Indian Journal of Marine Sciences Vol. 33 (4), December 2004, pp. 338-345

Nitrogen and phosphorus in water and sediments at Ria Lagartos coastal lagoon, Yucatan, Gulf of Mexico. David S. Valdes* & Elizabeth Real CINVESTAV, Centro de Investigación y de Estudios Avanzados del I. P. N., Km 6 Antigua Carretera a Progreso, A.P. 73, Cordemex, C.P. 97310, Merida, Yucatan, Mexico. * (Email: [email protected] Received 23 March 2004, revised 25 October 2004 Ria Lagartos, a hypersaline coastal lagoon in the Gulf of Mexico on the Yucatan peninsula was sampled three times in a year at 30 locations, in order to quantify the nitrogen and phosphorus available in the water column and sediments. Undisturbed samples of sediment were analyzed and incubated in the laboratory to estimate ammonium and phosphate fluxes between water and sediment together with nitrification and denitrification rates. The results indicated higher ammonium liberation (250 ± 149 µmol m-2 h-1) and nitrification (151 ± 132 µmol m -2 h-1), than denitrification (47 ± 33 µmol m -2 h-1). In contrast phosphate liberation from sediments was found to be low (0.67±1.31 µmol m -2 h-1). The high concentrations of organic matter (3.73±1.65 %), total nitrogen (99.14±62.73 µmol g -1 ), and phosphorus (4.42±1.82 µmol g-1) in the sediments of the lagoon, indicate that can be a sink for these elements. [Key words: Sediment, Ria Lagartos Mexico, nitrogen, phosphorus, nitrification, denitrification]

Introduction Marine plants require certain trace elements for growth, these nutrients are used until they become limiting and further growth is inhibited, the most important micronutrients are nitrogen and phosphorus1. In the coastal zone these nutrients are buried in the sediments and recycled to the water increasing their availability and producing blooms of phytoplankton and others plants2-4. The Ria Lagartos coastal lagoon, on the northern coast of the Yucatan peninsula in the Gulf of Mexico (Fig 1), is under pressure from human activities. When combined with the climate (low precipitation, high evaporation) and the geology (karstic) of the region5, this makes the lagoon prone to eutrophication, an increase in the nitrogen and phosphorus availability for primary producers that could bring negative effects like anoxic conditions in the water column6. The objective of this study was to determine the nitrogen and phosphorus budgets in both water and sediment, and the fluxes between these two phases through physical and biogeochemical processes. Materials and Methods The lagoon is very shallow (0.5-1.0 m), 80 km long, and has 94 km2 of surface, and the tide is diurnal

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with a range of 0.6 m in the open sea. Water circulation (lagoon-ocean) is through a natural mouth at the western end of the lagoon. There are also two other artificial inlets in the western region of the lagoon. The mean annual precipitation ranges between 500 to 700 mm from the western to eastern end of the lagoon, and annual evaporation is 2,000 mm. The weather and the fact that freshwater input from springs is low, causes the lagoon to be mainly hypersaline. There are four fishing villages on the shore of the lagoon: San Felipe, Rio Lagartos, El Cuyo, and Las Coloradas, the last of which contains a salt mining industry that uses the lagoon’s water. Surface water and recent sediments were sampled from 30 stations (Fig. 1) three times, to determine environmental changes between the winter season (north winds), the dry season and the rainy season (November 1994, March 1995 and August 1995). Water temperatures were measured with an YSI-33 meter. Surface seawater samples were taken with a Van Dorn bottle and stored in HDPE bottles at 4 /C for the transportation to the laboratory (dissolved oxygen samples were fixed in the field in BOD bottles). Analysis were made in our laboratory in Merida about a 100 km from the sampling site within the next 24 hours, pH was measured with a Beckman Phi-32 pH meter (NBS standards), suspended solids

