Dissolved organic matter dynamics in diverse freshwater ecosystems across Canada: long-term trends, transformations, and explanatory factors

Dissolved organic matter (DOM) is a key variable in freshwater ecosystems that influences numerous limnological properties and ecosystem services. While DOM dynamics have received considerable attention in certain parts of Canada, many regions remain underexplored or require updated analysis. This research examines long-term patterns of DOM in diverse freshwater ecosystems across Canada and determines the factors that best explain them. Rivers play a critical role in the global carbon cycle by connecting terrestrial and aquatic ecosystems and serving as important sites for DOM processing. However, DOM dynamics in semi-arid northern plains rivers are understudied, and simultaneous studies of multidecadal trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are scarce. Therefore, I analyzed four decades of DOC and DON patterns in two major rivers of the South Saskatchewan River (SSR) basin and assessed the impact of a large reservoir on riverine DOC properties. Contrary to many northern hemisphere studies, DOM concentrations did not show long-term increases. River discharge was a key factor influencing DOM patterns, with regional precipitation also playing a role in locations where runoff contribution is greater. These regional factors explained greater variability in DOM compared to global scale indices (e.g., Pacific decadal oscillation) due to their direct control on DOM. In an agriculture-dominated watershed like the SSR basin, agricultural land use likely influenced DOC and DON differently, which may have resulted in the observed asynchronous patterns between them. Additionally, a large reservoir, Lake Diefenbaker, reduced allochthonous DOC characteristics through photodegradation and increased autochthonous DOC characteristics. The findings illustrate: 1) the increase in DOM seen in many northern hemisphere waters is not present in prairie rivers, 2) controls on different DOM components may vary, and 3) large reservoirs may modify riverine DOC composition due to longer water residence time (WRT). Reservoirs are vital to meet the ever-increasing demands for freshwater in a warming climate. However, insights into reservoir DOM dynamics, particularly in temperate, semi-arid regions, remain limited. I investigated the variations in DOM properties in Lake Diefenbaker by analyzing eight years of DOM concentrations and composition through linear mixed effect modeling. Contrary to expectations, reservoir DOC concentration showed no correlation with inflow from the SSR, while DON increased with decreasing inflow. DOM optical indices (SUVA254 and E4:E6 ratio) and DOC:DON ratio revealed a pronounced influence of tributary flow on reservoir DOM composition, i.e., allochthonous characteristics increased with increasing flow, and autochthonous proportion increased with declining flow. A travel time corrected comparison of approximately the same water parcel along the reservoir length revealed that increasing WRT in downstream regions led to a significant transformation in DOM composition, favoring autochthonous characteristics. These autochthonous inputs likely offset the allochthonous DOC losses, which resulted in a stable DOC concentration throughout the reservoir. Additionally, the effect of a large aquaculture operation on reservoir DOM properties was investigated, but no effect was detected. These results have important implications for managing large reservoirs, particularly given the global increase in reservoir construction. The increase in DOC concentrations, known as brownification, has drawn considerable attention due to its implications for ecosystem dynamics. While past studies have documented the influence of recovery from acidification and climate change on long-term DOC trends, the emerging importance of these explanatory factors remains less understood. In addition, few studies have followed up on recent trends in sites that have undergone increases in DOC. Using a four-decade dataset, I investigated interannual variations in DOC and DON in 49 lakes across four eastern Canadian regions with a history of increases in DOC. I identified recent shifts in DOC patterns using LOESS smoothing and piecewise regression. I observed a stabilizing pattern or even a decrease in high acidification regions (Dorset and Nova Scotia), where increases in DOC were previously documented. At the low acid deposition region, IISD-Experimental Lakes Area, an increasing pattern in DOC stabilized in the early 2000s; however, DOC appears to be increasing again in recent years. My analysis identified precipitation and SO4 deposition as the primary explanatory variables for long-term patterns in DOC. However, because acid deposition has declined substantially, climate and local watershed factors are becoming increasingly influential, leading to the emergence of new DOC patterns. Long-term changes in DOC and DON were not always synchronous, as these were often correlated with different factors (e.g., DON with ammonium deposition). This resulted in observable shifts in DOC:DON ratios, indicative of changes in DOM composition. This research underscores the importance of ongoing monitoring in diverse regions because of the changing nature of environmental variables and new emerging trends.

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