Małgorzata Witak

Diatoms are regarded as reliable bioindicators for assessing heavy metal contamination in marine ecosystem including mercury. In the present study diatoms were used as bioindicators to reconstruct changes in the concentration of total mercury and its fractions in the Middle and Late-Holocene in the southern Baltic based on a 450 cm long sediment core. The goal of the study was to reconstruct changes in the concentration of Hg _tot and its fractions in relation to biodiversity of the diatom flora, climatic fluctuations occurring inferred from pollen and Pediastrum records and in relative abundance of sediment fractions. Hg _tot concentration increased from 4.6 ng g ⁻¹ (6300 cal. yr BP) to 353 ng g ⁻¹ (2021). Among the analyzed fractions HgS (⩽86.6%) dominated until ca. 4200 cal. yr BP after which a gradual increase in the proportion of Hg _F2 (⩽69.8%), was observed. The current study showed that over the last 6300 cal. yr BP the increase in Hg concentrations in the sediments of the Puck Bay has been influenced by human activities in addition to natural sources (atmospheric input, climatic fluctuations, storm events, diatoms productivity). About 4200–200 cal. yr BP the diatoms Schionodiscus oestrupii and Rhizosolenia hebetata f. hebetata enabled the identification eight time intervals during which climatic fluctuations influenced changes in sedimentary Hg _tot concentrations. Over the last ca. 200 cal. yr BP the highest Hg concentrations were associated with intensive human activity as evidenced by the anthropogenic diatom flora present at that time ( Cyclotella choctawhatcheeana , Thalassiosira levanderi ).

Storms have severe impacts on coastal communities and infrastructure. The southern Baltic Sea coast is particularly vulnerable to storm surges, as it predominantly features barrier-type coast, and extensive areas of coastal lowlands are situated within 1.5 m above sea level. Due to the short instrumental record, our knowledge of the complex issue of storminess is relatively limited. Therefore, it is very important to extend the timespan of storm activity records through the use of geological proxies. The variability of sediment grain size and benthic diatom species alien to the muddy sediments of the Puck Bay were used to reveal storm periods. This study presents a record of 21 storm periods dating back approximately 4700 years b2k. Particularly severe and frequent storms occurred during the transition from the Medieval Warm Period to the Little Ice Age (LIA), during the main phase of the LIA and the transition from the LIA to modern times. LIA storms were recorded in seabed sediments not only due to higher hydrodynamics, but also due to niveo-aeolian sand transport on sea ice during winter storms. Fourier spectral analysis shows a cyclic occurrence of storminess, with the most common frequencies of around 21–26, 59–74, 526–591, and 1180 years. These results are in good agreement with periodic variations in the atmospheric circulation over the North Atlantic, which are most likely related to variations in regional total solar irradiance.

Eutrophication is manifested by increased primary production leading to oxygen depletion in near‐bottom water and toxic cyanobacteria blooms. This is an important contemporary problem of the Baltic Sea and many other coastal waters. The present eutrophication is mainly ascribed to anthropogenic activity. To compare the present trophic state with that during past millennia, two sediment cores (50 cm long and ~400 cm long covering ca. 5,500 years) were taken from the Gulf of Gdańsk (southern Baltic Sea). The core subsamples were analyzed for phytoplankton pigments (chlorophylls and their derivatives, and carotenoids). In addition, carbon (C_tot, C_org, and δ¹³C), ¹⁴C dating, grain size, diatoms, and selected metals in the cores were analyzed to determine conditions in the depositional environment. The results indicated that there were high primary production periods in the past, during the Littorina Sea and the Roman Climatic Optimum, accompanied by oxygen deficiency in the near‐bottom water, most probably caused by climate warming. The ratio of 13²,17³‐cyclopheophorbide‐a enol, a labile degradation product of chlorophyll‐a, to the sum of other chloropigments‐a (CPPB‐aE/ΣChlns‐a) is proposed as a new paleoredox proxy. Heterocystous cyanobacteria blooms of an intensity similar to or even greater than at present also occurred in past millennia and were connected with climate warming. Hence, eutrophication must have occurred in the past, which means that natural factors have a substantial influence on it.

The paper presents the history of diatom studies carried out in the Gulf of Gdańsk and the Vistula Lagoon since the beginning of the 20^th century. One aspect of the research focuses on describing the structure of planktonic and benthic communities living in different parts of the area or occurring in their surface sediments as taphocoenoses. Other investigations concentrate on the fossil diatom flora, which is widely used in the research on the Late Glacial and Holocene to resolve paleogeographic, sedimentological and paleoecological problems.

This article presents results of diatom analysis of seven sediment cores taken from various depths of the Gulf of Gdańsk between the Hel Peninsula and the coast of Gdańsk and Gdynia. Further eight cores were drilled in the northern, southern and central part of the Vistula Lagoon. The present investigations enabled the determination of: (i) differentiation of diatom assemblage zones, (ii) correlation of palaeoecological units with reference to the following stages: Mastogloia Sea, Littorina Sea, Post-Littorina Sea, including the latest environmental changes associated with an anthropogenic factor, (iii) description of the defined diatom biofacies based on statistical analysis, (iiii) reconstruction of spatial and temporal development of both basins with the emphasis on the role of inflows of sea and river waters.

The aim of this study was to reconstruct paleoenvironmental changes of the SW part of the Gulf of Gdańsk. Three subbottom sedimentary cores were analyzed with respect to diatom flora. Post-Littorina Sea assemblages were noticed in the lower parts of the cores, whilst the present-day eutrophication of the gulf was evidenced in their superficial sediments, with high abundance of so-called ‘anthropogenic assemblage’ taxa. Shifts in the species composition might have been caused by intensified nutrient load, increasing organic matter concentrations and possible silica limitation.

Diatom analyses have been performed on a 425cm long Holocene marine sediment sequence from the North Atlantic south of Iceland and a 920cm sediment core spanning the last c. 7500 years from Skalafjord, Faeroe Islands. Additional core data include stable isotopes and results from magnetic susceptibility measurements, while chronostratigraphic control has been provided by AMS 14C measurements. The diatom records reveal distinct changes in North Atlantic surface circulation and climate that have been correlated with published terrestrial and marine records. Initial Holocene warming is dated at 9900 14C years BP, and after a Preboreal cold spell enhanced North Atlantic Current activity and warming prevailed from 9600 to 8800 14C years BP. From 8800 to 8000 "4C years BP the advection of warm Atlantic water masses weakened, presumably under atmospheric circulation conditions characterized by a dominating negative NAO. Prior to the Holocene Climatic Optimum (6000-5000 14C years BP) distinct hydrographic gradients existed in the northern North Atlantic that were probably most pronounced during the reported '8200 cal. years BP cold event'. After 5000 "4C years BP both core records indicate increased climate instability and periods of cooling ('Neoglaciation'), with enhanced cyclone activity affecting the northern North Atlantic particularly at around 4700, 4200, 3200, 2000, 1500 and 1000 14C years BP.