VALDES & REAL: N AND P IN WATER AND SEDIMENTS

were estimated by filtration of 1 liter of sample through a filter Whatman GF/C 7, dissolved oxygen using the Winkler method 8, and oxygen saturation calculated with the equations of Garcia & Gordon 9. Biochemical oxygen demand (BOD) was estimated by conducting incubations and measuring the decrease of dissolved oxygen7. Filtered water was analyzed for salinity using a salinometer (Kahlsico RS-9); ammonium, nitrite, nitrate, phosphate, silicate, and particulate nitrogen were quantified using the colorimetric techniques 8, with a Shimadzu 1200 UV/VIS spectrophotometer. A hand corer (45 mm i.d., 500 mm length, Wild Co. CA USA) was used to collect sediments (3 cores of each station), and stored in acrylic tubes. The pH and the redox potential of the sediment were measured by inserting the electrodes to a depth of 3 cm (following the recommendations of Brassard 10). The top 5 cm of the cores was sliced (two 2.5 cm sections) under nitrogen atmosphere and used to analyze the following parameters of the sediments: granulometry, porosity, organic matter, nutrients in the interstitial water, total nitrogen and total phosphorus. Law et al. 11 reported that most of the transformation processes occur in the top 5 cm of the sediment, for this reason we take this layer as the most representative for the water-sediment interactions. The porosity (M) of the sediments was determined by drying at 90 /C for 24 hours, the loss of weight divided by the wet weight was taken12 as M; the organic matter content was determined by the wet oxidation technique with an excess of dichromate

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and back titration with iron (II)13; the amount of sand, silt and clay (granulometry) was estimated suspending the sediment and measuring the density with a hydrometer 13; the nutrient concentrations in the interstitial water (2.5-5 cm) were assessed by extraction with water of the same station under nitrogen atmosphere14. The ammonium and phosphate flux (F) was estimated using Fick's first law corrected by porosity (M), and tortuosity 15 : F = -MD/ dC/dz; where dC/dz is the vertical gradient (the difference of concentration between the water column and the 2.5-5 cm interstitial water) and D the diffusion coefficient. The diffusion coefficients with tortuosity correction (D/), that were used in the calculations, were taken from Li & Gregory 16 : NH4+ = 19.8 X 10-6, and PO43- = 7.25 X 10-6 cm²/s. Total nitrogen and total phosphorus were determined by wet oxidation of the dried sediments (always the top 5 cm) with potassium persulfate in basic and acidic conditions, respectively 17. The second core was used for the assessment of denitrification rates following the methods described by Andersen18, measuring the rate of decrease in the nitrate concentration in an anaerobic incubation. The third core was preincubated and then incubated in oxygenated conditions to estimate the nitrification rates following the method of Sloth et al . 19. This involves the measurement of NH4+ flux before and after the addition of a nitrification blocker. This method has the advantage of stable and controlled O2 conditions in a closed microcosm, brief incubation times, calculation of rates

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in individual cores, and the use of an inhibitor (acetylene) that is known to be effective in very small concentrations. Results The Ria Lagartos water showed a wide range in most of the water parameters measured (Table 1). All data were analyzed with statistical/software 20 to identify correlations. The correlation was consider significant at a level of p<0.05. Salinity reached 147.52 psu in the inner zone of Ria Lagartos and had an average of 69.48 psu in the dry season. pH values changed from one season to other, they were higher in the November sampling (9.00 ± 0.29) than in March (8.28±0.21) and August (8.31±0.23). Dissolved oxygen levels ranged from less than 1 ml/l (in the inner zone) to more than 7 ml/l in the central zone. Oxygen saturation ranged from 8.6 to 168 %. For the dissolved inorganic nitrogen in water, the total ammonium presented concentrations above 2 micromolar (µM) all the sampling periods in the lagoon. Nitrite + nitrate were in low concentrations (<1 µM) except in the zone between stations 10 to 15 (>2 µM) where there are some ground water springs, with high levels of nitrate (we measured 7-70 µM in the spring water). Particulate nitrogen concentrations presented a positive gradient from the sea to the inner zone, where the levels increased to more than 200 µM (correlation with salinity was 0.93). The inorganic phosphorus in the water was at very low concentrations, (average of 0.09 µM). High concentrations of silicate were found because the ground water springs also contribute with these ions (levels of 43-145 µM in the springs) and also presented

