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Terrestrial and oceanic influence on spatial hydrochemistry and 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 Me´rida., Carr. Antigua a Progreso km. 6, C.P. 97310 Me´rida, Yucata´n, Me´xico
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abstract
Article history:
Terrestrial and oceanic influences like groundwater discharges and/or oceanic upwelling
Received 14 March 2010
define the hydrochemical and biological characteristics of near-shore regions. In karst
Received in revised form
environments, such as the Yucatan Peninsula (SE Mexico), the balance between these two
2 July 2010
influences on spatial and temporal scales is poorly understood. This study focused on near-
Accepted 14 July 2010
shore waters within 200 m offshore along the Yucatan coast. The trophic status and
Available online 23 July 2010
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
Keywords:
discharge, while the most important marine influence was related to seasonal changes in
Water quality
water turbulence. Spatial differences ( p < 0.05) were observed among salinity, light
Trophic status
þ extinction coefficient (k), NO 3 , NH4 , and Chl-a. Seasonal differences were observed for all
Near-shore waters
variables except for k. During the dry season, terrestrial influences are the dominant factor
Multivariate analysis
on near-shore hydrochemistry. The region around Dzilam exhibited the maximum influ-
Yucatan
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 oligomesotrophic; with a meso-eutrophic status in areas with documented anthropogenic impacts. Four hydrological zones were identified by a Canonical Variate Analysis (CVA) þ using salinity, NO 2 , k and NH4 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.
* Corresponding author. Tel.: þ52 999 9429462 (direct line); fax: þ52 999 9812334. E-mail addresses:
[email protected] (S.M. Morales-Ojeda),
[email protected] (J.A. Herrera-Silveira), jmontero@ mda.cinvestav.mx (J. Montero). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.07.046
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Abbreviations and notations SE CMW SGD NO 2 NO 3 NHþ 4 SRP N:P SRSi k
1.
Southeast Coastal marine waters submarine groundwater discharges nitrite (mmol l1) nitrate (mmol l1) Ammonia (mmol l1) soluble reactive phosphorus (mmol l1) Redfield ratio soluble reactive silica (mmol l1) light extinction coefficient (m1)
Introduction
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 region may depend on the input of pollutants from land to coastal waters, water residence 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 eutrophic conditions (Xua et al., 2004). Water quality and trophic status measurements of coastal and marine waters are often used to assess and evaluate their conditions for a better management of the coastal and marine 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 freshwater 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-
Chl-a O2 DOS D pH
TRIX DCA CVA PCA
chlorophyll-a (mg m3) oxygen (mg l1) dissolved oxygen saturation (%) salinity dilution (no units) inverse logarithm of the activity of the hydrogen ion Salinity (no units) Temperature ( C) Vollenweiner trophic index (no units) detrended correspondence analysis canonical variate analysis principal component analysis
Ojeda, 2009). Groundwater has been estimated to contribute 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 landuse, 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 importance of terrestrial and oceanic influences on the spatial heterogeneity and trophic status of marine 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 (NovembereFebruary). The predominant winds come from the BermudaAzores anticyclone; in general, the wind is of low intensity (0e15 km h1) 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 on marine coastal waters is via SGD, which is approximately 8.6 106 m3 km1 year1 (Hanshaw and Back, 1980). SGD is characterized by low salinity and high nitrate and silicate concentrations. The influence of
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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.
SGD on coastal ecosystems is a function of their proximity to the discharge and the type of discharge (point or non-point). Aquifer recharge occurs mainly during the rainy season (HerreraSilveira 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 common in this region, and are found in lagoons and near-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 subterranean water flow (Fig. 1). Human activities in land and coastal regions as eco-tourism, fisheries, salt extraction, port development, seasonal high-impact tourism, and cattle ranches have negatively impacted the water quality of the aquifer and consequently the SGD has 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 (
