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Terrestrial and oceanic influence on spatial hydrochemistryand trophic status in subtropical marine near-shore waters

Sara M. Morales-Ojeda, Jorge A. Herrera-Silveira*, Jorge Montero

Laboratorio de Produccion Primaria CINVESTAV-IPN, Unidad Merida., Carr. Antigua a Progreso km. 6, C.P. 97310 Merida, Yucatan, Mexico

a r t i c l e i n f o

Article history:

Received 14 March 2010

Received in revised form

2 July 2010

Accepted 14 July 2010

Available online 23 July 2010

Keywords:

Water quality

Trophic status

Near-shore waters

Multivariate analysis

Yucatan

* Corresponding author. Tel.: þ52 999 942946E-mail addresses: smorales@mda.cinvest

mda.cinvestav.mx (J. Montero).0043-1354/$ e see front matter ª 2010 Elsevdoi:10.1016/j.watres.2010.07.046

a b s t r a c t

Terrestrial and oceanic influences like groundwater discharges and/or oceanic upwelling

define the hydrochemical and biological characteristics of near-shore regions. In karst

environments, such as the Yucatan Peninsula (SE Mexico), the balance between these two

influences on spatial and temporal scales is poorly understood. This study focused on near-

shore waters within 200 m offshore along the Yucatan coast. The trophic status and

hydrochemical zones of the study area were determined as a function of physical and

nutrient data collected from 2005 to 2006. The main terrestrial influence was groundwater

discharge, while the most important marine influence was related to seasonal changes in

water turbulence. Spatial differences ( p < 0.05) were observed among salinity, light

extinction coefficient (k), NO3�, NH4

þ, and Chl-a. Seasonal differences were observed for all

variables except for k. During the dry season, terrestrial influences are the dominant factor

on near-shore hydrochemistry. The region around Dzilam exhibited the maximum influ-

ence of groundwater discharge estimated by salinity dissolution (d). During the rainy and

“nortes” seasons, there is a balance between oceanic and terrestrial influences. The trophic

status measured using the TRIX index, indicated that near-shore waters were mainly oligo-

mesotrophic; with a meso-eutrophic status in areas with documented anthropogenic

impacts. Four hydrological zones were identified by a Canonical Variate Analysis (CVA)

using salinity, NO2�, k and NH4

þ as the main discriminating variables. Zones I and II showed

almost pristine conditions, with well-balanced terrestrialeoceanic influences. In Zone III,

terrestrial influences such as groundwater discharges and inland pollution suggesting

human impacts were dominant respect to the effects of oceanic influences like upwelling

and sediment resuspension caused by winds and oceanic currents. Zone IV received

enhanced groundwater and associated nutrients. Anthropogenic activities have led to

ecosystem degradation but the speed at which this occurs depends on local and regional

characteristics. Therefore, this study has defined those characteristics so as to enact better

management policies.

ª 2010 Elsevier Ltd. All rights reserved.

2 (direct line); fax: þ52 999 9812334.av.mx (S.M. Morales-Ojeda), jherrera@mda.cinvestav.mx (J.A. Herrera-Silveira), jmontero@

ier Ltd. All rights reserved.

Abbreviations and notations

SE Southeast

CMW Coastal marine waters

SGD submarine groundwater discharges

NO2� nitrite (mmol l�1)

NO3� nitrate (mmol l�1)

NH4þ Ammonia (mmol l�1)

SRP soluble reactive phosphorus (mmol l�1)

N:P Redfield ratio

SRSi soluble reactive silica (mmol l�1)

k light extinction coefficient (m�1)

Chl-a chlorophyll-a (mg m�3)

O2 oxygen (mg l�1)

DOS dissolved oxygen saturation (%)

D salinity dilution (no units)

pH inverse logarithm of the activity of the hydrogen

ion Salinity (no units)

Temperature (�C)TRIX Vollenweiner trophic index (no units)

DCA detrended correspondence analysis

CVA canonical variate analysis

PCA principal component analysis

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45950

1. Introduction Ojeda, 2009). Groundwater has been estimated to contribute

In coastal waters, the bidirectional flux of materials and

energy between land and water, as well as the aquatic life and

human activities are closely related. This zone is conceived as

a region where complex interactions between terrestrial

variables (nutrients and sediments inputs) and oceanic factors

(tides and currents) take place (Valiela et al., 1992). These

interactions between forcing functions from the land and sea

show spatial and temporal variation, contributing to regional

or local vulnerability to human activities which threat the

water hydrochemistry (White et al., 2004).

The hydrochemistry of a specific regionmay depend on the

input of pollutants from land to coastal waters, water resi-

dence time, and the sources of fresh and/or marine water (e.g.

riverine, groundwater, upwelling, oceanic). In temperate

regions, the disturbance on the hydrodynamics and water

quality characteristics of coastal waters by chemical, physical

and biological stressors can cause shifts from oligo to eutro-

phic conditions (Xua et al., 2004).

Water quality and trophic status measurements of coastal

andmarine waters are often used to assess and evaluate their

conditions for a better management of the coastal andmarine

resources (Paerl, 2006). Traditionally, coastal hydrochemistry

assessments focused mainly on point sources of nutrients

from land, and less attention has been given to the nutrient

sources related to marine processes, or the interaction of both

land and oceanic sources in areas influenced by submarine

groundwater discharge (SGD) (Chen, 1996; Loubere, 2000;

Cullen, 2002).

One of the distinctive features of the Yucatan Peninsula is

the karstic substrate that allows rapid water infiltration and

negligible surface runoff. SGD is the main pathway of fresh-

water from land to coastal ecosystems in the Yucatan and

other places like Bosnia Herzegovina, Sierra de Libar, Spain,

Haute-Normandie region and France where karst is dominant

along the coast. However, little is known about the processes

and variables associated with the ecosystem health and the

relative importance of the terrestrial and oceanic influences

coastal tropical karstic regions (Calo and Parise, 2009; Andreo

et al., 2006; Valdes et al., 2007).

The highest freshwater input from SGD is usually found

near the shoreline. In the Yucatan Peninsula, SGD is the main

source of freshwater into the coastal ocean and it is canalized

and focused in small springs (Herrera-Silveira and Morales-

about 90% of the near-shore freshwater inputs to the north

coast of Yucatan, while coastal lagoons contribute 5%, surface

run off 4%, and harbors 1%. However, the main source of

freshwater and nutrients in the Yucatan coast is groundwater,

the differences in water quality are directly related to land-

use, residence time of water, and weather (Aranda-Cirerol

et al., 2006; Herrera-Silveira and Morales-Ojeda, 2009).

Another important feature of the SGD system found in

Yucatan is the “ring of cenotes” which is characterized by

large discharges of freshwater into the coast, mainly in areas

where the cenotes ring area meets the coastal line (i.e.,

localities of Celestun and Dzilam) (Pacheco Martinez and

Alonzo Salomon, 2003).

