Observations of a thin near surface layer in an estuarine environment: An exceptional bloom of the dinoflagellate Akashiwo sanguinea in the Lee estuary (Lough Mahon), Co. Cork, in September 2010 Shane O’Boyle a,n , Georgina McDermott b a Environmental Protection Agency, Office of Environmental Assessment, Richview, Clonskeagh Road, Dublin 14, Ireland b Environmental Protection Agency, Office of Environmental Assessment, John Moore Road, Castlebar, Co. Mayo, Ireland article info Available online 20 December 2012 Keywords: Phytoplankton Thin layer Water quality Estuary Dissolved oxygen Ireland abstract Observations on the occurrence of an exceptional phytoplankton bloom in the Lee estuary, Co. Cork, in September 2010 are reported. A thin layer of phytoplankton dominated by the dinoflagellate Akashiwo sanguinea (Dinophyceae), with a cell concentration of 16,900 cells ml 1 and chlorophyll a concentra- tion of 680 mgl 1 , was observed at a depth of 1 m. The layer extended over a horizontal distance of 3 km and was located in the estuarine pycnocline. Levels of dissolved oxygen supersaturation were elevated at 190 (%) saturation (15.9 mg O 2 l 1 ) and the highest biochemical oxygen demand associated with the bloom was greater than 25 mg O 2 l 1 . Using biovolume data to calculate carbon content, and a water exchange rate of 0.06 d 1 and dilution factor of 0.1, the potential oxygen demand following bloom collapse was estimated to be 7.22 mg O 2 l 1 . The potential impact of this thin layer on the water quality of the estuary is discussed. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction In recent years the occurrence of thin layers of phytoplankton in stratified marine waters has received greater attention, in part due to the development of technologies capable of resolving fine scale biological and physical features. The improved spatial characterisation of these features in stratified environments has contributed toward a better understanding of how these thin layers influence ecosystem dynamics and in particular primary productivity and the biogeochemical cycling of nutrients and other substances (Durham and Stocker, 2012). Furthermore, knowledge about the presence of thin layers of potential toxin- producing species has also improved the detection of harmful algal events (Gentien et al., 1995). Here, we provide a short communication on the occurrence of an exceptional thin layer of phytoplankton dominated by the dinoflagellate Akashiwo sanguinea (Hirsake) G. Hansen et Moestrup in the Lee estuary (Lough Mahon), Co. Cork in September 2010. The potential impact of this thin layer on water quality in the estuary is discussed. 2. Materials and methods Observations at eight stations in the Lee estuary (Lough Mahon) and Cork Harbour (Fig. 1) were carried out as part of the Irish Environmental Protection Agency’s national estuarine and coastal waters monitoring programme. Percentage dissolved oxygen (DO) saturation measurements together with tempera- ture, salinity and depth were recorded using a Hydrolab Data- sonde CTD. Fluorescence measurements were taken using a Cyclops (Turner Designs) fluorometer integrated to the CTD. Samples for the analysis of chlorophyll a, nutrients and BOD were collected using 2-litre Hydrobios Ruttner bottles. DO saturation measurements and water samples were taken at the surface and 0.5 m above the bottom. Composite phytoplankton samples con- taining equal volumes of surface and bottom sample, mixed together, were collected from the upper Lee estuary (LE150, LE160, LE170), Lough Mahon (LE 180, LE330, LE340) and Cork Harbour (LE380 and LE610). Discrete samples were also collected from a depth of 1 m at station LE180 and at the surface at station LE330 in response to elevated fluorescence readings. Samples for plankton analysis were preserved in Lugol’s iodine and stored in Sterilin tubes. Samples were counted using a Sedgewick Rafter cell and specific phytoplankton dimensions were taken on 20 randomly selected individual cells. DO measurements were calibrated on a daily basis using the water-saturated air method. Salinity measurements were cali- brated against KCl standards of known conductivity. Samples for Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2012.12.008 n Corresponding author. E-mail address: s.oboyle@epa.ie (S. O’Boyle). Deep-Sea Research II 101 (2014) 244–248