Seasonal dynamic and in situ division rates of the dominant Dinophysis species in Punic harbors of Carthage (Gulf of Tunis, South Mediterranean)

Abstract

This study investigates spatio-temporal variations of in situ division rate (μ) of two species of the genus Dinophysis. Based on a postmitotic index approach, estimates of μ from net-haul samples were compared during five daily cell cycles studies in three stations of the Punic harbors of Carthage (Gulf of Tunis). To study the annual cycle of Dinophysis spp., weekly sampling were carried out from March 2008 to June 2010. In addition, sampling over 24 h was conducted on three-cycle studies in late spring and two-cycle studies in autumn, seasons characterized by Dinophysis proliferations. We recorded important abundances of Dinophysis sacculus stein (2.25 × 10(4) cells/l, June 2008) and Dinophysis cf. acuminata (3 × 10(4) cells/l, June 2009). Cellular division was phased in D. sacculus stein and D. cf. acuminata, but the division time, required for the population cohorts to pass from one phase to another, and values of estimated division rates varied between seasons and stations for the same species. D. cf. acuminata exhibit moderate (0.22 day(-1)) to high (0.68 day(-1)) value of μ. In contrast, D. sacculus shows very low μ values (0.02-0.17 day(-1)) in autumn season to moderate values (0.21-0.35 day(-1)) in late spring. The results presented here confirm that the postmitotic index approach allowed estimated of μ at low field depths and concentrations (10(2)-10(3) cells/l).