Transformations of Organic Matter: The Carbon Cycle
The sea contains a substantial amount of carbon compared to that present in the atmosphere* or in terrestrial organisms (Table 10–1); only rock deposits contain more carbon, but these resources are not involved in the carbon cycle at ecologically meaningful time scales.
KeywordsOrganic Matter Salt Marsh Carbon Cycle Sulfate Reduction Particulate Organic Carbon
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- The relation of atmospheric CO2 to aqueous CO2 in the sea has received a lot of attention recently due to the continued increase in CO2 in the atmosphere attributable to the burning of fossil fuels (Keeling et al., 1976), and the consequent potential for disturbance of the world’s climate. Recall that CO2 is a major absorber of energy (Fig. 2–1), especially infrared radiation. Increases in CO2 might increase the temperature of the earth’s atmosphere by trapping heat in the so-called “greenhouse effect.” The ocean holds about 50 to 60 times as much CO2 as the atmosphere and is able to take up more CO2. It is not certain just how much of the CO2 added to the atmosphere will be taken up into the larger pool or what the effects of the increased temperature will be. In fact, some believe that the higher temperature will increase evaporation and will result in increased interception of energy such that temperatures will drop worldwide. The only conclusion agreed upon is that the effects of our altering the CO2 content of the atmosphere will have major consequences, and that the ocean will play a large role.Google Scholar
- Sediment traps are of remarkably variable design (Blomqvist and Hakanson, 1981). In general they are containers of various sizes and shapes open at the top and are moored at the desired depth to collect particles for certain periods of time. Decay of the collected organic matter is possible, especially at warm temperatures in shallow water, so poisons are often added to the traps to prevent decay. Animals that eat detritus may also be attracted to traps and may consume detritus; in poisoned traps, the attracted consumers may be killed, so that measurements of accumulation may be increased. The design and deployment procedures for sediment traps present several problems. The opening-to-height ratio changes the effectiveness of traps (Hargrave and Burns, 1979; Gardner, 1980, 1980a). The raising of traps to the ship may result in loss and alteration of the captured particles. Some sediment traps of different shapes and sizes may be especially susceptible to error in strong currents, since the containers themselves alter hydraulic flow lines such that under and over trapping can result and the faster the current the larger the effects. Particles of different size react differently in any given velocity of current, so it is not a simple matter to apply a correction factor to the trap based on flow velocity. One way to reduce the effects of currents is to use drifting traps that move along with a water mass, collecting particles as they go (Staresinic et al., 1978). A preliminary report of recent intercalibrations (Spencer, 1981) show that different traps gave very different measures of flux for one site in the Panama Basin of the Pacific, ranging from a few to about 200 mg m-2 day- of material. The size and shape of traps seemed less important than other unidentified sources of variation.Google Scholar
- Some of this so called “marine snow” does not sediment out of the water column and is perhaps the site of substantial amounts of primary production (G. Knauer, unpublished data). In addition to algal cells such aggregates are also enriched microenvironments for bacterial and protozoan populations, which are five orders of magnitude more abundant in marine snow than in the surrounding water (Caron et al., 1982).Google Scholar
- Fecal pellets of small copepods such as Paracalanus and Oithona nauplii may be 2 × 103 µm3, while a large zooplankton such as the euphausiid Meganyctiphanes norwegica makes pellets of 105 µm3 (Paffenhöffer and Knowles, 1979).Google Scholar
- It is possible for the sedimentation rate to be so high (Table 10–4) that some organic matter is buried before being completely decomposed (Müller and Suess, 1979). The material that is buried, even in such cases, is probably mainly organic compounds highly resistant to microbial attack.Google Scholar
- † This potential improvement of the quality of particulate organic matter due to microbial regrowth or colonization is important to many detritivores. For example, hydrobiid snails do not ingest fecal pellets until enough time has elapsed to allow microbial colonization (Levinton and Lopez, 1977).Google Scholar
- Bacterial production was about 29 g C m-2 per year, so the growth efficiency was about 60%, a high value compared to most heterotrophs (cf. Fig. 7–9). For bacteria using organic matter derived from kelp, Robinson et al. (1982) calculated growth (or conversion) efficiencies of 28 to 91%, while Hoppe (1978) believes that the range is most often between 60 and 70%Google Scholar
- This discussion applies to decay prompted by microbes during the decomposer phase; note in Figure 10–14 that the leaching phase is more or less the same under both conditions.Google Scholar