Ferrybox Measurements: a Tool to Study Meso-Scale Processes in the Gulf of Finland (Baltic Sea) Urmas Lips, Inga Lips, Villu Kikas and Natalja Kuvaldina Marine Systems Institute, Tallinn University of Technology Akadeemia tee 21 12618 Tallinn, Estonia Abstract- Ferrybox measurements are carried out in the Gulf of Finland (Baltic Sea) in a regular basis since 1997. Routines for data acquisition are developed enabling near real-time data delivery for operational models. Cross-gulf high-resolution temperature, salinity and chlorophyll a fluorescence profiles collected in 2007 are used to describe meso-scale variability of hydrophysical and -biological fields in the gulf. It is shown that higher values of chlorophyll a concentration are more often observed in the coastal areas and in the vicinity of a quasi- permanent salinity front in the central Gulf of Finland. I. INTRODUCTION Meso-scale physical features (fronts, eddies, upwelling, downwelling) are known to be determinant for biological production, retention and transport. To assess and quantify the influence of these processes on the functioning of pelagic ecosystem, measurements with high enough resolution, duration and extent have to be conducted. Conventional monitoring programs have too low resolution of sampling while special investigations using the research vessels are conducted episodically. Therefore, new methods such as remote sensing, measurements at autonomous buoy stations and voluntary platforms such as ferries have to be applied. Only using these methods we could be able to monitor the variability of environmental parameters in the spatial and temporal scales in order of 10 km and of a few days. The Gulf of Finland lies in the northeastern part of the Baltic Sea. It is an elongated basin with a length of about 400 km and a maximum width of 140 km. The large freshwater inflow in the eastern end of Gulf (the Neva River) leads to a surface-layer salinity decrease from 6 at its entrance to 1 in the easternmost area. The vertical stratification is characterized by a permanent halocline at depths of 60-80 m, and a seasonal thermocline, which forms at the depths of 10-30 m in summer. The long-term residual circulation in the surface layer of the Gulf is characterized by a relatively low speed and by a cyclonic pattern (e.g. [1,2] and references therein). According to the latter, the saltier water of the northern Baltic Proper intrudes to the Gulf along the Estonian coast and the seaward flow of fresher gulf water occurs along the Finnish coast. The circulation is more complex at time scales from days to weeks due to the variable wind forcing. A variety of mesoscale processes (fronts, eddies, upwelling/downwelling) have been observed [3,4,5]. Due to the dominating southwesterly winds, the Finnish coastal sea is one of the main upwelling areas in the Baltic Sea in summer, from May to September [6]. In case of easterly winds, upwelling events are observed in the Estonian coastal sea area near the southern coast of the Gulf (e.g. [7]). The nutrient concentrations in the surface layer of the Gulf of Finland reveal pronounced seasonal variation. Maximal dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations are observed in winter while in summer, the concentrations of DIN and DIP are usually close to the detection limit in the upper layer. However, higher concentrations are observed just below the seasonal thermocline [8]. The seasonal dynamics of phytoplankton species composition and biomass in the Gulf of Finland is characterised by spring bloom in April-May dominated by dinoflagellates (and diatoms), summer minimum from late May to late June and late summer bloom in July (or late June to mid August) dominated by cyanobacteria. The latter is often causing public concern about the status of the sea environment. The main aim of the present paper is to show how Ferrybox measurements can be used for the monitoring of meso-scale processes in the Gulf of Finland. On the basis of data collected from May to September 2007, the meso-scale dynamics in the study area and influence of observed processes to the chlorophyll a distribution in the surface layer will be described. II. MATERIAL AND METHODS A. Measurements Temperature (T), salinity (S) and chlorophyll a fluorescence data and water samples for chlorophyll a (Chl a) analysis are collected unattended on passenger ferries, travelling between Tallinn and Helsinki (Fig. 1) since 1997. In 2006 a new flow- through system (see Fig. 2; 4H-Jena, Germany) was installed onboard ferry Galaxy (Tallink Group). It is able to measure in addition to T, S and fluorescence also turbidity and nutrient concentrations. The system and sensors are kept clean by an acid washing procedure performed autonomously every evening. 978-1-4244-2268-5/08/$25.00 ©2008 IEEE