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.
Highlights
In oceanic ecosystems, most organic carbon originates from atmospheric CO2 diffusing into surface seawater where it is directly metabolized by primary producers through photosynthesis
Autumn was characterized by important changes in hydrological structure; the complete destratification of the water column occurred in early November
In 2009 the main wind event took place in mid-April with a maximum wind speed (9.4 m s -1) reached on the 11th, whereas in 2010 the maximum wind speed (11.0 m. s-1) was observed on 10th of June but other wind events occurred on early March (8.4 m. s-1) and in October and November (10.2 and 9.4 m.s-1, respectively) the latter being likely responsible for the de-stratification of the water column (Fig 3A)
Summary
Most organic carbon originates from atmospheric CO2 diffusing into surface seawater where it is directly metabolized by primary producers through photosynthesis. Phytoplankton play a major role as primary producers [1], contributing about 50% to the annual global net primary production [2] and form the base of marine food webs [3]. Oligotrophic surface waters, essentially host small ( 2 μm) size phytoplankton favoured by better efficiency in nutrient uptake [7,8]. This dominance difference linking cell size and trophic regime is the basis of a trophic index [9] relating the relative abundance of eukaryotic pico and nanophytoplankton Rpn, so that for instance Rpn >1 is indicative of an oligotrophic regime. The planktonic community structure, especially the primary producers, and the microbial food web, play a key function in the carbon cycle [11]
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