Bottom sediment, chlorophyll and pheophytin survey Plum Island Sound, Massachusetts, September 10 - 15, 2010


Tidal bars and tidal flats in shallow, mesotidal estuaries are subject to the action of tidal currents and waves. This complex forcing gives rise to large variations in bottom sediments and related benthic ecosystems. In this research we study the spatial relationships among tidal currents, waves, sediment characteristics, chlorophyll a, and pheophytin in the shallow bars and flats of Plum Island Sound, Massachusetts, USA. Waves and tides have a mild effect on the sand grain size and percent of silt and mud of these shallow deposits. Average wave conditions are more important than extreme events, like Nor’easters, in sediment sorting. The spatial distribution of pheophytin, an indicator of dead microphytobenthos, mimics that of silt and mud whereas chlorophyll a, a proxy for the live microphytobenthos stock, is greatest in sandy areas characterized by intermediate values of tidal velocity.Tidal currents and waves were computed using the numerical model Delf3D.-SWAN.
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Sediment samples for grain size, chlorophyll a, and pheophytin analyses were collected in exposed tidal bars and tidal flats in Plum Island Sound during low tide. The entire survey was carried out in three days in September 2010 (10 Sep 2010, 14 Sep 2010, and 15 Sep 2010) to minimize possible temporal variations in biological and parameters. The chosen period is just after the average peak in water temperature that usually occurs in the second week of August (NODC Coastal Water Temperature Guide).
To measure chlorophyll a and pheophytin, one sediment core was taken at each location , paired with the sediment sample, using a 50 cm3 syringe with the top cut off. Each sediment core was sliced in 1 cm sections for the top 5 cm of the core. Briefly, samples were stored on ice in the dark, transported to the lab, and frozen until analysis. For chlorophyll analysis samples were thawed, 25 mL of 92% acetone was added to each, and they sat overnight (∼12 h) on ice in a dark cooler. The next day samples were sonicated for 30 s, allowed to settle upright, and 2 mL of liquid only was pipetted into 3 replicate vials for absorbance measurement using a TURNER Trilogy laboratory fluorometer. For pheophytin analysis 25 μL of 0.300N HCl was added to each replicate, allowed to sit for 3 min, and then fluorometer measurement was repeated. Raw absorbance values were converted to concentration of chl a (μg cm−3). We chose to take two sediment samples at each location, one for sediment analysis and one to determine concentrations of chlorophyll and pheophytin, in order to maximize the number of sites visited in a single low tide, thus controlling for the possible temporal variability in biological and sedimentological parameters. We partially compensate for the lack of replicates with the high spatial density of measured points, implicitly assuming that the measured parameters are homogenous on a small-scale. Essentially this study provides a high spatial resolution, but a lower temporal resolution “snapshot” of chlorophyll and pheophytin distribution in this estuary.
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Data set complete for 2010
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