a high (r=0.81) correlation with salinity (Fig. 2). Suspended solids in the Ria Lagartos lagoon ranged from low levels in the mouth to very high in the inner zone; they reach 375 mg/l in one of the samples. BOD had a wide range, with the higher values in the eastern zone, where the suspended solids were also high. Surface sediments of the lagoon presented high porosity (above 0.6). The pH range in the sediment was from 7 to 9, except station 14 at pH 9.5. The redox potential was between -100 and -200 mV in most of the samples, meaning that the conditions were reducing. The organic matter in the sediments in most of the stations was between 3 and 4 %, although the general tendency was an increase of these levels from the area of the connection with the sea toward the inner zone of the lagoon (Fig. 3). Total nitrogen in the sediments showed lower concentrations in the zone near the sea, and higher concentrations in the inner zone. Exceptions were stations 2, 4 and 5 that presented high concentrations (Fig. 4), even though they are located near the inlet. Total phosphorus in sediments was higher in the inlet than in the inner zone (Fig. 5), showing an inverse behavior to the organic matter and nitrogen. Ammonium and phosphate concentrations in the sediment interstitial water (Table 2) were variable but higher than the respective water column concentrations (Table 1). Ammonification of organic nitrogen was high, and at stations 4 and 22, values reached to 400 µmol m -2 h-1 and the general trend was that the lowest ammonium fluxes were in the higher salinity zone (Fig. 6). Phosphate fluxes showed the same gradient, with the highest values ( 3.6 µmol m-2 h-1) at the stations in front of the towns of San Felipe and Rio Lagartos.

Table 1.- Average values of the most important water variables in Ria Lagartos during the three sampling periods. N.D. : Not detectable. Salinity (psu)

Ammonium (µM)

Nitrite (µM)

Nitrate (µM)

Phosphate (µM)

Particulate Nitrogen (µM)

Average Std. Dev. Minimum Maximum

60.05 33.90 28.39 147.52

3.24 1.31 1.49 8.16

0.12 0.12 N.D. 0.50

0.69 0.93 0.06 3.78

0.09 0.27 N.D. 1.43

16.8 37.9 0.5 216

Oxygen Saturation (%) 83.23 29.70 8.57 168.09

BOD (ml l-1)

Average Std. Dev. Minimum Maximum

Dissolved Oxygen (ml l-1) 3.39 1.45 0.39 7.50

Total suspended solids (mg l-1) 44.5 66.9 3.7 375.0

Organic suspended solids (mg l-1) 22.3 45.2 N.D. 270

Inorganic suspended solids (mg l-1) 22.2 28.0 0.9 194.0

1.36 0.59 N.D. 3.05

VALDES & REAL: N AND P IN WATER AND SEDIMENTS 80

27

70

Silicate (micro Molar)

341

19

30

Stations

26 28 29

20 21

60

22 18

50 15 14 16 13 10 92 12 11

40 30

23

24

25

17

45 783 6 1

20 10 0 20

40

60

80

100

120

140

Salinity (psu) Fig. 2.- Relationship silicate-salinity in the water samples of Ria Lagartos showing the increase of silicate concentration in the range of salinity 35 to 60 psu, a decrease in the range 60 to 90 psu and a second increase in the range 90-120 psu.

Organic matter (%)

8 7 6 5 4 3 2 1 0 1

4

7

10 13 16 19 22 25 28

Station number Fig. 3.- Organic matter in the sediments of Ria Lagartos. Average concentrations of the three seasons with standard deviation.

