A. Yilmaz

Transport and distribution of nutrients and chlorophyll-a by mesoscale eddies in the northeastern Mediterranean

Abstract The distribution of nutrient elements and chlorophyll- a in the Northern Levantine Basin (NLB) were investigated in some detail and are discussed together with the physical aspects of the region in the present study. The surface circulation pattern of the NLB was studied and the structure of the relatively large-scale Rhodes cyclonic gyre, which is located between Rhodes and Cyprus, was investigated for the same time period. The most important characteristics of this cyclonic gyre are the upwelling of nutrient-rich deep waters within the gyre and the reverse phenomenon at the peripheries. Anticyclonic circulation systems generally surround the Rhodes cyclonic gyre; the permanent ones are located in the southern part of the Rhodes gyre, in the Cilician Basin, and off Iskenderun Bay. The vertical distribution of nutrients in the water column shows completely reversed trends and the nutrient gradient ranges between 300–400 m in the central parts of the anticyclonic systems. Thus downwelling processes also occur in the NLB and the formation of Levantine Intermediate Water (LIW) observed in the NLB matches the chemical data presented here. This special vertical and spatial distribution of nutrients affects the distribution of the phytoplankton population, as the patches of primary producers are aggregated in the central parts of the Rhodes gyre. This is confirmed by the chlorophyll- a data and the unexpected content of neuston net collection in the same region. The concentration of chlorophyll- a was relatively high at offshore stations, such as in the central parts of the cyclonic gyres, where zooplankton, small shrimp and fish larvae, etc., were observed in large quantities.

The effect of cold- and warm-core eddies on the distribution and stoichiometry of dissolved nutrients in the northeastern Mediterranean

The nutrient distribution and phytoplankton production in the Levantine Sea of the eastern Mediterranean are principally determined by the duration and the intensity of deep winter mixing in the quasi-permanent anticyclonic and cyclonic eddies. In the seasons of stratification, a nutrient-poor aphotic layer is formed between the euphotic zone and the nutricline; interestingly, it consistently extends down to depths of about 29.0–29.05 isopycnal surfaces, but nearly vanishes in the core of the cyclonic Rhodes Gyre (RG) due to the upwelling of the Levantine deep water (LDW) up to the base of the euphotic zone. Accordingly, the nutricline is much sharper and shallower in the cyclonic RG; nevertheless, it is consistently established between the density surfaces of 29.00–29.05 and 29.15 throughout the basin. In the severe winters of 1992 and 1993, the upper 1000 m of the cyclonic Rhodes Gyre was occupied by the LDW with its associated chemical properties and abnormally high nutrient concentrations (NO3=3.8–4.7μM; PO4=0.14–0.16μM and Si=7.3–7.8μM) were observed in the euphotic zone. However, the surface nutrient concentrations of the anticyclonic regions were raised merely from the summer–autumn values of <0.02 and nearly 0.2μM to about 0.03 and 0.8μM for phosphate and nitrate, respectively. The molar ratios of nitrate to phosphate in the water column range between 5 and 20 in the euphotic zone but exhibit well-defined peak values (as large as 40–120) at the top of the nutricline (corresponding to nearly the depths of the 29.05 isopycnal surfaces) for most of the year. Such prominent maxima are the result of the apparent shift between the onsets of the nitracline and phosphacline due to as yet undefined factors. Below the nutricline the N/P ratios decrease regularly and reach an almost constant deep value (=28) over the basin. The mean ratio, derived from linear regression of the pooled phosphate and nitrate data from March 1991 to March 1994 is about 23.6, substantially higher than deep ocean values.

Characteristics of deep chlorphyll maximum in the Northeastern Mediterranean with respect to environmental conditions

