Reduction in body size, i.e., miniaturization, happens in different organisms as an adaption strategy under warming. Whether phytoplankton follow this size-temperature received little attention. Here we investigated the long-term change of phytoplankton body size in the coastal water of Hong Kong for 20 years. We tested the body size shift hypothesis (H1) and the species shift hypothesis (H2) contributing to the phytoplankton miniaturization. We found that miniaturization happened at the species level (supporting H1) rather than the community level, which could be due to the decreased proportion of small-sized phytoplankton at most stations (opposite to H2). More diatom species experienced miniaturization than dinoflagellates. Temperature dominated the change of phytoplankton body size, but there was a great difference in the response pattern within a certain phytoplankton group. Nutrients, especially phosphate, positively controlled the proportion of small species. Therefore, historical control on nutrient discharge may relieve the phytoplankton miniaturization at the community level in Hong Kong coastal waters. Our study provides a novel and detailed view of phytoplankton miniaturization, which will have profound indications on marine food web dynamics under global warming.

phytoplankton, miniaturization, coastal waters, global warming