S. Mažeika P. Sullivan

Bright lights, big city: influences of ecological light pollution on reciprocal stream–riparian invertebrate fluxes

Cities produce considerable ecological light pollution (ELP), yet the effects of artificial night lighting on biological communities and ecosystem function have not been fully explored. From June 2010 to June 2011, we surveyed aquatic emergent insects, riparian arthropods entering the water, and riparian spiders of the family Tetragnathidae at nine stream reaches representing common ambient ELP levels of Columbus, Ohio, USA, streams (low, 0.1-0.5 lux; moderate, 0.6-2.0 lux; high, 2.1-4.0 lux). In August 2011, we experimentally increased light levels at the low- and moderate-treatment reaches to 10-12 lux to represent urban streams exposed to extremely high levels of ELP. Although season exerted the dominant influence on invertebrate fluxes over the course of the year, when analyzed by season, we found that light strongly influenced multiple invertebrate responses. The experimental light addition resulted in a 44% decrease in tetragnathid spider density (P = 0.035), decreases of 16% in family richness (P = 0.040) and 76% in mean body size (P = 0.022) of aquatic emergent insects, and a 309% increase in mean body size of terrestrial arthropods (P = 0.015). Our results provide evidence that artificial light sources can alter community structure and ecosystem function in streams via changes in reciprocal aquatic-terrestrial fluxes of invertebrates.

In a state of flux: The energetic pathways that move contaminants from aquatic to terrestrial environments

Today, interest continues to grow in the potential ‘‘reterres-trialization’’ of contaminants as insects, animals, birds,reptiles, and other creatures move about or via the energeticpathways that quintessentially link aquatic and terrestrialecosystems.John Muir. Henry David Thoreau, Theodore Roosevelt.Rachel Carson. Preservationists, conservationists, politi-cians, scientists, authors, and many others have all impactedthe way we view and value our environments. The 1962publication of Carson’s Silent Spring not only awakened thepublic to the problem of environmental contaminants, butalso prompted a change in the lens through which scientistsview contaminated ecosystems. Despite the debates Carson’sbook sparked, she made it clear that she was encouragingresponsible and carefully managed use and awareness of thechemicals and how they impact the entire ecosystem

Relating stream physical habitat condition and concordance of biotic productivity across multiple taxa

To explore the potential of assessments of stream geomorphic condition and habitat quality in evaluating lotic productivity, we investigated concordance of stream biotic productivity (aquatic macroinvertebrates, crayfish, fish, and belted kingfishers (Ceryle alcyon)) and their physical habitat correlates in 18 streams in the Champlain Valley, Vermont, USA. Pearson correlation analysis indicated significant concordance between macroinvertebrate density and fish biomass (r = 0.76), between the density of macroinvertebrates in the orders Ephemeroptera, Plecoptera, and Trichoptera and fish bi- omass (r = 0.81), and between fish biomass and kingfisher brood weight (r = 0.54). We used principal component analysis followed by linear regression to investigate relationships between physical habitat condition and biotic productivity and to identify key components of physical habitat condition assessments. Our analysis supported the combined use of geomor- phic and habitat assessments as a comprehensive indicator of stream physical habitat condition. We found relationships be- tween habitat assessment scores and productivity measures of all taxa except crayfish, suggesting similar responses to physical condition across trophic levels. Our results encourage the use of additional taxa, in addition to widely used macroinvertebrate metrics, as indicators of the composite effects of physical habitat impairment in stream ecosystems.

Riparian swallows as integrators of landscape change in a multiuse river system: implications for aquatic-to-terrestrial transfers of contaminants.

Recent research has highlighted the transfer of contaminants from aquatic to terrestrial ecosystems via predation of aquatic emergent insects by riparian consumers. The influence of adjacent land use and land cover (LULC) on aquatic-to-terrestrial contaminant transfer, however, has received limited attention. From 2010 to 2012, at 11 river reaches in the Scioto River basin (OH, USA), we investigated the relationships between LULC and selenium (Se) and mercury (Hg) concentrations in four species of riparian swallows. Hg concentrations in swallows were significantly higher at rural reaches than at urban reaches (t=-3.58, P<0.001, df=30), whereas Se concentrations were positively associated with adjacent land cover characterized by mature tree cover (R(2)=0.49, P=0.006). To an extent, these relationships appear to be mediated by swallow reliance on aquatic emergent insects. For example, tree swallows (Tachycineta bicolor) at urban reaches exhibited a higher proportion of aquatic prey in their diet, fed at a higher trophic level, and exhibited elevated Se levels. We also found that both Se and Hg concentrations in adult swallows were significantly higher than those observed in nestlings at both urban and rural reaches (Se: t=-2.83, P=0.033, df=3; Hg: t=-3.22, P=0.024, df=3). Collectively, our results indicate that riparian swallows integrate contaminant exposure in linked aquatic-terrestrial systems and that LULC may strongly regulate aquatic contaminant flux to terrestrial consumers.

