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CLIMATE IMPACT ON LONG-TERM CHANGEABILITY OF THE BLACK SEA BIOLUMINESCENCE FIELD AND PLANKTON COMMUNITY CHARACTERISTICS.
2 Marine Hydrophysical Institute, NAS of Ukraine, 2, Kapitanskaya str., Sevastopol, Ukraine.
Abstract. Researches in the bioluminescence field vertical structure, amplitude characteristics and spatial distribution in the Black Sea have been conducted in IBSS, NAS of Ukraine, from 1965 untill present. Enough evidences are available to testify dramatic changes over the last decades in different Black Sea ecosystem components including bioluminescence field parameters. But still there is no direct evidences regarding the reasons for such transformations, as the variability of the Black Sea biophysical (bioluminescence) as well as biological characteristics at climatic time scales, i.e. at the scale of decade periods is not sufficiently studied now.
The most pronounced alterations are exhibited in vertical structure of the bioluminescence field in the centre of the Black Sea western halistase. Wide range of bioluminescence field vertical distribution and plankton community had been marked before 1980 followed by its sharp narrowing. It is illustrated by bioluminescence field lifting towards the surface layers in the following years. For example, if in 1976 intensive bioluminescence was observed up to the depth of 75 m, in 1984 the maximum of bioluminescent potential was found in the layer of 20-35 m, and in 1992 maximum bioluminescence values were registered only in narrow 15-20 m layer.
Connection of the North Atlantic Oscillation (NAO) index with the process of Cold Intermediate Layer (CIL) formation and its renewal is presented. The similar picture of the situation occurred in plankton community, namely, constant biomass reduction in one of the main Black Sea bioluminescents - Noctiluca scintilans. The same results were obtained by Russian researchers for the NW shelf of the Black Sea in 1967-1975 when N. scintilans abundance constituted 968,0 ind/m3, in 1980-1985- 10,2 ind/m3 and only 2,0 ind/m3 in 1989-1991 years.
Aims. To evaluate climate impact on long-term variability of the Black Sea plankton community by characteristics of the bioluminescence field.
the most dynamic in the Mediterranean basin. Its specificity is determined by weak water exchange with the adjacent seas through narrow straits, elongated vertical hydrogen - sulfide layer, intensive input of river discharges with great quantity of biogenic matter and different pollutants. All these factors exert negative effects on the Black Sea ecosystem, namely, its structure and functioning, productivity and biodiversity.
By present enough materials have been collected to testify dramatic changes in the Black Sea ecosystem components taking place over the last decades [2, 3, 4, 5]. But still no direct evidences have been obtained to explain the reasons for such transformations, as variability of hydrochemical and biological characteristics in the Black Sea waters at the climatic time scale, i.e. at the scale of several decades periods is not sufficiently studied [5, 6]. For example, decrease in average oxygen concentrations and decline in its stock in the Black Sea waters from the mid of 70-ties to the beginning of 90-ties have been registered . At the same time, starting from the end of 90-ties this negative tendency has leveled. The same trends are also recorded in the plankton community and in a number of biophysical characteristics of pelagial . This could have been the result of a tough anthropogenic load, but intensive input of biogenic matter together with river waters has not been still revealed . Thus there is a probability for another reason - climate change.
In fact, there is a correlation between variability of the Black Sea hydrophysical parameters and global processes of interaction existing in the system "Atmosphere - World ocean" [8, 9]. These processes caused changes in gyres movement trajectories and river discharge variations resulting in transformations in hydrodynamic characteristics in the NW Black Sea, where the main processes of CIL formation take place. Biotic properties of this layer in a number of cases determine vertical distribution of plankton community [7, 8, 10, 11].
Therefore, it seems to be of importance to test the hypothesis about the effect of climate change on plankton community characteristics and variability of the bioluminescence field in the Black Sea in the second half of the last century [7, 12, 13, 14].
Experiments. Studies in the bioluminescence field (BF) vertical structure have been conducted in the Black Sea from 1965 to 2008 (at more than 2500 stations). The Data base of Bioluminescence Field of the World ocean (ONR, 2003) , as well as the materials of biophysical monitoring, fulfilled in 2007-2008 in Sevastopol Bay and in the zone of five miles away were applied for the analysis. The methods of studies were based on instrumental measurements in situ in a real time scale (vertical sounding of bioluminescence intensity), as well as on its spatial conjunction and correlations with biological and hydrophysical characteristics of water masses [13, 14].
The photic zone temperature, which plays an important role in plankton biotope spatial structure formation , was studied as the background parameter of water column. The plankton community species composition, abundance and biomass of mass forms were investigated as pelagial biological characteristic.
All the measurements of bioluminescence in the Black Sea aquatorium, disregarding time of their fulfillment were accorded with those by hydrobiophysical complex "SALPA" , comprising measurements conducted by new generation device "SALPA-M". Besides BF characteristics, complex "SALPA-M" permits to carry out measurements of water column temperature and electric conductivity, hydrostatic pressure, turbidity and fluorescence of chlorofill "a" at a horizon of the device position.
Results and Discussion. During the last 25 years intensive climatic changeability has been observed in the north hemisphere . The period of 1980 - 1995 years was found under positive stage of NAO development. Difference in NAO indices values in 1970 and 1989 appeared to be about 5 -7 times more, than in previous 2 decades . After cold period of 1985 - 1987 (fig.1) air temperature in the Black Sea increased during the next decade with exclusively warm years .
Figure 1. Annual average air temperature at Ukrainian coastal meteorological stations for the period of 1950-2005.
