Modeling the distribution of Trichodesmium and nitrogen fixation in the Atlantic Ocean

Abstract

[1] In this paper we use a coupled, 3-dimensional, biological-physical model, which includes an explicit, dynamic representation of Trichodesmium, to predict the distribution of Trichodesmium and rates of N 2 -fixation in the tropical and subtropical Atlantic Ocean. It is shown that the model reproduces the approximate observed meridional distribution of Trichodesmium in the Atlantic and elevated concentrations in specific coastal and open ocean regions where this organism is known to occur. The model also appears to reproduce the observed seasonality of Trichodesmium populations at higher latitudes (highest concentrations in summer and fall), but this seasonal cycle may be too pronounced at low latitudes. High and persistent Trichodesmium concentrations and rates of N 2 -fixation are generated by the model in the Gulf of Guinea off of Africa. This unexpected finding appears to be confirmed by historical measurements. In general, increased Trichodesmium concentrations develop in regions where the mixed layer is relatively thin (resulting in high mean light levels) and dissolved inorganic nitrogen (DIN) concentrations and phytoplankton biomass are low for extended periods of time. The model-predicted Trichodesmium distributions are therefore very sensitive to the fidelity of the physical model's representation of mixed layer depth variability, and upwelling intensity, and the biological model's estimated DIN and phytoplankton concentrations. The model generates a three-step successional sequence where (1) high DIN concentrations due to upwelling and/or mixing stimulate phytoplankton growth, followed by (2) Trichodesmium growth after DIN depletion and phytoplankton decline, followed by (3) enhanced phytoplankton growth due to new nitrogen inputs from N 2 -fixation. This sequence develops in response to seasonal variations in mixing in the southwestern North Atlantic and in response to upwelling along the coast of Africa and the equator. We interpret this sequence as representing a diatom-Trichodesmium-flagellate succession, which is consistent with observed species successions off of northwest Africa and in the Gulf of Mexico. The results presented in this paper lead us to conclude that our model includes the primary factors that dictate when and where Trichodesmium and N 2 -fixation occurs in the Atlantic. Moreover, it appears that our model reproduces some of the major effects that diazotrophically-derived inputs of new nitrogen have on the pelagic ecosystem.

The fate of nitrogen fixed by diazotrophs in the ocean

“…At an average rate of 43 pmol N fixed trichome −1 d −1 , this represents a release rate of 1.3 to 28 pmol trichome −1 d −1 or (using an average colony size of 100 trichomes col −1 ) 13 to 280 pmol col −1 d −1 . These estimates agree well with the mortality rates calculated for Trichodesmium at BATS and in the equatorial Atlantic (2.1 to 2.5% d −1 ; Hood et al, 2001Hood et al, , 2004.…”

“…Elevated NH + 4 and/or DON concentrations have been observed in and around Trichodesmium blooms in the Arabian Sea (Devassy et al, 1978(Devassy et al, , 1979, Pacific Ocean (Karl et al, 1992(Karl et al, , 1997, the Gulf of Mexico (Lenes et al, 2001), along the coast of Australia (Glibert and O'Neil, 1999) and in aging Trichodesmium cultures . However, accumulation of dissolved organic matter (C or N) is not always observed or predicted, even in areas such as the Sargasso Sea, near the Bermuda Atlantic Time-series Study site (BATS) (Hansell and Carlson, 2001;Hood et al, 2001;Knapp et al, 2005), where N 2 fixation is suspected to be high based on geochemical arguments (Michaels et al, 1996;Gruber and Sarmiento, 1997). Nutrient concentrations within and around blooms may not always be high if the released N is rapidly taken up by organisms growing on and around colonies (e.g., see Sellner, 1992Sellner, , 1997, or by organisms co-occurring in the water column, as has been observed in the Gulf of Mexico (Mulholland et al, 2004b(Mulholland et al, , 2006.…”

Modeled contributions of three types of diazotrophs to nitrogen fixation at Station ALOHA

“…The potential contributions of these three diazotrophs to N 2 fixation were modeled as a function of field-measured cell abundances as well as measured and derived growth and biomass characteristics (Goebel et al, in revision). Consistent with laboratory experiments (Staal et al, 2007) and other published models of the cyanobacteria Trichodesmium (for example, Hood et al, 2001Hood et al, , 2002Hood et al, , 2004, we set N 2 fixation to be a function of lightdependent growth for all diazotrophs modeled. In addition, N 2 fixation was assumed to follow elemental stoichiometries (C:N) of cyanobacterial biomass.…”

“…The model also includes irradiance-dependent growth both vertically and in time. N 2 fixation for Trichodesmium has been shown to vary with irradiance in laboratory experiments (Staal et al, 2007) and this relationship has been parameterized in numerical models (Fennel et al, 2002;Hood et al, 2001Hood et al, , 2002Hood et al, , 2004. Fundamentally, it assumes that photosynthesis supplies the energy necessary to fix N 2 on a daily basis.…”

Microbial oceanography of anoxic oxygen minimum zones

“…One possibility is that the turbid highly productive waters typically overlying AMZs are not well suited to the growth of Trichodesmium-like and unicellular nitrogen-fixing cyanobacteria, which prefer open-ocean, oligotrophic, and calm conditions (62). It is not clear, however, why AMZ settings do not support other nitrogen fixers whose physiological traits are more compatible with the ambient physicochemical conditions.…”