Thomas R. Anderson

CARBON SEQUESTRATION IN ECOSYSTEMS: THE ROLE OF STOICHIOMETRY

The fate of carbon (C) in organisms, food webs, and ecosystems is to a major extent regulated by mass-balance principles and the availability of other key nutrient elements. In relative terms, nutrient limitation implies excess C, yet the fate of this C may be quite different in autotrophs and heterotrophs. For autotrophs nutrient limitation means less fixation of inorganic C or excretion of organic C, while for heterotrophs nutrient limitation means that more of ingested C will “go to waste” in the form of egestion or respiration. There is in general a mismatch between autotrophs and decomposers that have flexible but generally high C:element ratios, and consumers that have lower C:element ratios and tighter stoichiometric regulation. Thus, C-use efficiency in food webs may be governed by the element ratios in autotroph biomass and tend to increase when C:element ratios in food approach those of consumers. This tendency has a strong bearing on the sequestration of C in ecosystems, since more C will be di...

Metabolic stoichiometry and the fate of excess carbon and nutrients in consumers

Animals encountering nutritionally imbalanced foods should release elements in excess of requirements in order to maintain overall homeostasis. Quantifying these excesses and predicting their fate is, however, problematic. A new model of the stoichiometry of consumers is formulated that incorporates the separate terms in the metabolic budget, namely, assimilation of ingested substrates and associated costs, protein turnover, other basal costs, such as osmoregulation, and the use of remaining substrates for production. The model indicates that release of excess C and nonlimiting nutrients may often be a significant fraction of the total metabolic budget of animals consuming the nutrient‐deficient forages that are common in terrestrial and aquatic systems. The cost of maintenance, in terms of not just C but also N and P, is considerable, such that food quality is important even when intake is low. Many generalist consumers experience short‐term and unpredictable fluctuations in their diets. Comparison of model output with data for one such consumer, Daphnia, indicates that mechanisms operating postabsorption in the gut are likely the primary means of regulating excess C, N, and P in these organisms, notably respiration decoupled from biochemical or mechanical work and excretion of carbon and nutrients. This stoichiometrically regulated release may often be in organic rather than inorganic form, with important consequences for the balance of autotrophic and heterotrophic processes in ecosystems.

Reconciliation of the carbon budget in the ocean’s twilight zone

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink.

Stoichiometry: linking elements to biochemicals

Ecological stoichiometry is a useful tool for studying how the elemental composition of organisms and their food affects production, nutrient cycling, and food- web dynamics. Two analyses are presented here that show that the use of simple element ratios in stoichiometric calculations may in certain circumstances prove inadequate because of the influence in animal nutrition of biochemical aspects of diet. In the first, a stoichio- metric analysis of herbivores consuming food with varying carbon to nitrogen (C:N) ratios is undertaken, in which the intake of C is segregated into easily assimilated compounds and fiber. Two herbivore strategies emerge from the analysis, both as a means of minimizing limitation by C, not N: fiber eaters that consume high C:N food and have efficient fiber digestion, and selective feeders that consume low C:N food but that do not possess fiber- digesting enzymes. In the second example, the stoichiometric axiom that a single substrate, the one in least supply relative to demand, limits growth is used to identify potentially limiting essential amino acids in the diets of a range of animals. Large consumer-prey imbalances in amino acids were found in several cases, indicating that, at least in theory, growth should be strongly limited by individual amino acids rather than bulk N. In practice such limitation may be offset in consumers by physiological and other factors such as symbiotic relationships. The two analyses emphasize the simplicity of element stoichi- ometry, highlighting the need to consider biochemical and physiological arguments when undertaking stoichiometric studies of carbon and nutrient transfers in ecosystems.

Non-Redfield carbon and nitrogen cycling in the Sargasso Sea: pelagic imbalances and export flux

An ecosystem model embedded in a one-dimensional physical model is used to study the stoichiometry of carbon and nitrogen cycling at the Bermuda Atlantic Time Series site. The model successfully provides a budget for the processes contributing to the drawdown of dissolved inorganic carbon (DIC) that is observed in surface waters in the absence of detectable nitrate throughout much of the summer. The modeled drawdown is initially driven by export fueled by in situ N and accumulation of dissolved organic carbon, with continued DIC consumption after nutrient exhaustion resulting largely from nitrogen fixation and outgassing of CO2 to the atmosphere. The modeled export flux of organic C at 300 m is dominated by particles (81%), with a nevertheless significant fraction (19%) due to dissolved organic matter. The predicted combined C/N of particulate and dissolved export increases from 10.8 at 70 m to 14.3 at 300 m, because of preferential remineralization of N. In theory, at least as a first approximation, the ratio of net consumption of DIC and nutrients in the euphotic zone is equivalent to this C/N of export. However, the C/N of consumption during DIC drawdown averaged 23.5 (>10.8), indicating that this assumption is not always valid and C/N ratios of nutrient consumption cannot reliably be used to estimate the export ratio, which is difficult to measure directly. The work highlights the complex interplay between the cycling of C and N the upper ocean and the resultant export flux.

