|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Bahía de Chengue, Parque Natural Tayrona, Colombia
Instituto de Investigaciones Marinas y Costeras (INVEMAR), Apartado 1016, Santa Marta, Colombia
Bahía de Chengue is a small bay located on the Caribbean coast of Colombia. The shores are mainly steep and rocky and support poorly developed but diverse coral reef communities. At the inner portion of the bay there are sedimentary shallow bottoms with seagrass beds dominated by Thalassia and narrow but dense coastal fringes of small Rhizophora trees. Regional climate and oceanography are strongly determined by high coastal topography and trade wind incidence. There are two clearly distinct seasons: (1) a dry season (from December to April), when strong NE trade winds (mean 3.5 m s-1) reduce precipitation and generate an upwelling that transports water with low temperature (mean 25.5°C) from 100-200 m depth to the surface; and (2) a rainy season (from May to November), with low trade wind incidence (mean 1.5 m s-1), more than 80% of the total annual rain, reduced upwelling occurrence, and higher seawater temperature at surface (mean 28.0°C). Turbid currents of reduced salinity commonly enter the bay from the southwest during the rainy season. Chengue is part of a Natural Park but human activities in the area, although not extensive, have an impact on the marine ecosystem.
The CARICOMP site in Colombia is Bahía de Chengue (11°20'N and 74°08'W), a small bay (surface area 3.3 km2) in the Parque Natural Tayrona, located 14 km northeast of the city of Santa Marta on the Caribbean coast of Colombia (Fig. 1). The coastal topography is heterogeneous, with a steep relief due to the closeness of the Sierra Nevada de Santa Marta, the highest mountain system of Colombia (5,800 m above sea level). Consequently, the shore is basically a cliff composed of several types of metamorphic rocks (Doolan and McDonald, 1976), with numerous rocky headlands, islets, bays, and inlets. Also, the continental shelf is very narrow, and depths of 200 m are found only 2 km from shore. At the inner portion of area bays, there are sedimentary beaches, small coastal lagoons and, intermittently, the mouths of small rivers (Garzón-Ferreira and Cano, 1991).
|Fig. 1. Map of Bahía de Chengue,
indicating bathymetry, distribution of
principal coastal marine ecosystems and location of CARICOMP sampling
sites. Coral reef physical measurements were taken at site C1. Inset map
includes a section of the Caribbean coast of Colombia, showing the
locations of Bahía de Chengue, Santa Marta, and the mouth of the
The submarine topography and distribution of the major shallow marine ecosystems of Bahía de Chengue are shown in Fig. 1. Most of the bottom is sandy and deeper than 10 m. Coastal lagoons, mangroves, seagrass beds, and coral reefs have developed at the eastern and southern shores of the bay, where trade wind-induced wave action from the northeast is low (photo. 1), while rocky bottoms are the dominant ecosystem near shore in high wave energy zones, as in the NW shore of the bay (photo. 2). A detailed description of the sediments, hydrography, and biotic communities of the southern lagoon was provided by Alvarez-León et al. (1995). Due to coastal heterogeneity and the occurrence of wide gradients of exposure to prevailing waves from the northeast, there are extensive benthic communities and species. Therefore, marine biodiversity in Chengue is high, as suggested by the results of Velásquez (1987) who found 500 mollusk species when surveying less than 3 km2 of the bay. The work by Brattstrom (1980) may also be indicative of high marine biodiversity in the area of Santa Marta. He described nine stations in the rocky littoral zone (to 1 m depth) and reported at least 246 species (67 of algae and 179 of benthic invertebrates). Acero and Garzón (1987) recorded 372 species of reef fishes in the Santa Marta area.
|Photo. 1. Northwestern shore of Bahía
Chengue showing exposed metamorphic
rocks and strong wave action.
|Photo. 2. Fringes of Rhizophora
(red mangrove) in the southern lagoon of
Bahía de Chengue.
A single family of eight persons (five adults and three children) constitutes the permanent human population of Chengue, accompanied by six cows, one pig, two dogs, and two geese. This family subsists mainly on salt mining from the western lagoon, but also on fishing in the bay. Although they use wood for cooking, no major modification of the terrestrial vegetation cover around the bay is evident. At the end of March each year, an additional group of ten persons arrives at Chengue to work the salt mines for one and one-half months. Fishermen from nearby areas regularly work in Bahía de Chengue; unfortunately, they sometimes use dynamite. Each night, the lights of approximately six hook-and-line fishing boats are visible near the reefs. In daylight hours, the fishermen sometimes use gill nets or beach seines across the seagrasses and in the southern lagoon. Commercial reef fish populations are clearly overfished; thus, only small groupers, snappers, or grunts are observed. As no road has been opened to the bay, tourist visits are infrequent.