Abstract The vertical distribution of chlorophyll-a was recorded throughout the northern part of the Levantine Basin of the eastern Mediterranean and was related to patterns of the physical dynamics for October 1991–March 1994 period. A well developed deep chlorophyll maximum (DCM) was observed in the northern Levantine Basin (NLB), with concentrations greater than 1 μg/L at depths ranging from 45 to 100 m on average. Chlorophyll-a concentrations ranged between 0.01 (in surface waters, Oct. 1991) and 3.07 μg/L (in subsurface waters, March 1992). In general high concentrations of chlorophyll-a were observed in late winter. In cyclonic regions the depths of the DCM and the nutricline coincided and relatively high concentrations were observed at shallowe depths at relatively high percentages of surface light. In anticyclonic regions the DCM (at low level of concentration) were located at the base of the euphotic zone and much above the nutricline. Well defined DCM feature was not prominent since at most of the stations, uniform distributions of chlorophyll-a were observed in the euphotic zone during the cooler winter conditions in 1992. The chlorophyll concentrations were significantly high in this winter when compared with those of ordinary mild winters. Because of the relatively low chlorophyll-a concentration resulting most probably low phytoplankton biomass in the basin and low input of material from the land, a thick euphotic zone forms with an average value ∼ 80 m. Euphotic zone is nutrient depleted and the concentrations are close to detection limits (e.g. 0.02 μM for phosphate and > 0.05 μM for nitrate) and in general they do not show significant variations. Nutricline takes place in the euphotic zone in cyclonic regions. In anticyclonic regions, the main nutricline is deep (as deep as 600 m). In cooler winter conditions in 1992, very high concentrations of both nitrate and phosphate (almost equal to deep values) were observed in the euphotic zone in the cyclonic Rhodes region. In deep waters phosphate and nitrate concentrations stay almost constant at the levels of nearly 0.2 and 5.5 μM, respectively.

Organic carbon distribution in the surface sediments of the Sea of Marmara and its control by the inflows from adjacent water masses

Abstract The organic carbon contents and textural composition of a total of 166 surficial sediment samples (from 10 to 1226 m water depths) together with data on primary productivity rates and dissolved oxygen concentrations have been studied to investigate the main controls on the distribution of organic carbon buried within the modern sediments across the Sea of Marmara. The distribution of average annual primary production rates in the Sea of Marmara exhibits great lateral variations; the highest values are calculated for the southern shelf (161 gCm −2 year −1 ), the areas with high terrigenous input supplied by the southerly major rivers, and on the northeastern shelf (104 gCm −2 year −1 ) where organic- and nutrient-rich surface inflow from the Black Sea is prominent. The low primary productivities estimated for the southwestern shelfof the Sea of Marmara (64 gCm −2 year −1 ) suggest influences from the relatively organic- and nutrient-poor subsurface inflow from the Aegean or Mediterranean. Organic carbon contents in sediments from the northeastern (0.37–2.16%), northern (0.57–1.64%), southern (0.44–1.90%) and southwestern shelf regions (0.37–1.51%) all appear to be within the same range and show no direct relationship with surface productivity and oxygen deficiency in the Sea of Marmara. Production and accumulation of organic matter in the Sea of Marmara are believed to have been mostly affected by the inflow of relatively organic-rich Black Sea waters, by the southerly major rivers, and by inflow of organic-poor Aegean or Mediterranean waters. Lateral offshore transport in surface waters must have resulted in the decrease of organic carbon fluxes to the sediments.

Evaluation of SeaWiFS chlorophyll‐a in the Black and Mediterranean Seas

The performance of NASAs OC2 and OC4 algorithms to estimate chlorophyll‐a concentrations from SeaWiFS radiometric measurements on the global scale was tested in two contrasted bio‐optical environments, the Black Sea and the Mediterranean Sea. The in situ bio‐optical measurements were made during October 1999 at 25 stations. Comparisons of the in situ measurements with the concurrent SeaWiFS retrievals indicate that the OC2 and OC4 algorithms are not working satisfactorily in both seas. Case 2 waters dominate the Black Sea and the failure of the algorithms is expected. On the other hand, failure of the algorithms in the case 1 waters of the Mediterranean Sea may be due to their specific optical properties. Modifying the OC4 algorithm to include SeaWiFS information at 412 nm yields a better performance in the Mediterranean Sea without degrading performance in the Black Sea. Combining a local algorithm adapted to oligotrophic waters of the Mediterranean Sea and OC4 provides the best results overall.

Comparison of five humic acids

Abstract The ultimate analyses, u.v-visible absorption spectra, fluorescence spectra and i.r. spectra of five humic acids have been studied. One was obtained commercially from peat, two from organic-rich soils, one from an estuary and one from a marine sediment. The u.v. spectra were used to compare average molecular weights which varied from 800 to 7000 and increased with the atomic H C ratio. Fluorescence appeared to be due to poly-nuclear aromatic structures. All the humic acids showed maximum excitation at 360 nm and developed maximum emission in the range 430–455 nm but the humic acids from sediments showed an additional emission maximum at ~410 nm. Fluorescence from the peat humic acid was broad and secondary emission was observed with a maximum at 520 nm which was attributed to the formation of excimers. Fluorescence has been used to monitor the interaction of cations with humic acids in solutions of different ionic strengths which shows promise for distinguishing between metat—humate complex formation and the coagulation of a colloid. The structures of the humic acids are discussed in terms of the structures possessed by lignites.

