Phytoplankton gross primary production increases along cascading impoundments in a temperate, low-discharge river: Insights from high frequency water quality monitoring

Fabian Engel,Don C Pierson,Volker Kirchesch,Katrin Attermeyer,Helmut Fischer,Gesa A Weyhenmeyer,Ana I Ayala

doi:10.1038/s41598-019-43008-w

Abstract

Damming alters carbon processing along river continua. Estimating carbon transport along rivers intersected by multiple dams requires an understanding of the effects of cascading impoundments on the riverine metabolism. We analyzed patterns of riverine metabolism and phytoplankton biomass (chlorophyll a; Chla) along a 74.4-km river reach intersected by six low-head navigation dams. Calculating gross primary production (GPP) from continuous measurements of dissolved oxygen concentration, we found a maximum increase in the mean GPP by a factor of 3.5 (absolute difference of 0.45 g C m−3 d−1) along the first 26.5 km of the study reach, while Chla increased over the entire reach by a factor of 2.9 (8.7 µg l−1). In the intermittently stratified section of the deepest impoundment the mean GPP between the 1 and 4 m water layer differed by a factor of 1.4 (0.31 g C m−3 d−1). Due to the strong increase in GPP, the river featured a wide range of conditions characteristic of low- to medium-production rivers. We suggest that cascading impoundments have the potential to stimulate riverine GPP, and conclude that phytoplankton CO2 uptake is an important carbon flux in the river Saar, where a considerable amount of organic matter is of autochthonous origin.

Highlights

River systems play an important role in the carbon transport between terrestrial ecosystems, the atmosphere and the ocean, and in the global carbon cycle[1,2,3]
Mean gross primary production (GPP) along the studied river reach increased from 0.18 g C m−3 d−1 at the upstream site Guedingen to 0.63 g C m−3 d−1 at the station Lisdorf, and decreased slightly downstream (Table 2 and Fig. 2)
We suggest that the increase in GPP and Chla along the Saar is likely a consequence www.nature.com/scientificreports of the cumulative effect of the consecutive impoundments that increase water residence time (WRT), light availability and water temperature (WT), and stimulate GPP

Summary

Introduction

River systems play an important role in the carbon transport between terrestrial ecosystems, the atmosphere and the ocean, and in the global carbon cycle[1,2,3]. Compared to rivers in cold or tropical regions, phytoplankton biomass in many temperate, anthropogenically influenced river systems is high in relation to the total organic carbon (TOC) load[23,24]. This suggests that phytoplankton CO2 uptake plays a significant role in carbon spiraling (the combined processes of cycling and longitudinal transport25) along temperate rivers. The effect of smaller impoundments (i.e. impoundments with shorter WRT) on phytoplankton dynamics may be more subtle than the influences of large reservoirs[27] Despite their relatively small size, consecutive low-head dams were found to increase heterotrophic carbon processing along four lentic-lotic sections in a mid-size Mediterranean river[28]. We hypothesized that GPP and phytoplankton biomass increase along the studied river reach, since the hydro-morphological conditions in the dam headwaters favor phytoplankton growth

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Conclusion