## 2 Materials and Methods
### Fluorescence Excitation Light Source  
Some in situ sensor designs use Xenon flash (or pulsed) lamps due to their high energy output in the ultraviolet (UV) and visible regions of the light spectrum as well as their long lifespan [36]. For these reasons, the 'Kallemeter' also utilizes a Xenon flash lamp (PX-2, Ocean Optics, Largo, FL, USA), which emits pulsed light with a continuous spectrum in a fluorescence relevant excitation wavelength range from 220 nm to 750 nm. A pulse power of maximum 45 microjoules/pulse (with 5 \(\upmu\)s pulse duration at 1/3 height of pulse) and a lifetime of \(10^{9}\) pulses are expected for this 'expendable part' (estimated 230 days continuous operation @ 50 Hz pulse rate). A stable output from pulse to pulse is guaranteed for up to 100 Hz repetition rate, so the flash delay results in 10 milliseconds. Since the total intensity typically decreases slowly over the operating period [37], a measurement of the  
Figure 2: System components of the submersible EEMS sensor system. (**a**) System connected to an on-board unit and remote PC-control (optional). (**b**) Schematic structure of excitation–emission matrices (EEM) fluorescence spectrometer hardware and optical components (connected by optical fibers 1, 2, 3). (**c**) Internal close-up with electronics and optical components. (**d**) Close-up of the outer components of the EEMS sensor system. excitation spectrum is implemented to provide metadata of the EEMS sensor system status (presented in the subsection \"Quality status of the excitation wavelengths\"). Control of the Fluorescence Excitation Band  
A monochromator (Type CM 110, Spectral Products, Putnam, CT, USA) is used to separate a narrow excitation band from the spectrum of the Xenon flash lamp. A full width at half maximum (FWHM) effective bandwidth of 10 nm is realized (therefore no slits are used) and the excitation wavelengths are selectable from 200 nm to 750 nm in 1 nm steps (with ruled grating AG1200-00300-303). The monochromator provides a coupling and decoupling angle of 14.8\({}^{\circ}\) and results to 0.13 numerical aperture (NA). Quality Status of the Excitation Wavelengths  
A wavelength-calibrated reference spectrometer (USB2000+, Ocean Optics, Largo, FL, USA) controls the quality status of the excitation. The internally used grid provides a spectral sensitivity range from 200 nm to 800 nm with a fixed slit width of 100 \(\upmu\)m, and a possible integration time from 1 millisecond to 65 s. Reference data are recorded in parallel to the fluorescence measurements but are not included in the calculation of the EEM spectra. Instead, they only serve as metadata for monitoring the quality of the excitation light source state over the operation time. Water Sampling Flow-Through Cell  
Various conceptual instrument designs and principles for fluorescence detection hardware are available for different applications: right angle detection, intersecting cones, flow-through, and fiber optic designs [36]. The 'Kallemeter' sample flow-through cell design combines right angle detection (the commonly used 90\({}^{\circ}\) emission-excitation setup in luminescence spectrometry) with fiber optics and intersecting cones in a closed flow-through design, which is illustrated in Figure 3a. In addition, the measuring cell makes use of well-directed multireflective intersections of the optical fiber aperture to achieve multiple excitations and detection cone overlaps. Thus, the optical path within the water sample is enhanced and an effective increase of factor 2.5 of the fluorescence signal for the highest signal-to-noise ratio can be achieved (Appendix A, Table A1). Two mirror elements are used for this purpose, which are arranged opposite to the optical fiber terminations in a way that no direct fluorescence excitation light can enter in the optical fiber of the detector strand. A concave mirror (CM127-025-F01 - \(\mathcal{O}\)1/2\" UV Enhanced Al-Coated, f = 25.0, Thorlabs, Newton, NJ, USA) is used in (opposite) combination to the excitation cone.