Tracing Origin and Collapse of Holocene Benthic Baseline Communities in the Northern Adriatic Sea

Abstract

The shallow northern Adriatic Sea has a long history of anthropogenic impacts that reaches back many centuries. While the effects of eutrophication, overfishing, pollution, and trawling over recent decades have been extensively studied, the major ecological turnovers during the Holocene as a whole remain poorly explored. In this study, we reconstruct ecological baselines defining benthic ecosystem composition prior to major anthropogenic changes at four stations characterized by low sedimentation and millennial-scale time averaging of molluscan assemblages. We discriminate between natural and anthropogenic drivers based on (1) stratigraphic changes in the composition of molluscan communities observed in sediment cores and (2) changes in concentrations of heavy metals, pollutants, and organic enrichment. The four 1.5-m long sediment cores reach back to the Pleistocene-Holocene boundary, allowing for a stratigraphic distinction of the major sea-level phases of the Holocene. During the transgressive phase and maximum flooding, sea-level and establishment of the modern circulation pattern determined the development of benthic communities in shallow-water, vegetated habitats with epifaunal biostromes and, in deeper waters, with bryozoan meadows. After sea-level stabilization, the composition of these baseline communities remained relatively uniform and started to change markedly only with the intensification of human impacts in the late highstand, leading to a dominance of infauna and a decline of epifauna at all sites. This profound ecological change reduced species richness, increased the abundance of infaunal suspension feeders, and led to a decline of grazers and deposit feeders. We suggest that modern soft-bottom benthic communities in the northern Adriatic Sea today do not show the high geographic heterogeneity in composition characteristic of benthos prior to anthropogenic influences.

Ecological regime shift preserved in the Anthropocene stratigraphic record

“…B 287: 20200695 supplementary material, figure S1). Estimates of increment median ages, sedimentation rates and time averaging were based on shell ages (AAR calibrated by 14 C) of four molluscan species [34][35][36][37][38][39]. Time averaging corresponds to an inter-quartile age range in years (IQR) in approximately 10-30 cm thick units [34][35][36][37]40,41].…”

“…Estimates of increment median ages, sedimentation rates and time averaging were based on shell ages (AAR calibrated by 14 C) of four molluscan species [34][35][36][37][38][39]. Time averaging corresponds to an inter-quartile age range in years (IQR) in approximately 10-30 cm thick units [34][35][36][37]40,41]. Net sedimentation rate was approximately 0.3 cm y −1 during the transgressive phase (TST) and 1-2 cm y −1 during the highstand phase (HST) at the Po prodelta, 0.2-0.4 cm y −1 during the HST at the Isonzo prodelta, and approximately 0.01 cm y −1 during the TST and HST off Istria and in the Gulf of Venice [34][35][36][37]40,41].…”

“…Time averaging corresponds to an inter-quartile age range in years (IQR) in approximately 10-30 cm thick units [34][35][36][37]40,41]. Net sedimentation rate was approximately 0.3 cm y −1 during the transgressive phase (TST) and 1-2 cm y −1 during the highstand phase (HST) at the Po prodelta, 0.2-0.4 cm y −1 during the HST at the Isonzo prodelta, and approximately 0.01 cm y −1 during the TST and HST off Istria and in the Gulf of Venice [34][35][36][37]40,41]. The uppermost sediment core increments corresponding to twentiethtwenty-first centuries do not show any signs of change in the sedimentation rate [34].…”

Radiocarbon dating supports bivalve-fish age coupling along a bathymetric gradient in high-resolution paleoenvironmental studies

“…The targeted shells and otoliths are aragonitic (Degens et al, 1969). Although Corbula has conchiolin layers, which retard shell dissolution in waters undersaturated in calcium carbonate and increase shell strength, its taphonomic pathway does not dif-fer from bivalves with shell structure similar to Donax (Gallmetzer et al, 2019). Corbula and Donax ranged in length from 2.5 to 5.6 mm (median 3.7 mm) and from 3.0 to 16.0 mm (median 5.0 mm), respectively.…”

Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

“…The post-LGM sea-level rise induced significant changes in the sedimentary environments of the northern Adriatic Sea with terrestrial environments transitioning in paralic and later shallow marine environments [22,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. In the Gulf of Trieste, where our study area is located (Figure 1), the Late Pleistocene alluvial plain transitioned into a paralic environment until open marine conditions finally prevailed approximately 10,000 years ago [22,42,44,45,[55][56][57][58][59][60][61][62][63][64].…”