Daily and yearly variations of total inorganic carbon in a productive coastal area

Abstract Daily variations of total inorganic carbon, calculated from pH and total alkalinity measurements, have been monitored in the Bay of Calvi, above a Posidonia seagrass bed, during 24 campaigns over a 6-year period and covering the four seasons. The Posidonia and phytoplankton differ in their spatial distributions and seasonal activities. By making use of the way in which the observed parameters in both surface and bottom waters change as a function of time, (on both a daily and a seasonal basis) it is possible to distinguish the individual contribution of the two types of plant to the observed parameter changes. The data are sufficiently numerous to allow the testing of the variability of the daily mean values from year to year. It has been shown that the pH changes smoothly from 8·25 in winter to 7·9 in summer and shows good seasonal reproducibility. About one half of the seasonal change is due to the temperature dependence of pH. Total alkalinity and, as a consequence, inorganic carbon are less reproducible but show summer maxima despite sinks constituted by both the seagrass bed and the atmosphere. Except on a daily scale, carbon changes in the water column are not induced by primary production. The large range of in situ physico-chemical conditions makes the Bay of Calvi an ideal area for the study of processes related to the inorganic carbon cycle.