Land-Based Nutrient Enrichment of the Buccoo Reef Complex and

Fringing Coral Reefs of Tobago, West Indies

B. E. Lapointe 1 , R. Langton 2 , O. Day 2 , B. Bedford 1 , C. Hu 3 and Arthur Potts 4

1 Center for Marine Ecosystem Health, Harbor Branch Oceanographic Institute, 5600 US Highway One, Fort Pierce, FL, 34946 U.S.A.

2 Buccoo Reef Trust, Cowie’s Building , Auchenskeoch Road , C arnbee Junction, Carnbee, Tobago, W. I.

3 Institute for Marine Remote Sensing, College of Marine Science, University of South Florida, St. Petersburg, FL 33701 U.S.A.

4 Centre for Marine Sciences, Institute of Marine Affairs, Hill Top Lane, Chaguaramas, P.O. Box 3160, Carenage, Trinidad and Tobago, W.I.

*Corresponding author; Tel.: 772-465-2400 ext 276; fax: 772-468-0757

e-mail address:

Key words: coral reef, macroalgae, eutrophication, sewage, nitrogen, phosphorus, Orinoco

Abstract

Land-based runoff represents a major source of marine pollution in the Caribbean Sea. The fringing coral reefs of Tobago, including the Buccoo Reef Complex (BRC), can be affected locally by wastewater discharges and storm water runoff, as well as regionally by the floodwaters of the Orinoco River during the wet season. We examined the effects of land-based nutrient pollution by measuring dissolved water column nutrients (DIN, SRP), and chlorophyll a, C:N:P and d15N contents of macroalgae, and % cover of coral and macroalgae at a variety of sites on the Buccoo Reef and fringing reefs (FR) in the dry (April - June) and wet (September -October) seasons of 2001. Concentrations of DIN (mostly ammonium) increased at all stations from the dry to wet seasons, in contrast to SRP that decreased from the dry to wet season. These seasonal shifts in nutrient concentrations resulted in a significant increase in the DIN:SRP ratio from the dry to wet season. In April, C:N ratios of macroalgae were significantly lower in the BRC vs. FR sites (15.8 vs. 21.0), in contrast to higher N:P ratios in the BRC (49.5 vs. 34.6), indicating greater N-enrichment of the reefs within the Buccoo Reef itself. Overall, the d15N contents of macroalgae averaged + 6.05 ± 1.59 o/oo and did not vary with season, although the more remote and less populated sites off Little Tobago island did increase significantly from the dry to wet season. However, the d15N contents of macroalgae in the BRC (+ 6.62 o/oo) were significantly higher than the FR sites (+ 5.53 o/oo), indicating a higher degree of wastewater derived N on the Buccoo Reef. The % cover of coral correlated inversely with macroalgae among the reef sites, with the highest % cover of macroalgae occurring throughout the Buccoo Reef Complex. These data suggest that local inputs of sewage are a primary source of nutrient stress on Tobago’s reefs, especially throughout the Buccoo Reef, and that improved collection and treatment of wastewater could lead to improved water quality and reef health.

1. Introduction

Coral reefs are biologically diverse and economically valuable ecosystems that provide an array of ecological services to coastal communities throughout their range. Reef ecosystems occur in oligotrophic waters and are susceptible to eutrophication as a result of low level nutrient pollution (Johannes, 1975; NRC 2000). Studies in Kaneohe Bay, Hawaii, clearly showed how reef corals can be overgrown by macroalgae when small increases in nutrient concentrations (nitrogen and/or phosphorus) occur as a result of sewage pollution (Banner, 1974; Smith et al. 1981).

Pollution from land-based sources is now considered the single most important threat to the marine environment of the Caribbean and impediment to sustainable use of its resources (UNEP, 1994). Sources of nutrient pollution include sewage outfalls, septic tanks, storm water runoff, fertilizers, deforestation (Likens, 2001; Lapointe and Thacker 2002) and atmospheric deposition (Barile and Lapointe, 2005).

The island of Tobago, often described as the “Jewel of the Caribbean”, is located at the southern end of the Lesser Antilles off the coast of Venezuela. Tobago is primarily a volcanic island but with considerable coral reef growth, especially in the southwestern area where extensive coral reef development has resulted in a shallow limestone platform known as the Buccoo Reef Complex (BRC; Fig. 1). The Buccoo Reef Complex encompasses an area of ~ 7 km-2 and was officially designated as a marine protected area, The Buccoo Reef Marine Park, in 1973 (Laydoo, 1991). The Buccoo Reef Maine Park is the best example of contiguous coral reef – seagrass – mangrove ecosystem in the Republic of Trinidad & Tobago. The BRC is also a major economic asset, attracting some tens of thousands of visitors per year.

