global changes incidence on phytoplankton, with special

I.S.RIVERSLYON 2018

• 8 main significant genera with the highest biomass contribution of

Limnothrix, Oscillatoria and Aphanizomenon

• Possible relationships with physical-chemical parameters

• Rising water temperature, decreasing P, stable NO3-

• Discussion

• Time scale - phytoplankton biomass decreased along the years

together with cyanobacterial contribution.

• Longitudinal scale – cyanobacteria biomass increased along the

Loire: consequence of increased residence time, water

temperature and lateral inputs from slow flow tributaries

• Planktic cyanobacteria do not form blooms in the R. Loire,

contrary to earlier reports (Larroudé et al, 2013). On rare

occasions, some cyanobacteria reached high abundance in the

main river, but they were exported from slow-flowing tributaries

• Although part of the phytoplankton decrease may be explained

by significant P-limitation of growth, it is likely that predation by

benthic filter-feeders, particularly the invasive Asiatic clams

(Corbicula spp.), has been a key loss process for phytoplankton

in the R. Loire. This has been the case in other large European

rivers.

• More investigations are still necessary to tell apart the effects of

increased temperature, discharge variations, reduced

eutrophication and predation by invasive species on the

phytoplankton of the River Loire

• Introduction• Climate change and eutrophication are expected to affect phytoplankton

growth, with an increased risk of potentially harmful cyanobacterial

blooms

• Few papers reported an increase of cyanobacterial abundance in rivers,

that could be related to climate change

• In this study, based on phytoplankton monitoring data, we examine

long-term trends in phytoplankton development in the R.Loire, with a

focus on the contribution of cyanobacteria to total phytoplankton

biomass

• Material & Methods

• Results• Overall long term decrease of chlorophyll-a and cyanobacteria biomass

• Total phytoplankton biomass decreased highly since late 90’s

• Cyanobacteria biomass occurred in 38% of the samples

• Cyanobacteria were most of the time a very small fraction of total

phytoplankton (~1%), except on rare occasions

• 63 cyanobacteria taxa ; 2/3 of them have filamentous morphology

• Cyanobacteria were the most abundant in downstream station

(Montjean)

Global changes incidence on phytoplankton, with special reference to

cyanobacteria, in the R. Loire (France)

Impact des changements globaux sur le phytoplancton

et les cyanobactéries de la Loire (France)

Maria Leitao1; Andràs Abonyi4; Jean-Pierre Descy2; Anne Marie Lançon1; Camille

Minaudo3; Florentina Moatar3

Longitudinal change in the mean annual biomass of the main cyanobacteria taxa (1991-2011)

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Anabaena / Dolichospermum

Planktothrix

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Phormidium

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Aphanizomenon

Limnothrix

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Merismopedia

Microcystis

• Loire : 6 stations (km 138 – km 830)

• Monthly sampling: 1991 – 2011

(monitoring Loire-Bretagne Agency)

• Chemical analysis: standard methods

• Phytoplankton: Utermöhl &

biovolume for biomass

• Phytoplankton samples: n = 1 024

• Biomass calculated when the taxon > 4 individuals (>1% counting)

• Trends of nutrients were analysed in parallel with phytoplankton

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1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2005 2006 2007 2008 2009 2010 2011

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Biomasse C Cyano Biomasse C total Malvalette Chaumont Montjean