Large carbonate, bryozoan-serpulid constructions, made by Pentapora fascialis and Salmacina dysteri respectively, were found around karstic freshwater springs, called vruljas, in the Senj Archipelago (Velebit Channel, Croatia). In June 2002, several sites were investigated by SCUBA divers on the rocky cliffs of Grmac and Zdralova at depths ranging from 19 to 32 m. Mean colony diameter decreased with increasing distance from the vruljas: in the vicinity the mean diameter was 65.8 +/- 21 cm, at 2-m distance it was 40.4 +/- 8.2. Carbonate contribution was to a great extent due to the bryozoan (5,784 +/- 1,186 g(.)m(-2) CaCO3) rather than to the serpulid (383 +/- 218 g(.)m(-2) CaCO3). P. fascialis carbonate standing stock was remarkably high if compared with data from literature for shallow carbonate producers. The bryozoanserpulid constructions can be indicated as important, even if localised, contributions to the carbonate budget in the Adriatic Sea.