Microalgal blooms in a eutrophic estuary (Golden Horn, Sea of Marmara) following a remediation effort

Abstract

Abstract The present work describes microalgal blooms that occurred in a eutrophic estuary (Golden Horn, Sea of Marmara, Turkey) between October 2013 and September 2014 following a remediation effort. The relationships between bloom-forming microalgal species and environmental factors were investigated during the study period. The changing environmental conditions (e.g. increasing water transparency and salinity) after seawater transfer to the Golden Horn Estuary stimulated phytoplankton growth with dense algal blooms. Annual average values of Secchi depth, salinity and dissolved oxygen increased in comparison with those in an earlier study in 2009–2010. Nine microalgal species, which consisted of four diatoms, two dinoflagellates, one cryptophycean, one raphidophycean and one euglenophycean, formed the blooms with water discolorations during spring and summer. The species that reached the highest bloom density were Plagioselmis prolonga (62.4 × 106 cells l−1) among crytophyceans, Heterocapsa triquetra (21.8 × 106 cells l−1) among dinoflagellates and Skeletonema marinoi (39 × 106 cells l−1) among diatoms. The abundance of dinoflagellates and phytoflagellates increased particularly in the upper estuary when compared to diatoms and their rapid growth and bloom formation revealed that they have a wide range of tolerance to changing environmental conditions and a strong ability to compete with other species in this study area.