Conversion of Forest to Agriculture Increases Colored Dissolved Organic Matter in a Subtropical Catchment and Adjacent Coastal Environment

Abstract

Land-ocean dissolved organic matter (DOM) transport is a significant and changing term in global biogeochemical cycles which is increasing as a result of human perturbation, including land-use change. Knowledge of the behavior and fate of transported DOM is lacking, particularly in the tropics and subtropics where land-use change is occurring rapidly. We used Parallel Factor (PARAFAC) Analysis to investigate how land-use influenced the composition of the DOM pool along a subtropical land-use gradient (from near-pristine broadleaf forest to agri-urban settings) in Belize, Central America. Three humic-like and two protein-like components were identified, each of which was present across land uses and environments. Land-use mapping identified a strong (R 2 = 0.81) negative correlation between broadleaf forest and agri-urban land. All PARAFAC components were positively associated with agriurban land-use classes (cropland, grassland, and/or urban land), indicating that land-use change from forested to agri-urban exerts influence on the composition of the DOM pool. Humic-like DOM exhibited linear accumulation with distance downstream and behaved conservatively in the coastal zone whilst protein-like DOM exhibited nonlinear accumulation within the main river and nonconservative mixing in coastal waters, indicative of differences in reactivity. We used a hydrodynamic model to explore the potential of conservative humics to reach the region's environmentally and economically valuable coral reefs. We find that offshore corals experience short exposures (10 ± 11 days yr −1 ) to large (∼120%) terrigenous DOM increases, whilst nearshore corals experience prolonged exposure (113 ± 24 days yr −1 ) to relatively small (∼30%) terrigenous DOM increases. Plain Language SummaryThe transport of land-derived dissolved organic matter into the oceans plays a substantial and important role in the global carbon and nutrient cycles. Land-use change can alter the type and amount of material being transported, with widespread implications for downstream ecosystems. This is particularly true in the tropics and subtropics where land-use change is occurring most rapidly, and where research into its effects is often lacking. We investigated whether landuse had an effect on the type and amount of land-derived material found in a subtropical river system that is experiencing a rapid conversion from forest to agricultural and urban land-use. We found that streams draining agricultural and urban land contained more land-derived material than those draining forested land, and that a substantial fraction of this material reached the coastal environment. We estimated the frequency with which this land-derived material reached the region's environmentally and economically valuable coral reefs, and suggest that land-use-derived material reaches nearshore corals often and offshore corals rarely. FELGATE ET AL.

Environmental Calibration of Coral Luminescence as a Proxy for Terrigenous Dissolved Organic Carbon Concentration in Tropical Coastal Oceans

“…Reconstructing the full CDOM absorption spectrum has the potential to provide information about changes in optical water quality, which may contribute to a better understanding of the ecological effects of tDOM fluxes to coastal waters. Anthropogenic increases in the terrigenous CDOM flux to tropical coastal waters has recently been documented (Felgate et al., 2021; Sanwlani et al., 2022), and are likely to have led to ecologically relevant reductions in euphotic zone depth and spectral shifts in underwater irradiance (Martin et al., 2021). Such “coastal browning” due to increased terrigenous CDOM flux is thought to have contributed to ecological regime shifts off Norway, including kelp forest collapse (Aksnes et al., 2009; Frigstad et al., 2013), and possibly delaying the North Sea spring phytoplankton bloom (Opdal et al., 2019).…”

Environmental calibration of coral luminescence as a proxy for terrigenous dissolved organic carbon concentration in tropical coastal oceans

“…Reconstructing the full CDOM absorption spectrum has the potential to provide information about changes in optical water quality, which may contribute to a better understanding of the ecological effects of tDOM fluxes to coastal waters. Anthropogenic increases in the terrigenous CDOM flux to tropical coastal waters has recently been documented (Felgate et al, 2021;Sanwlani et al, 2022), and are likely to have led to ecologically relevant reductions in euphotic zone depth and spectral shifts in underwater irradiance (Martin et al, 2021). Such "coastal browning" due to increased terrigenous CDOM flux is thought to have contributed to ecological regime shifts off Norway, including kelp forest collapse (Aksnes et al, 2009;Frigstad et al, 2013), and possibly delaying the North Sea spring phytoplankton bloom (Opdal et al, 2019).…”

Characteristics of Dissolved Organic Matter in a Semi-closed Bay in Summer: Insights from Stable Isotope and Optical Analyses

“…Being surrounded by frequent human activity and diverse environments, estuarine DOM is provided with complex sources including primary productivity, river input, and sewage input (Hudson et al, 2007;Guo et al, 2014;Kinsey et al, 2018). As a matter of fact, more and more allochthonous DOM is imported into estuarine systems with the increase of anthropogenic activities like industrialization, urbanization, and agricultural practices (Boesch, 2002;Felgate et al, 2021). They will change the quality and quantity of DOM, which in turn impact the biogeochemical cycle, decomposition of organic matter, and recycling of chemical elements in sediments (Spilling et al, 2018;Felgate et al, 2021;García-Martín et al, 2021).…”

“…As a matter of fact, more and more allochthonous DOM is imported into estuarine systems with the increase of anthropogenic activities like industrialization, urbanization, and agricultural practices (Boesch, 2002;Felgate et al, 2021). They will change the quality and quantity of DOM, which in turn impact the biogeochemical cycle, decomposition of organic matter, and recycling of chemical elements in sediments (Spilling et al, 2018;Felgate et al, 2021;García-Martín et al, 2021). Due to the different sources of DOM, their conversion, retention, and output fluxes in biogeochemical processes including flocculation, photochemistry, and biodegradation in estuaries also differ (Carlson and Hansell, 2015;García-Martín et al, 2021).…”