Key points:• Differential sources of anthropogenic nitrogen and phosphorus inputs were quantified over 110 years in the St. Lawrence basin.• Our input models explain 87% and 71% of the variance in riverine N and P exports, respectively, across 23 watersheds and 3 decadal years • Regional legislations and local strategies reducing N and P fluxes are needed to address the eutrophication problem AbstractHuman activities have increased the flow of nitrogen (N) and phosphorus (P) over much of the Earth, leading to increased agricultural production, but also the degradation of air, soil, and water quality. Here, we quantify the sources of anthropogenic N and P inputs to 76 watersheds of the St. Lawrence Basin (SLB) throughout the 20th century using NANI/NAPI (net anthropogenic N/P input to watersheds), a mass balance modeling approach, and estimate the fraction of these inputs exported to adjacent rivers. Our results show that since 1901, NANI and NAPI increased 4.5-and 3.8-fold respectively with a peak in 1991 mainly due to high atmospheric N deposition and P fertilizer application. However the relative increase over the course of the last century was much higher in certain watersheds, particularly those where there was greater urbanization. Ranges in NANI and NAPI vary greatly among watersheds (110 to 9,351 kg N km-2 yr-1 and 0.16 to 1,938 kg P km-2yr-1, respectively in 2011) and are strongly related to riverine fluxes (R2 = 0.87 and 0.71 for N and P, respectively). Our results suggest that 22% of NANI (ranging from 11% to 68% across watersheds) and 17% of NAPI (ranging from 3% to 173%) are exported to rivers.Predominant sources of inputs vary spatially and through time largely due to changes in farming practices. By tracking the main sources of inputs to specific watersheds and through time, our work provides insights for N and P management. Reduction strategies will likely need to be watershed specific, although through time, our results clearly show the large-scale impact of targeted legislation.

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Phosphorus (P) plays a crucial role in both agricultural productivity and water quality. There has been growing recognition of the importance of 'legacy' P (that which has accumulated in watersheds over time), for understanding contemporary water quality outcomes; however, little is known about how different watersheds respond to cumulative P pressures. The "buffering capacity" concept describes the ability of watersheds to attenuate P loading to surface waters by retaining cumulative P inputs over time. To explore the role of various watershed characteristics in buffering capacity, we compared Net Anthropogenic P Input (NAPI) estimates to riverine total P flux across a thirty-year time span (1981-2011) in 16 large watersheds in southern Quebec, Canada. We used this historic P data to calculate indices describing long-and short-term buffering for these watersheds. We then examined the correlation between this buffering capacity and a set of key geochemical, hydrological, landscape, and socio-ecological variables that were hypothesized to influence P buffering dynamics. Both long-and short-term buffering metrics were most strongly correlated with hydrologic characteristics, indicating that watershed hydrology may be the most prominent characteristic in P buffering within watersheds. However, we found that considering estimates of long-term P accumulation along with biophysical characteristics of the watershed (including hydrology) predicts water quality better (R 2 =0.69) than either factor would alone (R 2 =0.35). Our findings provide a step towards improving models of watersheds' unique relationships to P pressure and can help guide management of historically agricultural landscapes with considerable amounts of legacy P. Manuscript Highlights Watershed buffering is a concept used to explore how watersheds respond to legacy P pressure Hydrology and landscape features play major role in watershed buffering 40 Combining P data and watershed characteristics gives a comprehensive picture of watershed buffering

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Balancing human well-being with the maintenance of ecosystem services (ES) for future generations has become one of the central sustainability challenges of the 21st century. In working landscapes, past and ongoing production-centered objectives have resulted in the conversion of ecosystems into simple land-use types, which has also altered the provision of most ES. These inevitable trade-offs between the efficient production of individual provisioning ES and the maintenance of regulating and cultural ES call for the development of a land-use strategy based on the multifunctional use of the landscape. Due to the heterogeneous nature of working landscapes, both protection and restoration actions are needed to improve their multifunctionality. Systematic conservation planning (SCP) offers a decision support framework that can support landscape multifunctionality by indicating where ES management efforts should be implemented. We describe an approach that we developed to include ES provision protection and restoration objectives in SCP with the goal of providing ongoing benefits to society. We explain the general framework of this approach and discuss concepts, challenges, innovations, and prospects for the further development of a comprehensive decision support tool. We illustrate our approach with two case studies implemented in the pan-Canadian project ResNet.