DEMOGRAPHY OF FRONDS OF SARGASSUM LAPAZEANUM FROM SOUTHERN BAJA CALIFORNIA, MEXICO, DURING EARLY STAGES

Abstract

In terms of biomass, Sargassum lapazeanum (Phaeophyceae, Fucales) is one of the most important seaweeds in La Paz Bay, on the southeastern coast of the Baja California Peninsula, Mexico. This species can be found year‐round in shallow subtidal sites. Standing biomass is lowest between fall and winter and highest between spring and summer. We are currently studying the annual demography of fronds as a necessary step to understanding the mechanisms of population regulation. Work is being done on a bed of about 90 m in length and three m in width. At this stage of abstract submission, we can report results for winter only. Random samples were collected in February and in March 2000 to estimate stand biomass, frond density, and size (frond length) structure. In addition, we labeled fronds with numbered plastic tags to estimate their rates of growth and of mortality during this period. Rates of recruitment were estimated from a combined analysis of the above. In February, fronds were all shorter than 6.5 cm. Between February and March, mean total frond density increased from 122 to 776 fronds m−2 (n = 30 quadrats). The mean recruitment rate was 667 fronds m−2, the mean growth rate was 0.5 cm day−1 (n = 60 fronds), and the mean mortality rate was 43 fronds m−2. From last year's preliminary observations, we expect to observe peaks of biomass and of reproduction in late spring, followed by negative growth rates and high mortality rates during summer.