Rainwater input modulates marine primary production across varied embayment nutrient levels

Rainwater alters seawater composition, impacting marine phytoplankton and microbes, but research on its effects under different nutrient conditions is limited. In this study, two separate shipboard incorporation incubations (NaH¹⁴CO₃ for primary production (PP) and ³H-thymidine for bacterial production (BP), respectively) were used to investigate the response of marine PP and BP to varying intensities of rainwater input in Daya Bay, a distinct embayment influenced by severe anthropogenic activities. The results showed that rainwater input could increase PP (9.5 % and 47.0 % for 2.5 % and 5 % rainwater addition groups), especially in areas under high nutrient loading areas. This effect is especially conspicuous for nano-diatoms and dinoflagellates species. Furthermore, the role of heavy rainfall (5 % rainwater addition) in augmenting nutrient supply bolsters primary productivity, particularly for the dominant species such as Chaetoceros. sp, Pseudo-nitzschia, and Thalassionema nitzschioides. Meanwhile, rainwater addition had a positive effect on BP, but inhibited bacterial respiration (BR) in the highest nutrient level. In contrast, rainwater addition promoted BR in mediate and less disturbed areas. The influence of rainwater input on bacterial growth efficiency (BGE) was modest, yet a discernible inhibitory effect was observed at the bay's mouth, where the seawater exhibits the lowest disturbance. While rainwater addition impeded the proliferation of autotrophic bacteria in the high nutrient level area, its effect was negligible at the low nutrient loading area. This study provides a collaboration or competition insight into the effects of atmospheric nutrient deposition on marine primary production processes in coastal areas affected by distinct levels of human activities.