Climatic warming and accompanying changes in the ecological regime of the Black Sea during 1990s

The Black Sea ecosystem is shown to experience abrupt shifts in its all trophic levels from primary producers to apex predators in 1995 - 1996. It arises as a manifestation of concurrent changes in its physical climate introduced by intensive warming of its surface waters as well as abrupt increases in the mean sea level and the net annual mean fresh water flux. The warming is evident in the annual-mean sea surface temperature (SST) data by a continuous rise at a rate of similar to 0.25 degreesC per year, following a strong cooling phase in 1991 - 1993. The most intense warming event with similar to2 degreesC increase in the SST took place during winters of the 1994 - 1996 period. It also coincides with 4 cm yr(-1) net sea level rise in the basin, and substantial change in the annual mean net fresh water flux from 150 km(3) yr(-1) in 1993 to 420 km(3) yr(-1) in 1997. The subsurface signature of warming is marked by a gradual depletion of the Cold Intermediate Layer ( characterized by T \u3c 8 °C) throughout the basin during the same period. Winters of the warming phase are characterized by weaker vertical turbulent mixing and upwelling velocity, stronger stratification and, subsequently, reduced upward nutrient supply from the nutricline. From 1996 onward, the major late winter-early spring peak of the classical annual phytoplankton biomass structure observed prior to mid- 90s was, therefore, either weakened or disappeared altogether depending on local meteorological and oceanographic conditions during each of these years. The effect of bottom-up limited unfavorable phytoplankton growth is reflected at higher trophic levels (e.g., mesozooplankton, gelatinous macrozooplankton, and pelagic fishes) in the form of their reduced stocks after 1995

Comparative analyses and modelling for regional ecosystems of the Black Sea

A zero dimensional ecosystem model is applied to the Black Sea to simulate the behaviour of its regional ecosystems. The modelling is guided by the analyses of the seasonal changes of mixed layer depth, nutrients and chlorophyll-a in the model regions, based on the available data in these areas of the Black Sea. Nutrients are continuously supplied to the adjacent coastal area by rivers, and in the other regions, mixed layer nutrient concentrations generally increase by mixing in winter. Despite the scarcity of data, it appears that the maximum chlorophyll-a concentration occurs in February-March in the central Black Sea, and in April-May in the peripheral regions, where the level is also an order of magnitude higher, as a result of riverine and coastal sources. The model successfully reproduces basic features of seasonal plankton and nutrient changes, and helps interpret the available observations and identify the factors creating the observed regional differences in productivity. The computed seasonal cycle of the chlorophyll-a compares well with the chlorophyll measurements in the central Black Sea. On the other hand, advection of nutrients is found to be important along the western and southern Black Sea coastal areas downstream of the river sources. Near the Bosphorus, reasonable agreement of model results and observations could only be ensured when the seasonal pattern of advection of river nutrients were taken into account. Better representation of the seasonal cycles including spring and autumn blooms are obtained with a nine compartment size fractionated model. Preliminary results are presented for a case including jelly organisms such as the ctenophore Mnemiopsis leidyi

Structure of Emiliania huxleyi blooms in the Black Sea surface waters as detected by SeaWIFS imagery

The temporal and spatial characteristics of coccolithoprid Emiliania huxleyi blooms in the surface waters of the Black Sea axe studied using the Sea-WIFS mean normalized water-leaving radiances data for 1998-2000. It is shown that the Black Sea consistently experiences high reflectance patches of coccolith platelets throughout the basin each year during the May-July period. Although the Black Sea is masked by clouds, the data also suggest enhanced activity for some period during autumn and early winter. Their spatial patterns resemble very closely the circulation system derived from the altimeter data, and exhibit pronounced differences between cyclones and anticyclones. The cyclonic cell, which covers the entire interior part of the basin, appears as a more favourable site for more intense bloom formation. This is related with its relatively shallower mixed layer thickness and stronger mixed layer average water leaving radiance

