Bi-decadal variability in physico-biogeochemical characteristics of temperate coastal ecosystems: from large-scale to local drivers

Coastal marine ecosystems, which play a crucial role in the biogeochemical and ecological functioning of the earth system, are highly sensitive to the combined effects of climate and human activities. Because of their location, coastal ecosystems are directly influenced by human activities. Thus, it remains challenging to assess the spatial and temporal scales at which climate influences coastal ecosystems. Twelve sampling stations distributed in eight ecosystems of Western Europe were monitored during two decades for physico-biogeochemical parameters (temperature, salinity, concentrations of dissolved oxygen, nutrients and particulate material). The study encompasses a large diversity of temperate coastal ecosystems regarding geomorphology, trophic status, tidal regime, river influence, turbidity, and so on. Time-series analysis coupled with standardised 3-mode Principal Component Analyses, Partial Triadic Analyses and correlations were used to assess bi-decadal variability and ecosystems trajectories, and to detect the large-scale, regional and local drivers. Overall results highlight two abrupt changes in 2001 and 2005. The bi-decadal changes are related to changes in large-scale and regional climate; detected through proxies of temperature and atmospheric circulation, as well as through river discharge. Ecosystem trajectories usually move towards an increase in temperature, salinity and/or a decrease in chlorophyll-a, nutrients and particulate matter. However, the magnitude of change, the year-to-year variability and the sensitivity to the 2001 and 2005 changes vary among the ecosystems. This study highlights the need to perform long-term time series and combining data sets as well as multi-ecosystem and local studies to better understand long-term variability of coastal ecosystems and its associated drivers.