The objective of this work was to determine the impor-

tance of terrestrial and oceanic influences on the spatial

heterogeneity and trophic status ofmarine subtropical coastal

waters of a karstic region as the northern Yucatan. This

research would provide a better understanding of the

connectivity between terrestrial and oceanic processes and

characteristics of subtropical karstic environments that will

further promote the establishment of ecological base lines for

coastal management and monitoring programs in similar

karstic coastal areas.

2. Materials and methods

2.1. Study area

This study was carried out along the northern Yucatan coast,

in the southwest Gulf of Mexico (Fig. 1). The coast is 365 km

long and represents only 3.3% of the entire Mexican coastline.

There are three well-defined climate seasons in this region:

dry season (MarcheMay), rainy season (JuneeOctober) and

nortes season when cold fronts dominate (NovembereFeb-

ruary). The predominant winds come from the Bermuda-

Azores anticyclone; in general, the wind is of low intensity

(0e15 km h�1) and comes from the southeast, and produces

low-energy waves. The tide is mixed semidiurnal with a range

from 0.4 to 0.8 m (Capurro, 2002).

The dominant terrestrial influence onmarine coastalwaters

is via SGD, which is approximately 8.6 � 106 m3 km�1 year�1

(Hanshaw and Back, 1980). SGD is characterized by low salinity

and high nitrate and silicate concentrations. The influence of

Fig. 1 e Distribution of sampling stations along the state of Yucatan’s coast showing only the names of the population zones

and inland disposition of the sinkholes ring.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5951

SGDoncoastal ecosystems isa functionof their proximity to the

dischargeandthe typeofdischarge (pointornon-point).Aquifer

recharge occurs mainly during the rainy season (Herrera-

Silveira et al., 1998; Herrera-Silveira and Comin, 2000). Diffuse

discharge (non-point) as seepage occurs when water infiltrates

through fractures in the calcareous rock, as occurs in Progreso

(Perry, 1990; Troccoli et al., 2004). Springs (point discharge) are

commonin this region, andare found in lagoonsandnear-shore

waters such as Dzilam. There is very little surface runoff due to

rapid infiltration of rain, however, there is some surface runoff

from mangrove fringe areas into coastal lagoons. The cenotes

ring also influences the subterraneanwaterflow (Fig. 1). Human

activities in land and coastal regions as eco-tourism, fisheries,

salt extraction, port development, seasonal high-impact

tourism, and cattle rancheshavenegatively impacted thewater

quality of the aquifer and consequently the SGDhas become an

important source of inorganic nutrients; registering spatial

differences of their load according with the spatial distribution

of the type and human activity intensity (Aranda-Cirerol et al.,

2006, 2010; Herrera-Silveira and Morales-Ojeda, 2009).

The main oceanic processes that affect the biochemistry

and hydrochemistry of coastal waters are the supply of

nutrients to the continental shelf arriving from the southern

Gulf of Mexico, and the Yucatan Current, which mainly

occurs during spring season (MarcheMay), when coastal

upwelling occurs near Cabo Catoche (east of the Yucatan

Peninsula) (Cochrane, 1969; Logan, 1969; Ruiz, 1979; Merino,

1997). Cold fronts (nortes) produce intense coastal currents

and tides which, in conjunction with hurricanes, favor

sediment resuspension and water clarity changes, and

intensify the export of nutrients and organic materials from

coastal lagoons and swamps to the marine coastal waters,

favoring fertilization of the area (Odum, 1972; Morales-Ojeda,

2004).

2.2. Sampling methodology

Surface samples were collected at twenty stations (S) at 3

different distances from the coast (50, 150 and 200 m) and five

times during different seasons from May 2005 to February

2006 (300 samples in total). Due to the shallowness of the shelf

(<3 m), the water column is mixed and no stratification has

been reported (Herrera-Silveira et al., 2004). Temperature (�C),salinity (expressed as dimensionless units), dissolved oxygen

(mg l�1), oxygen saturation (%), and pH measurements were

taken “in situ” with a multi-parameter probe (YSI 6600). Light

extinction coefficient (k, m�1) was recorded using a LICOR LI-

1000 spherical sensor. Samples were collected using 1 L Van

Dorn bottle and preserved in nalgene bottles in cold and dark

conditions until laboratory analysis. Water samples were

filtered through aMilliporemembrane (0.45 mmpore size), that

was later used for the Chlorophyll-a (Chl-a, mg/m3) extraction

methodwith 90% acetone. Chl-a concentrationwas calculated

with the equations of Jeffrey and Humphrey (1975). The

filtered water was used to analyze dissolved inorganic nutri-

ents (ammonia-NH4þ, nitrite-NO2

�, nitrate-NO3�, soluble reac-

tive phosphorus-SRP, and silica-SRSi, all expressed as

mmol l�1) as described by Strickland and Parsons (1972); dis-

solved inorganic nitrogen (DIN) was obtained by the addition

of NH4þ, NO2

� and NO3�.

2.3. Data analysis

To know if a salinity gradient could be observed in our sample

design, dilution plots (salinity vs nutrient concentrations)

were constructed as exploratory analysis. In case that no

differences between distances were detected, the samples

from each distance were considered as replicates for the

following data analysis.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45952

Area characterization was conducted through the identi-

fication of spatial and temporal variation of hydrochemical

(NH4þ, NO2

�, NO3�, SRP, SRSi) and trophic condition indicators

(Trophic index) (Vollenweider et al., 1998). Significant differ-

ences in variables among sites and/or sampling times were

identified through pair wise test using KruskaleWallis.

Since SGD is an important terrestrial influence on near-

shore hydrochemistry, salinity dilution (d) was used to esti-

mate the freshwater content of the marine near-shore waters

at each sampling site (Aranda-Cirerol et al., 2006); where dM is

the average of salinity dilution for the study period at each

sampling site; SM is the maximum salinity shown in the

course of time and SS is the sample salinity.

dM ¼ SS � SM=SS

Amultivariate analysis, which simplifies amultidimensional

space, was used to characterize the sampling sites according to

their hydrochemical characteristics. The first step was to

determine the gradient length with a DCA (detrended corre-

spondence analysis), which is an indirect ordination technique

that shows the maximum dispersion (using standard devia-

tions) of the sample scores. Since the first ordination axis was

<4 standard deviation, a linear behavior was identified, and so

an indirect method, like the Principal Component Analysis

(PCA) is recommended. The PCA was centered and standard-

ized (using a correlationmatrix) in order tominimize the effect

of the different variances of the measured variables, which are

in different scales (ter Braak and Smilauer, 2002). And so, PCA

was used as an exploratory technique to examine the main

“loading” or Pearson’s r correlation between the components

(i.e., component scores for each object) and the original vari-

able; high loading values indicate a strong correlation among

the variables and a particular component, therefore, the

loading could be used to examine which of the original vari-

ables had a strong contribution to each component (Quinn and

Keough, 2003). PCA was used to identify the variables that

explain the temporal hydrochemical behavior of the near-

shore Yucatan coasts.

