Three commercially available fluorometric instruments, designed for in-situ and real-time quantification of oil or oil compounds, were chosen for the experiments: UviLux and EnviroFlu-HC 500 for the FerryBox system, and C3 Submersible Fluorometer for the SmartBuoy. All three sensors were also tested in laboratory conditions.

The UviLux UV-fluorometer (Chelsea Technologies Ltd, UK) is an in-situ UV fluorometer. Oil detection is based on the measurement of PAH concentrations. The sensitivity of the sensor is 0.005 μg carbazole per liter, the calibrated range is 0.005–2,000 μg carbazole per liter, the excitation wavelength is 255 nm, and the emission wavelength is 360 nm.

The EnviroFlu-HC 500 submersible UV fluorometer (TriOS Optical Sensors, Germany) is an instrument designed for the measurement of PAHs in water. The fluorometer has a minimum detection limit of 0.3 μg phenantrene per liter, a calibrated range of 0–500 μg phenantrene per liter, an excitation wavelength 254 nm, and an emission wavelength of 360 nm.

The Turner Design C3 submersible fluorometer (Turner Design, USA) is manufactured according to users' requirements. C3 fluorometers come with a factory-installed temperature probe and can be configured with up to three optical sensors. Each optical sensor is designed with fixed excitation and emission filters. For the SmartBuoy experiment the C3 fluorometer was equipped with three sensors: a hydrocarbon sensor for crude oil with sensitivity of 0.2 μg pyrenetetrasulfonic acid (PTSA) per liter and linear range 0–1500 μg PTSA per liter, excitation light 325/120 nm and emission light 410–600 nm; a sensor for colored dissolved organic matter (CDOM) with minimum detection limit (MDL) 0.1 μg quinine sulfate per liter, linear range 0–1.5 μg quinine sulfate per liter, excitation wavelength 325–120 nm and emission wavelength 470–60 nm; a turbidity sensor with MDL 0.05 Nephelometric Turbidity Units (NTU) and range 0–1,500 NTU, excitation wavelength 850 nm, and emission wavelength 850 nm.

In order to examine selectivity and sensitivity of fluorometric sensors to oil, two separate laboratory experiments were performed in 2017. All experiments were performed in a dark climatic room set to a temperature of 16°C.

The first experiment examined the selectivity of the sensors, i.e., interferences to oil detection caused by interfering substances. Baltic Sea water (S; salinity 6.2 PSU) was used as a blank and as a solution for extraction of humic substances (H), cyanobacterial algae (A) and production of clay suspension (C).

Stock solutions (one of each) of clay suspension (from Baltic Sea's clayey sediment), seawater extract of humic substances (originating from decayed plant material) and a seawater extract of cyanobacterial algae (from dried Baltic Sea surface phytoplankton) were produced. Also, two batches of water accommodated fraction (WAF; W) of diesel oil were produced. Each extract was made by combining 100 ml of commercial winter-quality diesel oil with 1,000 ml of seawater under gentle stirring overnight. All solutions were prepared 1 week prior to measurements in the aquarium. Concentrations of PAHs were quantified using gas chromatography—mass spectrometry and aliphatic hydrocarbons (decane-tetracontane) using gas chromatography—flame ionization detection at SYKE laboratory center. No attempt was made to determine chemical composition of each interference solution in detail, instead the solutions mimicked natural constituents in Baltic Sea water.

Altogether six glass beakers (each 2,000 ml) were wrapped with black plastic except for the top section and filled with 1,200 ml of seawater. Aliquots of the clay suspension (50.0 ml), humic extract (50.0 ml), cyanobacteria algae extract (10.0 ml) and diesel WAF (50 ml) were sequentially added so that all combinations of the aforementioned solutions in seawater were achieved. The solutions were kept in slow stirring motion using a magnetic stirrer. In first three sets of measurements no WAF was added. WAF was applied in three last measurement sets. After addition of each solution the responses were measured using one sensor at a time at exactly 50 mm below the upper level of the beakers. The top part of the beakers and the sensor housing were wrapped with black plastic. Fluorescence responses were collected for 3–5 min with each sensor and the average and standard deviation of fluorescence for each period of data collection were calculated.

