Reconstructing Holocene primary productivity in the northern Adriatic Sea using δ15N of bivalves

Primary productivity is a critical parameter of marine ecosystems, yet in many coastal areas, it has been significantly altered by human activities. The Northern Adriatic Sea (NAS), a shallow epicontinental sea bordered by the Italian and Balkan peninsulas, exemplifies this phenomenon. In the 20^th century, eutrophication caused by substantial fertilizer use, industrial discharge, and high riverine input led to frequent algal blooms, bottom hypoxia, and mucilage events. Over the past three decades, however, environmental regulations and declining river discharge have reduced nutrient input, leading to decreased eutrophication.

These shifts in primary productivity have profoundly impacted marine communities. Understanding how communities respond to such changes is essential as climate change and anthropogenic pressures continue to shape the NAS. Fortunately, the NAS provides historical analogs due to marked fluctuations in freshwater, sediment, and nutrient input during the Holocene.

This study employs nitrogen stable isotope values (δ¹⁵N) in shell-bound organic matter of bivalves as a proxy for past primary productivity. δ¹⁵N is fractionated by primary producers and reflects nutrient dynamics within an ecosystem. As low-level consumers, bivalves offer δ¹⁵N values indicative of the food web base, providing a more stable proxy than primary producers, which are highly sensitive to short-term environmental fluctuations. The robust (sub)fossil record of bivalves allows correlations between changes in primary productivity and community turnovers over time.

Our research focuses on Varicorbula gibba, an infaunal filter feeder abundant in the NAS throughout the Holocene and increasingly dominant during 20^th-century eutrophication due to its opportunistic nature. The first step of this study involves calibrating δ¹⁵N values from live bivalves against water samples collected across a productivity gradient in the NAS. This calibration will assess how well shell-bound δ¹⁵N reflects variations in primary productivity along an onshore-offshore gradient.

δ¹⁵N analysis is conducted using the denitrifier method, wherein nitrogen species from bivalve and water samples are oxidized, bacterially transformed into N₂O, and analyzed via mass spectrometry. Understanding how δ¹⁵N of V. gibba relates to its environment enables us to extend this analysis to (sub)fossil specimens, reconstructing Holocene primary productivity changes and their ecological impacts.

By providing a historical baseline, this study offers valuable insights into the NAS's past ecosystem dynamics and serves as an analog for predicting future changes under ongoing environmental pressures.