Increased reliance of stream macroinvertebrates on terrestrial food sources linked to forest management intensity

Wildfire burn severity and stream chemistry influence aquatic invertebrate and riparian avian mercury exposure in forested ecosystems

Terrestrial soils in forested landscapes represent some of the largest mercury (Hg) reserves globally. Wildfire can alter the storage and distribution of terrestrial-bound Hg via reemission to the atmosphere or mobilization in watersheds where it may become available for methylation and uptake into food webs. Using data associated with the 2007 Moonlight and Antelope Fires in California, we examined the long-term direct effects of wildfire burn severity on the distribution and magnitude of Hg concentrations in riparian food webs. Additionally, we quantified the cross-ecosystem transfer of Hg from aquatic invertebrate to riparian bird communities; and assessed the influence of biogeochemical, landscape variables, and ecological factors on Hg concentrations in aquatic and terrestrial food webs. Benthic macroinvertebrate methylmercury (MeHg) and riparian bird blood total mercury (THg) concentrations varied by 710- and 760-fold, respectively, and Hg concentrations were highest in predators. We found inconsistent relationships between Hg concentrations across and within taxa and guilds in response to stream chemical parameters and burn severity. Macroinvertebrate scraper MeHg concentrations were influenced by dissolved organic carbon (DOC); however, that relationship was moderated by burn severity (as burn severity increased the effect of DOC declined). Omnivorous bird Hg concentrations declined with increasing burn severity. Overall, taxa more linked to in situ energetic pathways may be more responsive to the biogeochemical processes that influence MeHg cycling. Remarkably, 8 years post-fire, we still observed evidence of burn severity influencing Hg concentrations within riparian food webs, illustrating its overarching role in altering the storage and redistribution of Hg and influencing biogeochemical processes.

Effects of forest management on mercury bioaccumulation and biomagnification along the river continuum

Forest management can alter the mobilization of mercury (Hg) into headwater streams and its conversion to methylmercury (MeHg), the form that bioaccumulates in aquatic biota and biomagnifies through food webs. As headwater streams are important sources of organic materials and nutrients to larger systems, this connectivity may also increase MeHg in downstream biota through direct or indirect effects of forestry on water quality or food web structure. In this study, we collected water, seston, food sources (biofilm, leaves, organic matter), five macroinvertebrate taxa and fish (slimy sculpin; Cottus cognata) at 6 sites representing different stream orders (1-5) within three river basins with different total disturbances from forestry (both harvesting and silviculture). Methylmercury levels were highest in water and some food sources from the basin with moderate disturbance (greater clearcutting but less silviculture). Water, leaves, stoneflies and fish increased in MeHg or total Hg along the river continuum in the least disturbed basin, and there were some dissipative effects of forest management on these spatial patterns. Trophic level (δ¹⁵N) was a significant predictor of MeHg (and total Hg in fish) within food webs across all 18 sites, and biomagnification slopes were significantly lower in the basin with moderate total disturbance but not different in the other two basins. The elevated MeHg in lower trophic levels but its reduced trophic transfer in the basin with moderate disturbance was likely due to greater inputs of sediments and of dissolved organic carbon that is more humic, as these factors are known to both increase transport of Hg to streams and its uptake in primary producers but to also decrease MeHg bioaccumulation in consumers. Overall, these results suggest that the type of disturbance from forestry affects MeHg bioaccumulation and trophic transfer in stream food webs and some longitudinal patterns along a river continuum.

Are There Longitudinal Effects of Forest Harvesting on Carbon Quality and Flow and Methylmercury Bioaccumulation in Primary Consumers of Temperate Stream Networks?

Forest harvesting affects dissolved organic matter (DOM) and aqueous mercury inputs as well as the food web structure in small-headwater streams, but how these upstream changes manifest downstream is unclear. To address this uncertainty, we examined DOM quality, autochthony in the caddisfly Hydropsychidae (using δ² H), and methylmercury (MeHg) concentrations in stream water and the caddisfly along a longitudinal gradient (first- to fourth-order streams, subcatchments of 50-1900 ha) in paired partially harvested and reference catchments in central Ontario, Canada. Although measures of DOM quality (specific ultraviolet absorbance at 254 nm 2.20-11.62) and autochthony in caddisflies (4.9%-34.0%) varied among sites, no upstream-to-downstream differences in these measures were observed between the paired harvested and reference catchments. In contrast, MeHg levels in stream water (0.06-0.35 ng/L) and caddisflies (29.7-192 µg/kg dry wt) were significantly higher in the upstream sites but not the farthest downstream sites in the harvested catchments compared to the reference catchments. This suggests that while current mitigation measures used by forestry companies did not prevent elevated MeHg in water and invertebrates at smaller spatial scales (subcatchments of 50-400 ha), these upstream impacts did not manifest at larger spatial scales (subcatchments of 800-1900 ha). The present study advances our understanding of spatially cumulative impacts within harvested catchments, which is critical to help forest managers maintain healthy forest streams and their provisioning of aquatic ecosystem services. Environ Toxicol Chem 2022;41:1490-1507. © 2022 SETAC.