The logging rates used with fluorometer sensors during the experiment were 1 min (average of values collected at 0.5 Hz), 30 and 1 s for the UviLux, EnviroFlu-HC and Turner C3 sensors, respectively. Data from EnviroFlu-HC were logged into a laptop PC using the TriOS MSDA_XE software and the data from Turner C3 into a PC using Turner C-soft software. Data from UviLux were logged into a portable logger connected to automatic GSM network-based modem and sent via GSM network to Tallinn University of Technology once a minute. Altogether 1.32 million, 79,000 and 30,000 time points of data from Turner C3, Enviroflu-HC and UviLux were generated, respectively. The small dilution effect in WAF solutions, caused by the volume increase by the added interference solutions, was compensated by a corresponding multiplier factor in final calculations thus yielding oil-related fluorescence (ORF) presented. Normalized responses from each fluorometer were calculated by dividing observed responses from different solutions with responses obtained from seawater. No compensation was calculated for CDOM or turbidity values.

The second experiment for diesel WAF persistence and for method comparison lasted for 3 weeks. A 25-liter glass aquarium was used and placed over a black plastic sheet in a steel tank and filled with 21.8 liters of filtered seawater (salinity 6.2 PSU). Thermostatted (11°C) tap water was circulated in the exterior tank. The FLD sensors were fixed into laboratory stands so that the optical window of each sensor was at 10 cm below the surface of liquid (Figure 1). Gentle mixing was applied using an electricity-powered laboratory stirrer. The aquarium was covered by a black plastic wrapping except during water sampling and maintenance. The UviLux sensor needed to be moved further away from the two other sensors due to interference that was unfortunately not noticed before 7 days into the experiment. The data transmission, storage and sensors' acquisition rates were as described for the first experiment.

The persistence of diesel oil fractions in seawater and comparability between oil detection methods were measured in the second test. The methods were: the three fluorometric sensors, total oil monitoring method used by Finland, gas chromatography flame ionization detection (GC–FID; for aliphatic hydrocarbons) and gas chromatography—mass spectrometry (GC–MS; for PAHs including 1– and 2–methylnapthtalene). At S1–S9, water samples of 100 ml were collected for total oil HELCOM protocol and another 100 ml for the GC–MS and GC–FID analyses.

The Finnish HELCOM monitoring method for total oil analysis can be described briefly as follows: seawater is extracted with 10 ml of n–hexane under stirring. Fluorescence is measured using 310 and 360 nm excitation and emission wavelengths, respectively. Calibration is performed using solutions of Norwegian crude oil (Ekofisk) dissolved in n–hexane. Limit of quantification is 0.05 μg oil/l and analytical error is ±15%.

Aliphatic hydrocarbons (decane-tetracontane) were extracted using n-hexane, cleaned up using adsorbent, concentrated, separated using gas chromatography and quantified by flame ionization detection. The method has a limit of quantification of 100 μg total aliphatics/l and analytical error of 30 %. PAHs were extracted using n-hexane under stirring or solid phase extraction. The n-hexane solution was then concentrated and analyzed using GC–MS. The quantification limits of the method were between 0.005 and 0.01 μg/l, while the analytical errors varied between 20 and 40% depending on compound.

Normalized responses for each detection method were calculated by dividing observed responses with period S1 responses at given water sampling time points (S1–S9).

A FerryBox system developed by Marine Systems Institute at Tallinn University of Technology is used on board of the M/S Baltic Queen (Tallink Group, Estonia) (Figure 2), which commutes between Tallinn (Estonia), Mariehamn (Åland) and Stockholm (Sweden) (Figure 3). The ship route covers the western Gulf of Finland, Northern Baltic Proper, Southern Archipelago Sea, and Southern Åland Sea (Figure 3). One crossing, including a stop in Mariehamn harbor, is about 425 km long and takes approximately 17 h. The ship then returns after a 7 h stay in the destination harbor.

