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Changes in the hydrological characteristics of Chabihau coastal wetlands, Yucatan, Mexico, associated with hurricane Isidore impact

Eduardo Batllori * & José L. Febles

CINVESTAV, Research and Advanced Studies Center of I.P.N., Human Ecology Department., Km 6 Antigua Carretera a Progreso, A.P. 73, Cordemex, Merida, 97310, Yucatan, Mexico

*[E-mail: batllori@mda.cinvestav.mx ]

Received 22 May 2006, revised 14 May 2007

This paper describes changes in the hydrologic behavior of Chabihau coastal lagoon, Yucatan, Mexico, associated with the impact of Hurricane Isidore (2002) and the construction of hydraulic infrastructure in coastal highways, through the spatiotemporal analysis of the water physicochemical variables, from the 1999 flood season to the 2005 dry season. The coastal wetlands were subdivided into three areas: San Crisanto swamp in the west, Chabihau lagoon in the center, and Santa Clara swamp in the east. After the hurricane impact and construction of bridges in the coastal dune, stronger tide´s ebb into the Chabihau lagoon was recorded, changing it from a hyperhaline system to an euryhaline one. On the other hand, changes to hyperhaline conditions were observed in Santa Clara swamp during dry and flood seasons. After the hurricane, negative redox values were recorded throughout the entire Chabihau wetlands, in addition to a reduction in dissolved oxygen and pH, during both dry and flood seasons. This situation determined dominance of reductive processes in the three areas, with low temporal variability. If the salinization process continues in the Santa Clara swamp, changes may occur in the structure and composition of the mangrove forest.

[Key words: Coastal wetlands, hurricane Isidore, hydraulic infrastructure, Yucatan, Mexico, Chabihau wetland]

Introduction

The hydrologic conditions of coastal wetlands affect many abiotic factors, including those related with organic matter oxidation and reduction processes, nutrient and oxygen availability, water and sediment salinity1,2. This determines the flora and fauna that may develop in the ecosystem.

Consequently, this biological development alters the wetland hydrologic conditions, reinitiating the cycle with different levels of organization and complexity, depending on the level of inundation, input and output flows, geomorphology and sediment relief.

Coastal wetlands affected by ocean tides are generally more productive than those that are occasionally flooded, as well as alternation between dry and flood seasons may lead to optimal conditions for detritus decomposition1. On the other hand, the anaerobic condition that may develop in areas that are permanently flooded may considerably delay this decomposition process3. Nutrient cycling may also be very fast in situations of high water exchange, making many nutrients available for plants due to changes in pH and the soil redox potential4.

Coastal ecosystems in Yucatan present particular characteristics due to their karstic origin and the

absence of surface rivers. These are systems created by the dynamics of the sea currents that form a barrier island or coastal dune, and lagoon systems – open or enclosed – which, due to underground freshwater input through springs, acquire a brackish character in some sites, and hyperhaline conditions in others, as in the case of Chabihau wetlands. The most severe human induced problems that affect coastal wetlands in Yucatan are related to 1) road construction, 2) opening of ports, 3) population growth, 4) saltwater intrusion through coastal sandbar breaches, and 5) freshwater springs sedimentation. Due to these factors, Batllori et al.5 reported a loss of 1.3 km2/year of mangrove forest, from 1948 to 1991, in the Yucatan´s northwest coastal wetlands.

The main objective of this study is to describe changes in the hydrologic characteristics of the Chabihau wetlands, associated with the impact of hurricane Isidore and the construction of hydraulic infrastructure in coastal roads, through the spatiotemporal analysis of the water physicochemical variables, from the 1999 flood season to the 2005 dry season. This study is important to understand the spatial and temporal changes and the organization of the ecosystem to the new hydrological conditions, due

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to structural changes generated by the hurricane. Also, this environmental information is a key to understand the social adaptive response to the potential biological resources appropriation in the transformed ecosystem.

