Proxy profiles of organic pigments (a) and metal ratios (b-d) as measured in a long core covering the time indicated along the axis on the right side. Higher values of these proxies indicate lower oxygen levels in the bottom waters. The gray bands indicate warmer periods, as recognized by the abundance of warm-water species of diatoms, unicellular algae. Credit: Szymczak‐Żyła et al. [2019], Figure 5
Source: Paleoceanography and Paleoclimatology

Eutrophication leading to oxygen depletion in near-bottom water and toxic cyanobacteria blooms is an important contemporary problem of the Baltic Sea and many other coastal waters, with present eutrophication mainly ascribed to anthropogenic activity. Szymczak‐Żyła et al. [2019] compare the present trophic state of the Baltic with that during the past millennia in the southern Baltic Sea, using analysis of phytoplankton pigments (chlorophylls and their derivatives, and carotenoids), grain-size, diatoms and selected metals. The authors deduced that there were high primary production periods during past warm periods, accompanied by oxygen deficiency in the near-bottom water. Cyanobacteria blooms of an intensity similar to or even greater than at present occurred in past millennia, linked to warming. The authors thus conclude that natural factors contribute to eutrophication and low oxygen conditions in the Baltic Sea.

Citation: Szymczak‐Żyła, M., Krajewska, M., Witak, M., Ciesielski, T. M., Ardelan, M. V., Jenssen, B. M., et al. [2019]. Present and past‐millennial eutrophication in the Gulf of Gdańsk (southern Baltic Sea). Paleoceanography and Paleoclimatology, 34.

—Ellen Thomas, Editor in Chief, Paleoceanography and Paleoclimatology

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