Factors controlling water‐column respiration in rivers of the central and southwestern Amazon Basin

Abstract

We examined the factors controlling the variability in water-column respiration rates in Amazonian rivers. Our objectives were to determine the relationship between respiration rates and the in situ concentrations of the size classes of organic carbon (OC), and the biological source (C 3 and C 4 plants and phytoplankton) of organic matter (OM) supporting respiration. Rates were likely elevated in the former rivers, which were all sampled during low water, due to the stimulation of heterotrophic respiration via the supply of a labile, algal-derived substrate and/or the occurrence of autotrophic respiration. The organic composition of fine particulate OM (FPOM) of these rivers is consistent with a phytoplankton origin. Multiple linear regression analysis indicates that [FPOC], C : N FPOC ratios, and [O 2 ] account for a high amount of the variability in respiration rates (r 2 5 0.80). Accordingly, FPOC derived from algal sources is associated with elevated respiration rates. The d 13 C of respiration-derived CO 2 indicates that the role of phytoplankton, C 3 plants, and C 4 grasses in supporting respiration is temporally and spatially variable. Future scaling work is needed to evaluate the significance of phytoplankton production to basin-wide carbon cycling.

Evaluation of Primary Production in the Lower Amazon River Based on a Dissolved Oxygen Stable Isotopic Mass Balance

“…In the past, δ 18 O-O 2 values have been used to calculate the ratio of photosynthesis to respiration (P:R) under a steady state assumption for the Amazon River (Quay et al, 1995;Ellis et al, 2012). However, considering diel variability of d 18 O-O 2 was observed, a steady state assumption is not accurate.…”

“…It is typically assumed that d 18 O-O 2 should remain at steady state in the Amazon River mainstem considering the low levels of light penetration and limited primary production (Quay et al, 1995;Ellis et al, 2012). To test this assumption we measured O 2 saturation and d 18 O-O 2 throughout a day/night cycle in the center of the channel at Óbidos.…”

“…In fact, early measurements of primary production based on 14 C uptake indicated rates of primary production that are about an order of magnitude lower than respiration in the Amazon River mainstem, while clearwater tributaries in the basin's lowlands exhibited significantly higher rates of primary production (Wissmar et al, 1981;Benner et al, 1995). However, several studies have more recently found evidence for a higher relative contribution of primary production (and subsequent breakdown of algal OM) to ecosystem metabolism in the Amazon River based on the stable isotopic signature of dissolved oxygen (δ 18 O-O 2 ) and the presence of algal biomarkers in the mainstem (Quay et al, 1995;Mortillaro et al, 2011;Ellis et al, 2012). Similar observations of high rates of primary production and algal abundance have also been made in the mainstem of the Congo River, a similar large tropical system (Descy et al, 2017).…”

“…Past estimates of primary production in the Amazon River are based on a steady-state assumption (i.e., no diel variability in δ 18 O-O 2 ) (Quay et al, 1995;Ellis et al, 2012); however, if primary production is actually occurring at appreciable rates in the mainstem of the river this steady-state assumption should not hold true.…”

CO<sub>2</sub> Flux and its Relationship with Water Parameters and Biological Activity in the Ji-Paraná River (Rondônia State – Western Amazon)

“…The higher precipitation leads to the input of CO2 and 160 allochthonous organic matter from soil, riparian zone, and wetland, and carbon, which would be evaded directly from terrestrial systems, is outgassed from aquatic ecosystems (Butman and Raymond, 2011). Yet, the photosynthetic activity in Amazon rivers leads to high respiration rates through on a labile substrate for heterotrophs (Ellis et al, 2012), which helps the explanation of the positive flux values in the dry season, when the carbon inputs from terrestrial ecosystems are reduced. Therefore, autochthon carbon sustains the positive values of flux.…”