Stable isotopic variation in particulate organic matter and dissolved inorganic compounds in a northern fjord : implications for present and past environments

Ostrom, Nathaniel Emil (1992) Stable isotopic variation in particulate organic matter and dissolved inorganic compounds in a northern fjord : implications for present and past environments. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (9MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The principle objectives of this study were (1) to gain a detailed understanding of processes resulting in isotopic variation in water column organic matter and (2) to establish a relationship between the isotopic composition of water column material to that of underlying sediments. To facilitate this task nutrient and elemental abundance data and the stable isotopic composition of seston, sinking POM, sediments and inorganic compounds (DIC, NO₃ and NH₄⁺) was determined on a seasonal and spatial basis in a northern fjord, Conception Bay, Newfoundland. -- The concentration and isotopic composition of inorganic nitrogen in deep and pore waters provided insight into sources and cycling of these compounds. Ammonium in the porewaters obtained from the surface 20cm of sediment cores ranged in concentration from 34.7μM to 239.9μM. The nitrogen isotopic composition of porewater ammonium averaged 7.0‰ and was depleted in ¹⁵N by 2‰ relative to sediments. This difference indicates a small fractionation effect during the remineralization of ammonium from organic matter in sediments. Nitrate in water column samples had a very wide range in δ¹⁵N values of -6.2‰ to 7.9‰ and an average of 0.2 ± 3.6‰. The low δ¹⁵N of nitrate relative to pore water ammonium indicates that there is a large fractionation effect associated with nitrification (β = 1.0193) and that this reaction does not proceed to completion at the cold temperatures (<0ﾟC) present in the deep waters of this fjord. The presence of ammonium in deep waters and as a measurable flux from sediments confirms that this substrate is not completely consumed during nitrification. -- The δ¹⁵N of suspended POM varied from 5.2‰ to 20.2‰, with the highest values occurring in the spring at the base of the euphotic zone. High δ¹⁵N values may be a consequence of fractionation during peptide bond cleavage or deamination of proteins during partial degradation. Suspended POM δ¹³C values were narrower in range than δ¹⁵N and were between -26.7‰ and -21.9‰. Enrichments in ¹³c were frequently associated with the chlorophyll maximum during periods of high productivity. Concomitant decreases in the concentration of dissolved inorganic carbon and increases in seston δ¹³C values confirmed results of earlier research which found a inverse correlation between pCO₂ and the δ¹³C of phytoplankton. The carbon isotopic composition of DIC ranged from -4.1‰ to 1.6‰. Low δ¹³C-DIC values may be related to brief periods of enhanced respiration as a result of increased substrate concentration during the spring bloom. The higher δ¹³C-DIC values are typical for DIC equilibrated with atmospheric CO₂. Sediment trap δ¹⁵N and δ¹³C values were similar in range to suspended POM. The isotope values predicted by a model, which uses the isotopic composition of particulates and flux measurements, are similar to the average δ¹⁵N and δ¹³C determined for sediments in Conception Bay, 8.6‰ and -21.4‰, respectively. These results indicate that sediments are most similar in δ¹⁵N and δ¹³C to phytoplankton during periods of high primary production.

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