Suleyman Tugrul

Simulation of annual plankton productivity cycle in the Black Sea by a one‐dimensional physical‐biological model

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one-dimensional vertically resolved physical-biological upper ocean model, coupled with the Mellor-Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place sometime between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December–January and emerges as the winter bloom, or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January–March period. These bloom structures are similar to the year-to-year variabilities present in the data.

Nutrient and organic carbon exchanges between the Black and Marmara Seas through the Bosphorus Strait

Abstract Recent systematic chemical data, together with a new estimate of the water fluxes, permit the calculation of the total phosphorus (TP), nitrogen (TN) and organic carbon (TOC) exchanged between the Black and the Marmara Seas through the Bosphorus. Assuming the chemical concentrations of the exchanging waters to be constant on a yearly time scale, the estimated total annual fluxes are as follows. TP TN TOC Influx into the Marmara Sea 1.2 × 104 1.9 × 105 1.52 × 106 tons Influx into the Black Sea 1.0 × 104 0.6 × 105 0.35 × 106 tons The mainly river-borne annual flows of TN and TOC into the Sea of Marmara from the Black Sea are about three times those from the Marmara Sea into the Black Sea whereas the TP exchanges are comparable. The large TN export by Black Sea waters relative to the TP outflux is the result of the high N:P ratio of nutrients [primarily in the forms of nitrate, less labile dissolved organic nitrogen (DON) and ortho-phosphate] introduced by the polluted rivers to the northwestern Black Sea. The DON comprises about 75% of the TN inflow into the Marmara Sea whereas nearly 50% of the TP inflow is composed of dissolved inorganic phosphorus. Biologically labile nutrients exported from the Black Sea, corresponding to a new production of (3.5–4.9) × 105 tons C y−1 in the Marmara Sea, are almost compensated by the Bosphorus underflow as nitrate and phosphate primarily of biogenic origin.

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.

Identification of the oxic/anoxic interface by isopycnal surfaces in the black sea

The analysis of hydrochemical data collected in the Black Sea since 1987 shows that the upper boundary of the oxic/anoxic interface zone coincides with the nitrate maximum at the depths of σ t ⋍ 15.40 ± 0.10 isopycnal surface. Its lower boundary corresponds to the phosphate maximum depth at σt ⋍ 16.20 ± 0.05 isopycnal surface, independent of the geographical location and seasion. In the absence of the continuous pump cast measurement system, and when the oxygen and sulphide concentrations are too low to be measured with sufficient precision within the interface zone, such features of the oxygen-nitrate and H2S-phosphate correlations at specific density levels provide a direct and practical way to identify the oxic/anoxic interface zone. This, in turn, allows for a more precise and systematic water sampling for studying the complex biogeochemistry of the layer.

The chemical composition of Black Sea suspended particulate organic matter : pyrolysis-GC/MS as a complementary tool to traditional oceanographic analyses

Abstract A “traditional” description of the abundance and chemical composition of suspended particulate organic matter (POM) in open and coastal waters of the southern Black Sea in June 1996 has been confirmed and extended by pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) analyses. Py-GC/MS provided depth profiles of the relative concentrations of twenty three marker compounds characteristic of chlorophylls (CHL), lipids, carbohydrates (CBH) and proteins produced by thermal degradation of the POM retained on the filters. No terrestrial markers, characteristic of lignin or of plant waxes, were observed. Evidence was found for considerable changes in the chemical composition of POM in the water column from the surface down to the sulphidic water layer. In surface-mixed layer, both POC:CHL-a ratios and relative abundances of CBH markers were notably high, suggesting that the suspended POM was mainly composed of detritus. The profiles of both CHL and protein markers exhibit coherent maxima at the base of the euphotic zone, coinciding with the nutricline depth in the central cyclonic eddy, where the bulk POM possessed relatively low C:N ratios. Beneath the 0.1% light depth the absence of intact phytoplankton cells and the presence of bacteria and faecal pellets was accompanied by a change in the protein composition of the POM as shown by the changes in the ratio of pyrrole:indole markers. Lipid markers increased markedly from the euphotic zone into the oxycline and remained almost constant in the suboxic waters; they then decreased in the sulphidic interface, presumably due to consumption of lipids by anaerobic bacteria.

