Ivo Gallmetzer

Stratigraphic unmixing reveals repeated hypoxia events over the past 500 yr in the northern Adriatic Sea

In the northern Adriatic Sea and in most semi-enclosed coastal regions worldwide, hypoxia induced by eutrophication in the late 20 th century caused major die-offs of coastal marine organisms. However, ecosystem responses to hypoxia over longer centennial scales are unclear because the duration of direct observations is limited to a few decades and/or the temporal resolution of sedimentary archives is compromised by slow sedimentation and bioturbation. To assess whether perturbations of ecosystems by hypoxia recurred over centuries in the northern Adriatic Sea, we evaluate the timing and forcing of past hypoxia events based on the production history of the opportunistic, hypoxia-tolerant bivalve Corbula gibba , using 210 Pb data, radiocarbon dating, amino acid racemization, and distribution of foraminifers in sediment cores that capture the past 500 yr in the Gulf of Trieste. Unmixing the stratigraphic record on the basis of 311 shells of C. gibba , we show that the reconstructed fluctuations in abundance do not correlate with abundances in the raw stratigraphic record. We find that production of C. gibba has undergone major decadal-scale fluctuations since the 18 th century, with outbreaks corresponding to density of more than 1000 individuals per square meter. These outbreaks represent long-term phenomena in the northern Adriatic ecosystem rather than novel states characteristic of the 20 th century eutrophication. They positively correlate with centennial-scale fluctuations in sea-surface temperature, indicating that the hypoxia events were coupled with water-column stratification rather than with nutrient enrichment.

An innovative piston corer for large‐volume sediment samples

Abstract Coring is one of several standard procedures to extract sediments and their faunas from open marine, estuarine, and limnic environments. Achieving sufficiently deep penetration, obtaining large sediment volumes in single deployments, and avoiding sediment loss upon retrieval remain problematic. We developed a piston corer with a diameter of 16 cm that enables penetration down to 1.5 m in a broad range of soft bottom types, yields sufficient material for multiple analyses, and prevents sediment loss due to a specially designed hydraulic core catcher. A novel extrusion system enables very precise slicing and preserves the original sediment stratification by keeping the liners upright. The corer has moderate purchase costs and a robust and simple design that allows for a deployment from relatively small vessels as available at most marine science institutions. It can easily be operated by two to three researchers rather than by specially trained technicians. In the northern Adriatic Sea, the corer successfully extracted more than 50 cores from a range of fine mud to coarse sand, at water depths from three to 45 m. The initial evaluation of the cores demonstrated their usefulness for fauna sequences along with heavy metal, nutrient and pollutant analyses. Their length is particularly suited for historical ecological work requiring sedimentary and faunal sequences to reconstruct benthic communities over the last millennia.

Responses of molluscan communities to centuries of human impact in the northern Adriatic Sea

In sediment cores spanning ~500 years of history in the Gulf of Trieste, down-core changes in molluscan community structure are characterized by marked shifts in species and functional composition. Between the 16th and 19th century, a strong heavy metal contamination of the sediments, most notably by Hg, together with the effects of natural climatic oscillations (increased sedimentation and organic enrichment) drive community changes. Since the early 20th century up to 2013, the combined impacts of cultural eutrophication, frequent hypoxic events and intensifying bottom trawling replace heavy metal contamination and climatic factors as the main drivers. The pollution-tolerant and opportunistic bivalve Corbula gibba and the scavenging gastropod Nassarius pygmaeus significantly increase in abundance during the 20th century, while species more sensitive to disturbances and hypoxia such as Turritella communis and Kurtiella bidentata become rare or absent. An infaunal life habit and scavenging emerge as the dominant life strategies during the late 20th century. Down-core shifts in the proportional abundances of molluscan species and functional groups represent a sensitive proxy for past ecological changes and reveal a century-long anthropogenic impact as the main driver behind these processes in the northern Adriatic Sea, offering also a unique perspective for other shallow marine ecosystems worldwide.

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

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.

Molluscan benthic communities at Brijuni Islands (northern Adriatic Sea) shaped by Holocene sea-level rise and recent human eutrophication and pollution

The effects of and the interplay between natural and anthropogenic influences on the composition of benthic communities over long time spans are poorly understood. Based on a 160-cm-long sediment core collected at 44 m water depth in the NE Adriatic Sea (Brijuni Islands, Croatia), we document changes in molluscan communities since the Holocene transgression ~11,000 years ago and assess how they were shaped by environmental changes. We find that (1) a transgressive lag deposit with a mixture of terrestrial and marine species contains abundant seagrass-associated gastropods and epifaunal suspension-feeding bivalves, (2) the maximum-flooding phase captures the establishment of epifaunal bivalve-dominated biostromes in the photic zone, and (3) the highstand phase is characterized by increasing infaunal suspension feeders and declining seagrass-dwellers in bryozoan-molluscan muddy sands. Changes in the community composition between the transgressive and the highstand phase can be explained by rising sea level, reduced light penetration, and increase in turbidity, as documented by the gradual up-core shift from coarse molluscan skeletal gravel with seagrass-associated molluscs to bryozoan sandy muds. In the uppermost 20 cm (median age <200 years), however, epifaunal and grazing species decline and deposit-feeding and chemosymbiotic species increase in abundance. These changes concur with rising concentrations of nitrogen and organic pollutants due to the impact of eutrophication, pollution, and trawling in the 20th century. The late highstand benthic assemblages with abundant bryozoans, high molluscan diversity, and abundance of soft-bottom epi- and infaunal filter feeders and herbivores represent the circalittoral baseline community largely unaffected by anthropogenic impacts.

Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

Abstract. Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~ 25–50 years to ~ 10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. A.D. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

20th century increase in body size of a hypoxia-tolerant bivalve documented by sediment cores from the northern Adriatic Sea (Gulf of Trieste)

An increase in the frequency of hypoxia, mucilages, and sediment pollution occurred in the 20th century in the Adriatic Sea. To assess the effects of these impacts on bivalves, we evaluate temporal changes in size structure of the opportunistic bivalve Corbula gibba in four sediment cores that cover the past ~500 years in the northern, eutrophic part and ~10,000 years in the southern, mesotrophic part of the Gulf of Trieste. Assemblages exhibit a stable size structure during the highstand phase but shift to bimodal distributions and show a significant increase in the 95th percentile size during the 20th century. This increase in size by 2-3 mm is larger than the northward size increase associated with the transition from mesotrophic to eutrophic habitats. It coincides with increasing concentrations of total organic carbon and nitrogen, and can be related to enhanced food supply and by the tolerance of C. gibba to hypoxia.