|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Laguna de Celestún, Yucatán, México
Jorge A. Herrera-Silveira and Javier Ramírez-Ramírez
Centro de Investigación y de Estudios
Avanzados, IPN, Unidad Mérida. Carr. Ant., a Progreso km 6,
Apdo. Postal 73 CORDEMEX,
CP 97310, Mérida, Yucatán, México
Laguna de Celestún, on the northwest coast of the Yucatán Peninsula, is separated from the sea by a sand bar. The lagoon system is markedly affected by infiltration of cold nutrient-rich groundwater discharges and is estuarine in nature. There is an inner zone characterized by low salinity, high nitrate, and soluble reactive silica concentrations, and a seaward zone characterized by high salinity and low concentration of nutrients. The middle zone is characterized by intermediate salinity and higher concentrations of soluble reactive phosphorus and ammonium than those found in the remainder of the lagoon. Phytoplankton productivity and chlorophyll-a values show a seasonal pattern without regard to variations in salinity. The macrophyte community is represented by Chara fibrosa and Batophora oerstedi in the inner zone, Halodule wrightii and Chaetomorpha linum in the middle zone, and Halodule wrightii and Thalassia testudinum in the seaward zone. The mangrove community is represented on the sand barrier side of the lagoon by a small fringe of Rhizophora mangle, followed by Avicennia germinans, which dominates the area. Behind the fringe is a forest of Laguncularia racemosa mixed with sand dune vegetation. On the continental side of the lagoon, mangroves show a similar gradient from the seaward to the inner zone but with better structural development. There are no coral reefs in this area.
Laguna de Celestún is a macrophyte-mangrove co-dominated system situated at 20°45'N and 90°15'W on the western shore of the Yucatán Peninsula, southern Gulf of México. Although most coastal systems are inherently variable in time and space, macrophytes and mangroves dominate the lagoon, with strong seasonal pulses and spatial heterogeneity due to size, morphology, communication with the sea, and freshwater input.
|Fig. 1. Location of Laguna de
hydrobiological, groundwater discharge (GD), and
The northern portion of the Yucatán Peninsula contains over 148,000 ha of coastal lagoons; specific features differentiate these systems hydrologically from other lagoons. The extremly high permeability of the rock and the relatively featureless morphology in this area permit unrestricted filtration during and after rainfall, resulting in no surface runoff (Butterlin, 1958; López Ramos, 1974). Rainfall feeds a lens of fresh water that drains laterally towards the sea, forming coastal freshwater springs (Robles Ramos, 1950) in areas below mean sea level. Therefore, Laguna de Celestún can receive freshwater input from either direct rainfall or groundwater discharge (GD) but not from river discharge.
Celestún Port, a fishing village of 4,500 inhabitants, is located on the Gulf Coast one kilometer from the lagoon. Both the coastal zone and lagoon are heavily fished; this area is the second most important fishery region in Yucatán.
Laguna de Celestún is a long (22.4 km), narrow (0.5-2.4 km), and shallow (0.05 m to 3 m, mean = 1.2 m) feature located parallel to the coastline on the western shore of the Yucatán Peninsula. Contact with the sea is via a 410-m-wide entrance at the southern edge. The bottom of the lagoon is almost flat. A tidal channel (100 m wide, 15 km long) is the major bathymetric feature (Fig. 1). The lagoon is protected by a sand barrier, and it is a low-energy site because of a low tidal range (0.2-0.6 m; Instituto de Geofísica, 1990). However, marine processes such as storm tides and frontal systems can have a strong influence on the hydrological regime (Herrera-Silveira, 1993).
In 1979, the Mexican Goverment declared the lagoon a Wildlife Reserve because of its biological diversity, providing natural habitats for a variety of endangered species of birds and reptiles. It is particularly important as a nesting ground for the flamingo Phoenicopterus ruber ruber, (Barrios, 1988) and the king duck Chairina moschata. The sight of more than 40,000 birds has been a focal point for the development of tourist activities, which have steadily increased during the 1990s. Government economic policies emphasize the coastal zone as a mechanism to drive development, but little environmental information is available for decision making. We describe this CARICOMP site in the southern Gulf of Mexico and review what is known about the ecosystem.
