Vulnerability of copepod eggs in salinity and temperature gradients of the Marmara and Black Seas

Abstract

Mass density, sinking speed and hatching success of eggs in Calanus euxinus, Acartia clausi and A. tonsa have been examined in response to salinity and temperature of the Marmara and Black Seas. Eggs of C. euxinus from the Marmara and Black Sea populations (mean mass density of 1.036 to 1.040 g cm(-3)) laid in brackish (18 to 22 psu) upper layers of the Marmara Sea could sink to high-saline (similar to 38.5 psu) deep layers during embryonic development. Gradual salinity increase from 22 to 38.5 psu in laboratory experiments reduced egg hatching success in C. euxinus from the Marmara Sea from 97 +/- 4 to 47 +/- 11% (means +/- SD) while all eggs of the Black Sea C. euxinus did not hatch. Mean mass density of eggs in A. tonsa and A. clausi amounted to 1.062 and 1.044 g cm(-3), respectively. Eggs of A. tonsa with higher mass density and longer development time than those of A. clausi could descend to high salinity strata in the Marmara Sea, or to a cold intermediate layer in the Black Sea. On the contrary, sinking depth of eggs of A. clausi should not exceed 30 m both in the Black and Marmara Seas, as this increases predator pressure. Hatching success in A. tonsa eggs laid at 18 psu decreased from 77.7 +/- 18.5% at 20 degrees C to 46.7 +/- 21.5% at 7 degrees C, and reduced to 10.2 +/- 4.7% at 7 degrees C and 39 psu. Egg hatching success in A. clausi from the Black Sea did not depend upon temperature, whilst it decreased dramatically from 80 to 90% to 3.3 to 10% after salinity increase from 18 to 38 psu.