Picoplankton community structure before, during and after convection event in the offshore waters of the Southern Adriatic Sea

Abstract

Abstract. This paper documents the picoplankton community's response to changes in oceanographic conditions in the period between October 2011 and September 2012 at two stations belonging to the South Adriatic Pit (SAP). The recorded data include the community's abundance, composition, prokaryotic production rates and bacterial metabolic capacity. The sampling period included an intense sea cooling with formation of exceptional, record-breaking dense water. We documented an especially intense winter convection episode that completely diluted the core of Levantine intermediate waters (LIW) in a large area encompassing the SAP's center and its margin. During this convection event the whole picoplankton community had significantly higher abundances with a recorded picoeukaryotic peak at the SAP margin. In the post-convection phase in March, prokaryotic heterotrophic production strongly increased in the entire SAP area (up to 50 times; 456.8 nM C day−1). An autotrophic biomass increase (up to 5 times; 4.86 μg L−1) and a disruption of a close correspondence between prokaryotic heterotrophic biomass production and cell replication rates were observed only in the center of the SAP, which was not under the influence of LIW. At the SAP's margin such an effect was attenuated by LIW, since the waters affected by LIW were characterized by decreased concentrations of dissolved inorganic nitrogen, decreased autotrophic biomasses, and by increased bacterial biomass production balanced with cell replication rates as well as by the domination of Synechococcus among autotrophic picoplankton. The metabolic capacity was lowest in spring when autotrophic biomass largely increased, while the highest levels found in the pre-convection phase (October 2011) suggest that the system was more oligotrophic before than after the convection event. Furthermore, we showed that metabolic capacity is a trait of bacterial community independent of environmental conditions and tightly linked to cell replication and substrate availability. In contrast, the bacterial community composition appears to be strongly influenced by physico-chemical characteristics of waters (e.g., temperature and nutrients) and environmental forcing (e.g., convection and LIW). Our results showed that the two oceanographic phenomena of the Southern Adriatic, strongly relevant for the total production of the Adriatic Sea, winter convection and LIW intrusion, regulate the changes in picoplankton community structure and activities.

Bacterial Diversity in the South Adriatic Sea during a Strong, Deep Winter Convection Year

“…The diversity minimum in spring, during the maximum productivity, could be explained by the availability of a series of new ecological niches due to the production of phytoplankton-derived organic matter, in which spe- cialists could bloom (39). In addition, the maximum richness observed in summer could be explained by the intensification of the gyre (in summer, both stations were under the influence of LIW) that caused a stronger introgression of LIW, a possible source of new LIW-specific subclades (21,40).…”

“…The summer water column, characterized by a strong stratification, was also defined by a euphotic zone (depth, Ͻ200 m; T, 16.07 Ϯ 2.69°C; S, 38.85 Ϯ 0.07; Chl a concentration, Ͼ0.01 g liter Ϫ1 ) and a deep layer (depth, Ͼ200 m; T, 13.63 Ϯ 0.44°C; S, 38.72 Ϯ 0.06; Chl a concentration, Ͻ0.01 g liter Ϫ1 ). A further detailed description of the water masses, including nutrient concentrations, is given by Najdek et al (21). Briefly, nutrient concentrations were lower in the euphotic zone and higher in the deep layer, with the exception of those in winter, when a homogeneous nutrient distribution was observed in the mixed layer.…”

“…For the southern part of the Adriatic, total prokaryotic picoplankton abundances were determined during the winter convection episode in 2008, but no diversity estimation was provided (18,19). Also, several studies have reported on seasonal changes in picoplankton heterotrophic production and bacterial community metabolic capacity, suggesting the importance and influence of the deep convection event intensity on the annual cycle and productivity of this area (20,21). In the 2011-2012 win-ter season, a strong, deep winter convection event occurred in the southern part of the Adriatic, resulting in the highest, recordbreaking densities ever observed for this region (22,23).…”

“…In the 2011-2012 win-ter season, a strong, deep winter convection event occurred in the southern part of the Adriatic, resulting in the highest, recordbreaking densities ever observed for this region (22,23). Deep convective mixing reached down to a 600-m water depth, bringing phosphorus and nitrogen nutrients into the euphotic zone and thus triggering a bloom of photosynthetic microorganisms in early spring (21). We investigated for the first time in South Adriatic offshore waters (i) the diversity and seasonal dynamics of bacterial and archaeal communities and (ii) their response to a strong, deep winter convection event over a period of 1 year using a combination of 454 pyrosequencing of the 16S rRNA gene and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) techniques.…”

Report of the Croatian Committee of Geodesy and Geophysics on activities carried out between 2011 and 2014

“…The presence of two isolated circulation cells in the northern Adriatic during a spring cruise was detected on the basis of oceanographic data, including dynamic depths distribution, and confirmed by spatial variations in the structure of microbial community ). An intense winter convection event in the South Adriatic Pit resulting in increased bioproduction was documented as well (Najdek et al, 2014). long-term time series of physical and chemical parameters collected between 1960 and 2010 along the Palagruža Sill transect were investigated as well.…”