Study area

There are three urban localities in this wetland, San Crisanto, Chabihau and Santa Clara, connected by a road network in the coast as well as inland, which modifies water flows and sediment transport. Among the roads that section the lagoon, from north to south, there is one that joins San Crisanto with Sinanche, the one that goes from Chabihau to Yobain, and the Santa Clara – Dzidzantun road. The hydraulic infrastructure associated with these roads in flood areas includes 36 culverts of different dimensions (less than 0.45 m wide - most of which operate deficiently -) before year 1999, and eleven with 3.5 m wide bridges built after 2000. In fact, the first mentioned highway is established as a blockage of lateral water flow between the east side and the west side, creating very different environmental conditions due to variation in water salinity.

This ecosystem is quite vulnerable to hurricane impact. Both Hurricane Gilbert in 1988 and Hurricane Isidore in 2002 caused breaches in the sand barrier, connecting the sea with the lagoon in the eastern part of the Chabihau locality, changing the system from seasonal flooded to tide-dominated. This situation has brought on social and economical benefits, such as an increase in fishing for self-consumption - mainly shrimp in the winter.

Since June 1997, the local government constructed a floodgate system in coastal road (4 meters wide) near the village of Chabihau, which allows for controlled sea-water flow and the release of brackish or hyperhaline lagoon water, at the same time allowing for periodic harvesting of some species, under conditions of seasonal semi-captivity (shrimp, crab and fish). In 2003, two road bridges were built over the mouths that were opened by Hurricane Isidore in 2002, one 24-m wide in the eastern border of the urban area of the Chabihau locality, and the other, 12-m wide, near Santa Clara. In all of these sites there is now an important water exchange between sea and lagoon, in such a way those changes are expected in the water quality 6, in the ichthyofauna and in the vegetation structure, mainly of the mangrove, which is important as a refuge for species and productive base for strategic subsistence fisheries, both locally and regionally7.

The Chabihau coastal lagoon and swamp (salt flats and basin mangrove), extends for 17 km, from the San Crisanto to Santa Clara roads, and covers a surface area of approximately 45 km2, of which 34 km2 are occupied by the lagoon with permanent water (11.49 km2) and swamp (22.51 km2), and 11 km2 of flooded low forest and savannah. These wetlands are protected by a sand barrier with coastal dunes that barely exceed 300 ha (Fig. 1). The climate is hot and dry8, with a mean annual temperature of 26oC. Annual precipitation is 600 mm, while evaporation exceeds 1800 mm per year. Rainy season begins in May, with the rainiest months being June and September. A short dry period occurs in July and August, and starting from October, precipitation is reduced. The less humid months are February, March and April.

The prevailing wind direction is from the NE, E and SE, followed by winds from the north (October to March). Tides are diurnal with amplitudes < 1 m. The tide level is lower between March and August while the highest level occurs after October, related with the north winds.

In the southwestern part of the study area, 2 km from the coast toward Sinanche, natural terrain heights reach 1 m above mean tide (m) (high swamp);

toward the coast, extends a plain with average values

Fig. 1⎯Chabihau wetlands and sampling stations (1. High Swamp, 2. Middle Swamp, 3. Low Swamp; both East and West road side). Showing inlets opened by hurricane Isidore in 2002 (later spanned by bridges), towns and main roads.

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of 0.5 m (middle swamp). A depression of the land is observed in the east toward Chabihau, with values between –0.2 and –0.7 m (low swamp). In Santa Clara, topographic heights of –0.2 m predominate;

elevations of 0.3 m can be found in the entire northern extension and of 0.5 m in the southern portion. The sand barrier reaches heights near of 3 m, protecting the lagoon from the tide, waves and wind forces. The volume of water in this lagoon is approximately 17 × 106 m3.

Materials and Methods

In 1999, a hydrologic monitoring network was built, with 22 permanent stations distributed to the East and West (E and O) of the San Crisanto, Chabihau and Santa Clara wetland roads (Fig. 1), and located at different sites of the wetlands (high swamp -HS, middle swamp -MS, low swamp -LS, springs, floodgate and sea). The sampling was conducted on a monthly basis and in the morning. In order to measure the water depth, a calibrated wood beacon was installed in each site; an YSI model 85 multi-analyzer was used to record salinity (ranging from 0 to 80 psu,

±0.1), temperature (ranging from -5 to 65oC, ±0.1), and dissolved oxygen (ranging from 0 to 20 mg/L,

±0.3). Salinity values over 80 psu were estimated using an ATAGO refractometer (ranging from 0 to 28%, ±0.2). Values of over 280 psu were estimated with 50% dilution. Salinity values were classified according to Mitsch & Gosselink1 for marine and estuarine environments.

