Early anthropogenic transformation of the Danube-Black Sea system

Unravelling the anthropogenic pressures in deep waters of the N Iberian Peninsula in the last centuries through the study of sediment cores

The greatest degradation of marine ecosystems has occurred in the last two centuries, coinciding with changes in economic and production models such as the industrialisation. We reconstructed the recent history (last centuries) of marine pollutants (metals and microplastics) in the deep sea of the NW Mediterranean and the Cantabrian Sea (NE Atlantic), analysing 8 sites (cores) at depths between 87 and 1151 m with different levels of terrestrial/oceanic influence. The 210Pb dating showed higher sedimentation rates (MAR) in the upper slope and/or closer to the mainland (0.063-0.078 g/cm2/yr off Barcelona; 0.107 g/cm2/yr in Mallorca at 420 m depth) than in deeper and more open marine stations (MAR = 0.054-0.035 g/cm2/yr), including the Valencia seamount (VS). In terms of metal pollution history, Hg and Pb were good markers of industrial activity, at all stations, including those at 1100-1150 m off Mallorca and on the VS summit. As (arsenic) peaked in the late 19th-early 20th centuries, due to the use of coal as a fuel in steamboats. The role of other metals (e.g. V, Cr, Cd, or Cu) is interpreted locally, depending on the type of industry developed in each area. The rapid and widespread emergence and use of microplastics also made them a good historical marker for sediments. The lithogenic metals Li and Al were good tracers of natural changes (freshwater input, precipitation regime) as their concentration in deep sediments is linked to advective fluxes reaching the seafloor. Both showed a general decline after the middle of the 20th century (1960s), due to a reduction in rainfall and river discharge, as well as an increase in river damming. The observed changes can therefore be explained by a combination of natural variability and the impact of human activities.

A 600 years sediment record of heavy metal pollution history in the Danube Delta

Heavy metal pollution in the Danube Delta (in sediments, water and living organisms) has recently received increasing attention due to its impact on ecosystems health and water quality. However, long term records of heavy metal contamination are not available to date. In this study radiometric dating and geochemical analyses for major elements (Al, Fe, Ca and S) and metals (Cu, Zn, Pb, Ni, Cr and Cd) were performed on the top 4 m of a 9-m sediment core retrieved from the alluvial plain of Sulina distributary channel aiming to reconstruct the heavy metal geological background and contamination history and discuss the possible origins (natural vs. anthropogenic) of metals and the main factors driving their temporal variation. Chronological analysis revealed that the top 4 m of the core span the last ~600 years. Three distinct sediment units (U1: 400-200, U2: 200-140, U3: 140-15) were identified based on the downcore element concentration variation. The lower unit (400-200 cm, ~1450-1700 CE) shows an upward increase of Cu, Zn, Pb, Ni, Cr and Cd metal contents, which are strongly correlated with Al. Enrichment factor (EF) analysis indicates that metals detected in this unit are derived primarily from natural sources. In contrast, metals show elevated EF values within the middle (200-140 cm, ~1700-1770 CE) and upper unit (140-15 cm, ~1770 CE to present). The highest degrees of enrichment of Cu, Zn and Cd occur in the peat layer of the middle unit (U2) which displays higher organic carbon and sulfur contents, indicating that diagenetic enrichment of detected metals occurs under reducing condition. Overall, sediment contamination is moderate while the level increases with time. This study provides new insights into the metal contamination history of deltaic environments and yields baseline values for heavy metal contents in pristine sediments deposited prior to the onset of anthropogenic impact.

Lake Sedimentary DNA Research on Past Terrestrial and Aquatic Biodiversity: Overview and Recommendations

The use of lake sedimentary DNA to track the long-term changes in both terrestrial and aquatic biota is a rapidly advancing field in paleoecological research. Although largely applied nowadays, knowledge gaps remain in this field and there is therefore still research to be conducted to ensure the reliability of the sedimentary DNA signal. Building on the most recent literature and seven original case studies, we synthesize the state-of-the-art analytical procedures for effective sampling, extraction, amplification, quantification and/or generation of DNA inventories from sedimentary ancient DNA (sedaDNA) via high-throughput sequencing technologies. We provide recommendations based on current knowledge and best practises.

