  The dissolved organic matter (DOM) composition in river ecosystems could
reflect the human impacts on the river ecosystem, and plays an important role
in the carbon cycling process. We collected water and phytoplankton samples at
107 sites in the Dongjiang River in two seasons to assess the impact of the
sub-catchments land use structure on the DOM composition. The results showed
that (1) the forested sub-catchments had higher humic-like C1 (16.45%) and C2
(25.04%) and lower protein-like C3 (22.57%) and C4 (35.95%) than urbanized and
mixed forest-agriculture sub-catchments, while the urbanized sub-catchments
showed an inverse trend (4.54%, 15.51%, 33.97% and 45.98%, respectively). (2)
The significant variation in the proportion of C1 and C4 between the dry and
rainy seasons was recorded in both the forested and the mixed
forest-agriculture sub-catchments (p<0.01), but only C4 showed an obvious
seasonal variation in the urbanized sub-catchments (p<0.01). While the DOM
composition was mainly related to the proportion of urbanized land and forested
land year-round (p<0.01), it had stronger correlation with forested land in the
dry season and urbanized land in the rainy season. (3) No significant
correlation between the DOM composition and the agricultural land proportion
was found in either season (p>0.05). Our findings indicated that the DOM
composition was strongly dependent on the land use structure of the
sub-catchments and varied seasonally, but the seasonal variation pattern could
be disturbed by human activities in the extensively urbanized catchments.
Notably, the higher C4 proportion compared with those in temperate rivers
implied a larger amount of CO2 released from the subtropical rivers into the
atmosphere when considering bioavailability.
