Macroalgae reveal nitrogen enrichment and elevated N:P ratios on the Belize Barrier Reef.

Macroalgal blooms are increasing on the Belize Barrier Reef (BBR) as scleractinian coral cover declines. Although some have attributed this to reduced grazing, the role of land-based nutrient pollution has not been assessed. Nutrient enrichment was quantified through macroalgal tissue analysis from Belize City to the offshore fore reef and at several central BBR lagoon sites. These recent data were compared to baseline data from the 1980s. Significant nearshore-to-offshore gradients of %N, %P and δ13C in macroalgae all indicated land-based sources of these nutrients. Macroalgal δ15N values were generally enriched in nearshore waters where values matched those reported for human sewage. Notably, the N:P ratios of recent macroalgae measurements were elevated at all sites, more than two-fold higher than values from the 1980s (~30: 1 to 70:1). These results support the hypothesis that nitrogen enrichment from land-based sources has increased phosphorus limitation driving macroalgal blooms and coral stress on the BBR.

Nitrogen isotopic records of terrestrial pollution encoded in Floridian and Bahamian gorgonian corals.

Stable nitrogen isotope (delta(15)N) analysis has proven an effective "fingerprint" of sewage contamination in coral reef environments; however, short-term variability in nitrogen cycling and isotopic fractionation may obscure long-term trends. Here, we examine delta(15)N signatures in the organic endoskeletons of long-lived (20-40 years) gorgonian corals. Specimens were collected from relatively pristine reefs off Green Turtle Cay, Bahamas, and from reefs off southeast Florida heavily impacted by multiple sources of anthropogenic nitrogen. The delta(15)N of the most recently grown skeleton (branch tips) ranged from +2 to +3 per thousand at Green Turtle Cay, and +4.5 to +10 per thousand off Florida. These values closely match the delta(15)N of macroalgae collected from the same locations, indicating that gorgonian corals are isotopically similar to primary producers, and therefore suitable for assessing sources of dissolved inorganic nitrogen. Differences in the delta(15)N between younger and older skeleton indicated an overall decline of -0.34 +/- 0.06 per thousand (1 s.e) over the last 20 - 40 years at Green Turtle Cay, reflecting a possible increase in nitrogen fixation and/or atmospheric deposition of anthropogenic nitrogen. Off southeast Florida, there was an overall increase in delta(15)N over the same time period, reflecting increasing wastewater discharges from the rapidly growing population. These results highlight the usefulness of delta(15)N recorded in gorgonians and other long-lived organisms in assessing spatiotemporal patterns of nitrogen sources to coastal marine environments.

Land-based nutrient enrichment of the Buccoo Reef Complex and fringing coral reefs of Tobago, West Indies.

Tobago's fringing coral reefs (FR) and Buccoo Reef Complex (BRC) can be affected locally by wastewater and stormwater, and regionally by the Orinoco River. In 2001, seasonal effects of these inputs on water-column nutrients and phytoplankton (Chl a), macroalgal C:N:P and delta(15)N values, and biocover at FR and BRC sites were examined. Dissolved inorganic nitrogen (DIN, particularly ammonium) increased and soluble reactive phosphorus (SRP) decreased from the dry to wet season. Wet season satellite and Chl a data showed that Orinoco runoff reaching Tobago contained chromophoric dissolved organic matter (CDOM) but little Chl a, suggesting minimal riverine nutrient transport to Tobago. C:N ratios were lower (16 vs. 21) and macroalgal delta(15)N values higher (6.6 per thousand vs. 5.5 per thousand) in the BRC vs. FR, indicating relatively more wastewater N in the BRC. High macroalgae and low coral cover in the BRC further indicated that better wastewater treatment could improve the health of Tobago's coral reefs.