Montoya, Joseph P.
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ItemNatural abundance of ¹⁵N in particulate nitrogen and zooplankton in the Chesapeake Bay(Georgia Institute of Technology, 1990-07) Montoya, Joseph P. ; Horrigan, S. G. ; McCarthy, J. J. ; College of Sciences ; School of Biological SciencesSamples of dissolved inorganic nitrogen (DIN), particulate nitrogen (PN), and several species of zooplankton were collected at a series of stations in the main channel of the Chesapeake Bay, USA, during cruises in spring and fall 1984. The spatial and temporal variation in the natural abundance of ¹⁵N (δ¹⁵N) in each of these pools, in combination with measurements of the concentrations of DIN, PN, plant pigments, and the rates of biologically-mediated transformations of nitrogen, provide a number of insights into the dynamics of the nitrogen cycle in the Chesapeake Bay. During both spring and fall δ¹⁵N of surface layer PN showed no consistent Bay-wide pattern of distribution. Instead, the overall gradient of DIN concentrations along the axis of the Bay appears to be less important than local processes in determining the distribution of ¹⁵N in PN. The relationship between δ¹⁵N PN and δ¹⁵N of dissolved pools indicated that phytoplankton uptake was the dominant process acting on DIN in spring, but that microbially-mediated transformations of nitrogen dominated in fall. During both seasons δ¹⁵N of particulate and dissolved pools suggested that phytoplankton consume both NO₃ and NH₄⁺ roughly in proportion to concentration. The δ¹⁵N of the zooplankton species sampled generally increased with trophic level. The δ¹⁵N of the copepod Acartia tonsa was higher than that of PN by 4.2 ± 2.3 ‰ (X ± SD) in spring and 3.3 ±1.0 ‰ (X±SD) in fall. Similarly. δ¹⁵N of the ctenophore Mnemiopsis leidyi was higher than that of A. tonsa by 2.0 ± 2.6‰ (X±SD) in spring and 3.3 ±1.0‰ (X±SD) in fall. A reversal of the usual relationship between A. tonsa and M. leidyi occurred near the southern end of the Bay during spring, where δ¹⁵N of the copepod was greater than that of the ctenophore by as much as 4.9‰. In general, spatial variability of δ¹⁵N of all 3 of these trophic levels (PN, copepods, and ctenophores) was greater in spring than in all, suggesting that phyto- and zooplankton have a greater direct influence on the estuarine nitrogen cycle during spring. A comparison of the 2 transects conducted on each cruise demonstrates that δ¹⁵N of the PN and A. tonsa, but not that of M. leidyi, can change markedly on a time scale of roughly a week. Such changes clearly indicate that repeated sampling may be essential in studies of the natural abundance of ¹⁵N in dynamic planktonic systems such as that in the Chesapeake Bay.
ItemNitrogen fixation by Trichodesmium spp.: An important source of new nitrogen to the tropical North Atlantic Ocean(Georgia Institute of Technology, 2005) Capone, Douglas G. ; Burns, James A. ; Montoya, Joseph P. ; Subramaniam, Ajit ; Mahaffey, ; Gunderson, Troy ; Michaels, Anthony F. ; Carpenter, Edward J. ; College of Sciences ; School of Biological Sciences ; Georgia Institute of Technology. School of Biology ; Lamont Doherty Earth Observatory of Columbia University ; University of Hawaii at Manoa. School of Ocean and Earth Science and TechnologyThe broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth-integrated N₂ fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO₃ into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N₂ fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N₂ fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 X 10 ¹² mol N by Trichodesmium N₂ fixation alone. This input can account for a substantial fraction of the N₂ fixation in the North Atlantic inferred by several of the geochemical approaches.
