Effect of local hydroclimate on phytoplankton groups in the Charente estuary

Stéphane Guesdon, Sabine Stachowski-Haberkorn, Christophe Lambert, Beatriz Beker, Christophe Brach-Papa, Dominique Auger, Christian Béchemin
2016 Estuarine, Coastal and Shelf Science  
This study aimed to describe seasonal variations of phytoplankton abundances in relation to the physical and chemical (nutrients and metals) environment under the influence of freshwater input in the Charente river estuary (Marennes-Oléron bay, France) over three years, from 2011 to 2014. Phytoplankton abundances were determined using microscopy and flow cytometry. Considering high frequency temperature and salinity data, breakpoints in each series led to the identification of two local
more » ... matic periods: the first (2011 and early 2012) being warmer and higher in salinity than the second (from spring 2012 to the beginning of 2014). A multiblock PLS analysis highlighted the significant contribution of the physical environment (temperature, salinity and Photosynthetically Active Radiation (PAR)) on phytoplankton abundances. Two partial triadic analyses (PTA) were run in order to visualize seasonal variations of i) phytoplankton groups and ii) nutrients and trace elements, irrespective of spatial gradient: picoeukaryote occurrence showed a difference between year 2011 and the years 2012 and 2013 (as did cadmium, nickel and silica levels). However, both PTA revealed greater differences between year 2013 and the years 2011 and 2012, as shown by occurrences of cryptophytes, dinoflagellates and nanoeukaryotes, as well as copper and phosphate levels. These results showed a shift between the hydroclimate breakpoint and some phytoplankton responses, suggesting that their development and succession might depend on conditions early in the year. Finally, a STATICO analysis was performed on the paired PTA in order to examine the relations of phytoplankton with nutrients and metals more closely. Most phytoplankton groups were represented on the first axis, together with cadmium on the one hand, and nitrates, silica and nickel on the other. This 2 Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. analysis revealed the separation of phytoplankton groups on the second axis that represented phosphates and copper. Hydroclimatic conditions and the nature of freshwater inputs, especially phosphates and copper content, might be key factors driving phytoplankton structure in the Charente estuary. Graphical abstract Highlights ► Phytoplankton groups were studied in the Charente estuary for three years. ► The local hydroclimate (temperature and salinity) showed two distinct periods. ► Phytoplankton dynamics were mainly driven by their physical environment (temperature). ► The diatom spring bloom was fairly consistent. ► Phytoplankton structure could be linked to copper and phosphates. 2013; Marañón, 2015). The importance of understanding what factors drive phytoplankton communities and how they evolve is emphasized by their place in EU regulations (Water Framework Directive, WFD 2000/60/CE and Marine Strategy Framework Directive, MSFD 2008/56/CE) among the indicators of water mass ecological status. Lugoli et al. (2012) suggested the use of phytoplankton size-classes as an indicator of anthropogenic impact in marine and transition areas. However, as stated by Garmendia et al. (2013), many attributes of phytoplankton need to be considered before it is possible to develop a robust and sensitive indicator. There is thus a need to investigate whole phytoplankton communities, together with their physical and chemical environment, in order to define the baseline variations of all the parameters. Only such complete approaches will make it possible to discriminate for 'events' caused by environmental disturbances. In coastal areas, estuaries are transition areas between freshwater and marine ecosystems, subjected to strong anthropogenic pressure but achieving high productivity thanks to freshwater inputs. Among the most productive coastal areas on the French Atlantic coast, Marennes-Oléron bay (Région Poitou-Charentes, south-west France) is the top oyster producing area in France (Goulletquer and Héral, 1997): out of the 101 100 t of oysters produced in France in 2011/2012, 39 000 t were produced in Poitou-Charentes (CNC, 2014). This high oyster production relies mainly on primary production, which is largely due to phytoplankton. Nutrients are supplied by the Charente river, which discharges into the bay contributing about 90% of the freshwater input during summer (Ravail-Legrand et al., 1988). These nutrients were estimated to contribute annually to a primary production of 185 gC.m -2 .an -1 in the water column of Marennes-Oléron bay (Struski and Bacher, 2006), underlining their importance for phytoplankton development. The first aim of this study was to describe the seasonal variations of phytoplankton abundances in the transition area of the Charente estuary, during three years of monitoring Trace metals Samples were collected manually in 250-mL polyethylene bottles (acid cleaned) that were attached to a 2-m long plastic pole. Bottles were then stored in polyethylene bags in the dark at 4°C and brought back to the laboratory within 24 h in order to perform the extraction. Seawater samples were filtered using 0.45-µm pore size polycarbonate filters (acid cleaned, Nucleopore) under nitrogen pressure in a laboratory clean room (class 100). Filtrates were acidified (0.1%, ultrapure nitric acid 67-69%) and stored in polyethylene bags until analysis.
doi:10.1016/j.ecss.2016.08.035 fatcat:flo6rjwllrec7e2jycpzcl3z5e