Elemental composition of seston and nutrient dynamics in the Sea of Marmara

The Sea of Marmara, an intercontinental basin with shallow and narrow straits, connects the Black and Mediterranean Seas. Data obtained during 1991–1996 have permitted the determination of the elemental composition of seston in the euphotic zone and the N: P ratio of the subhalocline waters of the Marmara Sea. Since primary production is always limited to the less saline upper layer (15–20 m), of the Marmara Sea, the subhalocline waters of Mediteranean origin are always rich in nutrients (NO3 + NO2 = 8–10 µm, PO4 = 0.8–1.2 µm) but depleted in dissolved oxygen (30–50 µm) throughout the basin, yielding an - 02: N: P ratio of 178: 9: 1. Pollution of the surface waters since the 60s has modified the subhalocline nutrient chemistry slightly. In the euphotic zone, the N: P ratio of the seston changes from 5.9 to 9.5 between the less and more productive periods. Though the biology of the Marmara has changed significantly during the previous two decades, the close relationship observed between the elemental composition of the surface seston and the NO3: PO4 ratio of the subhalocline waters strongly suggests that during the whole year primary production throughout the basin and POM export to the lower layer remain nitrogen-limited. This suggestion needs to be confirmed by bio-assays, biological studies and sediment trap data from the upper subhalocline depths. Nonetheless, the counterflows in the Marmara basin possess relatively low N: P ratios in both dissolved and particulate nutrients and extend as far as the adjacent seas.

Variations in the Vertical Structure of Water Chemistry within the Three Hydrodynamically Different Regions of the Black Sea

High-resolution, basin-wide chemical data reveal that the Black Sea upper layer possesses distinct chemical features at specific density surfaces, but with some noticeable regional differences in their positions and magnitudes. The nitracline, is consistently established at smaller density surface relative to the phosphocline over the basin; thus, N:P molar ratios appear to be unexpectedly high (40–80) within the upper nutricline, especially in the anticyclonic regions (ACR). The ratio drops to levels of 4–8 at the nitrate maxima formed within the base of the main oxycline. Phosphate profiles display a prominent minimum within the suboxic zone of the cyclonic regions (CR), which weakens markedly within the coastal regions and rim current. Vertical distributions of chemical ratios have led to valuable information on the relative fluxes in the oxic/anoxic transition zone as well as the quality of data sets from different years.

Comparison of TOC concentrations by persulphate-UV and high-temperature catalytic oxidation techniques in the Marmara and Black Seas

Abstract Total organic carbon (TOC) concentrations were determined in the Marmara Sea and both oxic and anoxic waters of the Black Sea, using the Shimadzu high-temperature catalytic oxidation technique and the Technicon persulphate-UV oxidation method. The TOC values in the deep waters of the Marmara Sea ranged between 60 and 73 μ MC by the Shimadzu technique and from 40 to 50 μMC by the Technicon method. The TOC values obtained by both methods in the deep anoxic waters of the Black Sea varied between 105 and 130 μM. These values differ significantly from those in the literature, indicating that available TOC data are not accurate enough for reliable estimation of the carbon budget of the Black Sea. Storage of the anoxic water samples accompanied by freezing, acidifying or poisoning with HgCl 2 did not increase TOC recovery.

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.

Spatial Isopycnal Analysis of the Main Pycnocline Chemistry of the Black Sea: seasonal and interannual variations

The results of 1991–1994 basin-wide investigations have been analysed using the method of spatial isopycnal analysis. Seasonal and interannual variations of phosphates, nitrates, dissolved oxygen and the suboxic zoneare revised. The range and possible reasons of spatial and temporal variations in the density-dependent vertical profiles of nutrients and oxygen in the layer of the main pycnocline, in the thickness and the position of suboxic zone of the Black Sea are discussed.