Variations of phytoplankton community structure related to water quality trends in a tropical karstic coastal zone Cynthia A ´ lvarez-Go ´ ngora, Jorge A. Herrera-Silveira * CINVESTAV-IPN, Depto. de Recurso del mar, Unidad Merida, Carretera Antigua a Progreso km 6, C.P. 97310 Me ´rida, Mexico Abstract Phytoplankton community structure in coastal areas is a result of various environmental factors such as nutrients, light, grazing, tem- perature, and salinity. The Yucatan Peninsula is a karstic tropical region that is strongly influenced by submerged groundwater discharge (SGD) into the coastal zone. Phytoplankton community structure and its relationship with regional and local water quality variables were studied in four ports of the northwestern Yucatan Peninsula. Water quality was strongly related to SGD, and variations in phy- toplankton community structure were related to local nutrient loading and hydrographic conditions, turbulence, and human impacts. Our study provides an ecological baseline for the Yucatan Peninsula and serves as a basis for establishing monitoring programs to pre- dict changes at sites with high hydrological variation and in developing an early alert system for harmful toxic algal blooms. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Phytoplankton; Water quality; Eutrophication; Groundwater discharges; Harmful algal blooms; Yucatan Peninsula 1. Introduction Phytoplankton community assemblages are determined in part by their environment, exhibiting a complex behav- ioral in response to this influence. Phytoplankton commu- nity composition changes as a consequence of species succession, which occurs in response to new conditions encountered in the environment (Huisman et al., 2001). Predicting changes in species composition for a phyto- plankton community as an indicator of the variability of coastal water quality characteristics has promoted analysis of this community using different strategies, such as long- term monitoring of dominant species and their relation- ships with seasonal changes of environmental conditions (Jones et al., 2001; Roelke et al., 2003). Two important factors are recognized as controlling community structure of phytoplankton. The first is related to physical processes such as mixing of water masses, light, temperature, turbulence, and salinity, and the second is associated with nutrients. The study of phytoplankton community response to these variables is considered useful for interpreting hydrological variations in coastal areas (Troccoli et al., 2004). Different human activities in coastal areas generate wastes that cause changes in the natural hydrological con- ditions of the coastal system, inducing eutrophication. Phy- toplankton community structural changes are a good indicator of eutrophication effects, as it is recognized that phytoplankton composition is a natural bioindicator be- cause of its complex and rapid responses to fluctuations of environmental conditions (Livingston, 2001). However, to quantitatively understand species respond to water quality conditions, methodological approaches are required that are based on either long-term monitoring or the simultaneous analysis of sites that reflect differences in water quality variables as a consequence of the intensity and type of human activities (Harding, 1994; Marcela, 2003; Kelly-Gerreyn et al., 2004). The northern coast of the Yuca- tan Peninsula is environmentally a homogeneous region, and its karstic characteristic leads to the diffuse discharge of groundwater to the coastal zone (including mangroves 0025-326X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2005.08.006 * Corresponding author. Tel.: +52 99 99 812960; fax: +52 99 99 812334. E-mail address: jherrera@mda.cinvestav.mx (J.A. Herrera-Silveira). www.elsevier.com/locate/marpolbul Marine Pollution Bulletin 52 (2006) 48–60