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Algae and aquatic plants support river food webs through in-situ primary production. However, gross primary production (GPP) and ecosystem respiration (ER) are rarely evaluated in the context of river management or habitat restoration. We estimated daily GPP and ER during 2 growing seasons for 7 reaches in the Kootenai River and 1 reach in the Elk River, spanning 290 river km across British Columbia, Canada, and Montana and Idaho, USA. We characterized responses of GPP and ER to river management, including reaches with unregulated flow, regulated flow, nutrient addition, and habitat restoration. Downstream GPP and ER generally increased after changes in river management, and higher management intensity led to greater increases. GPP and ER followed a seasonal pattern with low initial values in spring, elevated values in mid-summer, and a return to low values in late summer and autumn. Timing and duration of the elevated period for GPP and ER also differed among reaches following changes in river management. Our results suggest that river management affects GPP and ER, likely through reducing turbidity and the frequency and magnitude of extreme flow events, nutrient additions, and enhanced floodplain connectivity, thereby altering the timing and amount of autochthonous carbon available to the food web. [ABSTRACT FROM AUTHOR]

This study analyzes 9 years of eddy-covariance (EC) data carried out in a Pacific Northwest Douglas-fir ( Pseudotsuga menzesii) forest (58-year old in 2007) on the east coast of Vancouver Island, Canada, and characterizes the seasonal and interannual variability in net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration ( Re) and primary climatic controls on these fluxes. The annual values (± SD) of NEP, GPP and Re were 357 ± 51, 2124 ± 125, and 1767 ± 146 g C m−2 yr−1, respectively, with ranges of 267–410, 1592–2338, and 1642–2071 g C m−2 yr−1, respectively. Spring to early summer (March–June) accounted for more than 80% of annual NEP while late spring to early autumn (May–August) was mainly responsible for its interannual variability (∼80%). The major drivers of interannual variability in annual carbon (C) fluxes were annual and spring mean air temperatures ( Ta) and water deficiency during late summer and autumn (July–October) when this Douglas-fir forest growth was often water-limited. Photosynthetically active radiation ( Q), and the combination of Q and soil water content ( θ) explained 85% and 91% of the variance of monthly GPP, respectively; and 91% and 96% of the variance of monthly Re was explained by Ta and the combination of Ta and θ, respectively. Annual net C sequestration was high during optimally warm and normal precipitation years, but low in unusually warm or severely dry years. Excluding 1998 and 1999, the 2 years strongly affected by an El Niño/La Niña cycle, annual NEP significantly decreased with increasing annual mean Ta. Annual NEP will likely decrease whereas both annual GPP and Re will likely increase if the future climate at the site follows a trend similar to that of the past 40 years. [ABSTRACT FROM AUTHOR]

• A flux variance similarity partitioning method was developed as a gap-filling model. • Results were compared with three established gap-filling models. • Gap-filled 18-year mean NEP values were similar among the four models. • There were large differences in annual photosynthesis and respiration values among the four models. • Accounting for light inhibition of daytime respiration in ecosystem models is critical. We propose a gap-filling model for carbon dioxide fluxes measured by eddy covariance (EC) that combines the flux variance similarity (FVS) partitioning approach with the artificial neural network (ANN) technique (FVS–ANN). 18 years of EC-measured net ecosystem production (NEP) of a Douglas-fir (Pseudotsuga menziesii) stand in British Columbia, Canada were used. FVS-partitioned fluxes were used as a training dataset for ANNs based on the environmental variables to model missing respiration and photosynthesis values. Results from the FVS–ANN model were compared with three established models using different partitioning and gap-filling approaches, namely the nighttime relationship, the daytime intercept, and an ANN-based model informed by the nighttime relationship (ANN–nighttime). In the gap-filled long-term NEP record, the 18-year mean NEP values were similar (between 225 and 239 gC m−2 year−1) and individual years showed small differences in annual NEP among the four models, demonstrating the reliability of estimated NEP when any of the models were used for gap-filling. However, the largest differences in annual gap-filled photosynthesis and respiration estimates among the four models in a given year were as large as 813 and 835 gC m−2 year−1, respectively. The FVS–ANN model consistently resulted in the smallest annual photosynthesis and respiration estimates, while the nighttime-based models resulted in the highest photosynthesis and respiration. The differences between the FVS–ANN and any of the other three models are explained by the different definitions of partitioned fluxes (i.e., values from the FVS–ANN are smaller than the other three models by leaf respiration). However, the differences may also be partly due to light inhibition of daytime respiration, which is known as the Kok effect that is believed to be adequately considered in the new FVS–ANN combination and partially in the daytime intercept model, but not in the other two models. The results suggest that previous EC-based estimates of global photosynthesis and respiration obtained by using nighttime relationships are potentially biased high. [ABSTRACT FROM AUTHOR]

• A mid-aged Douglas-fir stand was fertilized with urea at 200 kg ha−1 of N. • Fertilization increased NEP by an average value of 170 g m−2 y−1 of C over ten years. • Fertilization enhanced gross primary production only in the first two years. • Fertilization suppressed ecosystem respiration except in the first year after fertilization. • Fertilization caused water use to increase by an average of about 15%. Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest are the most productive managed forests in North America. Nitrogen (N) fertilizers are generally applied in this region to increase the rate of tree growth and consequently carbon (C) sequestration. However, the long-term effects of N fertilization on C and water exchanges of Douglas-fir forests are not clear. This study presents 15 years of eddy-covariance (EC) measurements of C and water fluxes above a pole-sapling-stage Douglas-fir stand on Vancouver Island, British Columbia, Canada, and determines how N fertilization with 200 kg ha−1 of urea-N affected these fluxes over the 10-year period after fertilization. A process-based forest growth model, 3-PG, was calibrated using EC-measured C and water fluxes during the 5 pre-fertilization years, and then used to predict these fluxes for the next 10 years assuming the stand was not fertilized. The modelled C and water fluxes for the pre-fertilization years agreed well with the measurements (with R2 values of 0.90, 0.89, and 0.89 for gross primary production (GPP), ecosystem respiration (R e) and evapotranspiration (ET), respectively) and only slightly underestimated GPP, R e , and ET by 4%, 1%, and 3%, respectively. The effects of N fertilization on C sequestration and water use (ET) were then obtained as differences between EC-measured (fertilized stand) and modelled (unfertilized stand) fluxes. Application of N fertilizer to this stand led to a short-term (first two years) increase in GPP followed by little change over the long term. R e increased over the short-term (first year), while it was appreciably suppressed over the long term. N fertilization resulted in an average increase in net ecosystem productivity by 170 g m−2 year−1 of C during 2007–2016 with interannual variation depending on annual weather conditions. N fertilization led to an average increase in annual water use of 15% (or 53 mm year−1). [ABSTRACT FROM AUTHOR]