To identify spatial zoning according to hydrochemical

variables, a cluster analysis was carried out using Euclidean

distances as similarity distances in a complete linkage

(furthest neighbour) clustering method, and represented in

a cluster tree. The included variables were station average

across all five sampling periods of temperature, salinity, light

extinction coefficient (k), oxygen saturation (%), NO2�, NO3

�,NH4

þ, SRP, SRSi, and Chl-a. The data were previously trans-

formed [log (X þ 1)]. The analyses were performed with the

Statgraphics Plus V 4.1 software package (Statistical Graphics

Corp., 1994e1999).

After the groups were identified by clustering, a Canonical

Variate Analysis (CVA) was used as a complement of the

clustering results, this approach testing which variables

discriminated best among the groups formed. Then, once no

multicollinearity was identified among the data (VIF < 20), the

CVA was run with Hill’s scaling, focusing on inter-variable

distances, with a full permutation model for the Monte Carlo

test with 999 unrestricted permutations. For the variable

selection, those with p values <0.05 were chosen. Canonical

Variate Analysis provided an objective assessment of the

similarity between the “a-priori” groups (formed by clustering)

as dummy variables (ter Braak and Smilauer, 2002). The above

approach was followed due to studies related to ecosystems

involve a multidimensional matrix that cannot be efficiently

analyzed through reductionist methods, which focus on the

properties of each isolated variable. Themultivariatemethods

allowed us to find relationships among response variables,

sample units or variables and sample units; for each case,

a specific method was required. DCA, PCA and CVA were

performed with the CANOCO V 4.51 software package (ter

Braak and Smilauer, 2002).

Trophic status of the Yucatan coast was estimated using

the TRIX index (Vollenweider et al., 1998). This index is a linear

combination of chlorophyll-a concentration, nutrient (DIN

and SRP) log scale concentrations, and dissolved oxygen

levels. This combination of four state variables describes

primary production (chlorophyll-a and oxygen), and nutri-

tional conditions (dissolved inorganic nitrogen and inorganic

phosphorus) (Giovanardi and Vollenweider, 2004). The TRIX

index was calculated as follows:

TRIX ¼ ðLog10½Chl� a$aD%O$DIN$P� þ xÞ=mWhere P¼ soluble reactive phosphorus (mg l�1); DIN¼ dissolved

inorganic nitrogen (mg l�1); Chl-a¼ chlorophyll-a concentration,

as mg l�1; aD%O ¼ absolute % of oxygen deviation from satu-

ration; x and m ¼ scale coefficients were introduced to fix the

lower limit value of the index and the extension of related

trophic scale from 0 to 10. This index was calculated for the

whole area and then for each particular zone. x ¼ 1.5 and

m ¼ 1.2.

In the original paper of Vollenweider et al. (1998), the

formulation of the index allowed the use of nitrogen and

phosphorus both in their mineral as in their total form, in the

case of nitrogen, as dissolved inorganic nitrogen (DIN) and

total nitrogen (TN), and in the case of phosphorus, as soluble

reactive phosphorus (SRP) and total phosphorus. In this study,

DIN and SRP were used and obtained an average difference of

0.5 TRIX units, that is to say that a TRIX value of 5 units

obtained using TN and TP, corresponds to a value of 4.5 units

using DIN and SRP, so this correction was applied.

Nevertheless, TRIX was mainly developed for coastal and

near-shore marine environments; it has been used in

ecosystems with different trophodynamic regimes, such as

coastal lagoons (e.g. Venice Lagoon; Bendoricchio and De Boni,

2005).

3. Results and discussion

3.1. Characterization

Almost all hydrochemical variables exhibited seasonal

differences (Table 1); for example, in the nortes season, cold

fronts reduced water temperature and light penetration.

Similar processes have been recorded in Florida Bay, where

shallowness favors sediment resuspension and turbidity

during windy events (Boyer et al., 1999). Water temperature

clearly displayed seasonal differences (Fig. 2a) with the lowest

mean value (21.8 �C) during the cold-front season (February),

Table 1 e The KruskaleWallis test for spatial and seasonal differences; p values are shown and the statistic H.

Spatial Temporal

Variable Median SE H p H p

Temperature (�C) 28.61 0.19 84.46 0.98 272.50 <0.05

Salinity 36.61 0.11 33.90 <0.05 177.40 <0.05

Light extinction coefficient as k (m�1) 1.12 0.05 54.45 <0.05 5.91 0.20

pH 8.24 0.01 18.08 0.52 212.67 <0.05

Dissolved oxygen saturation (%) 126.78 2.68 29.53 0.06 125.15 <0.05

Nitrite (mmol l�1) 0.22 0.02 15.94 0.66 191.8 <0.05

Nitrate (mmol l�1) 1.25 0.09 87.89 <0.05 80.32 <0.05

Ammonium (mmol l�1) 1.23 0.19 36.46 <0.05 50.65 <0.05

Dissolved inorganic nitrogen (mmol l�1) 3.46 0.25 30.64 <0.05 58.10 <0.05

Soluble reactive phosphorus (mmol l�1) 0.25 0.01 12.83 0.85 112.23 <0.05

Soluble reactive silica (mmol l�1) 9.7 0.44 29.54 0.58 132.90 <0.05

N:P ratio 17.03 4.06 17.39 0.06 82.61 <0.05

Chlorophyll-a (mg m�3) 3.11 0.25 44.44 <0.05 68.99 <0.05

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5953

and the highest mean value (31.1 �C) in the rainy season

(August) (Fig. 2a). Spatially, the lowest (27.6 �C) median

temperatures occurred in Chabihau (S10) and the highest

(29.5 �C) in Las Bocas 1 (S12) (Fig. 2b). The differences are likely

related to average depth (<2 m in Dzilam) and the occurrence

of SGD, which is characterized by low temperatures (<20 �C)(Herrera-Silveira, 1994; Aranda-Cirerol et al., 2006). In spite of

the fact that groundwater inputs reduce water temperatures,

it was similar to other subtropical coastal areas (Boyer et al.,

1999).

Salinity showed spatial and temporal variations. Season-

ally, mean salinity ranged from 35 during the end of the rainy

season (October) and early nortes season (November), to 37.8

during the dry season (May) (Fig. 2c). The lowest salinity

values were observed in the Dzilam region (from S10 to S15),

and the highest in the San Felipe-Rio Lagartos area (S16)

(Fig. 2d). High groundwater flow through the cenotes ring is

evident by an abundance of submarine springs at the inter-

section of the ring and the near-shorewaters in Xixim (S2) and

Las Bocas de Dzilam area (S12eS13), where salinity is highly

variable (Fig. 2d).