Climate and Oceanography
The Celestún region has a moderate annual variation in solar radiation, from 230 ly d-1 in December to 500 ly d-1 in August (Almanza and López, 1978). The climate in the region is hot semi-arid (Fig. 2): the annual mean temperature is 28.5°C, varying from 21°C in February to 35°C in August; the mean annual rainfall is 760 mm (data from 1960-1990, SARH, 1991). The two main seasons are the dry season with low rainfall (March-May, 0-50 mm), and the rainy season (June-October) with high rainfall (>500 mm), both with weak winds from the southeast (<15 km h-1). Futhermore, in this part of the Gulf of Mexico the period from November to February is known locally as the nortes season, which is characterized by strong north winds (>80 km h-1), little rainfall (20-60 mm), and low temperatures (<22°C) imposed by low pressure systems from the north (SARH, 1989). Additionally, Laguna de Celestún is affected by tropical storms and hurricanes from September through October.
|Fig. 2. Mean monthly variations in air
temperature and rainfall over
a period of thity years.
Annual mean rainfall = 760 mm
Annual mean temperature = 28.5°C
The hydrologic regime of the lagoon varies substantially during a year. Seasonal variations in environmental conditions are related to residence times and to fluxes of water and materials. In the dry season, high evaporation rates, low groundwater discharge (GD), and minor changes in mean sea level (MSL) limit fluxes toward the sea (1.1 x 106 m3 h-1) and moderate residence times (190 days); tide-GD co-dominate the flows. During the rainy season, GD-tide co-dominate the flows; the highest GD and greatest variations in MSL promote high ebb fluxes (2.5 x 106 m3 h-1) and lesser residence times (<150 days). During the nortes season, low GD and moderate variations in MSL reduce the water flux (0.6 x 106 m3 h-1). In addition, high sea level in the coastal zone due to frontal passages favors the longest residence time (>250 days). The water column of the inner zone is red in color because of tannins accumulated and not exported during the nortes season (Herrera-Silveira, 1993).
The hydrologic regime results in the transport of materials dominated by inorganic forms during the rainy season, by particulate material during the nortes season, and probably by organic compounds during the dry season. Seasonal variation in flux of each material results from changes in both water flux and seasonal concentration of nutrients due to the relationship between the GD and biogeochemical processes in the lagoon. The system traps NO3- and SRSi (soluble reactive silicate) in the innner zone during all three seasons and releases NH4+. SRP (soluble reactive phosphate) is trapped during both dry and rainy seasons but is released during the nortes season. Despite these seasonal and spatial changes in uptake and release of nutrients in the lagoon, the system functions primarily as an exporter of nutrients (Herrera-Silveira and Comín, 1995).
In terms of the functional classification proposed by Kjerve (1986), Laguna de Celestún exhibits a mix of characteristics of a choked and a restricted lagoon. There is a co-dominance of hydrologic cycle and tidal circulation, modified by wind forcing. As a choked lagoon, Celestún experiences seasonal water level changes, which exceed one meter, in response to the onset of the rainy season or related to wind forcing. Seiching and set-up/set-down cycles are particularly intense in response to frontal passages during the nortes season, and residence times are on the order of months. As a restricted lagoon, Celestún is located on a low wave energy coast with a small tidal range (0.3-0.7 m). Tidal water level and current variability are readily transmitted into the lagoon, almost without filtering. As a result, the lagoon has good tidal circulation, which is modified by wind forcing and freshwater discharge. Salinity fluctuates less dramatically (10-35) than in a choked lagoon (1-80); it is usually well mixed vertically. Fresh to brackish water is found near the freshwater discharge. During flood discharge, the entire restricted lagoon may turn fresh or brackish (Herrera-Silveira and Comín, 1995).
