High-frequency monitoring of phytoplankton dynamics within the European water framework directive: application to metalimnetic cyanobacteria

Abstract

Large, sub-alpine, stratified lakes are directly within the scope of the European Water Framework Directive (WFD) and need adapted monitoring systems. Moreover, anthropogenic eutrophication was frequently the main cause of their water quality degradation in the 20th century. This paper is primarily aimed at demonstrating how in situ sensor-equipped buoys could be the base of monitoring designs to support the WFD objectives. The core of this paper, mainly methodological, focuses on single-depth, high frequency (4 per hour) fluorescence measurements. It shows that the internal wave pattern provides additional information to the singledepth time series to assess phytoplankton dynamics in a stratified water column displaying strong, vertical biomass heterogeneity. The paper deals with the following three aspects: (1) definition of an indicator to determine whether or not the sensor actually

Impact of internal waves on the spatial distribution of Planktothrix rubescens (cyanobacteria) in an alpine lake

“…At the horizontal scale, there was contrast between P. rubescens biomasses in the northern and southern parts of the lake, that is, when high biomasses were found in the northern part, low biomasses were present in the southern part, and vice versa. We also found that the effect of internal waves persisted for several days after the end of a high-wind event, and this has been confirmed by recent data obtained by a buoy equipped with various different sensors (Le Vu et al, 2010) showing that internal waves can be detected throughout the stratified period of the water column. This finding suggests that P. rubescens is subjected to the impact of seiches throughout its development in the metalimnic layer from the lake from April to late September.…”

Can participatory approaches strengthen the monitoring of cyanobacterial blooms in developing countries? Results from a pilot study conducted in the Lagoon Aghien (Ivory Coast)

“…As a matter of consequence, many efforts have been made to develop and validate new innovative devices for the survey of cyanobacterial blooms. For example, probes equipped with chlorophyll-a/phycocyanin and other sensors that can be coupled with autonomous monitoring platforms are now available for punctual or continuous estimations of cyanobacteria biomasses in the water column [ 6 , 7 ]. In the same way, the use of remote sensing [ 8 ] and LIDAR [ 9 ] allows to estimate the horizontal and/or vertical distribution of the cyanobacterial biomass in a lake.…”

Automatic High Frequency Monitoring for Improved Lake and Reservoir Management

“…AHFM systems can assist in managing the provisioning of clean water in several ways (Table ). In water supply schemes, AHFM has been used to avoid the withdrawal of water rich in nutrients, sediment, metals and organic matter during floods , or from anoxic layers; the detection of organic matter, the presence of which leads to formation of disinfection byproducts during chlorination; and the identification and prediction of harmful cyanobacterial blooms . The information delivered by AHFM is used in different ways to assist management: it may be used to prompt grab sampling for confirmation of a threat, to switch to alternative unpolluted sources, to trigger an in situ remediation action (e.g., application of hydrogen peroxide for algal bloom control or to change the intake depth in reservoirs with several withdrawal structures).…”

“…Assessments of ecological status based on other variables, particularly dissolved oxygen concentration, can also be misleading unless AHFM is applied . In fact, ecological assessment using AHFM of chlorophyll a and cyanobacterial pigments is already in use in some countries …”

“…For instance, calculation of lake productivity and phytoplankton biomass needs to take into account strong diel and day-to-day changes . Eutrophication management is typically based on long-term monitoring schemes, but a high frequency sampling can provide a unique insight into the relationship between phytoplankton biomass and external and internal nutrient loads. , Moreover, low frequency sampling is inadequate for identifying potential threats, such as short-lived blooms of cyanobacteria and the fast water quality shifts promoted by sudden floods in reservoirs . These limitations may compromise our ability to understand how aquatic systems respond to, and recover from short-term perturbations, , and make clear the importance of new measurable variables and an enhanced temporal sampling resolution for extended periods of time.…”