Coarse and Fine Particulate Organic Matter Transport by a Fourth-Order Mountain Stream to Lake Bourget (France)

Transport of coarse particulate organic matter (CPOM) derived from forest litterfall has been hardly studied in rivers, unlike fine particulate organic matter (FPOM) or dissolved organic matter (DOM). Yet, many rivers are dammed or run into lakes, and there is growing evidence that CPOM accumulation in river delta participates substantially in ecological processes such as greenhouse gas emissions of lakes and reservoirs. We investigated the transport of CPOM and FPOM by the Leysse River (discharge from 0.2 to 106 m3 s−1) to Lake Bourget (France) in relation to aerial litter deposition, river network length, and discharge. Over a 19-month study period, the volume-weighted mean CPOM and FPOM concentrations were 1.3 and 7.7 g m−3, respectively. Most CPOM and FPOM transport occurred during major flood events, and there were power relationships between maximum discharge and particulate organic matter (POM) transport during these events. The annual export of CPOM (190 t AFDM) was 85% of the litter accumulation in autumn on permanent sections of the riverbed (224 t AFDM), which suggests that export is a major process compared to breakdown. Export of CPOM was 1.25 t yr−1 km−2 of the forested catchment area. This study highlights the need to account for long-range CPOM transport to describe the fate of litter inputs to streams and to quantify the organic matter input and processing in lakes and reservoirs.

Die hard in Lake Bourget! The case of Planktothrix rubescens

Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was clearly thought that such a proliferation was over. However, against all odds, blooms occurred again in 2016 and 2017 despite apparent very low phosphorus concentrations in surface waters of the lake. Aims of this study were thus to explain the reasons of this come back in order to propose scenarios likely to be helpful to stakeholders who need to know if such proliferations may occur again in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium since a minimum autumn/winter inoculum of P. rubescens was detected the year before. Then, the bloom, that was observed deeper than previous years, was associated to a conjunction of factors already well-known to favour the development of this very competitive species (i.e. mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc…). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, not observed, measured or taken into account here, a plausible scenario could be proposed and may be useful to deciders. One thing remains unclear: where do the cyanobacterium hides when it is not observed during the routine monitoring survey and thus from which place it could initiate its development (nearshore, in the pelagic zone, from the sediment?), unless it is simply not sampled and observed due to methodological bias.

Die hard in Lake Bourget! The case of Planktothrix rubescens reborn

Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was thought that its proliferation was over. However, blooms occurred again in 2016 and 2017 despite apparent low phosphorus concentrations in surface waters of the lake. We have attempted to explain the reasons for this come back in order to develop scenarios helpful to stakeholders who are concerned such proliferations may occur in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium provided a minimum autumn/winter inoculum of P. rubescens was detected the year before. The subsequent bloom was observed deeper than previous years and associated with a conjunction of factors known to favour the development of this species (i.e., mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc.). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, a plausible scenario is proposed. One thing remains unclear: where does this cyanobacterium “hide” when it is not observed during the routine monitoring surveys and from which place it could initiate its development (nearshore, the pelagic zone, or from the sediment?).