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

Abstract

Our understanding of how forest management practices affect the relative importance of autochthonous vs. allochthonous resource use by headwater stream food webs is relatively poor. To address this, we used stable isotope (C, N, and H) analyses of food sources and macroinvertebrates from 15 streams in New Brunswick (Canada) and assessed how different catchment conditions arising from the gradient in forest management intensity affect the contribution of autochthonous resources to these food webs. Aquatic primary production contributed substantially to the biomass of invertebrates in these headwater streams, especially for scrapers and collector‐gatherers (25–75%). However, the contribution of algae to food webs decreased as forest management intensity (road density and associated sediments, water cations/carbon, and dissolved organic matter humification) increased, and as canopy openness decreased. This trend was probably due to an increase in the delivery of organic and inorganic terrestrial materials (dissolved and in suspension) in areas of greater harvesting intensity and road density, which resulted in more heterotrophic biofilms. Overall, results suggest that, despite the presence of riparian buffers, forest management can affect stream food web structure via changes in energy flows, and that increased protection should be directed at minimizing ground disturbance in areas with direct hydrological connection to streams and at reducing dissolved and particulate matter inputs from roads and stream crossings in catchments with high degrees of management activity.

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

“…Though increases in autochthonous contributions to consumer diets occur after forest harvesting, likely due to increased delivery of nutrients and light to streams stimulating aquatic primary production or to stream-side vegetation removal limiting the subsidies of terrestrial detritus (Göthe et al, 2009), this was not observed in the present study or in other harvested headwater systems where decreased reliance on algae was found, likely from the greater inputs of terrestrial organic matter from harvesting residues and sediment deposition and heterotrophic biofilms at these sites (Erdozain et al, 2019;Göthe et al, 2009;Jonsson et al, 2018). The lack of increase in autochthony in consumer diets at the harvested sites was noted despite greater concentrations of total dissolved nitrogen (Supporting Table S1) and may be due to the presence of riparian buffers.…”

“…where ω is the per-trophic level contribution of dietary water to consumers and τ is the trophic level (τ = 1 for primary consumers, including hydropsychids). A ω of 0.20 was assumed based on previous studies (Erdozain et al, 2019;Solomon et al, 2009;Wang et al, 2009;Wilkinson et al, 2015). The relative contribution of aquatic organic matter to the diet of hydropsychids (percentage of algae in diet) was estimated using a two-source mixing model, according to Equation 3:…”

“…These strong land-water linkages also mean that disturbances in the catchment translate into impacts in lotic systems. More specifically, catchment disturbances from forest management impact aquatic ecosystem services such as flood control and the provision of clean water supplies, habitat for invertebrates and fish (Brogna et al, 2017;Furniss et al, 2010), and water quality and carbon cycling in small-headwater streams (Erdozain et al, 2018(Erdozain et al, , 2019.…”

Food web incorporation of marine-derived nutrients after the reintroduction of endangered inner Bay of Fundy Atlantic salmon (Salmo salar)

“…Such sources could be from forestry or other land use activities outside of Fundy National Park that have changed the isotopic composition of the aquatic food web. Forest harvesting impacts the stable isotope values of basal resources and subsequent trophic levels (Richardson 2008) by increasing the delivery of terrestrial organic and inorganic materials to aquatic systems and shifting the reliance of biota to more terrestrial carbon sources (Erdozain et al 2019). In addition, there is variability in nature that we captured but cannot fully explain.…”

Streams and riparian forests depend on each other: A review with a special focus on microbes

“…While relatively narrow buffer strip may (or may not; Vuori & Joensuu 1996) protect the stream from sedimentation, few rows of trees unlikely provide effective protection for natural values on the riparian zone. Furthermore, forestryassociated ground disturbance can affect aquatic food webs despite buffers (Erdozain et al 2019). If the aim is to develop a dispersal corridor, then the buffer zone should be long and continuous, but not necessarily wide (see Gundersen 2010).…”