ABUNDANCE AND PHOTOSYNTHETIC CHARACTERISTICS OF TRICHODESMIUM SPP ALONG THE ATLANTIC BARRIER-REEF AT CARRIE-BOW-CAY, BELIZE

Trichodesmium abundance and photosynthetic rates were determined acros s a transect of the Atlantic Barrier Reef at Carrie Bow Gay, Belize in May 1993 and May-June 1994. A fore reef station receiving oceanic wat er had a Trichodesmium abundance characteristic of the open Caribbean Sea (less than or equal to 10(2) colonies m(-3) and 10(4)-10(5) tricho mes . m(-3)). Several species were present; however, in all cases a sh arp decrease in abundance occurred across the reef to the coastal lago on. At this latter site, virtually no Trichodesmium was present, altho ugh a substantial bloom of the diatom Proboscia alata occurred in 1994 . Maximum photosynthetic rates and the initial light-limited slope ct were significantly lower in offshore, stations than in reef top popula tions. The increased photosynthetic rates and light-harvesting ability in reef-top populations, as well as the lack of significant photoinhi bition, indicated that the Trichodesmium abundance decrease was probab ly not due to a decline in photosynthetic capacity. Grazing and physic al removal rather than a physiological decline were likely mechanisms operating to reduce Trichodesmium abundance in these regions. These da ta suggest that the Belizean barrier reef is a sink for Trichodesmium transported in from offshore regions.