|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
La Parguera, Puerto Rico, USA
Jorge R. García, Christoph Schmitt, Craig Heberer, and Amos Winter
Department of Marine Sciences, University of Puerto Rico, Isla Magüeyes Laboratories, La Parguera, PO Box 908, Lajas PR 00667 USA
The insular shelf of La Parguera, on the southwest coast of Puerto Rico, is characterized by an extensive development of coral reefs, seagrass beds, and mangrove forests. The dry, warm, and relatively stable climate, low wave energy, high water transparency, relatively wide shelf, oligotrophic offshore waters, and low urban coastal development are some of the factors that contribute to the conditions of the marine ecosystem of La Parguera. Interactions among coral reef, seagrass, and mangrove communities provide for a highly productive, structurally complex, and biologically diverse ecosystem. Coastal development and associated anthropogenic impact, technologically advanced exploitation of fisheries, global climatic change, and natural events all have potentially detrimental effects on marine ecosystems and need to be analyzed from a regional perspective. We review and summarize information leading to a baseline characterization of the ecosystem of La Parguera.
La Parguera is a coastal village within the township of Lajas on the southwestern coast of Puerto Rico. Its insular shelf boundaries extend from Punta Montalva in the east (66°59'W) to Punta Tocón in the west (67°06'W) and from the coastline (18°01'N) to the shelf edge (18°07'N) (Fig. 1). The southwestern coast is a generally dry and warm region, classified as a subtropical dry forest life zone (Ewel and Whitmore, 1973). A chain of low hills, known as Sierra Bermeja, separates the coastal plain from the Lajas Valley. Sierra Bermeja acts as an important hydrographic boundary that confines the watershed of La Parguera to the southern slopes of the Sierra and to the relatively narrow coastal plain. The shelf is composed mainly of carbonates deposited during the Cretaceous (Almy, 1965) and flooded some 5,000 to 9,000 years ago due to eustatic sea level rise (Goenaga, 1988), thereby forming the neritic zone of La Parguera.
La Parguera is recognized for the exceptional value of its marine resources, which include two bioluminiscent bays (Bahía Fosforescente and Monsio José), a coastal mangrove fringe with several small lagoons, mangrove islands associated with coral reefs, seagrass beds, and perhaps the best developed, most extensive coral reef ecosystem of the island. Such attributes, and the significant improvement in transportation and infrastructure across the island, have transformed La Parguera from a mostly undeveloped and quiet fishing village to a center of tourism. Resorts, guest houses, and private vacation homes have proliferated over the past ten years, and the transient population has increased at least three-fold from approximately 35,000 visitors per year (NOAA/DNR, 1984) to more than 100,000. In order to halt chaotic deforestation of the natural semi-arid forest and mangrove coastline, the Puerto Rico Planning Board classified La Parguera as a Zone of Special Planning. In further recognition of the ecological value of its marine resources, La Parguera has also been designed as a Natural Reserve by the Department of Natural Resources. At present, there is a proposal for the establishment of a Marine Fishery Reserve at Turrumote Reef (Plan Development Team, 1990; García, 1990); a previous effort to establish a Marine Sanctuary Program (NOAA/DNR, 1984) was not accepted by the local community (Fiske, 1992). Field and laboratory research facilities of the Department of Marine Sciences, University of Puerto Rico Mayaguez Campus, are based on Magüeyes Island off La Parguera.
The coexistence and interdependence of coral reef, seagrass, and mangrove communities within the insular shelf of La Parguera result in a highly productive and structurally complex ecosystem with very high biodiversity. Coral reefs act as barriers to wave action and permit the establishment of seagrasses and fringing mangroves (Goenaga and Cintrón, 1979). In turn, seagrasses and mangroves contribute organic matter for coral nutrition and serve as important foraging and nursery habitats for coral reef fishes and other organisms. Each of these communities can be regarded as highly productive and taxonomically diverse. For example, mangrove lagoons function as nurseries for many juvenile coral reef fishes (Austin, 1971; Yáñez-Arancibia and Nugent, 1977; Gonzalez-Sansón, 1983), many of which are commercially important as adults (e.g., snappers, jacks, barracudas, and others). The lagoons are also the natural habitat of resident populations of, for example, snook, tarpon, ladyfish, mojarra, and sole that add to the structural complexity and diversity of the ichthyofauna in La Parguera. Likewise, seagrasses are particularly important foraging (transient) areas for coral reef fishes and endangered species such as manatees and green sea turtles (Gonzalez-Liboy, 1979) and, as well, provide a permanent niche for a highly diverse and abundant flora (Glynn, 1964; Matthews, 1967) and fauna (Gonzalez-Liboy, 1979; Vicente, 1992).
