Cyanobacteria Occurrence and Nitrogen Fixation Rates in the Seagrass Meadows of the East Coast of Zanzibar: Comparisons of Sites With and Without Seaweed Farms

Abstract

The occurrence and biological nitrogen fixation rates of epiphytic and benthic
diazotrophs were studied in seagrass meadows at sites with seaweed farms and at a control site
without seaweed farms from two locations, Chwaka Bay and Jambiani, along the east coast of
Zanzibar. Ten species of cyanobacteria were encountered and Lyngbya majuscula and Microcoleus
sp. were dominant in Chwaka Bay and Jambiani respectively. Cyanobacterial occurrence was
significantly higher in seagrass meadows without seaweed farms (P<0.02), but there was no
significant difference (P>0.05) in biomass (chlorophyll a). Biomass averaged 2.96±0.73 and
3.10±1.24 μg Chl a cm-2 in seaweed farms and 3.46±1.72 μg Chl a cm-2 at Chwaka Bay and
3.14±1.31 μg Chl a cm-2 at Jambiani in transects without seaweed farms. Nitrogenase activity
showed no significant differences between sites with and without seaweed farms, (P=0.66 Chwaka
and 0.75 at Jambiani). Fixation rates in sites without seaweed farms averaged 35.8±39.9 (Chwaka
Bay) and 13.1±12.7 (Jambiani) μmol of C2H4 produced/m2/hr, while with seaweed farms averaged
22.6±22.5 and 12.8±14.9 μmol C2H4 produced/m2/hr at the same sites. There were no significant
differences in nutrient levels between locations, sites with and without seaweed farms, or between
tidal levels except for nitrate, which was significantly higher (P=0.01) at Jambiani than at
Chwaka Bay. It was concluded that diazotrophs contribute a significant amount of exogenous
nitrogen to the seagrass ecosystem, but no significant differences could be found between sites
with or without seaweed farms.