Seeing the unseen: an introduction to underwater plankton imaging systems

Traditional plankton sampling methods are labor-intensive, time-consuming and often fail to resolve the fine-scale spatial and temporal variability that characterizes plankton ecosystems. Underwater imaging systems offer a powerful alternative, enabling in situ observations across a wide range of organism sizes and environmental conditions. However, imaging platforms differ substantially in illumination strategy, optical design, sampling volume, deployment mode and data requirements, and no single system is optimal for all ecological questions. Selecting the appropriate instrument is therefore a prerequisite for obtaining representative, interpretable and comparable datasets. Here, we provide a practical primer on underwater plankton imaging systems, synthesizing their core components, operational principles and ecological applications. We introduce a stepwise decision framework centered on the trade-offs among resolution, representativeness and data volume and show how study objectives, target organisms, environmental conditions, deployment platforms and data-processing capacity guide instrument choice. Case studies are used to demonstrate how the framework supports real-world applications ranging from near-real-time monitoring to fine-scale spatial mapping and community-composition assessment. Finally, we discuss how thoughtful instrument selection, standardized metadata and intercalibration can support interoperable and FAIR (Findable, Accessible, Interoperable, Reusable) plankton imaging datasets.