Seasonally the lowest salinity dilution d values (0.06) were

registered in May (dry season) and the highest values (0.13)

were observed in November (end rainy period) (Fig. 3e). High

values were calculated from Palmar (S3) to Progreso (S7) and

for Celestun (S1) in August, and in Xixim (S2) in February, as

previously observed by Aranda-Cirerol et al. (2006) (Fig. 2f).

The salinity dilution (d) showed little influence of ground-

water discharge in San Felipe (S16, d mean ¼ 0.06), Coloradas

(S18, d mean ¼ 0.07), and Rio Lagartos (S17, d mean ¼ 0.08),

whereas in Dzilam (S11) showed the largest groundwater

discharge (d mean ¼ 0.17), followed by Las Bocas (S12eS14,

d¼ 0.1) (Fig. 2f). The assessment of the salinity dilution (d) factor

could be non-precise due to the assumption of a constant end-

member value, or at least an end-member salinity is not

influenced by external variation. This is a problem in estuarine

areas where the end-member salinity station is set too close to

land, as occurs in this study. However, the results (Fig. 2e, f) are

a valuable indication of the fresh-marine water mix in this

region, characterized by submarine groundwater inputs.

Spatial differences in groundwater recharge and discharge

flows may be associated with geological variability along the

cenotes ring, due to high density of springs and breaks in sand

bars at the intersection of the ring with the sea as occurs in

Celestun and Dzilam (Perry et al., 1995; Pacheco Martinez and

Alonzo Salomon, 2003). Thus, these natural features affect

groundwater flow by diverting more than the 80% of the

groundwater flowing across the ring and discharging it to the

sea (Marin et al., 1990).

The spatial variability of coastal salinity has been observed

to be influenced by marine waters and point sources of

freshwater such as, rivers, creeks and springs (Boyer et al.,

1999; Kelble et al., 2007), suggesting that, in the Yucatan

coast, water exchange with the Gulf of Mexico, the Caribbean

Sea, point sources (springs), and non-point sources (mangrove

runoff) of freshwater, should exert strong influence on the

spatial variability and zoning of the hydrochemical conditions

observed in Yucatan near-shore waters.

According to the spatial distribution of temperature and

salinity, the main locations where SGD and runoff influence

near-shore hydrochemistry were El Palmar area (S2 and S3),

and from Chabihau (S10) to Punta Bachul (S15). Thus, is

probably that salinity variations are controlled by local

geomorphology, such as the cenotes ring and coastal features

as mangrove zones. The area with least influence of SGD and

runoff was the Sisal-Chuburna area (S4eS6), and from Ria

Lagartos (S17) to El Cuyo (S20), suggesting that, in this area,

local weather and oceanic influence (marine currents and cold

fronts) could be responsible for the variability in temperature

and salinity.

The lowest average light extinction coefficient (k) was

recorded in May (0.98 m�1), whereas the highest average k was

recorded in November (1.3 m�1). Although these differences

were not significant, low k values are related to stable water

conditions that occur during dry periods, while in the nortes

season high k values were the result of turbulence induced by

the strong winds that characterize this season (Troccoli et al.,

2004; Alvarez-Gongora and Herrera-Silveria, 2006; Aranda-

Cirerol et al., 2006) (Fig. 2g). Spatially, the highest k values

were associatedwith coastal lagoon inlets (stations S1, S6, S17),

harbor discharge (stations S7, S10, S20) and surface runoff

(stations S12, S13, S14) (Fig. 2h), while low k values were

recorded in areas with a greater coverage of submerged aquatic

vegetation, which reduce sediment resuspension, as in the

Fig. 2 e Seasonal and spatial variation of temperature, salinity, salinity dilutions (d), light of extinction coefficient, and

oxygen saturation in the study area. Seasonal variation are referred as May [ dry; August [ rain; October and

November [ “nortes”. For variables which show significant differences, different letters indicate differences ( p < 0.05)

from the pair wise test.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45954

Dzilam-San Felipe zone (S12eS16) (Aguayo, 2003; Cardoso et al.,

2004; Medina-Gomez and Herrera-Silveira, 2006). Similar

seasonal and spatial patterns of water transparency were

recorded in Florida Bay and coastal lagoons of the Gulf of

Mexico; in both cases, cold fronts, shallowness, and submerged

aquatic vegetation were associated with the observed light

extinction values (Boyer et al., 1997; Medina-Gomez and

Herrera-Silveira, 2003; Tapia et al., 2008).

High oxygen saturation values (up to 154%), which corre-

spond to high dissolved oxygen (up to 6.7 mg l�1), were

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5955

recorded during the dry season (May) and cold-front season

(February), while the lowest values (77.8%, 3.5mg l�1) occurred

at the end of the rainy season (November) (Fig. 2i). Spatially,

the Sacna and Telchac locations (S8eS9) showed the lowest

mean values (94%), which are probably related to respiration

processes resulting from organic matter export from Telchac

harbor (Fig. 2j). In spite of the nutrient inputs, shallowness,

and SAV coverage, the oxygen concentrations in the water

column does not show hypoxic symptoms as in other coastal

ecosystems with similar environmental settings (Breitburg,

1990; Bilkovic et al., 2006; Bong and Lee, 2008), however,

ecosystem metabolism should be determined in order to

establish the net balance between production and respiration

processes (Duarte et al., 2005).

Nutrients variability was related to spatial and temporal

inputs from SGD, as well as intensity of anthropogenic activ-

ities. The lowest mean NO2� concentration (0.04 mmol l�1) was

recorded during the rainy season, whereas the highest value

(0.77 mmol l�1) was observed in the dry season (Fig. 3a).

Spatially, the lowest concentrations (0.09 mmol l�1) were in

Celestun (S1), and the highest (0.41 mmol l�1), in the region

from Sacna (S8) to Punta Yalkubul (S13) (Fig. 3b). The high NO2�

values are related to nutrient export from harbors and coastal

lagoons. Nitrate and ammonium concentrations showed

similar spatial and seasonal trends related to the rainy season,

Fig. 3 e Seasonal and spatial variation of nitrogen compounds (

variation are referred as May[ dry; August[ rain; October and

differences, different letters indicate differences ( p < 0.05) from

at the end of which (November) the highest concentrations

(>10 mmol l�1, >4 mmol l�1, nitrate and ammonium respec-

tively) were recorded (Fig. 3c and e).