Ocean fertilization: a potential means of geoengineering?

The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.

Beyond the Durfee square: Enhancing the h-index to score total publication output

An individual’s h-index corresponds to the number h of his/her papers that each has at least h citations. When the citation count of an article exceeds h, however, as is the case for the hundreds or even thousands of citations that accompany the most highly cited papers, no additional credit is given (these citations falling outside the so-called “Durfee square”). We propose a new bibliometric index, the “tapered h-index” (hT), that positively enumerates all citations, yet scoring them on an equitable basis with h.The career progression of hT and h are compared for six eminent scientists in contrasting fields. Calculated hT for year 2006 ranged between 44.32 and 72.03, with a corresponding range in h of 26 to 44. We argue that the hT-index is superior to h, both theoretically (it scores all citations), and because it shows smooth increases from year to year as compared with the irregular jumps seen in h. Conversely, the original h-index has the benefit of being conceptually easy to visualise. Qualitatively, the two indices show remarkable similarity (they are closely correlated), such that either can be applied with confidence.

A one‐dimensional model of dissolved organic carbon cycling in the water column incorporating combined biological‐photochemical decomposition

A one-dimensional model incorporating separate labile, semilabile, and refractory fractions of dissolved organic carbon (DOC) is used to study the vertical distribution of dissolved organics in the ocean and the downward flux of organic carbon into the water column. The modeled vertical gradient of DOC is generated almost entirely by the semilabile fraction, which has predicted lifetimes of 0.4 years at the ocean surface increasing to 6.3 years at 1000 m, owing to diminishing bacterial numbers. Although predicted fluxes of DOC and sinking particles exiting the upper mixed layer are similar, labile and semilabile DOC are mostly degraded by bacteria before being mixed below 250 m. The simple one-dimensional scheme employed by the modeling may, however, underestimate downward transport of DOC by physical mechanisms because it does not capture three-dimensional processes such as subduction. The inclusion of a biologically inert refractory pool is a first step to try and incorporate the growing awareness that photochemical processes play an important role in the dynamics of organic carbon in the ocean. However, the predicted rate of change of refractory material in response to altered climatic forcing (e.g., ultraviolet radiation at the ocean surface) is so slow that it may not be necessary to include it dynamically in ocean models used to examine climatic change within the next 200 years.

Global fields of sea surface dimethylsulfide predicted from chlorophyll, nutrients and light

Abstract The major difficulty in estimating global sea–air fluxes of dimethylsulphide (DMS) is in interpolating measured seawater DMS concentrations to create seasonally resolved gridded composites. Attempts to correlate DMS with variables that can be mapped globally, e.g. chlorophyll, have not yielded reliable relationships. A comprehensive database of DMS measurements has recently been assembled by Kettle et al. [Global Biogeochem. Cycles 13 (1999) 399]. This database, which contains chlorophyll as a recorded variable, was extended by merging nutrients and light from globally gridded fields. A new equation was developed whereby DMS is predicted from the product of chlorophyll ( C , mg m −3 ), light ( J , mean daily shortwave, W m −2 ) and a nutrient term ( Q , dimensionless) using a “broken-stick” regression: DMS =a, log 10 (CJQ)≤s DMS =b[ log 10 (CJQ)−s]+a, log 10 (CJQ)>s where Q =N/( K N +N), N is nitrate (mmol m −3 ) and K N is the half saturation constant for nitrate uptake by phytoplankton (0.5 mmol m −3 ). Fitted parameter values are: a =2.29, b =8.24, s =1.72. Monthly maps of global DMS were generated by combining these equations with ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The resulting high DMS concentrations in high latitude, upwelling and shelf areas are consistent with observed patterns. Predicted global seasonally averaged mean DMS is 2.66 nM. The further application of gas transfer equations to these fields leads to estimates of globally integrated DMS fluxes from ocean to atmosphere of 0.86 and 1.01 Tmol S year −1 for two formulations of piston velocity. The simplicity of the new relationship makes it suitable for implementation in global ocean general circulation models. The relationship does not however resolve DMS variability in low-DMS areas, which constitute large tracts of the open ocean, and should therefore be used with caution in localized studies.

Climate sensitivity to ocean dimethylsulphide emissions

The production of dimethylsulphide (DMS) by ocean phytoplankton is hypothesized to form part of a feedback process on global climate. Changes in the DMS flux to the atmosphere cause changes to aerosols for cloud formation, leading to changes in the amount of radiation reaching the ocean, and hence on the planktonic production of DMS. This hypothesis has been investigated using a coupled ocean-atmosphere general circulation model (COAGCM) that includes an ocean ecosystem model and an atmospheric sulphur cycle. Ocean DMS concentrations are parameterised as a function of chlorophyll, nutrient and light. The results of several sensitivity experiments are presented showing significant global climate change responses to perturbations in ocean DMS production. A small negative feedback from climate change onto ocean DMS production is found and the implications are discussed.