The YSI pH100 analyzer was used to measure pH (ranging from -2 to 16, ±0.01) and redox potential (ranging from –1999 to 1250 mV, ±1). A total of 43 sampling events were conducted throughout the network from July 1999 to July 2005, with the exception of the year 2003, in which only one sampling event took place. This way, 26 sampling events occurred before Hurricane Isidore in September 2002 and 17 events after the hurricane.

The values were grouped into two seasons, in relation to flood periods (September-February) and dry (March-August) respectively. The September to February period corresponds to the end of rainy and the north wind seasons (fall-winter), when the greatest volume of groundwater discharging into the coastal system by springs and the highest ocean tide elevation are recorded 9-11. As a complement, rainfall data for the Telchac Puerto station, for the 1999-2005 period, were obtained from the Mexican National Water Commission.

The data were processed in Excel for windows (2002) for each station and per parameter, for the 43 events (5,676 data in total). Measures of central tendency such as mean, standard deviation and variation coefficient were taken. The log10- transformed data of each parameter and each area were submitted to a two-way analysis of variance, with seasons (dry or flood) and sites as factors. A Spearman nonparametric correlation analysis was conducted to determine the degree of association among sites. The information of each parameter was incorporated into the SURFER V.8 program, obtaining graphics, with their respective values in isolines.

Results

The general seasonal average of the data during the study period (June 1999 to July 2005) is shown in Table 1. Among other characteristics, the one that stands out is the wetlands hyperhaline character, in addition to it being a very shallow, warm water body, with oxygen levels near 4 mg/l. Water depth was the parameter with greatest seasonal variation (CV = 60.5%), followed by redox and water salinity values (49% and 46%, respectively).

The general spatial average of the data (1999 to 2005) is shown in Table 2. In terms of salinity, depth, temperature and redox, mean values were higher in the Eastern swamp portion of the wetland roads than in the Western swamp portion.

Table 1⎯Temporal average of hydrological parameters at Chabihau wetland and zones, Yucatan, 1999-2005 (Variation Coeficient %)

Chabihau wetlands San Crisanto swamp Chabihau lagoon Santa Clara swamp

Salinity (psu) 41.62 (46.2) 61.57 (70.9) 41.35 (33.4) 28.87 (64.9) Depth (cm) 20.16 (60.5) 10.23 (128.9) 18.02 (43.5) 26.75 (87.6) Temp (oC) 29.09 (2.5) 28.78 (13.1) 29.45 (10.2) 28.15 (10.7) O2 (mg/l) 4.36 (23.3) 3.29 (76.3) 5.19 (59.0) 3.56 (80.4)

pH 8.17 (2.7) 8.17 (8.1) 8.21 (9.3) 8.05 (8.4)

Redox (mV) -68.82 (49.4) -13.91 (651.7) -76.81 (325.4) -112.30 (218.3)

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Table 2⎯Spatial average of hydrological parameters at Chabihau wetlands, Yucatan, 1999-2005, (Variation Coeficient %).

Salinity

(psu) Depth

(cm) Temperature

(°C) O2

(mg/l) pH Redox

(mV)

EHS 62.82 (97.31) 5.68 (64.72) 29.70 (8.26) 3.06 (76.33) 8.00 (2.10) -137.00 (252.00) EMS 69.70 (51.18) 23.09 (80.12) 30.18 (4.08) 3.92 (39.60) 8.14 (1.35) -82.87 (81.00) ELS 64.56 (64.88) 29.09 (57.77) 29.59 (2.44) 4.54 (31.05) 8.18 (1.80) -59.89 (40.62) WHS 22.28 (107.55) 7.85 (47.62) 28.00 (6.47) 2.58 (23.04) 8.39 (7.04) -69.48 (136.20) WMS 27.56 (130.51) 18.24 (69.25) 28.41 (11.16) 4.46 (43.29) 8.04 (4.14) -64.82 (29.76) WLS 35.41 (83.33) 15.81 (13.68) 29.57 (4.95) 5.52 (18.74) 8.65 (0.55) -44.48 (15.65) Floodgate 39.33 (24.65) 43.92 (25.88) 28.80 (12.26) 4.72 (53.73) 8.17 (8.08) -87.56 (181.49) Spring 18.56 (41.33) 17.62 (51.88) 29.20 (2.20) 4.94 (12.27) 7.94 (1.17) -62.25 (91.70)

