Aquatic ecosystems are fuelled by biogeochemical inputs from surrounding lands and within-lake primary production. Disturbances that change these inputs may affect how aquatic ecosystems function and deliver services vital to humans. Here we test, using a forest cover gradient across eight separate catchments, whether disturbances that remove terrestrial biomass lower organic matter inputs into freshwater lakes, thereby reducing food web productivity. We focus on deltas formed at the stream-lake interface where terrestrial-derived particulate material is deposited. We find that organic matter export increases from more forested catchments, enhancing bacterial biomass. This transfers energy upwards through communities of heavier zooplankton, leading to a fourfold increase in weights of planktivorous young-of-the-year fish. At least 34% of fish biomass is supported by terrestrial primary production, increasing to 66% with greater forest cover. Habitat tracers confirm fish were closely associated with individual catchments, demonstrating that watershed protection and restoration increase biomass in critical life-stages of fish.

There is widespread evidence that aquatic consumers use terrestrial resources depending on the features of surrounding catchments.

Variation in habitat quality can have important consequences for fitness and population dynamics. For food-caching species, a critical determinant of habitat quality is normally the density of storable food, but it is also possible that quality is driven by the ability of habitats to preserve food items. The food-caching gray jay (Perisoreus canadensis) occupies year-round territories in the coniferous boreal and subalpine forests of North America, but does not use conifer seed crops as a source of food. Over the last 33 years, we found that the occupancy rate of territories in Algonquin Park (ON, Canada) has declined at a higher rate in territories with a lower proportion of conifers compared to those with a higher proportion. Individuals occupying territories with a low proportion of conifers were also less likely to successfully fledge young. Using chambers to simulate food caches, we conducted an experiment to examine the hypothesis that coniferous trees are better able to preserve the perishable food items stored in summer and fall than deciduous trees due to their antibacterial and antifungal properties. Over a 1-4 month exposure period, we found that mealworms, blueberries, and raisins all lost less weight when stored on spruce and pine trees compared to deciduous and other coniferous trees. Our results indicate a novel mechanism to explain how habitat quality may influence the fitness and population dynamics of food-caching animals, and has important implications for understanding range limits for boreal breeding animals.

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Riparian forests along urban streams are affected by flashy hydrological regimes and adjacent upland land‐use, and their downstream dynamics are altered by forest fragmentation and by frequent river network truncation. This can lead to lower riparian habitat quality, decreased dispersal along the river network and increased recruitment of novel plant communities including non‐native species. Compared to forested landscapes, our understanding of riparian plant community diversity and composition along urban river networks is poor. We investigated riparian plant communities and riparian habitat variables along seven river networks situated along an urban gradient in the Greater Toronto Area, Canada. Six land‐use metrics classifying the intensity of urbanisation on three spatial scales (sub‐catchment, upstream riparian buffer, site buffer) were tested to explain the variation in riparian plant communities along the river networks and to describe the differences in riparian habitat. Riparian plant diversity peaked at sites which had between 40% and 70% of built‐up land cover within sub‐catchments; however, these communities contained a large number of non‐native species compared to the forested streams. While total species richness and native species richness were more related to river network factors (catchment area), non‐native richness was also affected by land‐use within a site. Community composition was more related to local land‐use than catchment area. Geographical distance between sites (both overland and hydrological) was increasingly more important for community heterogeneity along the urbanisation gradient. Riparian habitat worsened along the urbanisation gradient, with higher disturbance and soil pH and lower soil organic content and bryophyte cover in the more urban catchments. Riparian habitat variables were also related to the position along the river networks. Our results provide important insights into how riparian plant communities and riparian habitat differ along a gradient of urbanisation. Compared to previous studies we investigated the patterns in riparian vegetation in the context of river networks, including a variety of land‐use metrics, and analysed data across different spatial scales. This revealed important trends in plant diversity, demonstrated that urban land‐use increases the occurrence of non‐native species, and showed that the land‐use at the local scale is a stronger determinant of plant assemblages compared to land‐use at broader scales. These novel findings can be applied to land management in order to mitigate the effects of urbanisation on riparian ecosystems and the functions they provide for adjacent streams and rivers.

Aquatic ecosystems depend on terrestrial organic matter (tOM) to regulate many functions, such as food web production and water quality, but an increasing frequency and intensity of drought across northern ecosystems is threatening to disrupt this important connection. Dry conditions reduce tOM export and can also oxidize wetland soils and release stored contaminants into stream flow after rainfall. Here, we test whether these disruptions to terrestrialaquatic linkages occur during mild summer drought and whether this affects biota across 43 littoral zone sites in 11 lakes. We use copper (Cu) and nickel (Ni) as representative contaminants, and measure abundances of Hyalella azteca, a widespread indicator of ecosystem condition and food web production. We found that tOM concentrations were reduced but correlations with organic soils (wetlands and riparian forests) persisted during mild drought and were sufficient to suppress labile Cu concentrations. Wetlands, however, also became a source of labile Ni to littoral zones, which was linked to reduced abundances of the amphipod H. azteca, on average by up to 70 times across the range of observed Ni concentrations. This reveals a duality in the functional linkage of organic soils to aquatic ecosystems whereby they can help buffer the effects of hydrologic disconnection between catchments and lakes but at the cost of biogeochemical changes that release stored contaminants. As evidence of the toxicity of trace contaminant concentrations and their global dispersion grows, sustaining links among forests, organic soils and aquatic ecosystems in a changing climate will become increasingly important.

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Headwater streams accumulate, process, and export organic materials for use in downstream environments. Decomposition of organic material, an important ecosystem function, may be sensitive to land cover changes in urbanizing regions since headwater stream processes tend to be tightly coupled with riparian and catchment characteristics. Headwaters represent 70–80% of total stream length in watersheds but are disproportionately converted to drainage infrastructure or buried with urban development. Cumulatively, this loss may result in substantial changes to physical and biological downstream processes. From a monitoring perspective, headwaters are largely ignored compared with fishable/navigable waterways for planning decisions, so their structural and functional variability is not well understood. Here, we engaged citizen scientists to contribute data on this variability and to evaluate the sensitivity of standardized cotton‐strip decomposition rates to multiscale factors across headwaters with varying landscape conditions in the Greater Toronto Area (York Region), Canada. These factors included stream, riparian vegetation, and catchment characteristics. We expected decomposition rates to be similarly sensitive to local‐ and catchment‐scale factors because of the strong links between headwater catchment and stream processes. We also expected a hump‐shaped distribution of decomposition rates across a gradient of urban cover, with stimulating effects at low to moderate cover but deleterious effects at high urban cover. We found that decomposition rate was most sensitive to local‐scale factors (e.g., strip burial, stream velocity, and both local upland riparian vegetation density and topography) rather than whole catchment properties. We did not find the expected hump‐shaped distribution with urban cover and suggest that more mechanistic studies are needed for understanding cotton‐strip decomposition to control for local factors in determining the scale at which decomposition rate is most sensitive to land cover change.