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

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+, 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 influence of groundwater discharge estimated by salinity dissolution (δ). 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+ as the main discriminating variables. Zones I and II showed almost pristine conditions, with well-balanced terrestrial–oceanic 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.