number of organisms having died during a specific period of 

 time and by specific death in connection with the whole 

 number of population of organisms (Odum. 1975). 



The importance of the present trend towards investigation 

 of marine ecosystems is evident. Results from the study of 

 mortality make it possible to estimate a number of functional 

 characteristics of pelagic communities (Beklemishev, 1969; 

 Vinogradov, 1970), parameters of biosedimentational processes 

 (Zhelezinskaya, 1969; Scott, 1977; Stepanov & Svetlichnii, 

 1978; Lebedewd etal.. 1982), and peculiarities of specific and 

 temporal distribution of plankton animals (Zhelezinskaya, 

 1968. 1969; Koval, 1978; Kulikov, 1990). Given the large 

 array of negative intTuences by environmental factors, as well 

 as anthropogenic ones, the process of mortality in hydrobenthic 

 research is important in studying the ecological consequences 

 of oceanic contamination (Sheehan, 1984; Izraele/a/., 1989). 

 In this context, it is necessary to note that the show of 

 anthropogenic effects should be preceded by determining the 

 background of natural variability. This is realized by means of 

 long-term baseline investigations in broad regions in the World 

 Ocean (Izrael&Tysban, 1983). Once sufficient data on marine 

 zooplankton mortality levels are accumulated, it is possible to 

 determine the approximate natural mortality levels in individual 

 areas of the ocean, thus making it possible to identify regions 

 of mass destruction of marine organisms. 



Investigations of A. F. Pasternak in the Black Sea 

 determined that the number of dead mesozooplanktonic 

 organisms averaged 5% of the total number of animals and 2% 

 of their biomass (Sazjin, 1985). Similar characteristics were 

 observed during a period of detailed ecological investigations 

 in the Baltic Sea where the average number of dead copepods 

 was about 5-6% of the numbers and biomass of animals 

 (Kulikov. 1990). Mass destruction of plankton was caused by 

 necrogenic factors from both anthropogenic and natural origins. 

 Intensive losses of marine organisms were discovered in areas 

 exposed to oil pollution (Vinogradov, 1970; Mironov, 1973), 

 as well as areas of unregulated waste discharge ( Grinbart ei al. , 

 1976). Areas of the Baltic Sea showing levels of sulphurated 

 hydrogen indicated high levels of death in copepods, averaging 

 up to 13% of the total numbers and biomass of the community 

 (Kulikov, 1990). Dead copepods found in frontal areas of 

 upwelling, close to the shore of northwest Africa, reach 1 6% of 

 total numbers (Weikert, 1977). Mortality conditions exist 

 where brackish- water and marine planktonic complexes formed 

 in the fronts oflarge rivers (Beklemishev, 1969; Koval, 1970a,b, 

 1984). 



Present investigations into the complex Bering and Chukchi 

 Sea ecosystems were conducted to detemiine the variables of 

 mortality in the population of mesoplankton communities. 

 They will also determine the disturbance areas showing 

 significantly higher concentrations of dead organisms, as well 

 as any reasons for the increases. 



Materials and Methods 



Mesozooplankton samples were collected using 30-1 plastic 

 Niskin bottles during the Third Joint US-USSR Bering & 

 Chukchi Seas Expedition aboard the Soviet research vessel 



Akademik Korolev {iu\y-August, 1988). Permanent depths of 

 sampling were 5, 10, 25, 45, 70, and 100 m. In the shallow 

 areas, the lower horizon was determined by the depth to bottom 

 of the station (Timoshenkova & Kulikov, 1988). 



Physiological state indicators for organisms were carried 

 out by means of painting samples with neutral or red dyes for 

 the duration of their lifetimes in accordance with Fleming and 

 Couchman"s( 1978)inethod(Crippen&Perrier, 1974). Samples 

 were brought up to a volume of 100 ml and were inserted into 

 glass jars with screw tops. Then 2 ml (0.05% ) of dye was added 

 to each sample ( 1 :2,000) and the capped jars were put into deep 

 water with wastewater extract for the period of 1 h. Samples 

 were then fixed using a neutral formula with a peak concentration 

 of 4%. Calculations measuring differences between dead and 

 live organisms were detemiined by means of a microscope 

 having a 2 x 8 magnification. During the course of study, the 

 following indices were used: number of dead individuals 

 (ind/m- and ind/m'); biomass of dead organisms (mg/m', 

 mg/m-); percent of number, identity, and biomass of dead 

 fraction versus the total number (dead and alive); identity and 

 biomass of the mesozooplankton community (%); ratio of the 

 number and biomass of the dead fraction versus total number 

 (dead and alive); and the biomass types of the total population 

 (%). Since mesozooplankton groups of 0.1-2.0 mm intervals 

 averaged 97% of all communities (of 0. 1-2.0 mm sizes) and 

 their total biomass was 35%, the priority was in the description 

 of the mesozooplankton state at all levels in the pelagic 

 community for the number of characteristics that most fully 

 accessed the situation in the investigated areas. 



Results 



The vertical distribution of quantitative indices of 

 necrozooplankton in the euphotic zone of the Bering and 

 Chukchi Seas were distinguished by type as well as the place 

 and location of the station in relation to the hydrographical 

 characteristics of water masses, degree of development of 

 zooplankton communities, depth of layers and their quantitative 

 extremes, etc. Zooplankton numbers differed during the 

 investigations from to 7,500 ind/m' (Station 1 1, 25 m), and 

 biomass reached 100.3 mg/m'. The corresponding contents of 

 dead marine organisms in zooplankton communities varied 

 from to 44.3% (Station 96, 25 m), and biomass varied from 

 to 49.9% (Station 32, 10 m). 



Some general features of distribution were found. Stations 

 that were situated in more than 2,500 m (Stations 2 and 108, 

 East and South Polygons) had high accumulations of dead 

 animal bodies, up to 2,800 ind/m', were related to the upper 

 warm layer (Fig. 1 ), and coincided with the accumulation of 

 the maximum number of live organisms at that depth. However, 

 the peak percentages of dead animals in the plankton were 

 deeper in the layer of the thermocline, the cold intermediate 

 layer, where their numbers reached 25% of the total zooplankton. 

 Similar vertical distribution characteristics of necrozooplankton 

 were found in the deep region of the continental shelf (East 

 Polygon). At other stations situated on the outer slope of the 

 eastern (East Polygon) and northwestern shelf regions, the 

 highest values of absolute and relative characteristics of the 



173 



