The sea-surface microlayer (hereafter microlayer), which forms the interface between the ocean and atmosphere, plays a key role in nutrient cycling and microbial dynamics. Coastal slicks, which are viscous and biogenic surface films, often serve as hotspots for microbial activity, yet their structure and interactions remain incompletely understood. Here, we investigated viral and microbial abundance, surfactants, dissolved organic carbon, and total dissolved nitrogen in slick microlayer and their corresponding underlying water in the coastal Baltic Sea. Dissolved organic carbon and total dissolved nitrogen showed minor enrichment in the microlayer, while surfactants were elevated in several slicks. Some microbial abundances were significantly higher in the slick microlayer, including prokaryotes and different phytoplankton size classes. Co-occurrence network analysis revealed clusters in the slick: surfactant-associated groups (nanophytoplankton groups, viruses) and surfactant-independent groups (cyanobacteria, picophytoplankton). Cyanobacteria were enriched in the slick microlayer relative to the slick underlying water, and their abundance in the microlayer was negatively correlated with total dissolved nitrogen and dissolved organic carbon. Viruses were positively correlated with prokaryotes in both the slick microlayer and slick underlying water, and additionally with microphytoplankton in the slick underlying water. Microscopic analysis of the > 100 μm slick fraction revealed a detritus-dominated matrix containing juvenile sporophytes, pennate diatoms, filamentous cyanobacteria, and a ciliate-dominated microzooplankton community. Overall, the microlayer within slicks in this eutrophic coastal system contained elevated microbial and viral densities, along with distinct patterns of correlations among community members, indicating that it can locally influence surface-layer biogeochemistry and ecology.