Total nitrogen (µmol g-¹)

250 200 150 100 50 0

1 3 5

7 9 11 13 15 17 19 21 23 25 27 29

Station number Fig. 4.- Total nitrogen in the sediments of Ria Lagartos. Average concentrations of the three seasons with standard deviation.

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In 5 of the 30 points studied, the nitrification rate estimated in the laboratory incubations, was not detected, but in the other 25 the rates were high with an average value over 150 µmol m -2 h-1, and the

highest ones in the intermediate zone of the lagoon (160-500 µmol m-2 h -1), where the conditions are more pristine (Fig. 6). For denitrification, the 30 samples presented detectable values, around

Table 2.- Average values of the most important sediment variables in Ria Lagartos during the three seasons sampled. Analysis was done in the top 5 cm section, ammonium and phosphate concentrations are from pore waters (2.5-5 cm). Redox potential was measured at 3 cm depth. N.D. : Not detectable. Nitrogen (µmol g -1)

Phosphorus (µmol g -1)

Ammonium (µM)

Phosphate (µM)

Redox potential (mV)

Average

3.73

99.14

4.42

490.5

3.72

-166.9

Std. Dev. Minimum

1.65 0.40

62.73 16.36

1.82 1.07

283.6 13.3

7.03 N.D.

46.4 -258.0

Maximum

7.85

281.20

12.32

1226

44.43

106.0

Total phosphorus (µmol g-¹)

Organic matter (%)

12 11 10 9 8 7 6 5 4 3 2 1 0 1

3

5

7

9 11 13 15 17 19 21 23 25 27 29

Station number

Transformation rates (µmol m² h-¹)

Fig. 5.- Total phosphorus in the sediments of Ria Lagartos. Average concentrations of the three seasons with standard deviation.

600 500

Ammonification Nitrification Denitrification

400 300 200 100 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Station number Fig. 6.- The nitrogen transformation processes: ammonium flux, nitrification and denitrification in the sediments of Ria Lagartos. Average of the three seasons.

VALDES & REAL: N AND P IN WATER AND SEDIMENTS 50 µmol m-2 h -1, with only two stations above 100 µmol m-2 h-1 (one of these with nitrification rate not detectable, Fig. 6). Discussion Evaporation is one of the most important processes in this lagoon, causing the increase of the concentration of some substances, but a decrease in others. Salinity was about 4 and 5 times more than Gulf of Mexico seawater. pH values showed a significative increment with salinity, a normal behavior in the hypersaline environments because of the increase of the concentrations of carbonate and sulphate21. Dissolved oxygen levels were higher in the zones with more aquatic vegetation (central one). Salinity plays an important role in the concentrations of this gas since the correlation between these parameters was high and significant (r= -0.74). Oxygen saturation varied widely, the minimum saturation levels were values found in the first hours of the day, meaning that oxygen falls during the night. For the dissolved inorganic nitrogen in the water column, the ammonium was found to be the most abundant compound, but compared with other coastal lagoons 22, 23 concentrations were low (by 5 to 10 times). Ammonium showed a positive correlation with salinity and negative correlation with dissolved oxygen. Ground water inputs, high in nitrite and nitrate, elevated the concentrations of these ions in the zone between stations 10 and 15. In all the stations both ions showed a negative correlation with salinity and positive correlation between them. Very high particulate nitrogen concentrations in the inner zone, relative to the seaward region, indicate that the nitrogen transformation processes (like ammonification, nitrification and denitrification) in the lagoon sediments were low. The inorganic phosphorus in the water was at very low concentrations, as usually found in karstic regions where the calcium carbonate traps the phosphates14, 15, 24 and inverse to the behavior in zones of igneous terrigenous materials 25 . We also observed a negative correlation of the phosphate concentrations with the water pH, which is in agreement with the process just mentioned, because at higher pH, higher is the carbonate concentration and then occurs more calcium carbonate precipitation together with phosphate. High concentrations of silicate were found in the water because of the ground water springs that are a source of these ions, which keep in solution long time 26; in this way, the silicate presented a high

