Thin layers are ubiquitous features of stratified coastal waters and play important roles in trophic interactions, material transport, and ecosystem functioning. Although these layers have been widely documented in the field, the behavioral mechanisms by how zooplankton respond to fine-scale density gradients and subsequently modify their surrounding environment remain poorly understood. Recent advances in high-resolution imaging provide new opportunities to directly resolve these processes at the individual and population scales.

In this study, we combined high-speed videography and schlieren visualization in controlled laboratory experiments to investigate how density stratification influences the swimming behavior, spatial organization, and environmental impacts of the coastal calanoid copepod Pseudodiaptomus annandalei. High-speed imaging was used to quantify individual swimming kinematics and population-level spatial distributions in a quasi-two-dimensional chamber under linear and step-like salinity stratifications. Complementary schlieren experiments were conducted to directly visualize density-interface deformation induced by copepod swimming.

High-speed behavioral tracking revealed that spontaneous aggregation occurred only in the presence of sharp density interfaces. Copepods remained randomly distributed under linear stratification (buoyancy frequency N = 0.4–0.8 s⁻¹), whereas step-like stratification with stronger density gradients (N = 1.0–2.4 s⁻¹) triggered pronounced aggregation even in the absence of prey or other biological cues. Aggregation became increasingly pronounced with increasing salinity gradient. Trajectory analysis further showed that copepods initially increased horizontal exploration after becoming trapped near the interface, and subsequently escaped from the aggregation by increasing cruising behavior while reducing sink–jump activity. Following these behavioral responses, the population gradually redistributed and eventually recovered a random Poisson spatial pattern.

Schlieren visualization provided direct evidence that active swimming continuously perturbs the density interface. Vortex-like structures generated by copepod motions were frequently observed near the interface, leading to the rapid emergence of a visible mixing layer. As swimming activity continued, the initially sharp density interface progressively broadened, consistent with the temporal weakening of copepod aggregation observed in the behavioral experiments.

Together, these complementary observations link individual behavioral responses to population-scale aggregation and the evolution of fine-scale density structures. Our results demonstrate how high-resolution imaging can reveal previously unresolved plankton–environment interactions and provide new mechanistic insights into the dynamic formation and dissipation of thin plankton layers in stratified coastal waters.

*This work was supported by the NSFC Grant No. 12102165.