Sea 34.37 (1.66) 28.34 (1.41) 5.52 (2.04) 7.98 (0.80) -11.06 (174.22)

Figures 2-4, on spatial-temporal distribution of the different parameters for each transect, show changes in the water physicochemical characteristics, particularly in the low portion of the swamp in Santa Clara, with an increase in salinity, whereas in the middle and low part of Chabihau, salinity decreased.

Another relevant aspect is that the levels of dissolved oxygen, pH and potential redox decreased, which could be related with a dominance of organic matter reduction processes over oxidation processes. San Crisanto was the area with less change.

In terms of average values before and after the hurricane, and the construction of bridges, differences in salinity and pH were found in all transects. In San Crisanto and Santa Clara, differences were observed in depth and temperature, while dissolved oxygen only showed differences in Santa Clara (Table 3, 4).

Before hurricane Isidore (2002), high redox potential values were observed, with very high variation coefficients. In Santa Clara, average redox values were positive in both dry and flood seasons.

Nevertheless, there were no differences in redox potential between dry and flood season in the three transects (Table 4). Before the hurricane, pH values were above 8.5 and only decreased in San Crisanto during dry season (with average values of 7.7), showing significant differences between seasons (Table 4). In Chabihau and Santa Clara, pH values remained above 8.4, in both dry and flood seasons.

Oxygen values were ≥4.0 mg/l, with highest values during dry season. None of the transects exhibited differences in oxygen between dry and flood seasons (Table 4). Greater variability in water depth levels was observed (as in Santa Clara). Temperature and depth exhibited differences in the three transects, between dry and flood seasons (Table 4). The San

Crisanto and Chabihau areas displayed hyperhaline conditions before the hurricane in both dry and flood seasons; Santa Clara had brackish water in both seasons, with lower values during dry season (26 psu). Before the hurricane, both San Crisanto and Santa Clara displayed differences in salinity between dry and flood seasons.

After the hurricane, negative redox values were recorded for all the areas during both dry and flood seasons, with low seasonal variability, and significant differences were not found between dry and flood seasons in all the areas (Table 4). The swamp water exhibited a decrease in pH, with values of 7 for both dry and flood seasons. In the Chabihau lagoon, water depth decreased; however, it increased in San Crisanto and Santa Clara, particularly in dry season, which could be controlled by sediment transport and tides. Significant depth differences were observed in all the areas and between seasons. A decrease in dissolved oxygen, down to 3 mg/l, was recorded, with high variability, and a significant drop in dry season.

Differences between seasons were only observed in the Santa Clara swamp. Average temperatures in San Crisanto increased in both dry and flood seasons.

Water salinity in Chabihau had marine conditions in both seasons, as in Santa Clara during flood season.

Differences were not observed between seasons, except for San Crisanto. Here, hyperhaline conditions were recorded only during the dry season, as in Santa Clara. The latter means that a salinization process began in the middle and low areas of the Santa Clara swamp. It is important to point out that the seasonal variation coefficients, per area, of all the parameters, were lower after Hurricane Isidore.

If we consider the dry season, before and after the hurricane, we can observe differences in water

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Fig. 2⎯San Crisanto transect. Temporal Distribution of A) water salinity (psu), B) Temperature (°C), C) Depth (cm), D) O2 (mg/l), E) pH and F) REDOX (mV) (1999-2003 and 2004-2005). [EHS: East High Swamp, EMS: East Middle Swamp, ELS: East Low Swamp and SEA].

temperature in San Crisanto, while Chabihau exhibits differences in salinity and pH. In Santa Clara, differences were observed in all the parameters except for redox potential. Now, regarding flood seasons before and after the hurricane, San Crisanto displayed differences in temperature, salinity, depth and pH;

Chabihau displayed differences in salinity, depth and pH; and Santa Clara only exhibited differences in salinity (Table 4).

