Tropical systems face increasing pressure from land-use change in developing countries as well as unknown effects of imminent climate change. The concentration and composition of riverine dissolved organic carbon (DOC) serves as an integrator of terrestrial processes on timescales relevant to land-use change and climate change. Whereas most tropical rivers are challenging to study due to their remote locations and limited infrastructure, this project will take advantage of existing infrastructure and ongoing studies in the Luquillo Critical Zone Observatory (CZO) in Puerto Rico. Specifically, the investigator will measure the composition and concentration of biomarkers for higher plants, lignin-derived phenols, on weekly samples over the course of one year from two rivers that drain catchments of similar vegetation but contrasting lithologies and erosion rates with the goal of characterizing seasonal, event-driven, and catchment differences in DOC composition and concentration. Results will be compare and contrasted with a similar dataset from a temperate agricultural watershed to investigate partitioning effects, the effects of hydrologic flowpaths during low and high discharge, and the use of carbon-normalized yields of lignin as an endmember for DOC sources. This project will help to establish carbon cycling baseline measurements within these rivers, hence the societal impact will be an increased ability to understand impending climate and land-use changes in sensitive tropical systems. In addition, this is an investigation of fundamental processes within watersheds that are likely to be generalizable to nearly any DOC studies on rivers, hence results will have broad impact toward future studies. Results from this project will be presented at scientific meetings and published in peer-reviewed journals. The PI, Hernes, teaches undergraduate and graduate classes that cover various aspects of the global carbon cycle, climate change, and river biogeochemistry, and data collected from this project will be used to demonstrate important processes that are susceptible to changing state factors. Incorporation of active research into course curriculums increases the enthusiasm of both the instructor and the students, frequently leading to undergraduate internships. One such undergraduate from a group under-represented in science will participate in the laboratory analyses of samples and will have the opportunity to present results at undergraduate research forums on campus. Results will be posted on research websites and data contributed to appropriate national and international data bases associated with the Luquillo CZO for widespread use.
Intellectual Merit Tropical rivers are responsible for two thirds of global fluxes of terrigenous organic matter to the oceans, yet because of their remote locations relative to most industrialized countries, they are poorly studied compared to temperate and even Arctic rivers. This study measured lignin biomarkers on weekly samples from two Puerto Rican rivers, the Mameyes and the Rio Icacos, over the course of ~9 months in order to investigate watershed controls on organic matter export via rivers. In both rivers, carbon-normalized yields of lignin increased with discharge, indicating higher organic matter contributions from vascular plants and lower contributions from microbial sources. Lignin biomarkers can be used to distinguish between conifer and deciduous trees as well as woody and nonwoody tissues, and the source signatures of the two rivers were essentially the same, indicating similar vegetation and sources of organic materials to both rivers. Thus, differences in diagenetic lignin parameters can be attributed to differences in processing within the two watersheds. For example, the Rio Icacos is a much flashier system with much higher sediment loads during storm events, which can lead to significant exchange of organic matter between the dissolved and particulate phase. Acid to aldehyde ratios for vanillyl and syringyl lignin phenols, (Ad:Al)v and (Ad:Al)s, are commonly used indicators of degradation. Plots of (Ad:Al)s vs. (Ad:Al)v for the two rivers reveal a significantly steeper slope for the Mameyes, which could reflect differences in the mineralogy between the systems, as the Mameyes watershed is underlain by volcaniclastic materials while the Rio Icacos is underlain by granodiorite. It is a commonly held assumption that base materials and soils should exert a strong influence on riverine organic matter compositions, but paired watershed studies are comparatively rare even in temperate climates. Hence, measureable differences between the two systems studied here demonstrate significant potential for future studies of landscape controls on river biogeochemistry. Broader Impacts Enormous amounts of carbon are stored in tropical biomass and soils that could significantly impact climate change if released in appreciable quantities through deforestation, urbanization, or changes in precipitation patterns and amounts. Therefore, it is critical to establish a baseline understanding of carbon dynamics in tropical systems on which to gauge future change. River chemistry responds quickly to changes in climate and land use and as such is an ideal canary for monitoring the impacts of these changes. This project points toward underlying mineralogy as one control on the release and composition of riverine carbon. Society in general relies on rivers for water, transportation, recreation, food, waste disposal, and other ecosystem services and functions. This line of research furthers our understanding of how rivers respond to various anthropogenic and environmental pressures, and helps to establish a baseline for a healthy river ecosystem. Healthy rivers are a benefit to everyone. This research project included participation of several people from underrepresented groups, including a female undergraduate student who is working toward a career in environmental sciences. Outcomes A manuscript detailing our research findings is currently in preparation for submission to a peer-reviewed journal. Findings from this research will be presented at scientific meetings in the near future, and will be utilized in the classroom for undergraduate and graduate instruction.
