Temporal variability in bacterioplanktonic abundance in coastal waters of the Northern Adriatic Sea

The purpose of this study is to analyse the heterotrophic bacterial abundance (HBA) distribution in the water column at a coastal site of the Gulf of Trieste (Northern Adriatic Sea), from 1999 to 2003, by adopting three different sampling frequencies: monthly, twice monthly, and daily. The HBA ranged from 1.6 to 54.6 x 108 cell l-1, showing a high variability of more than two orders of magnitude over the study period, with a mean annual variation of one order of magnitude and a 60% annual mean coefficient of variation. Similar seasonal patterns were observed every year: these were characterized by low bacterial abundances during the winter which increased in the summer. Intensifying samplings from monthly to twice monthly did not provide more detailed information on the seasonal HBA distribution but allowed us to detect sporadic HBA hot spots perhaps as the consequence of micro-environmental changes. The results of a daily sampling carried out for 4 weeks, during winter 2002, did not show any significant changes in HBA, ranging from 0.8 to 2.4 × 108 cell l-1. The highest variations between two consecutive days was of about twofold, and the CV over the period was lower than 30%. A principal-component analysis was used to compare HBA, temperature, salinity, dissolved organic carbon, and chlorophyll a. Different gradients related to the surface water hydrological and biochemical characteristics resulted from the long- and short-term study periods. The study of different timescales allowed us to obtain a complete view of HBA temporal distribution confirming an annual pattern that could be affected by sporadic HBA hot spots as the consequence of changes of local environmental conditions.