Climate and land-use as the main drivers of recent environmental change in a mid-altitude mountain lake, Romanian Carpathians

Recent decades have been marked by unprecendented environmental changes which threaten the integrity of freshwater systems and their ecological value. Although most of these changes can be attributed to human activities, disentagling natural and anthropogenic drivers remains a challenge. In this study, surface sediments from Lake Ighiel, a mid-altitude site in the Carpathian Mts (Romania) were investigated following high-resolution sedimentological, geochemical, environmental magnetic and diatom analyses supported by historical cartographic and documentary evidence. Our results suggest that between 1920 and 1960 the study area experienced no significant anthropogenic impact. An excellent correspondence is observed between lake proxy responses (e.g., growth of submerged macrophytes, high detrital input, shifts in diatom assemblages) and parameters tracking natural hydroclimate variability (e.g., temperature, NAO). This highlights a dominant natural hydroclimatic control on the lacustrine system. From 1960 however, the depositional regime shifted markedly from laminated to homogenous clays; since then geochemical and magnetic data document a trend of significant (and on-going) subsurface erosion across the catchment. This is paralleled by a shift in lake ecosystem conditions denoting a strong response to an intensified anthropogenic impact, mainly through forestry. An increase in detrital input and marked changes in the diatom community are observed over the last three decades, alongside accelerated sedimentation rates following enhanced grazing and deforestation in the catchment. Recent shifts in diatom assemblages may also reflect forcing from atmospheric nitrogen (N) deposition, a key recent drive of diatom community turnover in mountain lakes. In general, enhanced human pressure alongside intermittent hydroclimate forcing drastically altered the landscape around Lake Ighiel and thus, the sedimentation regime and the ecosystem’s health. However, paleoenvironmental signals tracking natural hydroclimate variability are also clearly discernible in the proxy data. Our work illustrates the complex link between the drivers of catchment-scale impacts on one hand, and lake proxy responses on the other, highlighting the importance of an integrated historical and palaeolimnological approach to better assess lake system changes.

The human ecology of the Danube Delta: A historical and cartographic perspective

We present a case of environmental transformation, in the Romanian Danube Delta, driven by the interplay of state power, technological intervention, geomorphological processes, and local practices. Through the presentation of a cartographic archive (1856-2017), together with participant observation and historical research, we detail the various stages of transformation in the deltaic environment and show the relative interplay of driving forces. We show that each transformation of the Delta is at the same time an imposition from without and an adaptation from within, a move of consolidation of state power and a resistance to being fully incorporated. We show how in the history of this particular environment, the main drivers of change pass from being of a geomorphological nature to being related to the use of state power. We detail three stages in the transformation of the delta, through which the conceptualization of, and interventions in, the environment, go from a borderland to be secured, to a rich exploitation ground, to an ecological marvel to be protected. We argue that this kind of analysis can be particularly relevant for the governance of protected areas.

World's largest dam removal reverses coastal erosion

Coastal erosion outpaces land generation along many of the world's deltas and a significant percentage of shorelines, and human-caused alterations to coastal sediment budgets can be important drivers of this erosion. For sediment-starved and erosion-prone coasts, large-scale enhancement of sediment supply may be an important, but poorly understood, management option. Here we provide new topographic measurements that show patterns and trends of beach accretion following the restoration of sediment supply from a massive dam removal project. River sediment was initially deposited in intertidal-to-subtidal deltaic lobes, and this sediment was reworked by ocean waves into subaerial river mouth bars over time scales of several months. These river mouth bars welded to the shoreline and then initiated waves of sediment accretion along adjacent upcoast and downcoast beaches. Although the downcoast shoreline has a high wave-angle setting, the sedimentation waves straightened the downcoast shoreline rather than forming self-organized quasi-periodic instabilities, which suggests that simple coastal evolution theory did not hold under these conditions. Combined with other mega-nourishment projects, these findings provide new understanding of littoral responses to the restoration of sediment supplies.

