Comparison of salt marsh creeks and ditches as habitat for nekton

Abstract

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.

Impacts from ditching salt marshes in the mid-Atlantic and northeastern United States

“…Greater isolation of ponds may mean their fish populations cannot be restocked if the ponds dry out or some other ecological catastrophe strikes. Knieb (1997) suggested all artificial aquatic habitats in marshes have natural functional analogs, and ditches act like tidal channels; however, Corman and Roman (2011) disagree, as their data showed distinct water quality differences. A comprehensive report on fish in marshes did not find any special role for ditches as habitat (Rountree and Able 2007).…”

“…One Long Island study found shallow ditches in a microtidal marsh (which had poor water quality) did not support as many fishes and species as creeks did (Corman and Roman 2011). When ditches were converted to ponds and more natural appearing channels in another Long Island marsh, there were significant increases in overall nekton populations (Rochlin et al 2012).…”

“…Ditching has been demonstrated to double channel edges in a marsh (Lathrop et al 2000), and crabs were found in increased numbers in ditched areas, possibly due to more burrowing opportunities (Rockel 1969). However, the generation of berms as a result of construction can be an impediment to marsh surface access (Reed et al 2006), and ditches generally have poorer water quality (Kuenzler and Marshall 1973;Corman and Roman 2011), although this is not generally documented. Still, some resident marsh fish (especially mummichogs, Fundulus heteroclitus) are tolerant of very poor water conditions (Knieb 1997), and ditches can give access to otherwise unexploited marsh surface productivity (Whalley and Minello 2002), including mosquito larvae (see above)…”

“…Since ditches result in less overall surface water area by draining ponds, there is a trade-off between a smaller area of ditches with greater edge habitat compared with a larger area of ponds and pools, with greater predation pressure in the ponds and pools. On balance, losses of high quality habitat in ponds are probably not mitigated by extra edge habitat associated with ditching, because water quality in the ditches is generally too poor to serve as adequate replacement habitat for the habitat lost by lowering the water table, although even poor water quality results in ditches being acceptable habitat for mummichogs (Corman and Roman 2011).…”

An assessment of the extent and social-ecological impacts of salt marsh mosquito ditching

“…Furthermore, the ditches act as additional tidal channels, thereby sometimes (but not always) reducing ponding on the marsh platform, which is a critical foraging habitat for a subset of marsh birds (Burger et al 1982, Erwin 1996, Trocki and Paton 2006. These habitat and hydrologic changes can not only affect birds, but also invertebrates and fishes at multiple levels of the marsh food web, thereby indirectly impacting birds at higher trophic levels (Bourn and Cottam 1950, Dreyer and Niering 1995, Corman and Roman 2011. Consequently, there is the potential for the marsh bird community to exhibit complex responses to ditching.…”