Long-term changes in abundance and diversity of tintinnids in the Gulf of Trieste (Northern Adriatic Sea)

Abstract

Abundance and composition of the planktonic tintinnid ciliates were studied in the Gulf of Trieste (Northern Adriatic Sea, NE Mediterranean Sea) from July 1998 to July 2016. Tintinnids were collected biweekly-monthly from the LTER station C1 (200 m offshore, 17.5 m depth) at four depths (surface, 5 m, 10 m and 15 m). The maximum tintinnid abundance reached 4476 ind. L-1 at surface in February 2016. The tintinnid community comprised a maximum of 35 species and was dominated by the genera Stenosemella, Tintinnopsis, Codonellopsis, Salpingella and Eutintinnus. The most abundant species were Stenosemella nivalis, Tintinnopsis nana, Codonellopsis schabi, Salpingella rotundata and Eutintinnus apertus. We found a species-specific correlation with the abiotic factors considered, i.e., temperature and salinity. Temperature was positively correlated with S. rotundata and E. apertus and negatively with S. nivalis. Salinity was negatively correlated with the majority of the detected species. Agglutinated species presented winter maxima while hyaline species showed higher abundance in summer-autumn. Some key species were present over the whole period studied. Significant differences within the water column were not seen in the species composition, but were seen in the relative abundances of the same species at different depths. Stenosemella nivalis, S. ventricosa and Tintinnopsis beroidea can be considered as keystone species in the area and their possible loss can be seen as a signal of changes in the structure of the entire planktonic system.