The diversity of benthic and pelagic populations associated with the different habitats found in La Parguera is also reflected in plankton community structure and trophodynamics. This occurs by introduction of meroplanktonic (larval) stages, which include almost the entire taxa of marine invertebrates and fishes. Onshore-offshore gradients of phytoplankton production associated with the high plankton production of mangrove lagoons (Burkholder and Burkholder, 1958; Margalef, 1957; Gonzalez-Lagoa, 1967; García and López, 1989; García, 1990) and offshore oceanic oligotrophic waters (Margalef, 1957; Gonzalez-Lagoa, 1967) add substantial richness to the taxonomic structure of neritic zooplankton. A pattern of lower diversity and higher abundance of holoplankton prevails inshore, whereas both the diversity and the abundance of meroplankton decline offshore and within mangrove channels and lagoons.
Coral reefs extend throughout a wide range of depths and distances from the coast in La Parguera and consequently are exposed to gradients of physical, chemical, and biologically interacting forces (e.g., wave energy, light penetration, temperature, salinity, nutrient availability, suspended sediments). These gradients affect the structure of the biological community within reefs (e.g., vertical coral zonation patterns) and between reefs (Morelock et al., 1977; Acevedo and Morelock, 1988). This variability in community structure within and between reefs promotes the biological diversity of coral reef-associated organisms. These changes in coral reef community structure introduce variable patterns of sedimentation adjacent to the reefs (Morelock et al., 1977), potentially influencing variability in benthic communities associated with different sediment types. The submerged shelf-edge reef of La Parguera is an important spawning site for coral reef fishes (Colin and Clavijo, 1988) and serves as a foraging area for pelagic (oceanic) predators. Such neritic-pelagic interaction contributes to ichthyofaunal biodiversity and local fisheries production.
The prevailing low wave energy, extensive insular shelf, and low rainfall and freshwater runoff along the southwestern coast of Puerto Rico are all important factors that contribute to the excellent growth potential of coral reefs, seagrass beds, and mangroves at La Parguera. The Lesser Antilles chain generally shields the area from both NNE (North Atlantic) and ESE swells; thus, wave action on the southern coast is normally produced by systems of relatively short fetch. Nevertheless, the northern Caribbean region, which includes the southern coast of Puerto Rico, is within the trajectory of tropical depressions, tropical storms, and hurricanes. These systems generate the highest waves in the region and affect coral reef formations and other coastal ecosystems. Where insular platforms are narrow, wave action from tropical storms is high and causes massive destruction.
The insular shelf of La Parguera extends 8-10 km offshore; a well developed coral reef formation exists at the border of the shelf (Morelock et al., 1977) and serves as a first barrier against wave action. Two other lines of barrier reefs provide further protection for the mangrove coastline and submerged seagrass beds of La Parguera. Nevertheless, storm-generated waves may play an important role in the distribution, structural complexity, and biodiversity of local coral reefs and associated communities (Yoshioka and Yoshioka, 1989).
The paucity of rainfall and the absence of large rivers along the SSW coast of Puerto Rico, combined with the oligotrophic ocean waters in the northern Caribbean contribute to the high level of water transparency on the insular shelf. This condition optimizes growth rates and maximizes the depth range and distribution of corals and seagrasses. Sediment runoff from rivers discharging upcurrent along the south coast of Puerto Rico and from intermittent streams in the watershed of La Parguera can be significant during and for a few days after heavy rainfall and affect water transparency. Locally induced inorganic turbidity is quickly diluted by flushing. Mesoscale processes such as the northwestern plume of the Orinoco River (Muller-Karger et al., 1989) may affect water transparency locally at times, due to seasonal increments in organic (planktonic) productivity associated with fronts (Yoshioka et al., 1985).
A series of "coral bleaching" events has taken place recently at La Parguera (Goenaga et al., 1989; Williams and Bunkley-Williams, 1989; Bunkley-Williams et al., 1991). In total, more than 60 species of corals have been affected (Williams and Bunkley-Williams, 1989). Extremes in climatological conditions, particularily water temperature, have been suggested as possible precusors to these bleaching events.
