The In Situ Plankton Assemblage eXplorer (IPAX): An Inexpensive Plankton Imaging System for Zooplankton Study

Pichaya Lertvilai, University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States
Studying plankton using in situ imaging has always been difficult because of the cost of the necessary hardware. If a lower cost alternative existed, with the desired resolution and capability to study organisms in the mm – cm range, it would avail a host of both research questions as well as educational opportunities. A recently developed system, the IPAX, is built around a Raspberry Pi computer and a small S-mount camera with optics capable of resolving features with better than 100 µm resolution with a field of view 5cm x 3cm. The illumination is achieved by using high-power LEDs together with a focusing lens. The entire system is self-contained, rated to 100 m depth, and contains enough batteries for 24 hours of continuous operation. The recorded data is encoded into an MPEG-4 format with a recorded size of 1640x1232 pixels at 30 frames per second. In 2019 dollars, the system components cost less than $400. In the first in situ deployment, the system was used to record migrating zooplankton for 14 nights on Coconut Island, Hawaii. Two IPAX units were used with one projecting white light and the other projecting 730nm light. The white light was used to observe phototactically attracted zooplankton while the red light was used to non-invasively observe them. Both systems successfully captured footage of zooplankton at multiple habitat types around the island. As observed, the recorded video footage temporally resolved the emergence time of different zooplankton taxa with its 10-minute sampling interval. Such low-cost system can further be built in multiple units to cover large spatial areas, allowing application to other research questions that require automated image collection with high temporal and spatial resolution.