Dramatic changes in a phytoplankton community in response to local and global pressures: a 24‐year survey of the river Loire (France)

Abstract

The impact of climate change and of other anthropogenic pressures on the structure and composition of phytoplankton communities of large European rivers remains poorly documented. Here we report the findings of a study of the changes in the phytoplankton community of the middle segment of the river Loire over the past 24 years. An attempt is made to distinguish between the impact of changes acting at the local scale and that of those acting more globally. A dramatic reduction in phytoplankton abundance was observed, particularly in the mid -1990s; this was concomitant with an increase in the relative proportion of cyanobacteria. At the same time, the phytoplankton community displayed increasing richness and diversity, and little change in its size structure. All these changes seem to be related to local changes, in particular to the reduction in phosphorus concentrations, as well as to changes in climate, throughout modifications in the river discharge and water temperature. Interestingly, herbicide contamination also appeared to be of particular importance in explaining the unexpected increase in the proportion of cyanobacteria in the phytoplankton community after the 1990s. These findings suggest that combinations of numerous anthropogenic pressures acting at different spatial and temporal scales have led to a mix of predictable and unpredictable changes occurring in the phytoplankton community of the river Loire, with probable consequences for the trophic networks in this river.

A large river (River Loire, France) survey to compare phytoplankton functional approaches: Do they display river zones in similar ways?

“…Besides physical reasons explained, this regional summer occurrence of low N:P ratio might be indicative for the whole section, being either explanatory or a consequence for the potamoplankton composition (Xie et al, 2003). The very distinct location of this low N:P ratio seems to delimit cyanobacterial blooms exceptionally in this upper Loire section, might be providing only the possibility for downstream transport, in contrast to recent findings at the middle Loire (Larroudé et al, 2013).…”

Poleward shift in large‐river fish communities detected with a novel meta‐analysis framework

“…However, other environmental changes are likely to have occurred locally over the same period and may also have contributed to the observed biological changes (Malmqvist & Rundle, 2002;Zajicek et al, 2018). Several studies demonstrated that improved water quality has ecological consequences for various organisms all along the trophic network, such as phytoplankton (Larroudé et al, 2013) and macroinvertebrates (Floury et al, 2013) in the Loire River or fish in various large rivers in the U.S.A. (Counihan et al, 2018) and probably also in France (Poulet et al, 2011). Several studies demonstrated that improved water quality has ecological consequences for various organisms all along the trophic network, such as phytoplankton (Larroudé et al, 2013) and macroinvertebrates (Floury et al, 2013) in the Loire River or fish in various large rivers in the U.S.A. (Counihan et al, 2018) and probably also in France (Poulet et al, 2011).…”

Identifying multiple stressor controls on phytoplankton dynamics in the River Thames (UK) using high-frequency water quality data

“…Flow and light intensity were identified as the key controls on bloom dynamics for agricultural streams in Illinois (Figueroa-Nieves et al, 2006) and the River Elbe, Germany (Hardenbicker et al, 2014). Other abiotic combinations that have been proposed include flow and water temperature (Desortova and Puncochar, 2011), flow, temperature and nutrient concentration (Larroude et al, 2013;van Vliet and Zwolsman, 2008), and flow, temperature and light (Balbi, 2000;Reynolds and Descy, 1996;Waylett et al, 2013). The impacts of invertebrate grazing (Lazar et al, 2012;Waylett et al, 2013;Whitehead et al, 2015), self-shading (Whitehead and Hornberger, 1984) and inhibition due to high light intensities (Whitehead et al, 1997) have also been postulated as a mechanism for limiting phytoplankton biomass and causing bloom cessation in modelling studies of the River Thames catchment, UK.…”