Phytoplankton blooms were affected by external environmental nutrient input, while the interspecific interactions in plankton (phytoplankton and mesozooplankton) under the nutrient pollution gradient remain largely unknown. Here, we systematically collected samples for 9 months (from April to December 2018) in the coastal waters of Xiaoping Island in China to analyze the planktonic community structure and identify the main driving environmental factors along the nutrient gradient from the sewage outlet to the open sea. The results indicated that there existed obvious seasonal and spatial variations in the planktonic community. Procrustes test analysis showed that temperature, transparency, dissolved oxygen, nitrate (NO₃-N), phosphate (PO₄-P), and silicate (SiO₃-Si) significantly affected the community compositions and diversity of plankton (p<0.05). Co-occurrence network showed that seasons and nutrients pollution had an important influence on the inter-specific interactions between phytoplankton and mesozooplankton. In different nutrient pollution gradients, diatom was the most associated with Copepods in Section 1 (9.38%), Section 2 (9.84%), and Section 3 (5.38%), respectively, and it was also associated with Planktonic larva in Section 1 (7.81%), followed by in Section 3 (4.30%) and 2 (1.64%). Dinoflagellates were associated with Chaetognatha only in Section 1 (4.69%). This study may provide new insights into the plankton dynamics and facilitate nearshore environmental management.