In the meteorological station of Telchac Puerto, located 6 km from Chabihau lagoon, lower precipitation was recorded in 2000 and 2001 (374 mm and 590 mm, respectively), when hyperhaline

conditions were observed in the lagoon, in dry as well as flood season. Subsequently, in 2002, accentuated by the effect of Hurricane Isidore, a precipitation of 860 mm was recorded, promoting the lowest salinity recorded in the lagoon. In 2004, higher rainfall was recorded (944 mm), however, the effect of the bridges and the increase in water exchange with the sea did not allow reaching the minimum salinity values recorded previously.

Discussion

The water temperature is a limiting factor for many aquatic organisms3 such as fish and mangroves, which

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Table 3⎯General average of hydrological parameters before and after hurricane Isidore, and by season, at the Chabihau wetlands (Variation Coeficient %)

Before hurricane 1999-2002 After hurricane 2002-2005

Average Dry season Flood season Flood season Dry season Average San Crisanto Transect

Salinity 50.45 (85.56) 60.14 (82.69) 43.35 (90.24) 30.94 (30.22) 49.56 (18.86) 40.80 (42.84) Depth 11.27 (74.62) 8.90 (83.33) 13.13 (69.97) 15.59 (33.27) 10.87 (47.74) 13.09 (55.66) Temperature 27.78 (11.72) 29.10 (8.72) 26.81 (13.32) 28.60 (5.06) 31.61 (4.57) 30.19 (8.14) O2 3.72 (40.24) 3.78 (41.15) 3.68 (36.03) 3.84 (36.22) 4.01 (34.70) 3.93 (38.06) pH 8.17 (6.57) 7.84 (7.47) 8.50 (2.02) 8.06 (8.10) 7.54 (8.65) 7.73 (11.32) Redox -4.88 1053.47) -2.83 1125.94) -6.24 (1030.51) -78.61 (143.88) -115.27 (98.12) -104.28 (92.66)

Chabihau Transect

Salinity 43.89 (21.03) 45.44 (20.32) 42.75 (21.93) 33.82 (27.92) 31.64 (14.77) 32.66 (21.90) Depth 19.36 (40.23) 15.4 (25.82) 22.46 (38.86) 17.82 (54.80) 14.47 (37.89) 16.04 (48.16) Temperature 29.43 (10.46) 31.39 (8.13) 28.00 (9.49) 28.00 (12.75) 30.81 (5.53) 29.49 (10.24) O2 5.20 (44.52) 5.56 (53.57) 4.94 (35.43) 5.29 (60.35) 3.90 (21.65) 4.56 (50.67) pH 8.49 (6.14) 8.50 (7.67) 8.48 (4.75) 7.99 (13.76) 7.58 (5.54) 7.73 (9.35) Redox -0.19 (320.90) 12.87 (357.08) -9.53 (284.72) -156.78 (21.71) -142.91 (78.40) -147.95 (60.18)

Santa Clara Transect

Salinity 28.36 (36.28) 26.99 (38.32) 29.37 (35.73) 31.11 (48.80) 44.18 (42.58) 38.03 (47.26) Depth 25.47 (53.28) 14.81 (46.52) 33.85 (33.91) 40.47 (30.90) 25.93 (43.20) 32.77 (41.75) Temperature 29.35 (10.00) 31.68 (4.90) 27.64 (9.06) 26.55 (12.42) 27.79 (3.27) 27.20 (8.68) O2 4.70 (58.05) 5.75 (63.90) 3.93 (37.18) 3.43 (53.46) 2.53 (46.25) 2.95 (52.07) pH 8.35 (4.76) 8.38 (6.51) 8.32 (2.50) 8.05 (10.01) 7.81 (7.80) 7.90 (8.41) Redox 24.81 (197.08) 18.55 (304.98) 29.29 (159.98) -90.71 (14.17) -232.68 (60.27) -185.35 (71.26)

Table 4⎯Analysis of Variance at Chabihau wetlands (* p=0.05 95 %).

Average before and after Dry vs Flood before Dry vs Flood after Dry before and after Flood before and after F Sig. F Sig. F Sig. F Sig. F Sig.

