A closer look at plankton: potential interactions inferred from centimeter-scale in situ observations

Plankton are essential to marine ecosystems, supporting food webs and mediating biogeochemical processes such as carbon export to depth. Their spatial distribution influences ecosystem dynamics and serves as an indicator of environmental change. Although drifting plankton could be expected to exhibit random distribution, numerous studies have revealed significant heterogeneity in their spatial patterns. However, very few studies targeted plankton distribution at the centimeter scale in situ, despite its importance for understanding biological processes. We argue that centimeter-scale distances in plankton could reveal potential ecological interactions. Using an extensive in situ dataset of 18 million planktonic organisms collected by the In Situ Ichthyoplankton Imaging System (ISIIS), which images multiple organisms simultaneously and preserves their positions in the water column, we analyzed centimeter-scale distances in plankton. By comparing observed distances with those expected under a random distribution, we assessed potential interactions at three levels: among all organisms, within plankton groups and across groups. Our results show that planktonic organisms exhibit non-random distributions at the centimeter scale, with smaller distances than expected, suggesting potential ecological interactions. Notably, distances up to 11 cm were the most informative, which is much larger than typical interaction distances in plankton. Additionally, observed distances were compatible with a simple attraction model. Finally, we propose the non-randomness of distances as a novel metric of interaction strength in plankton ecological networks and compare it against classical empirical or co-occurrence networks. These results offer new insights into in situ interactions and how they shape plankton distribution at centimeter scale. Significance statementThis study reveals that planktonic organisms exhibit non-random spatial distributions at the centimeter scale, highlighting the importance of ecological interactions in shaping their distribution at this scale. By analyzing an extensive in situ plankton imaging dataset, we introduce a novel metric of interaction strength based on the non-randomness of distances between organisms, and compare it to common interaction metrics. These findings challenge the traditional view of plankton as passive drifters by highlighting that their distribution at microscale is shaped not just by physical processes such as turbulence but also by ecological interactions. Author contributionsJOI contributed to data acquisition. TP processed the data under the supervision of JOI. TP, JOI and BBC designed the study. TP conducted the analyses under the supervision of MF, JOI and BBC. TP wrote the initial draft of the manuscript. All authors contributed to the interpretation of results, supported manuscript preparation and approved the final submitted version.