Grazing effects on nitrogen fixation in coral reef algal turfs |
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Authors: | S L Williams R C Carpenter |
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Institution: | (1) Department of Biology, San Diego State University, San Diego, California 92182–4624, USA, US;(2) Department of Biology, California State University, Northridge, California 91330, USA, US |
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Abstract: | This study addressed whether grazing by the sea urchin Diadema antillarum influenced rates of nitrogen fixation by algal turf communities on Caribbean coral reefs. Because the turfs were nitrogen-limited,
we also assessed whether newly-fixed nitrogen was important for supporting net primary productivity by the turfs. We measured
acetylene reduction in turfs grown in treatments excluding or including D. antillarum in the presence of other herbivores at 3 m water depth on Tague Bay forereef, St. Croix, U.S. Virgin Islands. These were
the first measurements of acetylene reduction on coral reefs under quasi-natural conditions of high water-flow and photosynthetic
oxygen generation. Rates of acetylene reduction under these conditions were as high as any measured previously in coral reef
communities (mean 7.6 nmol C2H4 cm−2 h−1). Algal turfs grazed by D. antillarum and other herbivores had chlorophyll-specific acetylene reduction rates up to three times higher than when D. antillarum was excluded. High rates of nitrogen fixation by the turfs were sufficient to meet <2% of the nitrogen required to support
net chlorophyll-specific primary productivity over 24 h. Grazer-mediated increases in nitrogen fixation do not appear responsible
for a parallel enhancement of net primary productivity. Algal turfs at this site must be dependent primarily on external sources
of nitrogen.
Received: 1 July 1997 / Accepted: 5 September 1997 |
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