Elevated Allochthony in Stream Food Webs as a Result of Longitudinal Cumulative Effects of Forest Management

The river continuum concept (RCC) predicts a downstream shift in the reliance of aquatic consumers from terrestrial to aquatic carbon sources, but this concept has rarely been assessed with longitudinal studies. Similarly, there are no studies addressing how forestry related disturbances to the structure of headwater food webs manifest (accumulate/dissipate) downstream and/or whether forest management alters natural longitudinal trends predicted by the RCC. Using stable isotopes of carbon, nitrogen and hydrogen, we investigated how: 1) autochthony in macroinvertebrates and fish change from small streams to larger downstream sites within a basin with minimal forest management (New Brunswick, Canada); 2) longitudinal trends in autochthony and food web length compare among three basins with different forest management intensity [intensive (harvest and replanting), extensive (harvest only), minimal] to detect potential cumulative/dissipative effects; and 3) forest management intensity and other catchment variables are influencing food web dynamics. We showed that, as predicted, the reliance of some macroinvertebrate taxa (especially collector feeders) on algae increased from small streams to downstream waters in the minimally managed basin, but that autochthony in the smallest shaded stream was higher than expected based on the RCC (as high as 90% for some taxa). However, this longitudinal increase in autochthony was not observed within the extensively managed basin and was weaker within the intensively managed one, suggesting that forest management can alter food web dynamics along the river continuum. The dampening of downstream autochthony indicates that the increased allochthony observed in small streams in response to forest harvesting cumulates downstream through the river continuum.

Forest management impacts on stream integrity at varying intensities and spatial scales: Do biological effects accumulate spatially?

The effects of forest harvesting on headwaters are quite well understood, yet our understanding of whether impacts accumulate or dissipate downstream is limited. To address this, we investigated whether several biotic indicators changed from smaller to larger downstream sites (n = 6) within three basins that had intensive, extensive or minimal forest management in New Brunswick (Canada). Biofilm biomass and grazer abundance significantly increased from upstream to downstream, whereas organic matter decomposition and the autotrophic index of biofilms decreased. However, some spatial trends differed among basins and indicated either cumulative (macroinvertebrate abundance, predator density, sculpin GSI) or dissipative (autotrophic index, cotton decomposition) effects downstream, potentially explained by sediment and nutrient dynamics related to harvesting. No such among-basin differences were observed for leaf decomposition, biofilm biomass, macroinvertebrate richness or sculpin condition. Additionally, results suggest that some of the same biological impacts of forestry observed in small headwaters also occurred in larger systems. Although the intensive and extensive basins had lower macroinvertebrate diversity, there were no other signs of biological impairment, suggesting that, overall, current best management practices protect biological integrity downstream despite abiotic effects.

Multiple stressors in small streams in the forestry context of Fennoscandia: The effects in time and space

In this paper we describe how forest management practices in Fennoscandian countries, namely Sweden and Finland, expose streams to multiple stressors over space and time. In this region, forestry includes several different management actions and we explore how these may successively disturb the same location over 60-100 year long rotation periods. Of these actions, final harvest and associated road construction, soil scarification, and/or ditch network maintenance are the most obvious sources of stressors to aquatic ecosystems. Yet, more subtle actions such as planting, thinning of competing saplings and trees, and removing logging residues also represent disturbances around waterways in these landscapes. We review literature about how these different forestry practices may introduce a combination of physicochemical stressors, including hydrological change, increased sediment transport, altered thermal and light regimes, and water quality deterioration. We further elaborate on how the single stressors may combine and interact and we consequently hypothesise how these interactions may affect aquatic communities and processes. Because production forestry is practiced on a large area in both countries, the various stressors appear multiple times during the rotation cycles and potentially affect the majority of the stream network length within most catchments. We concluded that forestry practices have traditionally not been the focus of multiple stressor studies and should be investigated further in both observational and experimental fashion. Stressors accumulate across time and space in forestry dominated landscapes, and may interact in unpredictable ways, limiting our current understanding of what forested stream networks are exposed to and how we can design and apply best management practices.

Forest management impacts on stream integrity at varying intensities and spatial scales: Do abiotic effects accumulate spatially?

Though effects of forest harvesting on small streams are well documented, little is known about the cumulative effects in downstream systems. The hierarchical nature and longitudinal connectivity of river networks make them fundamentally cumulative, but lateral and vertical connectivity and instream processes can dissipate the downstream transport of water and materials. To elucidate such effects, we investigated how a suite of abiotic indicators changed from small streams to larger downstream sites (n = 6) within three basins ranging in forest management intensity (intensive, extensive, minimal) in New Brunswick (Canada) in the summer and fall of 2017 and 2018. Inorganic sediments, the inorganic/organic ratios and water temperatures significantly increased longitudinally, whereas nutrients and the fluorescence index of dissolved organic carbon (DOC; indication of terrestrial source) decreased. However, some longitudinal trends differed across basins and indicated downstream cumulative (inorganic sediments, the inorganic/organic ratios and to a lesser extent DOC concentration and humification) as well as dissipative (temperatures, nutrients, organic sediments) effects of forest management. Overall, we found that the effects previously reported for small streams with managed forests also occur at downstream sites and suggest investigating whether different management practices can be used within the extensive basin to reduce these cumulative effects.

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.