III.9 Ecosystem Productivity and Carbon Flows: Patterns across Ecosystems
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III.9 Ecosystem Productivity and Carbon Flows: Patterns across Ecosystems Julien Lartigue and Just Cebrian OUTLINE 1. Nature of carbon budgets 2. Rationale and approach for studying patterns of ecosystem productivity and carbon flow 3. Patterns in ecosystem productivity and carbon flow 4. Conclusion The characterization and understanding of carbon flows in aquatic and terrestrial ecosystems are topics of paramount importance for several disciplines, such as ecology, biogeochemistry , oceanography, and climatology. Scientists have been studying such flows in many diverse ecosystems for decades, and sufficient information is now available to investigate whether any patterns are evident in how carbon flows in ecosystems and to determine the factors responsible for those patterns. In particular, a wealth of information exists on the movement of carbon through the activity of herbivores and consumers of detritus (i.e., decomposers and detritivores), two of the major agents of carbon flows in ecosystems. This chapter analyzes the transference of carbon through herbivory and decomposition in aquatic and terrestrial ecosystems, documents the nature and implications of salient patterns, and explains why those patterns emerge. GLOSSARY absolute decomposition. The amount (in g Cm2  year1 ) of detritus consumed by microbial decomposers (e.g., bacteria, fungi) and detritivores, which range from detritivorous micro-, macro-, and gelatinous zooplankton in pelagic systems to micro- (500 mm) in benthic and terrestrial systems decomposition rate. The proportion of detrital mass decomposed per unit time (e.g., day ), often estimated by fitting the following single exponential equation to the pattern of detritus decay observed in experimental incubations, DMt ¼ DMt0 e k(t  t0) , where k is the decomposition rate, DMt is the detrital mass remaining in the experimental incubation at time t, DMt0 is the initial detrital mass, and (tt0) is the incubation time detrital production. The amount (in g Cm2 year1 ) of net primary production not consumed by herbivores , which senesces and enters the detrital compartment detritus. Dead primary producer material, which normally becomes detached from the primary producer after senescence herbivory. The amount (in g Cm2 year1 ) of net primary production ingested or removed, including primary producer biomass discarded by herbivores net primary production. The amount (in g Cm2  year1 ) of carbon assimilated through photosynthesis and not respired by the producer nutrient concentration (producer or detritus). The percentage of nitrogen and phosphorus within producer biomass or detritus on a dry weight basis 1. NATURE OF CARBON BUDGETS Carbon enters the biotic component of an ecosystem when inorganic carbon, often carbon dioxide, is taken up and converted into organic compounds. With the rare exception of chemosynthetic organisms, the energy for this conversion comes from photosynthesis. Once inorganic carbon has been converted into organic compounds, it is considered fixed. This production of fixed carbon is known as primary production, and those organisms that can fix carbon are primary producers . Gross primary production is the entire amount of carbon fixed by a primary producer. Net primary production is gross primary production minus the organic compounds that have been broken down during respiration to fuel cellular processes within the primary producer. It is the fixed carbon measured by net primary production that becomes primary producer biomass and part of the producer carbon pool (figure 1). This fixed carbon will then either remain as producer biomass, be consumed by herbivores, or enter the detrital pathway and become part of the detrital carbon pool. The import or export of detritus can also alter the amount of carbon in the detrital pool, but regardless of the source of the detritus, detrital carbon will either be recycled by decomposers and detritivores or stored as refractory carbon. In both aquatic and terrestrial ecosystems, the transfer of fixed carbon from primary producers to herbivores and decomposer/detritivores provides major pathways for the flow of energy and nutrients. As a result, these transfers have consequences not only for carbon storage but also for nutrient recycling and herbivore and decomposer/detritivore populations. In assessing these transfers, it is important to recognize that they can be viewed in absolute as well as proportional terms. Absolute size refers to the amount or magnitude of the transfer measured in units of producer carbon often over space and time (i.e., g C m2 year–1 ), whereas proportional size refers to the percentage of net primary production consumed by herbivores or the percentage of detrital mass consumed per unit time by decomposers and detritivores. When regarded as an absolute flux, herbivory sets limits to...