ItemNitrogen stable isotope dynamics in the central Baltic Sea: influence of deep-water renewal on the N-cycle(Georgia Institute of Technology, 1997-11-17) Voss, Maren ; Nausch, Günter ; Montoya, Joseph P. ; College of Sciences ; School of Biological SciencesThe vertical profiles of NO₃-, NH₄+, O₂, and H₂S as well as the isotopic composition of particulate nitrogen and NH4+ were sampled yearly over a 5 yr period in the Gotland Basin to follow biochemical changes in N-cycling resulting from an inflow of saltwater. The water column has a pronounced interface at 80 to 120 m depth which separates warm (13°C) brackish surface waters (salinity 7 psu) and the underlying cold winter water layer from more saline (9 to 11 psu) bottom waters originating from irregularly occurring inflow events of oxygenated, nitrate-rich North Sea water masses. Anoxic conditions usually exist in the deep stagnant waters, where nutrients only occur as ammonia, which reaches concentrations of up to 30 µmol l-1. In spring 1993 large amounts of nitrate- and oxygen-rich water were transported into the deep waters of the Gotland Basin, thus displacing the stagnant deep water body. With the inflow, oxygen and nitrate concentrations rose by 3 ml l-1 and more than 10 µmol l-1 respectively. During the following years the concentrations of oxygen in the near bottom layer decreased again. The isotope signature of the suspended particles in the layer below 120 m reflects these changes: in 1993 the mean stable nitrogen isotope value in the anoxic water was at 1.1o/oo. We assume bacterial incorporation of ammonia to be the mechanism producing isotopically light particles. A fractionation factor calculated for ammonia uptake of 11 ‰ supports this hypothesis. During the following years the particles in the oxygenated water column were around 8o/oo which is characteristic for microbially degraded material. The surface sediment of the central Gotland Sea has a low isotope signal of 3 to 4o/oo. These findings might have consequences for the interpretation of sediment δ15N data where low isotope contents are usually taken as an indicator of high nutrient concentrations in surface waters.
ItemUse of stable isotopes to investigate individual differences in diets and mercury exposures among common terns Sterna hirundo in breeding and wintering grounds(Georgia Institute of Technology, 2002-10-22) Nisbet, Ian C. T. ; Montoya, Joseph P. ; Burger, Joanna ; Hatch, Jeremy J. ; College of Sciences ; School of Biological Sciences ; Rutgers University. Dept. of Biological Sciences ; University of Massachusetts. Dept. of BiologyWe measured variations in stable isotope signatures (δ¹³C and δ¹⁵N) and concentrations of mercury (Hg) in breast feathers from pairs of common terns Sterna hirundo and their chicks at a breeding site in Buzzards Bay, northwestern Atlantic Ocean. By collecting 2 sets of feathers from the same adult birds, we compared values of δ¹³C, δ¹⁵N and Hg in feathers grown in the wintering area in the South Atlantic Ocean (‘southern’ feathers) and in the breeding area (‘regrown’ feathers). Regrown feathers had lower δ¹³C, higher δ¹⁵N and higher Hg than southern feathers. Values of δ¹³C , δ¹⁵N and Hg were much more variable in adults than chicks. Within families, δ¹³C and δ¹⁵N were correlated between parents and chicks; Hg was correlated between male and female parents. Among regrown feathers, Hg was positively correlated with δ¹³C and negatively correlated with δ¹⁵N. These findings suggest that high individual exposure of common terns to Hg results from consumption of inshore prey at low trophic levels in restricted parts of Buzzards Bay and that members of pairs have similar diets in the breeding season but not in winter. They demonstrate the power of stable isotope analyses in revealing individual differences in foraging habits, diet and contaminant exposure in generalist predators.
