A decline in molluscan carbonate production driven by the loss of vegetated habitats encoded in the Holocene sedimentary record of the Gulf of Trieste.

Adam Tomašových,Borut Mavrič,Alexandra Haselmair,Ivo Gallmetzer,Darrell S Kaufman,Martin Zuschin

doi:10.1111/sed.12516

Adam Tomašových, Borut Mavrič + Show 4 more

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https://doi.org/10.1111/sed.12516

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Abstract

Carbonate sediments in non‐vegetated habitats on the north‐east Adriatic shelf are dominated by shells of molluscs. However, the rate of carbonate molluscan production prior to the 20th century eutrophication and overfishing on this and other shelves remains unknown because: (i) monitoring of ecosystems prior to the 20th century was scarce; and (ii) ecosystem history inferred from cores is masked by condensation and mixing. Here, based on geochronological dating of four bivalve species, carbonate production during the Holocene is assessed in the Gulf of Trieste, where algal and seagrass habitats underwent a major decline during the 20th century. Assemblages of sand‐dwelling Gouldia minima and opportunistic Corbula gibba are time‐averaged to >1000 years and Corbula gibba shells are older by >2000 years than shells of co‐occurring Gouldia minima. This age difference is driven by temporally disjunct production of two species coupled with decimetre‐scale mixing. Stratigraphic unmixing shows that Corbula gibba declined in abundance during the highstand phase and increased again during the 20th century. In contrast, one of the major contributors to carbonate sands – Gouldia minima – increased in abundance during the highstand phase, but declined to almost zero abundance over the past two centuries. Gouldia minima and herbivorous gastropods associated with macroalgae or seagrasses are abundant in the top‐core increments but are rarely alive. Although Gouldia minima is not limited to vegetated habitats, it is abundant in such habitats elsewhere in the Mediterranean Sea. This live–dead mismatch reflects the difference between highstand baseline communities (with soft‐bottom vegetated zones and hard‐bottom Arca beds) and present‐day oligophotic communities with organic‐loving species. Therefore, the decline in light penetration and the loss of vegetated habitats with high molluscan production traces back to the 19th century. More than 50% of the shells on the sea floor in the Gulf of Trieste reflect inactive production that was sourced by heterozoan carbonate factory in algal or seagrass habitats.

Highlights

Depositional models of temperate carbonate ramps with heterozoan skeletal assemblages in semienclosed water bodies are primarily based on environments in the Mediterranean Sea (Betzler et al, 1997, 2011; Pomar et al, 2004; Wilson & Vecsei, 2005; Tropeano & Spalluto, 2006)
It remains unclear whether communities inhabiting mixed or carbonate sediments were affected by similar dynamics and whether these shifts were associated with reduced carbonate production, for example, owing to high mortality of large bivalves (Hrs-Brenko, 1980; Peharda et al, 2006; Mautner et al, 2018), loss of macroalgal and seagrass habitats (Falace et al, 2010; Orlando-Bonaca & Mavric, 2014; Ivesa et al, 2016) and/or reduced carbonate preservation owing to eutrophication-driven degradation of carbonate shells (Hallock, 1988; Lescinsky et al, 2002)
Age unmixing and comparison of sea floor and living assemblages reveal that the rate of molluscan production in the southern Gulf of Trieste declined over the past two centuries due to the loss of species associated with vegetation and loss of epifaunal suspension-feeders, with present-day molluscan production being largely dominated by deposit and detritus feeders

Summary

Introduction

Depositional models of temperate carbonate ramps with heterozoan skeletal assemblages in semienclosed water bodies are primarily based on environments in the Mediterranean Sea (Betzler et al, 1997, 2011; Pomar et al, 2004; Wilson & Vecsei, 2005; Tropeano & Spalluto, 2006). Ecological and palaeoecological analyses performed in the northern Adriatic Sea detected that the composition of benthic communities inhabiting siliciclastic muds changed towards the dominance of opportunistic species during the 20th century (Crema et al, 1991; Barmawidjaja et al, 1995; Gallmetzer et al, 2017; Tomasovych et al, 2018) It remains unclear whether communities inhabiting mixed or carbonate sediments were affected by similar dynamics and whether these shifts were associated with reduced carbonate production, for example, owing to high mortality of large bivalves (Hrs-Brenko, 1980; Peharda et al, 2006; Mautner et al, 2018), loss of macroalgal and seagrass habitats (Falace et al, 2010; Orlando-Bonaca & Mavric, 2014; Ivesa et al, 2016) and/or reduced carbonate preservation owing to eutrophication-driven degradation of carbonate shells (Hallock, 1988; Lescinsky et al, 2002). Such criteria are insufficient when a significant decline in production occurred over the past decades or few centuries

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