engleski

Relationships within the microbial food web in the northern Adriatic Sea

Processes within the microbial food web play an important role in the energy-matter transfers throughout the marine ecosystems. Heterotrophic nanoflagellates (HNF, 2-20 ľm in diameter) are considered to be the primary bacterivores in most of the aqautic ecosystems. There are also strong evidences of them as herbivores, which is connected with the available autotrophic versus heterotrophic prey biomass. Concerning the significant abundance of cyanobacteria, photosynthetic prokaryotes, in coastal waters, it is necessary to compare their relation to HNF with the relation between HNF and heterotrophic bacteria, because of the different roles of those two bacterial groups in the cycling of nutrients and organic matter. The data presented in this study were collected during a 12-year-period (1990-2001) at five depths of six stations, along the profile Rovinj (Croatia) - Po River Delta (Italy). Heterotrophic bacteria, cyanobacteria and HNF counts were performed by epifluorescence microscopy using the fluorochrome DAPI staining technique for heterotrophic bacteria, primulin staining technique for HNF and autofluorescence of cyanobacteria. The studied area is characterized by marked eutrophication gradients, due to high nutrient and organic matter contributions of the Po River. It is also characterized by periodic hypertrophic formation of giant organic aggregates in the water column and kilometres long gelatinous surface layers (mucilage events) in late spring and summer. This phenomenon occurred with increasing frequency during the last two decades. The aim of this study was to compare the relationships of HNF with heterotrophic bacteria and cyanobacteria, in years with (1991, 1997, 2000 and 2001) and without mucilage events. Each of the two groups of years is divided in five seasons, which were identified according to the prevailing processes in each of those seasons in the northern Adriatic ecosystem. The six stations were divided into four groups, according to the bacterial abundance gradient along the profile. The results show that HNF were tightly coupled to the cyanobacteria more often than to heterotrophic bacteria, in all seasons, especially in the years with the mucilage event. In the July-August and September-November seasons, both bacterial groups mostly were controlled by HNF, much more than in the other three seasons. However, in the season July-August cyanobacteria were correlated with HNF in all years and regions (R=0.612-0.229, p=0.000-0.007), while, in contrast, heterotrophic bacteria show correlations only in the years without mucilage events (R=0.512-0.367, p=0.000-0.002), although their biomass, as well as the cyanobacterial, were both higher in that season compared to the other seasons, in the years with the events (het. bacteria: 35.3 - 66.2 ľg C L-1, cyanobacteria: 16.9 - 37.5 ľg C L-1). This difference was also obvious in the February-April and May-June seasons, when again cyanobacteria were positively correlated to HNF in most of the cases in the years with mucilage events (R=0.569-0.223, p=0.000-0.018). Also, the ratios of the cyanobacterial biomass in the total bacterial biomass were higher (up to ~70%) and the cyanobacterial abundances were significantly larger in the years with mucilages, already from early spring, preceding the triggering of the events. The obtained results indicate that relationships within the microbial food web varied significantly among the seasons and the years with and without mucilage events, pointing out an important role of cyanobacteria and their relationship with heterotrophic nanoflagellates in the northern Adriatic.

Bacteria; Cyanobacteria; Heterotrophic nanoflagellates; northern Adriatic

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