Evidencing the Impact of Climate Change on the Phytoplankton Community of the Mediterranean Sea Through a Bioregionalization Approach

Abstract

The Mediterranean is a miniature ocean where most of the processes documented in the global ocean are encountered (Lejeusne et al., 2010). Indeed, important global features are present in it, but on smaller spatial and temporal scales (Bethoux et al., 1999) such as the thermohaline circulation forced by dense water formation in winter. In addition, several features of the Mediterranean make it a hotspot of marine biodiversity (Coll et al., 2010) and potentially vulnerable to climate change.At regional scale, the Mediterranean Sea plays a role of sentinel with regard to global warming since it is very sensitive to climate change (Giorgi, 2006). Indeed, the Mediterranean Sea was one of the first ocean places where a warming trend was observed in the deep-water temperatures in the western basin and attributed to global warming (Bethoux et al., 1990). The Mediterranean Sea Surface Temperature (SST) has experienced an intensive and continuous warming since the mid-1980s, which is expected to increase throughout the 21st century under present climate scenarios (Somot et al., 2008). Several studies have shown an increase of the mean Mediterranean SST in the last three decades. Based on the 4 km Advanced Very High Resolution Radiometer (AVHRR) Pathfinder SST data set, Nykjaer ( 2009) estimated a mean warming trend of 0.03 ± 0.008°C/year in the western Mediterranean Sea and 0.05 ± 0.009°C/year in the eastern basin from 1985 to 2006. Similar results were found by (López García & Camarasa, 2011) from 1985. Based on Reynolds' SST reanalysis (Reynolds et al., 2007, Shaltout and Omstedt (2014) found a mean warming trend of 0.035 ± 0.007°C/year during a 31-year period (1982-2012) over the Mediterranean Sea. And recently,

High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry (1999–2019)

“…Most Level 1 and 2 metrics are spatially evaluated for 16 Mediterranean sea subbasins (Figure 1), in the open sea region (defined as the area with a depth greater than 200 m), and for the full water column in selected layers, i.e., 0-10, 10-30 m (for certain metrics these layers are merged into a single layer, namely, 0-30 m), 30-60, 60-100, 100-150, 150-300, 300-600, and 600-1,000 m. In the coastal areas, the reanalysis accuracy can be assessed only for surface chlorophyll using the CMEMS satellite product and for other variables and a few subbasins according to in situ data availability. The 16 subbasins synthesize the heterogeneity in the Mediterranean Sea into homogeneous areas, considering previous bioregionalization analyses (D'Ortenzio and Ribera d'Alcalà, 2009;Ayata et al, 2018;Di Biagio et al, 2020;El Hourany et al, 2021;Novi et al, 2021).…”

Long‐term variability of the coastal ocean stratification in the Gulf of Naples: Two decades of monitoring the marine ecosystem at the LTER–MC site, between land and open Mediterranean Sea

“…For example, the recent work of Xiu et al (2018) illustrates the case of the California Current System, where wind and eddy activities in this specific area play a complex role in the redistribution and response of biological communities to nutrients supply. Each regional area is governed by specific physical and biogeochemical characteristics, establishing them as bioregions (El Hourany et al, 2021) whose variability evolves with global warming. Systems in a Mediterranean climate are expected to become warmer and drier with climate change, whereas estuaries in these regions are predicted to experience variability in freshwater flows, with consequences such as ‘ marinisation ’ and hypersaline conditions (Hallett et al, 2018).…”

Seasonal and Long-Term Variability of the Mixed Layer Depth and its Influence on Ocean Productivity in the Spanish Gulf of Cádiz and Mediterranean Sea

“…However, these results should be taken cautiously as the length of the chlorophyll time series obtained from satellite could not have the length needed to discern between decadal variability and long-term changes (Dutkiewicz et al, 2019;Hammond et al, 2020). Some works have shown that the total phytoplanktonic biomass has not changed in the Mediterranean Sea since the end of the twentieth century, or even it had increased, but its composition could have changed with a substitution of large cells, such as diatoms, by small-size phytoplankton (Marty et al, 2002;El Hourany et al, 2021). The analysis of chlorophyll concentration time series does not allow us to discern this possibility.…”

“…Ribera D'alcalà et al (2004) analyzed time series of phytoplankton in a coastal station in the Gulf of Naples from 1984 to 2000, and also found a decrease of the size of phytoplanktonic cells, but in this case the overall biomass also decreased. Mazzocchi et al (2012) observed a negative trend for the chlorophyll concentration at the same location from 1984. El Hourany et al (2021 also found negative trends for the surface chlorophyll concentrations in the Mediterranean Sea, using satellite-derived time series over the period 2003-2020, but these trends were not statistically significant.…”