ABSTRACT
Annual growth increments formed in bivalve shells are increasingly used as proxies of environmental variability and change in marine ecosystems, especially at higher latitudes. Here, we document that well-replicated and exactly dated chronologies can also be developed to capture oceanographic processes in temperate and semi-enclosed seas, such as the Mediterranean. A chronology is constructed for Glycymeris pilosa from a shallow embayment of the northern Adriatic and extends from 1979 to 2016. The chronology significantly (p < 0.05) and positively correlates to winter sea surface temperatures, but negatively correlates to summer temperatures, which suggests that extreme winter lows and extreme summer highs may be limiting to growth. However, the strongest and most consistent relationships are negative correlations with an index of the Adriatic-Ionian Bimodal Oscillating System (BiOS) for which positive values indicate the inflow of the ultraoligotrophic Eastern Mediterranean waters to the Adriatic. In contrast, the substantial freshwater flows that discharge into the Adriatic do not correlate to the bivalve chronology, emphasizing the importance of remote oceanographic processes to growth at this highly coastal site. Overall, this study underscores the potential of bivalve chronologies to capture biologically relevant, local- to regional-scale patterns of ocean circulation in mid-latitude, temperate systems.
ABSTRACT
Month-to-month fluctuations in the abundance of bacteria and heterotrophic nanoflagellates (HNF) and bacterial production, as well as various chemical (nutrients, oxygen) and physical (salinity, temperature) parameters were analysed at a station located in the open middle Adriatic Sea during one decade (1997-2006). Being influenced by both coastal waters and open Adriatic circulation in the surface layer, and by the deep Adriatic water masses in the deep layers (100 m), this station is quite suitable for detecting the environmental changes occurring in the open Adriatic Sea with respect to the circulation of its water masses and their long-term changes and anomalies. Multivariate methods were used to identify seasonal and inter-annual changes of the investigated parameters, associating observed changes to the changes in Adriatic water masses and circulation regimes. The analyses showed that bacterial abundance and production were controlled by different water mass dynamics during 1997-2001 compared to 2002-2006 period, particularly noticeable in different seasonal patterns of biological parameters. The interplay between North Adriatic Dense Water (NAdDW) and Levantine Intermediate Water (LIW) resulted in a change in the available nutrients (NAdDW is poor in orthophosphates), and as a consequence different bacterial abundance and production. A few periods were examined in detail, such as 2004, when LIW inflow was particularly strong and was accompanied by an increase of bacterial and HNF abundances, as well as of bacterial production.
