CDOM is the colored dissolved organic matter. What makes the fresh water higher in CDOM concentrations than the ocean?
Well,
Enrichment mechanism occurs in the land water bodies, lagoon-wet land and estuaries system, while the Dispersion mechanism found in open sea environment.
Regards
In freshwater and inshore waterbodies CDOM comes from primary production and also substantially from a variety of terrestrial sources like agriculture, forestry, wastewater treatment. The supply can out-strip the capacity of microbes to process it, and the fraction that is resistant to degradation can remain near the surface because these water bodies are relatively shallow.
In the open ocean, waters are far deeper and oligotrophic, so there's less CDOM to be produced per m3 (mainly by primary production), and bacteria rapidly degrade labile/semilabile components in surface waters while refractory CDOM can be transported to the deep ocean.
Most of the coloured dissolved organic matter (CDOM) occurring in freshwater and estuarine waters has been leached from organic rich soils. Humic substances, which result from the incomplete decomposition of terrestrial plant material, make up the bulk of this CDOM. As CDOM is carried by rivers from land to ocean, a sequence of processes act on these substances. First, flocculation affects a large portion as it crosses the salinity gradient between freshwater and seawater, so that portion is effectively removed as it settles out of the water. Secondly, sunlight is, by definition, absorbed by CDOM and the absorption of certain radiations may lead to photo-degradation into components which are no longer optically active, i.e. no longer coloured. Thirdly, marine microbes may also contribute to CDOM degradation, releasing enzymes that break down the photodegraded components into a form they can absorb. All in all, these processes explain why CDOM is much more abundant in freshwater than in seawater, even though there are also sources of CDOM in the ocean such as biological production.
I would think the only factor that could result large differences in CDOM abundance between fresh/estuarine and open oceanic waters: "their proximity to the terrestrial environment and the processes the occurs within/around each of these systems". As mentioned in the previous answers, CDOM can enter into the aquatic systems from allochthonous sources (e.g., terrestrial sources such as runoff from soil and wetlands etc. and anthropogenic inputs such as water water discharge, agricultural etc.) and autochthonous sources (e.g., primary production). Although several factors/processes such as precipitation/evaporation, land cover around lake, drainage area, water residence time, season (and many more), photo-bleaching, microbial degradation can affect CDOM supply and their abundance in a water column; allochthonous sources generally dominates autochthonous sources likely because of more than one process work collectively in controlling CDOM supply to the system. On the other hand, autochthonous sources (e.g., primary production) are often limited by several environmental factors, but their contribution to bulk CDOM can be notable in extreme cases (e.g., Cyanobacteria bloom in lakes).
As we move to estuary, terrestrial (allochthonous) CDOM enters mainly via rivers and streams where they are already diluted by freshwater in a river and salt water in an estuary. Also, as mentioned above, the processes such as floccuation & sedimentation, photo-bleaching, and microbial degradation remove a large portion of CDOM in addition to dilution in a water column. This could be a potential reason that estuary generally have lower CDOM concentrations than inland water bodies. [There are exceptions, such as the head Apalachicola Estuary and the Carrabelle River (black/dark brown water) that have low tidal influence and are surrounded by highly productive wetlands/vegetation cover, thus, have CDOM abundance similar to inland water bodies (Joshi et al., 2017)]
In Open Ocean, authchthonous sources are the major drivers of CDOM abundance with minimum or no influence of allochtonous sources due to its remoteness to land. Additionally, CDOM extinction processes are still present. Thus, CDOM abundance is the smallest compared to the other two systems.
I hope this answers your question.
Regerds,
Ishan Joshi
Article Assessing chromophoric dissolved organic matter (CDOM) distr...
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