Recovery from multi-millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity.

Niels A. G. M. van Helmond,Francien Peterse,Guillaume Fontorbe,Annika Vollebregt,Bryan C. Lougheed,Caroline P. Slomp,Daniel J. Conley

doi:10.1002/lno.11575

Abstract

Enhanced nutrient input and warming have led to the development of low oxygen (hypoxia) in coastal waters globally. For many coastal areas, insight into redox conditions prior to human impact is lacking. Here, we reconstructed bottom water redox conditions and sea surface temperatures (SSTs) for the coastal Stockholm Archipelago over the past 3000 yr. Elevated sedimentary concentrations of molybdenum indicate (seasonal) hypoxia between 1000 b.c.e. and 1500 c.e. Biomarker‐based (TEX86) SST reconstructions indicate that the recovery from hypoxia after 1500 c.e. coincided with a period of significant cooling (∼ 2°C), while human activity in the study area, deduced from trends in sedimentary lead and existing paleobotanical and archeological records, had significantly increased. A strong increase in sedimentary lead and zinc, related to more intense human activity in the 18th and 19th century, and the onset of modern warming precede the return of hypoxia in the Stockholm Archipelago. We conclude that climatic cooling played an important role in the recovery from natural hypoxia after 1500 c.e., but that eutrophication and warming, related to modern human activity, led to the return of hypoxia in the 20th century. Our findings imply that ongoing global warming may exacerbate hypoxia in the coastal zone of the Baltic Sea.

Highlights

Dissolved oxygen concentrations are decreasing in marine settings globally, leading to hypoxia (O2 < 2 mg L−1), which is detrimental to most marine life (Diaz and Rosenberg 2008; Schmidtko et al 2017)
Our results underline that the timing of past coastal hypoxia in the Baltic Sea differs from that in offshore areas, and that changes in climate and salinity were instrumental in its cause
Recent model sensitivity studies have shown that anthropogenic nutrient loading was key for the development of modern hypoxia in offshore areas of the Baltic Sea (Meier et al 2018)

Summary

Introduction

Dissolved oxygen concentrations are decreasing in marine settings globally, leading to hypoxia (O2 < 2 mg L−1), which is detrimental to most marine life (Diaz and Rosenberg 2008; Schmidtko et al 2017). Widespread oxygen depletion in the Baltic Sea is not limited to the modern human-induced hypoxic interval, Such a depletion is well-described for the Baltic Proper, where two previous intervals of widespread oxygen depletion, coinciding with the Holocene Thermal Maximum (HTM, 6000–2000 B.C.E.) and the Medieval Climate Anomaly (MCA, 800–1300 C.E.), were identified in sediment records (Zillén et al 2008; Jilbert and Slomp 2013). During both intervals, climatic warming was likely a key driver of hypoxia (Kabel et al 2012; Papadomanolaki et al 2018). In a recent study of diatom assemblages for three locations along the Swedish east coast, capturing the last 1000 yr, no evidence for increased productivity was observed before 1800 C.E. (Norbäck Ivarsson et al 2019)

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