The biological state of the Baltic Sea 2017

Dating to 1979, the HELCOM time series on species composition, biomass and abundance of phyto- and zooplankton as well as macrozoobenthos from Kiel Bay to the Eastern Gotland Basin was continued in 2017.

The phytoplankton spring bloom started its development already in February in Kiel Bay and even in the central Arkona Basin, but it occurred in the Eastern Gotland Basin only in May 2017. It was dominated by Dictyocha speculum in the Belt Sea, by Skeletonema marinoi in the Arkona Basin and by Mesodinium rubrum in the Bornholm and Eastern Gotland Basins. The development of cyanobacteria, observed by satellite imagery, started in the northern Baltic Proper at beginning of July and lasted exceptionally long in the western Baltic by end of August. The autumn bloom was well developed in the Belt Sea with Ceratium spp. dominating in Kiel Bay and Pseudo-nitzschia spp. dominating in the Bay of Mecklenburg.

The chlorophyll a concentrations were highest (12.9 mg m-3) during the autumn bloom in Lübeck Bay.

The seasonal pattern of vertical export of particulate organic matter in the Arkona Basin in 2017 showed a clear succession of algal blooms and increased diversity in diatoms and dinoflagellates. The total annual flux for single elements in 2017 amounted to 580 mmol C, 76 mmol N, 91 mmol Si and 2.8 mmol P m^-2 a^-1 at a mass flux of 56 g dry mass m^-2 a^-1. However, these relatively high fluxes are also attributed to resuspension events. The mass weighted δ15N signature of 6.1 ‰ documents a lower nitrogen fixation than in the former years.

The zooplankton development was characterised by a recovery of the historically low stock size recorded in 2015-2016. This is particularly based on increasing abundances of cladocera in the Arkona Basin, and of cyclopoid and calanoid copepods in all areas. Due to a late occurrence of rotifers only in May, the seasonal development occurred late in 2017. Meroplankton was generally scarce. The high diversity of the zooplankton observed in preceding years was continuously present. A total of 63 species were recorded including the anthomedusae Lizzia blondina and Staurosarsia gemmifera, which are recorded the first time in the Kiel Bay and the Bay of Mecklenburg.

The 142 species found in the macrozoobenthos mark a high diversity, mainly driven by the high species number in the Kiel Bay and the Fehmarnbelt. The oxygen supply in bottom waters in the current year was not always higher than 2 ml/l at all stations, but basically no oxygen depletion was observed. Due to the good hydrographic conditions and the larval supply from the Kattegat area a fast recovery of the impoverished area around the island of Fehmarn was caused. Depending on the region, the abundances ranged from 242 to 8.790 ind./m², and the biomass (ash free dry weight) from 1.4 g/m² to 59.3 g/m². Some marine species were observed again since a long time or for the first time at all. Sixteen species of the German Red List (Categories 1, 2, 3 and G) were observed at the 8 monitoring stations. With five, the number of invasive species in 2017 was low. Two neozoan amphipod species, Grandidierella japonica and Melita nitida, were observed at the monitoring stations for the first time.

Dr. Norbert Wasmund, Dr. Jörg Dutz, Dr. Falk Pollehne, Dr. Herbert Siegel, Dr. Michael L. Zettler