamander larvae and thus in a net 100 % import of the salamander biomass into the pond food webs. Indirect effects further accelerate this net import of matter into the aquatic habitat, e.g. the feeding of salamanders on aquatic insect larvae with the emergence of terrestrial adults—thus preventing export—and on terrestrial organisms that fall on the water surface (supporting import). This study demonstrates that the adaptation of salamanders to pond reproduction can alter food web linkages across ecosystem boundaries by enhancing the flux of materials and energy from terrestrial (i.e. forest) to the aquatic (i.e. pond) habitat.     

Shifts in reciprocal river‐riparian arthropod fluxes along an urban‐rural landscape gradient

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Climate change modifies the size structure of assemblages of emerging aquatic insects

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The regional and global significance of nitrogen removal in lakes and reservoirs

Human activities have greatly increased the transport of biologically available nitrogen (N) through watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and reservoirs) have the potential to act as important sinks for this reactive N as it is transported across the landscape because they offer ideal conditions for N burial in sediments or permanent loss via denitrification. However, the patterns and controls on lentic N removal have not been explored in great detail at large regional to global scales. In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N year^?1 from watersheds globally. Small lakes (<50 km²) were critical in the analysis, retaining almost half (9.3 Tg N year^?1) of the global total. In model runs, capacity of lakes and reservoirs to remove watershed N varied substantially at the half-degree scale (0–100%) both as a function of climate and the density of lentic systems. Although reservoirs occupy just 6% of the global lentic surface area, we estimate they retain ~33% of the total N removed by lentic systems, due to a combination of higher drainage ratios (catchment surface area:lake or reservoir surface area), higher apparent settling velocities for N, and greater average N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of NiRReLa suggests that, on-average, N removal within lentic systems will respond more strongly to changes in land use and N loading than to changes in climate at the global scale.