San Crisanto Transect

Salinity 7.545 0.007 * 8.157 0.005 * 28.001 0.001 * 1.821 0.127 7.703 0.001 * Depth 13.006 0.000 * 5.708 0.018 * 2.621 0.113 3.881 0.054 8.413 0.005 * Temperature 19.715 0.000 * 14.009 0.000 * 14.243 0.000 * 8.690 0.004 * 9.992 0.002 *

O2 1.345 0.248 1.698 0.195 0.310 0.581 0.517 0.475 1.283 0.260

REDOX 0.042 0.838 0.483 0.495 0.262 0.636 0.558 0.489 0.013 0.909

pH 5.309 0.024 * 12.377 0.001 * 0.574 0.454 1.335 0.255 5.058 0.031 *

Chabihau Transect

Salinity 23.235 0.000 * 0.012 0.912 0.340 0.561 10.319 0.001 * 12.424 0.001 *

Depth 1.319 0.252 30.143 0.000 * 0.855 0.358 0.367 0.546 4.302 0.040 *

Temperature 0.004 0.949 62.832 0.000 * 18.245 0.000 * 2.697 0.103 0.000 0.989

O2 0.929 0.336 0.002 0.960 2.105 0.150 2.604 0.109 0.012 0.912

REDOX 1.423 0.239 0.015 0.902 0.727 0.427 0.034 0.856 0.019 0.892

pH 47.495 0.000 * 0.036 0.849 6.717 0.011 * 46.659 0.000 * 4.925 0.030 * Santa Clara Transect

Salinity 5.271 0.023 * 40.594 0.001 * 2.930 0.090 15.110 0.001 * 9.046 0.000 * Depth 5.390 0.022 * 19.940 0.000 * 7.358 0.009 * 9.257 0.003 * 2.249 0.138 Temperature 11.314 0.001 * 41.181 0.000 * 4.966 0.029 * 60.056 0.000 * 2.091 0.152

O2 5.601 0.019 * 2.857 0.094 7.406 0.008 * 12.803 0.001 * 0.020 0.887

REDOX 0.831 0.581 0.024 0.878 0.588 0.499 0.361 0.563 0.011 0.916

pH 9.212 0.003 * 0.179 0.674 0.742 0.393 5.142 0.027 * 2.985 0.091

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Fig. 3⎯Chabihau transect. Temporal Distribution of A) water salinity (psu), B) Temperature (°C), C) Depth (cm), D) O2 (mg/l), E) pH and F) REDOX (mV) (1999-2003 and 2004-2005). [EHS: East High Swamp, WHS: West High Swamp, EMS: East Middle Swamp, WMS: West Middle Swamp, ELS: East Low Swamp, FLG: Floodgate and SEA].

present problems when growing in temperatures of over 42oC. The effect of the increased temperature, particularly in the San Crisanto area, may play a role in certain important reactions such as the accelerated decomposition of organic matter, nitrification, and biological processes such as germination and the development of mangrove seedlings12,13.The salinity gradient and the water level, contribute to the distribution of different species throughout the

wetlands, while their seasonal variations determine the vegetation composition14,15.

The high concentrations of salt may inhibit enzymatic activity, protein synthesis and respiration rate 16. Red mangrove (Rhizophora mangle) can grow in deeper water with sediment salinity under 40 psu;

however, black mangrove (Avicennia germinans) can grow in saline substrata of more than 50 psu and in shallower areas with short hydroperiod14,17.

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Fig. 4⎯Santa Clara transect. Temporal Distribution of A) water salinity (psu), B) Temperature (°C), C) Depth (cm), D) O2 (mg/l), E) pH and F) REDOX (mV) (1999-2003 and 2004-2005). [EHS: East High Swamp, WHS: West High Swamp, EMS: East Middle Swamp, ELS:

East Low Swamp and SEA].

Critical parameters for the development of mangroves4,12,18,19 are 1) nutrients (nitrogen and phosphorus), 2) superficial and soil salinity, 3) temperature, 4) rainfall and evapotranspiration, 5) tides, 6) geomorphology and 7) topography.

However, Odum & Johannes20 comment that radicular gas exchange is the mangrove forest´s “Achilles heel”. Siltation and soil salinity interferes with both the forest´s nutrient cycling, and gas exchange between the rhizosphere and the water column or atmosphere21.