ItemBiology and ecology of newly discovered diazotrophs in the open ocean(Georgia Institute of Technology, 2009-10-30) Montoya, Joseph P. ; Zehr, Jon ; College of Sciences ; School of Biological Sciences ; University of California, Santa Cruz
ItemExtensive bloom of a N₂-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean(Georgia Institute of Technology, 1999-08-20) Carpenter, Edward J. ; Montoya, Joseph P. ; Burns, James ; Mulholland, Margaret R. ; Subramaniam, Ajit ; Capone, Douglas G. ; College of Sciences ; School of Biological Sciences ; University of Maryland Center for Environmental Sciences. Chesapeake Biological Laboratory ; USC Wrigley Institute for Environmental StudiesWe encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South America in autumn 1996. Each diatom cell contained the heterocystous, N₂-fixing cyanobacterial endosymbiont Richelia intracellularis. Surface Richelia heterocyst (and filament) densities increased from <100 to >10⁶ heterocyst l⁻¹ in the bloom. Total abundance ranged from 10⁶ heterocyst m⁻² outside the bloom to over 10¹⁰ heterocyst m⁻² within the bloom. Rates of primary production averaged 1.2 g C m⁻² d⁻¹, higher than typical for oligotrophic open ocean waters. N₂ fixation during the bloom by the Richelia/Hemiaulus association added an average of 45 mg N m⁻² d⁻¹, to the water column. The relative importance of NH₄⁺ uptake over the course of the bloom increased from 0 to 42% of total N uptake by the Hemiaulus/Richelia association. N2 fixation by Richelia exceeded estimates of 'new' N flux via NO₃ diffusion from deep water and, together with additional N₂ fixation by the cyanobacterium Trichodesmium, could supply about 25% of the total N demand through the water column during the bloom. Suspended particles and zooplankton collected within the bloom were depleted in ¹⁵N, reflecting the dominant contribution of N₂ fixation to the planktonic N budget. The bloom was spatially extensive, as revealed by satellite imagery, and is calculated to have contributed about 0.5 Tg N to the euphotic zone. Such blooms may represent an important and previously unrecognized source of new N to support primary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates that heterocystous cyanobacteria can also make quantitatively important contributions of N in oceanic water column environments.
ItemTrophic-level interpretation based on δ¹⁵N values: implications of tissue-specific fractionation and amino acid composition(Georgia Institute of Technology, 2004-01-30) Schmidt, Katrin ; McClelland, James W. ; Mente, Eleni ; Montoya, Joseph P. ; Atkinson, Angus ; Voss, Maren ; College of Sciences ; School of Biological Sciences ; University of Aberdeen. Dept. of Zoology ; British Antarctic Survey ; Marine Biological Laboratory (Woods Hole, Mass.)Stable nitrogen isotope ratios are routinely used to disentangle trophic relationships. Several authors have discussed factors in addition to diet that might contribute to variability in δ¹⁵N of consumers, but few studies have explored such factors in detail. For a better understanding of tissue-specific differences in δ¹⁵N, we examined postlarval euphausiids across a variety of seasons and regions in the Southern Ocean. The concentration and δ¹⁵N of individual amino acids were analysed to account for both the biochemical and physiological underpinnings of the observed bulk δ¹⁵N. Euphausiids showed consistent d15N differences of 1 to 2 ” between the digestive gland and abdominal segment, and between reproductively active males and females. These differences in bulk δ¹⁵N were accompanied by variations in relative proportions of amino acids (up to 5 mol %) and their δ¹⁵N (up to 11‰). Aspartic acid and glutamic acid had the strongest influence on bulk δ¹⁵N, due to their high abundance and variable δ¹⁵N values. Differences in relative proportions and/or δ¹⁵N of glycine alanine were also important for bulk δ¹⁵N values. Isotopic variations in amino acids between gender and tissues were explained by dominant internal processes such as protein synthesis or degradation for energy supply, and by differences in amino acid pool sizes. Despite the offset in bulk δ¹⁵N between females and males, several lines of evidence suggested that their trophic levels were similar. Thus, specific amino acid composition and metabolism may confound trophic level interpretations of bulk δ¹⁵N values. Micronekton are normally analyzed whole in isotopic studies, and we suggest that their analyses should be restricted to comparable tissues such as muscles.
ItemEstimating the contribution of microalgal taxa to chlorophyll a in the field-variations of pigment ratios under nutrient- and light-limited growth(Georgia Institute of Technology, 1998-08-06) Goericke, Ralf ; Montoya, Joseph P. ; College of Sciences ; School of Biological SciencesCellular concentrations of chlorophylls and carotenoids were measured in nutrient- and light-limited cultures of marine microalgae to determine the utility of accessory pigments as proxies for the biomass of specific groups of microalgae in the ocean. In most species, concentrations of chlorophyll a (chl a) and photosynthetically active pigments varied linearly with growth rate in nitrate-limited continuous cultures or with the logarithm of the irradiance in light-limited and light-sufficient batch cultures, as has been observed before. Rates of pigment-concentration change as a function of irradiance or growth rate did not covary with rates of maximum growth. Concentrations of carotenoids covaried with chl a in most species analyzed; intraspecies variations of chl a-carotenoid ratios were usually smaller than variations of chl a:b or chl a:c ratios. These results were used to critically evaluate the assumptions underlying iterative methods used to determine the contribution of different algal taxa to chl a from ratios of chl a and accessory pigments. Estimates based on chl a:b or chl a:c ratios are prone to error because these ratios can vary by up to an order of magnitude among species and within species as a function of irradiance, thus violating an assumption of the iterative methods. Instead, such methods should rely on ratios of chl a and photosynthetically active carotenoids. Using simple models and field data from the Chesapeake Bay, USA, we showed that iterative methods are either prone to error when different populations of microalgae co-vary or do not give discrete solutions. As an alternative we suggest methods that rely more strongly on empirically determined pigment ratios.