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correlation (r=0.81) with salinity that suggests a concentration rise caused by evaporation or other inputs and a decrease between salinities 60-100 psu (stations 19-25; Fig. 2) caused perhaps by precipitation. Suspended solid levels were very high (the average concentration was 44.5 ± 66.9 mg l-1 ) but in the inner zone of the Ria Lagartos lagoon this level raised to more than 300 mg l-1 ; the inorganic-organic proportion of these solids was near 1:1, meaning that both are important. BOD had a wide range, from low levels (< 1 ml l-1) in stations near the sea, to high values (> 3 ml l-1 ) in the eastern zone, where the suspended solids were also high, because of the isolation of the waters. Surface sediments of the lagoon were highly porous; since the principal components were very fine sand, silt and clay (correlation of porosity with clay was 0.63). The redox potential values indicated that denitrification and sulphate reduction were occurring in these sediments 27, 28. For this region the organic matter in the sediments is reported14 to be approximately 5%, in almost all the stations of Ria Lagartos this was the concentration, although the general tendency was an increase of these levels from the area of the connection with the sea toward the inner zone of the lagoon, suggesting that there are processes leading to accumulation of organic matter in these sediments. Statistical analysis showed that where the pH of water was high, the organic matter in sediment was also high. Total nitrogen in the sediments showed similar behavior to organic matter (correlation between the two parameters was high, 0.86), lower concentrations in the zone near the sea, and higher concentrations in the inner zone. Exceptions were stations near the connections with the sea, which presented high concentrations; this could be due to the proximity of the two fishing towns that are on the lagoon, San Felipe and Rio Lagartos. Statistical analysis showed a significant inverse correlation between total phosphorus in the sediments with salinity, organic matter and total nitrogen. The decrease of phosphorus with increasing salinity is in agreement with the relation found by Millero et al. 24 because of the variations in the concentrations of bicarbonate ions, which changes the adsorptiondesorption of phosphate (Millero et al.24 found that at low salinity the adsorption of phosphate by calcite is higher than at high salinity, relevant in Yucatan because of its karstic origin). The ratio N:P and the ratio C:P, had a positive correlation (r = 0.93) and also separated the stations in three groups (Fig. 7). The first

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INDIAN J. MAR. SCI.VOL. 33, No 4, December 2004 70 29

Nitrogen-Phosphorus

60 30

50

25

Stations

40 4

30 20

2

19 17 26 27

23

28 24

22

18 21 16 20 15 1413 12 10 8 5 11

9 7 1

10

6 3

0 -20

20

60

100

140

180

220

Carbon-Phosphorus Fig. 7.- The correlation and the separation of stations with the total nitrogen-total phosphorus ratio versus carbon-total phosphorus ratio in the sediments of Ria Lagartos. Values are averages of the three seasons. Table 3.- Average fluxes and transformation rates in the sediments of Ria Lagartos, during the three seasons sampled. [ N.D. : Not detectable]. Ammonium flux (µmol m -2 h-1)

Phosphate flux (µmol m -2 h-1)

Nitrification (µmol m -2 h-1)

Denitrification (µmol m -2 h-1)

Average

250.0

0.67

150.5

47.4

Std. Dev.

149.4

1.31

131.8

33.0

Minimum

5.4

N.D.