Locations of the CARICOMP sampling stations in Bahía de Chengue are shown in Fig. 1. Physical measurements of the coral reef environment are being made 50 m to the west of site C1, and a Hobo data logger for permanent temperature recording was placed at a depth of 9 m (under a coral head) in this site. Two additional Hobos have been installed: one at a depth of 1.5 m in the seagrass bed site, and one buried 10 cm below the sediment surface (exposed at low tide) at the mangrove site (20 m behind the outer margin of the Rhizophora fringe). Sampling procedures for physical weekly measurements at seagrass and mangrove stations are as described in the CARICOMP methods manual. Daily site measurements are made at the Instituto de Investigaciones Marinas (INVEMAR), which is located 30 m above sea level on a coastal rocky cliff at Punta de Betín, Santa Marta.
Climate and Oceanography
There exist no meteorological data from Chengue, but climatological data are available from Santa Marta (Salzwedel and Muller, 1983) (Table 1). Personal observations during 13 years in the area indicate that the general climate as well as oceanographic conditions are similar at both locations, being strongly dominated by the trade winds that blow from the northeast (known locally as "brisa"). There is a dry season from December to April, when the trade winds are strong (mean wind velocity 3.5 m s-1, maximum up to >30 m s-1) and precipitation is low (<20% of the annual total). The rest of the year (May-November) is known as the rainy season, characterized by lower trade wind incidence (mean wind velocity 1.5 m s-1) and higher precipitation (52% of total annual rainfall during September-November). There usually is an intermediate, weak dry season during June and July. If the wind does not blow from the northeast, calm conditions prevail. A strong southwest wind, the "vendaval," occurs occasionally, principally during the rainy season. Seasonally, air temperature varies insignificantly (Table 1). The annual mean is 28°C. Between 1976 and 1981, the mean annual precipitation was 347 mm; the range was 169 to 676 mm. The total number of sunshine hours per year has been estimated at 2,900 (Salzwedel and Muller, 1983). Total precipitation in 1993 was 318 mm at Punta de Betín, Santa Marta, of which more than 50% fell in May (Fig. 2).
|Table 1. Summary of meteorological and hydrological conditions in Santa Marta, Colombia.|
|Dry Season (Dec.-Apr.)||Rainy Season (May-Nov.)|
|Air temperature (°C)||27.6 28.1||27.9||26.7 28.5||27.7|
|Wind direction NE (%)||72 93||85||62 87||70|
|Calm (%)||6 25||14||12 37||27|
|Mean wind velocity (m s-1 )||2.1 5.1||3.5||0.8 2.9||1.5|
|Max wind velocity (m s-1 )||12 22||16.7||6 18||9.9|
|Sunshine (hr d-1 )||7.1 9.7||8.8||5.8 8.8||7.6|
|Precipitation (mm)||9 174||64||121 523||283|
|Number of rainy days||1 6||3.0||16 40||27.5|
|Water temperature (°C)||24.8 26.4||25.5||27.4 28.6||28.0|
|Salinity ()||36.1 37.4||36.7||25.1 36.4||35.7|
|Oxygen content (ppm)||4.4 4.6||4.5||4.4 4.8||4.6|
|Oxygen saturation (%)||94 101||97||99 109||103|
|pH||8.15 8.26||8.20||8.18 8.33||8.25|
|Mean values and their respective ranges given for each season are calculated from monthly means except for precipitation, which result from sums; compiled from several sources and years (between 1966 and 1983) by Salzwedel and Muller (1983).|
Coastal oceanographic conditions also show some seasonal variation. During the dry season, upwelling occurs nearshore due to currents transporting waters from depths of 100-200 m to the surface (Bula-Meyer, 1977; Ramírez, 1983). As a result, typical water temperatures are considerably lower and salinity higher from December to April (Table 1; Fig. 3). Also, a slight increase in dissolved inorganic nutrients in surface waters during the dry season has been reported. Ramírez (1990) recorded nitrate concentrations four times greater for December-April in two bays in the Santa Marta area. At Bahía de Chengue, surface water temperature and salinity were measured weekly at three CARICOMP stations (Fig. 3) during 1993. Except for December, mean monthly temperature was highest at the inner station (mangrove lagoon), intermediate at the seagrass station, and lowest at the outer station (coral reef), ranging from 25.5°C in January to 28.8°C in May at the coral reef (annual mean 27.2°C). With respect to salinity, the highest values were found in the lagoon during all months except May (which was the rainiest month in 1993; Fig. 2) when salinity dropped to a minimum and was almost identical at the three stations (33.3-34.0). Solano (1987) recorded colder waters in Chengue during the dry season of 1985 when the mean monthly temperature reached a minimum of 24.1°C in March, and also lower salinity (31.4 in November).
