Abstract

Representing a significant portion of the world’s second largest coral reef complex and encompassing a World Heritage site, the Belize Barrier Reef Complex (BBRC)) has experienced increasing blooms of macroalgae in recent decades. Researchers suggest that this is not the result of overfishing of herbivorous fishes, as many coral reef biologists have previously suggested, but more likely related to external factors, significantly nutrient enrichment. Because historical nutrient data for seawater and macroalgae were collected in the BBRC in the 1980s, we re- sampled the same sites at South Water Caye (SWC) marine reserve (e.g. Man-O-War Caye), and various sites at Glovers Reef (GR) marine reserve (e.g. Middle Caye) in June 2017 and 2018. Seawater dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) concentrations increased from undetectable concentrations in the 1980s to values of 0.5 – 1.0 µM DIN and 0.02-0.05 µM SRP at several reef sites, indicating nutrient enrichment of reef waters. Elevated macroalgal tissue C:N ratios from 22 to 44 occurred at reef sites, suggesting nitrogen limitation; however, lower C:N ratios of 13 to 16 occurred at Middle Caye, GR and Man-O-Way Caye, SWC indicating nitrogen enrichment at these sites. Stable nitrogen isotope values (d13N) were elevated (+3 to + 7 o/oo) at Middle Caye and Man-O-War Caye compared to lower values at reef sites (-0.5 to + 2 o/oo), pointing to significant nitrogen enrichment from humans and seabirds, respectively. These comparative nutrient data support recent suggestions that nutrient enrichment is a significant factor driving macroalgal blooms, declines in hard coral and loss of broader ecosystem services generated by the BBRC.

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