10.2

4.8

Maximum

661.2

8.86

529.7

153.8

group had low nitrogen and high phosphorus, and comprised by the stations of the inlet zone and near to the villages (1, 7, 3, 6, 9, 2 and 5). The second one was formed by the stations with intermediate values of nitrogen and phosphorus (most of the stations). And the third group was formed by stations with high N:P (above 30) and C:P ratios above 140. In this last group were the stations of the inner zone of Ria Lagartos (22, 23, 24, 25, 28, 29 and 30). These results indicate that a phosphorus source exists, that could be the Gulf of Mexico water, or the sewage of the fishing villages. Most of this phosphorus is trapped in the sediments of the seaward zone. The estimated flux of ammonium and phosphate ions was from sediment to the water column. The general trend was that the lowest fluxes for both ions were in the higher salinity zone because their interstitial water concentrations were lower. As expected, the phosphate flux had a direct correlation with the total phosphorus concentration in the sediment. The inverse correlation found, between this flux and the sediment pH, is explained by the increase

of the adsorption of phosphate at the pH range 7.4 8.6 in calcareous sediments reported by Millero et al .24. Nitrification rate in the water-sediment interface was high but variable. Ammonium flux and nitrification were correlated (r = 0.42), meaning that in sediments with more available ammonium, the nitrification rates were higher 29. Dissolved oxygen and nitrification also presented a positive correlation (r = 0.53) and the statistical analysis separated the stations of the east (inner zone) that have low oxygen levels and low nitrification rates, from the rest of the stations. The estimated denitrification rates were lower than nitrification rates by a factor of 3, and the average ammonium liberation was almost two times the average nitrification (Table 3) The large differences between the three processes: ammonium production, nitrification and denitrification, indicated that the recycling of nitrogen is very large and the output is small. This explains the large amount of nitrogen that was found in the water and sediments of the lagoon. Even though phosphorus was present in the sediments

VALDES & REAL: N AND P IN WATER AND SEDIMENTS in reasonable concentrations, the phosphate liberation was so low, meaning that most of this element is trapped by the sediments; the lagoon is a sink for phosphorus. Even though Ria Lagartos lagoon has very few effects of industries and other human activities26, the small water exchange with the Gulf of Mexico and the poor input of freshwater, makes the lagoon prone to eutrophication. The high concentrations of nitrogen and phosphorus in the sediment, together with the low denitrification rates found in this study, are important indicators that this lagoon must be protected and conserved. Acknowledgment We thank to the “Comision Nacional de Biodiversidad” of Mexico (Conabio) for the support for this study. We thank to the “Secretaria del medio ambiente y recursos naturales” of Mexico (Semarnat) for help in sampling. The following people participated in sampling: Rodrigo Valdes Real, Miguel Herrera Rodriguez, Hector Hernandez Arana and Laura del Carmen Sanchez Garcia. References 1

Millero F J, Chemical oceanography 2nd , (CRC Press, Inc, Boca Raton, Florida, USA) 1996, pp. 469. 2 Justic D, Rabalais N N & Turner R E, Stoichiometric nutrient balance and origin of coastal eutrophication. Mar Poll Bull, 30 (1995) 41-46. 3 Van-Raaphorst W & Kloosterhuis H T, Phosphate sorption in superficial intertidal sediments. Mar Chem , 48 (1994) 1-16. 4 Arenas F V & De La Lanza G, El metabolismo como determinante de intercambio de nutrientes en sedimentos ricos en materia orgánica en una laguna costera. Cienc Mar , 16 (1990) 45-62. 5 Perry E, Velasquez-Oliman G & Marin L, The hydrogeochemistry of the karst aquifer system of the northern Yucatan Peninsula, Mexico. Int Geol Rev, 44 (2002) 191-221. 6 Corredor J E & Morell J, Assessment of inorganic nitrogen fluxes across the sediment-water interface in a tropical lagoon. Estuar Coast Shelf Sci, 28 (1989) 339-345. 7 Stirling H P, Chemical and biological methods of water analysis for aquaculturalists, (Institute of Aquaculture. University of Stirling. Stirling Scotland) 1985, pp. 89. 8 Strickland J D H & Parsons T R, A practical handbook of seawater analysis, Bulletin 167, (Fisheries Research Board of Canada, Ottawa), 1972, pp. 310. 9 Garcia H E & Gordon L I, Oxygen solubility in seawater: Better fitting equations. Limnol Oceanogr, 37 (1992) 1307-1312. 10 Brassard P, Measurement of Eh and pH in aquatic sediments, in Manual of physico-chemical analysis of aquatic sediments, edited by A Mudroch, J M Azcue & P Mudroch (CRC Press, Inc. Lewis Publishers), 1997, pp. 47-69.

