Sediment chlorophyll-a, pheophytin, biogenic silica, and carbohydrate content (EPS) from the Rowley River mudflat in September 2012, Rowley, MA


<p>Tidal flats are critical components of coastal estuarine ecosystems characterized by high rates of benthic primary productivity and biogeochemical cycling. In order to investigate the impact of anthropogenic nutrient loading on tidal flat biogeochemistry we carried out a two-week fertilization experiment. Throughout the course of the study we conducted two light-dark, whole-core incubations and took measurements of three indicators of microphytobenthos activity in addition to quantifying the resident eastern mud snail (Ilyanassa obsoleta) population.</p>

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<p>In order to assess the response of microphytobenthos and sediment nutrient cycling to increasing nitrogen loads, we conducted a fourteen-day fertilization study of these tidal flat sediments from September 12-26, 2011. We established two 1x6 m sediment transects perpendicular to the tidal line and 4 m apart (Fig. 1). The upstream transect on the received daily fertilization in the form of 70 μmol L-1 sodium nitrate (NaNO3) added to filtered river water. We applied the fertilizer to the sediment surface with a fertilizer sprayer (SoloPRO) at a rate of 1.1 L min-1 giving a total N load over the two week period of 385 μmol d-1. The concentration was chosen to match the concentration added in a long-term fertilization study of the nearby salt marsh tidal creeks (Trophic cascades and interacting control processes in a detritus-based aquatic ecosystem, TIDE, Deegan et al. 2007; Johnson et al. 2009). Additionally, this was the maximum concentration approved through permitting. When fertilizing we were careful to spray the sediment surface long enough to supply treatment, but not so long as to wash away the top layer of fine, flocculent sediment. The transect down river from the experimental plot, and 4 m away served as a control, and each transect was divided into six 1x1 m sub plots for the purpose of sampling. Samples for analysis of sediment chlorophyll a (chl a), biogenic silica (BSi), and extracellular carbohydrates (EPS) were collected every other day of the experiment in order to monitor the spatial and temporal variability of the tidal flat MPB community. Samples for chl a were collected in duplicate from each subplot using 1 cm sub cores. The top 0.5 cm of sample was then frozen until analysis. To analyze for chl a and pheopigments, frozen sediment samples were thawed, sonicated and extracted in 25 ml of 90% acetone overnight (Dalsgaard et al. 2000). Following extraction, samples were centrifuged and 2 ml aliquots were analyzed for chl a and pheophytin fluorescence (Trilogy Fluorometer, Turner Designs). Samples for BSi were also collected using 1 cm subcores with the top 0.5 cm frozen (-4&deg;C) until analysis. BSi analysis was done using the wet alkaline method, which involves digesting approximately 30 mg of dry sediment in 1% Na2CO3 solution at 85&deg;C for five hours (Conley and Schelske, 2001). During digestion, subsamples of the solute were taken at after 3, 4, and 5 h. These subsamples were then colorimetrically analyzed on a SEAL Auto-analyzer, and BSi concentration was determined by linear extrapolation to the y intercept through the three sample points in order to correct for any dissolution of mineral silicates (DeMaster 1981). Sediment samples for extracellular carbohydrates (EPS) were collected using a Cryolander (modified from (Wiltshire et al. 1997). This collection method flash freezes the sediment surface (in this case to a depth of approximately 0.5 cm) using liquid nitrogen without distorting the sediment surface. Frozen samples were sliced in half and stored as replicates at -80&deg;C until analysis. Frozen samples were then analyzed for 2 separate extracellular carbohydrate fractions, a water-extractable or colloidal fraction, and an ethylenediaminetetraacetic acid (EDTA)-extractable fraction. Frozen sediments were extracted in Milli-Q water and 0.1M Na2EDTA for colloidal and EDTA-extractable fractions, respectively, as described by (de Brouwer et al. 2000). The colloidal fraction was extracted by adding 5-55 mg of frozen sediment sample to 400 μL of Milli-Q water, which was then extracted in a 30&deg;C water bath for 1 h. After extraction samples were centrifuged at 6000 x g for 5 min and the supernatant was analyzed for colloidal EPS. The EDTA-extractable fraction was then extracted from the remaining sediment pellet by adding 500 μL of 0.1M Na2EDTA, and allowing samples to incubate at room temperature overnight. After extraction, samples were centrifuged again, and the supernatant was analyzed for EDTA-extractable EPS. Carbohydrate quantities were determined using the phenol-sulfuric acid assay (Dubois et al 1956) quantified spectrophotometrically (CaryBIO 100) using a glucose (0-2mM) external standard. Quantities of both fractions are therefore reported in glucose equivalents (μg g-1).</p>


<p>One time data collection, no update Version 01: January 20, 2015, new data and metadata to comply with importation to Drupal and LTER PASTA. Used MarcrosExportEML_HTML (working)pie_excel2007.xlsm 1/15/15 4:26 PM for QA/QC to EML 2.1.0</p>


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