High nitrate concentrations in Yucatan coast are associ-

ated with SGD and wastewater derived from human activities

(Herrera-Silveira, 1994; Herrera-Silveira et al., 2004; Aranda-

Cirerol et al., 2006), with the highest values (4e12 mmol l�1)

in Dzilam (S10eS13), where SGD from springs has been

observed, and in Sisal (S5), where a shrimp farm was estab-

lished in 2000 (Fig. 3d). In Yucatan, the main sources of nitrate

for groundwater are the excess in use of fertilizers and the

inadequate disposition of manure (Pacheco et al., 2002;

Aranda-Cirerol et al., 2010), some other factors which

control the pollution are dissolution for local recharge, the

water flux through the regional aquifer, denitrification in

unsaturated zones, as well as the hydrologic condition (Lewis

et al., 1980).

The high concentrations of ammonium were probably

related to surface runoff (S2, S13) and seepage discharges in

areas with a greater population density, such as Progreso (S7)

and San Felipe (S16) (Fig. 3f). The SGD could be a source of NH4þ

due to the organic matter decomposition in areas where the

marine currents are of low energy (Spiteri et al., 2008), as

happened in Progreso due to the harbor. However, in

temperate regions such as Waquoit Bay (USA), the loading of

nitrates, nitrites and ammonia) in the study area. Seasonal

November[ “nortes”. For variables which show significant

the pair wise test.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45956

nitrogen from watersheds could come from wastewater

fertilizer use and atmospheric deposition (Valiela et al., 1997).

Inorganic nitrogen sources in the near-shore waters have

been related mainly to terrestrial inputs; high nitrate

concentrations have been associated with groundwater

polluted with fertilizers and manure (Valiela et al., 2000),

whereas ammonium has been related to coastal urban areas

and aquaculture activities (Paez-Osuna, 2001; Bricker et al.,

2003). In Yucatan, both source could be responsible of the

inorganic nitrogen inputs due to the urban, agriculture

a farms developments lack wastewater treatment systems

(Herrera-Silveira et al., 2004; Aranda-Cirerol et al., 2010).

Average SRP concentration was higher (0.45 mmol l�1)

during the dry season (Fig. 4a) and was probably related to

water masses from the Cabo Catoche upwelling. Thus, only

one peak in SRP was recorded during the dry season

(Merino, 1997). Upwelled water only affects the eastern

region from El Cuyo to Las Bocas (S20eS12); and so, the high

SRP concentrations recorded in the western region from

Fig. 4 e Seasonal and spatial variation of phosphates, silicates

differences, different letters indicate differences ( p < 0.05) from

Progreso to Celestun (S7eS1) could be related to inputs from

runoff (S3), harbors (S11), or wastewater from the shrimp

farm in Sisal (S5), as well as SGD in coastal areas with high

population densities, where septic tank effluent infiltrates

the aquifer and discharges into the coast, as in Progreso (S7)

(Fig. 4b) (Alvarez-Gongora and Herrera-Silveria, 2006;

Aranda-Cirerol et al., 2006). Organic matter degradation is

often the major source of groundwater NH4þ and SRP (Spiteri

et al., 2008). The values reported for SRP in this study were

similar to those reported for karstic regions where high

alkalinity favored the precipitation and low concentrations

(<0.5 mmol l�1 of SRP; Cravo et al., 2003; Jones et al., 2003),

such as in near-shore waters from Aqaba Gulf, Israel

(0.48 mmol l�1) and Florida (Shellenbarger and Monismith,

2006). Nevertheless, high SRP concentrations have been

reported during harmful algal blooms (HABs) events

(6.44 mmol l�1) (Yentsch et al., 2008).

It is important to remark that the biogeochemistry of SRP in

SGDnutrientfluxes tocoastalwatersarestronglyaffectedbythe

and Chlorophyll-a. For variables which show significant

the pair wise test.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5957

redox conditions of the freshwater and seawater (Spiteri et al.,

2008), these processes has not been investigated in the region.

In Yucatan Peninsula, nitrates and silicates (SRSi) are

enriched in SGD and have been used as a tracer of freshwater

input to thecoastal ecosystems (Smithetal., 1999).Thiscouldbe

possible because in the tropics the rate of rockmineralization is

high and in the karstic regions chemical reactions are respon-

sible of silica dissolution from the marine calcareous rock

(White and Blum, 1995; Stonestrom et al., 1998; Asano et al.,

2003). Seasonal variation was identified with the highest mean

of SRSi concentration (16.8 mmol l�1) during the dry season,

possible related to water masses from the Cabo Catoche

upwelling; while the highest mean value in late rainy season

(>14 in November) was probably due to SGD, which is more

intense during this tiem period (Fig. 4c). Spatially, the highest

mean concentrations (>10 mmol l�1) occurred Dzilam (S10eS15)

where there is enhanced SGD from springs and runoff into

harbors and coastal lagoons (S4, S9, S10, S12) (Fig. 4d).

Nitrogen (N) and phosphorus (P) are potentially limiting

nutrients in estuarine and near-shore waters (EEA, 1999;

USEPA, 2001). In general, the plant-growth limiting nutrient

in freshwater ecosystems is phosphorus; whereas in coastal

waters, the limiting nutrient is frequently nitrogen (EEA, 1999;

Ryther andDunstan, 1971; Fisher et al., 1992; Nixon et al., 1996;

Vitousek et al., 1997). In the Yucatan coast, the N:P ratio

exhibited seasonal changes from less than 15 during the dry

season to 60 during the nortes season (Fig. 5a,c). Spatially, the

N:P ratio did not show a clear pattern; however, as the values

were in general higher than the Redfield ratio, it is probable

that P acts mainly as the phytoplankton-growth limiting

nutrient (Fig. 5b,d). It has been reported that in karst regions

such as the Yucatan Peninsula, SRP shows affinity for calcium

carbonate, which makes it less available in the water column

Fig. 5 e General and detailed scheme of seasona

(Vuorio et al., 2005). Nitrogen can be limiting for some regions

or during some seasons due to N loss by denitrification in

polluted areas. This can often cause changes in the N:P ratio

and ecological community structure, which may lead to HABs

when N is suddenly increased (Slomp and Van Cappellen,

2004). This phenomenon has been observed seasonally in

the Yucatan coast (Herrera-Silveira and Morales-Ojeda, 2009).

Average Chl-a concentrations showed seasonal trends,

with low average values during the early rainy season (August,

2.1 mg m�3) and high values during the end of the same

season (November 6.96 mg m�3), which could be related to

enhanced nutrient inputs during this period (Fig. 4e). Spatially,

the highest Chl-a concentrations (>10 mg m�3; Fig. 4f) were

measured around coastal lagoon inlets (S7, S9, S13) or harbors

(S6, S11, S20) and where there are known point sources of

groundwater. Nevertheless, the high Chl-a concentrations

found at stations S2, S8, S15 and S19 are likely associated with

phytoplankton blooms, which occur in areas influenced by

diffuse groundwater discharges, low light extinction and

relatively high nutrient availability, which frequently occurs

during the rainy season (Troccoli-Ghinaglia, 2001; Herrera-

Silveira and Morales-Ojeda, 2009).