Holocene paleodepositional changes reflected in the sedimentary microbiome of the Black Sea

Subsurface microbial communities are generally thought to be structured through in situ environmental conditions such as the availability of electron acceptors and donors and porosity, but recent studies suggest that the vertical distribution of a subset of subseafloor microbial taxa, which were present at the time of deposition, were selected by the paleodepositional environment. However, additional highly resolved temporal records of subsurface microbiomes and paired paleoenvironmental reconstructions are needed to justify this claim. Here, we performed a highly resolved shotgun metagenomics survey to study the taxonomic and functional diversity of the subsurface microbiome in Holocene sediments underlying the permanently stratified and anoxic Black Sea. Obligate aerobic bacteria made the largest contribution to the observed shifts in microbial communities associated with known Holocene climate stages and transitions. This suggests that the aerobic fraction of the subseafloor microbiome was seeded from the water column and did not undergo post-depositional selection. In contrast, obligate and facultative anaerobic bacteria showed the most significant response to the establishment of modern-day environmental conditions 5.2 ka ago that led to a major shift in planktonic communities and in the type of sequestered organic matter available for microbial degradation. No significant shift in the subseafloor microbiome was observed as a result of environmental changes that occurred shortly after the marine reconnection, 9 ka ago. This supports the general view that the marine reconnection was a gradual process. We conclude that a high-resolution analysis of downcore changes in the subseafloor microbiome can provide detailed insights into paleoenvironmental conditions and biogeochemical processes that occurred at the time of deposition.

Exceptionally high levels of lead pollution in the Balkans from the Early Bronze Age to the Industrial Revolution

The Balkans are considered the birthplace of mineral resource exploitation and metalworking in Europe. However, since knowledge of the timing and extent of metallurgy in southeastern Europe is largely constrained by discontinuous archaeological findings, the long-term environmental impact of past mineral resource exploitation is not fully understood. Here, we present a high-resolution and continuous geochemical record from a peat bog in western Serbia, providing a clear indication of the extent and magnitude of environmental pollution in this region, and a context in which to place archaeological findings. We observe initial evidence of anthropogenic lead (Pb) pollution during the earliest part of the Bronze Age [∼3,600 years before Common Era (BCE)], the earliest such evidence documented in European environmental records. A steady, almost linear increase in Pb concentration after 600 BCE, until ∼1,600 CE is observed, documenting the development in both sophistication and extent of southeastern European metallurgical activity throughout Antiquity and the medieval period. This provides an alternative view on the history of mineral exploitation in Europe, with metal-related pollution not ceasing at the fall of the western Roman Empire, as was the case in western Europe. Further comparison with other Pb pollution records indicates the amount of Pb deposited in the Balkans during the medieval period was, if not greater, at least similar to records located close to western European mining regions, suggestive of the key role the Balkans have played in mineral resource exploitation in Europe over the last 5,600 years.

A History of Pebbles and Silt – Fluvial Sediment Transport, Hydropower and Technical Expertise at the Austrian Danube and its Tributaries

The paper investigates experts’ perceptions of hydropower, sediment regime, and their interaction in the 20th century with an environmental historical approach, based on various case studies at both the Danube River and one of its tributaries, and on a review of contemporary literature authored by engineers. Results show that questions of sediment continuity have engaged planners of hydropower plants since the advent of this technology, and decisions were at any time influenced by multiple interests (navigation, electricity demand, nature conservation). In such an intricate fluvial landscape, phenomena like reservoir sedimentation and riverbed incision can be approached as “legacies” of past technical interventions, which limit the options of current and future river management.

