Ultraphytoplankton basin-scale distribution in the eastern Mediterranean Sea in winter: link to hydrodynamism and nutrients

M. Denis,V. Martin,M. Thyssen,B. Manca,F. Vidussi

doi:10.5194/bg-7-2227-2010

Abstract

Abstract. The basin-scale distribution of ultraphytoplankton (<10 μm) was determined in the upper 200 m of the eastern Mediterranean Sea during the winter season. Four clusters were resolved by flow cytometry on the basis of their optical properties and identified as Synechococcus, Prochlorococcus, pico- (<3 μm) and nanoeukaryotes (3–10 μm). Synechococcus was the most abundant population (maximum abundance of about 37 000 cells cm−3) and contributed up to 67.7% to the overall ultraphytoplanktonic carbon biomass, whereas the contribution of Prochlorococcus never exceeded 6.5%. The maximum integrated carbon biomass was 1763, 453, 58 and 571 mg C m−2 for nanoeukaryotes, picoeukaryotes, Prochlorococcus and Synechococcus respectively. Water mass properties were analyzed on the basis of temperature and salinity distributions in order to account for the general circulation and locate the main hydrodynamic structures (fronts, gyres, transition between western and eastern basins). The effect of the main hydrodynamic structures and nutrients on the ultraphytoplankton distribution was investigated. No positive correlation between nutrients and phytoplankton could be established when considering large scales. However, below 50 m depth, nutrient ratios between particular stations were correlated to corresponding density ratios. In contrast, significant relationships were found between Synechococcus abundance and density, resulting from the impact of a gyre in southern Adriatic basin and a thermohaline front in the Ionian basin. A significant relationship was also found between picoeukaryotes and salinity in the comparison of western and eastern Mediterranean Sea.

Highlights

Hydrodynamism in the oceans is known to affect phytoplankton distribution
The horizontal maps at surface were constructed for the temperature, salinity and density fields in order to illustrate the spatial distribution of the Atlantic Water (AW), LSW (Levantine surface waters) and their dynamics, as well
In spite of the large seasonal variability of the upper dynamics indicated by the model simulations (Roussenov et al, 1995), which have shown a prevalent eastward flow pattern of AW in winter, in January 1995 the salinity pattern revealed instead that both the western and northern Ionian basin were under the influence of the low-salinity AW

Summary

Introduction

Hydrodynamism in the oceans is known to affect phytoplankton distribution. Meso-scale hydrodynamic structures such as fronts, eddies and gyres control the biomass and primary production (McGillicuddy et al, 1998) as well as phytoplankton composition Hydrodynamic structures and circulation can influence directly via vertical motion the phytoplankton size structure (Rodriguez et al, 2001). At the same time hydrodynamic structures drive nutrients or modify the light environment and indirectly control phytoplankton biomass and composition (Vidussi et al, 2001). The Mediterranean Sea is an ideal region to study physical-biological relationships as different hydrodynamic structures (mesoscale of 10–100 km) occur at relative small scales which can be simultaneously sampled for physical and biological parameters (Claustre et al, 1994; Rodriguez et al, 2001; Vidussi et al, 2001)

Methods

Results

Conclusion