Abstract

Phase-shifts from coral to macroalgal dominated reef systems have been well documented throughout the wider Caribbean region. Although some biologists consider herbivore loss through disease and overfishing to be the causal factor, recent reef surveys in Belize indicated that expanding macoalgal blooms are not related to loss of herbivorous fishes. To assess the possible role of long-term nutrient enrichment, macroalgae from several mangrove and reef sites in the Belize Barrier Reef Complex (BBRC) were re-sampled in June 2017 and compared with previous tissue nutrient (carbon:nitrogen:phosphorus; C:N:P) data collected in the late 1980s. Comparative sampling sites included Man-O-Way Cay, Twin Cays, Tobacco Reef, and Curlew Reef. In addition, the 2017 sampling included collection of macroalgae from a variety of habitats at Glover's Reef that are experiencing macroalgal blooms. The dried macroalgal tissue was processed and analyzed for stable carbon and nitrogen isotopes, as well as C:N:P ratios. The comparative nutrient data showed significant increases in the N:P and C:P ratios at all sites since the late1980s, with parallel decreases in the C:N ratio. These data provide preliminary evidence of long-term nitrogen enrichment of the BBRC, which could explain the expanding macroal-gal blooms without loss of herbivorous fishes. Macroalgae in the lagoon and fore reef habitats at Glover's Reef also had high N:P and C:P ratios, suggesting that this offshore location is also experiencing nitrogen enrichment. Stable nitrogen isotope values in the macroalgae were dependent on location, with some sites reflecting enriched values typical of wastewater, and other sites more depleted values typical of nitrogen fixation, fertilizers, and atmospheric deposition.

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