The similar pattern was also observed in the north Atlantic: winter water surface temperature turned out averagely low, and in1986 it reached the lowest registered value for all the periods of studies starting from the end of 19-th century [21, 22]. To compare the previous and the following years considerable shift towards the south of the Golfstream northern boundary was marked during 1986-1988. [2з]. Another situation occurred in winter 1989, when intensive advection of warm and humid air from the north Atlantic led to warm winter in western and northern Europe .
It is known, that changeability of CIL warmth stock mainly depends on index of winter severity, evaluated as the sum of negative air temperatures in winter [10, 20]. Analyzing long trend of average year temperature in the CIL core (fig.2) it is important to notice, that it was high before the beginning of 1980-ties that testify a period of weak renewal of CIL waters. From 1985 to 1995 the layer appeared again intensively renewed due to cooling of surface waters in severe winters.
Before 1975 salinity in the layer 0 - 50 m was found to have mainly positive anomalies, and after 1975 - negative ones . These factors facilitated formation of CIL waters with increased density, which in its turn endeavors their influence and penetration into open and central regions of the sea. The level of CIL development can serve as the criteria of biotic properties of the coastal or open sea regions under studies , as CIL affects oxygen concentration in the surface layer 0 - 100 m (fig.3).
In 1957 we can clearly observe a tendency enhancing antigyre activity due to soft climate and weakening of general water circulation at sea.Vast oxygen "hollows" were registered practically in all the zones of the antigyres quazistationary position. In 1985 there was quite another situation: oxygen spatial distribution appeared almost even and at less depths.
Figure 3. Oxygen distribution (liM/1) in 0 - 100 m layer in 1957 (weakening of general water circulation in the sea) - upper picture, and in 1985 - lower picture.
- 2,0 ind/m3, in 1992-1993 - 3,6 ind/m3 . These data clearly demonstrate, that in the period of positive NAO meanings, when the surface water temperature in the Black Sea increases, N. scintillans exists under unfavorable conditions for the development.
- 20 m (fig 4C). It is interesting to note, that in 40 - 50-ties, when the similar formation of exclusively cold CIL core took place (fig 2), similar phenomena was observed in the period of 1980-1995: lower zooplankton habitat boundary went up to the surface .
Figure 4. Variability of the euphotic layer bioluminescence at the centre of the Black Sea western chalistase in 1976 (A), 1983 (B) and 1989 (C) at night time.
As BF characteristics are expressive index of the plankton community functional state, it seemed to be important to conduct comparative evaluation of bioluminescence amplitude parameters while monitoring Sevastopol coast in 1965 - 1966 and in 2007 - 2008. But as all the BF measurements in the middle of 60-ties of the last century were executed exclusively at night time and current monitoring was conducted at the day time as well, interpolation of the day measurements with those made at night was carried out (fig.5).
Thus it can be concluded that plankton community sustains stability despite of tough anthropogenic load in 70-80ss of the last century, biological pollution (particularly invasion of ctenophora Mnemiopsis leidyi), global warming, etc. Also, we should note that there is shift of the spring maximum of phytoplankton development and intensity of BF towards earlier periods and autumn maximum towards later periods. We link these processes with global warming, that we can see at fig.6. Really, the average temperature of the 0-50 m layer in monitoring at 2007-2008 was close to 1.00C at autumn and close to 0.8°C at spring more that the same time in monitoring at 1965-1966 (fig.6).
Figure 6. Temperature while monitoring in 1965-1966 and 2007-2008.
Thus on the background of climatic changes, all other factors, influencing the development of the Black Sea pelagic community appears to be the derivatives of more significant and effective process. For example, anticyclonic gyres, developing in the Black Sea during negative NAO values, put CIL to the depth of 100 - 120 m thus creating quazistationary chain of oxygen "hollows", which are the main areals for many pelagic species of the cold - like complex.
observe intensive development of phytoplankton in the thin surface layer, general decrease of plankton size and increase of water reservoir trophity. On the whole these factors result in structural reconstruction of pelagic community and change in taxonomic composition of the Black Sea dominant species. That is why prolonged periods of anticyclonic activity weaken, conditioned by high NAO indices that led to replacement of cold-water species by those of warm -water and reconstruction of pelagic community.
In the periods of high NAO meanings mass appearance of warm -water species - aliens in Atlantic and in all the European seas takes place . It can be clearly demonstrated by dynamics of biomass variations of cold - water species Calanus finmarchicus in the north Atlantic, the abundance of which is in antiphase with NAO index variations . The same situation exists with the Black Sea bioluminescents, which for the most part do not belong to warm - liking species. Thus with increase of density and oxygen stratification in 0 -75 m, they concentrate in the middle of thermocline and form very narrow habitation layer: lower - oxygen lacking CIL, higher - water, warmed over temperature optimum. Therefore to the end of prolonged period of positive NAO meanings, in 1992 bioluminescence layers in deep waters were registered only at 15 - 20 m, that drastically differed from soft climate period of 70 - 80-ties.
Thus long-term changeability of water circulation, caused by climate change, determines tendency and dynamics of plankton community structural transformations which are reflected in alterations of bioluminescence field intensity - the indicator of the Black Sea pelagic community ecological state.
1. Variability of the Black Sea plankton community is determined by long-term changeability of the climatic processes.
2. Shifts of spring and autumn maxima of the Black Sea phytoplankton development and bioluminescence characteristics were registered between 1965 - 1967ss and 2007-2008ss are connected with global warming.
3. The functional state of the Black Sea luminous plankton community nowadays it is practically identical to its functional state which is attributed to the mid of 60 - ties of the last century.
Acknowledgements. We present our sincere gratitude to all scientific staff of the IBSS Biophysical Ecology Department for active discussion of the materials obtained, and for valuable remarks.
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