Influence Of Hypoxia On The State Of The Population Of The Black Sea Mussels
INFLUENCE OF HYPOXIA ON THE STATE OF THE POPULATION OF THE BLACK SEA MUSSELS
N.M. Shurova
Odessa Branch Institute of biology of Southern Seas, National
Academy of Sciences of Ukraine
Recently in the northwestern part of the Black Sea hypoxia in the near-bottom layers of the water has frequently occurred (Zaitsev, 1993). Although here it may be accidental in nature, it has a strong effect on structure of bottom communities (Salsky, 1977; Losovskaya, 1987) causing the death of many hydrobiont species, especially of attached organisms: mussels, barnacles, etc. In the course of evolution these organisms adapted, and were able to isolate themselves from unfavourable environmental conditions. However, this kind of isolation can’t be lengthy. Experimental works (Theed, 1973) show that at 10°C, 15 day isolation of molluscs caused 15-25% death of the organisms. More than 20 days isolation caused more than 50% death of mussels.
A duration of hypoxia in the near-bottom layer of waters of the north-western Black Sea is usually from one to three weeks (Zaitsev, 1993). So far as hypoxia is observed in summer-autumn seasons, when temperature of waters fluctuated from 14 °C to 25 °C, it led to a mass mortality of mussels.
The aim of the present investigation is studying the influence of the greatest simultaneous mortality of molluscs caused by the hypoxia on the population characteristics of the Black Sea mussels.
The samples of the mussel, Mytilus galloprovincialis Lam. (both alive and just dead) were collected in Odessa Bay (northwestern part of the Black Sea) at depths of 6,5 and 15,0 m, in the zone between the Danube river delta and Dniester river estuary at depths of 20 - 24 m in the end of hypoxia, in August - September 1987, 1990, 1992. The density, biomass, size-age and phenotypic structures were estimated for mussel settlements both before hypoxia and after it. Individual age mussels were revealed by the methods of sclerochronology on the basis of counting of annual bands in the inner nacreous shell layer (Shurova, Zolotarev, 1988). Phenotypic groups of mussels
were detected by analysis of the colour of the outer prismatic layer of shells (Shurova, Zolotarev, 1990). Heterozygote deficiency was calculated as
D = Do/De – 1 ,
where Do - number of heterozygotes observed, De - number of heterozygotes expected under Hardy-Weinberg equilibrium. The coefficient of reproduction has been estimated as the sum total of products of average fecundity and survival for each age class of the mussels (Begon et al., 1986). The parameters of the average biomass, density, production and reproduction, recruitment, mortality, average age, life span of the mussels from the north - western part of the Black Sea (for 1984 - 1995) was used in analysis of the influences of the area of hypoxic zones on the mussel population.
Hypoxia, which was observed in during more than two weeks in September 1990 at depths of 6 - 15 m in Odessa Bay, decreased the average density of mussel settlements almost 3 times, and the biomass - twice. Differences in the level of mortality of size groups of the mussels were revealed (Fig. 1). A direct relationship has been established between mortality of adult molluscs and the length of their shells from 30 to 75 mm (n = 10, r = 0,947). In the group of juvenile mussels (8-25 mm length) this relationship is reverse (n=8, r = – 0,906). Differences in the level of mortality of size groups of the mussels were revealed in all samples collected in August 1987 and in September 1992, when hypoxia in the near-bottom water layers occurred. Analogous differences of mortality have been revealed for age groups of the mussels. The average age of mussels in the settlements after hypoxia (in September 1990) has decreased by 25%, making up 1,3 years. The decrease in the amount of molluscs of older age groups in which fecundity is markedly higher than in younger has led to a decrease almost threefold average amount of offsprings per one female.
Some changes in phenotypic structure of the mussel settlements have been noticed. In during prolonged hypoxia of September 1992, when mortality of the mussels makes up more than 50%, the moiety of the molluscs of the phenotype B (blue-violet) in the mussel settlements had increased (p = 0,95) from 42% to 48%. In the juvenile group of the mussels highest mortality of the heterozygotes (striped mussels) was revealed (p = 0,999). Here, mortality of the phenotype A (eight-brown mussels) also was higher (p = 0,99), than mortality of the phenotype B (blue-violet). As a result of this hypoxia the index of the heterozygote deficiency in the mussel settlements has risen by 18% (p=0,99) reaching –0,68 towards the end of hypoxia. Less prolonged hypoxia in September 1987, when mortality of mussels was only 25%, phenotypic differences of the mollusc mortality had not been discovered.
Fig. 1. The mussel mortality in different size groups as a result of hypoxia in the near-bottom water layers in the north-western Black Sea
Influences of the area of hypoxic zones in the north-western Black Sea for biomass (Fig. 2), production, reproduction (Fig. 3), mortality, average age of the mussels, percentage of juvenile mussels in the population of the molluscs were analysed. The correlation between mortality coefficients of the mussels, percentage of young molluscs in the population and the surface area of hypoxic zones in the north - western Black Sea (r = 0,990 and 0,991, accordingly) has been established. The relationship between surface area of hypoxic zones and mean age of molluscs, coefficient of their reproduction were reverse (r = –0,978 and –0,968, accordingly).
Thus, hypoxia in the near - bottom layers of the water in the north - western part of the Black Sea has a negative influence on the state of mussel population. If hypoxia was continued for more than two weeks, very high mortality of the mussels was observed. As a result, the mussel abundance and biomass was lowering. In the last ten years the abundance and biomass of the mussels in this region has decreased more than twofold. At this time the area of the hypoxic zones in the north - western part of the Black Sea increased almost tenfold. Now, it is more than 40000 km2.
Higher mortality of the large molluscs in during prolonged hypoxia is a cause for constant rejuvenation of the mussel settlements. So, in the last ten years the average age of the mussels of the north-western part of the Black Sea was decreased more than twofold. The life span of molluscs was decreased from 26 to 9 years. A decrease in the mussel population of the amount of molluscs of older size-age groups, which have fecundity markedly higher than younger mussels, is the reason for lowering the coefficient of their reproduction. As compared with 1985 this parameter was reduced more than tenfold.
Fig. 2. Mean biomass (B, g·m-2) of the mussels at different surface area of hypoxic zones in the near-bottom water layers in the north-western Black Sea (S, 103 km2)
Fig. 3. Mean age (Tm, year) and reproduction coefficient (R, 106) of the mussels at different surface area of hypoxic zones in the near-bottom water layers in the north-western Black Sea (S, 103 km2)
As a result of hypoxia, the phenotypic structure of the mussel settlements in the north-western Black Sea was changed. The index of heterozygote deficiency for population of the mussels of this region in the period of 1989 - 1994 has risen twofold, reaching D = – 0,58.
References
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