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

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.

Dermal denticle assemblages in coral reef sediments correlate with conventional shark surveys

“…wave energy) processes can mix sediments (Flessa, Cutler, & Meldahl, ; Kosnik, Hua, Kaufman, & Zawadzki, ; Olszewski, ). This mixing is a key process affecting death assemblage composition, as it blends new inputs from the living community with older buried cohorts (Kidwell, ; Tomašových, Gallmetzer, et al, ; Tomašových & Kidwell, ). For this reason, the amount of time‐averaging should be resolved and, depending on the study aims, constrained.…”

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

“…Boxplots show median ages and 25 th and 75 th percentiles, whiskers denote age minima and maxima. Down-core changes in shell ages at Piran (Gouldia minima, Corbula gibba) and Brijuni (Timoclea ovata) are reported in Tomašových et al (2019) and Schnedl et al (2018).…”

“…Several paleoecological studies have demonstrated effects of overfishing, eutrophication, hypoxia, and pollution on ecosystem composition in the northern Adriatic over the past few centuries based on historical and sediment core data (Barmawidjaja et al 1995;Sangiorgi and Donders 2004;Ferretti et al 2013;Kowalewski et al 2015;Vidović et al 2016;Gallmetzer et al 2017;Tomašových et al , 2019. However, quantitative analyses of benthic assemblages in sediment cores capturing time intervals preceding the past 500 years are rare, and the major ecological turnovers in the northern Adriatic during the entire Holocene remain poorly explored, especially in the north-eastern part (Fiorini and Vaiani 2001;Scarponi and Kowalewski 2007;Rossi and Vaiani 2008;Lotze et al 2011;Coll and Lotze 2016).…”

“…This was the lucinid Lucinella divaricata for Venice, the venerid Gouldia minima for the two Piran stations (in addition, for Piran 2, shells of the corbulid Corbula gibba were dated), and the venerid Timoclea ovata for Brijuni. For C. gibba, the calibration is described in , for G. minima in Tomašových et al (2019), and for T. ovata in Schnedl et al (2018). The calibration for L. divaricata is presented below.…”

“…In total, 564 specimens of G. minima (Piran 1 and 2), 232 specimens of C. gibba (Piran 2), 300 of T. ovata (Brijuni), and 341 of L. divaricata (Venice) were used in shell-based core geochronology. AAR measurements were calibrated with 14 C ages of 13 specimens of G.minima (Tomašových et al 2019), 12 of L.-divaricata (this study), and 13 specimens of T.-ovata , respectively. Median age and interquartile age range (corrected for calibration age error according to Dominguez et al 2016) were computed for each increment.…”