COS 117-5 - Temporal variation and the reliability of primary production in rivers

Abstract: Gross primary productivity (GPP) is a fundamental measurement in ecosystem ecology and many animals have adapted so that critical events in their life-history overlap in space and time with peaks in GPP. Modeling and sensor innovations in the last decade have enabled estimation of daily GPP for rivers. Here, we evaluate year-to-year variability in GPP for 59 rivers from across the continental United States over a fourteen year period (2008-2021). We compared the rates of annual production and the degree of interannual variation in annual GPP across rivers, and we contrasted these rates with those derived from terrestrial ecosystems within the AmeriFlux network. To understand variation in ecosystem phenology, we looked at within and across system variation in productivity regimes. For each river we identified the day, week, and month of maximum productivity for each year, and we evaluated each year’s productivity regime using the metrics of mean, CV, skewness, kurtosis, AR1 coefficient, amplitude, and phase.

Annual GPP rates ranged from 35 - 6,316 g C m^-2 y^-1, with only 8 of 59 records showing a significant linear trend over the 13 year time series, with GPP increasing in 2 increasing and declining in 6 rivers. Despite few instances of linear change, there was considerable year-to-year variability in GPP across many of the sites. Cumulative light and discharge variance explained a small (R²=0.13) but significant fraction (p< 0.05) of the annual variation in GPP across rivers. On average, rivers are less productive than terrestrial ecosystems, but the range in riverine GPP encompasses the range of annual GPP rates in terrestrial ecosystems and rivers have higher interannual variation than their terrestrial counterparts. Productivity in rivers showed wide variation within each year. The median earliest date of 50th percentile GPP was 202 ± 99.8, with a range from 5 to 365. The median number of days per year with greater than the 75 percentile GPP was 81 ± 10.0 and ranged from 38 to 91. We show broad variability in high productivity rivers, at short and long time scales, and posit this variability acts to alter the synchrony of animal phenology that relies on GPP as a food resource. Changes in annual riverine GPP can be useful to identify ecosystem-scale changes to climate change.