Establishing ecological baselines for benthic communities in the southern Gulf of Trieste (northern Adriatic Sea) using molluscan death assemblages

Abstract

The northern Adriatic Sea is one of the most degraded marine ecosystems worldwide and has been under anthropogenic influence for many centuries. Reconstructing pre-human impact benthic communities is vital to establish baseline conditions, allowing a better understanding of how anthropogenic activities and climate warming have influenced and altered marine ecosystems over time. This historical context helps to assess the extent of anthropogenic influence, guiding future conservation efforts and management strategies. Our primary goal is to uncover the composition of pre-human impact benthic communities on the shallow shelf along the Slovenian coast (Gulf of Trieste) and to reconstruct the natural state of these habitats. This investigation involved systematic grab sampling of subtidal soft substrates across three distinct areas, encompassing eight stations in the Bay of Koper and Bay of Piran, at water depths ranging from seven to ten meters. We focus on analyzing the composition of mollusc death assemblages, time-averaged accumulations of empty shells present in the uppermost mixed layer of seafloor sediments. Due to high durability of these skeletal remains and generally low sedimentation rates in shelf environments, death assemblages integrate shells over time spans of decades to millennia, making them valuable natural archives of past community states. Our results show that samples collected near the major port of Koper are strongly dominated by the opportunistic and stress-tolerant bivalve Varicorbula gibba, and show significantly lower species richness compared to stations located further away from this potential source of environmental disturbance. Moreover, assemblages from the protected area of the Debeli rtič are compositionally distinct from those of other areas. These patterns likely reflect the combined effects of differential levels of anthropocentric impacts, as well as differences in substrate characteristics and the magnitude of time averaging of the death assemblages among sampling stations. The obtained community data will be cross-checked with live assemblage data based on the systematic benthic monitoring efforts conducted over the last 15 years (2005–2020) to ascertain changes in the mollusc community. The examination of modern and historic data will provide a robust long-term baseline that is essential for understanding and quantifying the impacts of significant human-induced modern stressors on the northern Adriatic marine ecosystem.