|Fig. 3. Mean monthly surface salinity
and temperature, and Secchi disk visibility at CARICOMP
(mangrove wetland, seagrass bed, and coral reef) in Bahía de Chengue during 1993.
Water transparency was measured weekly at Chengue as part of CARICOMP sampling during 1993. A Secchi disk (30 cm in diameter, 50% black and 50% white on the upper surface) was used, vertically at the coral reef and horizontally above the seagrass bed. Disk visibility distance (at which the disk no longer can be seen) varied irregularly, indicating low values in both seasons (Fig. 3). Monthly means ranged from less than 8 m in January and May to more than 15 m in March and October at the coral reef, and from less than 4 m in January and May to more than 7 m in September at the seagrass site.
Turbidity, which is notably higher in the southeastern part of the bay, increases because of local drainage from the southwest during the rainy season, resuspension of sediments due to wind-induced wave action during the dry season, and daily tidal currents from the southern lagoon. Sedimentation rates have been estimated from six traps (PVC tubes 25 cm long, 5 cm in diameter) placed for two-month periods at depths of 9-11 m, 0.5 m above the coral surface at the reef site. Mean rates, obtained in 1994/1995 from dried sediments smaller than 1 mm, varied between 0.53 and 1.55 mg cm-2 d-1. The highest values were obtained in January and February of each year when trade winds were strong. Solano (1987) reported much higher sedimentation rates at the same site during August-October 1985: 5.09-33.3 (mean 14.4) mg cm-2 d-1. These large differences could be due to different sampling methodologies, as Solano (1987) used small vials (9.2 cm long, 2.2 cm in diameter) as traps fixed on sandy substrates for 7-14 days.
The local tide is mixed, mainly diurnal with a form number of 1.7 and a mean range of 17 cm (Kjerfve, 1981). The entire variation between high and low tidal levels during an entire year was only 25 cm (Brattstrom, 1980).
Garzón-Ferreira and Cano (1991) mapped the shoreline red mangrove (Rhizophora mangle) fringes of Bahía de Chengue (Fig. 1). They found three additional mangrove species in nearby less swampy terrain: Avicennia germinans, Conocarpus erectus, and Laguncularia racemosa. Of these, A. germinans is the only tree that forms monospecific stands of considerable size landward of the Rhizophora fringes. The red mangroves are twisted and form a dense, intricate but narrow forest. CARICOMP plots for monitoring mangrove structure and function were established along the southeastern shore of the southern lagoon, where Rhizophora stands appear to be most luxuriant. Data obtained from these plots in 1995, according to CARICOMP protocols, show Rhizophora trees reaching heights of 13 m (mean 7.2), with trunk diameters of 30 cm (mean 11.5), basal areas of 47.4 m2 ha-1 (mean 42.8), and densities of 3,800 trees ha-1 (mean 3,467). Interstitial water salinity at the plots fluctuated between 36.4 and 49.2 (mean 42.8). Reyes (1991) described the root epifaunal community of the red mangrove (Rhizophora mangle) in Chengue, reporting 119 species from the southern lagoon and 140 species from the seaside fringe of the southern shore of the bay.
Five species of seagrasses occur in Bahía de Chengue (Garzón-Ferreira and Cano, 1991). Thalassia testudinum beds are most extensive but are restricted to the shallow (less than 3 m deep) and quiet waters of the southern portion of the bay (Fig. 1). Within these beds there exist monospecific patches of Syringodium filiforme and, less frequently, Halodule wrightii and Halophila baillonis, although the latter species may also appear mixed with Thalassia. Plants of all four species grow sparsely to a depth of 5 m in sediment patches of the adjacent coral fringes. In deeper waters of the eastern bay, there are also wide stands of Halophila decipiens, which grow more densely below a depth of 15 m.