Climate and Oceanography
Caribbean surface waters are part of the North Atlantic anticyclonic surface gyre that controls currents of the subtropical North Atlantic. Under the influence of trade winds, north and south equatorial water masses flow into the Caribbean and farther, through the Yucatan Channel into the Gulf of Mexico (Pickard and Emery, 1982). The current system of the Caribbean Sea undergoes seasonal fluctuations influenced by changes in the position of the inter-tropical convergence zone (ITCZ). During the winter, when the thermal equator is farthest south, the ITCZ is located between 0° and 5°S, and waters of the tropical Atlantic flow with increased strength westward into the Caribbean. In summer, the thermal equator shifts north, the ITCZ is located between 6°and 10°N, and surface waters in the Caribbean are influenced by increasing precipitation. This is also the time of year when the north equatorial counter-current (NECC) is established and surface waters of the equatorial Atlantic are displaced to the east (Busalacci and Picaut, 1983; Richardson and McKee, 1984).
The zonal shift of the ITCZ is responsible for the change from wet to dry seasons in the Caribbean. In the dry season (February to May), the ITCZ is near the equator; in the rainy season (August to October), the ITCZ is at its most northerly position in the Caribbean (Etter et al., 1987). The seasonal change is mirrored in the change of surface water salinity; however, precipitation affects salinity only indirectly. The main contribution to buoyancy in the Caribbean is the discharge from three rivers: the Amazon (average outflow 17.3 x 104 m3 s-1;), the Orinoco (3.9 x 104 m3 s-1; Meade et al., 1983), and the Magdalena (0.8 x 104 m3 s-1; Milliman and Meade, 1983). These discharges increase silica concentrations (3-5 µm), decrease salinity (Froelich et al., 1978; Morrison and Nowlin, 1982) as well as chlorophyll pigments (Muller-Karger et al., 1989), and increase loading of terrestrial materials onto the Caribbean floor (Milliman and Meade, 1983). Seasonal changes in Caribbean surface salinity directly influence the water mass transport of the Gulf Stream. Periodic fluctuations in water transport are aso linked to wind speed changes in the tropical-subtropical trade wind zone.
La Parguera is located on the southwestern coast of Puerto Rico in the subtropical climate belt influenced by easterly trade winds during 90% of the year. However, by the time the moisture-laden trade winds have crossed the island and reached La Parguera, most of the moisture has been lost. Therefore, La Parguera is one of the driest and hottest areas along the coast of Puerto Rico; the average annual rainfall 1961-1990 was 74.52 cm (Table 1), compared to 132.74 cm at San Juan. The "rainy season" occurs during the fall (average 35.61 cm), the "dry season" occurs in winter (average 9.12 cm). The highest one-day rainfall 1961-1990 was 35.31 cm on September 17, 1975 (Table 1). Most of the high rainfall amounts are caused by tropical storms that stall in the northeastern Caribbean. Occasional cold fronts in winter, which may sometimes be associated with large amounts of rain in Puerto Rico, seem not to affect the southwestern corner of the island. Total precipitation amounts vary from year to year (Fig. 2). The lowest annual rainfall 1960-1991 was 40.94 cm in 1977, the highest was 123.57 cm in 1960.
|Table 1. Historical
monthly mean rainfall record from the Isla Magüeyes
climatological station in La Parguera
|Fig. 2. Histroical trends in total
annual rainfall measured at the
climatological station, Magüeyes Island, La Parguera.
The average mean air temperature at La Parguera 1960-1991 was 27.0°C (Table 2). This value is similar to all coastal locations in Puerto Rico. However, the average annual maximum of 31.8°C is nearly 1.4°C higher than at San Juan, while the average annual minimum of 22.5°C is 1.3°C lower than at San Juan. The average annual maximum, minimum, and mean temperatures do not change appreciably throughout the year winter months are only 2.2°C cooler than summer months. The highest temperature recorded 1960-1991 was 41.1°C in October 25, 1991, and the lowest, occurring on a number of occasions, was 15.7°C. The high of 41.1°C is unusual, as the maximum average for October is 32.3°C.
