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1. Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) of samples collected from rivers draining onto the West Florida Shelf and submarine groundwater wells on the Western coast of Florida from Nov 2022 to Mar 2024

   https://doi.org/10.26008/1912/bco-dmo.989225.1  

   Knapp, Angela N (December 2025, Biological and Chemical Oceanography Data Management Office (BCO-DMO))

   The West Florida Shelf (WFS) is oligotrophic, with inorganic N and P concentrations typically at or below detection limits, and yet significant rates of primary productivity, including blooms of the cyanobacterial diazotroph Trichodesmium spp. as well as the harmful algal species Karenia brevis, are observed there. Consequently, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are thought to be the primary sources of assimilative nutrients on the WFS. Here we report measurements of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations made on samples collected quarterly from rivers draining onto the WFS as well as from submarine groundwater wells on three transects extending from the Florida Nature Coast in the north to Venice Headland in the south. Elevated concentrations of TDN (>12 µM) and TDP (>0.5 µM) were observed in riverine and groundwater samples, as well as in the shallow, nearshore region, while to the west concentrations of both TDN and TDP decreased to values typically associated with oligotrophic North Atlantic waters (4 to 5 µM TDN and 0.05 to 0.1 µM TDP). 

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2. Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) of samples collected on the West Florida Shelf during STING II cruise EN704 on R/V Endeavor from Jul 1 to Jul 12, 2023

   https://doi.org/10.26008/1912/bco-dmo.986861.1  

   Knapp, Angela N (December 2025, Biological and Chemical Oceanography Data Management Office (BCO-DMO))

   The West Florida Shelf (WFS) is oligotrophic, with inorganic N and P concentrations typically at or below detection limits, and yet significant rates of primary productivity, including blooms of the cyanobacterial diazotroph Trichodesmium spp. as well as the harmful algal species Karenia brevis, are observed there. Consequently, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are thought to be the primary sources of assimilative nutrients on the WFS. Here we report measurements of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations made on samples collected on a cruise in July of 2023 aboard the R/V Endeavor. Elevated concentrations of TDN (>12 µM) and TDP (>0.5 µM) were observed in the shallow, nearshore region, while to the west concentrations of both TDN and TDP decreased to values typically associated with oligotrophic North Atlantic waters (4 to 5 µM TDN and 0.05 to 0.1 µM TDP). 

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3. Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) of samples collected on the West Florida Shelf during STING I cruise AE2305 on R/V Atlantic Explorer from Feb 20 to Mar 05, 2023

   https://doi.org/10.26008/1912/bco-dmo.986688.1  

   Knapp, Angela N (December 2025, Biological and Chemical Oceanography Data Management Office (BCO-DMO))

   The West Florida Shelf (WFS) is oligotrophic, with inorganic N and P concentrations typically at or below detection limits, and yet significant rates of primary productivity, including blooms of the cyanobacterial diazotroph Trichodesmium spp. as well as the harmful algal species Karenia brevis, are observed there. Consequently, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are thought to be the primary sources of assimilative nutrients on the WFS. Here we report measurements of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations made on samples collected on a cross-shelf cruise in Feb-Mar 2023 aboard the R/V Atlantic Explorer. Elevated concentrations of TDN (>12 µM) and TDP (>0.5 µM) were observed in the shallow, nearshore region, while to the west concentrations of both TDN and TDP decreased to values typically associated with oligotrophic North Atlantic waters (4 to 5 µM TDN and 0.05 to 0.1 µM TDP). 

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4. Dissolved organic matter stimulates N2 fixation and nifH gene expression in Trichodesmium

   https://doi.org/10.1093/femsle/fnaa034  

   Benavides, Mar; Duhamel, Solange; Van Wambeke, France; Shoemaker, Katyanne M; Moisander, Pia H; Salamon, Ellen; Riemann, Lasse; Bonnet, Sophie (February 2020, FEMS Microbiology Letters)

   ABSTRACT Mixotrophy, the combination of heterotrophic and autotrophic nutrition modes, is emerging as the rule rather than the exception in marine photosynthetic plankton. Trichodesmium, a prominent diazotroph ubiquitous in the (sub)tropical oceans, is generally considered to obtain energy via autotrophy. While the ability of Trichodesmium to use dissolved organic phosphorus when deprived of inorganic phosphorus sources is well known, the extent to which this important cyanobacterium may benefit from other dissolved organic matter (DOM) resources is unknown. Here we provide evidence of carbon-, nitrogen- and phosphorus-rich DOM molecules enhancing N2 fixation rates and nifH gene expression in natural Trichodesmium colonies collected at two stations in the western tropical South Pacific. Sampling at a third station located in the oligotrophic South Pacific Gyre revealed no Trichodesmium but showed presence of UCYN-B, although no nifH expression was detected. Our results suggest that Trichodesmium behaves mixotrophically in response to certain environmental conditions, providing them with metabolic plasticity and adding up to the view that mixotrophy is widespread among marine microbes. 

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5. Constraining the sources of nitrogen fueling export production in the Gulf of Mexico using nitrogen isotope budgets

   https://doi.org/10.1093/plankt/fbab049  

   Knapp, Angela N; Thomas, Rachel K; Stukel, Michael R; Kelly, Thomas B; Landry, Michael R; Selph, Karen E; Malca, Estrella; Gerard, Trika; Lamkin, John (August 2021, Journal of Plankton Research)

   Moisander, Pia (Ed.)

   Abstract The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradients in upper ocean dissolved organic N and suspended particulate N concentration and/or δ15N were found that would indicate significant N2 fixation fluxes accumulated in these pools, consistent with low Trichodesmium spp. abundance. Finally, comparing the δ15N of sinking particulate N captured within vs. below the euphotic zone indicates that during late spring regenerated N is low in δ15N compared to sinking N. 

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