Variation of stream metabolism along a tropical environmental gradient

Abstract

Stream metabolism is affected by both natural and human-induced processes. While metabolism has multiple implications for ecological processes, relatively little is known about how metabolic rates are influenced by land use in tropical streams. In this study, we assessed the metabolic characteristics and related environmental factors of six streams located in a transition area from Cerrado to Atlantic Forest (São Carlos/Brazil). Three streams were relatively preserved, while three were flowing through more agriculturally and/or urban impacted watersheds. Surface water samples were analyzed for biological and physico-chemical parameters as well as discharge and percentage of canopy cover. Metabolism was determined through the single-station method to estimate gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) with BAyesian Single-station Estimation (BASE). Nutrient concentrations tended to be higher in impacted versus preserved streams (e.g., average total phosphorus between 0.028-0.042 mg L-1 and 0.009-0.038 mg L-1, respectively). Average canopy cover varied between 58 and 77%, with no significant spatial or seasonal variation. All streams were net heterotrophic (ER exceeded GPP) in all sampling periods. GPP rates were always lower than 0.7 gO2 m-2 d-1 in all streams and ER varied from 0.6 to 42.1 gO2 m-2 d-1. Linear Mixed-Effect models showed that depth, discharge, velocity and total phosphorus are the most important predictors for GPP. For ER, depth, velocity and canopy cover are significant potential predictors. Canopy cover was the main light limiting factor and influenced stream metabolism. Our findings reinforced the concepts that shifts in the shading effect provided by vegetation (e.g., through deforestation) or changes in discharge (e.g., through land use conversion or water abstractions) can impact freshwater metabolism. Our study suggests that human activities in low latitude areas can alter tropical streams’ water quality, ecosystem function, and the degree of riparian influence. Our data showed that tropical streams can be especially responsive to increases of organic matter inputs leading to high respiration rates and net heterotrophy, and this should be considered to support management and restoration efforts.

Nitrogen and Phosphorus Uptake Dynamics in Tropical Cerrado Woodland Streams

“…Above a molar Redfield ratio of N:P of 16:1 [62], P is expected to limit algal growth (N is in excess), and below this ratio there is an N deficit (P is in excess). The authors of [58] presented molar TN:TP ratios of 44 to 195 in the stream water for the same reaches we studied here, which would suggest P limitation. The lack of a relationship between SRP uptake metrics and DIN concentrations further supports P limitation as the increased availability of N should not facilitate SRP uptake if phosphorus is the primary limiting nutrient.…”

“…Forested heterotrophic headwater streams, such as the streams we studied [58], typically have a considerable nutrient retention capacity (i.e., short S w-amb and high U amb and V f-amb [10,14]). Several studies have also reported substantial NO 3 -N, NH 4 -N, and SRP retention in pristine tropical headwater streams [40,41,57,59].…”

Nutrient uptake in a simplified stream channel: Experimental manipulation of hydraulic residence time and transient storage

“…In our study reach, the relatively high NO 3 − background concentration (approximately 300 μg L −1 ) can thus be a result of the nitrification process and loading from the broader watershed. In addition, the dissolved oxygen concentrations in the studied stream (usually between 5.5 and 7.5 mg L −1 , Saltarelli et al, 2018) probably favoured the nitrification process. Ribot et al (2013) found that nitrification in a high-N stream (mean ambient NO 3 − + NH 4 + concentration of approximately 800 μg L −1 ) was lower (0.2%) than in a low-N stream (mean ambient NO 3 − + NH 4 + concentration of approximately 200 μg L −1 ; 0.2-7.6%).…”

Agriculture influences ammonium and soluble reactive phosphorus retention in South American headwater streams

“…Particularly, the lack of differences in canopy cover between the studied stream types probably influenced our results, because canopy cover significantly drives metabolic rates (Bunn, Davies, & Mosisch, ). We also speculate that higher ER rates in the low‐intensity agricultural sites result from higher channel complexity and potential differences in the composition and abundance of benthic biofilms (e.g., algae, bacteria, and fungi) (Saltarelli et al, ), although we did not include biofilm characteristics in this study. Also, herbicides and siltation can be important issues in agricultural streams as turbidity can absorb light and sediments can scour algae, as highlighted in other studies (e.g., see Wantzen & Mol, ).…”

“…The streams were typical meandering channels with alternating runs and pools. Benthic chlorophyll‐a is usually not greater than ~35 mg m −2 (Saltarelli et al, ). Four other sites are located in the State of Minas Gerais, in the Campo das Vertentes region (municipalities of São João del‐Rei, Tiradentes, Prados, and Resende Costa), in the Cerrado/Atlantic Rainforest transition.…”