1950-1952 



Benng Sea 



1958-1960 



■"''^^ CHUKCHI ^ 



D 



r. 1984 



1988 ^^^^^X^ 



1958-6 



Benng Sea 





^-3 



-4 



Fig. 7. Long-slanding variations of distribution of Mfcu/ana/ame/tosararfiato. 

 Maconui cakarea et al., biocenoses in the Gulf of Anadyr and 

 neighboring waters of the Bering Sea. Biocenoses distribution in 

 various years: A-1950-1952 (Vinogradova, 1954); B-1958-1960 

 (Neiman, 1963); C-1984 (report on the 47th cruise of the R/\ 

 Akademik Korolev); dashed line-present habitation of Nucuhma 

 lamellosa radiala in 1988; D-gradual displacement of the northern 

 boundary o( NucuUma lamellosa radiata biocenosis for 30 years in the 

 northwestern direction (indicated with arrow); \-Macoma cakarea; 

 2-Nuculana lamellosa radiata. i-Ophiura sarsi; 4-Yoldia 

 traciaefonnis. 



In the eastern part of the Chukchi Sea (Stations 54, 63, and 

 64), which is presently dominated by various species of 

 polychaetes, echiurids, and amphipoda, a great number of 

 empty shells of Macoma were found, which testifies to a former 

 biocenosis that might have been dominated by these mollusks. 



There is also a tremendous increase in the average biomass 

 of Macoma biocenosis in the Gulf of Anadyr that during the last 

 38 years has increased from 455 g m ' (Vinogradova. 1954) to 

 612 g m - (our data). Our values of average biomass in the 

 regions studied are 2-2.5 times higher than that recorded 50 

 years ago (Makarov, 1937). 



It is hard to provide an unambiguous answer to the problem 

 of variations within the same biocenosis and replacement of 

 biocenoses. Most probably, there is a whole group of biotic and 

 abiotic factors influencing changes in faunal populations and 

 biomass. Let us consider the replacement of Macoma biocenosis 

 with Nuculana biocenosis. It is quite obvious from the data 

 presented by Makarov (1937) that most of the samples 

 dominated by Leda {Nuculana) were collected in the coldest 

 regions (below 0°C), both in the Chukchi and Bering Seas. At 

 the same time, most often Macoma dominates in regions where 

 bottom water temperature is above 0°C (though not in every 

 case). Taking into account the direction of expansion of 

 Nuculana biocenosis (which is a more coldwater species than 

 Macoma). one could draw a conclusion that the cold spot 

 expanded northward to the Gulf of Anadyr. In addition. 



according to Deryugin and Ivanov ( 1 937), the center of the cold 

 Anadyr spot in 1933 was located in the region of the present 

 location of the Nuculana biocenosis. 



Another possibility for a change in faunal composition 

 could be preference by the dominant fauna for certain sediment 

 grain size or chemical composition. Nuculana prefers a muddier 

 sediment regime than Macoma (Scarlatto, 1981); it was also 

 noted that the sediment dominated by Nuculana smells of 

 hydrogen sulphide (i.e., there is a shortage of oxygen for 

 mollusks). Nuculaces, which includes Nuculana, Tellinacea. 

 and Macoma. incorporates species that are pocket detritus 

 feeders. Still, \f Macoma can use gills to change its feeding 

 mode to sestonophagy, nuculanae are most likely to feed on 

 particles of food from the seabed by collecting them using 

 labial palpus. Their gills perform only the function of respiration 

 (Kuznetsov, 1984). Itispossiblethatmonofunctionality of the 

 gills of Nuculana under conditions of oxygen deficiency gives 

 them an advantage over Macoma. It is also possible that 

 Nuculanae, like similar deepwater species, use the energy 

 released from vital functions of sulfur bacteria; it may also give 

 them the advantage over Macoma in case of inhabiting sludge 

 contaminated with hydrogen sulphide. 



Continuing our examination of biocenoses replacement, 

 we would like to discuss the general mechanisms of biocenosis 

 biomass variation both in the case when dominant species are 

 preserved and in the case when they are replaced with other 

 species. During the last 20 years, due to long-standing 

 observations of benthic biocenoses in various water areas, we 

 have accumulated a lot of facts testifying to time variation of 

 benthic biomass (Antipova, 1973, 1975; Antipovaefa/., 1974; 

 Rachor&Gerlach, 1975;Klimova, 1977;Golikove?a/., 1986). 

 Masse ( 1972) was the first person to distinguish three types of 

 time variation of benthic organism's biomass. We will discuss 

 the distribution of various types of dominant species' 

 replacement in relatively stable ( nonseasonal ) biocenoses based 

 on two types: reversible and irreversible. 



Irreversible replacement occurs either in cases of 

 developing siltation by sediments (e.g., in the Gulf of Possiet; 

 Kobyakov, 1962; Golikov era/., 1986) or in cases of pollution 

 (the Baltic Sea). Reversible types of replacement of dominant 

 species with a long life cycle can occur with considerable 

 variation of abiotic conditions in the environment (considerable 

 change of temperature, salinity, etc. ) that in most cases leads to 

 a complete destruction of biocenosis. When normal 

 environmental conditions are restored, the consequent 

 succession in the long run will restore the previous biocenosis. 

 Some of the shallow water biocenoses of the south Far East 

 undergo similar changes; these biocenoses are characterized 

 by a complete elimination of dominant species due to drastic 

 desalination of water that occurs every 2-3 years. 



Additionally, reversible types of change of the dominant 

 species may occur due to the change of biotic conditions — for 

 instance, due to an invasion of a large number of predators that 

 almost completely devour the dominant species. One or 

 several subdominant species that are not affected by predators 

 then become dominant; after predators leave the region, the 



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