Temperature, salinity, and pH, along with copepod traits were measured inter-daily (i.e., on average each 4 days) in an upwelling and temperate estuary in the coastal Southeast Pacific. The purpose of the study was to evaluate the phenotypic plasticity of local copepod populations to natural extreme low pH conditions. Temperature and salinity were measured with CTD (Ocean Seven 305 Plus in the estuary system, and SeaBird SBE19 Plus in the upwelling location). Samples for pH measurements were collected with an oceanographic bottle. pH was measured potentiometrically, and calibrated with Tris buffer at 25 °C. Adult females of the copepod species Acartia tonsa (Copepoda, Calanoidea) were sampled with a WP2 plankton net. Cephalothorax length was measured under a stereomicroscope. Egg production was estimated individually over 24 h incubation.

Further information on POC method: Food concentration was determined as chlorophyll-a (Chl, µg Chl L-1) concentration, and then Chl concentration was converted to carbon units by using a Chl:C ratio=120, according to Vargas and González (2004), for the same study area. Triplicate samples were filtered onto a GF/F filter. Chl was extracted for 24 h in 90% acetone v/v and measured in a TD Turner fluorometer (Strickland and Parsons, 1972).