Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


The land-­‐ocean margin is characterized by a shallow water column, which allows light to reach the benthos and supports a diverse community of benthic autotrophs. One group of benthic autotrophs, consisting of benthic diatoms, cyanobacteria and other photosynthetic microorganisms living near the sediment surface (i.e., benthic microalgae) accounts for a substantial amount of this primary production. Benthic microalgae are also tightly coupled to carbon and nutrient cycling processes carried out by the sediment bacterial community. Benthic microalgae exude complex biopolymers, called extracellular polymeric substances (EPS), which consist mainly of carbohydrates, but can contain proteins and nucleic acids. EPS serves multiple functions for the benthic microalgae including: sediment attachment and stabilization and provides a labile substrate that may facilitate the tight coupling between benthic microalgae and sediment bacteria.

A novel experimental apparatus, called the perfusionator, was used to examine carbon and nitrogen cycling through the benthic microalgal and sediment bacterial communities during a 48-­‐day field experiment. Dual stable isotopic tracers (H13CO3-­‐ and 15NH4+) were added to the porewater within the perfusionators to trace carbon and nitrogen cycling through inorganic and organic pools in order to assess the role of benthic microalgae in sediment nutrient cycling and sequestration. Physical and biological processes were characterized by: (1) installing covers over half of the perfusionators on day 14 to dampen physical mixing and sediment reworking, (2) using glass beads to track sediment mixing and reworking within the perfusionators, (3) continuous measurements of current speeds using an acoustic Doppler velocimeter and (4) monitoring a suite of environmental variables. The dominant feature of the experimental period was a harmful algal bloom (chl a concentrations peaked on day 14 at 107 μg L-­‐1) dominated by Cochlodinium polykrikoides, which reduced biomass of benthic microalgae.

Sediment total organic carbon and nitrogen (TOC and TN), benthic microalgae isolated through Ludox extractions, and EPS were enriched in 13C and 15N during the labeling period. Two fractions of EPS were extracted from the sediments, a hot water (HW-­‐EPS) or intracellular fraction and a hot bicarbonate (HB-­‐EPS) or extracellular fraction. Concentrations of HB-­‐EPS were higher than HW-­‐ EPS, but the HW-­‐EPS showed enrichment in 13C before the HB-­‐EPS, consistent with its association with photosynthetic pathways inside the cell. The carbohydrate content of the EPS was higher later in the experiment when sediment chlorophyll concentrations were lower. The carbohydrate composition of the EPS indicated that a higher proportion of material was probably derived from sources other than benthic microalgae including bacterial sources. Increased concentrations of phaeopigments in surface sediments corroborated increased contributions from decaying or degraded material, likely derived from the harmful algal bloom. Overall, this field experiment traced carbon and nitrogen through the benthic microalgal community in a shallow coastal system, and captured the response of the system to a harmful algal bloom event. This experiment provided new insights about carbon and nitrogen dynamics within shallow systems and benthic microalgae community responses to key environmental events.



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