The results indicate that the Yucatan coast is strongly

influenced by SGD and similar to other coastal aquifers

worldwide that are influenced by anthropogenic impacts, it is

becoming contaminated with nutrients from diverse sources

(e.g. fertilizer, manure, wastewater; Valiela et al., 1992).

However, it is poorly understood why the chemical composi-

tion of SGD discharge not only depends on the landward

freshwater sources, but also on the rates of groundwater flow

and the biogeochemical reactions that occur in the region of

the coastal aquifer where freshwater and seawater interact

(Moore, 1999).

l and spatial variation of redfield ratio (N:P).

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45958

Nutrient dynamics and Chl-a concentrations along the

northern coast of the Yucatan suggests that terrestrial inputs

can be divided into distinct zones such as: 1) areas affected by

runoff (mangrove areas, coastal lagoons inlets or harbors); 2)

locations with high population density where wastewater

inputs are greater; 3) locations with aquaculture activities and

4) areas where the cenotes ring intersects the coast and SGD is

enhanced. On the other hand, oceanic influence of the near-

shore ecosystem mostly shows seasonal variations in nutrient

dynamics. During the nortes season, remineralization

processes and sediment resuspension occur, the latter favoring

a reduction of the water clarity; whereas during the dry season,

upwellingwaters fromCabo Catoche are an important nutrient

source. Finally, during the rainy season, nutrient inputs to the

coast increased, particularly in locations with a higher pop-

ulation density, where septic tank effluent infiltrates the

aquifer and reaches the coast via SGD as seepage (Progreso),

and in areas where there is enhanced SGD from springs (Dzi-

lam). These terrestrial and oceanic influences play an impor-

tant role in the spatial and seasonal hydrochemical condition

of the northern cost of the Yucatan Peninsula, in consequence,

site-specific or region-specific patterns are observed, which

could be used to establish a framework for management

strategies and monitoring programs.

3.2. Spatial and temporal pattern analysis

Principal Component Analysis (PCA) was performed on the

entire data set to identify relationships between different

Table 2 e PCA results and seasonal and gentral interpretations.

Timeperiod

Principalcomponent

Explainedvariance

(%)

Mainvariables(loadings)

Process related

Annual PC1 23 Sal (-0.45)

SRSi (0.42)

NH4þ (0.36)

Inland: Ground

water influence

PC2 17 SRP (0.5)

Temp (0.38)

NO2� (0.37)

Water column

biogeochemistry

Nortes PC1 43 Sal (0.41)

SatO2 (0.4)

Temp (-0.39)

Harbor activities

impact

PC2 13 k (0.58)

SRP (0.54)

NO3� (-0.40)

Nutrient

resuspension

and lagoon

exportation

Dry PC1 23 SRSi (0.46)

k (0.43)

NO2� (0.37)

Inland: Ground

water influence

PC2 17 Chl-a (0.54)

SRP (0.53)

NO3� (0.34)

Water column

production

Rainy PC1 18 SRSi (-0.53)

NO2� (-0.47)

Sal (0.44)

Diffuse organic

imput

PC2 15 Temp (0.65)

NO3� (-0.45)

Chl-a (-0.31)

Inland: Ground

water influence

variables. Table 2 shows the differences between time periods

(annual, nortes dry and rain) in terms of the two first

components contribution and main variables loadings.

The PCA performed annually showed that PC1 was related

with salinity, NH4þ and SRSi, constituents usually associated

with SGD. This has been observed previously (Aranda-Cirerol

et al., 2006; Herrera-Silveira and Morales-Ojeda, 2009) were

low salinities and high SRSi concentrations indicating SGD

influence in Yucatan region. High SRSi content in SGD can be

the result of the dissolution of biogenic carbonate from the

calcareous rock, which is indeed constituted by deposition of

diatoms skeletons which occurred when the Yucatan penin-

sula was submerged (Herrera-Silveira, 1996; Smith et al., 1999).

High NH4þ concentrations may be the result of a non-effective

removal of NH4þ mechanism in oxygen-limiting environments

such as subterranean aquifers (Spiteri et al., 2008). For the

annual PC2, main variable association according to the load-

ings (SRP, NO2� and temperature), indicated possible water

column biogeochemistry processes like decomposition,

hydrolysis, nitrification denitrification. SRP in karst is often

precipitated in apatite but the availability depends on the redox

conditions (Spiteri et al., 2008). The adsorption process is very

fast, almost instantaneous, phosphate will adsorb easily onto

carbonate materials in fresh water (springs), but des-adsorb-

tion from carbonate materials when exposed to seawater

(Froelich, 1988). As a result, the dissolved concentrations of

phosphate in estuaries tend to be higher than in freshwaters.

The biogeochemical reactions that occur in sites influenced by

SGD is not well understood, however we were able to deter-

mine that this areas do indeed show seasonal biogeochemical

differences. This suggests that SGD and water column biogeo-

chemistry are the most important phenomena explaining the

annual hydrochemical variability (Edwards andWithers, 2008).

During the cold-front season (nortes) turbulence generated

by strong winds decreased water light penetration. Negative

impacts from harbors activities like wastewater inputs could

be magnified during the turbulence caused by cold front-

events. In the case of nortes, the PC1main variables according

to loadings (Table 2) indicated that nutrient resuspension and

export from harbors and coastal lagoons might be occurring.

The barrier island which separates coastal lagoons and

marshes from the sea along the Yucatan coast is vulnerable to

breaching during this time period which may cause water

from the coastal lagoons and swamps to flows into the coast,

leading low-oxygen, high nutrient concentrations and

hypersaline waters to the near-shore. The main variables of

the PC2 according to loadings (Table 2) are related to the

organic matter inputs from coastal lagoons, which can

become part of the suspended material during turbulence

events caused by waves and winds during the nortes season

(Troccoli et al., 2004; Aranda-Cirerol et al., 2006).

During the dry season, stable weather conditions dominate

the Yucatan landscape promoting homogeneous conditions in

the water column (Herrera-Silveira and Morales-Ojeda, 2009).

According to PC1, themain variable loadings are related to the

SGD influence on near-shore hydrochemistry, even if the flow

is lower in this season to produce turbulence or stratification

of water column, the penetration of light is high. In this

condition, the human impacts are indeed more notorious and

evident because of the high variability of NO2� concentrations.

Table 3 e Comparison of TRIX along different marine coastal waters.

