Provenance, concentrations and nature of sedimentary organic nitrogen in the gulf of Maine

Abstract

Factors controlling the provenance, concentrations, and nature of sedimentary organic matter (SOM), particularly the nitrogenous fraction, were examined for sites throughout the Gulf of Maine and two of its estuaries. Stable nitrogen isotope data corroborate previously reported Br: C ratios in indicating that macroalgae are important contributors to organic nitrogen pools in estuarine sediments. Significant contributions of terrigenous organic carbon in both the estuaries and the deep basins of the open Gulf are not accompanied by significant contributions of terrigenous organic nitrogen. Variations in concentrations of organic carbon, nitrogen, chlorophyll, and enzymatically hydrolyzable protein throughout the Gulf and its estuaries can be explained largely by variations in the delivery rate of organic matter to the benthos and the specific surface area of minerals in the sediments. Chlorophyll and protein concentrations represent relatively fresh organic matter and exhibit greater sensitivity to organic delivery variations than do total organic carbon or nitrogen. A strong, linear relationship between organic carbon and surface area is consistent with monolayer adsorption of SOM on mineral surfaces, which appears to inhibit degradation of the SOM. Amino acid compositions are remarkably constant throughout the Gulf, with large differences seen only with elevated tyrosine in estuaries and elevated basic amino acids and methionine in the open Gulf. Leucine and glutamic acid correlate with chlorophyll concentrations in open Gulf sites. Enzymatically hydrolyzable protein comprises small to major fractions of total hydrolyzable amino acids. Amino acid-nitrogen: total nitrogen correlates inversely with total nitrogen.