The water column does not exhibit stratification because of mixing processes due to low mean depth (1.2 m) and wind mixing. During a one-year sampling cycle, water temperature varied 9°C, with the highest values occurring in June (31.4°C) and the lowest in February (22.4°C). The maximum difference between air and water temperatures occurs during the rainy season, due to the input of groundwater with lower temperatures than found in the lagoon (Table 1).
|Table 1. Mean values of various parameters (±1 standard error in parentheses) in Laguna de Celestún, groundwater discharge (GD) and seawater (SW) during a one-year cycle (data from Herrera-Silveira, 1993).|
|Key: Temp, temperature; DO, dissolved oxygen; S, salinity; NO3-, nitrates; NO2-, nitrites, NH4+, ammonium; SRP, soluble reactive phosphate; SRSi, soluble reactive silicate.|
The horizontal salinity gradient in the lagoon was observed over one year. In the inner zone, salinity was always lower than 20, while in the seaward zone it was greater than 30 (Fig. 3). The lowest salinity (19.6) was observed in October after the rainfall peak. During the nortes season, mean salinity was <25 throughout the lagoon. The highest salinity was observed at the end of the dry season, with mean concentrations of 30 occurring in May. Mean pH values follow an inverse relationship to mean salinity: high values (8.3) in the inner zone during the rainy season and low values (6.8) during the dry season in the middle segment of the lagoon (Herrera-Silveira, 1993).
Annual dissolved oxygen concentrations ranged from 2.5 mg l-1 in the inner zone to >8.5 mg l-1 in the seaward zone during the sampled year; the highest values occurred during the nortes season. The highest NO3- concentrations were observed in the inner zone (>40 µM); NO3- values increased in all zones of the lagoon at the start of the rainy season. During the nortes season, the highest concentration (10 µM) was observed in the middle of the lagoon. Nitrate concentrations decreased from the inner zone of the lagoon seaward at all times during the annual cycle. Ammonium concentrations in the lagoon were higher than those in the inflowing groundwater and seawater (Table 1). The highest NH4+ concentrations were observed in July (15 µM) and February (11 µM) in the central part of the lagoon.
The spatial and temporal distributions of SRP differed from those of NO3- and NH4+. SRP concentrations were higher (>9 µM) in the inner and middle zones of the lagoon during the nortes season (November-February) than during the rest of the year. The maximum SRP (2 µM) was observed in the central part of the lagoon at the beginning of the rainy season. Thus, the highest concentrations of SRP evidently are not associated with groundwater discharge. SRSi concentrations ranged from <1 to 280 µM. They followed the same spatial and temporal patterns as NO3-, decreasing from the inner zone to the seaward zone and increasing between the dry and rainy seasons. During the nortes season, SRSi fluctuated between <10 µM and >200 µM.
Based on the results of multivariate analysis, three zones are identified in the lagoon according to variations in physical and chemical characteristics present during a year. The inner part of the lagoon is characterized by low salinity and high NO3- and SRSi. Clearly, it is strongly affected by groundwater discharge. As expected, the seaward zone of the lagoon is characterized by high salinity and low nutrient concentrations. The zone in the middle of the lagoon is characterized by intermediate values of salinity, as expected; however, concentrations of SRP and NH4+ are higher than in the rest of the lagoon as a consequence of non-conservative behavior of these compounds, probably due to biological processes (Herrera-Silveira, 1994). This zonation represents a typical pattern for coastal lagoons with one seawater inlet and freshwater discharge occurring at the landward end (Kjerfve, 1986; Guélorget and Perthuisot, 1992).
The data indicate differences between the three seasons (dry, rainy, nortes) and the three zones (inner, middle, seaward), revealing spatial and seasonal patterns. The differences between seasons and groups were due to the coupling of the intensity and frequency of external factors, such as rainfall, winds, frontal systems, and biogeochemical processes, including primary production, mineralization, conservative and non-conservative behavior of nutrients, fertilization, and bioturbation. If these patterns are found to repeat from year to year, a microsuccession process can be inferred (Herrera-Silveira, 1993).
