DON as a source of bioavailable nitrogen for phytoplankton

Relative to inorganic nitrogen, concentrations of dissolved organic nitrogen ( DON) are often high, even in regions believed to be nitrogen-limited. The persistence of these high concentrations led to the view that the DON pool was largely refractory and therefore unimportant to plankton nutrition. Any DON that was utilized was believed to fuel bacterial production. More recent work, however, indicates that fluxes into and out of the DON pool can be large, and that the constancy in concentration is a function of tightly coupled production and consumption processes. Evidence is also accumulating which indicates that phytoplankton, including a number of harmful species, may obtain a substantial part of their nitrogen nutrition from organic compounds. Ongoing research includes ways to discriminate between autotrophic and heterotrophic utilization, as well as a number of mechanisms, such as cell surface enzymes and photochemical decomposition, that could facilitate phytoplankton use of DON components

Nitrogen uptake by phytoplankton and bacteria during an induced Phaeocystis pouchetii bloom, measured using size fractionation and flow cytometric sorting

Uptake of inorganic and organic nitrogen (N) by phytoplankton and bacteria was investigated during a mesocosm study conducted in Raunefjord, Norway in April 2005. One mesocosm was fertilized with nitrate and phosphate at a ratio of 16:1 and maintained in the light, while one unamended light mesocosm served as a control. Dissolved nutrients, phytoplankton and bacterial biomass, and phytoplankton community composition were monitored throughout the 26 d experiment. Uptake of (15)N-labeled ammonium and nitrate, and dual-labeled ((15)N and (13)C) urea and dissolved free amino acids (DFAA) was measured for phytoplankton and bacteria using 2 methods: size fractionation into 0.2-0.8 and \u3e 0.8 pm size classes, and flow cytometric sorting based on chlorophyll autofluorescence. Prior to fertilization, dissolved inorganic N concentrations were low and comprised similar to 5% of total dissolved N. Added nitrate was completely utilized in the amended mesocosm within 10 d, stimulating a large bloom of colonial Phaeocystis pouchetii. Ammonium contributed over half of total measured N uptake by phytoplankton and bacteria in both enclosures, while nitrate and urea each supplied roughly 10 to 25%. Overall, DFAA were a negligible N source to phytoplankton but contributed 11 % to total bacterial N uptake. Bacterial uptake represented a significant portion of total uptake of all N forms, especially urea and DFAA. Comparison of the 2 methods for measuring phytoplankton versus bacterial uptake demonstrates how the use of 0.8 mu m filters can lead to significant overestimation of phytoplankton N uptake due to the retention of bacterial biomass

Blooms of \u3cem\u3eKarenia Brevis\u3c/em\u3e (Davis) G. Hansen & Ø. Moestrup on the West Florida Shelf: Nutrient Sources and Potential Management Strategies Based on a Multi-Year Regional Study

Identification and quantification of the nutrient sources supporting large, extended duration Karenia brevis blooms on the West Florida Shelf (WFS) in the eastern Gulf of Mexico are critical steps for effective bloom management and mitigation. Previous research had identified multiple (\u3e 12) potential nutrient sources available to K. brevis blooms on the WFS, which vary with bloom stage, location, biomass and bloom toxicity. This current study newly identified and quantified additional nitrogen (N) sources including water column nitrification, photochemical nutrient production, pelagic unicell N2 fixation by diazotrophs other than the colonial cyanobacterium Trichodesmium, and remineralization from seasonal Trichodesmium biomass decay and microzooplankton grazing (and estimated regeneration). Newly identified phosphorus (P) sources include remineralization from Trichodesmium biomass decay and microzooplankton grazing. In estuarine environments, benthic nutrient flux, mixotrophic consumption of picoplankton, nutrient release from zooplankton and microzooplankton grazing, photochemical nutrient production, and nitrification all can contribute up to 100% of the N and/or P requirements of small (\u3c 105 cells L−1) K. brevis blooms. During average estuarine flow years, combined estuarine sources contribute up to 17 and 69% of the N and P needs of these blooms, however local estuarine contribution can increase to 100% for exceptional, high flow years. In coastal and offshore environments, regenerated nutrient sources become increasingly important to blooms, with zooplankton excretion, nitrification, decay and regeneration of nutrients from dead fish and pelagic N2 fixation potentially providing 100% of bloom N and P needs. During the largest observed coastal blooms (14.0 × 106 cells L−1) N2 fixation and release and decay of seasonal Trichodesmium bloom biomass were the only sources of N and P that were completely sufficient to support blooms of that magnitude. Given the complexity of K. brevis bloom dynamics, the multiple available nutrient sources on the WFS and the importance of regenerated N forms in supporting blooms, efforts to reduce potentially controllable nearshore nutrient inputs should be undertaken with the understanding that while they may lead to enhanced coastal water quality, they may not have an immediate impact on the frequency or magnitude of nearshore K. brevis blooms. Additionally, time lags in ecosystem responses or differences in the time scales on which various process operate may require multi-year assessments to determine how effective management practices are in relation to K. brevis blooms. Timely red tide related monitoring products that allow for effective focusing of monitoring needs for short-term prediction of impacts and targeted communication of scientific results to the public and stakeholders, remains the most effective means of K. brevis management