Conclusions
Station #2
1. The distribution of sands, silts and clays in the surficial sediment (0-0.5 cm) at station #2 showed very little change over the 11-year monitoring period except for two cruises (W7 and W8 conducted in October 1991 and February 1992). Excluding from these two cruises, the mean textural percentages were 97.6 % sand, 1.5% silt and 0.9 % clay and all samples were classified as well to moderately sorted fine sands. There appeared to be no seasonal differences in grain-size distribution.
2. A somewhat finer grain size distribution was observed in samples collected on cruise W7 (October 1991). The duplicate samples from cruise W8 (February 1992) contained widely varying sand percentages (98.5% and 83.1%). We speculate that the W7 results are within the natural variability for field samples. The single low sand concentration (and high within station variability) on cruise W8 may reflect the impact of a major storm that struck coastal Massachusetts on October 31, 1991. However, the storm response at this station is not clear. Samples collected in February 1993 (W11) after another intense storm of longer duration, which occurred December 11-16,1992, showed no measurable change of grain-size distribution.
3. The layer from 0-0.5 cm contains a complex of organic-mineral aggregates bound by an elastic mucus-like membrane. Observations by light microscope confirmed that quartz composed up to 95% of the aggregates with small and variable amounts of fecal pellets, carapaces, worm tube fragments, and amorphous organic material. The weight percent of aggregates varied between 1- 2% during the monitoring period and showed no seasonal trends.
Fecal pellets retained on the 2-phi sieve (0.25-mm) were the most abundant organic component of the aggregates, and they were a minor component of the aggregate total. These pellets were produced by a mixed assemblage infauna including polychaetes, amphipods, and isopods.
Station #3
1. The surficial sediment was muddier and more variable with time at station #3 than at station #2. This station is primarily silty in texture. Sand varied from 10% to 52%, silts from 40% to 70%, and clays from 5% to 47% over the entire monitoring period. No seasonal trends were evident.
2. The samples collected from station #3 on February 1992 (W8), after the Halloween storm of 1991, showed virtually no change in the overall grain-size distribution from previous sampling cruises. However, samples collected after the storm on December 1992 showed an increase in concentrations of clay from about 10% to 45%.
The difference in the response of sediment texture of the sample at station #3 to different major storms may be related to the duration of the December 1992 storm, which generated waves higher than 5 m for 1.7 days longer than the storm in October 1991. The increase in clay content measured in February 1992 was accompanied by increases in concentrations of silver and Clostridium perfringens, both indicators of sewage particles (Bothner and others, 2001).
These observations suggest that new sediment was transported to station #3 from areas closer to Boston Harbor by the December 1992 storm.
Down-core textural characteristics in two cores from the same cruise and station (W15, cores 3-1 and 3-3) were similar in magnitude but maxima and minima in mud percentages occurred at different depths. The mud differences may reflect local variability in bioturbation and/or accumulation processes. The texture in a 50 cm core collected on cruise W24 was also variable with depth, and the core included small amounts of gravel sized particles.
3. The weight percent of organic-mineral aggregates in the sediment interval from 0 to 0.5 cm averaged between 5% and 15% over the 11-year monitoring period, excluding spring and late summer sampling intervals following the major storm of December 1992. Although quartz was the major mineral grain of the aggregates, fecal pellets tended to be the major component of the organic fraction. Fecal pellets were smaller and more abundant than those found at station #2 and were typically retained on the 4-phi sieve (0.0625-mm). In the summer and fall of 1993 (cruises W12 and W13), aggregates (mostly fecal pellets) increased to 27% and 54%, respectively, of the total sediment weight. We speculate that the storm of December 1992 deposited a new supply of fine-grained, organic-matter-rich sediments that supported higher populations of benthic infauna over the following summer.
Aggregate analysis with depth in cores collected on cruise W15 showed that the highest concentrations were restricted to the top few centimeters and decreased dramatically with depth. A more gradual decrease with depth was observed from a core collected on cruise W24.