Environment and development
in coastal regions and in small islands
colbartn.gif (4535 octets)

Coastal region and small island papers 3

Bermuda

Struan R. Smith

Bermuda Biological Station for Research, Inc., 17 Biological Lane, Ferry Reach, St. George, Bermuda GE01

Bermuda is an isolated subtropical coral reef ecosystem in the North Atlantic. Shallow water carbonates cover an atoll-like volcanic seamount. The islands are emergent aeolianite limestone dunes that lie along the southeastern margin of the seamount. Extensive zones of coral reefs, dominated by massive corals (Diploria spp., Montastraea spp. and Porites astreoides), surround the central North Lagoon. Within the lagoon there is a complex of shallow patch reefs that support a higher diversity of corals, dominated by branching species (Oculina spp., Madracis spp.) and the hydrozoan Millepora alcicornis. Seagrass beds are distributed throughout the patch reefs, the inshore basins, and the shoreward margin of the outer rim reef. Three species of seagrasses, Thalassia testudinum, Syringodium filiforme, and Halodule bermudensis, are often intermixed or form monospecific beds. Mangrove forests have been reduced by foreshore development to small pockets and fringing communities, except at Hungry Bay. Rhizophora mangal and Avicennia germinans are the only species present. Seawater temperature fluctuates seasonally within the lagoon (14-31C) and on the outer reef (18-29C), which is moderated by the surrounding Sargasso Sea. Salinity remains close to oceanic values (36.5‰) due to low run-off from the porous limestone islands. Despite a high population density, human impacts are limited to over-fishing (now controlled through legislation), nutrient and trace metal loading of the inshore basins, and ship groundings.

Introduction                                                                                             

Bermuda is a unique island ecosystem located at a high latitude (32N, 65W) in the North Atlantic. The warming influence of the nearby Gulf Stream moderates the air and water temperature, allowing the development of subtropical marine and littoral communities (coral reefs, seagrass beds, mangrove forests). The islands of Bermuda are emergent aeolian dunes, formed during Pleistocene interglacial periods, located along the southeastern edge of the Bermuda Pedestal (Garret and Scoffin, 1977; Mackenzie and Vacher, 1975; Morris et al., 1977).

The volcanic pedestal is covered by Pleistocene and Holocene carbonates, 15 to 100 m thick, and the upper surface is about 665 km2 in size of which only 50 km2 is covered by the islands. The bulk of the upper surface of the pedestal is a shallow lagoonal system (<20 m deep) with extensive shallow reefs, seagrass beds, and deeper muddy basins, referred to as the North Lagoon (Fig. 1). The islands form protected nearshore areas with further seagrass development and isolated pockets of mangroves. Extensive reef zones are developed at the seaward margin and flanks of the pedestal (Fig. 1). The outer reefs and lagoon areas, <20 m deep, are referred to collectively as the Reef Platform (Logan, 1988).

Bermuda platform
Fig. 1. Map of the Bermuda Platform, showing the outer reef zones
surrounding the central North Lagoon, which contains numerous patch
reefs (e.g. Brackish Pond Flats and the Crescent) and muddy depressions.

The marine biota is Caribbean in origin, with reduced species diversity and relatively little endemism (Sterrer, 1986). The cool winter water temperatures (14-18C) are believed to be the limiting factor for the survival of some Caribbean species, although the degree of isolation of Bermuda (about 1200 km from The Bahamas and Florida) may limit larval dispersal from the Caribbean (Logan, 1988).

The islands were colonized in the early 17th century and extensive development has taken place, primarily since 1940. At present, the population is about 60,000, distributed throughout the islands. The economy is based principally on tourism, international business, and finance (Hayward et al., 1981). The main settlements are the city of Hamilton in the center of the island and the town of St. George at the eastern end. Both urban areas have centralized sewage systems that discharge directly into the marine environment with little treatment; the Hamilton outfall is located on the South Shore and the St. George system empties into the North Lagoon.

