Effects of wrack accumulation on Spartina alterniflora, Jamaica Bay Wildlife Refuge, New York city

A common natural source of disturbance in Jamaica Bay salt marshes is the local accumulation of wrack, composed primarily of Spartina alterniflora and Phragmites australis. Mats of wrack accumulate throughout the marsh, killing or damaging the plants beneath them. The objective of this study was to observe the effect of wrack cover upon Spartina alterniflora in monocultures of this species as assessed by the following experiments. Five arrays, each of 8 permanent experimental plots were delineated within dense near-monocultures of S. alterniflora. Each array, save one randomly selected plot designated as the control, was completely covered with wrack to a depth of 10-15cm on 24 August and 5-7 October, 2003. In mid-November 2002 and monthly from June through November 2003, one randomly selected plot in each array was uncovered for sampling. Numbers of shoots, cover, and culm height remained nearly the same after the brief period of coverage in 2002, but declined drastically in wrack-covered plots during the 2003 season; in plots uncovered in August and thereafter of that year, means of fewer than 3 Shoots/m(2) remained compared to nearly 400/m(2) in control plots. Those few shoots remaining in the experimental plots from August on were mostly less than 10cm high, compared to about 1.22.2m in the controls, and covered on average 3 percent or less of the surface, in contrast to the complete cover in control plots. Bertness et al. [5] observed the marsh Snail Littoraria irroalia selectively grazing on S. altern flora at a Sapalo Island, Georgia, salt marsh. They have reported that large populations of Lifforaria can destroy an S. alfern flora marsh. During our study, no snails were observed on S. alterniflora during the growing season of 2003. Snail grazing may not be a factor in the death and destruction of S. altern flora salt marshes at Jamaica Bay. We have found that wrack has a strong impact on the growth and survival of S. alterniflora, as measured by percent mortality and delayed recovery after wrack removal. However, the distribution of wrack in the marsh is limited in area; less than 5 percent of the marsh is covered at any particular time. Thus, wrack would not appear to be one of the major causes of the rapid marsh loss that is occurring in Jamaica Bay.