HORIZONTAL TRANSFER OF MATTER BY A CAVE-DWELLING MYSID

The role of a cave-dwelling mysid population as a matter carrier was e valuated in a cave of the Medes Is. (NW Mediterranean) during 1958-89. Hemimysis speluncola (LEDOYER, 1963) is a greg arious mysid whose swa rms migrate daily from the inner end of the cave-where they remain dur ing the day-to the exterior where they feed during the night. Fecal pe llet composition, pellet egestion and pellet decomposition were measur ed in order to evaluate the transfer of matter by mysids. Based on the strong daily behavioural rhythms of these mysids, special importance was attached to the sampling frequency (every 2 h). In order to assess seasonal variability, four daily cycles were evaluated within a year. In situ incubations were carried out to determine changes on the eges tion rates, morphology, weight and composition (AA, C:N ratios) of pel lets in the course of a day. The number of pellets deposited inside th e cave was estimated using containers distributed along the cave botto m by SCUBA divers. Fecal pellets showed an amorphous composition, wher eby diatom frustules, dinoflagellate loricae and coccolithoporids were very scarce. About 25% of the body weight were estimated to be egeste d daily as fecal pellets, suggesting a detritivorous feeding habit. Th e population oscillated seasonally between 1 and 12 millions. Individu als egested between 1.6 and 3.5 pellets a day into the cave, each aver aging from 9.4 to 11.9 mu g DW, 0.5 to 1.0 mu g C and 0.046 to 0.27 mu g N. Therefore, the population carried daily about 20-407 g DW POM, 2 -21 g C and 0.5-2.7 g N from outside to inside the cave. Pellets decom posed very quickly; between 20 to 50% of both C and N were released fr om pellets in less than 2 h after egestion. Oxidation of pellets theor etically consumes an amount of oxygen which agrees with the high BOD v alues previously reported for this cave. Marine caves are generally vi ewed as strictly oligotrophic systems; dense mysid populations, howeve r, could strongly modify the trophic relationships in marine caves.