Atmospheric deposition of biologically relevant trace metals in the eastern Adriatic coastal area

Highlights

• Simultaneous trace metal (TM) determination in atmospheric and sea surface samples.

• Aerosol and bulk deposition TM levels impacted by local/long-range human activities.

• BB (biomass burning) and dust events may cause most of the atmospheric TM deposition.

• Sea surface microlayer enrichment of TM followed BB events at coastal Adriatic site.

• BB followed by precipitation impacted TM levels and partitioning within sea surface.

Abstract

Aerosol (PM₁₀), bulk deposition, sea surface microlayer (SML) and underlying water (ULW) samples were collected simultaneously during a field campaign at the middle Adriatic coastal site between February and July 2019, to assess the impact of atmospheric deposition (AD) of biologically relevant trace metals (TM) (Zn, Cu, Co, Ni, Cd and Pb) on the sea surface responses in an oligotrophic coastal region. Anthropogenic emissions from continental Europe, alongside local/regional domestic heating, likely affected the concentrations of Zn, Cd and Pb in aerosols during winter-early spring, while traffic emissions during the tourist season impacted Ni, Co and Cu aerosol concentrations. Additionally, open-fire biomass burning (BB) episodes caused considerable TM concentration increases, while Saharan dust intrusion in spring led to a 10-fold increase in Co concentrations in PM₁₀ samples. These intensive episodes significantly affected the bulk deposition fluxes of TMs, showing that a small number of such extreme events, common to Mediterranean coastal areas, could be responsible for most of the AD. Enrichments and concentrations of total TMs in SML samples collected following BB events indicated that such events, along with high precipitation, influenced TM partitioning in surface water layers. We estimated that AD represents a significant source of TM to the shallow middle Adriatic coastal area, highlighting the need to further explore the atmosphere-sea surface links, to expand our understanding of the biogeochemistry of these important micronutrients and pollutants, including their impact on the aquatic community.

Introduction

Atmospheric deposition (AD) is recognized as a significant, and in some cases dominant, pathway by which anthropogenic and natural material is transported from the continent to coastal and open seas (Guerzoni et al., 1999). Once deposited through dry and wet processing, atmospheric aerosols provide the aqueous ecosystems with an external source of typical macro (N and P) and micronutrients (Fe), but also trace metals (TM) such as Zn, Co, Ni, Cd, Cu, Pb and Mn, whose importance is being acknowledged due to their impact on marine organisms (Baker et al., 2016; Bonnet and Guieu, 2006; Browning et al., 2017; Desboeufs et al., 2018; Hassler et al., 2012; Mahowald et al., 2018). TMs are essential to the aquatic microbial community as they limit or co-limit phytoplankton growth, act as enzymatic co-factors in various biochemical cycles, and participate in the uptake and metabolism processes of nitrogen, silicate, carbon, and phosphate (Lee et al., 1995, Morel and Price, 2003).

The AD influx of TMs may be particulary important for oligotrophic environments, which account for up to 60% of the global ocean (Maranon et al., 2003). The Mediterranean Sea is a semi-enclosed oligotrophic sea that is under constant influence of anthropogenic and natural emissions brought by air masses from Europe, Africa and Asia (Kanakidou et al., 2011) and also receives the highest rate of aeolian material in the global ocean (Guerzoni et al., 1999).

An important source of crust-dominated aerosols containing macro- and micronutrients such as P, Fe, Zn, is Saharan dust, which is transported to the Mediterranean basin mainly in spring and summer in the form of non-continuous dust pulses, (Guieu et al., 2002; Guerzoni et al., 1997; Ridame et al., 1999). Despite the general paradigm that TMs from anthropogenic material are more soluble compared to those from lithogenic material, there are large uncertainties associated with TM solubility in atmospheric deposition and in marine surface waters (Baker et al., 2016; Chance et al., 2015; Fishwick et al., 2018; Mahowald et al., 2018), implying a potentially significant impact of dust TMs on the biogeochemistry of Mediterranean surface waters.

