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Montoya,
Joseph P.
Montoya,
Joseph P.
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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.We 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.
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ItemAn extensive bloom of the diazotrophic cyanobacterium, Trichodesmium, in the Central Arabian Sea during the spring intermonsoon(Georgia Institute of Technology, 1998-10-22) Capone, Douglas G. ; Subramaniam, Ajit ; Montoya, Joseph P. ; Voss, Maren ; Humborg, Christoph ; Johansen, Anne M. ; Siefert, Ronald L. ; Carpenter, Edward J.We encountered an extensive surface bloom of the N2 fixing cyanobacterium Trichodesmium erythraeum in the central basin of the Arabian Sea during the spring inter-monsoon of 1995. The bloom, which occurred during a period of calm winds and relatively high atmospheric iron content, was metabolically active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by N2 fixation could account for a major fraction of the estimated 'new' N demand of primary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N2 fixation to the organic nitrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 x 106 km2, or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N2 fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N2 fixation may account for an input of about 1 Tg N yr-1. This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N2 fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence.
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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.Cellular 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.
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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.The 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.
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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.Samples 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.