Regional Groundwater and Storms Are Hydrologic Controls on the Quality and Export of Dissolved Organic Matter in Two Tropical Rainforest Streams, Costa Rica

Abstract

A paired‐watershed approach was used to compare the quality and fluxes of dissolved organic matter (DOM) during stormflow and baseflow in two lowland tropical rainforest streams located in northeastern Costa Rica. The Arboleda stream received regional groundwater (RGW) flow, whereas the Taconazo stream did not. DOM quality was assessed with absorbance and fluorescence and stable carbon isotope (δ13C‐DOC) values. RGW DOM lacked detectable fluorescence and had specific ultraviolet absorption (SUVA254) and absorbance slope ratio (SR) values consistent with low aromaticity and low molecular weight material, respectively. We attributed these properties to microbial degradation and sorption of humic DOM to mineral surfaces during transport through bedrock. SUVA254 values were lower and SR values were higher in the Arboleda stream during baseflow compared to the Taconazo stream, presumably due to dilution by RGW. However, no significant difference in SUVA254 or SR occurred between the streams during stormflow. SUVA254 was negatively correlated to δ13C‐DOC (r2 = 0.61, P < 0.001), demonstrating a strong linkage between stream DOM characteristics and the relative amounts of RGW flow and local watershed runoff containing soil and throughfall C sources. Mean DOC export from the Taconazo stream during the study period was 2.62 ± 0.39 g C m−2 year−1, consistent with other tropical streams, yet mean DOC export from the Arboleda stream was 13.79 ± 2.07 g C m−2 year−1, one of the highest exports reported and demonstrating a substantial impact of old RGW from outside the watershed boundary can have on surface water carbon cycling.

DOC Transport and Export in a Dynamic Tropical Catchment

“…Intermittent baseflow conditions were characterized by variable DOC, ranging from 0.89 to 4.97 mg/L, with a mean value of 3.41 ± 0.97 (1σ). Mean annual carbon flux was 6.7±0.1 g C · m -2 · yr -1 in concordance with recent values reported by Osburn et al (2018) in Costa Rica (1.9 -13.79 6.7±0.1 g C · m -2 · year -1 ) and with other tropical regions (Table 1). CDOM ranged from 2.3 up to 31.2 m -1 , while A 254 / DOC ratio (i.e., the specific UV absorption known as SUVA 254 ) varied from 2.6 to 4.3 L mg · C -1 · m -1 .…”

“…These factors have a great influence on the ability to store SOC (Dubinsky et al, ) and on the bioavailability of organic matter related‐nutrients (i.e., N, P, and trace elements) in aquatic food webs (Brandão et al, ). In such ecosystems, rainfall seasonality has been identified as one of the main drivers of allochthonous carbon transport to aquatic environments (Osburn et al, ; Suhett et al, ). Exported DOC (ranging from ~1.1‐2.7 Pg C/year from terrestrial ecosystems to inland waters and oceans; Aufdenkampe et al, ; Regnier et al, ¸ Khadka et al, ) is further decomposed (both biologically and photochemically) to CO 2 (~1.2 Pg C/year) downstream from the headwater forests and can be easily degassed within the surface water network before even reaching the coastal regions.…”

“…Grieve et al () in a pioneering work, reported an increased trend in the organic matter content from 18 to 49% in a transect from 100 to 2,600 m above sea level (asl), coupled with a decreased trend in clay content (from 80 to 10%) and free iron ratio (from 0.3 to 0.1) in central Costa Rica, indicating a clear potential of SOC reservoirs in high‐elevation landscapes. More recently, Osburn et al () quantified the quality and fluxes of DOC during storm and base flows for two locations in a tropical rainforest in northeastern Costa Rica. The authors determined one of the largest DOC export fluxes in a forested tropical catchment (13.79 ± 2.07 g C · m ‐2 · year ‐1 ).…”

Regional diversity of complex dissolved organic matter across forested hemiboreal headwater streams

“…Counter-intuitively, the more hydrophobic, saturated constituents remained dissolved in the groundwater, and were proportionally of higher abundance in headwater streams with a greater deep groundwater contribution. This removal mechanism preferentially removes the same carboxyl and aromatic functionality as photo-degradation, and leads to changes that resemble those found in samples that have been processed in surface waters 7 , 12 , 46 , 52 . With respect to surface water runoff generation, the timescale of groundwater residence time is generally confined to <1 yr in this region 26 , whereas water residence times in lakes can reach tens of years 9 .…”

“…In Sweden, first and second order streams make up 78% of the total length of all rivers 5 . In organic carbon terms, headwater streams are absolutely dominated by terrestrial dissolved organic matter (tDOM) that has been previously altered in soils and groundwater 12 , 13 , 21 . Primary production and other in-stream processes are highly constrained by the short water residence time in low order streams 22 , and only begin to significantly affect organic matter in streams at higher order stream level 23 , 24 .…”