Decadal shifts of coastal microphytoplankton communities in a semi-enclosed bay of NW Mediterranean Sea subjected to multiple stresses

Abstract

Long-term evolution of microphytoplankton communities remains poorly studied in anthropized coastal zones submitted to multiple stressors. Here, we investigate decadal (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) microphytoplankton community changes, focusing on abundance and biovolume of major taxa related to both local abiotic conditions (rainfall rate, temperature and salinity) and regional convection events (wintering deep mixing) in the highly urbanized and semi-enclosed Toulon Bay (NW Mediterranean Sea). Results showed that persistent variations in local rainfall regime were followed by major changes in microphytoplankton community composition. Wet period (P 2 ) (increase of wintering precipitations observed between late 2008 and early 2015) was associated to an increase of large heterotrophic dinoflagellates and disappearance of dominant diatom taxa, while dry periods (2005-2008 (P 1 ) and 2015-2017 (P 3 )) promoted diatoms, microflagellates and small mixotrophic/heterotrophic dinoflagellates including potentially toxic species. Concomitance between intense deep mixing events, reported in open Ligurian Basin (particularly during winters 2005 and 2006) and higher values in the total microphytoplankton abundance and in spring diatom abundance regardless of rainfall conditions, presents this meso-scale process as the main fertilization mechanism in Toulon Bay. Although no change was detected in the chlorophyll a concentration during the 2006-2017 period, its trend was negatively correlated to the total microphytoplankton abundance. This negative relation as well as a change of size in dinoflagellates suggested a shift in the primary producer nature, from large autotrophic cells (diatoms and microflagellates) to smaller ones, driven by a runoff intensification. Finally, different communities composition were observed during both dry periods (i.e. diatoms-dominated and autotrophic microflagellate-dominated communities during P 1 and P 3 , respectively), suggesting another environmental driver of change for phytoplankton communities of this coastal ecosystem.

Dissolved organic carbon and surface active substances in the northern Adriatic Sea: Long-term trends, variability and drivers

“…), such findings are an important support for the observed increasing SAS trend in our study. Similar changes in primary production and related processes are also reported for the semi-enclosed Toulon Bay (NW Mediterranean Sea), as a result of changes in rainfall rates, temperature, salinity and regional convection events (wintering deep mixing) (Serranito et al, 2019).…”

Phenological shuffling of major marine phytoplankton groups over the last six decades

“…Alternatively, the declining abundance of dinoflagellates in warmer temperatures could be due to parasitic infections such as fungal infections, which can accelerate the termination of a bloom and result in a lower peak population (Frenken et al, ). Serranito, Jamet, Rossi, and Jamet () also noted long‐term changes in the size of dinoflagellates at a site in the Mediterranean, linked to changing nutrient levels. The complexity of understanding the mechanisms underpinning long‐term changes in phytoplankton populations is further complicated by the fact that, even among a group such a dinoflagellates, we recorded major differences in the trends of both phenology and abundance across individual taxa.…”

Ecological succession of pico- and nanophytoplankton in a coastal bay of NW Mediterranean sea

“…In LiB, no clear seasonal variation of Shannon index was observed. However, diversity was significantly higher in LiB than LaB and thus, confirmed by RFD analysis (with more month described as in LiB than in LaB), contrary to the previous results obtained at the same site for zooplankton and phytoplankton suggesting that diversity was higher in LaB (Jamet et al, 2005;Rossi and Jamet, 2009;Serranito et al, 2019). Such contradiction could be induced by the dominance of SYN in the Bay of Toulon: in LaB, 20 SYN largely drive the community's dynamic; in LiB, SYN was still very abundant but due to high trace metals concentration, it became less competitive than in LaB, and other groups, as PICOeuk, became more abundant.…”