Variability of ultraplankton composition and distribution in an oligotrophic coastal ecosystem of the NW Mediterranean Sea derived from a two-year survey at the single cell level

Abstract

Ultraplankton [heterotrophic prokaryotes and ultraphytoplankton (<10 μm)] were monitored weekly over two years (2009 & 2010) in a coastal area of the NW Mediterranean Sea. Six clusters were differentiated by flow cytometry on the basis of their optical properties, two heterotrophic prokaryote (HP) subgroups labelled LNA and HNA (low and high nucleic acid content respectively), Prochlorococcus, Synechococcus, autotrophic picoeukaryotes and nanoeukaryotes. HP represented an important component of the microbial assemblage over the survey with relatively small abundance variation through seasons. The carbon biomass ratio HP/ultraphytoplankton averaged 0.45, however this ratio exceeded 1 during spring. Ultraphytoplankton biomass made about 50% of the total autotrophic carbon estimates but this contribution increased up to 97% and 67% during the 2009 and 2010 spring periods respectively. Within ultraphytoplankton, nanoeukaryote represent the most important ultraphytoplankton group in terms of autotrophic carbon biomass (up to 70%). Picoeukaryote maximum abundance occurred in winter. Synechococcus was the most abundant population (maximum 1.2 x 10 5 cells cm-3) particularly in spring where it represented up to 54% of ultraphytoplankton carbon biomass. The warmer winter-spring temperatures and the lengthening of the stratification period created a favorable situation for the earlier appearance of Synechococcus and its persistence throughout summer, paralleling Prochlorococcus development. Prochlorococcus was dominant over summer and autumn with concentrations up to 1.0 × 10 5 cells cm-3. While the abundance of Synechococcus throughout survey was of the same order as that reported in western Mediterranean Sea, Prochlorococcus was more abundant and similar to the more typical oligotrophic and warm waters. The abundance variation of the ultraplankton components through the survey was relatable to variations in the hydrological and nutrient conditions.

Fig 1. Location of the study area with the sampling station Point B (43°41.10 N, 7°19.00 E) in the Ligurian basin (Northwestern Mediterranean Sea).

Schlitzer, R., Ocean Data View, http://odv.awi.de, 2016.

Fig 2. Vertical distribution of the physical variables collected weekly along the upper 75 m at Pt. B from January 2009 to December 2010.

(A) Temperature (°C) and temperature anomalies, (B) Salinity (PSU) and salinity anomalies and (C) fluorescence (UF) and fluorescence anomalies. The anomaly graphs represent the standard deviation of a given year with respect to the median year values of 13 years (1995–2008) for each variable.

Fig 3

Meteorological and hydrological variables during the study period: (A) wind speed (ms^-1); (B) solar radiation(Jm²) with were collected on a daily basis; red lines represent the weekly average. (C) The stratification index was calculated as the average density-excess difference between 0 to 75 m (Sigma-t kg m^-3). MIX (mixed winter period), SSM (spring semi-mixed period), STRAT (summer stratified period), ASM (Autumn semi-mixed period). The transition from mixed to semi-mixed period was considered when there is a change of 0.05 in the stratification index and from semi-mixed to stratified period when the index reach 0.1. In gray, variations of integrated total Chl a (mg m^-2) during the studied period. Dots represent the DCM depth (successively 0, 10, 20, 30, 50, 75 m).

Fig 4. Vertical abundance (cells cm⁻³) of HP (Heterotrophic procaryotes) and ultraphytoplankton during the study period along the upper 75 m.

Four autotrophic clusters are described namely Prochlorococcus, Synechococcus, picoeukaryotes and nanoeukaryotes. Black dots illustrate the 6-sampling depth. The coloured bars at the top represent the duration of the different hydrological periods (MIX SSM, STRAT, ASM) based on the calculated stratification index and on the ACP (see Methods for detail).

Fig 5. Principal Components Analysis (PCA) ACP-Plan 1/2, applied to data from 6 January 2009 to 28 December 2010.

The 42-active data set consisted of 101 week values described by 42 biological variables: HP, Synechococcus, Prochlorococcus, picoeukaryotes, nanoeukaryotes abundances, fluorescence intensity and the Chl a concentration at 6 discrete depths (0, 10, 20, 30, 50, 75 m). Illustrative variables are concentrations of nutrients (NO₂^-, NO₃^-, PO₄^3-, Si(OH)₄, POC concentrations. In red the year 2009, in blue 2010. N = 7420. The blue zone in the figure corresponds to the seasonal variability between 2009 and 2010 highlight by the ACP.

Fig 6. PCA variables factor map representing projection of variables on the plane defined by the first two principal components.

The cycle of correlation represents the changes of the biological and biogeochemical parameters over time.