All FerryBoxes are flow-through systems where the water is taken in from an inlet in a vessel hull and then pumped into a measuring circuit containing sensors (Petersen, 2014). On M/S Baltic Queen the water was taken from the ship's sea chest and pumped continuously through the FerryBox system at a rate of 9 l/min. The opening of the chest is situated about 4 m below the waterline. Parameters were measured with 1-min intervals, giving an approximately 0.5 km spatial resolution along the fairway, depending on the ship's speed. A typical transect contained roughly 1,000 records for each variable.

The UviLux and EnviroFlu-HC fluorometers, were installed in parallel to the FerryBox system (Figure 2B), which recorded additional real-time seawater properties. Conductivity (salinity) and temperature was measured with a High-Precision Pressure Level Transmitter Series 36XiW (KELLER AG, Switzerland), turbidity was measured with a Seapoint Turbidity Meter (Seapoint Sensors Inc., USA) and pCO2 was measured with an OceanPack^TM pCO2 analyzer (SubCtech, Germany).

We made four maintenance visits to M/S Baltic Queen in order to clean the FerryBox system and the sensors. These visits were made roughly every 2 weeks (March 3, March 20, April 3, April 15) during our 2-month experiment (February 21 to April 21, 2018). The maintenance consisted of washing the system with a solution of oxalic acid and a manual cleaning of the optical sensors. The FerryBox also has a automatic cleaning system, which was not functional during the experiment. Similar automatic acid-washing cleaning method is applied to prevent biofouling on a FerryBox traveling between Tallinn and Helsinki (Lips et al., 2016).

Measurements from the sensors in the FerryBox were collected by a datalogger (RTCU-MX2i pro by Logic IO) on board M/S Baltic Queen. The datalogger includes a modem and a GPS, and adds the position and a time stamp to the measurement before sending the data to an on-shore FTP server of the Marine Systems Institute using the GSM/GPRS protocol. The data were sent in real-time (every minute).

A publicly available, web-based, user interface was built to visualize the data online, where the ship's track, real-time position and gathered FerryBox data was available in real-time (Figure 4) (TalTech, 2017). The web-based user interface is also equipped with different options to view historical data: the user can select parameters or time periods, and construct a map view and 2D graphs of multiple parameters. Data are also available in a tabulated form and parameters can be viewed in color-coded view along the ship's track. The data that were provided in real-time during the experiment is still available on the web-page.

The SmartBuoy, developed by Meritaito Ltd (Finland), is a combination of a polyethylene spar buoy with mobile and/or satellite communication technology and a versatile selection of monitoring sensors (Figure 5). The Turner Design C3 fluorometer was installed inside of the buoy in a vertical monitoring well with an open flow through a pipe, enabling continuous sea water exchange. The monitoring depth was 2–3 m depending on the sea level. Concentration of CDOM and turbidity values were measured simultaneously with the hydrocarbon measurements. To ensure high data quality, the C3 sensor was equipped with a mechanical wiper cleaning all three optical lenses before taking the measurements. In addition to the water quality monitoring, the SmartBuoy also collected data about current speed and direction at the depth of 7 m to identify spreading direction of potential oil spills. Furthermore, the significant wave height was calculated from roughly 8.5-min time series, measured by a pressure sensor (Aanderaa Wave and Tide Sensor 5218).

Water quality and sea state sensors were connected to a datalogger (Aanderaa SmartGuard 5300) that was programmed to operate the sensors with an 1-h measurement and data transmission interval. Real-time data from the SmartBuoy were transmitted over satellite modem to a server, where it were subjected to an automatic data quality control before being visualized online.

The SmartBuoy buoy was moored on a junction of the main merchant shipping lanes in the Gulf of Finland (Figure 3) for a 2-month measurement period (October 25 to December 24, 2018). Vessels navigating on the north–south direction shipping lane between Helsinki and Tallinn were passing close to the buoy, and vessels on the east–west direction shipping lane passed the buoy further south. The SmartBuoy was visited once during the monitoring period for datalogger reprogramming and manual maintenance of the sensors.

In the first laboratory experiment we investigated how the sensors reacted to the addition of interference solutions. In the absence of interference solutions, Turner C3 showed the largest absolute fluorescence values (primary data), followed by Enviroflu-HC and Uvilux. The data from Turner C3's PAH and CDOM channels had occasional spikes, but overall the data stability from each channel of the detectors were good. When WAF was added to the seawater, the relative change in signal strength (normalized response) was largest for UviLux (4.48-fold), followed by EnviroFlu-HC (2.62-fold) and Turner C3 (1.1-fold) (Table 1).