A riverscape perspective on habitat associations among riverine bird assemblages in the Lake Champlain Basin, USA

The riverscape perspective recognizes the heterogeneous habitat types within the stream corridor as a single, integrated ecological unit operating across spatial scales. Although there is ample evidence that the riverscape notion is appropriate in understanding the physical phenomena of stream corridors, significantly less attention has focused on its ecological ramifications. To this end, we surveyed riverscape habitat variables and bird community characteristics in the Champlain Valley of Vermont, USA. From the data collected, we used information theoretic methodology (AICc) to model relationships between bird community attributes and key habitat variables across the riverscape. Our models with the greatest support suggest that riverine bird communities respond to a suite of characteristics; representing a variety of riverscape habitats at the in-stream, floodplain, and riparian levels. Channel slope, drainage area, percent conifers, and in-stream habitat condition were among the most influential variables. We found that piscivores are potentially important indicators of riverscape condition, responding to a host of variables across the riverscape. Our results endorse a holistic approach to assessing and managing the mosaic of patches in the riverscape and suggest that a riverscape approach has significant conservation potential.

Relative effects of local- and landscape-scale environmental factors on stream fish assemblages: Evidence from Idaho and Ohio, USA

Despite increased attention to spatial considerations in catchments, the importance of both environmental factors and spatial context in understanding stream fish assemblages is not fully resolved. This study explored the relative influences and spatial relationships of environmental factors at landscape (i.e., catchment) and local (i.e., reach) scales on characteristics of stream fish assemblages including species richness (S), diversity (H’, evenness, and 1/D), density and biomass, and composition (% top carnivores and % benthic insectivores). We conducted this research in two geographic regions (northern Idaho and Ohio, USA) characterized by distinct environmental attributes. Using a partial constrained ordination approach, we found that pure spatial factors explained 26 % of fish assemblage variation in Ohio (OH) and 18 % in Idaho (ID). Shared (spatially-structured) environmental variables explained more variation in fish assemblages in ID (28.6 %) than in OH (20.7 %). The influence of pure (nonspatial) environmental characteristics accounted for nearly 24 % of the variation observed in fish assemblages in OH catchments, whereas pure environmental factors accounted for 31 % of assemblage variation in ID. Within the pure environmental component, the influence of local, landscape, and joint (i.e., combined effects of environmental variables from both spatial scales) effects accounted for relatively equal amounts of assemblage variation for both study regions. However, local-scale variables were slightly more important in ID, whereas joint influences were more important in OH. Taken as a whole, our results suggest that a complex suite of spatial and environmental factors influenced fish assemblages but that the relative importance of these components differed in each of our study areas. The strong influence of spatial patterns in our results reveals the importance of integrating spatial context into studies predicting fish assemblage characteristics. The contrasting findings between geographic regions encourage additional research efforts that address regional variability in multi-scale environmental influences on

Pathways and consequences of contaminant flux to Acadian flycatchers (Empidonax virescens) in urbanizing landscapes of Ohio, USA

A prevalent environmental contaminant, mercury (Hg) is mobile and persistent in aquatic systems, where it often occurs in its bioavailable form methylmercury. Because methylmercury can bioaccumulate in aquatic insects and then transfer to terrestrial food webs, riparian consumers reliant upon aquatic emergent insects, should be disproportionately affected. Using the aerial insectivore Acadian flycatcher (Empidonax virescens) as a focal species, we examined (1) the extent to which total Hg loads in breeding flycatchers affected body condition and reproductive output and (2) potential pathways of contaminant flux in 19 riparian forest fragments distributed across an urban-to-rural landscape gradient in Ohio, USA. From April-August 2011-2012, we collected blood samples from adult (n=76) and nestling (n=17 from 7 nests) flycatchers, monitored their annual reproductive success (i.e., total number of fledglings), and sampled water, sediment, and aquatic emergent insects at each site. Hg concentrations in adult flycatcher blood (47 to 584 μg/kg, x¯=211.8, SD=95.5) were low relative to published advisory levels and not related to body condition. However, even at low concentrations, blood Hg was negatively related to reproductive success, with a 0.83 decline in the number of fledglings per μg/kg (loge) increase of blood Hg. Adult flycatchers had 11× greater concentrations of blood Hg than their offspring. Hg levels in flycatcher blood were not predicted by Hg concentrations in sediment, water, or aquatic emergent insects, with the exception of rural landscapes alone, in which flycatcher Hg was negatively related to sediment Hg. In addition to illustrating the difficulty of predicting exposure pathways that may vary among landscape contexts, our study provides evidence that even trace levels of contaminants may impair reproductive success of free-living songbirds.

The energetic contributions of aquatic primary producers to terrestrial food webs in a mid-size river system.

