Repaired shell damage among Ilyanassa trivitatta in Boston Harbor, Massachusetts, before and after sewage abatement

Keleher, Frances V.
Tom Wessels, MA
Department of Environmental Studies
Repaired shell damage frequencies were estimated for Ilyanassa trivitatta before and after sewage abatement in Boston Harbor, Massachusetts at four stations: T1, located nearest the abatement site (42º20.95'N, 70º57.81'W), is just off the tip of Deer Island; T3 (42º19.81'N, 70º57.72'W) is off the tip of Long Island; T5 (42º20.38'N, 70º57.64'W) is between the tips of Deer and Long Island; and T8, located the farthest away from the abatement site (42º17.12'N, 70º57.75'W), is located in Hull Harbor. In September 1991, Battelle scientists conducted the before abatement survey of Boston Harbor benthic communities. Sewage abatement occurred in December 1991. Therefore the four after abatement sampling dates were April and August of 1992 and April and August 1993. Three questions were investigated in this study: 1) Does repaired shell damage frequency in Boston Harbor differ before and after sewage abatement? 2) Do each of the four stations in Boston Harbor significantly differ from each other in repair frequency? and 3) Do the spring and late summer survey dates have significantly different repair frequencies? Two way analysis of variance of all four stations indicated no significant difference in recent repaired shell damage (0-45º counterclockwise from aperature) frequencies at all stations before and after sewage abatement (DF = 3, p>.25). However, when looked at separately, stations T1, T3, and T5 had significant changes in repair frequencies among the five survey dates (T1, ?2=27.791, p=.001; T3 ?2=40.676, p=.001; T5 ?2=17.637, p=.01); T8, however, did not. These differences may be seasonal results because the chi-square analysis of all April vs. August station repair frequencies were significant ?2=28.924, p=.001). Furthermore, these seasonal differences (April vs. August survey dates) in repair frequency were significant at stations T1 (?2=19.297, p=.001), T3 (?2=27.938, p=.001) and T5 (?2=10.36, p=.001) when each station was tested independently. T8 showed no effect of seasons on repaired shell damage frequencies. The fact that T8 showed no difference in seasonal repair frequency, while the stations closer to the abatement site did, may indicate that sewage abatement in Boston Harbor results in pronounced seasonal differences in predator activity upon Ilyanassa trivitatta. Chi-square analysis of August survey dates versus the September survey date to determine if repaired shell damage frequencies were effected by sewage abatement when compared seasonally was significant at station T8 only (?2=5.218, p=.05). More data in the before sewage abatement condition is needed to address the questions proposed in this study. Furthermore, it is suggested that long term monitoring of Boston harbor would generate more data, thereby providing a more accurate profile of benthic populations, predator-prey relationships, and the effect of sewage abatement on repaired shell damage of the benthic mollusk, Ilyanassa trivitatta.

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