During a one-year cycle, chlorophyll-a values ranged from <1 to 28.5 mg m-3 (Fig. 4). Chlorophyll-a levels were lowest (3.1 mg m-3) during the dry season and the end of the nortes season (0.88 mg m-3), when GD was low, temperatures dropped, and the water column became less transparent. The highest chlorophyll-a concentrations were observed in the middle zone of the lagoon during the rainy season (14-28.5 mg m-3). In the inner zone, a lower peak of chlorophyll-a (11.5 mg m-3) was observed early in the nortes season (Fig. 4). There is no relationship with salinity variations. The annual range of net phytoplankton productivity in the lagoon is 0.22-1.9 g C m-2 d-1, within the range observed in other Gulf lagoons (0.1-3.3 g C m-2 d-1; Vannucci, 1969; Day et al., 1982) that receive input from rivers. Also, these other lagoons are quite different from Celestún with respect to GD, particulate matter, and SRP input. The levels of SRP in the GD of Laguna de Celestún and seawater were low (<1 µM), but high levels (2-9 µM) were observed in the middle zone. In addition, the shallowness of Celestún (1.2 m) suggests intense coupling between sediments and water column; thus, remineralized nutrients are probably available to the water column through this pathway. The average phytoplankton productivity and chlorophyll-a levels showed the same seasonal pattern that is characterized by two peaks, one in July and one in December; the former is related to the May-September rainy season and the latter to the November-February nortes season. The first peak is the maximum for both for chlorophyll-a (18.82 mg m-3) and productivity (>900 mg C m-3 d-1), reached after a period of increasing chlorophyll-a and productivity associated with the beginning of the rainy season. The second peak is lower, and the period of relatively higher chlorophyll-a and productivity is shorter than during first peak.
No information is available with regard to species composition of the phytoplankton community. Nevertheless, spatial heterogeneity will be less as compared to other lagoons because the spatial salinity range is relatively narrow (12-18) and, as freshwater enters the lagoon via groundwater discharge, there is no contribution to the community from continental aquatic ecosystems.
The spatial distribution of macrophytes in Laguna de Celestún is heterogeneous. Chara fibrosa forms very dense stands in the inner zone of the lagoon, and mixed stands of Ch. fibrosa and Batophora oesterdi, are found in the central part of the inner zone. The shoal grass Halodule wrightii and the green algae Chaetomorpha linum form dense mixed stands in the middle zone. The seaward zone is occupied mostly by H. wrightii in the lagoon and by Thalassia testudinum outside the lagoon. Other algae (Caulerpa cupresoides, Hypnea musciformes, Dyctiota sp., Cladophoropsis membranoce, Cladophora sp., among others) are found near the mouth of the lagoon, but their coverage and seasonal changes comprise less than 5% of the entire biomass of macrophytes.
During a one-year cycle (Fig. 5), the total biomass of macrophytes ranged from 50 to >1,000 g m-2 dry weight. The two distinct biomass peaks observed during the year were due to different species: the peak in August was due mainly to H. wrightii (721 g m-2); the second, in January, was due to Ch. fibrosa (320 g m-2). Batophora oesterdi showed the same general temporal pattern as Ch. fibrosa but with less biomass (90-145 g m-2). The mean total biomass of Halodule wrightii increased during the rainy season to a peak of about 700 g m-2 dry weight in August. A second, shorter, growing period was observed during the nortes season, with a peak of 308 g m-2. The below-to-above ground ratio shifted from 0.3 in August to 1.6 in December, a growth strategy that produces seeds during good conditions and takes up new zones and/or supports unfavorable water column conditions (low transparency, low temperatures). The mean biomass of Chaetomorpha linum ranged from 20 to 212 g m-2 and showed two distinct high biomass periods: (1) from the end of the dry season to the beginning of the rainy season (95-280 g m-2), before the H. wrightii growth period; and (2) during the nortes season (62-135 g m-2), before the second annual period of growth of H. wrightii. There is no information available on Thalassia testudinum.
|Fig. 5. Mean seasonal changes in the
biomass (g m-2 dry weight) of
submerged macrophytes in Laguna de Celestún during a one-year cycle.
