Sampling strategy

The ecosystems monitored by SOMLIT are distributed over mainland France’s seafront. This diversity allows for the study of numerous gradients and assess drivers from a wide variety of origins : geomorphology (types of geosystem, depth of the water column, surface area, etc.), catchment area (surface area, land use) and continental influence (river flows and their load of dissolved and particulate material), climate and meteorology, trophic status (oligotrophic, mesotrophic, partially eutrophic), turbidity, etc. Some ecosystem types are monitored at several stations (2 – 3), to provide a better understanding of the shore-ocean gradient and its multidecadal evolution.

The parameters used allow to understand the evolution of the ecosystems by identifying modifications at several levels:

  • impact of the climate (temperature, salinity, etc. ): water inputs, hydrodynamics, seasonality of thermal changes

  • influence of allochthonous, marine and continental inputs (salinity, pH, nutrients, parameters describing the particulate organic matter): changes in river flows, particulate inputs, bentho-pelagic coupling (resuspension and hydrodynamics), marinisation of systems

consequences on ecosystem functioning (dissolved oxygen, pH, nutrients, parameters describing particulate organic matter and biology (microphytoplanktonic and pico-nanoplanktonic biomass)): modification of the ecosystem’s metabolism (dissolved and particulate flows, quality of particulate organic matter) and importance of the involved biological compartments.

The fortnightly sampling frequency tracks the seasonality of processes. The complementary high-frequency measurements (automated in situ measurements every 10 to 20 minutes) carried out by the SNO Coast-HF at the SOMLIT sites provide insights into exceptional events, which are rare and of short duration. Finally, the multiparametric CTD profiles describe the entire water column. The SOMLIT network’s sampling strategy can be summarised as surface sampling (all parameters: hydrological series and pico-nanoplankton series) and water column profiles (CTD series), at high tide (for ecosystems on the English Channel and Atlantic coasts), every two weeks. This strategy can be enhanced locally by increasing the frequency (i.e. weekly), sampling at several depths and/or adding parameters. Some of these additional data are part of the official SOMLIT perimeter and are made available to users. See station descriptions for details. Due to the very specific nature of the Gironde Estuary (large, highly turbid macrotidal estuary), the sampling strategy had to be adapted to a (approximately) monthly sampling of all parameters (hydrological series) at high and low tide, at the surface and at the bottom in three sites. The water column profile (CTD series) is restricted to the most downstream site at high tide. As the Gironde estuary is highly turbid, the identification and enumeration of pico-nanoplankton is not possible.