III.9 Ecosystem Productivity and Carbon Flows: Patterns across Ecosystems Julien Lartigue and Just Cebrian OUTLINE by fitting the following single exponential equation to the pattern of detritus decay observed in experi- 1. Nature of carbon budgets mental incubations, DM ¼ DM eÀ k(t À t0), where 2. Rationale and approach for studying patterns of t t0 k is the decomposition rate, DM is the detrital ecosystem productivity and carbon flow t mass remaining in the experimental incubation at 3. Patterns in ecosystem productivity and carbon time t,DM is the initial detrital mass, and (tÀt )is flow t0 0 the incubation time 4. Conclusion detrital production. The amount (in g CÁmÀ2ÁyearÀ1)of net primary production not consumed by herbi- The characterization and understanding of carbon flows in vores, which senesces and enters the detrital com- aquatic and terrestrial ecosystems are topics of paramount partment importance for several disciplines, such as ecology, bio- detritus. Dead primary producer material, which nor- geochemistry, oceanography, and climatology. Scientists mally becomes detached from the primary producer have been studying such flows in many diverse ecosystems after senescence for decades, and sufficient information is now available to herbivory. The amount (in g CÁmÀ2ÁyearÀ1) of net investigate whether any patterns are evident in how carbon primary production ingested or removed, including flows in ecosystems and to determine the factors respon- primary producer biomass discarded by herbivores sible for those patterns. In particular, a wealth of infor- net primary production. The amount (in g CÁmÀ2Á mation exists on the movement of carbon through the yearÀ1) of carbon assimilated through photosyn- activity of herbivores and consumers of detritus (i.e., de- thesis and not respired by the producer composers and detritivores), two of the major agents of nutrient concentration (producer or detritus). The per- carbon flows in ecosystems. This chapter analyzes the centage of nitrogen and phosphorus within pro- transference of carbon through herbivory and decomposi- ducer biomass or detritus on a dry weight basis tion in aquatic and terrestrial ecosystems, documents the nature and implications of salient patterns, and explains why those patterns emerge. 1. NATURE OF CARBON BUDGETS Carbon enters the biotic component of an ecosystem when inorganic carbon, often carbon dioxide, is taken GLOSSARY up and converted into organic compounds. With the absolute decomposition. The amount (in g CÁmÀ2Á rare exception of chemosynthetic organisms, the en- yearÀ1) of detritus consumed by microbial decom- ergy for this conversion comes from photosynthesis. posers (e.g., bacteria, fungi) and detritivores, which Once inorganic carbon has been converted into organic range from detritivorous micro-, macro-, and ge- compounds, it is considered fixed. This production of latinous zooplankton in pelagic systems to micro- fixed carbon is known as primary production, and (<100 mm), meio- (100–500 mm), and macrofauna those organisms that can fix carbon are primary pro- (>500 mm) in benthic and terrestrial systems ducers. Gross primary production is the entire amount decomposition rate. The proportion of detrital mass de- of carbon fixed by a primary producer. Net pri- composed per unit time (e.g., dayÀ), often estimated mary production is gross primary production minus Copyright © 2009. Princeton University Press. All rights reserved. May not be reproduced in any form without permission from the publisher, except fair uses permitted under U.S. or applicable copyright law. EBSCO Publishing : eBook Academic Collection (EBSCOhost) - printed on 3/6/2017 3:26 PM via UNIV OF SOUTH ALABAMA AN: 331277 ; Carpenter, Stephen R., Levin, Simon A..; The Princeton Guide to Ecology Account: s4595122 Productivity and Carbon Flows 321 Net primary ducers to herbivores is not complete, and only a frac- production tion of producer biomass ingested becomes herbivore biomass. When herbivory is considered as the per- centage of net primary production removed, its impli- cations for the impact of herbivores on carbon and Producer Herbivory nutrient recycling and storage as producer biomass in biomass Detrital the ecosystem become apparent. If herbivores remove a production Decomposition large percentage of net primary production, only a small percentage of the carbon fixed and nutrients Degradable Export Import detrital mass taken up by producers is available for accumulation as Refractory producer biomass. In such cases, herbivores have the accumulation potential to exert significant control on carbon and Refractory detrital mass nutrient storage by producers, which is commonly re- ferred to as top-down regulation (see chapter III.6). Likewise, as the percentage of net primary