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Long-term evolution of the algal blooms in Romanian and Bulgarian Black Sea waters
Bodeanu, N.; Moncheva, S.; Ruta, G.; Popa, L. (1998). Long-term evolution of the algal blooms in Romanian and Bulgarian Black Sea waters. Cercet. Mar. = Rech. Mar. 31: 37-55
In: Cercetari Marine = Recherches Marines. Institutul Naţional de Cercetare-Dezvoltare Marină Grigore Antipa: Constanta. ISSN 0250-3069, more
Peer reviewed article  

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  • Bodeanu, N.
  • Moncheva, S., more
  • Ruta, G.
  • Popa, L.

    A synthesis quantitative data of main microalgal species and total phytoplankton in the Romanian and Bulgarian waters during the last two decades is presented. The algal blooms were of higher densities and frequencies during the 1980's in comparison to the 1990's, closely related to the intensification (the 80-ies) and the consequent reduction of eutrophication (the 90-ies) in the western Black Sea. Historical data from the relative pristine period of the ecosystem (before 1970) and the period of onset anthropogenic pressure (1971-1980) were used for comparison, thus providing information on the long-term trends of phytoplankton evolution over a period of about 40 years. A total number of 49 monospecific algal blooms occurred at the Romanian littoral in the 1980's and 31 at the Bulgarian littoral, many of them of an exceptional amplitude (the abundance of blooming species was within the range 50 million-1 billion cells.l-1). The bloom intensification has been associated with some other features characterizing phytoplankton growth: an increase of numerical density, of most algal species, an increase of non-diatoms proportion in the quantitative structure of microalgal communities, an increase of total phytoplankton abundance and biomass, the latter becoming superfluous to the consumers. During the recent period (1991-1997) the number of blooms decreased (20 at the Romanian littoral and 25 at the Bulgarian one), only a few species attaining densities exceeding 50 million cells.l-1. Concurrently, the share of algal species with increasing densities dropped, so did the number of mass species with exceptional abundance and the total phyplankton biomass. The reduction in the magnitude of non-diatom blooms and the trend of replacing them by the growth of other blooming species belonging especially to diatoms have induced modifications in the total phytoplankton quantitative structure, becoming more similar to that from the period before the onset of the anthropogenic eutrophication. The reduction of algal blooms and total phytoplankton abundance in the recent period (1991-1997) coincided well with the general decrease of nutrients and the remarkable increase of the Si:P ratio in the coastal environment. The decrease of algal blooms together with the increase of diatoms proportion and the trend of shifting the annual maximum of abundance from summer to spring and autumn are considered signs of an evolution towards the normal status of phytoplankton community. These positive signs, concomitant with some fragile tendencies of improvement of the ecological conditions in the area, are still uncertain as both nutrient and total phytoplankton biomass -despite their general reduction- remain considerably higher in comparison to the period before the cultural eutrophication.

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