Microbial communities and biogeochemistry in an area of <i>Engraulis encrasicolus</i> spawning in the Sicilian Channel

Abstract

Attempts to merge marine food webs and marine biogeochemical research have recently been developed in response to the need to understand how global change will impact marine ecosystems and resources (Salihoglu et al., 2013; Painting, et al., 2013). Among the multiple drivers that influence ecosystem dynamics, Link et al. (2012) focused on three main processes that affect marine fish productivity: biophysical (environmental), exploitative (fisheries), and trophodynamic (species interactions) factors. Previous paper underlined the relationships among microbial parameters and physical and chemical conditions of waters (Zaccone et al., 2012). With the aim of providing an ecosystem approach to fisheries management, the monitoring of biological and environmental variables was carried out in the frame of the Bansic12 cruise performed in the Sicily Channel (Ibleo-Maltese platform) during July 2012. The Sicily Channel is a spawning area of Engraulis encrasicolus (Linnaeus, 1758), a pelagic species of commercial interest (Basilone et al., 2013). As a consequence, fish eggs and larvae together with microbial assemblages (virio-, autotrophic and heterotrophic pico-, nano- and microplankton) were quantified in terms of abundance and biomass (Andrade et al., 2003; Caroppo et al., 2010; Fonda Umani et al., 2010; La Ferla et al., 2012). To give a more comprehensive picture of microbial role in supporting the flux of biogeochemical elements in the study area, rates of microbial metabolic processes (primary and secondary production, enzymatic hydrolysis of dissolved and particulate matter, and community respiration) were also quantified (La Ferla et al., 2005; La Ferla et al., 2010; Caruso et al., 2014). In addition, the hydrology, the trophic parameters and correlated indices were analyzed to contribute to the understanding of plankton communities dynamism (Leonardi et al., 2009). The euphotic layer was characterized by the presence of the typically oligotrophic MAW (Modified Atlantic Water), flowing eastward along the Sicilian coast. Low nutrient concentrations were recorded, particularly regarding the phosphorus, whose values were close to the detection limit. The diffuse oligotrophy of the area was confirmed by high N/P ratios as well as by low Chl-a values. Particulate Organic Carbon (POC) concentrations were low, as well as those of Particulate Nitrogen (PN). Peaks of POC and PN were recorded at the Deep Chlorophyll Maximum (DCM) or below the thermocline (Leonardi et al., 2014). As regards biological parameters, virus counts were low with a mean value of 1.68E+05 ± 7.0E+04 VPL ml-1 and prokaryotic biomass ranged between 24.9 and 113.0 µg C L-1, mainly composed by curved rods and coccobacilli (range size: 0.160 – 0.459 µm3). Low nucleic acid cells (LNA) prevails over high nucleic acid cells (HNA) with a HNA/LNA mean ratio of 0.56, similar to that occurring in surface layer of several pelagic Mediterranean areas (La Ferla et al., 2012). The picophytoplanktonic biomass (range 0.29–26 µg C L-1), was mostly due to the presence of coccoid cyanobacteria, belonging to the genus Synechococcus. Total nanoplankton biomass (range 0.11-1.71 µg C L-1) showed the highest values at the DCM or below the thermocline. Nanoplankton was represented, among the others, by the dinoflagellates Amphidinium carterae and Heterocapsa niei. The coccolithophorid Emiliania huxleyi and flagellates of uncertain taxonomic identification were also abundant. Microphytoplankton abundance ranged between 6.1 and 40.5 E+03 cells L-1, while biomass between 0.28 and 2.43 µg C L-1. Diatoms and dinoflagellates were the main components of the community, composed by 24 and 30 species, respectively. Concerning microzooplankton, eggs, nauplia and metazoan larval stages prevailed in terms of biomass in almost all samples. Heterotrophic dinoflagellates were the second more relevant group while aloricate Ciliates and Tintinnids did not significantly contribute to the total biomass. Biomass maxima were generally observed at the surface or at the DCM, but never exceeded 16.0 µg C L-1. Finally, anchovy eggs and larvae mainly occurred in the 0-10/0-25 m depth intervals, with highest densities of 4.2 eggs m-3 and 2.6 larvae m-3. The rates of production, hydrolysis and degradation activities were quite low, in agreement with the general oligotrophy of the area and in agreement with this findings, picophytoplankton contribution to total production amounted to over than 65%. In terms of metabolic activities, different spatial distributions were observed between the autotrophic and heterotrophic components. In particular, heterotrophic metabolism showed high values in the layer located below the thermocline and above the DCM. Concerning the enzymatic activities, total leucine aminopeptidase activity showed the highest rates, followed by alkaline phosphatase and beta-Glucosidase (Caruso et al., 2014). Distribution of the dissolved enzymatic activities acting on proteins varied widely along the column, accounting for 5 to 90% of the total enzymatic activity and always prevailed on dissolved b-GLU. Total enzymatic activity rates were comparable to those obtained in the oligotrophic zones of Ionian and Mediterranean Sea (Zaccone et al., 2012), while the analysis of the dissolved fraction was the first report for the Mediterranean. In conclusion, the multidisciplinary scientific approach used in this study depicted a complex picture of the study area evidencing a high heterogeneity and dynamism of plankton communities, probably associated to peculiar hydrology of this ecosystem. On the whole, the study area appeared to be characterized by a relatively efficient microbial food web. However, low trophic conditions were stressed by all the biological and biochemical patterns, suggesting an important role of heterotrophic processes in this area in the examined summer period. Consequently, how and how much the microbial web sustains fish reproduction and larval survival need a more comprehensive analysis and will be focused in further research