Like most marine protected areas in the Caribbean region, creation of the Buccoo Reef Marine Park has not protected the area from water quality degradation. Many coastal dwellings in the contiguous Buccoo Village area, for example, have septic tanks and “soak-away” systems built in the carbonate-rich limestone that overlay the volcanic geology. This sewage disposal technology provides little removal of nitrogen, which is highly mobile in carbonate-rich systems and flows subsurface, via groundwater, into coastal waters (Lapointe et al. 1990, Costa et al. 2000, Lapointe and Thacker 2001). Similarly, secondarily treated sewage, a technology that does not remove nutrients sufficiently to protect coral reef ecosystems, enters Buccoo Bay and the Bon Accord Lagoon via several sewage treatment plants that service subdivisions near Buccoo Village (Coral Garden Estates) and in Bon Accord. Sewage pollution in Buccoo Bay presents a significant risk to public health as indicated by consistently elevated concentrations of coliform bacteria and fecal streptococci (John 1996).

Although there has long been concern about sewage pollution in the Buccoo Reef Complex, little is known about the dynamics by which nutrient pollution fosters eutrophication and degradation of these coral reef communities. Kenny (1976) provided one of the first qualitative studies of coral communities in the BRC and noted that “care should be taken not to increase run-off from the adjacent land and to restrict the entry of pollutants”. Laydoo and Heileman (1987) sampled effluents from sewage treatment plants servicing subdivisions on the watershed of the BRC and recommended upgrading the treatment facilities throughout the Buccoo Reef watershed. In 1994, the Institute of Marine Affairs completed a management plan for the Buccoo Reef Marine Park (IMA, 1994) that specifically recognized sewage impacts (i.e. fecal coliform contamination) in the inshore waters of Buccoo Bay, but the plan did not address the chronic ecological impacts of nutrient pollution of the coral reef communities.

Like the Buccoo Reef Complex, fringing reefs around Tobago’s coast could also be receiving increasing nutrient loads from poorly or inadequately treated sewage effluents. Both large and small communities rely on “soak-aways” as on the BRC catchment or less than adequate (i.e. tertiary level) treatment of waste water. Where centralized collection and treatment systems for sewage have been developed they only provide only secondary-level treatment at best (no nutrient removal) and are usually inadequate for the end-volume, resulting in nutrient-rich discharges into rivers or wetlands that eventually flow into coastal waters.

To specifically assess the chemical and ecological impacts of sewage pollution in the Buccoo Reef Complex and the fringing reefs around Tobago, we undertook a water quality sampling and coral reef monitoring project in 2001. The study involved measurement of water column dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP), and chlorophyll a during the dry (April - June) and wet (September – October) seasons. This sampling design was necessary to address the seasonal effects of the Orinoco River floodwaters that impact Tobago’s coastal waters during the wet season (Laydoo, 1991). Underwater video of benthic reef communities was used to quantify cover of hard corals, octocorals, macroalgae, turf algae, coralline algae, and sponges. Stable nitrogen isotope ratios (d15N) in benthic macroalgae were used to discriminate among natural (N-fixation) versus anthropogenic (e.g. sewage) nitrogen sources fueling algal blooms and eutrophication in the seasonal samplings.