|Table 2. Historical trends in monthly maximum, minimum, and mean air temperature measurements at the Isla Magüeyes climatological station, La Parguera.|
|Averages (°C)||Daily Extremes (°C)|
There exists no long-term salinity record for La Parguera. During the two-year period 1971-1972, Froelich et al., (1978) recorded a 1.0 change in annual salinity at a station located 34 km south of Puerto Rico. Salinities were depressed in late October through early November, coinciding with advected Amazon River water (Muller-Karger et al., 1989) rather than local rainfall and runoff. Glynn (1973) measured near-daily surface salinities between 1960 and 1966 at a nearshore site and found that the inter-annual range of monthly means varied from 1.5 to 2.0, with a depression in salinity occurring from October to January. Yoshioka et al. (1985) measured monthly salinity values at a number of stations over the southern shelf of Puerto Rico from January through November of 1980. Salinity fluctuations at the water surface were no greater than 0.5 and were also depressed in October and November. Since corals live in an exposed area on the shelf, the salinity measurements taken by Yoshioka et al. (1985) are the most representative. In contrast, surface salinities at the CARICOMP reef monitoring site in La Parguera (Turrumote Reef) averaged 35.2 (Table 3).
|Table 3. Biweekly hydrological measurements at
Turrumote Reef, La Parguera,
|Surface Temp (°C)||Surface Salinity ()||Secchi (m)|
Sea surface temperature (SST) and rainfall measurements have been collected since 1957 by the Department of Marine Sciences of the University of Puerto Rico at Mayaguez; NOAA facilitated the operation in 1976. The data set is reliable only after 1964 when time of measurement was standardized to within several hours. However, to reduce the effect of daily variations, only readings collected between 0700 and 0900 hours are used, representing 85% of all readings after 1964. Figure 3 shows the timing of the high temperature peaks and the duration of warm events throughout the year. Monthly means of measured temperatures between 1957 and 1992 illustrate intra-annual and inter-annual variations. The data show no warming trend for the duration of the record, in agreement with other data from the Caribbean (Atwood et al., 1992). During the 35 years illustrated, August 1958, September 1987, and September 1990 had mean monthly temperatures above 30°C. Since 1987, every year has had at least three months with mean temperatures above 29°C. Surface water temperatures at Turrumote Reef (fore-reef) averaged 28.4°C and ranged from 26.0°C to 29.5°C (Table 3).
|Fig. 3. Historical trends in monthly
mean water temperatures measured at
the climatological station, Magüeyes Island, La Parguera.
According to Almy (1969), coral reefs in La Parguera originated from erosion and deformation of Upper Cretaceous limestones (with interbedded mudstones and volcanic rocks) into a WNW-ESE trending syncline. The northern limb of the syncline is the Sierra Bermeja, and the southern limb is a platform of lower relief represented by the coral reefs on the shelf. The rise in sea level associated with the last Pleistocene glaciation (Wisconsin) flooded the lower limestone ridges on the shelf, providing appropriate sites for coral growth and subsequent reef development (Glynn, 1973; Goenaga and Cintrón, 1979). Cross-shelf seismic profiles (Morelock et al., 1977) support the theory of Kaye (1959), which states that the reefs developed on drowned calcarenite cuestas formed as eolianite structures parallel to the coastline during the Wisconsin glaciation. Substrate, depth, and water transparency conditions in La Parguera allowed for extensive development of coral reefs during the mid-Holocene (Vicente, 1993).
Two distinct lines of emergent reefs align east-west, parallel to the coastline, and divide the insular shelf of La Parguera into inner, middle, and outer shelf zones (Morelock et al., 1977). There are many other smaller submerged patch reefs dispersed throughout the shelf, as well as a large submerged reef at the shelf edge. Altogether, it has been estimated that coral reefs occupy about 20% of the La Parguera insular shelf (Morelock et al., 1977). Margarita Reef, the westernmost in the second line of emergent reefs, is the largest of the "island reefs," with a maximum underwater extension of 4.2 km. The shelf-edge reef is located at 20 m and has a "buttressed" appearance, with channels cut into the slope down to 30 m (Morelock et al., 1977).
Almy and Carrion-Torres (1963) reported 35 species of scleractinian corals, and at least 30 other species of shallow-water octocorals have been reported from La Parguera (Yoshioka and Yoshioka, 1989). A generalized pattern of coral zonation at the fore-reef of emergent coral reefs in La Parguera was described by Acevedo and Morelock (1988). Zonation is similar to other Caribbean coral reefs, except that the Lithothamnium ridge found elsewhere is replaced by Millepora in La Parguera.
