We collected horizontally detailed and extensive in situ chlorophyll a data from the coastal Baltic Sea (SW Finland), covering the ice-free season of an annual cycle. The spatially detailed and extensive in situ Chl-a data were collected by an automated underway measurement system equipped with an optical Chlorophyll a sensor. The system was constructed by Luode Consulting Inc. and installed in a rigid inflatable boat, or RIB (Brig N610H) with 0.4 m draft. The system consisted of seven sequential components as follows: The water intake was placed at 0.5m depth, facing forward right below the left-hand stern. Since the water intake was the lowest part of the whole system, its minimum operation depth was also 0.5 m. After passing the pump at 29 L/min, the water was led through a lamellar debubbler before entering a 0.5-L cylindrical chamber enclosing an EXO2 sonde equipped with a dual-channel fluorescence sensor (Xylem Inc., USA) for integrated Chl-a detection, incorporating also cyanobacterial contribution by applying two excitation wavelengths; 470±15 and 590±15 nm and one emission detection wavelength at 685±15 nm. From the sonde, the water passed by a faucet for water collection before being led back to the sea from an outflow pipe at the right-hand rail of the boat. Time and GPS position were tracked continuously for geolocating the Chl-a measurements by using an EXO Handheld unit (Xylem Inc., USA) connected to the sonde with a data transmission cable. Here we demonstrate the spatial patchiness of chlorophyll a in coastal waters. We analyzed the spatial heterogeneity of calibrated chlorophyll a data by using multiple statistical approaches, and quantified the chlorophyll a patches using a rolling average filter.
