The abundance pattern and species diversity of phytoplankton in the surface waters of the Golden Horn Estuary were investigated between August 2011 and July 2012 in relation to environmental factors. Seventy-eight phytoplankton taxa (38 diatoms, 30 dinoflagellates and 10 phytoflagellates) belonging to eight taxonomic classes were identified in bottle and net samples. Phytoplankton abundance increased in spring and summer (from March to August) and reached its highest (10,429 × 103 cells L−1) during the bloom of Heterosigma akashiwo (Raphidophyceae) in the middle and upper estuary in late May. In general, phytoplankton abundance was higher in the middle estuary indicating more suitable conditions for phytoplankton growth, while species richness and diversity was higher in the lower estuary. The most abundant species were Skeletonema marinoi and Thalassiosira sp. among diatoms; Scrippsiella trochoidea among dinoflagellates; Plagioselmis prolonga and Heterosigma akashiwo among phytoflagellates. Diatoms were more abundant in the lower and middle estuary, while dinoflagellates and phytoflagellates in the upper estuary. The main factors causing the spatio-temporal variation of phytoplankton in the study area were temperature, salinity, water transparency and nutrients. Water transparency in the upper estuary is mostly influenced by organic and inorganic matter carried by two streams. As a result, this area should be considered a potential risk area for future algal blooms.

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This study reports the first evidence of domoic acid (DA), an algal neurotoxin produced by the genus Pseudo-nitzschia, from plankton net samples collected in the Sea of Marmara in December, 2010 and February, 2011. DA concentrations of plankton net samples were analyzed by high-performance liquid chromatography (HPLC), using the fluorenylmethoxycarbonyl fluorescence derivatization technique (detection limit 0.2 ng DA). The biotoxin concentrations in samples from coastal waters varied between 0.96 and 5.25 µg DA/mL. We also investigated possible correlations between physicochemical parameters and DA concentration. The DA levels appear to be correlated negatively with silica and nitrite concentrations for both sampling periods. These data may be used to evaluate the probability of finding similar conditions in coastal waters of the Sea of Marmara in order to determine the potential risks to local aquaculture and fisheries.

Abstract:In the period of February-May 2012, harmful algal blooms (HABs) along with environmental factors were investigated biweekly in the Golden Horn Estuary at the northeast of the Sea of Marmara and a dense bloom of the raphidophyceaen Heterosigma akashiwo (Y.Hada) Y.Hada ex Y.Hara & M.Chihara was observed in late May. H. akashiwo abundance increased gradually from the lower estuary to the upper estuary. The dense bloom of H. akashiwo in late May occurred at the temperature of 20.2°C and salinity of 16.4 psu and its abundance reached to 10.4×10 6 cells L -1 . When compared to mid-May, the mean temperature of surface water increased 4.50°C, while salinity decreased 2.30 psu in late May. Secchi depth values decreased rapidly from the lower to the upper estuary (6.00 m to 0.50 m). It is considered that a rapid increase in temperature, a decrease in salinity, and high nutrient concentrations in combination with low water circulation at the upper estuary were the causes of the bloom of H. akashiwo. Fish mortality or other harmful effects in environment were not observed during the H. akashiwo bloom. But, these events may create a potentially toxic risk for the study area in the future.