Comparison of Carbon Biogeochemical Processes in Terrestrial and Aquatic Ecosystems

The synthesis and degradation of organic matter in the ocean are significantly distinct from those in terrestrial ecosystems. The phytoplankton provides for a large part of photosynthetic organic matter. The dry mass of phyto-plankton is three orders of magnitude less than global terrestrial mass, whereas the annual production is only about 3 times smaller. This can be related to the much faster life cycles of plankton organisms in comparison with the terrestrial vegetation.

Let us consider the renewal of terrestrial and oceanic organic matter. The terrestrial biomass might be assessed as (2400-2500) x 109t of dry organic matter and annual production as (170-175) x 109t. These values present a period of 13-15 years for complete renewal of organic matter. In the oceans, the problem is much more complicated. The various authors give eight- to tenfold discrepancy in the existing estimates of phytoplankton productivity and biomass. It is estimated also that phyto-plankton mass cycle takes 1-2 days to be completed. Taking this into account, one can reasonably consider that the renewal of the total biomass in the global ocean takes about 1 month. Based on modern assessments, the annual production of photosynthesis varies from (20-30) x 109 to 100 x 109t of organic carbon and the average values are (50-60) x 109t Co. Furthermore, one can hypothesize that the plankton-synthesized organic matter is almost completely assimilated in subsequent upper food webs. Thus, the organic precipitation would not exceed 0.1 x 109t. These calculations present the annual uptake of terrestrial and oceanic living organisms of about 440 x 109t CO2 or 120 x 109t Co. Most of this amount recycles into the ocean and atmosphere.

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