Our goal was to test the potential of ALAN to shift fish assemblage composition in urban headwater streams. We found that the range of lux trespassing from ALAN into our sites had no observable effects on urban headwater stream fish assemblages. However, it is widely recognized that a suite of collinear anthropogenic impacts including hydrologic variability (“flashiness”), fluvial geomorphic adjustment, and nutrient and contaminant pollution are common stressors for small urban streams. These combined effects typically result in reduced species richness and are symptomatic of the “urban stream syndrome” (Walsh et al. 2005). Owing to the positive scaling between urban development and artificial lighting at night, ALAN is also collinear with the multiple stressors of urban streams (Hölker et al. 2021), thereby confounding potential effects of ALAN. Influences of ALAN on biotic communities are particularly difficult to isolate in urban streams because the regional species pool becomes restricted to highly tolerant, cosmopolitan fish species that can cope with combinations of multiple stressors (Sulliván et al. 2021).
Indeed, the effects of ALAN may be subtle compared to those of more classically identified stressors identified in the urban stream syndrome (Walsh et al. 2005, Sullivan et al. 2019b). Many studies, especially those with experimental approaches, provide evidence of the influence of ALAN at multiple levels of biological organization ranging from gene expression to ecosystem functions (Meyer and Sullivan 2013, Brüning et al. 2016, Amichai and Kronfeld-Schor 2019, Sullivan et al. 2019a). Here, we provide evidence that habitat structure (depth) had the largest explanatory effects for urban stream fish assemblages and may have overwhelmed potential influences of ALAN. Stream geomorphic features are well recognized as an important factor in structuring stream fish assemblages (Sullivan et al. 2006). In fact, also working in urban streams of the CMA, Rieck and Sullivan (2020) documented temporal and spatial linkages between multiple stream hydrogeomorphic characteristics (e.g., sediment size, channel incision) and fish assemblages. Not surprisingly, fish α-diversity increased with stream depth in our study, presumably because deeper stream sites provide more habitat volume, complexity, and critical refugia (i.e., from summer dry-downs and temperature extremes) compared to our shallower stream sites. These results re-iterate the need to not only manage chemical water quality, but also fluvial geomorphic forms and processes by conserving natural hydrology and riparian zones (Sweeney and Newbold 2014).
The effects of phosphorus concentrations were generally negative in our study as both species richness and α-diversity scaled negatively with at least one phosphorus variable and mean fish mass scaled negatively with orthophosphate. For our dataset, TP and orthophosphate were not correlated (df = 78, r = -0.12, p = 0.290; Table 1). The negative relationship between phosphorus and diversity and fish biomass may be indicative of nutrients functioning as system stressors at elevated concentrations (King and Richardson 2007, Ouyang et al. 2018). In urban settings, nutrient enrichment is typically collinear with other stressors, thus, we cannot conclude that these effects on species richness and α-diversity were directly due to nutrient effects or whether differences in nutrient concentrations indicate differential degrees of urban impacts. However, the relationship between orthophosphate and mean fish mass was relatively weak overall, with most of the variation explained by site-level differences.
Some species (Bluegill Sunfish, Mottled Sculpin, and Central Stoneroller; Fig. 4B) showed potential patterns in raw values of relative abundance across reach types and may warrant further investigation, but high variation in responses and relationships with covariates precluded any significant effects. ALAN might be expected to advantage Bluegill Sunfish by extending the dawn and dusk foraging windows (Zapata et al. 2019), when they are most active (Werner 1969). Conversely, for benthic invertivores such as Mottled Sculpin and Central Stonerollers, even small increases in illumination of the benthos may disrupt camouflage and make them both more vulnerable to predation and less effective in ambushing their prey (e.g., Keyler et al. 2019). Predation risk associated with ALAN, in particular, has been documented for both freshwater (i.e., Nelson et al. 2021) and marine (e.g., Bolton et al. 2017) fishes. Meyer and Sullivan (2013) hypothesized that ALAN can prompt reductions in mean body size and higher abundances of armored grazers (e.g., glossosomatid caddisflies), which could reduce the accessibility of macroinvertebrate prey for benthic insectivorous fishes. Of note, Mottled Sculpin are a species of conservation concern in Ohio, highlighting the potential for ALAN to impact rare and endangered species (e.g., Shier et al. 2020).
