Phytoplankton Pigment Time Series from SO RadeMicro at Coastal Station Point B, Bay of Villefranche-sur-Mer, Ligurian Sea, NW Mediterranean

[Lien vers la fiche source] The Bay of Villefranche-sur-Mer is 2 miles long and 0.8 miles wide, characterized by great depth (> 300 m at its mouth) and the absence of a continental shelf. This site, considered oligotrophic to mesotrophic, is located in a heavily urbanized area, subject to seasonal anthropogenic influences (cruise ships, intense yacht traffic, tourism). Thanks to the absence of a continental shelf and the northerly current, organisms living offshore reach the coast via offshore water masses entering the bay. In 1957, the Observatoire Océanologique de Villefranche-sur-Mer (OOV) initiated a hydrological monitoring program at a fixed station "Point B" (43°41’N / 7°19’E; 80 m depth) in the Bay of Villefranche-sur-Mer. Since then, weekly sampling has been conducted throughout the year across various depths of the water column, providing consistent temporal coverage. The OOV—now integrated into the Institut de la Mer de Villefranche (IMEV) — maintaining this long-term oceanographic observation effort. Over the decades,the range of monitored oceanographic parameters has expanded to include of approximately fifteen oceanographic variables at six different depths within the water column. It includes, temperature, salinity, nutrients, pH, isotopes, suspended solids, colored organic particles (COP), non-organic particles (NOP), and fluorescence-based chlorophyll a. Cytometry was included in 2009, followed by high-performance liquid chromatography (HPLC) in 2012. Monitoring programs for zooplankton and micro phytoplankton communities have also been implemented at Point B. This initiative has expanded to include additional observation labelled programs such as SNO SOMLIT (www.somlit.fr) PHYTOBS, and REPHY, as well as internal IMEV observation programs (SO) such as RadeHydro, RadeMicro and RadeZoo. Observations are conducted using vessels from the French Oceanographic Fleet or stationary platforms, with personnel from IMEV and contributions from the Laboratoire d’Océanographie de Villefranche (LOV). Water samples for pigment analysis were collected weekly as part of the long-term observational services (RadeHydro, SOMLIT). Sampling was performed using Niskin bottles at fixed depths—typically 1, 10, 20, 30, 40, 50, and 75 meters, or alternatively at 1 and 50 meters—depending on cruise objectives and weather conditions. For each sampled depth, in average 1500mL of water were filtered through Whatman GF/F glass fiber filters for the determination of pigment concentrations of the total phytoplankton community. Filters were stored at -80 °C for subsequent HPLC pigment analysis. Samples were subsequently analyzed for phytoplankton pigments by the national analytical service SAPIGH (Service d’Analyse des Pigments par HPLC). The SAPIGH platform (based at IMEV, Villefranche-sur-Mer) is certified by INSU-CNRS. It is dedicated to HPLC measurement of phytoplankton pigments (chlorophylls and carotenoids) in the marine environment (https://lov.imev-mer.fr/web/facilities/sapigh/). Briefly, pigments were extracted in 100% methanol, disrupted by sonication, and clarified by filtration using GF/F filters (Whatman). HPLC analysis was performed on the same day using an HPLC 1200 system (Agilent Technologies). Detection wavelengths were set as follows: carotenoids and chlorophylls c and b at 450 nm, chlorophyll a and its derivatives at 676 nm, and bacteriochlorophyll a at 770 nm. The complete analytical procedure is described in Ras, J., Uitz, J., and Claustre, H. (2008). Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: Comparison between in situ and predicted data. Biogeosciences, 5, 353–369.The data treatment involved assigning quality control codes to detected peaks: QA = 1 (good), QA = 2 (acceptable), and QA = 3 (qualitative). Pigments with concentrations below the limit of detection were reported as “LOD.” This long-term, multi-parameter monitoring program in the Villefranche-sur-Mer Bay aims to study fluctuations in marine communities on multi-decadal scales, providing a critical overview of their responses to climate variability and anthropogenic pressures. Through consistent data collection, it contributes to understanding the impacts of environmental change on coastal ecosystems. It remains one of the longest-running continuous coastal time-series programs in the Mediterranean.