ItemGeophysical and geochemical signatures of Gulf of Mexico seafloor brines(Georgia Institute of Technology, 2005-05) Joye, S. B. ; MacDonald, I. R. ; Montoya, Joseph P. ; Peccini, M. ; College of Sciences ; School of Biological Sciences ; Georgia Institute of Technology. School of BiologyGeophysical, temperature, and discrete depth-stratified geochemical data illustrate differences between an actively venting mud volcano and a relatively quiescent brine pool in the Gulf of Mexico along the continental slope. Geophysical data, including laser-line scan mosaics and sub-bottom profiles, document the dynamic nature of both environments. Temperature profiles, obtained by lowering a CTD into the brine fluid, show that the venting brine was at least 10°C warmer than the bottom water. At the brine pool, two thermoclines were observed, one directly below the brine-seawater interface and a second about one meter below the first. At the mud volcano, substantial temperature variability was observed, with the core brine temperature being either slightly (~2°C in 1997) or substantially (19°C in 1998) elevated above bottom water temperature. Geochemical samples were obtained using a device called the "brine trapper" and concentrations of dissolved gases, major ions and nutrients were determined using standard techniques. Both brines contained about four times as much salt as seawater and steep concentration gradients of dissolved ions and nutrients versus brine depth were apparent. Differences in the concentrations of calcium, magnesium and potassium between the two brine fluids suggests that the fluids are derived from different sources or that brine-sediment reactions are more important at the mud volcano than the brine pool. Substantial concentrations of methane and ammonium were observed in both brines, suggesting that fluids expelled from deep ocean brines are important sources of methane and dissolved inorganic nitrogen to the surrounding environment.
ItemPlasticity of N:P ratios in laboratory and field populations of Trichodesmium spp.(Georgia Institute of Technology, 2006-03-29) Krauk, Jamie M. ; Villareal, Tracy A. ; Sohm, Jill A. ; Montoya, Joseph P. ; Capone, Douglas G. ; College of Sciences ; School of Biological Sciences ; USC Wrigley Institute for Environmental Studies ; University of Southern California. Dept. of Biological Sciences ; Georgia Institute of Technology. School of BiologyWe followed changes in N:P ratios in batch cultures of the planktonic marine cyanobacterium Trichodesmium (IMS 101) grown in 2 different media and in field populations from 4 different oceanic regions. Cultures grown on low P media showed a rapid rise in N:P ratio upon depletion of phosphate. Ratios exceeding 125 were reached in 1 experiment before attaining stationary phase. A transect across the North Atlantic Ocean along 32°N showed a monotonic decrease in the N:P ratio of field collected colonies, dropping from about 60:1 on the western side of the basin to about 30:1 on the eastern side. A second cruise sampled colonies and surface slicks in waters along the north coast of Australia, where ratios of N:P were generally lower than in the North Atlantic, ranging from 11:1 to 47:1 with an average of 22:1. A comparison of rising and sinking colonies collected at 8 stations in the Gulf of Mexico shows a higher mean N:P ratio among sinking colonies than floating colonies. Overall, the average N:P in the Gulf of Mexico was about 68:1. N:P ratios of Trichodesmium around the Hawaiian Islands were very consistent between 2 consecutive years of sampling, with an average colony N:P for both years of about 38:1. Our research demonstrates high variability in the cellular N:P in Trichodesmium both in the laboratory and in the field. Trichodesmium N:P ratio may provide an index to the relative severity of P limitation in these diazotrophs. Geochemical and ecological modeling efforts which rely on using the N:P ratio of diazotrophs in deriving nitrogen fixation rates should account for the variability of these ratios in situ.