System Site Tropic status Author

Lagoon Venice, Italy 4e5 (between good and moderated) Melaku Canu et al. (2003)

Lake Varna Bulgarian Black Sea 6 (bad) Moncheva et al. (2002)

Lake Beloslav Bulgarian Black Sea 6.7 (bad) Moncheva et al. (2002)

Bay Varna Bulgarian Black Sea 5.3 (bad) Moncheva et al. (2002)

Coastal (Cape) Galata Bulgarian Black Sea 3.99 (moderate) Moncheva et al. (2002)

Coastal (Cape) Kaliakra Bulgarian Black Sea 4.6 (moderate) Moncheva et al. (2002)

Coastal waters Zone I (S1eS4 and S14eS16). Yucatan Min 2.41 This study

Max 5.79 (good to moderate)

Zone II (S17eS20) Yucatan Min 2.75

Max 6.02 (good to bad)

Zone III (S5, S7e10) Yucatan Min 2.43

Max 5.86 (good to moderate)

Zone IV (S11eS13) Yucatan Min 2.57

Max 5.05 (good to moderate)

Different TRIX formula has been used.

Fig. 6 e Spatial and seasonal variation of TRIX in the study

area.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5959

This inorganic nitrogen form is an intermediate step of the

biological nitrification-denitrification process of waste waters

(Ciudad et al., 2005). The main variables in PC2 according to

the loadings (Table 2) suggested that organic matter decom-

position processes were ongoing favoring high NO2� þ NO3

�

concentrations, SRP availability, and high Chl-a concentra-

tions suggesting high productivity in the water column.

Finally, during the rainy season themain factor controlling

the hydrochemical water properties was SGD, even though

the aquifer normally recharges this time of the year. The

variables associated to PC2 suggested organic matter resus-

pension and decomposition of allochtonous matter coming

from mangroves (Herrera-Silveira and Morales-Ojeda, 2009),

which occurs when turbulence and surface external fertil-

ization take place. SGD can also contribute to the increased

nutrient availability during these time periods.

3.3. Trophic condition

The TRIX index is derived using measurement of chlorophyll-

a, oxygen, nitrogen and phosphorus; and has been used to

assess the trophic status of coastal waters (Vollenweider et al.,

1998; Coelho et al., 2007; Giovanardi and Vollenweider, 2004).

As an example, the Italian legislation uses the TRIX index for

the classification of the coastal waters condition (Casazza

et al., 2002). The TRIX index has been used as a diagnostic

tool to study variability of ecosystems for lagoon systems

(Solidoro and Cossarini, 2001), marine coastal waters

(Moncheva et al., 2002; Giovanardi and Vollenweider, 2004),

bays, and lakes (Moncheva et al., 2002) (Table 3).

In Yucatan, previous analysis of trophic status based on

a study carried out in Celestun, Sisal, Progreso, and Dzilam,

showed a TRIX index results ranging from 4 to 6 (meso-

eutrophic), which was highly correlated with SRP (Aranda-

Cirerol et al., 2006). In this study, which includes more

sampling sites, the TRIX index indicates a general oligo-

mesotrophic condition for the near-shore waters of the

northern Yucatan Peninsula (Fig. 6a). Spatial differences were

observed, with some sites having unique conditions, such as

Sisal (S5) which receive wastewater discharges from a shrimp

farm, and Chabihau (S10) which receives brackish water from

a swamp connected artificially to the coast. The Dzilam region

(S11, S12, S13) showed a meso-eutrophic condition, which

could be considered as “normal” trophic status due to natural

nutrient inputs from groundwater discharge (Alvarez-

Gongora and Herrera-Silveria, 2006; Aranda-Cirerol et al.,

2006). From Celestun (S1) to Punta Baz (S4), the trophic

status was mesotrophic, probably as a result of nutrient

inputs frommangrove runoff and longer water residence time

(Cochrane, 1969; Monreal-Gomez et al., 2004), which could

stimulate nutrient assimilation by phytoplankton (Herrera-

Silveira, 1993). The meso-eutrophic condition estimated

from Progreso to Telchac (S7eS9) was related to high

ammonia and phosphate concentrations, both associated

with areas of high human population density that lack

adequate wastewater treatment. Finally, the San Felipe-Rio

Lagartos region (S14eS20) was meso-oligotrophic, with low

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45960

nutrient concentrations as a result of the influence of the low

nutrient Caribbean waters (Lapointe and Thacker, 2002).

Seasonally, the north coast of the Yucatan Peninsula was

eutrophic, mainly during the Cabo Catoche upwelling inten-

sity peak in May (Merino, 1994; Cardenas-Palomo et al., 2010).

During this time terrestrial nutrient inputs also increase due

to peak in local tourism and consequently population incre-

ments which lead to an increase in wastewater discharge to

the aquifer. However, in November eutrophic conditions were

also registered (Fig. 6b), probably related to the turbulence,

nutrient and organic matter resuspension and remineraliza-

tion favored by windy conditions during this time of the year

(Ragueneau et al., 2002; Aranda-Cirerol et al., 2006).

This study is one of the first to develop a trophic charac-

terization and condition in subtropical near-shore waters

influenced by groundwater discharges. However, to establish

a trophic classification and reference values, the compilation

of a long-term dataset is required, which captures seasonal,

inter-annual variability, and terrestrial and oceanic changes

related to climate change.

3.4. Hydrologic zones

As a result of the classification analysis, four zones of hydro-

chemical affinity (Z IeZ IV) were identified (Fig. 7). Zone I (Z I)

grouped stationswith hydrochemical similarities but spatially

non-continuous, including stations located in the western

region from S1 (Celestun) to S4 (Punta Baz), and stations in the

eastern area from Dzilam to San Felipe (S14eS16).

Zone II includes eastern stations from S17 to S20 (Rio

Lagartos-El Cuyo). The grouping of the clusters indicates

similarities between Z I and Z II, in spite of being located in

geographically distant areas, both zones seems to be in

good hydrochemical and trophic conditions (Figs. 3, 4 and

Fig. 7 e Representation of hydrological affinity zones and the c

annual cycles in the coast of the state of Yucatan. Furthest neigh

and the squared Euclidean as a distance metric.

6). Both zones are characterized by high salinity and rela-

tively low nutrient concentrations, with the exception of

SRP. Zone II was influenced by the Caribbean Sea and by the

Gulf of Mexico, in response to this, the microalgal

community composition of the eastern coast of Yucatan

differs from the rest of the Yucatan coastal area as reported

by Troccoli et al. (2004) and Alvarez-Gongora and Herrera-

Silveria, 2006.

Z II and Z I are close and exhibited good conditions. Z II was

influenced by the Caribbean Sea with low nutrient concen-

trations, and Z I was influenced by the Gulf of Mexico. Both

zones showed high concentrations of organic matter coming

from coastal lagoons and also from submerged aquatic vege-

tation, however, they both were less affected by anthropo-

genic activities which could favor a good quality status of the

near-shore area being supported for phytoplankton structure

communities (Alvarez-Gongora and Herrera-Silveria, 2006).

