Eutrophication history and organic carbon burial rate recorded in sediment cores from the Mar Piccolo of Taranto (Italy)

Abstract

During the second half of the twentieth century, coastal lagoons in densely populated regions experienced eutrophication due to excessive nutrient inputs. Detrimental effects, including hypoxia/anoxia and harmful algae blooms, have occurred in many Mediterranean lagoons, but their trophic evolution is poorly documented. The lack of adequate monitoring data can partly be offset by examining sedimentary records. In the Mar Piccolo, a lagoon comprising two basins near Taranto (Italy), eutrophication has followed population growth and pollution resulting from naval activities and massive industrialisation. Based on ²¹⁰Pb-dated sediment cores, continuous in situ density profiles obtained with computed tomography, organic carbon (OC) and total nitrogen (TN) content and OC and TN isotopic signatures, this paper reconstructs the history of eutrophication, discusses the sources of organic matter and provides an estimate of the OC burial rate before and during the eutrophic period. OC burial increased in the period 1928-1935 and peaked in the decade 1960-1970. OC and TN content were still high in the surface sediments collected in 2013, despite partial diversion of sewage outfalls in the period 2000-2005. The divergent δ¹³C and δ¹⁵N signatures of the two basins during the eutrophic period suggest they were affected by different nutrient sources. The OC burial rate during the eutrophic phase (≈ 46 g m^-2 y^-1) was close to the world median value for lagoon sediments, and was about twice the burial rate recorded in the preceding oligotrophic phase.

Keywords: Burial rate; Computed tomography; Eutrophication proxies; Lagoon; Organic carbon; Sediment; Total nitrogen.

Fig. 1

Map of the Mar Piccolo, showing the location of cores, bottom current field (after De Pascalis et al. 2016), mussel-growing areas and sewage outfalls (past and still active after 2005) according to Caroppo et al. (2016). Sampling site nomenclature follows that of the RITMARE project (Cardellicchio et al. 2016)

Fig. 2

Depth profiles for in-situ dry density (ρ^s), nutrient element content and related isotopic signatures (0–50 cm depth) in dated cores from 1^st basin (core 1F) and 2^nd basin (core 2C). For non-sampled intervals, ρ^s values were inferred from CT (AIP) measurements. Time scales are derived from ²¹⁰Pb dating, assuming constant mass accumulation rates (MAR in g cm⁻² y⁻¹). The corresponding mean sediment accumulation rates (SAR, in cm y⁻¹) are also given. Dashed horizontal lines depict the probable depth and corresponding date of the onset of eutrophication

Fig. 3

History of population growth in Taranto (ISTAT 2022) compared to sediment organic carbon content in the 1^st (core 1F) and 2^nd (core 2C) basins

Fig. 4

a CT images of the upper 50 cm of sediment cores and the corresponding in situ dry density (ρ^s) profiles reflecting the development of eutrophication in three cores from the 2^nd basin of Mar Piccolo. The dark sections correspond to low density, OM-rich intervals, while the light-coloured intervals correspond to high-density sediments or shell fragments. The depths of the magnetic susceptibility peaks (χ peaks) are depicted with light blue dots and the flood layer sections are marked with arrows. b Black and white photograph of the upper section of core 2C cut in half longitudinally showing the abundance of OM with the presence of filamentous algae remains. c Time scale for core 2C obtained with the ²¹⁰Pb method. d Schematic representation of the evolution of eutrophication and related effects in the 2^nd basin, based on our observations and literature data. e Detailed CT profile (mean of 4 profiles, in maximum intensity projection (MIP) setting) and enlarged CT image from core 2C revealing density changes to the sedimentary sequence that was formed as a result of the historic flood in 1883. f Grain-size spectra of samples collected at four depth intervals with respect to the sedimentary sequence shown in (e); the bimodal distribution in the 44–46 cm sample reflects the heterogeneity of the sampling interval (sand and underlying silty clay)

Fig. 5

Organic carbon, total nitrogen content, OC/TN ratio, δ¹³C, δ¹⁵N and CaCO₃ content in sediment cores from the Mar Piccolo

Fig. 6

Plots of δ¹⁵N against δ¹³C, δ¹⁵N against the OC/TN ratio and δ¹³C against the OC/TN ratio in samples grouped by depth (upper and lower sections) and basin (1^st and 2^nd), as defined in the text. Symbols marking SOM 0–3 cm (sediment organic matter at depths of 0–3 cm) show mean values with standard deviations obtained from surface sediments by Bongiorni et al. (2016) in June 2013 (J) and April (A) 2014 from two sites in the 1^st and 2^nd basins. Symbols marking SOM 0–2 cm show mean values with standard deviations obtained from surface sediments in this study (sampled in June 2013, 7 sites in the 1^st basin and 3 sites in the 2^nd basin)

Fig. 7

Box plots of measured variables showing medians, quartiles, minima and maxima of measured parameters for all samples, upper and lower sections of sediments (as defined in the text) and upper and lower sections of sediments in the 1^st and 2^nd basins of the Mar Piccolo. Numerical statistics are shown in Table S4.1 in Supplementary Information