Estimation of cellular carbon content based on the cell biovolume of microalgae from a eutrophic estuary (Sea of Marmara, Türkiye).

The cellular carbon content based on the cell biovolume of a total of 61 microalgal species determined in a eutrophic estuary (Golden Horn, Sea of Marmara) was estimated in seawater samples taken during two different sampling periods. Cell biovolume according to geometric dimensions of the cells was then converted to phytoplankton carbon using an appropriate conversion factor. The range of diatom biovolume, in which the majority had small cell sizes (<50 µm), was much wider than that of dinoflagellate biovolume, in which the majority had large cell sizes (>50 µm). The cell biovolume and carbon content ranged from 35 to 4.88 × 105 µm3 and 5 to 1.18 × 104 pgC cell-1 for diatoms and from 3.66 × 102 to 8.68 × 105 µm3 and 55 to 8.14 × 104 pgC cell-1 for dinoflagellates, respectively. The mean carbon density for diatoms and dinoflagellates (excl. Noctiluca scintillans) varied between 0.027 and 0.099 pgC µm-3 0.096 and 0.136 pgC µm-3, respectively. The mean cell carbon content and carbon density of dinoflagellates (6.73 × 103 pgC cell-1 and 0.115 pgC µm-3) were approximately 10 and 2 times greater than those of diatoms, respectively. The carbon content of the other phytoflagellates was lower, whereas their carbon density was higher. As a result, the findings from this study will provide a significant contribution to the assessment and estimation of carbon biomass during algal blooms in this study area.

Toxic marine microalgae and noxious blooms in the Mediterranean Sea: A contribution to the Global HAB Status Report.

We review the spatial distribution of toxic marine microalgal species and the impacts of all types of harmful algal events (Harmful Algal Blooms, HABs) in the Mediterranean Sea (MS), including the Black Sea, the Sea of Marmara, coastal lagoons and transitional waters, based on two databases compiled in the Ocean Biogeographic Information System (OBIS). Eighty-four potentially toxic species have been detected in the MS (2,350 records), of which 16 described from these waters between 1860 and 2014 and a few suspected to have been introduced. More than half of these species (46) produce toxins that may affect human health, the remainders ichthyotoxic substances (29) or other types of toxins (9). Nevertheless, toxicity-related events are not frequent in the MS (308 records in 31 years), and mainly consist of impacts on aquaculture, caused by the dinoflagellates Dinophysis and Alexandrium, along with a few actual shellfish poisoning cases. Pseudo-nitzschia blooms are widespread, but domoic acid in shellfish rarely exceeds regulatory levels. Fish kills are probably less sporadic than reported, representing a problem at a few places along the southern MS coasts and in the Ebro River Delta. Since the last decade of the 20th century, blooms of the benthic dinoflagellates Ostreopsis cf. ovata have regularly occurred all along rocky shores of the MS, at times with human health problems caused by toxic aerosol. New records of Gambierdiscus and Fukuyoa, until now reported for the westernmost and easternmost MS coasts, raise concerns about the risk of ciguatera, a syndrome so far known only for subtropical and tropical areas. Recent discoveries are the dinoflagellates Vulcanodinium rugosum, responsible for the presence of pinnatoxins in French lagoons' shellfish, and the azaspiracid-producers Azadinium spp. Mucilages and discolorations have a major impact on tourism in summer. Reports of toxic species and HABs have apparently increased in the MS over the last half century, which is likely related to the increased awareness and monitoring operations rather than to an actual increase of these phenomena. Indeed, while the case of Ostreopsis appears as a sudden upsurge rather than a trend, no actual increase of toxic or noxious events has so far emerged in intensively studied areas, such as the French and Spanish coasts or the Adriatic Sea. Moreover, some cases of decrease are reported, e.g., for Alexandrium minutum blooms disappearing from the Harbour of Alexandria. Overall, main HAB risks derive from cases of massive development of microalgal biomass and consequent impacts of reduced coastal water quality on tourism, which represents the largest part of the marine economy along the MS coasts.

Changes in biodiversity of the extremely polluted Golden Horn Estuary following the improvements in water quality.

Long-term biological data supported by physicochemical parameters were evaluated to investigate the biodiversity of the Golden Horn Estuary from the past to the present. Limited observations dating back to 60 years ago indicated the existence of a diverse community in this small estuary. Unfortunately, in parallel with the increase in unplanned settlements and industry around the Golden Horn, pollution stress increased since the 1960s. Preliminary studies in the 1990s indicated survival of only a couple of pollution-resistant species, in the relatively cleaner lower estuary. Following the intensification of rehabilitation studies in 1998 and particularly after the opening of the floating bridge at the mid estuary; a remarkable day-by-day recovery in marine life has begun with the improving water quality. Nutrient concentrations decreased markedly; while water clarity significantly increased. Fecal coliform values decreased 10(3) fold. Phytoplankton composition changed and dense blooms of eukaryotic phytoplankters frequently occurred. Hydrogen sulfide almost completely disappeared even during the warmest periods of the year and dissolved oxygen concentrations increased. All results clearly depicted that the Golden Horn ecosystem shifted to eutrophic conditions from an anoxic environment. SCUBA dives in 2002, documented the level of diversification of life in the Golden Horn. All appropriate substratums were intensely covered by macrobenthic forms until the Halic Bridge and filter feeders dominated the plankton-rich ecosystem. Achieving the diversity of 1940s is not possible since the Black and Marmara seas, influencing water quality of the Golden Horn, are also suffering from anthropogenic impacts and are far less diverse than their rich diversity in 1940s. However, the Golden Horn is a good example that even the most polluted ecosystems can recover when appropriate measures are taken.