Turrumote Reef is located 3.5 km south off Punta Papayo, Lajas (17° 56.2'N; 67°1.2'W; Fig. 1). Adjacent emergent reefs are Corral (~0.8 km to the north) and Media Luna (~2.0 km to the west). Turrumote Reef runs east-west with a longitudinal extension of 1.8 km; its emergent extension is 0.5 km along the E-W axis, and its total surface area is ~1.3 km2. This reef is situated on an isolated platform with its base at 20 m. The base substrate is covered with sandy silt sediments. Several other submerged reef platforms, rising to ~5 m from the surface, are located close to Turrumote to the southeast, including "Pinnacles." A small patch reef lies to the northeast, and a hard ground, low relief platform known as "Turrumote Ridge" is found to the southwest. The emergent section of Turrumote Reef is shaped like a horseshoe, forming a shallow reef lagoon at its center; this section is partially vegetated with red mangroves. Turrumote is farther offshore than any other reef in La Parguera.
The fore-reef is characterized by a long reef crest, an abrupt slope that includes several terraces, and a deep reef zone of irregular topography featuring massive coral formations at the base. At its easternmost section, the fore-reef extends 0.8 km to the south as a low-relief platform dominated by soft coral. Elsewhere, the fore-reef has horizontal extensions of less than 100 m from the surf zone to the reef base. The back-reef of Turrumote lacks the typical sandy-silt substrate leading to turtlegrass beds (Thalassia testudinum) that is found on many other reefs in La Parguera. Instead, the back-reef presents substantial hermatypic, soft coral, and hydrocoral (Millepora) development on a mostly sandy substrate.
The reef crest (depth 0-5 m) of Turrumote is a zone of energetic wave action dominated by encrusting biota: firecoral, Millepora spp., and branching coral, Acropora palmata. A dense algal turf composed of an assemblage of red and filamentous green algae covers most of the non-living hard substrate. The encrusting sponge Anthosigmella varians is found in multiple, small to intermediate patches (max 1.5 m diameter). Zoanthids, mostly Palythoa caribbea, and encrusting soft coral (Erythropodium sp.) are also common at the reef crest. Topographic relief ranges from 2 to 3 m and is influenced by the presence of large colonies of A. palmata .
The reef slope (5-13 m) is an area of intermediate wave and surge action and good light penetration. Using CARICOMP methodology, five chain transects, 10 m long, were surveyed at the fore-reef slope of Turrumote Reef. The data on percent linear cover by sessile-benthic taxa for 1994 are summarized in Table 4. Massive corals, mainly Montastraea annularis dominate linear cover with a mean of 41.2%. Branching coral (mostly Porites porites) represents an additional 6.6%, for a total of 47.8% of live coral cover at a depth of 10 m. Bell-shaped colonies of M. annularis are generally of large size, ending with extensive overhangs on the sides. Some of these colonies measure more than 4 m in diameter and are contiguous with other colonies, creating sections of live coral that exceed 10 m in length. In addition to M. annularis, other massive coral species that are present in excellent condition at the reef slope of Turrumote include Montastraea cavernosa, Dendrogyra cylindricus, Siderastrea spp., and Diploria spp. Algal turf follow live corals in linear cover, averaging 27.2% along the surveyed transects (range 13.9-38.3%). A highly diverse and abundant population of gorgonians represents another important component of the benthos at the reef slope.
|Table 4. % linear cover by sessile-benthic biota at the
fore-reef slope of Turrumote Reef at a depth of 10 m,
La Parguera, 1994.
The deep fore-reef (13-20 m) is a zone of low wave and surge energy. Reef rugosity is irregular, influenced by the growth of massive scleractinian corals, particularly Montastraea annularis. As on the fore-reef slope, Montastraea colonies grow with extensive lateral projections, creating ledges and overhangs. Many massive coral colonies are overgrown by an algal turf, an encrusting soft coral (Erythropodium sp.), or a combination of both. The most common massive coral species at the deep fore-reef are M. annularis , M. cavernosa, Diploria spp., Siderastrea spp., and Dendrogyra cylindricus. A profuse development of soft coral colonies is also found at the deep fore-reef of Turrumote.
