Analysis of the ecosystem structure of laguna alvarado, western gulf of mexico, by means of a mass balance model

B3.1

Estuarine, Coastal and Shelf Science 72 (2007) 155e167 www.elsevier.com/locate/ecss

Analysis of the ecosystem structure of Laguna Alvarado, western Gulf of Mexico, by means of a mass balance model
´ ´ ´ V.H. Cruz-Escalona*, F. Arreguın-Sanchez, M. Zetina-Rejon
Av. Instituto Politecnico Nacional, Centro Interdisciplinario de Ciencias Marinas, Departamento de Pesquerıas y BiologıaMarina, ´ ´ ´ Col. Playa Palo de Santa Rita El Conchalito, A. P. 592, La Paz, Baja California Sur, C. P. 23096, Mexico Received 12 May 2006; accepted 21 October 2006 Available online 15 December 2006

Abstract Alvarado is one of the most productive estuary-lagoon systems in the Mexican Gulf of Mexico. It has great economic and ecological importance due to high fisheries productivity and because itserves as a nursery, feeding, and reproduction area for numerous populations of fishes and crustaceans. Because of this, extensive studies have focused on biology, ecology, fisheries (e.g. shrimp, oysters) and other biological components of the system during the last few decades. This study presents a mass-balanced trophic model for Laguna Alvarado to determine it’s structure and functional form,and to compare it with similar coastal systems of the Gulf of Mexico and Mexican Pacific coast. The model, based on the software Ecopath with Ecosim, consists of eighteen fish groups, seven invertebrate groups, and one group each of sharks and rays, marine mammals, phytoplankton, sea grasses and detritus. The acceptability of the model is indicated by the pedigree index (0.5) which range from 0 to 1based on the quality of input data. The highest trophic level was 3.6 for marine mammals and snappers. Total system throughput reached 2680 t kmÀ2 yearÀ1, of which total consumption made up 47%, respiratory flows made up 37% and flows to detritus made up 16%. The total system production was higher than consumption, and net primary production higher than respiration. The mean transfer efficiency was13.8%. The mean trophic level of the catch was 2.3 and the primary production required to sustain the catch was estimated in 31 t kmÀ2 yrÀ1. Ecosystem overhead was 2.4 times the ascendancy. Results suggest a balance between primary production and consumption. In contrast with other Mexican coastal lagoons, Laguna Alvarado differs strongly in relation to the primary source of energy; here theprimary producers (seagrasses) are more important than detritus pathways. This fact can be interpreted a response to mangrove deforest, overfishing, etc. Future work might include the compilation of fishing and biomass time trends to develop historical verification and fitting of temporal simulations. Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: energy flows; trophic structure; Ecopath model; coastallagoon; Mexico; Alvarado Lagoon

1. Introduction Coastal lagoons are recognized as highly productive ecosystems that are used by numerous organisms for feeding, growth, reproduction and refuge (Allen and Horn, 1975; Day ´˜ and Yanez-Arancibia, 1982; Day et al., 1989). These ecosystems benefit humans in terms of food production (i.e. fisheries), transportation and recreation (NOAA, 1990). Theannual economic value of coastal fisheries in the U.S. Gulf

* Corresponding author. E-mail address: vescalon@ipn.mx (V.H. Cruz-Escalona). 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.10.013

of Mexico stands around 650 million U.S. dollars (NOAA, 1990). The complexity of the oceanographic conditions that characterize coastal lagoons, as well astheir multiple uses and biological diversity, necessitates a holistic approach to assess and manage their living resources. In highly dynamic ecosystem, the resilience of the food web depends on how energy flows through the system (Hunter and Price, 1992). Many coastal lagoon food webs appear to be highly resilient, as they remain generally intact despite the challenges of an extremely dynamic...