Tidal hydrology and habitat utilization by resident nekton in Phragmites and non-Phragmites marshes

We compared nekton densities over a range of measured flooding conditions and locations within Phragmites australis and Spartina alterniflora (salt marsh cordgrass) at the Charles Wheeler Salt Marsh, located on the lower Housatonic River estuary in southwestern Connecticut. Nekton were sampled on nine spring high tide events from May to October 2000 using bottomless lift nets positioned between 0-5 and 10-20 m from the creek edge. Flooding depth, duration, and frequency were measured from each vegetation type during each sampling month. Benthic macroinvertebrate density was also measured within each vegetation type in May, July, and September. Frequency of flooding was 52% lower and flooding depth and duration were also significantly reduced in R australis relative to S. alterniflora. A total of 4,197 individuals representing 7 species, mostly Palaemonetes pugio (dagger-blade grass shrimp) and Fundulus heteroclitus(common mummichog), were captured. P pugio densities were significantly greater in S. alterniflora as were benthic macroinvertebrate density and taxa richness during May, but not during June or October. Total fish density was not significantly different between P australis and S. alterniflora and was independent of location on the marsh. Significantly more juvenile F heteroclitus were collected within S. alterniflora relative to P australis in June and July, suggesting that recruitment of this species may be lower in P australis habitat. Fish density generally did not vary predictably across the range of flooding depth and duration; there was a positive relationship between flooding depth and fish density in S. alterniflora. The measured reduction in flooding frequency (52%) within R australis at the Housatonic site would result in an average total monthly fish use, expressed as density, of 447 ind m(-2) for P. austratis and 947 ind m(-2) for S. alterniflora. When R australis expansion results in reduction of flooding frequency and duration, nekton community composition can change, access to the marsh surface is reduced twofold, and nursery habitat function may be impaired.