Water column nutrients and sedimentary denitrification in the Gulf of Maine

Abstract

Sedimentary denitrification acts to remove nitrogen from both the sediments and water column in continental shelf ecosystems, so that in enclosed shelf areas where water residence times are long (about a year in the Gulf of Maine), significant rates of sedimentary denitrification might lower inorganic nitrogen concentrations. We examined this using a basin-wide suite of hydrographic and nutrient data collected in mid-summer. Total inorganic nitrogen (TIN) concentrations (nitrate + nitrite + ammonium) were highest in the aged North Atlantic continental slope waters found at depth in the Jordan Basin on the eastern side of the Gulf. Phosphate and silicate concentrations were moderately high in these waters. On the western side of the Gulf, Wilkinson Basin receives much less of the nutrient-rich slope waters. In these deep waters, TIN concentrations were lower and phosphate and silicate concentrations higher than in the Jordan Basin. In the intermediate and deep waters, TIN/PO 4 ratios averaged 19 on the eastern side but only 15–16 on the western side of the Gulf. Partially isolated regions within the Gulf had even lower TIN/PO 4 ratios. The differences in this ratio suggested the occurrence of either a non-stoichiometric reduction in nitrogen or an enrichment in phosphate between the eastern and western sides of the Gulf. The relationship between phosphate and silicate was identical in open waters on both sides of the Gulf, indicating that the difference in the TIN/PO 4 ratio was due to a loss of nitrogen. A parameter, delta-N, quantified the non-stoichiometric nitrogen loss and was defined as, ΔN = α[PO 4] − [TIN], where [PO 4] and [TIN] were the measured concentrations in an individual water sample and α was the average TIN/PO 4 ratio in the Jordan Basin. Within and below the Maine Intermediate Waters (50–120 m), the distribution of ΔN showed the removal of 2–4 μgat N l −1 in most of the waters west and south-west of the Jordan Basin. ΔN was greater in waters close to the sediments suggesting that sediments were the site of nitrogen removal. The overall rate of denitrification was estimated to be 31.2–46.8 × 10 9 gat N y −1 based on the average ΔN, the water volume within the Gulf and the water residence time. When normalized to sediment area, this rate was 0.80–1.21 pgat N cm −2 s −1. The classical inorganic nitrogen budget of the Gulf of Maine, based on inflow rates and nutrient concentrations of the source waters, was reassessed and found to be unbalanced by 43.1 × 10 9 gat N y −1 with TIN inputs (138.6 × 10 9 gat N y −1) exceeding advective TIN losses (−95.5 × 10 9 gat N y −1). Organic matter burial and net organic nitrogen export at the rate of 1.5 % of the primary productivity could account for 23 % of the imbalance (−10.0 × 10 9 gat N y −1). Denitrification, at the aerial rate of 0.85 pgat N cm −2 s −1 accounted for the remainder (−33.1 × 10 9 gat N y −1).