The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect

Abstract

One of the major conundrums in oceanography for the past 20 y has been that, although the total flux of dissolved organic carbon (OC; DOC) discharged annually to the global ocean can account for the turnover time of all oceanic DOC (ca. 4,000-6,000 y), chemical biomarker and stable isotopic data indicate that there is very little terrestrially derived OC (TerrOC) in the global ocean. Similarly, it has been estimated that only 30% of the TerrOC buried in marine sediments is of terrestrial origin in muddy deltaic regions with high sedimentation rates. If vascular plant material--assumed to be highly resistant to decay--makes up much of the DOC and particulate OC of riverine OC (along with soil OC), why do we not see more TerrOC in coastal and oceanic waters and sediments? An explanation for this "missing" TerrOC in the ocean is critical in our understanding of the global carbon cycle. Here, I consider the origin of vascular plants, the major component of TerrOC, and how their appearance affected the overall cycling of OC on land. I also examine the role vascular plant material plays in soil OC, inland aquatic ecosystems, and the ocean, and how our understanding of TerrOC and "priming" processes in these natural systems has gained considerable interests in the terrestrial literature, but has largely been ignored in the aquatic sciences. Finally, I close by postulating that priming is in fact an important process that needs to be incorporated into global carbon models in the context of climate change.

Fig. 1.

The global carbon cycle (units are in PgC or Pg C⋅y⁻¹). Sources of inventories and fluxes are refs. –, , , ; diagram modified from Drenzek (114).

Fig. 2.

Hydrolysis of high molecular weight OM to mono/oligomers (blue) by a variety of endo-acting (gold and red) and exo-acting (green, orange) extracellular enzymes. The identity of organisms producing enzymes, structural specificity of enzymes, size spectrum of hydrolysis products, and identity of organisms consuming substrates are not specified in this diagram. This highlights some of the major unknown factors in the early steps of carbon transformation in many aquatic systems. Modified from Arnosti (28).

Fig. 3.

Three different priming effects involving LOM and ROM, respectively. These proposed priming effects are mainly derived from work in soils. Modified from Guenet et al. (86).

Fig. 4.

Sources of possible priming substrates in different terrestrial and aquatic environments. Modified from Raymond et al. (114).