The fate of marine autotrophic production

The fate of photosynthetic carbon in marine ecosystems dominated by different types of primary producers was examined by compiling published reports on herbivory, autotrophic respiration, decomposition, carbon storage, and export rates as fractions of net primary production (NPP) in ecosystems dominated by different types of autotrophs (i.e. oceanic and coastal phytoplankton, microphytobenthos, coral reef algae, macroalgae, seagrasses, marsh plants, and mangroves). A large fraction (>40%) of the NPP of marine ecosystems is decomposed within the system, except for microphytobenthos (decomposition, ∼25% of NPP). Herbivory tends to be highest for microalgae (planktonic and benthic, >40% of NPP) and macroalgae (33.6±4.9% of NPP) and is somewhat less for higher plants. Microphytobenthos export on average a much higher proportion of their NPP than do other microalgal communities, whereas marine macrophytes, except marsh plants, export a substantial proportion (24.3-43.5% on average) of their PP. The fraction of NPP stored in sediments is 4-fold greater for higher plants (∼10-17% of NPP) than for algae (0.4-6% of NPP). On average, ∼90% of the phytoplankton NPP is used to support local heterotrophic metabolism (i.e. grazed or decomposed). This fraction is even higher in oceanic communities. Mangrove forests, and to a lesser extent seagrass meadows and macroalgal beds, produce organic carbon well in excess of the ecosystem requirements, with excess photosynthetic carbon (i.e. export rate plus storage) in these ecosystems representing ∼40% of NPP. Extrapolation of these results to the global ocean identifies marine angiosperms, which only contribute 4% of total ocean NPP, as major contributors of the NPP stored (30% of total ocean carbon storage) and subsequently buried in marine sediments. Consideration of burial of NPP from marine angiosperms should lead to estimates of total burial of marine NPP that exceed current estimates by 15-50%.