If we consider the water’s most saline conditions, recorded in Santa Clara after the construction of the bridges in 2003, where vegetation with greatest structural development was observed, we can expect changes in the mangrove forest structure and composition, with stress symptoms and characteristics similar to those recorded in Chabihau.

Before Hurricane Isidore and the construction of bridges, the water of Chabihau wetlands was under aerobic conditions, particularly in Santa Clara, and during dry season in the rest of the areas. This may be

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related to 1) rate of oxygen transport through the water – atmosphere – soil interface, 2) a small population of oxygen consuming organisms, 3) the photosynthetic production of oxygen by algae inside the water column or at the bottom, 4) surface mixing by springs and 5) winds.

Phosphorus is vital for life; however, it may not be available for plants and micro-consumers due to precipitation of insoluble phosphate with ferric iron, calcium and aluminum under aerobic conditions or conditions of pH over 8 (ref. 1,23) (this is particularly important in a karstic high alkaline water, like in Yucatan Peninsula), thus, the low biological productivity in the lagoon before the hurricane.

Valdes & Real 22 mention that for the case of Chelem Lagoon, on the central coast of Yucatan, phosphorus showed significant correlation (r = -77, p ≤ 0.05) with pH, indicating that phosphorus precipitation and dissolution are regulated by pH.

Main topobathymetric changes (2000 and 2004) in the lagoon and littoral area of Chabihau, and the distribution of surface sediment, before and after Hurricane Isidore, were recorded. The hurricane moved large amounts of mud (10 to 20 cm thick in some sites), making the lagoon slightly deeper toward Santa Clara and San Crisanto, exporting large amounts of fine sediment and organic matter to the littoral area with the tides – a washing process that continues until today and have a strong effect on the system productivity on the mid- and long-term. This explains why these two areas increased in water depth after hurricane. Before the hurricane, the salinity of the lagoon water discharge to the sea, through the floodgate, was 70 psu in the dry season; however, sea- water salinity was never > 41 ups. After the hurricane, in the same season, this value never raised to 41 and 37.6 psu in the floodgate and sea, respectively.

Zaldivar23, found important relationships between the mangrove community structure and the physical and chemical characteristics of the sediment in the Chuburna-Sisal swamp, located in the northwest of the Yucatan coast and connected to the sea by two water floodgates, since Hurricane Gilbert (1988).

Salinity (r= −0.72, p≤0.05) and redox potential (r= −0.61, p≤0.05) were the sediment characteristics that best explained the mangrove trees’ height, density and basal area. This author also found highly reduced conditions (–389 and –10 mV), mainly during flood season, while in dry season the values tended to be close to zero.

This is quite similar to Chabihau wetlands which, after hurricane Isidore and the construction of bridges, showed reduced conditions throughout the year. With the redox values recorded in Chabihau wetlands, one may expect conditions where iron is predominantly found in a reduced form, just like nitrogen, with active transformation of sulfur and carbon1,24. This could lead to high toxicity of free sulfur when in contact with plant roots, particularly R. mangle, since A. germinans is more tolerant25. Phosphorus solubility is therefore increased under anaerobic conditions or with pH values lower than 7. In the case of Chelem Lagoon, Valdes & Real2 found a direct and significant correlation between the presence of phosphorus and chlorophyll-a concentrations in the water. This phosphorous availability may increase the biological productivity in the lagoon, and this could be related with capture of fish species by inhabitants of the three localities. More than 8 tons of shrimp have been collected, and the ichthyofauna, for example, increased from 18 species before the hurricane to 34 species after, particularly euryhaline species, and a wide variety and quantity of aquatic birds.

Acknowledgement

Authors are thankful to the local government of Yucatan and the National Science and Technology Council (CONACyT) for supporting the project:

Evaluation of Socio-environmental Changes in the Chabihau Wetlands, Caused by Hurricane Isidore and Prevention Strategies to cope with Future Meteorological Phenomena (2003-2004). The National Water Commission (CONAGUA) provided climatic data for Telchac. Physicochemical data from July 1999 to February 2000 correspond to samplings conducted by Rocio Rendis Ruz. Jorge Novelo helped with field work and data organization.

References

1 Mitsch W & Gosselink J, Wetlands, (Van Nostrand Reinhold, New York, USA).1993, pp. 722.

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References

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