Human impacts on the marine environment have been relatively limited. Fishing pressure on fish, conchs, and lobsters on the reefs has increased throughout the 20th century and stocks have declined. A reasonably comprehensive management plan was introduced in 1990, including restricted areas and endangered species protection, to stabilize harvesting efforts (Butler et al., 1993). The inshore coastal lagoons receive nutrient inputs derived from agriculture and sewage via both terrestrial runoff and groundwater seepage, and also trace metal contaminants (Bodungen et al., 1982; Jickells et al., 1986). Direct sewage inputs appear to have limited impact due to good flushing at the outfall sites. Foreshore development has been extensive, with the creation of commercial and naval port facilities. Also, shipping channels have been dredged and blasted through the North Lagoon for access to the capital city of Hamilton at the center of the island and to the naval bases at the western end of the island. The creation of an airfield by dredging and filling a large area in Castle Harbour eliminated coastal mangroves, caused coral mortality, and altered the ecology of the lagoon (Dodge and Vaisnys, 1977; Dryer and Logan, 1978).

Climate and Oceanography                                                                      

Bermuda’s climate and the oceanographic conditions on the Reef Platform and surrounding Sargasso Sea have been studied intensively for decades (Anon., 1984; Morris et al., 1977). A comprehensive summary of many types of data pertaining to the Bermuda marine environment is given by Morris et al. (1977).

The high latitude of Bermuda (32N) makes it susceptible to strong low-pressure systems from North America, bringing an average of 4-6 strong gales per winter (Anon., 1984; Garret and Scoffin, 1977). High seas (5-20 m) are generated over a long fetch and impinge on the outer reefs, qualifying them as high energy zones. The lagoonal reefs experience 1-2 m waves during gales. Hurricanes and tropical storms hit or pass by every 2-5 years and also produce extreme wave conditions on the outer reefs.

Air temperature fluctuates strongly with the seasons, dropping to 8C with the passage of strong winter lows, while summer highs range to 35C (Table 1; Anon., 1984). Spring and summer air flow is generally from the SE to SSW and usually less than 15 knots. ENE-N winds are common in the autumn; winter air flow is from SW-W-NW, often >15 knots, and frequently gale force (35 knots). Rainfall averages 147 cm per year and is evenly distributed throughout the year. Cloud cover varies seasonally, with more clear days in summer (35-40%) than winter (15-20%). Solar insolation ranges from a July average of 640 g cal–1 cm–2 d–1 to a December average of 240 g cal–1 cm–2 d–1.

Table 1. Summary of CARICOMP climatic, oceanographic, and biological data (average values).
  Maximum Monthly
(Sampling Period)
Minimum Monthly
(Sampling Period)
Cumulative rainfall 547 mm (09/1995) 19 mm (05/1993)
Air temperature 35.0C (07/1995) 7.8C (02/1993)
Sea temperature, reef site 29.2C (07/1994) 18.7C (02/1995)
Sea temperature, seagrass site 29.2C (07/1994) 18.7C (02/1995)
Secchi horizontal distance, reef site 50.0 m (05/1993) 11.0 m (10/1995)
Secchi horizontal distance, seagrass site 25.3 m (04/1995) 7.3 m (08/1995)
 
  Range of Averages Sampling Period
Low High Low High
Mangrove total litter fall (g m–2 month -1) 15.00 250.00 01/1993 09/1993
Mangrove biomass (Cintron & Novelli) (kg m–2) 10.17 10.96 08/1993 09/1994
Thalassia leaf production rates (g-1 m–2 d-1) 0.52 1.48 09/1995 04/1994
Thalassia biomass (above+below) (g m–2) 658.00 1083.00 09/1993 04/1994
Total coral coverage (%) 16.70 24.20 09/1994 08/1993
Total algal coverage (%) 8.30 30.40 09/1995 09/1993

The North Lagoon and outer reef zones differ slightly in their oceanographic conditions (Boden and Kampa, 1953). The water temperatures on the outer reefs range from 18 to 28C, moderated by the passage of Gulf Stream eddies in the surrounding Sargasso Sea (Hela et al., 1953). Due to shallow depths, the North Lagoon warms and cools over a larger range; winter lows may reach 14C, climbing to 31C in the summer. There appear to have been record-setting highs in the past six years (Table 1; Cook et al., 1990, 1993), but this may be the result of more frequent sampling and detection of short-term peaks.

The lagoonal waters are exchanged over the outer reef zones by semi-diurnal tides with a mean range of 0.75 m and a spring range of 1.0 m. Salinity varies little from the 36.5 ppt recorded in the North Lagoon and the outer reef zones. The more enclosed inshore basins do show modest departures (~ 35.8 ppt) due to groundwater seepage, run-off, and rainfall.

