Lightframe On-sight Key species Investigation (LOKI)

The art of imaging tiny plankton species on-the-fly

Authors: Jan Schulz^1,4, Kristina Barz¹, Dirk Mengedoht¹, Thomas Hanken², Heiko Lilienthal², Norbert Rieper², Jan Hoops², Kurt Vogel³, Hans-Jürgen Hirche¹

• ¹ Alfred Wegener Institute for Polar and Marine Research, www.awi.de
• ² iSiTEC GmbH, www.isitec.de
• ³ MedeaAV Multimedia GmbH, www.medea-av.de
• ⁴ IMARE-Institute for Marine Resources, www.imare.de

Plankton forms the basis of the food chain in limnic and marine waters and is a fundamental characteristic of ecosystem dynamics. Rapid mapping of plankton abundance in combination with taxonomic and size composition is very important for aquatic environmental research, but difficult or impossible to accomplish using traditional techniques. Common sampling strategies, like towed plankton nets, have method-borne shortcomings like low resolution, intrinsic depth integration and time consuming sample analysis. Thus, their use is limited for studying distribution patterns on adequate scales in the vicinity of hydrographic fronts and clines, although such structures determine to a high degree community assemblages and trophic interactions. To overcome these limitations a new imaging device was developed for sensing tiny objects floating in clear liquids. The device is now established under the name Light frame On-sight Key species Investigation (LOKI, Figure 1a).

Figure 1: Overview and set-ups of the LOKI device for different applications. a) Overview of the main parts. b) Light-ring application for free sample through-flow. In the ring centre a disc of collimated light illuminates particles from all sides simultaneously to avoid casting shadows (Schulz 2007). c) Schematic view of the light ring setup. d) The imaging unit for moored applications during maintenance. e) Schematic view of the image head for moorings. f) Imaging unit equipped with a net for concentrating samples, through-flow chamber and cod end for additional sample collecting. g) Schematic view of the combined net and imaging unit.

Objects floating in water with a relative movement to the observation windows are exposed on-the-fly. To account for different questions a variety of imaging units were designed (Figure 1b-g). They cover the entire application range for detection of objects from low to high densities, for use on towed platforms aboard of research vessels, on moorings and in the laboratory. Units are operated with 1.3 to 4 megapixel GigE cameras. Imaging runs with 15fps at full resolution. Frames trigger a high power LED flash units allowing exposure times <100µs. Image quality is sufficient for taxonomic identification on species or at least genus level (e.g. doi://10.1594/PANGAEA.708230).
Each image frame is processed in real time and Areas Of Interest (AOI) are extracted in the underwater unit (Figure 2, upper panel). Time stamps are assigned to every AOI and allow association with recorded environmental parameters. Environmental information from arbitrary sensors are gathered by a newly developed Multi-Sensor-Carrier-System (MUSECS, Figure 2 lower panel) and recorded geospatially- and time-referenced. MUSECS is designed to attach any desired customary sensor automatically to the system via universal interface. The interface uses industrial standard CAN-Bus technology and requires no system update after attachment of predefined sensor types to the device. Final data handling is managed by a SQL database backend. This easily allows formulation of queries to investigate relationships between recorded objects and hydrographic information. Modules for manual and automatic classification further ease processes of initial classification. Communication with the underwater unit is managed by multi-frequency modems over simple coax cables. A data signal is modulated onto the supply voltage of 600VAC and allows transmission rates of up to 1MBit TCP/IP signals over cable lengths of 10km. Internationally stipulated sustainable data storage is achieved by an import option for the World-Data-Centre MARE database PANGAEA.