Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea

Abstract

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000–4000 B.P. during the Littorina transgression, (2) ca. 1400–700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions.

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

“…2), coinciding with the MCA-LIA transition. The temporal pattern is similar to the development of hypoxia in the Baltic Proper Funkey et al, 2014;Jilbert et al, 2015;Dijsktra et al, 2016;Hardisty et al, 2016;Papadomanolaki et al, 2018) and in the Danish straits (van Helmond et al, 2017), where warm climatic phases during the Holocene have been characterized by declining oxygen levels. Yet, we note that the amplitude of these changes appears markedly less pronounced at our study site, which is most likely attributed to the lack of permanent halocline in this shallow coastal region.…”

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

“…2), coinciding with the MCA-LIA transition. The temporal pattern is similar to the development of hypoxia in the Baltic Proper Funkey et al, 2014;Jilbert et al, 2015;Dijsktra et al, 2016;Hardisty et al, 2017) and in the Danish Straits (van Helmond et al, 2017), where warm climatic phases during the Holocene have been characterized by declining oxygen levels. Yet, we note that amplitude of these changes appears markedly 25 less pronounced at our study site, which is most likely attributed to the lack of permanent halocline in this shallow coastal region.…”

Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

“…1). The Baltic Sea is experiencing widespread hypoxia and severe cyanobacteria blooms, which are linked to enhanced external inputs of nutrients into the coastal waters from agriculture and waste water (Gustafsson et al 2012;Funkey et al 2014;Carstensen et al 2014). In the past, large parts of the Baltic Proper were also hypoxic after the intrusion of seawater into the freshwater Ancyclus lake and the establishment of the Littorina Sea (A/L transition; ∼ 8000 year before present (BP); Andrén et al 2011).…”