Satellite-detected early summer coccolithophore blooms and their interannual variability in the Black Sea

Interannual variability of the prevalent early summer coccolithophore blooms within surface waters of the Black Sea was studied by means of satellite-based bio-optical observations. Two coccolith detection algorithms, tested for the Black Sea conditions, were found to provide comparable spatial coccolith patterns consistent with the corresponding true color images. Reliability of the algorithms is also supported by several sets of time-series measurements in different parts of the basin. An analysis of 6 year-long series of OCTS and SeaWiFS images from 1997 onwards shows major phytoplankton bloom activity every year in early summer. These blooms are often dominated by dense populations of coccolithophores but with some year-to-year variations in their spatial coverage, intensity, duration and timing of their maximum expanse over the basin. Moreover, limited coccolith coverage noted in the historical CZCS data may imply an increasing contribution of coccolithophores to the summer phytoplankton community from the early 1980s to 1990s. It is consistent with almost an order of magnitude increase in phytoplankton production, and the current view of dramatic shifts in taxonomic composition from diatoms to coccolithophores and dinoflagellates, as a part of transformations that took place in the Black Sea biogeochemistry and ecosystem structure under changing anthropogenic and climate forcing

Structure of Emiliania huxleyi blooms in the Black Sea surface waters as detected by SeaWIFS imagery

The temporal and spatial characteristics of coccolithoprid Emiliania huxleyi blooms in the surface waters of the Black Sea axe studied using the Sea-WIFS mean normalized water-leaving radiances data for 1998-2000. It is shown that the Black Sea consistently experiences high reflectance patches of coccolith platelets throughout the basin each year during the May-July period. Although the Black Sea is masked by clouds, the data also suggest enhanced activity for some period during autumn and early winter. Their spatial patterns resemble very closely the circulation system derived from the altimeter data, and exhibit pronounced differences between cyclones and anticyclones. The cyclonic cell, which covers the entire interior part of the basin, appears as a more favourable site for more intense bloom formation. This is related with its relatively shallower mixed layer thickness and stronger mixed layer average water leaving radiance

Is there any relationship between phytoplankton seasonal dynamics and the carbonate system?

Production of calcium carbonate by marine calcifying organisms has been shown to decrease under increasing CO2. This effect appears to be driven by a decrease in [CO32?]. The modelling study here described aims at investigating whether the success of a marine calcifying phytoplankton species, the coccolithophore Emiliania huxleyi, may be tied to [CO32?]. The work highlights the complex interactions between the carbonate system variables and spring blooms, and the possibility of a link to the competition between calcifying vs. non-calcifying species on the Bering Sea shelf. We find that the strong seasonal cycle in [CO32?] is driven primarily by carbon drawdown during spring blooms. The interesting outcome of this work is the fact that E. huxleyi bloom timings always coincide with periods of high [CO32?], which is consistent with studies showing coccoliths malformations and a slowdown in calcification at low [CO32?]. Whether the condition of high [CO32?] can be considered a crucial ecological factor for the success of E. huxleyi, however, remains an open and important question needing further investigation

Interannual variability of the early summer coccolithophore blooms in the Black Sea: impacts of climatic factors

Interannual variability of the early summer (May-June) coccolithophore blooms within surface waters of the Black Sea was studied by means of satellite-based bio-optical observations. The performance of two coccolithophore detection algorithms were tested for Black Sea conditions, and were found to provide comparable spatial patterns consistent with the corresponding true color images. An analysis of six, year-long OCTS and SeaWiFS imagery from 1997 onwards points to the presence of a major phytoplankton bloom in every early summer season. Blooms are dominated by densely populated coccolithophore algae within the entire basin, except during 2001. In the early summer of 2001, the coccolithophore activity was limited to the northeastern coastal zone, and the bloom in the rest of the basin was formed by non-coccolithophore groups, as suggested by their relatively strong chlorophyll signature. More coccolithophore over, limited coccolithophore abundance noted in the historical CZCS data suggests substantial differences in terms of spatial coverage and total biomass from the early 1980s to the late 90s. The increasing contribution of coccolithophores to the early summer phytoplankton community structure during the last decade is also consistent with the current view of dramatic shifts in taxonomic composition from diatoms to coccolithhophores and flagellates, as a part of transformations that took place in the Black Sea biogeochemistry and ecosystem structure under changing anthropogenic and climate forcing during the 1980s and 1990s, respectively