All sensors reacted when interference solutions were added to seawater. The addition of suspended clay to seawater slightly attenuated the PAH channel's fluorescence signal in Turner C3 and Enviroflu-HC, while slightly enhancing it in UviLux (Figure 6, left half, S vs. SC). When suspended clay was added to the seawater-WAF solution, the signal was slightly attenuated in Enviroflu-HC and UviLux, and slightly enhanced in Turner C3 (Figure 6, right half, SW vs. SWC). In all combinations, the changes were at most 12% (0.88–1.04).

The sensors were more sensitive to the presence of humic substance extracts, with the PAH-signals increasing between 1.05- and 1.8-fold in all sensors (compared to seawater). The enhancement was strongest in Turner C3 and weakest in EnviroFlu-HC (Figure 6, left half, S vs. SH). In the seawater-WAF solution the added humic extracts enhanced the PAH signal in Turner C3 (1.67-fold), while slightly attenuating the signal in the other sensors (Figure 6, right half, SW vs. SWH).

Clearly the largest effect on the PAH fluorescence signal was observed after adding algae extract. The signal was enhanced in all combinations of sensors and solutions. Importantly, the enhancement caused by the algae extract was stronger than the PAH signal of the WAF-solution without any interference solutions, which means that the presence of algae can interfere strongly with the sensors ability to detect oil compounds in the water. The enhancements when adding the algae extract to the seawater were roughly between 3- and 8-fold, with UviLux showing the largest change and EnviroFlu-HC showing the smallest change (Figure 6, right half, SW vs. SWA). When the algae extract was added to the seawater-WAF solution, the PAH signal was enhanced roughly 2- to 3-fold in all sensors, with Turner C3 showing the strongest change in signal strength (Figure 6, right half, SW vs. SWA).

In the second laboratory experiment we studied how long the sensors can detect oil compounds in seawater, while also comparing detection methods. Lagging software caused several gaps in the Turner C3 data and one gap in the EnviroFlu-HC data. UviLux was relocated after 165 h when we noted that it did not react clearly to the third addition of WAF (WAF3; Figure 7), having large subsequent oscillations in the signal. This behavior was caused by optical interference by the two other sensors. Despite that there was substantial suppression in UviLux responses during the suppressed period, the sensor overall did show slight responses even during this period. These small responses occurred simultaneously when adding WAF 1 and WAF 3 (but not when adding WAF 2), and were also simultaneous with HELCOM and Enviroflu-HC responses (Figure 7). After UviLux was relocated, it provided data with a considerably higher response and less noise. For response calculations we therefore only used the UviLux data gathered after the relocation.

The responses from all sensors and the standard HELCOM oil monitoring method indicated that fluorescent compounds originating from diesel oil were detectable for at least 20 days since the start of the experiment (Figure 7). The temporal evolution in PAH-related responses measured by the EnviroFlu-HC and UviLux sensors were in line with the HELCOM method, especially after the readjustment of the sensors. In contrast, Turner C3 showed slightly increasing fluorescence between 100 and 505 h while the signal from the other two sensors decayed and leveled off.

Table 2 summarizes the responses obtained from sensor-based fluorescence right before water samples (GC methods) were collected at events S5 and S7. Event before sample S7 should serve as the primary point for response comparison since it was taken after the readjustment of sensor locations (R). The relative responses compared to initial state S1 of the fluorescence-based methods, during 165–505 h were (from strongest to weakest): HELCOM, UviLux, EnviroFlu-HC and Turner C3 (Figure 7 including responses at each S event).

Several PAH molecules were good indicators for diesel-originating contamination (Table 2). All napthtalenes provided pronounced responses also at S1 (strongest to weakest): 1-methylnaphtalene (1-MN), naphthalene (NAF) and 2-methylnaphtalene (2-MN) (data not shown). Also, anthracene, acenapthtene and phenantrene were sensitive and showed a similar temporal evolution as the FLD-based responses and the methylated napthtalenes (1/2-MN).

