Abstract

Land-based discharges of nutrients from deforestation, sewage and agricultural activities are a rapidly growing threat to Canobean coml reefs and Marine Protected Areas (MPA's). To assess localized sewageproblems in Tobago, we conductedan integrated water quality and coral reef monitoring program in the dry season between April and June, 2001. The study included a variety of fringing reef sites around Tobago, including the Buccoo Reef Complex (BRC) that was established as an MPA in 1973. The study involved:\i) measurement of water column dissolved inorganic nitrogen (DIN = ammonium plus nitrate plus nitrite), soluble reactive phosphorus (SRP), and chlorophyll-a (chl-a),\ii) digital video monitoring of benthic reef communities to quantify cover of hard corals, octocorals, macroalgae, turf algae, coralline algae, sponges, and\iii) determination of d15N ratios (=13N/14N) in benthic macroalgae at the reef sites to discriminate among natural versus anthropogenic nitrogen sources.\Low DIN:SRP ratios (<5:1) throughout the study area indicated DIN rather than SRP-limitation of algal growth. DIN (~1 µm mostly ammonium) and chl-a (up to 0.6 ug/l) within the BRC were high compared to more oligotrophic fringing reefs around Tobago and were indicative of eutrophication. Cumulative impacts of land-based nutrient eorichment in the BRC was also evidenced by relatively low cover of hermatypic (reef-forming) comls and high cover of macroalgae, turf algae, octocomls, and the zoanthid Palythoa that was physically overgrowing reef corals. Schools of mobile, herbivorous fishes (parrotfish, smgeonfish) were abundant in the BRC because of protection from reef fishing in this MPA. The d15N values of macroalgae in the BRC ranged from a high of +11.8 %o in Buccoo Bay near a sewage outfall to +5.3%o at Coml Gardens on the outer reef, values within the range reported for macroalgae growing on sewage nitrogen. Elevated DIN and chl-a concentrations combined with relatively high d15N values of macroalgae at other fringing reef sites off southwest Tobago were indicative of sewage pollution from upland urban centers and tourist resorts around Scarborough, Crown Point, and Mt. Irvine. The lowest DIN and chl-a concentrations and d15N values (± 2.5 %o) of macroalgae of the entire study were at Black Jack Hole off Little Tobago Island, a site characteristic of oligotrophic, background nitrogen sources and a lower degree of sewage pollution.

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