Seasonal Variation of Eutrophication in Some Lakes of Danube Delta Biosphere Reserve Liliana To ¨ro ¨k 1 , Zsolt To ¨ro ¨k 1 , Elfrida M. Carstea 2* , Dan Savastru 2 ABSTRACT: To understand the trophic state of lakes, this study aims to determine the dynamics of phytoplankton assemblages and the main factors that influence their seasonal variation. Sampling campaigns were carried out in three lakes from the Danube Delta Biosphere Reserve. Spectral analysis of specific phytoplankton pigments was applied as a diagnostic marker to establish the distribution and composition of phytoplankton taxonomic groups. Fluorescence spectroscopy was used to quantify changes in dissolved organic matter (DOM). The relative contribution of the main phytoplankton groups to the total phytoplankton biomass and the trend of development during succession of the seasons showed that cyanobacteria could raise potential ecological or human health problems. Moreover, fluorescence spectroscopy revealed that Cryptophyta and cyanobacteria were the main contributors to the protein-like components of DOM. It was concluded that fluorescence could be used to provide a qualitative evaluation of the eutrophication degree in Danube Delta lakes. Water Environ. Res., 89, 87 (2017). KEYWORDS: aquatic ecosystem, algae, dissolved organic matter, fluorescence spectroscopy. doi:10.2175/106143016X14733681696248 Introduction Phytoplankton is a main indicator of an increasing trophic state in aquatic ecosystems and the increasing biomass is the first response to nutrient pollution and the most visible effect of eutrophication. General analyses and reviews over the past two decades have identified that in the case of most shallow freshwater ecosystems, huge amount of algal biomass and frequent algal blooms have been recorded, which are an important sign for an eutrophic or hypereutrophic aquatic ecosystem (Coops et al., 2008; Huang et al., 2014; Isenstein et al., 2014; Pinto et al., 2014; To¨ro¨k, 2011). The development of phytoplankton in shallow lakes, as those investigated in the present study, depends on the following regulatory factors: water temperature, light availability, nutrient availability, flushing, sedimentation, resuspension, humic acids, and zoo- plankton grazing (Scheffer, 1998). Seasonal variation of phytoplankton is one of the most important factors that influence the trophic state of water and contribute to the well-being and conservation of the species present in the trophic chain (Oosterberg et al., 2000). Moreover, seasonal changes in species composition due to disturbance caused by eutrophication, favors the emergence of opportunistic species such as cyanobacteria. Cyanobacteria are the Earth’s oldest oxygenic photoautotrophs, being the most obvious and troublesome algal species due to their contemporary ecological ‘‘success’’ and increasingly frequent harmful cyanobacterial blooms, which have had major impacts on aquatic ecosystems (Paerl and Otten, 2013). Studies made over recent decades have highlighted physical and chemical seasonal drivers that provide favorable conditions for cyanobacteria development. The World Health Organization (WHO) expressed concerns about the effects of cyanobacteria and provided guidance for monitoring and management for the safety of population, while the European Commission, through its Water Framework Directive, offered an ecological approach to the water resources manage- ment in Europe. The World Health Organization established the thresholds for chlorophyll-a (chl-a) of 10 lg/L (Alert Level 1) for increased odds of irritative or allergenic effects, and 50 lg/L (Alert Level 2) for increased probability of irritative symptoms and toxic impacts, both under conditions of cyanobacterial dominance (Chorus and Bartram, 1999; Poikane et al., 2011). Due to the large number of lakes inside the Matita-Merhei hydrological sub-unit of the Danube Delta and the isolation due to large area cover by reed, which is by far the dominant species in the Danube Delta, covering almost 160 000 ha (Oosterberg et al., 2000), a small number of lakes have been included in the monitoring program of water quality in Danube Delta Biosphere Reserve. However, gaps regarding ecological processes are considered by the researchers or authorities. To fill the gaps and to improve the knowledge on phytoplankton variation, three 1 ‘‘Danube Delta’’ National Institute for Research and Development, 165 Babadag Street, Tulcea 820112, Romania; liliana.torok@ddni.ro. 2 National Institute of R&D for Optoelectronics, INOE 2000, Magurele, 077125, Romania. * National Institute of R&D for Optoelectronics, INOE 2000, Magurele, 077125, Romania; e-mail: elfrida.carstea@inoe.ro. //titan/production/w/waer/live_jobs/waer-89/waer-89-01/waer-89-01-27/layouts/waer-89-01-27.3d  6 December 2016  1:20 pm  Allen Press, Inc. Page 87 WATER ENVIRONMENT RESEARCH  January 2017 87