View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NERC Open Research Archive GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L15610, doi:10.1029/2009GL039719, 2009 Click Here for Full Article High numbers of Trichodesmium and diazotrophic diatoms in the southwest Indian Ocean Alex J. Poulton,1 Mark C. Stinchcombe,1 and Graham D. Quartly1 Received 22 June 2009; accepted 7 July 2009; published 8 August 2009. [1] We observed high cell numbers of large (>50mm) nitrogen, which is then available to the nutrient-starved nitrogen-fixing phytoplankton to the south and east of non-diazotrophic components of the community [Letelier Madagascar in February 2005. The distribution of the and Karl, 1996; Villareal, 1990, 1991, 1992; Mulholland nitrogen-fixing (diazotrophic) taxa found was markedly and Bernhardt, 2005]. In low latitude ecosystems, different: Trichodesmium was most abundant (500– Richelia/diatom blooms are a mechanism for rapid opal 1000 trichomes LÀ1) in the waters at the southern tip of and organic carbon export [Scharek et al., 1999a, 1999b], Madagascar; while high numbers of diazotrophic diatoms while Trichodesmium fuels upper ocean microbial produc- (125–500 cells LÀ1) were limited to the southeast of tion [Dore et al., 2008]. Madagascar. Using literature values of nitrogen-fixation per [3] The aim of this study was to assess the abundance of cell, we estimate potential rates of nitrogen-fixation for large (>50 mm) diazotrophic phytoplankton during a Trichodesmium (south of Madagascar) of 1–5 mmol N hydrographic survey of the waters to the south and east of mÀ2 dÀ1 and for diazotrophic diatoms (east of Madagascar) Madagascar (Figure 1) [Quartly, 2006]. We observed high of 0.4–2.4 mmol N mÀ2 dÀ1. These cell-based estimates numbers of two diazotrophic taxa in the area: Trichodesmium highlight the potential for significant nitrogen-fixation in and the diatom Rhizosolenia spp., the latter of which we the southwest Indian Ocean. High numbers of diazotrophic observed to contain several trichomes of Richelia (here- diatoms in subtropical waters to the east of Madagascar after termed Richelia/Rhizosolenia). From measurements may have important implications for the biogeochemistry of the abundance of these two diazotrophs, in combination of the austral phytoplankton bloom in this region. with published values of their N2-fixation per cell, we esti- Citation: Poulton, A. J., M. C. Stinchcombe, and G. D. mate potential rates of N2-fixation for these taxa in the south- Quartly (2009), High numbers of Trichodesmium and west Indian Ocean. These observations represent the first diazotrophic diatoms in the southwest Indian Ocean, Geophys. in situ information on the abundance and distribution of Res. Lett., 36, L15610, doi:10.1029/2009GL039719. diazotrophs around Madagascar, and highlight the potential role of N2-fixation in the biogeochemistry of the austral 1. Introduction phytoplankton bloom. [2] Planktonic organisms which fix atmospheric N2 (diazotrophs) have a critical role in oceanic production 2. Methods and the marine nitrogen cycle [Capone et al., 2005; [4] Water samples for the analysis of large (>50 mm) Mahaffey et al., 2005; Dore et al., 2008]. One of the most diazotrophic phytoplankton were collected during a hydro- conspicuous diazotrophic species is the colony-forming graphic survey (28/01/05–18/02/05) of the Madagascar cyanobacterium, Trichodesmium, which is found throughout Ridge and Madagascar Basin onboard the RRS Discovery tropical and subtropical oceans and often forms large-scale (Figure 1) [Quartly, 2006]. Water samples were collected surface blooms [Capone et al., 2005; Mahaffey et al., 2005; from either the ship’s non-toxic underway seawater supply Dore et al., 2008]. Another important diazotroph is the (inlet depth 5 m) every 2–4 hrs during transit, or from a cyanobacterium, Richelia intracellularis (hereafter Riche- surface (5 m) Niskin bottle from a small number (n = 24) of lia), which is an endosymbiont of several diatom genera CTD casts. Underway samples were collected every 1–2 (e.g., Hemiaulus and Rhizosolenia)[Venrick, 1974; Villareal, hrs for measurements of chlorophyll-a (n = 322) and 1990, 1991, 1992]. The role of Richelia in marine biogeo- macronutrients (nitrate, phosphate, silicate; n = 334), and chemistry is poorly understood due to a lack of measure- every 2–4 hrs for large (>50 mm) diazotrophs (n = 100). ments of its distribution, abundance or N2-fixing potential [5] Diazotroph abundance was measured on large volume [Mahaffey et al., 2005]. However, time-series observations (10 L) water samples, which were slowly concentrated from the subtropical North Pacific have recognised the down to 20 mL by gentle removal of seawater through importance of Richelia/diatom associations in the annual a50mm nylon mesh, and preserved with 2% acidic Lugol’s export of organic matter and biogenic silica from low latitude solution in 25 mL