This study presents the longest microbial time-series analyzed to date using high-resolution 16S rRNA tag pyrosequencing of samples collected monthly over 6 years at a temperate marine coastal site off Plymouth, UK. The data revealed strong seasonal patterns in bacterial community diversity, with winter peaks in diversity across all years. Environmental variables explained more variation in seasonally predictable bacteria than did data on protists or metazoan biomass. Day length alone explained over 65% of the variance in community diversity. The results suggest that seasonal changes in environmental variables are more important than trophic interactions. Microbial association network analysis showed that correlations in abundance were stronger within bacterial taxa rather than between bacteria and eukaryotes or between bacteria and environmental variables. The study also identified the Alphaproteobacteria as the most abundant class, with the Rickettsiales (SAR 11) and Rhodobacteriales being the most frequently recorded OTUs. The results indicate that seasonal succession patterns of marine surface water bacterial communities in temperate regions may be conserved across different biomes. Environmental factors, rather than interactions with eukaryotes, were better at explaining seasonal variance in bacterial community composition. The study highlights the importance of environmental factors in shaping microbial community structure and diversity, and shows that the seasonal cycle is consistent across years. The results suggest that nutrient concentrations, physical parameters, and biology all demonstrate significant influence in an extraordinarily complex matrix. The annual day length cycle explained most of the variability in the seasonal pattern of species diversity. The study also identified that the most common and abundant bacterial OTUs have temporally defined niches, while the most variable OTUs have niches that can be defined temporally as well as by nutrient pulses and changes in currents. The results suggest that biological factors may be more important in defining the fine-grain community structure than physical factors. The study provides insights into the dynamics of microbial communities in marine environments and highlights the importance of long-term temporal observations in understanding these dynamics.This study presents the longest microbial time-series analyzed to date using high-resolution 16S rRNA tag pyrosequencing of samples collected monthly over 6 years at a temperate marine coastal site off Plymouth, UK. The data revealed strong seasonal patterns in bacterial community diversity, with winter peaks in diversity across all years. Environmental variables explained more variation in seasonally predictable bacteria than did data on protists or metazoan biomass. Day length alone explained over 65% of the variance in community diversity. The results suggest that seasonal changes in environmental variables are more important than trophic interactions. Microbial association network analysis showed that correlations in abundance were stronger within bacterial taxa rather than between bacteria and eukaryotes or between bacteria and environmental variables. The study also identified the Alphaproteobacteria as the most abundant class, with the Rickettsiales (SAR 11) and Rhodobacteriales being the most frequently recorded OTUs. The results indicate that seasonal succession patterns of marine surface water bacterial communities in temperate regions may be conserved across different biomes. Environmental factors, rather than interactions with eukaryotes, were better at explaining seasonal variance in bacterial community composition. The study highlights the importance of environmental factors in shaping microbial community structure and diversity, and shows that the seasonal cycle is consistent across years. The results suggest that nutrient concentrations, physical parameters, and biology all demonstrate significant influence in an extraordinarily complex matrix. The annual day length cycle explained most of the variability in the seasonal pattern of species diversity. The study also identified that the most common and abundant bacterial OTUs have temporally defined niches, while the most variable OTUs have niches that can be defined temporally as well as by nutrient pulses and changes in currents. The results suggest that biological factors may be more important in defining the fine-grain community structure than physical factors. The study provides insights into the dynamics of microbial communities in marine environments and highlights the importance of long-term temporal observations in understanding these dynamics.