Partitioning Inorganic Carbon Fluxes from Paired O2 - CO2 Gas Measurements in a Neotropical Headwater Stream, Costa Rica

The role of rivers and streams in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO2 evasion (FCO2) to the atmosphere is high. Stream C cycling is understudied in the tropics compared to temperate streams, and tropical streams may have among the highest FCO2 due to higher temperatures, continuous organic matter inputs, and high respiration rates both in-stream and in surrounding soils. In this paper, we present paired in-stream O2 and CO2 sensor data from a headwater stream in a lowland rainforest in Costa Rica to explore temporal variability in ecosystem processes. Further, we estimate groundwater CO2 inputs (GWCO2) from riparian well CO2 measurements and assess all fluxes to examine the relative contributions of sinks and sources of dissolved inorganic C (DIC) to a headwater stream. Paired O2 - CO2 data reveal stream CO2 supersaturation driven by groundwater CO2 inputs and large in-stream production of CO2. Areal fluxes in our study reach show FCO2 is supported by both GWCO2 inputs and in-stream metabolism and the seasonality in GWCO2 reflects the hydrology of the site. Using a mass balance approach, we show FCO2 is the dominant loss of DIC from the stream, greater than dissolved exports, and is sustained by both internal production of DIC and terrestrial inputs of DIC. Our results underscore the importance of tropical headwater streams as large contributors of greenhouse gases to the atmosphere among inland waters and show of this C derives from both in-stream and terrestrial sources.

Spatial Variations in the Distribution of Benthic Macroinvertebrate Functional Feeding Groups in Tropical Rivers

Functional feeding group (FFG) is an approach that classifies macroinvertebrates based on their utilization of organic matter food resources. Across streams and rivers, variations in the distribution of FFGs reflect the unequal distribution of food resources, which are affected by varying environmental conditions and disturbances to the ecosystem. In the tropics, the distribution of FFGs does not follow the pattern observed in temperate streams. This study aims to (1) determine the FFGs present in selected Philippine tropical streams, (2) assess the variations in FFG distributions and how the distributions vary across space, and (3) evaluate how FFG distributions are affected by certain environmental factors and habitat structure. Benthic macroinvertebrates were collected in 2010 from five sites and assigned to their primary FFG based on published literature. Across study sites, the macroinvertebrates collected were classified into gathering collectors (total n = 4,640), scrapers (n = 1,670), filtering collectors (n = 1,545), predators (n = 632), and shredders (n = 270). Within and between sites, these FFGs varied significantly. Variations explained the upstream-downstream and north-south variations in the mean of FFG abundance in width and depth of the stream, discharge, temperature, pH, riparian vegetation, and habitat stability and variability. This study reveals that the FFG approach is a useful bioassessment tool in tropical aquatic systems. However, there is still a need for verification of the findings in other streams and seasons.

Experimental desiccation indicates high moisture content maintains hyporheic biofilm processes during drought in temperate intermittent streams

Droughts are expected to become more common with climate change resulting in more frequent occurrences of flow intermittency in temperate streams. As intermittency has deleterious effects on fluvial microbial biofilms, there is a need to better understand how droughts affect the microbial functioning and thereby nutrient and organic matter processing in temperate stream ecosystems. Here, the hyporheic zone is of particular importance as it has been shown to be a hot spot for biogeochemical activity under flow intermittence. This study evaluates how drought duration affects microbial biofilm dynamics in the hyporheic zone of intermittent temperate streams. To do so, we used outdoor hyporheic flumes that were subject to periods of drought ranging from 4 to 105 days. Sediment was sampled before and during the drought, and at several occasions after rewetting. Samples were analyzed for extracellular enzymatic activity, bacterial respiration, and bacterial abundances including live to dead cell ratios. The high moisture content remaining in the hyporheic zone of the flumes allowed for the sustained microbial functioning during drought, regardless of drought duration. This can be attributed to cooler temperatures in these climate zones and shading by riparian forests. The high moisture content inhibited the local habitat and community changes that the biofilm might have undergone during more severe desiccation. However, the change in the hyporheic flow regime (flow cessation and resumption) may stimulate microbial processing in these moderate drought conditions. We suggest that the hyporheic zone may act as a buffer against drought and the factors determining this buffer capacity, such as sediment characteristics and climatic regions, need to be analyzed in more detail in future.

The impacts of drought on the microbial states and processes in the hyporheic zone of temperate streams

<p>Intermittent streams as well as extreme events are expected to become more common as the climate changes. Therefore, it is important to understand how drought affects the biofilms that are essential for nutrient and DOM processing within streams. Previous work has largely focused on Mediterranean streams. This project evaluates how drought affects the state and processes of the microbial biofilms in the hyporheic zone of temperate intermittent streams. The experiment was conducted with outdoor experimental hyporheic flumes (5 m long, 0.6 m wide, 1.2 m deep) that were allowed to fall dry for periods ranging from 4 to 100 days. Sediment was sampled before drying, during the drought, and at several time points after rewetting. Samples were analyzed for extracellular enzymatic activity, respiration, bacterial growth, live to dead cell ratios, bacterial abundances, and extracellular polymeric substances.</p><p>Extracellular enzymatic activities remained unaffected by the drought in the hyporheic zone but showed an increase on the surface during the dry phase. Upon rewetting, the enzymatic activities generally fell to pre-drought levels on the surface. Extracellular polymeric substances also remained unaffected by drought in the hyporheic zone. However, surface values for extracellular polymeric substances showed a similar pattern to enzymatic activities during the longer dry periods (70+ days) and subsequent rewetting. These results indicate that the hyporheic zone retained enough moisture in the sediment to continue functioning, while the surface sediment was impacted by the loss of flowing water.</p>