Extreme Variations of pCO2 and pH in a Macrophyte Meadow of the Baltic Sea in Summer: Evidence of the Effect of Photosynthesis and Local Upwelling

The impact of ocean acidification on benthic habitats is a major preoccupation of the scientific community. However, the natural variability of pCO 2 and pH in those habitats remains understudied, especially in temperate areas. In this study we investigated temporal variations of the carbonate system in nearshore macrophyte meadows of the western Baltic Sea. These are key benthic ecosystems, providing spawning and nursery areas as well as food to numerous commercially important species. In situ pCO 2 , pH (total scale), salinity and PAR irradiance were measured with a continuous recording sensor package dropped in a shallow macrophyte meadow (Eckernförde bay, western Baltic Sea) during three different weeks in July (pCO 2 and PAR only), August and September 2011.The mean (± SD) pCO 2 in July was 383±117 µatm. The mean (± SD) pCO 2 and pH tot in August were 239±20 µatm and 8.22±0.1, respectively. The mean (± SD) pCO 2 and pH tot in September were 1082±711 µatm and 7.83±0.40, respectively. Daily variations of pCO 2 due to photosynthesis and respiration (difference between daily maximum and minimum) were of the same order of magnitude: 281±88 µatm, 219±89 μatm and 1488±574 µatm in July, August and September respectively. The observed variations of pCO 2 were explained through a statistical model considering wind direction and speed together with PAR irradiance. At a time scale of days to weeks, local upwelling of elevated pCO 2 water masses with offshore winds drives the variation. Within days, primary production is responsible. The results demonstrate the high variability of the carbonate system in nearshore macrophyte meadows depending on meteorology and biological activities. We highlight the need to incorporate these variations in future pCO 2 scenarios and experimental designs for nearshore habitats.