Date of Award

Summer 8-18-2017

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

David W. Townsend

Second Committee Member

Lawrence M. Mayer

Third Committee Member

Neal R. Pettigrew

Abstract

It has been historically documented that deep, nutrient-rich, offshore slope waters that enter the Gulf of Maine through the Northeast Channel are the principal source of nutrients to the region. Silicate (Si(OH)4) and nitrate (NO3), both supplied by these advective inputs, are potentially limiting nutrients for phytoplankton production. In recent years, a closer examination of nutrients in the region has revealed that relative proportions of silicate and nitrate vary considerably with space and time. Additionally, there are cases where silicate concentrations in the bottom waters exceed expected values for any slope water source. The purpose of this thesis is to determine the extent to which nutrient variability in the region may be caused by internal recycling of organic material (i.e., chemical breakdown and dissolution of diatoms) versus variability in water mass sources, which can be identified by temperature and salinity properties. This thesis examines the results from a single research cruise conducted in October 2016 in the Gulf of Maine with a focus on stations where silicate concentrations in the bottom waters exceeded 15 μM. Nutrient samples were collected at 44 stations and compared to temperature, salinity, and turbidity profiles. Additionally, suspended sediment in near- bottom waters was collected on filters at all stations, analyzed for biogenic silica, and photographed using an SEM. The results show that silicate is more likely to be elevated in the bottom waters west of Jordan Basin and that the internal recycling of silica material is more likely causing the elevated silicate than the mixing of silicate rich water masses. The near-seafloor turbid layers in these stations are thinner than in the eastern gulf and experience less intense mixing. While the results of this thesis begin to explain some degree of intra-gulf nutrient variability, they do not resolve the documented instances where elevated silicate was observed in Jordan Basin, which is near the Northeast Channel point of entry for deep slope waters to the gulf.

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