The chlorophyll a survey is an annual assessment of the Chlorophyll a concentration in micrograms per litre found in surface waters in coastal and transitional (estuarine) waters around Northern Ireland. Spot samples are taken at established sample sites and returned to the laboratory for analysis. The dataset contributes to Water Framework Directive (WFD) classification, as well as providing a eutrophication indicator under Nitrates Directive (ND), and Urban Waste Water Treatment Directive (UWWTD). Selected samples are submitted to the MERMAN database for inclusion in the CSEG (Clean Seas Evidence Group) work programme. The data held in MERMAN fulfils the UK's mandatory monitoring requirements under the Oslo and Paris Convention (OSPAR), Joint Assessments and Monitoring Programme (JAMP). Within DAERA, this survey forms part of the Marine Phytoplankton and Chlorophyll work programme (MM(YY)-14).