The southwestern coast of Puerto Rico contains 996 ha of mangroves, representing 15.3% of the total mangrove area in Puerto Rico (Martinez et al., 1979). The original area of mangroves in Puerto Rico was estimated at 30,000 ha; by 1975, only half of that area remained. Unfortunately, the destruction of mangrove areas continues despite political resolutions and laws (Lugo, 1988). Some of the emergent portions of the shelf reefs at La Parguera are colonized by mangroves. The degree of exposure to the incoming waves limits mangrove development on these offshore islands (Yoshioka, 1975). Red mangrove, Rhizophora mangle, is the dominant species on island reefs; a few white mangroves (Laguncularia racemosa) are also present. Mangrove development is greatest in zones of intermediate wave energy. On the exposed outer cays, the strong surf does not allow deposition of the fine sediments needed for the growth of red mangroves. On the middle shelf zone, waves and currents are strong enough to maintain a constant flow of water, yet allow for accumulation of fine sediments. Consequently, red mangroves prevail at these middle shelf reefs. The inner shelf reefs are not subject to enough wave energy to maintain adequate flushing; consequently, these reefs normally have strong transverse salinity gradients. Salt builds up in the center of these islands and enables the succession of red mangroves by the more salt-tolerant black mangrove (Avicenna). Prolonged accumulation of salt eventually leads to the death of the black mangroves.
The Pitahaya mangrove forest, the largest and best developed mangrove stand in the southwestern corner of Puerto Rico, reaches 1.2 km inland and stretches 6.5 km along the coast between Punta Guayacan and Punta Pitahaya (Fig. 1). The area is characterized by a well-developed fringe forest of red mangrove and a less developed basinal forest that includes red, white, and black mangrove species. The structural characteristics of the mangrove forests at La Parguera are summarized in Table 5. Differences in mean plot stand height and trunk diameter are observed within and between mangrove sites. The tallest stands and greatest mean trunk diameter correspond to red mangroves of fringe forest type. The basinal mangrove stand at Pitahaya has lesser stand heights and trunk diameters. Farther inland, vegetation gives way to barren salt flats (Martinez et al., 1979). Inshore islands and fringing (coastal) mangroves are separated by channels and embayments. Large areas between the mangroves are covered by turtle grass (Thalassia testudinum) and associated algae (e.g., Dictyota divarica, Laurencia obtusa, Caulerpa verticillata, and Acanthophora spicifera). The upside-down jellyfish (Cassiopeia frondosa), the starfish (Oreaster reticulatus), and the sea cucumber (Ludwigothuria mexicana) are some of the dominant faunal components in the embayments and channels (Cerame, 1973), along with a diverse ichthyofauna of juvenile marine fishes and resident estuarine populations (Austin, 1971; García, 1981). The red mangroves bordering the channels, bays, and islands protrude with drop and prop roots into the sediments. This root system provides a unique habitat for a variety of sessile organisms. Typical epifaunal species are sponges (Tedania ignis, Callyspongia, Ircinia, Chondrilla, Haliclona), hydroids, sea anemones (Aiptasia and Bartholomea), oysters (Crassostrea rhizophorae), tunicates (Clavelina spp., Ascidia nigra, Ecteinascidia turbinata), and polychaetes (Sabellastarte magnifica and Sabella melanostigma) (Matthews, 1967). The structure and composition of this epifaunal community seem to vary due to different hydrodynamic regimes (Yoshioka, 1975).
|Table 5. Structural characteristics of mangrove forests
in La Parguera; all values are from live stems
(after Cintrón et al., 1978).
|Stand Type*||Mean diameter at breast height (cm)||Number of species||
No. of Trees With Trunk Diameter
|Basal Area (m)||
Stand Height (m)
|>2.5 cm||>10 cm||>2.5 cm||>10 cm|
|Caballo Blanco Reef|
|*Stand Type: I = island; F = fringe; B = basin.|
Seagrass meadows and associated macroalgae are highly productive habitats that provide living space, foraging grounds, and refuge from predators for populations of invertebrates and fishes, many of which are commercially valuable species. The extensive seagrass beds that occur in southwestern Puerto Rico, in close proximity to some of the islands most pristine coral reef and mangrove habitats, provide nursery and feeding grounds. In addition to providing basic nutrients, primary productivity, and stable habitats, these beds provide essential foraging grounds for such endangered marine species as the West Indian manatee, Trichechus manatus, and the green sea turtle, Chelonia mydas.