2. Materials and methods

2.1 Study sites and samplin g rationale - A total of six sites in the Buccoo Reef Complex were sampled to assess water quality and benthic community structure along an inshore to offshore gradient, to address the importance of land-based nutrient discharges. The sites included a sewage outfall emptying into Buccoo Bay, a site immediately off the shore in a location near the house where Prince Margaret honeymooned that we refer to as Princess Reef and off Buccoo Point, near Buccoo Village, all in the inner BRC; Walkabout Reef, where glass-bottom boat tour operators allow people to disembark and walk on the backreef, and Nylon Pool, both in the middle of the BRC; and Coral Gardens and Outer Reef in the offshore areas of the BRC (Fig. 1). We predicted that the inshore sites of the Buccoo Reef Complex would show the strongest evidence of nutrient enrichment from land-based runoff and that there would be a decreasing impact with distance from shore. In addition, a variety of fringing reefs around Tobago’s Caribbean and Atlantic coasts were sampled, which included a series of well known dive sites: Mt. Irvine “Wall”, the Maverick (a sunken ship-artificial reef), Diver’s Dream, a site off shore from the Tobago Vanguard Hotel, Diver’s Thirst, Culloden, Arnos Vale, Englishman’s Bay, Three Sisters and two sites off Little Tobago Island -- Kelliston Drain and Black Jack Hole. We hypothesized that nutrient enrichment would be relatively minimal off Little Tobago Island, the site furthest offshore and flushed with oceanic “blue water” waters of the Guyana Current and conversely, greater nutrient enrichment on the fringing reef sites more directly impacted by land-based runoff from Scarborough and the more urbanized areas of southwest Tobago, which includes sites off the Vanguard Hotel, Mt. Irvine, the Maverick, Diver’s Dream and Diver’s Thirst. Finally, we sampled all sites in both the dry and wet seasons of 2001 (Fig. 2) to assess the role of land-based storm water runoff in driving seasonal shifts in nutrient concentrations and eutrophication.

2.2 Analysis of seawater for DIN, SRP, and chlorophyll a - SCUBA divers collected water samples from the various reef sites between April 10 and June 20, 2001 in the dry season and between September 24 and October 23, 2001 during the wet season. Water temperatures and depths at the reef sites were measured using Oceanic Datamax Pro Plus dive computers. Divers used clean, one liter polyethylene Nalgene bottles to collect replicate (n = 4) seawater samples ~ 0.5 m off the bottom at the various reef sites. The water samples were held on ice in the dark in a cooler until return to shore where aliquots of the samples were filtered through a 0.45 mm GF/F filter and frozen. The samples were analyzed for DIN ( = NH4+plus NO3- plus NO2-) and SRP on a Bran and Luebbe TRAACS Analytical Console at Harbor Branch Oceanographic Institution’s (HBOI) Environmental Laboratory in Ft. Pierce, FL. The methods for collection, handling, and processing of all nutrient samples followed a quality assurance/quality control protocol developed by HBOI’s Environmental Lab. This plan prevented problems associated with sample contamination and excessive holding times. Salinity of the water samples was measured to ± 1.0 psu using a Bausch and Lomb refractometer. The GF/F glass fiber filters used for filtering the water samples were frozen and analyzed for chlorophyll a (chl a) as a measure of phytoplankton biomass. The filters were extracted for 30 minutes using 10 ml of dimethyl sulfoxide and then with an added 15 ml of 90% acetone at 5 oC overnight. The samples were measured fluorometrically before and after acidification for determination of chlorophyll a and phaeopigment concentrations. Fluorescence measurements were made using a Turner Designs 10-000R fluorometer equipped with a infrared-sensitive photomultiplier and calibrated using pure chlorophyll a.

2.3 Analysis of macroalgae for C:N:P and d 15 N – SCUBA divers collected replicate (n = 2) composite samples of macroalgae from the various reef sites into nylon mesh bags. At least 3-6 separate plants were collected for each composite sample of each species to ensure representativeness. Immediately following collection, the macroalgae were cleaned of debris and transferred to plastic zip-loc baggies and held in a cooler during transport to the lab. In the lab, the samples were identified, rinsed briefly (3-5 s) in deionized water to further remove debris. The plants were placed in plastic drying dishes and dried in a lab oven at 65 oC for 48 h. The dried macroalgae were ground to a fine powder using a mortar and pestle and stored in plastic vials until analysis. Samples of dried, powdered macroalgae were analyzed for stable nitrogen isotope ratios with a Carlo-Erba N/A 1500 Elemental Analyzer and a VG Isomass mass spectrometer using Dumas combustion (by Isotope Services, Los Alamos, New Mexico). The standard used for stable nitrogen isotope analysis was N2 in air. d15N values, expressed as o/oo (ppm), were calculated as [(Rsample/Rstandard) – 1] x 103, with R equal to 15N/14N. Subsamples of the powdered macroalgae samples from the dry season sampling were analyzed for C:N:P contents at Nutrient Analytical Services, Chesapeake Biological Laboratory, University of Maryland System, Solomons, MD. Tissue C and N were measured on an Exeter Analytical, Inc. (EAI) CE-440 Elemental Analyzer, whereas P was measured following the methodology of Asplia et al. (1976) using a Technicon Autoanalyzer II with an IBM-compatible, Labtronics, Inc. DP500 software data collection system (D’Elia et al. 1997).