Changes in precipitation and land use influence carbon (C), nitrogen (N) and phosphorus (P) exports from land to receiving waters. However, how these drivers differentially alter elemental inputs and impact subsequent ecosystem stoichiometry over time remains poorly understood. Here we quantified long-term (1979-2020) trends in C, N, and P exports at three sites along the mainstem of a north temperate river, that successively drains forested, urban, and more agriculturally impacted land-use areas. Riverine N and to a lesser degree C exports tended to increase over time, with major inter-annual variation largely resolved by changes in precipitation. Historical increases in net anthropogenic N inputs on land (NANI) also explained increases in riverine N exports, with about 35% of NANI reaching the river annually. Despite higher Net anthropogenic P inputs, NAPI, over time, P exports tended to decrease at all riverine sites. This decrease in P at the forested site was more gradual, whereas a precipitous drop was observed at the downstream urban site, following legislated P removal in municipal wastewater. Changes in historical ecosystem stoichiometry reflected the differential elemental exports due to natural and anthropogenic drivers and ranged from 174: 23: 1 to 547: 76: 1 over the years. Our work shows how C, N, and P have responded to different drivers in the same catchment over the last four decades, and how their differential riverine exports have influenced ecosystem stoichiometry.

Given the important role of protists in trophodynamics and major biogeochemical cycles, identifying which factors influence the distribution of their biomass and species composition is a central tenet in oceanography. However, understanding the drivers of that distribution from a functional trait perspective would allow us to better link protistan biogeography to ecosystem function. Here we evaluated the distribution of protistan functional traits across the Labrador Sea during spring over three consecutive years. More variability in the biogeography of protistan functional traits was explained across water masses, and among years than taxonomic composition. Furthermore, patterns in trait variability were more apparent when site-specific timing of peak chlorophyll-a was considered. By recreating bloom phenology, we found that approximately 20 days prior to peak, mixotrophs were replaced by autotrophs of different size classes, supporting the critical role of bloom timing in structuring protistan community trait succession with consequences on modelling of ecosystem function.

Large rivers can retain a substantial amount of nitrogen (N), particularly in submerged aquatic vegetation (SAV) meadows that may act as disproportionate control points for N retention in rivers. However, the temporal variation of N retention remains unknown since past measurements were snapshots in time. Using high frequency measurements over the summers 2012-2017, we investigated how climate variation influenced N retention in a SAV meadow at the confluence zone of two agricultural tributaries entering the St. Lawrence River. Distinctive combinations of water temperature and level were recorded between years, ranging from extreme hot-low (2012) and cold-high (2017) summers (2 ˚C and 1.4 m interannual range). Using an indicator of SAV biomass, we found that these extreme hot-low and cold-high years had reduced biomass compared to hot summers with intermediate levels. In addition, change in main stem water levels were asynchronous with the tributary discharges that controlled NO3- inputs at the confluence. We estimated daily N uptake rates from a moored NO3- sensor, and partitioned these into assimilatory and dissimilatory pathways. Measured rates were variable but among the highest reported in rivers (median 576 mg N m-2 d-1;, range 60 – 3893 mg N m-2 d-1) and SAV biomass promoted greater proportional retention and permanent N loss through denitrification. We estimated that the SAV meadow could retain up to 0.8 kt N per year and 87% of N inputs, but this valuable ecosystem service is contingent on how climate variations modulate both N loads and SAV biomass.