Rivers are increasingly recognized as providing nutritional subsidies (i.e., energy and nutrients) to adjacent terrestrial food webs via depredation of aquatic organisms (e.g., emergent aquatic insects, crayfish, fish) by terrestrial consumers. However, because these prey organisms assimilate energy from both aquatic (e.g., benthic algae, phytoplankton, aquatic macrophytes) and terrestrial (e.g., riparian leaf detritus) primary producers, river subsidies to terrestrial consumers represent a combination of aquatically and terrestrially derived energy. To date, the explicit contribution of energy derived from aquatic primary producers to terrestrial consumers has not been fully explored yet might be expected to be quantitatively important to terrestrial food webs. At 12 reaches along a 185-km segment of the sixth-order Scioto River system (Ohio, USA), we quantified the relative contribution of energy derived from aquatic primary producers to a suite of terrestrial riparian consumers that integrate the adjacent landscape across multiple spatial scales through their foraging activities (tetragnathid spiders, rove beetles, adult coenagrionid damselflies, riparian swallows, and raccoons). We used naturally abundant stable isotopes (13C and 15N) of periphyton, phytoplankton, macrophytes, and terrestrial vegetation to evaluate the energetic contribution of aquatic primary producers to terrestrial food webs. Shoreline tetragnathid spiders were most reliant on aquatic primary producers (50%), followed by wider-ranging raccoons (48%), damselflies (44%), and riparian swallows (41%). Of the primary producers, phytoplankton (19%) provisioned the greatest nutritional contribution to terrestrial consumers (considered collectively), followed by periphyton (14%) and macrophytes (11%). Our findings provide empirical evidence that aquatic primary producers of large streams and rivers can be a critical nutritional resource for terrestrial food webs. We also show that aquatically derived nutrition contributes to both shoreline and broader-ranging terrestrial consumers and thus may be an important landscape-scale energetic linkage between rivers and upland habitats.

Stream foodweb δ13C and geomorphology are tightly coupled in mountain drainages of northern Idaho

Abstract. The relative influence of landscape vs local processes on variation in stream C and energy sources to lotic consumers remains unresolved, but is a key component in understanding pathways through stream food webs. I investigated potential linkages between the distribution of &dgr;13C signatures of primary producers (periphyton), terrestrial organic matter (detritus), and secondary consumers (aquatic invertebrates and fish) and: 1) drainage area and 2) stream reach geomorphology in 3 mountain watersheds of northern Idaho, USA. When considered alone, drainage area explained variation in the &dgr;13C signatures of periphyton (R2  =  0.30) and sculpin (R2  =  0.15) only. Subsequent General Linear Models (GLMs) showed that stream morphology explained significant variation in &dgr;13C of periphyton (79%), Diptera (93%), sculpin (51%), and trout (63%). Drainage area was not significant in these models, a result indicating that once &dgr;13C signatures are explained as a function of geomorphic type, the contribution of drainage area becomes less important. On the whole, consumer &dgr;13C signatures related more strongly to periphyton &dgr;13C than detrital &dgr;13C, though this relationship varied among geomorphic types. Collectively, results indicate that the morphology of a stream reach (including factors such as substrate size and storage elements, floodplain development, etc.) may be an important factor in the assimilation and distribution of C in food webs in forested mountain streams.

Distribution and trophic dynamics of riparian tetragnathid spiders in a large river system

Development and agriculture are increasingly encroaching into riparian areas, with largely unknown effects on nearshore arthropods, which are important components of linked aquatic–terrestrial food webs. To assess the environmental determinants of the distribution and trophic dynamics of riparian spiders of the family Tetragnathidae, we characterised riparian habitat, collected emergent aquatic insects, and surveyed spiders in developed and rural landscapes of the Scioto River system, Ohio, USA, which provided a range of riparian land cover, nearshore vegetation types and habitat complexity. We also estimated the trophic position (TP) of Tetragnathidae and the proportion of energetic and nutritional subsidies derived from benthic algae (EBA) using naturally abundant carbon (C) and nitrogen (N) stable isotopes. Model-selection results revealed that tetragnathid spider density (1.57–3.80individualsm–1) was more sensitive to differences in overhanging vegetation than to those in aquatic food resources (i.e. emergent aquatic insects). Tetragnathidae TP, which averaged 3.16 across all 12 study reaches (range: 2.35–3.98), was largely driven by canopy density, shoreline shape, percentage overhanging vegetation and emergent-insect density. Emergent-insect density was the strongest driver of tetragnathid spider EBA (0.04–0.54, µ=0.24). Our study reinforced the notion that riparian spiders ecologically link aquatic and terrestrial ecosystems. In particular, our results further current understanding of the mechanisms affecting riparian spider distribution and trophic dynamics, particularly in the context of larger stream and river systems, given that the propensity of related research has occurred in small streams.