All primary producers (phytoplankton and macrophytes) in Laguna de Celestún have the same seasonal pattern of productivity: two periods of relatively high productivity during the rainy and nortes seasons (March-August and November-February), separated by periods of low productivity during the dry season (March-April) and during the last months of the rainy season (September-October). This pattern is similar to that observed in other lagoons, whether tropical (Day et al., 1982; Flores-Verdugo et al., 1988) or temperate (Morgan and Kitting, 1984). In all cases, the covariance of enviromental factors (temperature, nutrient input) contributes to the two peaks observed. However, differences between the productivities of the different groups of plants studied in Laguna de Celestún, both temporally and spatially, provide clues to explain this pattern. The primary productivity of the inner zone of the lagoon and of the rest of the lagoon correspond to different groups of plants. C. fibrosa and B. oesterdi are restricted to the inner zone. Ch. linum and H. wrightii are mostly restricted to the middle and seaward zones of the lagoon, with T. testudinum and other macroalgae in the adjacent coastal zone. The spatial heterogeneity in distribution and abundance of primary producers is common in coastal lagoons and is mostly related to salinity gradients (Flores-Verdugo et al., 1988; Sand-Jensen and Borum, 1991). The salinity gradient along the longest axis of the lagoon is maintained year round, and probably is the major factor controlling spatial distribution of macrophytes in the lagoon.
The observed space-time changes in biomass of phytoplankton and macrophytes suggest a shift in competition capacity driven by N:P ratio, salinity gradient, and temperature (Wium-Andersen et al., 1982; Sand-Jensen and Borum, 1991). Another probable factor is the accumulation of toxic compounds, such as tannins from decomposition of mangrove leaves (the reddish color of the water in the inner zone during this period; Herrera-Silveira, 1993). These compounds have demonstrated negative effects on microbiota (Lee et al., 1990).
Laguna de Celestún shows a pattern of primary production that is quite different from other lagoons along the Pacific coast and the Gulf of México (Day et al., 1982; Villalobos-Figueroa et al., 1984; Flores-Verdugo et al., 1988), In other regions, phytoplankton grows during the rainy season and, later, macrophytes increase in biomass. In Laguna de Celestún, the primary producers follow a distinctive sequential order of growth: the Ch. linum population develops during the late dry season; after this, phytoplankton biomass increases, followed by H. wrightii, Ch. fibrosa, and B. oesterdi during the rainy season. A similar order of biomass peaks is observed during the nortes season (Fig. 5).
As in other coastal lagoons, Celestún shows an apparent spatial and seasonal primary production succession between phytoplankton and macrophytes (Flores-Verdugo et al., 1988). The changes in primary production and spatial heterogeneity, supported by climatological patterns in the northern Yucatán Peninsula, affect the biomass changes of the different producers and its relative importance in space and time in the lagoon. The patterns suggest different optimum conditions for phytoplankton and macrophytes. During the dry season, the remineralization process should support a high biomass of Chaetomorpha linum, but during the rainy season, the nutrient input from groundwater discharge and remineralization support phytoplankton blooming. The increase in water temperature and uptake of nutrients from sediments by H. wrightti support the high biomass during this period. During the nortes season, strong climatological oscillations attenuate remineralization and loss of organic matter from middle and seaward zones to the coastal area, limiting the growth of phytoplankton and seagrasses. However, the inner zone is less affected by these phenomena, where Ch. fibrosa reaches peak development.
This suggests that there are two scales of factors operating separately and/or together to cause the seasonal succession of primary producers in Laguna de Celestún: (1) interaction between species as competition or allelopathic behavior; (2) the relationship between life cycles of the primary producers and both external factors (temperature, groundwater discharge, and frontal systems) and internal processes (mineralization, bioturbation).
The mangrove forest shows differing structural development along a horizontal gradient and also differences between its barrier and continental sides related to soil characteristics (texture, organic mater, salinity), competition with other vegetation, and human development.