production Figure 1. Diagram of carbon flow into and out of the producer and consumed increases, so does consumer-driven recycling detrital pools in an ecosystem. (Adapted from Cebrian, 1999) of carbon and nutrients in the ecosystem. It is impor- tant to mention that, when diverse ecosystems are compared, absolute consumption and percentage of the organic compounds that have been broken down net primary production consumed are not always re- during respiration to fuel cellular processes within the lated. Ecosystems with high net primary production primary producer. may support large absolute consumption by herbi- It is the fixed carbon measured by net primary pro- vores, which may still represent a small percentage of duction that becomes primary producer biomass and that high net primary production, in comparison with part of the producer carbon pool (figure 1). This fixed ecosystems with lower net primary production sup- carbon will then either remain as producer biomass, be porting less absolute consumption but a larger per- consumed by herbivores, or enter the detrital pathway centage of net primary production lost to herbivores. and become part of the detrital carbon pool. The im- As is the case for herbivory, decomposition can also port or export of detritus can also alter the amount of be viewed as an absolute flux or as a proportion of carbon in the detrital pool, but regardless of the source detrital mass decomposed per unit time (i.e., decom- of the detritus, detrital carbon will either be recycled by position rate). When considered as an absolute flux, decomposers and detritivores or stored as refractory decomposition corresponds to the amount of detritus carbon. consumed by microbial decomposers and detritivores. In both aquatic and terrestrial ecosystems, the This consumption leads to the reduction of particulate transfer of fixed carbon from primary producers to and dissolved detritus into simpler and simpler con- herbivores and decomposer/detritivores provides ma- stituents and, ultimately, to nutrient mineralization. jor pathways for the flow of energy and nutrients. As a Much like herbivory, decomposition, when regarded as result, these transfers have consequences not only for an absolute flux, is indicative of the potential levels of carbon storage but also for nutrient recycling and decomposer and detritivore production maintained in herbivore and decomposer/detritivore populations. the ecosystem because only a fraction of the carbon In assessing these transfers, it is important to rec- ingested by decomposers and detritivores is metabo- ognize that they can be viewed in absolute as well as lized into biomass of these organisms. When decom- proportional terms. Absolute size refers to the amount position is viewed as the proportion of detrital mass or magnitude of the transfer measured in units of decomposed per unit time, its implications for how fast producer carbon often over space and time (i.e., g C mÀ2 carbon and nutrient flow through the detrital pathway year–1), whereas proportional size refers to the per- become apparent. Ecosystems whose decomposition centage of net primary production consumed by her- rate is high tend to have faster nutrient and carbon bivores or the percentage of detrital mass consumed recycling rates and store less carbon in their detrital per unit time by decomposers and detritivores. pools regardless of any large differences in detrital pro- When regarded as an absolute flux, herbivory sets duction. It is worth mentioning that, when diverse limits to the level of herbivore production maintained ecosystems are compared, higher values of absolute in an ecosystem. Because of herbivore respiration and decomposition do not always equate to higher de- herbivore egestion of nonassimilated producer bio- composition rates. Ecosystems with low detrital pro- mass, the transfer of fixed carbon from primary pro- duction may have high decomposition rates, yet small Copyright © 2009. Princeton University Press. All rights reserved. May not be reproduced in any form without permission from the publisher, except fair uses permitted under U.S. or applicable copyright law. EBSCO Publishing : eBook Academic Collection (EBSCOhost) - printed on 3/6/2017 3:26 PM via UNIV OF SOUTH ALABAMA AN: 331277 ; Carpenter, Stephen R., Levin, Simon A..; The Princeton Guide to Ecology Account: s4595122 322 Communities and Ecosystems absolute decomposition, when compared with other grams of carbon or ‘‘g C.’’ Last, researchers need to ecosystems with high detrital production, low decom- ensure that the conclusions obtained from multistudy position rates, and large absolute decomposition. data sets are not compromised by the uncertainty that results from compiling values from studies that use different methods,