The interplay between tectonics, sediment dynamics and gateways evolution in the Danube system from the Pannonian Basin to the western Black Sea

Understanding the natural evolution of a river-delta-sea system is important to develop a strong scientific basis for efficient integrated management plans. The distribution of sediment fluxes is linked with the natural connection between sediment source areas situated in uplifting mountain chains and deposition in plains, deltas and, ultimately, in the capturing oceans and seas. The Danube River-western Black Sea is one of the most active European systems in terms of sediment re-distribution that poses significant societal challenges. We aim to derive the tectonic and sedimentological background of human-induced changes in this system and discuss their interplay. This is obtained by analysing the tectonic and associated vertical movements, the evolution of relevant basins and the key events affecting sediment routing and deposition. The analysis of the main source and sink areas is focused in particular on the Miocene evolution of the Carpatho-Balkanides, Dinarides and their sedimentary basins including the western Black Sea. The vertical movements of mountains chains created the main moments of basin connectivity observed in the Danube system. Their timing and effects are observed in sediments deposited in the vicinity of gateways, such as the transition between the Pannonian/Transylvanian and Dacian basins and between the Dacian Basin and western Black Sea. The results demonstrate the importance of understanding threshold conditions driving rapid basins connectivity changes superposed over the longer time scale of tectonic-induced vertical movements associated with background erosion and sedimentation. The spatial and temporal scale of such processes is contrastingly different and challenging. The long-term patterns interact with recent or anthropogenic induced modifications in the natural system and may result in rapid changes at threshold conditions that can be quantified and predicted. Their understanding is critical because of frequent occurrence during orogenic evolution, as commonly observed in the Mediterranean area and discussed elsewhere.

Historical ecology of riverine fish in Europe

The temporal dynamic of riverine ecosystems and their fish communities and populations has been addressed in ecological theory and management for several decades. A growing number of case studies on the historic development especially of European and North American rivers have been published. Nonetheless, a theoretical debate about the contributions and limits of historical approaches and interdisciplinary co-operation is lacking. This article presents a brief overview of the role of history in river and fish ecology and suggests historical ecology as a scientific field that can offer a framework for future research. Based on case studies compiled in this special issue on the "Historical ecology of riverine fish in Europe", we draw conclusions on long-term changes of fish communities, on fisheries, aquatic ecosystem management and past habitat alterations and the potential of archaeological remains and written sources to study them. We discuss how modelling of historical fish data can help elucidate the effects of climate change and human influences on rivers and fish. Finally, we account for the necessity to consider appropriate spatial and temporal scales. In conclusion we call for future comparative studies on continental and global scales and methodological development, which can benefit especially from recent advances in marine historical ecology. We suggest that future interdisciplinary studies of ecologists, hydrologists, historians and archaeologists can reveal the history of riverine ecosystems as socio-ecological systems, addressing both their natural dynamics and human dimension. Such an endeavor can also support developing management plans for habitat restoration and conservation against the background of global change.

Radiometric, SEM and XRD investigation of the Chituc black sands, southern Danube Delta, Romania

The black sand of the Chituc marine sand bank, northern of the city of Navodari (Romania), presents anomalous high radioactivity. Field measurements recorded in some places dose rate up to 200 nSv/h, significantly overpassing the average value of 44 ± 20 nSv/h along the entire Southern sector of Romanian Black Sea shore. Gamma ray spectrometry performed on both Slanic-Prahova Underground Low Background Laboratory and Geological Institute of Romania Radiometric Facilities showed with clarity the dominance of (228)Ac radioisotope in the 50 microns fraction together with the (226)Ra and traces of (40)K. No significant amount of anthropogenic (137)Cs was identified. Based on radiometric as well as on SEM-EDAX and XRD determinations we come to the conclusion that the evidenced radioactivity could be attributed to both uranium and thorium series in the zircon and monazite fractions and to a lesser extent to potassium in the feldspars.

