Recorded abundances of microplastics in the environment vary by several orders of magnitude, with a range of environmental factors being argued to have an impact (or no impact) on these abundances. In order to develop a more detailed understanding of how environmental factors might influence abundances of microplastics in estuaries, surface microlayer (SML) and bulk water sampling was employed to assess daily and seasonal variation in microplastics abundance at two locations in Southampton Water, U.K. High variation in suspected microplastic concentration was observed with both sampling methods, and at both locations. Suspected microplastic abundance in the SML ranged between 1 and 356 suspected microplastics/L. Observed concentrations were seasonally higher at the more sheltered River Itchen site, and more variation was seen at this site, as compared to the more exposed estuarine site at Mayflower Park. The wide range in suspected microplastic abundance suggests that repeated sampling of locations over a period of time under a range of meteorological and oceanographic (i.e. tidal) conditions is necessary to reflect the variation of abundances at any given site, and to enable an accurate assessment of microplastic abundance and fluxes in the estuarine system. While a variety of meteorological and oceanographic conditions were recorded over the four months sampled, no one variable appeared to be a driving factor determining microplastic abundance in Southampton Water. This supports previous (non microplastic-related) studies in Southampton Water which define it as a well-mixed system. A significant difference was found between suspected microplastic abundances sampled using SML and bulk water sampling, with the SML-specific method (glass plate sampling) more clearly highlighting inter-site differences. This suggests that the SML glass plate method is a more sensitive method for sampling microplastics from estuarine waters.