A narrow fringe of red mangrove (Rhizophora mangle) on the barrier side is followed by black mangrove (Avicennia germinans), which exhibits the greatest importance values (IV = 63%). Behind the fringe, white mangrove (Laguncularia racemosa) mixes with sand dune vegetation. From the seaward zone to the inner zone, tree heights increase from 10.2 m to 12.2 m and the IV of R. mangle shifts from 24% to 31%. In the marginal forest, the mean basal area is 22 m2 ha-1, the mean complexity index is 7.
On the continental side of the lagoon, the mangrove forest shows a better structural development than on the barrier side but with a similar horizontal gradient landward. In the seaward zone (Fig. 6A), the forest is dominated by A. germinans (IV = 74.6%) with a mean height of 11 m, a basal area of 21 m2 ha-1, and a complexity index of 15. In the central zone of the lagoon (Fig. 6B), the dominance shifts to R. mangle (69.6%) followed by A. germinans (29%), with a mean height of 13 m, a basal area of 23 m2 ha-1, and a complexity index of 16.5. Finally, in the inner zone (Fig. 6C), the forest reaches its greatest structural development, dominated by R. mangle (74%), followed by L. racemosa (20%), with a basal area of 36 m2 ha-1, a mean height of 15 m, and a complexity index of 21 (Herrera-Silveira, 1993). Changes of basal area and complexity index indicate the gradient of mangrove forest type from fringe forest in the seaward zone, to basinal forest in the middle zone, to riverine forest in the inner zone.
|Fig. 6. Diagramatic mangrove
vegetation profile in Laguna de Celestún:
A seaward zone
B middle zone
C inner zone
On the barrier side of the lagoon, the soil is sandy, poor in organic matter, and salt-stressed. A. germinans is hardy under these conditions and dominates this side. Additionally, the mix with dune vegetation favors the competence of this species. On the continental side of the lagoon, the forest is large, extending >5 km from the lagoon. Its configuration is heterogeneous because the dominant species changes according to the spatial gradient, from A. germinans in the seaward zone to R. mangle in the inner zone. The capacity of A. germinans to occupy zones with the lowest flood and highest salinity variations explains the high importance values in the seaward zone. However, increasing groundwater discharge in the middle and inner zones favors a shift of dominance to R. mangle; in the inner zone, L. racemosa is replaced by A. germinans because of its tolerance of low salinity (Pool et al., 1975; Snedaker, 1982).
The mean density of trees in the mangrove forests of Celestún (1,826 trees ha-1) is less than the mean reported previously (2,503 trees ha-1; Pool et al., 1975; Brown and Lugo, 1982; Twilley, 1982; Flores-Verdugo et al., 1990). The basal area is not related to mean height, probably because Celestún is susceptible to hurricanes, which have a "pruning" effect.
Changes in soil salinity, from 2 in the inner zone to 60 in the seaward zone (Trejo, 1986), could have favored the spatial differences in structural development of the mangrove forest. The low nutrient input from groundwater and the seasonal change yield low litterfall production (Snedaker and Snedaker, 1984). The high litterfall production in the inner zone is related to the salinity gradient (Pool et al., 1975). However, mangrove development is poor farthest inland where A. germinans forms a dwarf forest.
Laguna de Celestún is an ecosystem in which frontal systems, rainfall, and tides influence the development of gradients in hydrological and biological characteristics. The seasonality and distribution of groundwater discharge is important in establishing a longitudinal salinity gradient, which is the main factor affecting distribution and organization of primary producers through dispersion of nutrients and particulate material. Temporal and spatial distribution patterns are related to the frequency and intensity of rains, tides, and meteorological forces. Thus far, research efforts been have concentrated on the middle and inner zones of the lagoon. Information is still sparse on primary production in the seagrasses and mangrove forests of the inlet zone. Only through systematic studies and standardization of methodology will it be possible to obtain an overall view of this system, detect both short-term and long-term trends, and develop a reliable database to understand and manage the ecosystem efficiently (Shaffer-Novelli et al., 1990).
We thank the staff of Marine Ecology Section of CINVESTAV-Mérida, and DUMAC-Yucatán for providing field support. Financial support was provided by CONACYT and CINVESTAV-Mérida.
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