Trophic Characteristics of the Sapanca Lake (Turkey)

Systematic measurements of optical properties, concentrations of major and minor chemicals and primary production throughout 1989-1992 enabled us to identify the oligotrophic state of Sapanca lake, Marmara region, Turkey. Lake waters which have low concentrations of major anions and cations (total = 49 μM) overturn every February-March, ventilating the bottom waters and enriching the surface waters with nutrients. Surface waters cool down to 6.5 °C by late winter and then warm steadily to 26 °C by late summer, while temperatures in deep waters range between 6.5 and 10.0 °C throughout the year. When the seasonal thermocline develops, the dissolved oxygen profiles exhibit a subsurface maximum in the thermocline, while in the hypolimnion water, the content varies seasonally from 11.5-12.0 ppm (350-375 μM) in March to 0.5-1.7 ppm (16-56 μM) in late autumn. Surface nitrate concentrations vary markedly with season, from < 0.15 μM in summer to 5.7 μM in early March, whilst the bottom water concentrations range from 13.5-14.0 μM in late autumn to 5.7 μM after the winter overturn. Phosphate concentrations are always less than 0.1 μM throughout the entire water column. Subsurface chlorophyll-a maximum descends in late summer to 20-25 m depths, where the light intensity is less than 1% of the surface value. Below 10-15 m depths, corresponding to the upper thermocline, primary productivity is very low. The range was from 35 to 93 mg C/ m2 day-1 during 1989-1991, consistent with the values in other oligotrophic lakes

Phytoplankton composition and environmental conditions of a mucilage event in the Sea of Marmara

The composition and abundance of phytoplankton and the corresponding environmental conditions were investigated during a mucilage event (October 2007-February 2008) in the Sea of Marmara. The mucilage producers Gonyaulax fragilis, Skeletonema costatum, and Cylindrotheca closterium were identified as abundant species. Thallassiosira rotula was also identified in large numbers. The maximum number of G. fragilis was observed at 83,600 cells L(-1) in November 2007 during the first sampling occasion on the surface layer of Izmit Bay, and T rotula was the most abundant diatom species, with 131,040 cells L(-1) in the same period. G. fragilis was recorded at 96,250 cells L(-1) in the dense mucilage-containing water samples collected from Degirmendere (Izmit Bay) in January 2008, and C. closterium was the dominant (161,250 cells L(-1)) diatom species in the same sample. Species diversity values (H'(log2)) increased from November to February, displaying maximum values of 2.5-3.5 in February at almost all stations (depths of 0.5 and 10 m), caused by the balanced increase of diatom species and their individual numbers in the total phytoplankton abundance. The surface layer waters of Izmit Bay exhibited severely low nitrogen to phosphorus ratio (N:P) values (0.1-14.4) compared to the Redfield ratio (16), which could be supported by the fact that the limiting nutrient for the Sea of Marmara is nitrogen. Even though there is a lack of data from before the mucilage period, the data obtained during the lifespan of the event indicated that a mixture of several phytoplankton species might have played a key role in the mucilage formation. The high organic carbon content of the mucilage (24% org-C) and the surrounding waters support the possibility that the event could have been related to organically rich cellular exudates of phytoplankton