The lagoonal and inshore basins are more turbid than the surrounding oceanic waters, due to particle resuspension and higher water column productivity (Bodungen et al., 1982; Jickells et al., 1986; Morris et al., 1977). Extinction coefficients are about 0.05 for the oceanic waters and range between 0.13 and 0.57 on the Reef Platform. Recent Secchi data show clear seasonal patterns at both the lagoonal seagrass site and the outer reef site, with generally clearer conditions in late spring/early summer, followed by a decline in water clarity through summer and fall (Table 1).

A water quality monitoring program (BIWI) has collected monthly surface water samples from the inshore waters and in the North Lagoon since 1977 (Bodungen et al., 1982; Jickells et al., 1986; Morris et al., 1977). The program measures dissolved nutrients (NO2+NO3, NH4, PO4), pigment concentrations, temperature, and Secchi disc depths. Significant nitrogen enrichment was found in the inshore basins but not in the North Lagoon. One site (Site 3A) is located about 5 km east of the North Seagrass Site.

Climatic data are collected at two locations. Rainfall data are collected on St. David’s Island as part of the AEROCE program. Air temperature data are collected at the Naval Oceanographic Command Facility, U.S. Naval Air Station, also on St. David’s Island (see Fig. 1). oceanographic data are collected at the Hog Breaker Reef Site, the North Seagrass Site, and the Hungry Bay Mangrove Site.

Mangrove Ecosystems                                                                            

Mangrove communities in Bermuda are the northernmost in the Atlantic and are limited in diversity and development (Thomas and Logan, 1992; Thomas, 1993). Bermuda’s mangrove swamps have been reduced dramatically due to foreshore development, particularly in the last century (Sterrer and Wingate, 1981; Thomas and Logan, 1992). Approximately 16 ha of mangroves remain, perhaps less than half the pre-Colonial amount, and ~6 ha of the total are found associated with landlocked anachialine ponds (Thomas and Logan, 1992).

Mangrove swamp development was never very extensive due to the steeply sloped shoreline and lack of estuarine environments. Most of the current mangroves are classified as fringing communities and are composed of only two species, Rhizophora mangale and Avicennia germinans, that exist as narrow stands along the shore (Thomas and Logan, 1992). The distribution of the mangroves on Bermuda is disjunct, due to the character of the coastline. Most of the mangrove communities are small (< 1 ha) and there are no zonation patterns. Hungry Bay is the largest swamp (2.9 ha), with a creek system that has been channelized since the 1950s (Fig. 2). The seaward margin of the Hungry Bay swamp has retreated significantly due to sea-level rise over the past 100 years (Ellison, 1993).

Hungry Bay - mangroves
Fig. 2. Map of the mangrove forest at Hungry Bay (after J. Ellison, 1993). The locations
of the four study plots are shown, as well as the principal channels within the forest.

The CARICOMP mangrove study is carried out in Hungry Bay (3217.3'N; 6445.5'W) on the southern shore of the island. Four 10 x 10 m vegetation study plots are located along the central axis of the swamp (Fig. 2) and the water sampling station is located near the mouth of the main drainage canal. CARICOMP results from 1992-93 indicate that salinity in the creek may drop to 33 ppm. The mangrove trees at the seaward margin are under stress, with reduced diameters, heights, and litter production compared to the plots in the interior of the swamp (CARICOMP data). Also, no seedlings are found at the seaward margin though they are abundant within the swamp. Ellison (1993) has demonstrated that the margin of the swamp is below mean sea level due to erosion and rising sea level, and this has contributed to the extensive retreat of the margin over the past 90 years.

Seagrass Ecosystems                                                                             

Seagrass beds are distributed throughout the Reef Platform but they are generally limited in extent and have not been well studied (Thomas and Logan, 1992). The dominant species are Thalassia testudinum and Syringodium filiforme, which can form large monospecific stands. The most extensive Thalassia beds are located off the southwestern end of the island. A large bed of Syringodium is located along the North Shore (Knap et al., 1991; Smith et al., 1995). In the inshore basins, Thalassia and Syringodium usually occur in mixed stands along with a third species, Halodule bermudensis. These beds are generally small due to the limited shallow water areas surrounding the relatively deep inshore basins. Several small bays and sheltered coasts have been mapped (Morris et al., 1977; Thomas and Logan, 1992). Seagrass beds are found associated with the patch reefs in the North Lagoon, either contained within the cellular and mini-atoll reefs (Logan, 1988) or on their flanks. Their distribution within the North Lagoon reef complex is patchy and has not been described. T. testudinum and S. filiforme are the most abundant species in these beds, occurring as either mixed beds or monospecific stands.

