Comparison of salt marsh creeks and ditches as habitat for nekton

Salt marshes are dynamic systems supporting a diverse assemblage of resident and transient nekton (free-swimming fish and decapod crustaceans). Within a marsh, many sub-habitats are used by nekton, including ditches and natural creeks. While the use of natural creeks is well documented, the role of ditches as habitat for nekton remains less well known. The present study describes the nekton-support function of this prolific marsh sub-habitat, and compares it to tidal creeks. Sampling was conducted in the summer of 2008 in a microtidal, polyhaline salt marsh on Fire Island, NY, USA. Ditches and creeks were found to have different nekton communities. Creeks had significantly higher species richness and represented nearly all ditch species. Prolonged anoxia (up to 18 h) in ditches is suggested as a significant factor contributing to differences in the creek and ditch nekton community. Upper portions of ditches and creeks differed for resident nekton species, with higher species richness and density of the mummichog Fundulus heteroclitus in the upper portion of creeks than in the upper portion of ditches. Additional differences were seen in the nekton community composition of open and naturally plugged ditches, where the densities of individual species (such as F. heteroclitus, which was more abundant in open ditches) also differed. Ditches are a major habitat type in salt marshes of the northeastern USA, such as at the Fire Island marsh site studied here, and as efforts are considered to remove ditches from marshes as a habitat restoration technique, their nekton support function should be considered.