The number of biomass burning (BB) events from wildfires increased drastically in recent decades throughout the Mediterranean area, causing severe economic and environmental damage (Pausas et al., 2004). Estimates of future climate change impacts suggest that the burned area and carbon emissions from BB will increase, particularly in the Mediterranean basins, the Balkan regions and Eastern Europe (Migliavacca et al., 2013). Consequently, external AD inputs should become even more important for oligotrophic Mediterranean regions with increased aerosol loads, including plumes of Saharan desert dust (Moulin et al., 1997) and a shallower mixed layer depth, due to increasing temperatures.

The majority of data related to AD impacts generated in the Mediterranean so far have been conducted in its western and eastern regions (e.g., Desboeufs et al., 2018; Eker-Develi et al., 2006; Guieu et al., 2010; Heimburger et al., 2010), while only a limited number of studies are related to the Adriatic Sea sub-basin, and are generally restricted to its northern part (Contini et al., 2012; Rossini et al., 2001, 2005; Čačković et al., 2009). The Adriatic Sea is under the combined influence of local, regional and long-distance sources of natural and anthropogenic emissions. North Adriatic is considered one of the most eutrophic parts of the Mediterranean due to strong riverine inputs (Milliman et al., 2016; Siokou-Frangou et al., 2010). In contrast, the middle and southern Adriatic present oligotrophic regions (Milliman et al., 2016; Cukrov et al., 2008; Ljubešić et al., 2007), where AD is expected to significantly affect primary production (Richon et al., 2017, 2018). Oligotrophic Adriatic regions are impacted by Mediterranean arid conditions with frequent transport of Saharan dust, and similar to the rest of the Mediterranean, exhibit a high to very high risk of wildfires during summer (Bakšić et al., 2015), effects of which remain unexplored.

AD onto the sea surface cannot be completely understood without considering the interfacial processes within the sea surface microlayer (SML). The SML is the top 1000 μm of the sea surface, governing all exchange processes between the atmosphere and the sea. It is a unique environmental niche, described as a gelatinous film that provides a home for complex microbial community (Cunliffe and Murrell, 2009; Stolle et al., 2020). The accumulation of organic matter (OM) in the SML facilitates the accumulation of TMs through complexation with organic ligands and physical interactions with particulate matter (Cunliffe et al., 2010, 2013; Hunter and Liss, 1981). There are many strategies that microorganisms use to successfully uptake TMs under TM-limiting conditions as well as mitigate the detrimental effects of increased TM concentrations. This is particularly demanding within the SML, where the interconnections between physical, biological and chemical processes such as AD impacts, OM production, (photo)transformation and degradation become especially relevant and are still surprisingly poorly characterized (Baker et al., 2016; Engel et al., 2017; Mahowald et al., 2018; Stolle et al., 2020). A better understanding of the increasingly important processes affecting the air-water region can only be achieved through studies that consider both atmospheric and sea surface compartments as an integrated whole (Engel et al., 2017).

Therefore, this study focused on the AD impacts of biologically relevant trace metals (Zn, Cu, Co, Ni, Cd and Pb) on the complex sea surface responses of an oligotrophic coastal region, considering the SML at the air-water interface. A comprehensive dataset, comprising TM concentrations in ambient aerosol particles, bulk deposition and sea surface, differentiating between the SML and underlying water, was obtained during a field campaign conducted from February to July 2019 on the middle Adriatic coastal site. This study aimed to: (i) provide new insights into the variability of aerosol TM levels in relation to different seasons, air-mass impacts and special events, such as open-fire BB emissions and Saharan dust inputs typical of the coastal Mediterranean area, (ii) identify the magnitude and temporal variability of atmospheric TM deposition fluxes to the coastal region, (iii) better understand the controlling factors and processes that can impact TM concentrations and enrichments at the air-water interface in the coastal environments. To the best of our knowledge, this work is the first study of its kind ever conducted in the Adriatic Sea area and paves the way to a better understanding of the linkage between the ocean and the atmosphere. Knowledge of the interactions between different environmental compartments, taking into account the role of environmental interfaces, represents a fundamental feature for understanding the impacts of various pressures on the environment in general.