The two CARICOMP seagrass sites are located within a mini-atoll reef (Crescent Reef) in the center of the North Lagoon, south of the north shipping channel, in an area known as The Crescent (Fig. 1 and Fig. 3). Water depths off the reef reach 15-18 m. The North Seagrass Site (3224'04"N; 6447' 57"W) consists of a broad belt of T. testudinum adjacent to the eastern reef, which grades into an extensive mixed stand of S. filiforme and H. bermudensis. The South Seagrass Site (3224'10"N; 6448'20"W) is a monospecific stand of T. testudinum. The seagrass beds are surrounded by banks of reef that may shallow to 0.5 m depth but average 2-3 m in depth. Water depth at both seagrass beds is about 5 m. Shoot densities of T. testudinum are low (790 190 [standard error] m–2) and blade lengths short (5-11 cm), perhaps indicative of the relatively exposed condition of these beds. These sites appear to be pristine and undisturbed by human activities, apart from the CARICOMP study.

Hog breaker reef site Twin Breaker reef site
North seagrass site South seagrass site
Fig. 3. Detailed maps of the two reef monitoring sites and the two seagrass
study sites.

Thalassia beds are found at the lagoonward edge of the outer reef zone of the North Lagoon, known as the Rim Reef (see Fig. 3 and Fig. 1). These beds are small (<0.5 ha), patchy in distribution, and occur in relatively deep water (8-10 m). They are an important habitat for protected populations of the Queen Conch, Strombus gigas (Berg et al., 1992).

Quantitative studies on seagrasses are limited primarily to the nearshore beds (Patriquin, 1973; Pitt, 1992; South, 1983; Smith et al., 1995). Shoot densities for T. testudinum and S. filiforme range from 300 to 1600 per m2. Leaf production estimates for T. testudinum range from 0.8 to 1.7 g m–2 d–1 dry weight (Pitt, 1992; CARICOMP data). Higher leaf growth rates are found in the nutrient-enriched inshore areas compared to the lagoonal beds (Pitt, 1992). Intensive studies of T. testudinum and S. filiforme beds along the North Shore have been carried out as part of an environmental monitoring study (Knap et al., 1991; Smith et al., 1995). Syringodium leaf production rates are 0.3-0.6 g m–2 d–1 dry weight (Smith et al., 1995).

Coral Reef Zones                                                                                   

Extensive reef zones are developed at the margin and on the shallow flanks of the Bermuda Pedestal (Fig 1; Garret and Scoffin, 1977; Logan, 1988). The Rim Reef is a shallow (3-10 m) zone, about 0.5-1 km wide, that separates the ocean from the North Lagoon. This zone is reduced on the narrow southeastern edge of the pedestal adjacent to the islands, where a nearly continuous linear sequence of emergent algal-vermetid reefs ("boilers") separate the nearshore reef zone from the ocean (Meischner and Meischner, 1977). Seaward of the northern Rim Reef and southern boiler reefs is the extensive Main Terrace, sloping between 15-30 m, that surrounds the entire pedestal and may be up to 3 km in width. Below the Main Terrace reefs is the deep fore-reef that slopes sharply off and terminates at about 60 m (Fricke and Meischner, 1985). Within the North Lagoon is an extensive array of patch reefs varying in size and configuration (Garret et al., 1971; Logan, 1988), interspersed with deep (15-18 m) muddy basins.

The Rim and Terrace Reefs have similar coral assemblages, made up of a few species, but coverage ranges from 25% in the former zone to 50% in the latter zone (Dodge et al., 1982; Logan, 1988). Diploria strigosa, D. labyrinthiformis, Montastraea franksi sensu (Weil and Knowlton, 1994), M. cavernosa, and Porites astreoides are the dominant corals in these zones, along with the hydrozoan Millepora alcicornis. Less common species include Stephanocoenia michelini, Favia fragum, Agaricia fragilis, Madracis decactis, Siderastrea spp., Scolymia cubensis, and Isophyllia sinuosa. The deep fore-reef community is composed of Montastraea spp., A. fragilis, S. michelini, and Madracis spp. (Fricke and Meischner, 1985).

The lagoonal patch reefs have a similar Diploria-Montastraea-Porites community structure with lesser coverage (<20%) (Dodge et al., 1982; Garret et al., 1971). However, the patch reefs closer to the island and within Castle Harbour support a different community of primarily branched species (Madracis decactis, M. mirabilis, Oculina diffusa) that grow on the vertical sides of the reefs (Dryer and Logan, 1978; Logan, 1988). This reef community may have developed as the result of higher sedimentation rates close to shore and the degree of protection from wave energy. Coral diversity is greatest on these reefs, along with other sessile invertebrates and benthic algae.

