Potential contribution of terrestrial plants in the marginal sea food web: Evidence from in situ diet of the pelagic copepods in the South China Sea

Understanding the in situ diet of zooplankton is fundamental for evaluating their ecological role and their contribution to biogeochemical cycles in the ocean. Yet, knowledge of copepod feeding in open and marginal seas remains incomplete, particularly regarding non-traditional food sources. Here, we investigated the dietary composition of the pelagic copepod Pareucalanus attenuatus in the South China Sea using high-throughput DNA metabarcoding of gut contents and environmental seawater. Our results show that P. attenuatus maintains an omnivorous-herbivorous diet dominated by dinoflagellates and green algae, but with a striking and unexpected contribution from terrestrial plants (Magnoliopsida, primarily Stellaria media). Terrestrial plant DNA occurred in more than 75 % of gut samples, ranking second in relative read abundance (10.15 %), and exhibited strong feeding selectivity (Jacobs' D = 0.99). These findings provide the robust evidence that terrestrial plant material constitutes not merely incidental detritus but a consistent and potentially significant supplementary carbon source for copepods in a marginal sea. This discovery underscores an overlooked terrestrial-marine trophic linkage that may alter our understanding of zooplankton nutrition, secondary production, and the functioning of the biological pump in the marginal sea.