Altered mangrove wetlands as habitat for estuarine nekton: are dredged channels and tidal creeks equivalent?

Abstract

hasty decisions are often made regarding the restoration of “altered” habitats, when in fact the ecological value of these habitats may be comparable to natural ones. To assess the “value” of altered mangrove-lined habitats for nekton, we sampled for 1 yr within three Tampa Bay wetlands. Species composition, abundance, and spatial distribution of nekton assemblages in permanent subtidal portions of natural tidal creeks and wetlands altered by construction of mosquito-control ditches and stormwater-drainage ditches were quantified through seasonal seine sampling. Results of repeated-measures analysis of variance and ordination of nekton community data suggested differences in species composition and abundance between natural and altered habitat, though not consistently among the three wetlands. In many cases, mosquito ditches were more similar in assemblage structure to tidal creeks than to stormwater ditches. In general, mosquito ditches and stormwater ditches were the most dissimilar in terms of nekton community structure. These dissimilarities were likely due to differences in design between the two types of ditches. Mosquito ditches tend to fill in over time and are thus more ephemeral features in the landscape. In contrast, stormwater ditches are a more permanent altered habitat that remain open due to periodic flushing from heavy runoff. Results indicate that environmental conditions (e.g., salinity, current velocity, vegetative structure) may provide a more useful indication of potential habitat “value” for nekton than whether the habitat has been altered. The type of ditching is therefore more important than ditching per se when judging the habitat quality of these altered channels for fishes, shrimps and crabs. Planning should entail careful consideration of environmental conditions rather than simply restoring for restoration’s sake. Mangrove systems provide habitat for numerous species of estuarine fishes and invertebrates, many of which are ecologically or economically valuable (Thayer et al., 1978; weinstein, 1979; Odum et al., 1982; Thayer et al., 1987; hettler, 1989; Sheridan, 1992; kneib, 1997; Ley et al., 1999). due to human population growth in coastal regions, mangroves have experienced a substantial reduction in coverage since 1950 with worldwide losses approaching one third (Alongi, 2002). Anthropogenic alteration of mangroves is evident in the landscape along the gulf Coast of Florida in the form of dredged channels, spoil mounds, and water-control structures. The ecological consequences of such habitat alteration, however, are not always as apparent. Understanding the fundamental differences between natural vs altered wetlands, and how and why faunal assemblages vary between habitats, is necessary to evaluate the effects of alteration on the ecological functioning of the system. In an effort to repair the effects of wetland disturbance, wetland restoration and creation have become widespread management tools over the last few decades (Lewis, 1992; weinstein et al., 2000; Alongi, 2002; warren et al., 2002; Lewis, 2005). however, the creation of a functional wetland system often proves more of a challenge than simply creating wetland structure (i.e., tidal creeks, ponds, salt barrens). A thorough understanding of faunal ecology, including that of the nekton communities (i.e., fishes, shrimps, BULLETIN OF MARINE SCIENCE, vOL. 80, NO. 3, 2007 840 crabs) that utilize wetland habitats, is necessary for the successful creation of a functional wetland. Many of the wetlands in the Tampa Bay estuary on the gulf coast of Florida have been subjected to dredge-and-fill activities since the mid-1900s (Johnston, 1981; Lewis and Estevez, 1988; Clark, 1992), while the overall coverage of the bay’s wetlands has been reduced by at least 44% over the last century (Lewis et al., 1985). Current wetland-restoration plans in the bay seek to restore areas that have been modified for mosquito control, stormwater drainage, and agriculture. however, hydrological changes induced by human activities such as dredge-and-fill and even restoration efforts may affect biotic components such as fish communities (Freeman et al., 2001; Bunn and Arthington, 2002; Roy et al., 2005). whereas little is known of the composition and structure of nekton communities in shallow-wetland habitats of Tampa Bay, even less is known about comparative nekton use of natural and altered wetlands. Thus, fish-community data provide a useful baseline to assist resource managers with pre-restoration planning and post-restoration assessment. To determine fish-community response to habitat alteration and to provide baseline community conditions from which to plan and assess wetland-restoration projects, we initiated a three-year sampling program in several Tampa Bay wetlands. Objectives were to: (1) compare habitat characteristics of naturally formed tidal creeks and man-made ditches; and (2) compare spatial use of natural and man-made mangrove habitat by nekton relative to habitat characteristics. here we report results from the first year of that ongoing study.