Главная Zooplankton investigations in stokman gas condensate deposits in summer 2005
Zooplankton investigations in stokman gas condensate deposits in summer 2005 Печать E-mail

DVORETSKYV.G. Murmansk Marine Biological Institute RAS, Russia

Development of hydrocarbonic raw material deposits involves all components of the abiotic and biotic environments.

Water hydrobionts and first of all, a plankton are affected the greatest negative influence at development of sea shelf zones. (Patin, 1997). It is necessary to note especially what exactly the conditions of this biotic component relate to sea ecosystem productivity including structure and stocks of commercial fishes and invertebrates. All this defines importance of studying of plankton communities. Zooplankton constitutes a central component of marine ecosystems, providing a link between phytoplankton production and higher trophic levels.

The knowledge of a modern zooplankton condition is a basis for construction of multilevel system of monitoring of a biological component of sea complexes in Shtokman gas condensate deposits (SGCD), especially during development and exploitation. In particular, zooplankton can serve as the precise indicator of deterioration of a condition of an environment at possible accidents (Patin, 2001). Besides changes of quantitative parameters (biomass and total abundance) due to destruction zooplankton at performance of the planned procedures of development form a basis for calculation of damage for fish stocks and cost calculation of the actions directed on their restoration or indemnification. The purpose of work was the description of the zooplankton condition during the summer period.

License platform of the STGD is located in boreal polar province (the Arctic water ecosystem) (Timofeev, 1997). 64 kinds of the zooplankton was described for this area, from them 34 is the copepods, among which Calanus finmarchicus and Oithona similis are revealed the greatest abundance (Shirokolobova, 1994), 11 is Decapoda (Timofeev, 1997). Zooplankton reaches greatest development during the summer due to growth of copepods Pseudocalanus, Oithona, appendicularians Fritillaria and Oikopleura. In the beginning of the autumn period (September) increase rate of predatory invertebrates: fine jellyfishes (Rathkea, Obelia, Aglantha digitate, Tiaropsis multicirrata) and Ctenophora (Bolinopsis, Pleurobrachia) (Scientifically-methodical approaches ..., 1997).

Long-term dynamics of zooplankton productions depend on intensity of inflow of the Atlantic waters (it determines quantity of C. finmarchicus) and plankton amount decreasing by fishes (Timofeev, 1997, 2001). The greatest annual values of a biomass which can essentially vary depending on a complex of the meteorological factors observable in this or that year are characteristic for a summer season. The biomass of the license site water area reaches up to 100 mg/m3.

Basic elements of food base of commercial fishes of area is Calanus and euphausiids. Pattern of seasonal distribution C. finmarchicus on depths is (Fomin, 1978; Timofeev, 1997): in the winter the copepods are on the big depth; in the beginning of spring there is C. finmarchicus rising to a surface; in the end of spring copepods are spawning then Calanus fall to bottom layers where die or is eaten by predators. Within summer and prior to the beginning of autumn the copepod current year generation fall on greater depths owing to rise in temperature of water of the top layer up to 6-7 °C, their growth stops and coloration (with red on yellow-white) changes; in the end of summer (since the middle of August) C. finmarchicus starts to make daily vertical migrations; by the end of autumn crustaceans concentrate in the bottom channels, daily vertical migrations gradually stop. Minimal number C. finmarchicus is usually observed in the beginning of spring (1-10 ind/m3), maximal - in the beginning of summer (100-1500 ind/m3) (Fomin, 1978). The contribution to general biomass С finmarchicus during the various periods of year in the given area can reach 90 % and above (Fomin, 1978).

Euphausiid seasonal distribution (Thysanoessa inermis and 77 raschii) is similar to Calanus seasonal distribution. Though euphausiid average quantity for the Barents Sea constitutes 3.6-4.0 ind/m3, mass aggregations on northwest and southeast banks of Barents sea are much more dense. There are quantitative abundance data only for winter and the spring-and-summer periods: in dense aggregations number of euphasiids can exceed 1000 individuals in 1000 m3 (Drobysheva, 1988).

Complex expedition MMBI to area SGCD has been organized in July, 2005. Zooplankton samples were collected by Upstein planktonic net-towing (168 um mesh). One depth strata was sampled: 100-0 m. Speed of net turning up was about 1 m/s. All samples were fixed in 4 % formaldehyde. Total 5 samples at 5 complex stations have been collected.

Sorting and identification of the zooplankton were carried out according to the standard procedure (Management on methods ..., 1980).

26 taxa of pelagic organisms were revealed, but at each single station of those the taxa quantity varies from 10 up to 15. The maximum taxa number (15) is observed in the center of a license platform (st. 25), the minimal abundance (10) is presented in northern part of investigated area (st. 29).
Copepods prevail at all stations, making 68.0 - 99.5 % from an aggregate number of zooplankton. Rotatoria (Synchaeta) were subdominants (abundance) only on st. 21, making 23.3 % from total zooplankton number. Percentage of non-crustacean zooplankton did not exceed 1-4 % from total parameters. Among copepods dominate group was O. similis at two stations (st. 21, st. 25), nauplii of copepods prevail on st. 23 and 27, on st. 29 - С finmarchicus. Nauplii of copepods subdominante on st. 29, C. finmarchicus - on st. 25 and O, similis - on st. 23 and 27, Similar distribution of adults and copepodit stages of Copepoda as a whole can be estimated as typical for a polar province. Most seldom were amphipods Tnemisto abyssorum (st. 25), Pleurobrachia spp. (Ctenophora) on st. 21, Limacina helicina (Pteropoda) on st. 27. On st, 29 larval decapod stages (Paralithodes camchaticus zoea) has been found.

The total number of zooplankton varied in significant limits: from 1611 up to 3411 mg/m. The highest parameters of zooplankton abundance were fixed at st. 23 (531 mg/m) situated in a southern part of the license platform whereas in northern part (st. 27) were marked the least quantities of planktonic organisms - 81 mg/m3. Average value has made 313 mg/m3.

The contribution of various zooplankton groups in formation of a biomass is distinguished. Copepods had the greatest part in a total biomass of each station. In investigated zone С finmarchicus and C. glacialis provided 95.0 % of all biomass, the rate of O. similis was 3.3 % (fig. 2).

Distribution of zooplankton abundance in area SGCD is subordinated to some principles. For example, the tendency of increase in zooplankton number with depth enhancement is precisely looked through. Besides despite of the established polar day, zooplankton abundance was more in the samples collected at morning and evening o'clock. The last can be caused by that, first, zooplankton makes daily vertical migrations, rising to a surface at night (Timofeev, 1996); secondly, at night filtration efficiency index of planktonic nets raises (Harris et al., 1999).

The estimation of a degree of similarity of stations on number and taxa composition of zooplankton has shown structure, which all stations have high taxa similarity - values of Sorensen indexes were from 60.0 up to 88.9 %.

Thus, the structure and biological characteristics of zooplankton community during the summer period (July, 2005) in area of license zone allow to make a conclusion, that all typical features of development, and also spatial and time distribution of zooplankton, do not fall outside the limits revealed before long-term parameters (zooplankton abundance variability, beginning of spawning in populations of mass species, etc.).

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