Robert A. Armstrong

A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals

Abstract In simulation studies of the oceans role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux F OC ( z ) at any depth z can be predicted from the flux of organic carbon F OC ( z 0 ) at some near-surface depth z 0 . Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.

Assessment of skill and portability in regional marine biogeochemical models: Role of multiple planktonic groups

[1] Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models’ performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical onedimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll-a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross-validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross-validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model-data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.

Progress made in study of ocean's calcium carbonate budget

Many of the uncertainties in diagnostic and prognostic marine carbon cycle models arise from an imperfect understanding of the processes that control the formation and dissolution of calcium carbonate (CaCO3). On the production side of the equation, the factors that control the abundances of calcifying phytoplankton or zooplankton are largely unknown. On the dissolution side, changes in the depth of CaCO3 saturation horizons for both calcite and aragonite may produce large-scale changes in dissolution of shelf and slope sediments and reefs, with potentially significant implications for atmospheric carbon dioxide concentration and climate change, as well as for coralline organisms themselves. In recent years, concern about the long-term fate of anthropogenic CO2 in the oceans has re-ignited scientific interest in the fundamental abiotic and biotic processes that control the marine CaCO3 budget, since biological CaCO3 production and export are important mechanisms by which carbon is exported from the oceans surface to its abyss. CaCO3 precipitation releases CO2 to solution, while CaCO3 dissolution takes up CO2 from solution.

Competition, Seed Predation, and Species Coexistence

In 1970 Janzen proposed that species-specific seed predation, by eliminating establishment of seedlings near adult trees of the same species, should lead to a regular dispersion of trees and to increased species diversity in tropical forests. Seed predation would lead to increased species diversity by setting upper bounds to the densities of individual species: if each extant species reached its predation-determined limit and yet more canopy space was available, individuals of additional species would be able to invade. Here I derive a patch model of Janzens mechanism that is simple in form yet captures its spatial flavor. The model predicts that this mechanism will tend to stabilize any number of competing species. As a corollary, the model predicts that Janzens mechanism can lead to increased diversity without leading to regular spacing of individuals.

Monitoring Ocean Productivity by Assimilating Satellite Chlorophyll into Ecosystem Models

Satellite color imagery is currently the only data source that can be used to monitor long-term, global-scale changes in ocean biology. The ability to monitor ocean ecosystem processes is important not only because oceanic biological resources have direct value to mankind, but also because ocean biology plays a major role in the global carbon cycle (Siegenthaler and Sarmiento 1993). The effects of biological processes on seasonal and interannual changes in surface ocean pCO2 are very large and are extremely difficult to monitor with in situ measurements, since shipboard measurements can provide only limited coverage of the world ocean. In addition, long-term changes in ocean circulation may occur in response to greenhouse warming (Manabe and Stouffer 1993); and it is likely that such changes will have a significant impact on biological systems, and hence on the ocean-atmosphere CO2 balance (e.g., Sarmiento and Orr 1991).

The effects of disturbance patch size on species coexistence

A central problem in community ecology lies in determining how the frequency and spatial extent of disturbance events affect community structure. In an empirical study of competition in fungi ( Armstrong, 1976 ), I showed that the ease with which species are able to coexist can depend strongly on the disturbance regime. In the present paper, I use a model of two species competing for space to show that this observation was not due entirely to idiosyncrasies of the species studied; in particular, I show that as disturbance patch size increases relative to the dispersal abilities of the species involved, coexistence becomes more difficult. The model also predicts that coexistence should occur more readily as the level of seed production by the subordinate species increases relative to seed production by the dominant species, and as the growth rate and dispersal ability of the subordinate species increase relative to those of the dominant species. The present model may be enhanced for use in field situations, yielding quantitative predictions of conditions under which fugitive species will be able to coexist.

DIFFERENCES IN GROWTH AND PHYSIOLOGY OF MARINE SYNECHOCOCCUS (CYANOBACTERIA) ON NITRATE VERSUS AMMONIUM ARE NOT DETERMINED SOLELY BY NITROGEN SOURCE REDOX STATE1

The preference of phytoplankton for ammonium over nitrate has traditionally been explained by the greater metabolic cost of reducing oxidized forms of nitrogen. This “metabolic cost hypothesis” implies that there should be a growth disadvantage on nitrate compared to ammonium or other forms of reduced nitrogen such as urea, especially when light limits growth, but in a variety of phytoplankton taxa, this predicted difference has not been observed. Our experiments with three strains of marine Synechococcus (WH7803, WH7805, and WH8112) did not reveal consistently faster growth (cell division) on ammonium or urea as compared to nitrate. Urease and glutamine synthetase (GS) activities varied with nitrogen source in a manner consistent with regulation by cellular nitrogen status via NtcA (rather than by external availability of nitrogen) in all three strains and indicated that each strain experienced some degree of nitrogen insufficiency during growth on nitrate. At light intensities that strongly limited growth, the composition (carbon, nitrogen, and pigment quotas) of WH7805 cells using nitrate was indistinguishable from that of cells using ammonium, but at saturating light intensities, cellular carbon, nitrogen, and pigment quotas were significantly lower in cells using nitrate than ammonium. These and similar results from other phytoplankton taxa suggest that a limitation in some step of nitrate uptake or assimilation, rather than the extra cost of reducing nitrate per se, may be the cause of differences in growth and physiology between cells using nitrate and ammonium.

On the quantitative theory of reproductive effort in clonal plants: refinements of theory, with evidence from goldenrods and mayapples

SummaryRecently I proposed a quantitative theory which predicts the partition of resources between vegetative growth and seed production in highly rhizomatous clonal plants (Armstrong 1982, 1983). My basic premise was that this partition should be controlled by basic geometric properties of clonal growth. My conclusions were that the ratio of resources expended on seeds and rhizomes should be relatively constant in time and space, and that the value of this ratio should be predictable from a knowledge of the allometric relationships among certain morphological characters.In the present paper I first refine this theory to yield explicit ramet-level predictions directly applicable to clonal species with densely-packed canopies. These predictions are then tested using observations on goldenrods (Solidago altissima) and mayapples (Podophyllum peltatum). In the Solidago studies, the ratio of infructescence weight to total rhizome weight was found to be asymptotically constant for the larger ramets in a clone, confirming an important prediction of the theory. A second prediction of the theory, that the ratio of infructescence weight to total rhizome weight should be constant across clones, was not confirmed using the goldenrod data. This observation may simply be due to measurement biases. An alternative hypothesis is that the prediction of this theory constitute an r-limit strategy, and so are applicable only in the limit of density independent growth. Data from Sohn and Policansky (1977) on mayapples support this latter interpretation.

Corrigendum to “Trajectories of sinking particles in the Sargasso Sea: modeling of statistical funnels above deep-ocean sediment traps” [Deep-Sea Research I 44, 1519–1541]

Corrigendum to ‘‘Trajectories of sinking particles in the Sargasso Sea: modeling of statistical funnels above deep-ocean sediment traps’’ [Deep-Sea Research I 44, 1519–1541] D.A. Siegel*, R.A. Armstrong a Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106-3060, USA Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794-5000, USA

On the concavity of island biogeographic rate functions

Abstract The concavity of the curves for colonization and extinction of species as a function of the species number on an island follows simply and directly from the assumption that there be dispersion in the specific probabilities of colonization and extinction, probabilities which are assumed independent of the total number and identities of the resident species on the island. Thus, a non-interactive model of the behaviors of individual species can account for the gross features of island biogeographic systems.