Coral growth rates appear to be seasonal, with faster growth in the summer months but reduced for some species compared to Caribbean congeners (Table 1; Logan and Tomascik, 1991). Higher growth rates for several coral species are found within the lagoonal reefs compared to the outer reef zones (Logan et al., 1994).

Overall, Bermuda’s reefs are in good health, despite repeated coral bleaching episodes (Cook et al., 1993). The reefs have suffered direct human impact (ship groundings, dredging) but these have generally been limited in extent (Cook et al., 1993). The potential effects of the over-harvesting of reef fishes have been mitigated by a new management plan that eliminates the use of non-selective traps and creates no-fishing zones (Butler et al., 1993). Recent monitoring has noted the recovery of some fish stocks (Luckhurst, 1994).

The CARICOMP reef sites are located on the northern rim reef about 12 km from the island (Fig. 1 and Fig. 3). The Hog Breaker site (3227'32"N; 6449'54"W) and the Twin Reefs site (3227'51"N; 6448' 56"W) have been used for extensive investigation of coral recruitment, mortality, algal abundance, and fish grazing activity since 1986 (Smith 1988, 1990, 1992; Hog Breaker = Smith’s WC and Twin Reefs = Smith’s EC). Monitoring of coral bleaching on permanent transects at both sites has been carried out since 1990 (Cook et al., 1993). Neither site shows any evidence of human interference apart from low-impact scientific endeavors (photography, algal collection, installation of marking stakes).

The reef sites are near the center of the Rim Reef zone, about 100-150 m shoreward from the transition of the Rim Reef to the Main Terrace; thus, these sites are exposed to oceanic swell and storm waves. Both sites are characterized as a bank of reef, average depth 7-9 m, interspersed with sediment-filled depressions at about 10 m depth. The reef surface is a fairly rugose relief of 1-2 m, due in part to the large sizes (0.5-1.5 m diameter) of the main framework-builder, Diploria spp. (Smith 1988). An unusual feature is the presence of occasional biogenic carbonate pillars up to 4 m in height and 1-2 m in width, with sparse coral cover (Logan, 1988).

The coral communities at both sites are the Diploria-Montastraea-Porites assemblage typical of the Rim Reef. Gorgonian corals are common, primarily Pseudoplexaura spp., Plexaura spp., Eunicea spp., Pseudopterogorgia spp., and Gorgonia ventalina. Other common sessile invertebrates are coralliomorpharians, zoanthids, and anemones. Large erect sponges are rare. Reef turf algae are primarily Polysiphonia spp., Ceramium spp., Herposiphonia secunda, and Sphacelaria sp. The most common macroalgae are Laurencia obtusa and Dictyota bartayresii, although Ceramium nitens becomes seasonally dominant in the summer months (Smith, 1990).

Interaction of CARICOMP Sites and Relationship to the Caribbean Sites  

The three CARICOMP study sites on Bermuda are separated physically and do not have any direct interactions. Bermuda’s study sites are geographically unrelated to the Caribbean, with different climatic and oceanographic regimes. However, Bermuda’s role as an outlier ecological system may be a valuable point of reference for future changes that may occur in the Caribbean.

Acknowledgements                                                                                 

The Bermuda CARICOMP program is supported by a grant from the MacArthur Foundation and the Marine and Atmospheric Programme at the Bermuda Biological Station for Research, which is partially supported by grants from the Bermuda Ministry of the Environment. I would like to thank Dr. Anthony Knap, Director of BBSR, for his support of the project. Thanks also to J. Ellison, the Earth Watch Challenge Award Scholars (L. Rymarquis, S. Brown, S. Rosen, S. Morby, J. Last, T. Klein), R. Morin, V. Mattin, D. Marsh, A. Holland, T. Murdoch, S. Keyes, T. Warren, D. Hayward, C. Bosch de Noya, I. Kuffner, D. Hellin, G. Levi, S. McKenna, J. Bell, B. White, H. Litzenberger, and F. Minors for assisting with the establishment and monitoring of the CARICOMP sites. Thad Murdoch prepared the illustrations. Contribution Number 1466 from the Bermuda Biological Station for Research, Inc.

References                                                                                              

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