Small headwater streams are often characterized by dense canopy cover during warm seasons when aquatic organisms are most active. Canopy cover can mitigate or block ALAN trespass, an effect that has been seen in terrestrial studies with bats (Straka et al. 2019). The capacity for canopy cover to mitigate ALAN should vary by ecosystem type and size, with dense canopies of terrestrial ecosystems in temperate and humid climates having the largest capacity. Small headwater streams embedded in forests or surrounded by intact riparian buffers should support sufficiently dense canopy to largely mitigate ALAN effects. As streams widen, the ratio of closed-canopied water from shoreline riparian vegetation to open-canopy water decreases, potentially allowing more ALAN trespass (e.g., unobstructed ALAN from streetlights on bridges over large water bodies). This is, of course, true for the well-documented effects of ALAN in coastal regions that lack canopy between coastal developments and water (Kamrowski et al. 2013), but large rivers, lakes, and reservoirs might also have increased vulnerability due to a lower closed- to open-canopy water ratio. However, the benefits provided by canopy can be incorporated into physical infrastructure with increased usage of dark-sky friendly luminaire designs that limit light trespass with directional shielding and targeted hours of operation (Kardel 2012, Zapata et al. 2019).
Our mean illuminance values were in line with values reported in other ALAN studies focusing on small stream with intact riparian zones (Meyer and Sullivan 2013, Perkin et al. 2014a). However, these mean illuminance values (Table 1) – even during the winter leaf-off period – are lower than illuminance values reported by other ALAN studies conducted in more open habitats, where effects of ALAN have been observed. For example, Davies et al. (2012) report 19.29 lx directly under street lights and 3.02 lx between street lights in their study of terrestrial invertebrate communities in Cornwall, United Kingdom. Riley et al. (2012) measured lx above the water's surface in a river in Hampshire, UK and report 14 lx directly near street lighting and 2 lx farther away. Meyer and Sullivan (2013) state that in highly urbanized areas of the CMA, lux levels ranged from 8–12 (range 7.5–12.2, mean ± SD = 1.5 ± 1.5 lx) in mostly unbuffered streams, although they used streams with intact riparian zones (0.1-4.0 lx) in their study. Illuminance from light sources rapidly and non-linearly decreases with distance due to the inverse-square law. Street lighting at the sites used in this study was typically positioned on roads and not directly over the water surface. Given this spatial effect and the presence of heavily canopied riparian zones at our sites, it is not surprising that we, and others (Meyer and Sullivan 2013, Perkin et al. 2014a), observed low levels of ALAN in headwater streams that may not have been strong enough to elicit fish-assemblage responses.
Site as a random factor explained a large component of variation for each response variable. Such large site effects might be expected to be largely explained by high β-diversity between subcatchments. The high β-diversity may be due to multiple underlying, unmeasured variables including, for example, differences in hydrological regimes, local-scale geological features, riparian vegetation characteristics, or upstream-downstream connectivity that would enable recolonization post-extirpation (e.g. artificial channels, culverts, and low-head dams limiting regional dispersal in CMA; e.g., Davis et al. 2017). The large site-level effect reinforces the notion that, in addition to the potential effects of ALAN, multiple stressors are at play in urban streams.
In general, the effects of ALAN are complex because biological responses have been observed at a wide range of illuminances (e.g., < 1 to > 100 lx) and spectral composition, often with species-specific effects. Effects on behavior, physiology (e.g., melatonin), and predator-prey interactions are particularly evident, but effects on biodiversity and community responses have been less consistent overall (see meta-analysis: Sanders et al. 2021), despite prominent examples in the literature (e.g., Davies et al. 2012, Grubisic et al. 2017, Bennie et al. 2018). In urban streams, the collinear impacts of urban land use strongly reduce species richness to a subset of tolerant fish species from the overall regional species pool (Booth et al. 2016, Vietz et al. 2016). While this conundrum makes teasing out the effects of collinear stressors on fish communities in natural systems, large-scale manipulative studies that are costly and logistically prohibitive are still needed (Hölker et al. 2021). However, we provide evidence that the effects of observed ALAN illuminance (Table 1) on small stream species diversity do not appear to be as extreme as other factors commonly associated with urbanization (e.g., hydrologic variability, fluvial geomorphic adjustment, contamination). This is not to say that ALAN does not exert community-to-ecosystem level effects, as other studies have clearly shown (Bennie et al. 2015, Manfrin 2017, Manfrin et al. 2017, Sullivan et al. 2019a), but simply that detection of such effects in heavily canopied streams, where ALAN illuminance tends to be relatively low (Meyer and Sullivan 2013, Perkin et al. 2014a), may be more difficult.