2017-08-30T02:06:56

To investigate the coupling between carbon (C) and phosphorus (P) cycling in a human-altered stream, we conducted a whole-ecosystem manipulation of the labile dissolved organic carbon (DOC) pool in a nitrate (NO { 3 )-rich stream in the midwestern United States. For 6 d, we increased stream DOC by , 1 mg L 21 through a continuous addition of sodium acetate. On the sixth day of the addition, ammonium (NH z 4 ) was increased by , 130 mg N L 21 to examine the potential for nitrogen (N) to mediate coupled C and P cycling. Of the added DOC, 85% was retained within the treatment reach, which increased ecosystem respiration with respect to the reference reach. Alkaline phosphatase activity (APA) increased from day 1 to day 6; however, water column P uptake only increased on day 6 concurrent with the NH z 4 addition. Gross primary production decreased during the DOC addition relative to the reference reach, yet seemed to recover on day 6 (NH z 4 addition). These results suggest that during the DOC addition, heterotrophs out-competed autotrophs for N and that sediment-sorbed P sustained the heterotrophic community while P uptake from the water column was dominated by autotrophs. Because APA and P uptake were stimulated by the simultaneous DOC and NH over NO

In the tropical rainforest at La Selva Biological Station in Costa Rica, regional bedrock groundwater high in dissolved carbon discharges into some streams and wetlands, with the potential for multiple cascading effects on ecosystem carbon pools and fluxes. We investigated carbon dioxide (CO 2 ) and methane (CH 4 ) degassing from two streams at La Selva: the Arboleda, where approximately one third of the streamflow is from regional groundwater, and the Taconazo, fed exclusively by local groundwater recharged within the catchment. The regional groundwater inflow to the Arboleda had no measurable effect on stream gas exchange velocity, dissolved CH 4 concentration, or CH 4 emissions but significantly increased stream CO 2 concentration and degassing. CO 2 evasion from the reach of the Arboleda receiving regional groundwater (lower Arboleda) averaged 5.5 mol C m À2 d À1 ,~7.5 times higher than the average (0.7 mol C m À2 d À1 ) from the stream reaches with no regional groundwater inflow (the Taconazo and upper Arboleda). Carbon emissions from both streams were dominated by CO 2 ; CH 4 accounted for only 0.06-1.70% of the total (average of both streams: 5 × 10 À3 mol C m À2 d À1 ). Annual stream degassing fluxes normalized by watershed area were 48 and 299 g C m À2 for the Taconazo and Arboleda, respectively. CO 2 degassing from the Arboleda is a significant carbon flux, similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of catchment connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance our understanding of carbon source/sink status and overall terrestrial ecosystem carbon budgets.Regional groundwater flow is a natural hydrogeological process by which groundwater moves long distances beneath surface topographic divides, possibly recharging in one watershed and discharging in another many kilometers away [Tóth, 2009;Schaller and Fan, 2009;Smerdon et al., 2012;Pacheco, 2015], thus creating the potential for relatively long-distance subsurface transport of C between watersheds and ecosystems. In addition to biogenic contributions from the surface, dissolved CO 2 and CH 4 in regional groundwater can originate from volcanic degassing, nonvolcanic escape of gases from the upper mantle, intrusive magma chambers, carbonate-bearing rocks in the crust, hydrocarbon accumulations [Mörner and Etiope, 2002], and remineralization of ancient sedimentary organic matter [Lovley and Anderson, 2000;Park et al., 2009;Liu et al., 2014]. Aquifers transporting high concentrations of dissolved C have been identified in numerous places around the world, for example, Portugal [Cruz and Amaral, 2004], the Canary Islands [Marrero et al., 2008], OVIEDO-VARGAS ET AL.

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.

Respiration in streams is controlled by the timing, magnitude, and quality of organic matter (OM) inputs from internal primary production and external fluxes. Here, we estimated the contribution of different OM sources to seasonal, annual, and event-driven characteristics of whole-stream ecosystem respiration (ER) using an inverse modeling framework that accounts for possible time-lags between OM inputs and respiration. We modeled site-specific, dynamic OM stocks contributing to ER: autochthonous OM from gross primary production (GPP); allochthonous OM delivered during flow events; and seasonal pulses of leaf litter. OM stored in the sediment and dissolved organic matter (DOM) transported during baseflow were modeled as a stable stock contributing to baseline respiration. We applied this modeling framework to five streams with different catchment size, climate, and canopy cover, where multi-year time series of ER and environmental variables were available. Overall, the model explained between 53% and 74% of observed ER dynamics. Respiration of autochthonous OM tracked seasonal peaks in GPP in spring or summer. Increases in ER were often associated with high-flow events. Respiration associated with litter inputs was larger in smaller streams. Time lags between leaf inputs and respiration were longer than for other OM sources, likely due to lower biological reactivity. Model estimates of source-specific ER and OM stocks compared well with existing measures of OM stocks, inputs, and respiration or decomposition. Our modeling approach has the potential to expand the scale of comparative analyses of OM dynamics within and among freshwater ecosystems.