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11 Law C S, Rees A P & Owens N J P, Temporal variability of denitrification in estuarine sediments. Estuar Coast Shelf Sci, 33 (1991) 37-56. 12 Corredor J E & Morell J, Inorganic nitrogen in coral reef sediments. Mar Chem, 16 (1985) 379. 13 Buchanan J B, Sediment analysis, in Methods for the study of marine benthos. Second Edition, edited by N A Holme & A D Mcintyre, (Blackwell Scientific Publications, Oxford), 1984, pp. 41-65. 14 Valdes D S & Real E, Ammonium, nitrite, nitrate and phosphate fluxes across the sediment water interface in a tropical lagoon. Cienc Mar, 20 (1994) 65-80. 15 Berner R A, Early diagenesis -A theoretical approach-, ( Princeton University Press, Princeton, N J ), 1980, pp. 241. 16 Li Y H & Gregory S, Diffusion of ions in seawater and in deep-sea sediments. Geochim. Cosmochim. Acta, 38 (1974) 703-714. 17 Parsons T R, Maita Y & Lali C M, A manual of chemical and biological methods for seawater analysis, (Pergamon Press, London) 1984, pp. 173. 18 Andersen J M, Rates of denitrification of undisturbed sediment from six lakes as a function of nitrate concentration, oxygen and temperature. Arch Hidrobiol, 80 (1977) 147-159. 19 Sloth N P, Nielsen L P & Blackburn T H, Nitrification in sediment cores measured with acetylene inhibition. Limnol Oceanogr, 37(1992) 1108-1112. 20 StatSoft, Statistica for windows, (SatSoft Inc., USA), 1994, pp. 3911. 21 Bricker O P, Redox potential: Its measurement and importance in water systems, in: Water analysis, Vol 1, Inorganic species, edited by R A Minear & L H Keith, (Academic Press Inc, Florida, USA), 1982, pp. 55-79. 22 Valdes D S & Real E, Variations and relationships of salinity, nutrients and suspended solids in Chelem coastal lagoon at Yucatan, Mexico. Indian J Mar Sci, 27 (1998) 149-156. 23 Tran K C, Valdes D S, Euan J, Real E & Gil E, Status of water quality at Holbox Island, Quintana Roo State, México. Aquat Ecosys Health Manag, 5 (2002) 173-189. 24 Millero F J, Huang F, Zhu X, Liu X & Zhang J, Adsorption and desorption of phosphate on calcite and aragonite in seawater. Aquat Geochem, 7 (2001) 33-56. 25 Windom H & Niencheski F, Biogeochemical processes in a freshwaterseawater mixing zone in permeable sediments along the coast of Southern Brazil. Mar Chem, 83 (2003) 121-130. 26 Smith S V, Marshall-Crossland J I & Crossland C J, Mexican and Central American coastal lagoon systems: Carbon, nitrogen, and phosphorus fluxes (Regional Workshop II), LOICZ Reports & Studies No. 13, (LOICZ IPO, Texel, The Netherlands) 1999, pp. 115. 27 MacIntyre F, in: Ocean Science, (W H Freeman and Company, San Diego, California, USA), 1977, pp. 104. 28 LyimoT J, Pol A & Op-den-Camp H J M, Methane emission, sulphide concentration and Redos potential profiles in Mtoni mangrove sediment, tanzania, West Indian Ocean J Mar Sci, 1 (2002) 71-80. 29 Valdes D S & Real E, Variation of nitrification rates in Chelem lagoon, Yucatán, México. Indian J Mar Sci, 28 (1999) 424-428.