Of the twelve recognized genera of seagrasses distributed throughout the temperate and tropical coastal zones of the world, four are found in the Caribbean and Puerto Rico. These four genera are represented by seven local species: turtle grass, Thalassia testudinum (Banks ex Konig); the sea vines Halophila decipiens (Ostenfeld), H. baillonis (Ascherson), and H. engelmannii (Ascherson); manatee grass, Syringodium filiforme (Kutzing); the shoal grass Halodule wrightii (Ascherson); and the widgeon grass Ruppia maritima (Linnaeus). Of these species, Thalassia testudinum, Syringodium filiforme, Halophila decipiens, and Halodule wrightii inhabit the insular shelf zones on both the Atlantic and Caribbean coasts of Puerto Rico as well as the nearby islands of Vieques and Culebra. The sea vines Halophila baillonis and H. engelmannii have been found only in restricted localities on the northern and southeastern coasts of Puerto Rico. Halophila decipiens, although patchy in distribution, can be quite abundant when encountered and appears to tolerate low salinities (<20) and turbidity (Secci <1 m), enabling this vine to invade polluted environments (Vicente et al., 1980). Widgeon grass (R. maritima) has been found only in enclosed lagoons with little or no oceanic influence (Detres, 1988).
Large areas of Thalassia on the reef flat are periodically exposed to air, resulting in increased temperature and desiccation. Glynn (1968) observed that Thalassia leaves do not survive when temperatures reach their upper tolerance levels of 35-40°C but that rhizomes of the plants are apparently unaffected, due primarily to the thermal buffering effect of fine-grained sediments covering the underground portion of the plants. Tidal fluctuations accompanying strong spring tides can cause extreme temperature shifts that, coupled with desiccation, kill vast quantities of leaves, which then are shed by the plants. The process occurs sporadically throughout the year and seems to be an integral part of the ecology of the seagrasses, with no apparent negative long-term effect on the population as a whole.
Large seagrass beds are established in the La Parguera area, with Thalassia and Syringodium being the most abundant and widely distributed seagrasses over the insular shelf and also in the back-reef zones of middle shelf reefs. The most extensive seagrass beds are found within the 2-m depth contour, fringing the red mangrove coastline. Mangrove forests border almost the entire southwestern coastline, and mangrove islets are common inside the inner shelf (Cintrón et al., 1978). The distribution of Thalassia near the offshore reefs and mangrove islets is generally restricted to the lee (north) side of these formations and commonly occurs in association with coral species such as fire coral (Millepora complanata), staghorn coral (Acropora cervicornis and A. prolifera), and finger coral (Porites spp). Seagrasses are absent from the exposed reef fronts. On the inundated central portion of the reef flats and in the shallow lagoon side of the reefs, Thalassia develops among the coral rubble and sand, providing a rich feeding ground for diurnal reef residents. Prominent grazers in the seagrass beds include the long-spined sea urchin (Diadema antillarum); the variegated sea urchin (Lytechinus variegatus); the West Indian sea egg (Tripneustes ventricosus), the Queen conch (Strombus gigas), the green sea turtle (Chelonia mydas), and a sizable assemblage of herbivorous fishes.
Reproduction in seagrasses occurs vegetatively (rhizome propagation) and sexually (incomplete dioecious flowers). Male and female Thalassia flowers begin to appear in March, reaching peak abundance in April and May, after which time flower production begins to wane. The season normally ends in June. The percentage of shoots with reproductive structures (buds, flowers, or fruits) varies from 4% to 54% during May. Inter-annual variation is dependent on factors such as wave energy, sediment load, turbidity, and water temperature, all of which affect the success of the reproductive season (Vicente et al., 1980).
There are considerable temporal and spatial fluctuations in total biomass of the seagrasses. Thalassia has low biomass (100 g m-2 dry weight) in areas where mangrove growth is prolific, thereby reducing the amount of available light, whereas biomass under high light intensity may reach 5,800 g m-2 dry weight (Vicente, 1992). For our purposes, "standing crop" refers to the above-sediment living component (including calcareous epiphytic algae), whereas "biomass" refers to the above- and below-sediment (roots, rhizomes, and short shoots) components.