Thalassia beds grow over calcareous sediments, mainly coarse sands. Coral rubble and small colonies of live sponges and corals of the genera Manicina, Siderastrea, Millepora, Diploria, Porites, and Cladocora are common in these beds, as are the sea urchins Tripneustes ventricosus and Lytechinus variegatus. The calcareous algae Halimeda opuntia is abundant and, in many places, is the dominant live component covering the bottom of Thalassia beds (Garzón-Ferreira and Cano, 1991). The shrimp community associated with the seagrass beds was studied by Puentes (1990), who found 26 species and registered maximum values of leaf Thalassia density and biomass in March and minimum values in November (1140-3140 leaves/m2, 48.8-221.2 g dry weight/m2). CARICOMP biomass and productivity measurements of Thalassia were done in March 1994. Estimates of total biomass (including below ground material) fluctuated between 631 and 1831 (mean 1101) g dry weight/m2, green leaves representing less than 10% in average. Areal productivity was estimated at 1.71-5.36 (mean 3.64) g m-2 d-1 dry weight, with a turnover rate of 2.69-7.95% (mean 4.01) per day.
Werding and Sánchez (1989) reviewed the information on corals in the Santa Marta area, indicating that reef development is impaired by continental run-off, lack of adequate substrate for reef settlement, and low water temperatures due to the local upwelling, although coral communities are diversely structured and rich in species. These investigators described eight types of coral communities in the bays of Parque Natural Tayrona, and discussed specific species composition and form of colonies. The most outstanding factor in determinating variation in community structure is degree of exposure to the predominant wave impact along the shore of the bay. Coral communities from Chengue have been described by Werding and Erhardt (1976), Solano (1987), and Garzón-Ferreira and Cano (1991).
Thirty-one species of hermatypic corals plus three hydrocorals of the genus Millepora are known in Bahía de Chengue. Although corals are present along the coastal rocky belt of the bay, only in areas protected from direct northeasterly ocean wave action have they grown sufficiently to modify the bottom morphology as true coral reefs (Fig. 1). Two main types of coral reefs are identified in Chengue: narrow, short coastal fringes growing over the belt of metamorphic rocks in the northern deep half of the bay, and extensive fringes growing away from shore on sedimentary flats of the bay. Although some Acropora palmata can be found in shallow water, the coastal fringes are dominated by massive and encrusting corals (mainly Diploria, Montastraea, and Colpophyllia), which form a reef slope extending to a depth of 15-25 m, <70 m from shore. In contrast, the inner fringes of the bay have extensive shallow reef flats, dominated by foliaceous and branching corals, which become reef slopes covered with massive corals at depths of 6-8 m. The southeastern inner fringe at depths between 3 and 6 m is dominated by large and dense Agaricia tenuifolia stands, with wide patches of Madracis mirabilis on the upper reef slope (6-10 m). The western inner fringe is more exposed to wave action and has a shallow zone dominated by Acropora palmata (1-4 m), with abundant Millepora squarrosa and Palythoa caribbea on the upper reef crest. As in many other areas of the Caribbean, shallow water coral formations in Chengue Bay have suffered considerable mortality among branching (A. palmata, A. cervicornis, Porites porites) and foliose corals (A. tenuifolia) over the last decade (Garzón-Ferreira and Cano, 1991).
CARICOMP coral reef stations in Chengue are located at depths between 9 and 12 m at two coastal sites separated by 400 m (Fig. 1). Due to the short horizontal extent of the coral communities, the CARICOMP random method for location of transects could not be applied in Chengue. Transects were placed arbitrarily following depth contours, trying to maintain the same community type. The coral community at the northern site (C2) is exposed to swells, which can reach considerable heights on days of strong trade winds; as a result, branching or foliaceous corals are almost absent. The dominant forms are small to medium-sized coral heads, and live coral cover is high (mean 42% at five CARICOMP transects in 1993). Site C1 is rarely disturbed by waves and Acropora palmata is abundant in shallow water. The coral community at depths of 9-12 m is dominated by medium-sized coral heads, but it also includes some delicate species e.g., Leptoseris cucullata and Agaricia tenuifolia; live coral cover is lower (mean 26% at five CARICOMP transects in 1993). At present, algae are the most important component covering the reef surface at both sites.
The Instituto de Investigaciones Marinas y Costeras (INVEMAR) and the Ministerio del Medio Ambiente have supported CARICOMP activities in Colombia. Students Alberto Rodríguez, Guillermo Díaz, Jaime Nivia, Néstor Ardila, and Mariana Escovar have been of great help in ensuring the success of the project.
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