Optical properties of mineral dust outbreaks over the northeastern Mediterranean

Ground-based aerosol optical measurements were conducted within the framework of the Aerosol Robotic Network (AERONET) program at the IMS-METU site at Erdemli (36degrees33'N, 34degrees15'E) along the Turkish coast of the northeastern Mediterranean from January 2000 to June 2001. The measurements were used to identify and define predominant regional aerosol optical properties, with an emphasis on mineral dust intrusion events. Dust storms affecting the region primarily originate from the central Sahara in spring, the eastern Sahara in summer, and the Middle East/Arabian peninsula in autumn. Summer and autumn dust intrusions usually occurred at higher altitudes (above 700 hPa), whereas urban-industrial aerosols from the north over the Balkan region, Ukraine, and Anatolia were transported to the region at lower altitudes. In addition to a drastic increase in the aerosol optical thickness, in some cases up to 1.8, the dust episodes were characterized by (1) a sharp drop in the Angstrom coefficient to values near zero, (2) a high-scattering with single-scattering albedo greater than 0.95+/-0.03, and the real part of the refractive index around 1.5+/-0.5, both of which acquire slightly higher values at longer wavelengths, (3) a lower absorption given by the imaginary part of the refractive index less than 0.002, and (4) an almost neutral spectral dependence of these parameters. Dust particles possessed a bimodal size distribution with typical volume mean radii of 2.2 mm and 0.08 mum for coarse and fine size fractions, respectively, and corresponding volume concentrations of about 1.0 and 0.1 mum(3) mum(-2) of dust particles. It was apparent that the Saharan and Middle East desert dusts differ in their absorption index values (0.0015 and 0.0005, respectively). The difference is likely a result of their contrasting mineralogies

Analysis of the change in mineral dust optical properties over the Eastern Mediterranean with source location using SEAWIFS imagery

Mineral dust from surrounding and regions is the major aerosol in the Mediterranean atmosphere during spring and summer. The Eastern Mediterranean is particularly interesting because mineral dust may come from three different sources, originate from the central Sahara in spring, the eastern Sahara in summer and the Middle East/Arabian peninsula in autumn. We obtain the best agreement between SeaWiFS and Sun-Photometer derived aerosol optical thickness by considering, the optical properties dust transported from different sources

Is there any relationship between phytoplankton seasonal dynamics and the carbonate system?

Production of calcium carbonate by marine calcifying organisms has been shown to decrease under increasing CO2. This effect appears to be driven by a decrease in [CO32-]. The modelling study here described aims at investigating whether the success of a marine calcifying phytoplankton species, the coccolithophore Emiliania huxleyi, may be tied to [CO32-]. The work highlights the complex interactions between the carbonate system variables and spring blooms, and the possibility of a link to the competition between calcifying vs. non-calcifying species on the Bering Sea shelf. We find that the strong seasonal cycle in [CO32-] is driven primarily by carbon drawdown during spring blooms. The interesting outcome of this work is the fact that E. huxleyi bloom timings always coincide with periods of high [CO32-], which is consistent with studies showing coccoliths malformations and a slowdown in calcification at low [CO32-]. Whether the condition of high [CO32-] can be considered a crucial ecological factor for the success of E. huxleyi, however, remains an open and important question needing further investigation. (c) 2005 Elsevier B.V. All rights reserved