Long-Term Responses of Nutrient Budgets to Concurrent Climate-Related Stressors in a Boreal Watershed

Contribution of water erosion to organic carbon and total nitrogen loads in agricultural discharge from boreal mineral soils

While organic carbon (OC) in agricultural mineral soils is widely studied in terms of soil carbon sequestration and gaseous emissions, discharge-induced OC loss from soil is still poorly understood and estimations of boreal soil OC loads within water erosion are lacking. Loss of organic matter from arable soils is a concern for surface water quality, climate change and soil productivity. The main aim of this study was to quantify the role of water erosion in total OC and nitrogen (N) loads exported in agricultural discharge from boreal mineral soils under various cultivation practices. Surface water and subsurface drainage were collected near-continually over 2 years in two clayey and one sandy soil in Finland. Eroded sediment was mechanically separated by centrifugation from all discharge samples to detect sediment OC% and N% by dry-combustion method. Dissolved OC and N concentrations in selected discharge samples were measured with high-temperature catalytic oxidation of unfiltered supernatant. A multiple linear regression model was used to study the significant factors affecting dissolved, sediment and total OC loads. In the clayey soils, the sediment OC (2-24 kg ha-1 y-1) and N (0.2-1.1 kg ha-1 y-1) export accounted for up to 35 % and 20 % of the annual discharge-induced total loads of OC (19-85 kg ha-1) and N (2-8 kg ha-1), respectively. In the sandy soil, erosion was negligible and dissolved loads of 17-35 kg OC ha-1 y-1and 4-7 kg N ha-1 y-1 were detected. Subsurface drainage exported most of the sediment-associated OC and N loads from clayey soils. For the total OC loads, the distribution varied between the discharge routes, while the total N loads were mostly exported in subsurface drainage in both soil types. Sediment OC and N exports were related to soil plowing and discharge intensity, while dissolved OC loss was promoted by high surface soil OC%. Our results also indicated that a single cultivation practice may affect sediment and dissolved loads in opposite ways. These findings can be used to complement carbon budget estimations for mineral agricultural soils, and to assess soil management effects on terrestrial organic matter loading to boreal surface waters.

Multiscale spatial analysis of headwater vulnerability in South-Central Chile reveals a high threat due to deforestation and climate change

Headwaters represent an essential component of hydrological, ecological, and socioeconomical systems, by providing constant water streams to the complete basin. However, despite the high importance of headwaters, there is a lack of vulnerability assessments worldwide. Identifying headwaters and their vulnerability in a spatially explicit manner can enable restauration and conservation programs. In this study, we assess the vulnerability of headwaters in South-Central Chile (38.4 to 43.2°S) considering multiple degradation factors related to climate change and land cover change. We analyzed 2292 headwaters, characterizing multiple factors at five spatial scales by using remote sensing data related to Land Use and Cover Change (LUCC), human disturbances, vegetation cover, climate change, potential water demand, and physiography. We then generated an index of vulnerability by integrating all the analyzed variables, which allowed us to map the spatial distribution of headwater vulnerability. Finally, to estimate the main drivers of degradation, we performed a Principal Components Analysis with an Agglomerative Hierarchical Clustering, that allowed us to group headwaters according to the analyzed factors. The largest proportion of most vulnerable headwaters are located in the north of our study area with 48.1 %, 62.1 %, and 28.1 % of headwaters classified as highly vulnerable at 0, 10, and 30 m scale, respectively. The largest proportion of headwaters are affected by Climate Change (63.66 %) and LUCC (23.02 %) on average across all scales. However, we identified three clusters, in which the northern cluster is mainly affected by LUCC, while the Andean and Coastal clusters are mainly affected by climate change. Our results and methods present an informative picture of the current state of headwater vulnerability, identifying spatial patterns and drivers at multiple scales. We believe that the approach developed in this study could be useful for new studies in other zones of the world and can also promote Chilean headwater conservation.

Dissolved organic carbon in eastern Canadian lakes: Novel patterns and relationships with regional and global factors

Long-term patterns in dissolved organic carbon (DOC) concentrations in 49 eastern Canadian lakes from four sites were re-examined with a ~ 35-year (~1980-2015) dataset. The study sites were Dorset (number of lakes, n = 8), Experimental Lakes Area (ELA, n = 4), Kejimkujik (n = 26) and Yarmouth (n = 11). Lake DOC patterns were synchronous within each site. However, comparisons of DOC patterns across sites showed that they were synchronous only between the Kejimkujik and Yarmouth locations. Hence, these two sites were pooled into a single Nova Scotia site (NS). Increases in DOC concentration were evident in Dorset, Ontario from 1988 (r² = 0.78, p < 0.001) and NS from 2000 (r² = 0.43, p = 0.006). DOC at the ELA in northwestern Ontario had a different pattern compared to the other sites, i.e., DOC had increased earlier (1983-2000), and then, unlike Dorset and NS, neither an increase nor decrease was detected between 2001 and 2015 (p = 0.78). Precipitation and sulfur deposition explained the greatest variance in DOC patterns at the Dorset and NS sites (i.e., precipitation: 21-49% and sulfur deposition: 24-54%). Precipitation was the most important driver of DOC at the ELA. Our results indicate that all the sites have gone through a process of increasing DOC, but at different times. The stabilizing pattern at the ELA since 2001 may suggest that DOC concentrations in ELA lakes have reached, or are approaching a new equilibrium, a phenomenon that was not observed at the other sites. Also, the increase in DOC was not always associated with declining sulfur deposition (e.g., ELA). Therefore, we conclude that there was considerable variation in DOC patterns across this large geographic region of Canada and potential drivers of these patterns were not consistent across these diverse sites.

The Influence of Climate Change on the Restoration Trajectory of a Nutrient-Rich Deep Lake

Nutrient reduction in impacted lowland freshwater systems is ecologically and culturally important. Gaining a greater insight into how lakes respond to lowering nutrient loads and how climate-driven physical limnology affects present and future cycling of available nutrients is important for ecosystem resource management. This study examines the nutrient decline in a hypereutrophic freshwater lake (Rostherne Mere, Cheshire, UK) 25 years after sewage effluent diversion, a uniquely long-term analysis of a recovering nutrient-rich deep lake. Using nutrient, phytoplankton, climate and catchment hydrological monitoring, the contemporary lake system is compared to previous studies from 1990 to 2002. Nutrient change since point source load diversion showed annual average and maximum phosphorus (P) concentrations decreased significantly for the first 10 years (1992: ~ 600 µg P L−1; 2002: ~ 200 µg P L−1), but have since stabilised due to a substantial legacy sediment P internal load. Dissolved inorganic nitrogen (DIN) concentrations have not substantially changed since diversion, resulting in the alteration of the DIN/SRP ratio from a system characterised by N limitation (N:P ~ 5), to one predominantly P limited (N:P > 20). Nutrient changes over this time are shown to drive ecological change, especially in the cyanobacterial and algal communities. Furthermore, very high-resolution monitoring of lake inflow and outflow (every 5 min during 2016) shows that water residence time at this lake is significantly shorter than previously estimated (~ 0.8 years compared to previous estimates of ~ 1.6–2.4 years). Together with long-term data demonstrating that the stratification period at Rostherne Mere has increased by 40 days over the last ~ 50 years (due to later autumnal mixing), we show that a rapid rate of epilimnetic flushing together with a long stratification period substantially reduces the available epilimnetic P during the summer cyanobacterial bloom. This is of growing importance for many such lakes, given widespread climate-driven lengthening of stratification and a national trend of decreasing summer rainfall (decreasing seasonal flushing) but more intense summer storm events (resulting in short-term flushing events).