The data set comprises measurements of temperature, salinity, oxygen, chlorophyll and nutrients from two locations near Port Erin, Isle of Man. Sea surface temperature has been measured at Port Erin breakwater (54 05.113N, 04 46.083W) on a twice-daily basis from 1904 to the present day, accompanied by twice-daily sea surface salinity measurements since 1965. Since 1954, further measurements have been taken at the Cypris station in Port Erin Bay, 5km west of Port Erin (54 05.5N, 004 50.0W). The Cypris data have been collected at frequencies ranging from weekly to monthly depending on season, boat availability and weather, and comprise measurements of temperature at 0, 5, 10, 20 and 37m since 1954; salinity, dissolved oxygen and phosphate at 0 and 37m since 1954; silicate at 0 and 37m since 1958; nitrate and nitrite and 0 and 37 m since 1960; chlorophyll a at 0 m since 1966; ammonia at 0 and 37m since 1992; total dissolved nitrogen at 0 and 37m from 1996-2005; and total dissolved phosphorus at 0 and 37m from 1996-June 2002. At Port Erin temperatures were recorded using a Meteorological Office issue thermometer from the mid-1900s to October 2006. Since then Vemco temperature autologgers and Star-Oddi DST CTD loggers have been deployed from the Port Erin lifeboat slip and are exchanged on an approximately monthly basis. Until November 1961 temperature was recorded in degrees Fahrenheit; these data have since been converted to degrees Celsius. At the Cypris station water samples were collected with either a Nansen-Pettersen or an NIO bottle from 1954-2005. The Nansen-Pettersen bottle was used in conjunction with an insulated thermometer, while the NIO bottle was used in conjunction with a mercury reversing thermometer. From 2006 onwards an RTM 4002 X digital deep sea reversing thermometer has been used with an NIO bottle. Salinity was determined by titration against silver nitrate until 1965, thereafter using inductively coupled salinometers (Plessey 6230N until June 1998; Guildline Portasal from July 1998). Nutrients are estimated colorimetrically and dissolved oxygen is determined by the Winkler technique. Until 2006 chlorophyll-a was estimated using the trichromatic methods recommended by SCOR-UNESCO Working Group 17. Since that year the spectroscopic methods of Aminot & Rey (2000) have been used. Dissolved nitrogen and phosphorus were measured using the persulphate digestion method adapted from Valderama (1981). The Cypris station data are frequently split into the Cypris I (D. John Slinn) data set comprising data from 1954-1992 and the Cypris II data set from 1992-present. Data from the Port Erin and Cypris stations are sometimes known collectively as the ‘Port Erin Bay’ data set. Data from Port Erin Bay form part of the Isle of Man GAL Coastal Monitoring Sites network, which is described in a separate EDMED entry. The data were collected by the Port Erin Marine Laboratory (part of the University of Liverpool) until its closure in 2006. Sampling has since been taken over by the Isle of Man Government Laboratory. The data are managed by the British Oceanographic Data Centre.