Concentration of fluoranthene did not change over the experimental period, and Dibenz[a.h]anthracene showed a declining trend compared to the initial seawater (Figure 8). These substances (in addition to acenapthtylene, fluoranthene, dibenzo[a,h]anthracene and pyrene) were poor proxies for WAF contamination in the seawater (Figure 8).

The concentration of aliphatic hydrocarbons followed a similar pattern as those of FLD, methylated napthtalene and the more sensitive PAHs. The PAH and aliphatic hydrocarbon concentrations also had a similar evolution pattern as the values derived using the HELCOM fluorescence method. Interestingly, the increase in aliphatic hydrocarbon concentrations between S8 and S9 showed a similar increase as the HELCOM method; this change was not visible in any other parameter (Figures 7, 8).

The real-time transmission and online presentation of the FerryBox data worked well, but data gaps occurred systematically in the Baltic Proper due to loss of the GSM network. These gaps typically lasted a maximum of a few hours. An example is presented in Figure 4. The complete data was recovered from the datalogger after the fact. Altogether 58 ship voyages were analyzed. The PAH-transects were visually checked for peaks that would have indicated an oil pollution. No such anomalies were found during the 2-month measurement period the UviLux and EnviroFlu-HC fluorometers were installed to the FerryBox system.

The detected PAH values stayed between 1 and 2.6 μg carbazole/l for the UviLux and 12.4–25.5 μg phenantrene/l for the EnviroFlu-HC (Figures 9A,B). The detected hydrocarbon values were lowest in the sea areas near Åland and highest in the Stockholm archipelago, where the effect of the natural background of organic carbon from river waters is greater (Figures 9A,B). From 20.5°, longitude eastward, the PAH values were quite homogeneous. Some variation can be seen e.g., between 22°, and 23° longitude (trips 19–23), showing up more clearly in the UviLux data (Figures 9A,B). We surmise that the detected values do not reflect concentration of carbazole, phenantrene or oil in seawater. This interpretation is supported by the chemical monitoring (HELCOM/EU MSFD) and analysis of the Baltic Sea, which has found concentrations below 1 μg oil hydrocarbons/l rather constantly (Pikkarainen and Lemponen, 2005; FIMR and Olsonen, 2007). These low values are considered to be typical for seawater without significant oil pollution (Bícego et al., 1996; Zanardi et al., 1999).

The FerryBox and the accompanying sensors were maintained and cleaned frequently. Nevertheless, biofouling still impacted the quality of the measured data as can be seen from the decreasing values between the maintenance visits (Figures 9A,B). The field data suggests that EnviroFlu-HC is affected more strongly by the fouling than UviLux.

The obtained values ranged between 790 and 1,250 during the monitoring period. The variation pattern was nearly identical with the variation of collected colored organic carbon (CDOM) values, varying between 600 and 890 (Figure 9). The measured data had systematic gaps. On the 12th of December a maintenance visit to the boy was made to reprogram the datalogger and manually clean the sensors.

The correlation between the collected hydrocarbon dataset and the CDOM data set collected by the C3 sensor on SmartBuoy was very high (Figure 9D). No anomalic spikes indicative of oil-contamination were present during the test period. The C3 sensor on SmarBuoy was equipped with a mechanical wiper, which cleaned all three optical lenses prior to measuring. No significant reduction in data quality due to biofouling was detected compared to the FerryBox where the detected values gradually decreased after the cleaning (Figure 9). The linear relationship between the two variables changed after the maintenance on 12 December. This change was probably caused by the readjustment of the mechanical wipers (Joose Mykkänen, Personal communication). Nonetheless, the correlation between the variables remained high (Figure 9D).

During the experimental period, the buoy tolerated several events with significant wave heights over 2 m. The maximum significant wave height reached almost 3 m, which is estimated to be exceeded at this location roughly 1% of the times (Tuomi et al., 2011; Björkqvist et al., 2018). During the monitoring period the current speed reached 39 cm/s, with the mean value being 12 cm/s. The mean yearly current speed (for depth 0–7.5 m) in the area is expected to be under 10 cm/s (Westerlund et al., 2018).