For Thalassia growing in an industrial estuary that receives thermal effluent from a power plant, primary production values have been reported as 0.43 to 2.61 g m-2 dry weight per day (Vicente, 1992). Biomass values for the same bay were reported as 330 g m-2 dry weight. Primary production is likely to be higher in less polluted environments of Puerto Rico. Higher productivity values (up to 18.9 g m-2 dry weight per day ) have been measured for Thalassia in other parts of the Caribbean (Buesa, 1972): an average of 70 t yr-1 ha-1 of Thalassia. Gonzalez-Liboy (1979) summarized Thalassia biomass and standing crop values (g m-2 dry weight) for various sites scattered throughout Puerto Rico, with means for southwestern Puerto Rico in the range 229-586 g m-2 for standing crop estimates and 532-1,986 g m-2 for biomass estimates. Burkholder et al. (1959) reported 80-450 g m-2 dry weight as a range of means for biomass estimates taken at several sites in Puerto Rico. Delgado-Hyland (1978) reported standing crops for Thalassia in the La Parguera area as in the range 96-480 g m-2 dry weight.
In summary, the seagrass beds of southwestern Puerto Rico appear to be in good condition and serve as a key component, intimately and functionally associated with the coral reef and mangrove ecosystems, in providing important nursery and foraging grounds for many commercially important fish and invertebrate populations. In order to ensure the long-term health and sustainability of these nearshore marine ecosystems, future development plans will have to take into consideration the physical and biological requirements of the seagrass beds and offer protection whenever possible, as well as restoring the original mangrove fringe where it has been cut or destroyed.
Human Utilization and Impact on Marine Resources
Approximately 100 fishermen concentrate their efforts within the insular shelf of La Parguera, evenly split between commercial and recreational fishing. Fish traps (arrow type) and monofilament gill nets are the main methods used by the commercial fishermen. A recent survey determined that 1,050 traps are utilized in the commercial fishing effort. Recreational fishing involves hook and line, flycasting, jigging, trolling, and spearfishing. Both commercial and recreational fishing here have suffered a sharp decline in catch per unit effort (CPUE), consistent with the decline seen for all of Puerto Rico (Appeldoorn et al., 1992). The direct and indirect implications of changes in ichthyofaunal community structure upon sessile-benthic communities are not well established.
The mangrove areas of La Parguera have been, and continue to be affected by several anthropogenic factors. For example, mangrove trees were cut down during the construction of 200 summer houses along the shoreline. Strips of salt marsh were filled to provide access to these houses, altering normal tidal flow throughout the mangroves. Boat traffic in the channels and embayments perturbs the sediments covering mangrove root systems.
Another important factor influencing coastal ecosystems is the sewage treatment plant (STP) that is located between La Parguera and Isla Guayacan. This plant discharges an average of 228,000 liters of secondary sewage effluent per day into percolation ponds at the landward margin of the coastal mangrove fringe. The mangrove area subject to the intrusion of effluent waters covers at least 0.123 km2 (Corredor and Morell, 1994). Investigations of surface nitrate concentrations indicated that some nitrates do not percolate completely into the sediments and thus reach the seaward fringe in effluent streams. However, experiments revealed that microbial communities in mangrove sediments are capable of depurating 10 to 15 times the nitrate added by the STP effluent (Corredor and Morell, 1994). The mangroves themselves seem to respond favorably to nutrient additions, as can be seen around the STP where the heights of the trees are much greater than in undisturbed areas. Nervertheless, the mangrove root community appears to be less diverse near the STP.
Deforestation is a common practice in construction of homes, camping areas, and other projects within the watershed. Such development has increased drastically during the last ten years, leading to alterations in the natural pattern of rainfall drainage and runoff and to an increase in sediments reaching coastal waters.
La Parguera is probably the single most popular tourist destination on the southwestern coast of Puerto Rico. Boat traffic can be quite heavy, especially on weekends and during the summer. Many tourists utilize the nearshore mangrove islets and associated seagrass beds as a psuedo-beach area, anchoring their boats in the lee of the islets. Thus, these habitats at times are subject to heavy boat and foot-traffic, with ensuing trampling effects in addition to propellor and anchor scars. According to Zieman (1976), although Thalassia is highly productive and capable of vigorous regeneration, it does not recover rapidly when injuries extend down to the root-rhizome system. Tracks from propeller damage have been observed to persist in Thalassia beds for more than five years.
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