engleski

Potential for developing multispecies chronologies in the Mediterranean Sea – the case study of Pag Bay, Adriatic Sea

Sclerochronology is the study of calcified structures, including fish otoliths, bivalve shells and corals, to reconstruct the past events. During the last decades, it has become increasingly evident that bivalve shells can increase our knowledge about climatic and environmental changes, and can, due to a long life span of some bivalve species, extend for decades and centuries into past. This study was conducted in the Pag bay, location for which a first bivalve chronology for the Adriatic Sea was recently developed. Samples of two bivalve species, Glycymeris bimaculata and Callista chione, were collected by skin and SCUBA diving from 3-5 m depth in a period from May 2014 to July 2016. G. bimaculata is a large and long-lived bivalve that can attain size in length of over 80 mm and age of over 55 years. The other target species, C. chione is a commercially important bivalve whose life span can extend over four decades. We analyzed 53 samples of G. bimaculata, which ranged in length from 82 to 98 mm and 50 shells of C. chione, which ranged in length from 64 to 79 mm. For each shell, length, height, width and dry shell weight were measured before every sample was cut around the hinge and embedded in epoxy resin. Embedded samples were cut along the maximum growth axis, grinded, polished and etched and acetate peel replicas of shell sections were prepared. Series of images for each specimen were taken using Zeiss microscope and camera, and composite images were prepared using Image Pro Premier program. List year method was used for visual crossdating of images. Measurements of increment widths were then taken continuously along maximal growth axis using Image Pro Premier. Crossdating was verified using program COFECHA and dendrochronology software package ARSTAN was used to construct chronologies for both G. bimaculata and C. chione. C. chione chronology was also developed with the short- lived species approach to permit a comparison with the results obtained from ARSTAN. Constructed chronologies were compared to each other and to available environmental data. In particular, results were related to the mean monthly seawater temperature and the mean monthly salinity, both taken from NEMO model. Mean monthly precipitation, provided by the Meteorological and Hydrological Service of the Republic of Croatia, and monthly values of absolute dynamic topography (ADT) of the northern Ionian Sea were also analyzed. In particular, the Ionian Sea has been chosen because it present san inversion of the Ionian surface circulation that influences the thermohaline proprieties in the Adriatic Sea. Growth increment data for the period from 1997 to 2016, derived from 26 individuals were used for develop the chronology in G. bimaculata. In C. chione analyzed with the short-lived species approach, the chronology was developed - from 14 individuals – from 1998 to 2014, while a chronology from 1992 to 2013 was developed from 34 individuals. Both species presented a strong ontogenetic pattern. Chronologies obtained through ARSTAN were statistically robust, as EPS values were higher than 0.85, with an exception for three years in C. chione. Chronologies for two analyzed species did not correlate significantly with each other, indicating that probably the different factors drive their growth. Moreover, it was observed that G. bimaculata growth can be explained by the salinity during coldest months and ADT_BMU values from March to August – with exception of May. C. chione chronology did not correlate with any of the analyzed environmental factors. Finally, this study confirmed the potential of G. bimaculata as a potential sclerochronological archive in the Adriatic Sea. It also provides the evidence that growth of individual C. chione shells is synchronous within a population and that future studies should try to determine which environmental factors drive shell growth of this commercially important species.

bivalve, sclerochronology, growth, shell, Adriatic, Pag, Glycymeris bimaculata, Callista chione

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