Holocene ecosystem shifts and human-induced loss of Arca and Ostrea shell beds in the north-eastern Adriatic Sea

Abstract

The molluscan assemblages in a sediment core from the north-eastern Adriatic show significant compositional changes over the past 10,000yrs related to (1) natural deepening driven by the post-glacial sea-level rise, (2) increasing abundance of skeletal sand and gravel, and (3) anthropogenic impacts. The transgressive phase (10,000-6000 BP) is characterized by strongly time-averaged communities dominated by infaunal bivalves. During the early highstand (6000-4000 BP), the abundance of epifaunal filter feeders and grazers increases, and gastropods become more important. Epifaunal dominance culminates during the late highstand (4000-2000 BP) with the development of extensive shell beds formed by large-sized Arca noae and Ostrea sp. bivalves. This community persists until the early 20th century, when it falls victim to multiple anthropogenic impacts, mainly bottom trawling, and is substituted by an infauna-dominated community indicative of instability, disturbance and organic enrichment. The re-establishment of this unique shell-bed ecosystem can be a goal for restoration efforts.

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

“…Both stations share a coarseningupward trend in grain size and an up-core increase in average shell size ( Fig. 6; Mautner et al 2018). The proportion of mud in the deeper part of the core (150-75 cm) is 70-80% and decreases to less than 20% in the uppermost 25 cm in favor of a sand and gravel fraction consisting mainly of mollusk shells, with the addition of echinoderm and crustacean fragments and foraminiferal tests.…”

“…Changes in molluscan composition at Piran 2 and Brijuni were analyzed by Mautner et al (2018) and Schnedl et al (2018).…”

“…With the establishment of the modern circulation system in the northern Adriatic at the end of the TST7 ,000-6,000 years ago (Asioli et al 1996), stronger bottom currents led to an increased winnowing of finer particles, to reduced sedimentation rates and thus to stabilization of the seafloor favoring colonization by epifauna . Once established, epifauna-dominated communities were enhanced by positive feedback effects such as (1) the positive dependence of larval survivorship on the abundance of adult conspecifics and (2) taphonomic feedback processes by which dead hard parts of epifauna facilitate the settlement of other epifauna and inhibit burrowing activities of infauna at the same time (Kidwell and Jablonski 1983;Mautner et al 2018). These developments culminate with the establishment of extensive banks with Arca and Modiolus between 4,000 and 2,000 years ago.…”

“…Both Piran stations fall within this area. However, with the modernization of fisheries between the two world wars (boats were increasingly equipped with motors, and new, larger and more destructive trawls were employed), the Arca beds were soon completely eradicated, and by the 1950s the last Arca communities had vanished from northern Adriatic soft bottoms (Fortibuoni 2009;Lotze et al 2011;Mautner et al 2018). A compilation of catch data for Arca noae in the northern Adriatic illustrates the population crash around 1950 and the dramatic resource exploitation in the preceding years (Fig.…”

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].…”