Long-term hydrochemical changes and “Harmful Algal Blooms” in the Arctic Lake Imandra

The large arctic Imandra Lake is located in the industrial Murmansk region, Russia. Since the 2000s it has regular Harmful Algal Blooms. Significant changes in hydrochemical indices were recorded in 1985—2017. The ratio of the main ions has changed, the pH and alkalinity have increased. The indicators of water salinity, the total content of P and N, and their mineral forms have decreased. The concentrations of Zn, Cu, Ni и SO42– have decreased as well. The decrease trends in the content of macronutrients in waters contradict traditional concepts and the role of mineral P and N in the development of HAB. This indicates more complex mechanisms of flowering in this arctic reservoir, which are being discussed.

Long-Term Eutrophication and Dynamics of Bloom-Forming Microbial Communities during Summer HAB in Large Arctic Lake

Harmful algal blooms (HABs) in arctic lakes are recent phenomena. In our study, we performed a long-term analysis (1990–2017) of the eutrophication of Lake Imandra, a large subarctic lake, and explored the biodiversity of bloom-forming microorganisms of a 2017 summer HAB. We performed a 16Sr rRNA metabarcoding study of microbial communities, analysed the associations between N, P, C, and chlorophyll concentrations in the lake water, and developed models for the prediction of HABs based on total P concentration. We have demonstrated that blooms in Lake Imandra occur outside of optimal Redfield ratios and have a nonlinear association with P concentrations. We found that recent summer HABs in a lake occur as simultaneous blooms of a diatom Aulacoseira sp. and cyanobacteria Dolichospermum sp. We have studied the temporal dynamics of microbial communities during the bloom and performed an analysis of the publicly available Dolichospermum genomes to outline potential genetic mechanisms beneath simultaneous blooming. We found genetic traits requisite for diatom-diazotroph associations, which may lay beneath the simultaneous blooming of Aulacoseira sp. and Dolichospermum sp. in Lake Imandra. Both groups of organisms have the ability to store nutrients and form a dormant stage. All of these factors will ensure the further development of the HABs in Lake Imandra and the dispersal of these bloom-forming species to neighboring lakes.

Lake Imandra depression in the Late Glacial and early Holocene (Kola Peninsula, north-western Russia)

The paper summarizes the evidence of litho-, biostratigraphy and 14C dating of sedimentary sequences studied in natural outcrops and bottom deposits in small lakes, as well as data on coastal morphology in the depressions of Ekostrovskaya and Babinskaya Imandra, the southern sub-basins of Lake Imandra. Lithological, 14C and diatom data suggest that the brackish-water reservoir followed by the fresh-water one existed in the Ekostrovskaya Imandra depression during the Younger Dryas chronozone prior to 11,400cal. yr BP. The Fennoscandian Ice Sheet margin is assumed to have been located in the Lake Imandra basin, covering western Babinskaya Imandra earlier than c. 10,250 cal. yr BP. The early Holocene c. 11,400–8,500 cal. yr BP was marked by a significant westward retreat of the ice margin in the western Lake Imandra depression and adjacent areas, and an extensive fresh-water pra-Imandra Lake basin was formed there. At the end of the Preboreal, earlier than c. 9,210–8,500 cal. yr BP, the pra-Imandra Lake coastline was at least 16–18 m higher than the modern one, as can be assumed according to coastal morphology and lithostratigraphical data. The coastline of that reservoir changed, water square slightly reduced, and isolated small lakes emerged on coasts during the early Holocene.

ОСОБЕННОСТИ ЛЕТНЕГО ПРОСТРАНСТВЕННОГО РАСПРЕДЕЛЕНИЯ ФОСФОРА, АЗОТА И ХЛОРОФИЛЛА-а В КРУПНОМ ЭВТРОФИРУЕМОМ АРКТИЧЕСКОМ ОЗЕРЕ ИМАНДРА (МУРМАНСКАЯ ОБЛАСТЬ) В СВЯЗИ С МАССОВЫМ РАЗВИТИЕМ ФОТОСИНТЕЗИРУЮЩИХ МИКРООРГАНИЗМОВ

The lake Imandra is a major Arctic water basin where harmful algal blooms (HABs) are observed regularly starting from the turn of the 20th and 21st centuries. HABs occur in the lake at significantly lower temperatures and phosphorus (P) and nitrogen (N) levels than in lakes located at lower latitudes. With regard to the above, the interrelationships between and the patterns of the spatial distributions of P, N and chlorophyll-a (Chl-a) in the upper layers of the lake have been considered. The complex outline of and strong gravity currents in the lake combined with potent focal anthropogenic sources of P and N determine the gradients of biogenic elements levels in the lake. Their examination suggest that HABs development is limited mutually by the levels of N and P and by their stoichiometric proportions. At the levels of total P and N (Ptot and Ntot) below 17 µg/L and 150 µg/L respectively, HABs probability is low. At Ptot>17 µg/L, HABs are limited by N rather than P. At Ntot>180 µg/L, HABs probability becomes increased. In the major part of the lake, N is mostly organic and NO3– levels are low, NH4+ being predominant among the inorganic forms of N. This makes conditions for the development of green algae and for the predominance of cyanobacteria. At [NO3-]:[NH4+] within 0.1 to 1.0, the probability of cyanoHABs is high. HABs development is promoted by [NO3-] > 2.5 µg/L upon [РО43-] > 3.1 µg/l, the N:P ratio being above 9.7. The ascertaining of the critical levels of N- and P-containing compounds will be useful in HABs forecasting and in understanding of HABs development in Arctic water basins.