Unusually High Solar Flux Triggered Extreme Productivity Patterns During the Spring Bloom 2018 in the Central Baltic Sea suggesting Vertical Nutrient Shuttling: a Glimpse into a Potential Future?

Gregor J Rehder1, Jens Daniel Müller2, Henry C Bittig3, Mati Kahru4, Seppo Kaitala5, Bernd Schneider2, Simo-Matti Siiriä6, Laura Tuomi6 and Norbert Wasmund7, (1)Leibniz Institute for Baltic Sea Research, Rostock, Germany, (2)Leibniz Institute for Baltic Sea Research, Department of Marine Chemistry, Rostock, Germany, (3)Leibniz Institute for Baltic Sea Research, Department of Physical Oceanography and Instrumentation, Warnemünde, Germany, (4)Univ California San Diego, La Jolla, CA, United States, (5)Finnish Environment Institute, Helsinki, Finland, (6)Finnish Meteorological Institute, Marine Research Unit, Helsinki, Finland, (7)Leibniz Institute for Baltic Sea Research, Department of Biological Oceanography, Warnemünde, Germany
Abstract:
The Baltic Sea is one of the largest brackish water systems on Earth, connecting 85 Million people from 9 nations. It underwent strong eutrophication in the 1970s and 1980s, but dedicated international management actions under the “Baltic Marine Environment Protection Commission” (HELCOM) aim to alleviate the anthropogenic pressures. Recently, increasing awareness arises that climatic changes might counteract the pan-Baltic effort for ecosystem restoration of the Baltic Sea.

The year 2018 was characterized by unusal meteorological conditions over Northern Europe, recently referred to as the 2018 European heatwave, which led to the highest sea surface temperatures in some areas of the Baltic Sea ever recorded. A closer look reveals that the most extreme deviation from long-term averaged monthly solar irradiation over the Baltic Sea actually occurred in May.

These unusual meteorological conditions in spring 2018 had dramatic consequences on the development of the spring bloom, the major productive period in the central Baltic Sea. Based on recent and long-term data from the ICOS (Integrated Carbon Observation System) voluntary observing ship Finnmaid, Finnish BGC-Argo floats, HELCOM research vessel based monitoring, and remote sensing, we deduce the following sequence of events and mechanisms:

(1) rapid surface warming led to the development of a shallow thermocline, complete depletion of inorganic nutrients in the upper 15 m, but still considerable loads of nitrate and phosphate below the mixed layer, by mid-April; (2) until mid-May, nitrate got completely depleted down to 60m depth, despite the persistent strong and stable thermocline; (3) carbon system observations and vertical Chl a data show that the productivity was focused in the mixed layer, where pCO2 dropped down to 40 μatm, indicating unpreceded high carbon fixation in the upper layer; (4) dominance of the dinoflagellate Peridiniella catenata suggests that the bloom was sustained by vertical shuttling of nitrate towards the mixed layer, leading to a spatial decoupling of the uptake of nitrate, phosphate and inorganic carbon.

The findings demonstrate the potential of climate-driven changes in the major biogeochemical functioning of the Baltic Sea. The work also shows the enormous potential to increase our knowledge of marine ecosystems by a combined use of large scale Earth Observation Infrastructures and conventional monitoring schemes.

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