Jay A. Brandes

A global marine-fixed nitrogen isotopic budget: Implications for Holocene nitrogen cycling

[1]xa0A nitrogen stable isotopic model was constructed in order to constrain the Holocene marine-fixed nitrogen budget. The primary sources and sinks considered were riverine and atmospheric sources, nitrogen fixation, sedimentary and water column denitrification, and sediment burial. The source budget was found to be insensitive to changes in nitrogen fixation rates, and thus could not be used to constrain this term. However, the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification. If the isotopic value of marine-fixed nitrogen has not changed during the Holocene, as supported by sedimentary records, then in order to balance the isotopic value of sinks and sources, approximately 280 Tg N yr−1 of sedimentary denitrification is required. If such a high rate of denitrification has been sustained throughout the Holocene, it implies that present-day estimates of marine nitrogen fixation are grossly underestimated. It also implies that the marine nitrogen budget has a residence time of less than 2000 years.

Stable isotopic composition of nitrate in Lake Taihu, China, and major inflow rivers

Identification of sources of nutrient pollution is a first step towards remediation of eutrophication in aquatic ecosystems. The stable isotope nitrogen-15 (15N) is a natural indicator of nitrogen (N) source and biogeochemistry. We sampled Lake Taihu, a hyper-eutrophic lake in eastern China, and major inflow rivers during winter and spring of 2004 to determine concentration and δ15N of nitrate (NO 3 - ). Nitrate concentrations in rivers and the lake were higher, in most cases, in spring than in winter. δ15N of NO 3 - was not correlated with NO 3 - concentration, indicating that concentrations alone are insufficient to describe N sources. Results show that riverine N inputs in winter are influenced by discharge of human sewage into rivers and the lake. In spring, however, wastewater inputs to the lake appear to be balanced by fertilizers, atmospheric, and/or N2 fixation sources. Rain NO 3 - concentrations were seasonally high and isotopically enriched compared to potential sources, indicating that rain may be a significant or even dominant source of N to the lake during the rainy season. δ15N values show that urbanized areas of the lake have more sewage-derived N than those areas dominated by agriculture, aquaculture, or industry. This observation has important implications for human health, since Lake Taihu is a source of drinking and irrigation water as well as fish for human consumption.

A high sensitivity, low volume HPLC method to determine soluble reactive phosphate in freshwater and saltwater

Abstract A reverse phase High Performance Liquid Chromatography (HPLC) method was developed to detect soluble reactive phosphate (SRP) in marine and freshwater samples. This technique is scalable over a range of sample volumes from 10 μl to 10 ml, requires minimal sample preparation, and responds linearly from 1 nM to 100 μM ( R 2 >0.964). Standard Murphy–Riley molybdenum blue reagents were used to react with phosphate in samples for analysis. Solid phase extraction tubes were used to clean the mixed complexing reagent prior to combining the reagent with samples. The method was applied to freshwater samples collected in July 2001 along an Andean mountain stream located in the Eastern Cordillera (∼2400 m) of Peru and marine samples collected in January 2002 within the low marsh of the estuarine Nueces Delta System, Texas Coastal Bend. For method comparison, field samples were analyzed using a standard manual spectrophotometric method. Field sample phosphate concentrations produced from each method were nearly identical. This method offers the advantages of low sample volume (1 ml or less) with high sensitivity, precision, and potential automation.