Temporal variability of the microbial food web (viruses to ciliates) under the influence of the Black Sea Water inflow (N. Aegean, E. Mediterranean)

Abstract

Τhe entire pelagic microbial food web was studied during the winter-spring period in the frontal area of the North Aegean Sea. Abundance of viruses, heterotrophic bacteria, cyanobacteria, auto- and hetero-trophic flagellates, and ciliates, as well as bacterial production, were measured at three stations (MD1, MD2, MD3) situated along a N-S transect between the area directly influenced by the inflowing Black Sea water and the area covered by the Levantine water. Samples were collected in December 2009, and January, March, April, and May 2011. Station MD1 exhibited the highest values of abundance and integrated biomass of all microbial groups and bacterial production during all months, and MD3 the lowest. Bacteria dominated the total integrated biomass at all stations and months, followed by cyanobacteria, auto-, hetero-trophic flagellates and ciliates. On a temporal scale, the microbial food web was less important in March as all microbial parameters at all stations showed the lowest values. After the phytoplankton bloom in March, the heterotrophic part of the microbial food web (mainly) strongly increased, though the intensity of the phenomenon was diminished from North to South. Pico-sized plankton was found to be heterotrophic whereas nanoplankton was autotrophic. It seems that the influence of the Black Sea water on station MD1, permanent throughout the study period of early winter to late spring, was reflected in all microbial populations studied, and produced a more productive pelagic food web system, with potential consequences for the upper trophic levels.

Zooplankton community dynamics in the N. Aegean front (E. Mediterranean) in the winter spring period

“…The role of food availability for the population increment of C. typicus is also evident from the lower numbers encountered at station MD2 and even less at MD3, as well as from the CCA results, since the species was positively related to chl α. In addition, the availability of ciliates as food declined gradually from MD1 to MD3, especially in the upper layer (Giannakourou et al, 2014). The above observations are in accordance with the statement that C. typicus is not tolerant to starvation (Dagg, 1977).…”

“…The increased numbers of the herbivore C. helgolandicus among large mesozooplankters at stations MD1 (April and May) and MD2 (April) should be also related to the availability of large diatoms in March (Peliz, 2013) and ciliates in April (Giannakourou et al, 2014). Its absence at station MD3 is most likely due to the paucity of phytoplankton and ciliates (Peliz, 2013;Giannakourou et al, 2014), since it has been found to be abundant in the less oligotrophic regions of the Mediterranean Sea (Bonnet et al, 2005), especially in upwelling and frontal areas (Boucher, 1984).…”

“…The minima values of nauplii and microcopepods in March, at all stations, coincided with the lowest abundance values of small and large sized copepods. Α decline of food availability could have occurred between January and March, affecting metazooplankton dynamics, since bacteria, phytoplankton cells and protozoa abundance values were found to be lower in March compared to January (Peliz, 2013;Giannakourou et al, 2014). Maxima values of nauplii and all size fractions of copepods coincided only at station MD2.…”

“…This seems to be true at station MD1, always positioned within the thermohaline front, which is submitted to lateral advection and to temperature and salinity variability, according to the seasonality of circulation and the involved water masses (BSW and Levantine water) characteristics (Poulos et al, 1997;Zervakis & Georgopoulos, 2002). Seasonal variability of BSW volume, the nutrients concentration and characteristics (organic/inorganic and stochiometry) (Polat & Tugrul, 1996;Tugrul et al, 2002) should affect primary and bacterial production, phytoplankton and the composition of microbial communities (Peliz, 2013;Giannakourou et al, 2014) and indirectly control the abundance and composition of zooplankton through a bottomup effect (Verheye & Richardson, 1998). The influence of the different water masses and their physical and trophic (in terms of chl α) characteristics, was clearly confirmed by the CCA results, since salinity, temperature and chl α accounted for the formation of the first axis of the small mesozooplankton samples analysis.…”

“…Most likely, the great quantity of large diatoms in March at MD1 enhanced the population growth of C. typicus in the March-April period; this copepod was found selecting larger phytoplankton cells among phytoplankton (Tomasini & Mazza, 1979). Interestingly, at station MD1, C. typicus was also very abundant in May; maintenance of the population should be supported by the increased number of ciliates in April-May (Giannakourou et al, 2014) and of dinoflagellates in May (Peliz, 2013). According to Calbet et al (2007), this copepod prefers ciliates or dinoflagellates when both phytoplankton and microzooplankton are available, displaying inconsistent selective behaviour (Broglio et al, 2004).…”