Record-breaking salinities in the middle Adriatic during summer 2017 and concurrent changes in the microbial food web

Abstract

Oceanographic measurements carried out in the middle Adriatic during summer 2017 revealed anomalous conditions in both physical and microbial properties. High salinities were observed throughout the entire water column, with an ‘inverse’ salinity profile in August and a maximum in the surface layer, recorded for the first time in the middle Adriatic. Surface salinity of 39.02 recorded in August was 2.5 standard deviations above the long-term average (1961–2016). The observed salinity distributions are the result of both local and remote drivers, whereby the North Ionian cyclonic gyre controlled by the Adriatic-Ionian Bimodal Oscillating System has been responsible for the overall above-average salinities since 2011. Yet, local factors present in 2017, such as strong evaporation caused by extremely high air temperatures, lack of precipitation and low river discharges, combined with a decrease in horizontal transport estimated from the Regional Ocean Modeling System simulations, contributed substantially to the observed surface salinity anomaly. The decrease in horizontal advection was conjoined with high values of repelling barriers in the fields of the finite-time Lyapunov exponent. Documented physical conditions were synchronized with considerably lower bacterial production and abundance of the most studied picoplankton groups in comparison to values during the last decade. The exception was the euryhaline organism Synechococcus, whose abundance was 88% higher than the average in the study area. Nutrient content and chlorophyll-a concentrations followed regular seasonal cycles during 2017, with typical low values pointing to salinity as a possible driver of the observed changes in the microbial food web. Following ongoing climate change and future projections, these documented anomalous physical and microbiological conditions may become more frequent in the Adriatic Sea.