Seasonal patterns of biomass, growth and reproduction in Dictyota cervicornis and Stoechospermum polypodioides (Dictyotales, Phaeophyta) on a shallow reef flat in the southern Red Sea (Eritrea)

Abstract

Seasonal patterns in thallus length, biomass, reproduction, total biomass m(-2) and size structure were monitored in populations of Dictyota cervicomis and Stoechospermum polypodioides on a shallow reef flat in the southern Red Sea. These tropical reef flats are exposed to extreme temperatures of about 34 degrees C in summer and to temperatures of about 25 degrees C in winter. Both species showed peaks in length, biomass and reproduction in winter; macrothalli were absent in summer. Thallus length, biomass and the proportion of reproductive thalli showed a strong negative correlation with seawater temperature. Young thalli first appeared in November, following a drop in seawater temperature from about 33 to 31 degrees C. Macroscopic thalli had disappeared in May, when temperatures had reached the same values as those at the start of the growth season (33 degrees C). When reef substrata collected in summer were kept at sub-ambient temperatures (29-31 degrees C), macroscopic thalli of D. cervicomis developed. Size structure varied over time, probably because of non-synchronous development and tissue loss among thalli. Reproduction was size-dependent. We conclude that D. cervicomis and S. polypodioides have highly seasonal patterns of growth and reproduction related to the seasonal variation in the environment, especially temperature.