Factors Affecting Community Structure of Mangroves Associated With Point Bars and Islands in a Costa Rican Estuary.

Abstract

A conceptual model of relationships between mangrove community structure and their physico-chemical environment is developed for point bars and islands in the Tempisque-Bebedero estuarine system, Pacific coast, Costa Rica. The model identifies key variables whose relationships to mangrove forest structure are the focus of extended field and mesocosm studies. Point bars and islands are very dynamic features constantly modified by hydrology and sediment dynamics. In the Tempisque-Bebedero system, they are additionally modified by extensive agricultural activities. High riverine sediment loads, associated with irrigation and seasonal wind-driven resuspension, result in significantly higher sediment deposition in lower intertidal zones; upper intertidal zones are less affected due to their elevation and less frequent flooding. This study shows significant relationships between mangrove forest structure and position along the intertidal gradient. Laguncularia racemosa is the pioneer species colonizing emerging mud banks, and dominates lower intertidal zones in terms of basal area and stem density. Rhizophora racemosa, Avicennia germinans, and A. bicolor dominate basal area of upper intertidal zones. The correlation between forest structure, hydrology and sediment deposition implies that mangroves in this region are distributed according to inter-specific differences in tolerances to inundation and sedimentation dynamics. Soil physico-chemical variables changed significantly across the intertidal gradient, implying that edaphic factors are important explaining the observed mangrove distributional patterns. Laguncularia dominates the lower intertidal zone characterized by lower soil stability and "richer" sods than the upper zone. Riverine source of nutrient-rich, low salinity water and sediments is important and explains the high structural development of these mangrove communities. The effect of hydrology and biotic factors in species distribution are important during the early phases of mangrove development by determining species-specific ability of establishment. Compared to Avicennia, Laguncularia propagules had higher establishment in mud banks, implying that Laguncularia 's specific propagule characteristics are important in determining this species' successful colonization of lower intertidal zones. Intensive crab predation in upper intertidal zones, however, restricts its distribution in upper intertidal zones. Mangrove structure in the Tempisque-Bebedero system is therefore sensitive to a combination of physical and biological factors, which deserve attention considering the impact of further alterations to this estuarine system.