Fluvial response to climate variations and anthropogenic perturbations for the Ebro River, Spain in the last 4,000 years

Fluvial sediment discharge can vary in response to climate changes and human activities, which in return influences human settlements and ecosystems through coastline progradation and retreat. To understand the mechanisms controlling the variations of fluvial water and sediment discharge for the Ebro drainage basin, Spain, we apply a hydrological model HydroTrend. Comparison of model results with a 47-year observational record (AD 1953-1999) suggests that the model adequately captures annual average water discharge (simulated 408 m(3)s(-1) versus observed 425 m(3)s(-1)) and sediment load (simulated 0.3 Mt yr(-1) versus observed 0.28 ± 0.04 Mt yr(-1)) for the Ebro basin. A long-term (4000-year) simulation, driven by paleoclimate and anthropogenic land cover change scenarios, indicates that water discharge is controlled by the changes in precipitation, which has a high annual variability but no long-term trend. Modeled suspended sediment load, however, has an increasing trend over time, which is closely related to anthropogenic land cover variations with no significant correlation to climatic changes. The simulation suggests that 4,000 years ago the annual sediment load to the ocean was 30.5 Mt yr(-1), which increased over time to 47.2 Mt yr(-1) (AD 1860-1960). In the second half of the 20th century, the emplacement of large dams resulted in a dramatic decrease in suspended sediment discharge, eventually reducing the flux to the ocean by more than 99% (mean value changes from 38.1 Mt yr(-1) to 0.3 Mt yr(-1)).

Evolution of the plankton paleome in the Black Sea from the Deglacial to Anthropocene

The complex interplay of climate shifts over Eurasia and global sea level changes modulates freshwater and saltwater inputs to the Black Sea. The dynamics of the hydrologic changes from the Late Glacial into the Holocene remain a matter of debate, and information on how these changes affected the ecology of the Black Sea is sparse. Here we used Roche 454 next-generation pyrosequencing of sedimentary 18S rRNA genes to reconstruct the plankton community structure in the Black Sea over the last ca. 11,400 y. We found that 150 of 2,710 species showed a statistically significant response to four environmental stages. Freshwater chlorophytes were the best indicator species for lacustrine conditions (>9.0 ka B.P.), although the copresence of previously unidentified marine taxa indicated that the Black Sea might have been influenced to some extent by the Marmara Sea since at least 9.6 ka calendar (cal) B.P. Dinoflagellates, cercozoa, eustigmatophytes, and haptophytes responded most dramatically to the gradual increase in salinity after the latest marine reconnection and during the warm and moist mid-Holocene climatic optimum. According to paired analysis of deuterium/hydrogen (D/H) isotope ratios in fossil alkenones, salinity increased rapidly with the onset of the dry Subboreal after ~5.2 ka B.P., leading to an increase in marine fungi and the first occurrence of marine copepods. A gradual succession of dinoflagellates, diatoms, and chrysophytes occurred during the refreshening after ~2.5 ka cal B.P. with the onset of the cool and wet Subatlantic climate and recent anthropogenic perturbations.

Man made deltas

The review of geochronological and historical data documents that the largest southern European deltas formed almost synchronously during two short intervals of enhanced anthropic pressure on landscapes, respectively during the Roman Empire and the Little Ice Age. These growth phases, that occurred under contrasting climatic regimes, were both followed by generalized delta retreat, driven by two markedly different reasons: after the Romans, the fall of the population and new afforestation let soil erosion in river catchments return to natural background levels; since the industrial revolution, instead, flow regulation through river dams overkill a still increasing sediment production in catchment basins. In this second case, furthermore, the effect of a reduced sediment flux to the coasts is amplified by the sinking of modern deltas, due to land subsidence and sea level rise, that hampers delta outbuilding and increases the vulnerability of coastal zone to marine erosion and flooding.