The dynamics of nutrient enrichment and primary production related to recent changes in the ecosystem of the Black Sea

The data collected in the southern Black Sea during the period of July 1997 to September 1998 showed that light penetrated into the upper 20-35 m, with a downward attenuation coefficient varying between 0.1 and 0.25 m -1 . The chlorophyll-a concentration for the euphotic zone ranged from <0.5 to 1.5 µg l -1 . Coherent sub-surface chlorophylla maxima formed near the base of the euphotic zone at all study points and a secondary one was observed at very low level of light (<0.1% of the surface light) in the nearshore regions. Production rates were estimated in between 62 and 785 mg C m -2 d -1 in this period. Bioassay experiments (using extra nitrate, ammonia, phosphate, silicon and iron) showed that under optimum light conditions the phytoplankton population is nitrate limited in open waters. Phosphate seems to control the growth in the nearshore regions of the southern Black Sea. Silicon concentration also influenced the phytoplankton growth since the majority of the population was determined to be diatoms.

Abundance and Elemental Composition of Particulate Matter in the Upper Layer of Northeastern Mediterranean

Suspended particulate (POC PON, PP) profiles obtained in 1991-1994 indicate the existence of characteristic subsurface maxima near the base of the euphotic zone in the cyclonic Rhodes gyre and its peripheral waters in the Northeastern Mediterranean. Interestingly the N:P of the bulk seston was reasonable during stratification seasons when the surface water was relatively poor in phosphate; but the ratio was unexpectedly low (N:P=6-12) in the late winter of 1992 when the surface layer of Rhodes gyre was occupied with nutrient rich deep waters.

Distribution of Planktonic Primary Production in the Black Sea

Planktonic primary production data from the entire Black Sea are reviewed for the last two decades. Surface and vertical profile of data are spatially and seasonally compared for different significant areas (north-western shelf, western and southern coast, eastearn and western halistatic zones). High production rates, especially in the coastal waters of the NW and W Black Sea exhibit large inter- and intraseasonal variations. Such values clearly reflect meso- and eutrophic feature of these waters. There are major annual spring (diatoms) and autumn (coccolithophorids) blooms, followed in recent years by additional summer (dinoflagellates, coccolithophorids) blooms. The major primary producers are usually Skeletonema costatum, Chaetoceros curvisetus, Peridinium trochoideum, Exuviaella cordata and Prorocentrum micans. Factorial analyses reveal very high correlation coefficients between chlorophyll a concentration and salinity, primary production and salinity, and between chlorophyll a concentration and primary production. The annual cycle of plankton dynamics in the central Black Sea is being studied by means of an one-dimensional vertically resolved physical-biological upper ocean model, involving interactions between inorganic N (NO3, NH4), phytoplankton and herbivorous zooplankton, and detritus.

Pelagic tar in the Mediterranean Sea

Abstract Floating tar samples were collected, using neuston nets, in 101 stations in the Mediterranean Sea in August–September, 1987, by research vessels of Cyprus, Germany, Israel, and Turkey. The distribution of the tar content indicates that the most tar contaminated sea is in the northeast between Cyprus and Turkey and in the Gulf of Sirte off the coast of Libya, where the mean tar content was 1847 and 6859 μg m−2, respectively. The least polluted areas were the western Mediterranean, 236 μg m−2, and the northern Ionian Sea as far east as halfway between Crete and Cyprus with mean tar concentration of 150 μg m−2. Strongly heterogeneous but overall intermediate mean values of 1347 and 876 μg m−2 were found in the Levantine Basin west and south of Cyprus, respectively. A comparison between pelagic tar data collected in 1969, in 1974 and our data shows a sharp decline in tar concentration with time, from 37,000 μg m−2 in 1969 to 9700 μg m−2 in 1974 and to 1175 μg m−2 in 1987. The geographical distribution of tar may be explained by the severe reduction in the activity of oil terminals in Israel, Lebanon and Syria which causes a reduction in tar content in the southeastern Mediterranean. The increased activity of the oil terminal in Iskenderun Bay, Turkey, and the activity of oil loading at the terminals in Libya still leave a high level of tar pollution in the Mediterranean water off these coasts. However, recent technologies in the oil shipping industry, international conventions on oil pollution of the Mediterranean, and harsher steps to administer the anti pollution laws by various countries in the Mediterranean have caused a general decline of this problem.