glass vials. The abundance of ecosystems [Scharek et al., 1999a, 1999b; Dore et al., 2008]. Trichodesmium colonies, individual trichomes and diatom Both Trichodesmium and Richelia are thought to release cells (LÀ1) were determined in the full preserved volume fixed nitrogen as either ammonium or dissolved organic using a 25 mL Bogorov tray and binocular microscope [after Tyrrell et al., 2003]. Colonies of Trichodesmium were converted into trichome numbers assuming each colony 1National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, Southampton, UK. consisted of 200 trichomes [e.g., Letelier and Karl, 1996; see also Carpenter et al., 2004]. The most prominent Copyright 2009 by the American Geophysical Union. diatoms in the samples were of the genera Rhizosolenia, 0094-8276/09/2009GL039719$05.00 L15610 1of4 L15610 POULTON ET AL.: DIAZOTROPHS IN THE SW INDIAN OCEAN L15610 14 ng C cellÀ1 [after Kovala and Larrence, 1966] and a carbon to Chl ratio of 100]. 3. Results 3.1. Regional Oceanography [8] The East Madagascar Current (EMC) flows down the eastern coast of Madagascar along the narrow continental shelf (Figure 1) [Tomczak and Godfrey, 1994; Quartly et al., 2006]. The interaction between EMC, Madagascar Ridge and local wind patterns cause year-round upwelling around Figure 1. Cruise track superimposed on regional bathy- the southern tip of Madagascar [Tomczak and Godfrey, metry and surface currents. Sampling locations are: MozB, 1994; Quartly et al., 2006]. Where the coast veers west, at Mozambique Basin; MadR, Madagascar Ridge; and MadB, the southern end of Madagascar, the path of the EMC is Madagascar Basin. Red/white stripped arrows show major poorly constrained (see Figure 1). Although the mean EMC currents in the region, with the East Madagascar Current flow is to the west, some of the surface water is caught by (EMC) from atlas and divergence south of Madagascar and eddies and turned cyclonically to the north along the west cyclonic eddy determined from ship-borne measurements coast of Madagascar, or anti-cyclonically to flow east at [Quartly, 2006]. 28°S[Tomczak and Godfrey, 1994; Quartly et al., 2006]. Such flow patterns are particularly evident in ocean colour imagery (Figure 2) [Quartly and Srokosz, 2004]. À3 with cell dimensions of 200–800 mmby40–60mm (n = 36), [9] A large phytoplankton bloom (>1 mg Chl m ) has and each diatom cell contained 1–2 trichomes of Richelia. been observed in austral summer from satellite images of As some of the diatom cells were smaller than the mesh size the subtropical southwest Indian Ocean, in close proximity used (50 mm), we may have slightly underestimated their to Madagascar [Longhurst, 2001; Srokosz et al., 2004; Uz, abundance. The size and appearance of the diatom cells 2007; Wilson and Qiu, 2008]. The bloom develops close to under the light microscope resembled the larger form of Madagascar in January–February, and persists until April in Rhizosolenia clevei (var. clevei)[Tomas, 1997], although the the Madagascar Basin. The Madagascar bloom exhibits full taxonomic affiliation cannot be confirmed at this time. considerable interannual variability in terms of intensity and No other diazotrophic diatom genera (e.g., Hemiaulus) were size, and was only evident as disconnected filaments of observed in the samples, and the (>50 mm) diatom elevated Chl during 1998, 2001, 2003, 2005 and 2007 community in the Madagascar Basin was solely comprised [Longhurst, 2001; Srokosz et al., 2004; Uz, 2007; Wilson of Rhizosolenia. and Qiu, 2008]. [6] We have estimated the potential magnitude of N2- [10] The Chl concentrations found during our study fixation associated with Trichodesmium from trichome were between 0.1 and 0.3 mg mÀ3, with elevated values abundance in a similar way to Tyrrell et al. [2003]: limited to the Madagascar Basin and over the Madagascar assuming a nitrogen content of 0.7 nmol N per trichome, a Ridge (Figure 2a). These Chl measurements confirm the nitrogen-specific N2-fixation rate of 4 mmol N (mmol constrained extent of the 2005 austral bloom [Uz, 2007], À1 À1 3 trichome N) h , a 12 hr period of N2-fixation, and a with only moderate increases (0.2–0.3 mg mÀ ) in Chl mixed layer depth of 50 m. For Richelia/Rhizosolenia,we relative to the low values found in the surrounding 3 estimated the potential magnitude of N2-fixation using subtropical waters (0.1 mg mÀ ) (Figure 2a). Our size- Richelia cell numbers and the measurements of Villareal fractionated Chl measurements show that 60–80% of the [1990, 1991, 1992]: we assumed a ethylene production rate Chl in the Madagascar Basin was associated with cells À1 À1 of 9.5 pmol ethylene (trichome) h , a 3:1 acetylene larger than 5 mm in diameter, while only 30–40% of Chl reduction to N2-fixation ratio [Mahaffey et al., 2005], a 12 hr was found in this fraction elsewhere in the study region period of N2-fixation, a 50 m mixed layer and 2 Richelia (e.g., Madagascar Ridge).