The Zone III is also spatially discontinuous, including Sisal

location (S5) and the region of Sisal and from Progreso to

Telchac (S5, S7eS10). This zone was characterized by rela-

tively high salinity and nutrients concentrations (Figs. 3, 4

and 8); the latter is related to anthropogenic activities and

corresponds to the regionwith the greatest number of harbors

and human density (Progreso-Chicxulub). In addition, the

artificial inlets which connect mangrove swamps with the sea

(Telchac-Chabihau) play an important role modifying physic

and chemical environmental characteristics of lagoons and

near-shore waters, then changes in the structure of primary

producers as phytoplankton, SAV and mangrove (Herrera-

Silveira and Morales-Ojeda, 2009).

The ZIV is a group formed by the Dzilam region (S11eS13),

which is characterized by low salinity and high nutrient and

Chl-a concentrations, in this zone occurs the discharge zone

of the“cenotes ring” and springs inside marine near-shore

luster analysis of hydrological parameters of 2005e2006

bour (complete linkage) was used as the clustering method

Fig. 8 e In the CVA the first axis explained 44% and the

second explained 76%. Groups were singificantly

characterized by salinity ( p [ 0.0001, F [ 12.4), NO2

( p [ 0.0001, F [ 10.8), NH4 ( p [ 0.0005, F [ 5.9) and

k ( p [ 0.0005, F [ 7.2).

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 4 5961

waters are widely distributed throughout this area (Troccoli

et al., 2004; Aranda-Cirerol et al., 2006).

The prior groups formed by cluster analysis were proved in

a canonical variate analysis (CVA) where the multivariate

normality was absent. The CVA results indicated that the total

inertia or variation of the CVA analysis was 1.61 this value

indicate no multicollinearity. The inertia or heterogeneity

explained by salinity was 32.8%, for NO2� was 24.5% for k was

14.7 and for NH4þ was 11.2%. The total inertia explained was

83.3%, then, this analysis was helpful for identification of

linear combination of hydrochemical characteristics that best

discriminates the four hydrologic zones (Z IeZ IV). The value

for all canonical axes was significant ( p ¼ 0.0001, F ¼ 13.45),

this mean that all in conjunction have an effect over the

zonation. The results indicate differentiation along the axes

corresponding to different environmental gradients in

salinity, NO2�, k and NH4

þ. The first axis explained 44% and the

second explained 76%. The selection of variables was per-

formed using permutation in a Monte Carlo test 9999 permu-

tations under reduced model (Fig. 8).

The arrows in Fig. 8 represent the gradients and its length

the variation. Zone I (from Celestun S1 to Punta Baas S4, Sisal

S6 and from S14 to S16) was characterized by salinity and

ammonium concentrations which lead to significant inter-

group variation as well as low NO2� concentration. The results

support that this zone reflects the effect of high organic

matter inputs from coastal lagoons; this area is also charac-

terized by extensive areas covered by submerged aquatic

vegetation (Aguayo, 2003), low human impact and population

density, recognized as a well-preserved area in the Yucatan

coast, where co-dominance of terrestrial and oceanic controls

has not been changed yet (Herrera-Silveira and Morales-

Ojeda, 2009).

The CVA shows that Z III was characterized by high

concentrations of nitrite, a nutrient associated with waste-

waters. In the case of Sisal (S5), high nitrite concentrations

could be related to the shrimp farm, which is not presently

operating though some effects may persist. This is why this

zone represents the highest anthropogenic perturbation,

where the balance between ocean and land controls influ-

ences on near-shore hydrodynamics is disrupted, with inland

activities influencing much of the region, and consequentially

magnifying ocean effects such as hurricanes or nortes winds.

Excess of nitrogen is the major cause of ecosystem dete-

rioration, both on land and in water and it is particularly

difficult to amend due to multiple sources, the complexity of

nitrogen, transformations and the necessity of anaerobic

conditions for its return to the atmosphere as elemental

nitrogen via denitrification (Brush, 2009). In the near-shore

Yucatan, SGD are the main source of nitrogen. If the inland

wastes inputs follow without any treatment, the eutrophica-

tion could be a problem along the coast as is observed in the

region of Progreso (Aranda-Cirerol et al., 2006; Herrera-Silveira

and Morales-Ojeda, 2009).

Low salinity, high nutrient and Chl-a concentrations in Z III

are related to springs (SGD), which are geomorphic charac-

teristics related to the cenotes ring and widely distributed

throughout this area (Troccoli et al., 2004; Aranda-Cirerol

et al., 2006). The CVA triplot shows that Z IV which is con-

formed fromDzilam to P. Yalkubul (S11eS13) is defined by low

salinity concentrations and that its centroid does not overlap

with other groups. This area is within a natural basin of the

cenotes ring where there is enhanced SGD and associated

nutrient input to coastal waters (Fig. 8); therefore, terrestrial

controls are dominant in this area.

4. Conclusions

Terrestrial and oceanic influences act on a regional scale,

where intensity and type of human activities are added. These

lead to spatial, ecological and water quality differences along

the near-shore Yucatan coast.

This study demonstrates that the hydrochemical condi-

tions of the Yucatan coast exhibit four distinct hydrologic

zones, however, these are not spatially continuous. This

finding supports our hypothesis that each site has a specific

hydrochemical response associated with the relative impor-

tance of both regional and local terrestrial and oceanic influ-

ences. The type and intensity of human activities are also an

important factor affecting these ecosystems, and any char-

acterization of near-shore area must take all these variables

into account.

SGD is the main terrestrial influence and occurs as springs

or diffuse seepage. Oceanic influences include turbulence and

water currents, which change seasonally, especially during

the cold-front season (nortes) and upwelling intensification

period.

Since groundwater recharge occurs inland where human

activities aremost intense, the SGD provides a mechanism for

anthropogenic derived constituents to reach the coast and

modify the biogeochemical processes. Increase in these

activities could threaten the ecosystem dynamics of non-

impacted zones, potentially modifying nutrient concentra-

tions and/or ratios in adjacent near-shore waters, modifying

their trophic state, leading to eutrophication and harmful

algae blooms.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 4 9e5 9 6 45962

Acknowledgements

We would like to thank Primary Production Lab staff, CIN-

VESTAV-IPN, Merida. For field and laboratory assistance we

thank Javier Ramirez and IleanaOsorio.We thank Inia Soto for

English edition review. This was supported by CONACYT

FOMIX-YUC 66223 and 21336.

r e f e r e n c e s

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Alvarez-Gongora, C., Herrera-Silveria, J.A., 2006. Variations ofphytoplankton community structure related to water qualitytrends in a tropical karstic coastal zone. Marine PollutionBulletin 52, 48e60.

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