Abstract
AbstractThe seasonality of gross primary production (GPP) in streams is driven by multiple physical and chemical factors, yet incident light is often thought to be most important. In Arctic tundra streams, however, light is available in saturating amounts throughout the summer, but sharp declines in nutrient supply during the terrestrial growing season may constrain aquatic productivity. Given the opposing seasonality of these drivers, we hypothesized that “shoulder seasons”—spring and autumn—represent critical time windows when light and nutrients align to optimize rates of stream productivity in the Arctic. To test this, we measured annual patterns of GPP and biofilm accumulation in eight streams in Arctic Sweden. We found that the aquatic growing season length differed by 4 months across streams and was determined largely by the timing of ice‐off in spring. During the growing season, temporal variability in GPP for nitrogen (N) poor streams was correlated with inorganic N concentration, while in more N‐rich streams GPP was instead linked to changes in phosphorus and light. Annual GPP varied ninefold among streams and was enhanced by N availability, the length of ice‐free period, and low flood frequency. Finally, network scale estimates of GPP highlight the overall significance of the shoulder seasons, which accounted for 48% of annual productivity. We suggest that the timing of ice off and nutrient supply from land interact to regulate the annual metabolic regimes of nutrient poor, Arctic streams, leading to unexpected peaks in productivity that are offset from the terrestrial growing season.
Highlights
IntroductionSeasonal variation in terrestrial nutrient supply to streams is widespread (Stoddard 1994; Sponseller et al 2014), as is evidence for nutrient limitation of benthic algal growth (Keck and Lepori 2012) and gross primary production (GPP) (Mulholland et al 2001; Bott et al 2006)
We evaluated the drivers of annual gross primary production (GPP) and biomass accrual (Chl a on tiles) during the open water season using partial least square regressions (PLSR) with the R package “pls” and leave-one-out cross-validation
We estimate that only 52% of annual aquatic GPP was produced during summer months/terrestrial growing season (June to August, 5B) with ca. 46% occurring during the shoulder periods (May, September, and October). In this first assessment of continuous, annual metabolism for Arctic or sub-Arctic streams, we show that nutrient availability is important for the seasonality of GPP and the cumulative annual productivity
Summary
Seasonal variation in terrestrial nutrient supply to streams is widespread (Stoddard 1994; Sponseller et al 2014), as is evidence for nutrient limitation of benthic algal growth (Keck and Lepori 2012) and GPP (Mulholland et al 2001; Bott et al 2006). Despite these observations, nutrient supply has not been incorporated into seasonal GPP models, in part because technological and/or logistical constraints often prevent pairing nutrient data with continuous metabolism records (Ulseth et al 2018; Mejia et al 2019). For the vast, remote regions of Earth drained by nutrient poor waters, temporal variability in terrestrial supply of nitrogen (N) and phosphorus (P) may act as an overlooked driver of GPP seasonality
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