A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

Abstract

Abstract. The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated the depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygenation since the beginning of the 1900s was fueled by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.

Legacy Effects of Eutrophication on Modern Methane Dynamics in a Boreal Estuary

“…In a recent study, Raateoja and Kauppila (2019) studied eutrophication development of three estuaries in Northern Baltic, including Pojo Bay, from the 1970s onwards. In accordance with other studies of SW Finland archipelago areas (Conley et al 2011;Jokinen et al 2018), they did not find any evidence of oligotrophication in the recent past, but note that the two large lakes in the catchment of Mustionjoki may buffer changes in coastal nutrient levels, by delaying land-to-sea nutrient transfer. Bryhn et al (2017) also argue that direct nutrient loading reduction to coastal catchments rarely controls the recovery of these systems from eutrophication, due to import of nutrients from the open Baltic Sea.…”

“…Like many other coastal regions of the Baltic Sea, the Pojo Bay estuary and the adjacent archipelago suffered from anthropogenic eutrophication during the twentieth century, leading to the expansion of seasonally hypoxic bottom waters in stratified areas (Conley et al 2011;Jokinen et al 2018). In the Pojo Bay estuary itself, there was a brief recovery period in the early 1980s, but oxygen conditions soon deteriorated again and the estuary suffered from extensive hypoxia in the 1990s (Malve et al 2000).…”

Barriers and Bridges in Abating Coastal Eutrophication

“…These characteristics combine to create a brackish, tideless sea with year-round thermal stratification that is highly susceptible to eutrophication. Stratification isolates the deeper basins, making them vulnerable to year-round hypoxia that developed during the 20th century and is unprecedented during at least the prior 1,500 years (Jokinen et al, 2018). The hypoxic basins are influenced not only by nutrient-driven productivity, but also periodic intrusions of more saline waters from the North Sea.…”

“…These feedbacks spread the loss of eelgrass into neighboring areas. Recovery from hypoxia in basins in the Archipelago Sea of Finland is made difficult both by postglacial uplift, that further isolates basins and results in remobilization of organic matter as the wave base shifts and the reduced ice cover prolongs the period experiencing surface waves, as well as by warming surface waters (Jokinen et al, 2018). Regionally specific processes affect recovery, which can be idiosyncratic.…”

Identifying areas prone to coastal hypoxia – the role of topography

“…The lack of oxygen alters the structure and functioning of benthic communities (Nilsson and Rosenberg, 2000;Gray et al, 2002;Karlson et al, 2002;Valanko et al, 2015), disrupts bioturbation activities (Timmermann et al, 2012;Villnas et al, 2012Villnas et al, , 2013, changes predator-prey relationships (Eriksson et al, 2005) and may lead to mass mortalities of benthic animals (Vaquer-Sunyer and Duarte, 2008). Hypoxia does not only affect organisms of the seafloor, but also influences biogeochemical cycling and benthic-pelagic coupling (Gammal et al, 2017).…”