Drivers of dissolved inorganic carbon dynamics in tropical streams: insights from the Munnar Critical Zone Observatory, Western Ghats, India

Temperate headwater streams with closed canopies rely on inputs of terrestrially derived organic matter to provide the major energy basis for their food webs. Microbial colonization, or conditioning, makes leaf litter more nutritional and palatable to stream detritivores, but few studies have investigated the relative importance of litter source to macroshredders in tropical streams. We determined the source (terrestrial, aquatic, or aerial), quantity, and species composition of allochthonous inputs into the Quebrada Prieta, a tropical headwater stream in Puerto Rico, as a first step toward understanding the importance of conditioning history to rates of tropical leaf-litter processing by decapod consumers. Fresh leaves of 4 common species of leaves were treated by exposing them to different conditions for 2 wk. These exposure treatments (conditioning histories) represented routes by which leaves might enter streams and included submersion (aquatic input), incubation on the streambank soil (terrestrial input), and suspension above the ground (aerial input). Conditioned leaves were placed in small experimental microcosms with or without shrimp (Xiphocaris elongata) for 20 d. Shrimp significantly increased the rate of decomposition of all leaf species independent of conditioning history. Conditioning history had little effect on breakdown rates independent of the presence of shrimp. One species (Rourea surinamensis) had faster mass loss when the leaves were conditioned as aquatic inputs rather than as terrestrial or aerial inputs. Our results indicate that conditioning history has little effect on the ability of some macroconsumers to alter detrital foodweb dynamics in tropical streams. Tropical stream ecosystems may function differently from temperate ecosystems because of the dominance of large detritivores such as shrimps.

Organic Matter Budgets for Streams: A Synthesis

Large wood recruitment, retention and mobilization in low-order streams of the Brazilian Savanna

Riparian vegetation typically provides substantial allochthonous material to aquatic ecosystems where micro-organisms can play an important role in organic matter degradation which can support consumer biomass. We examined the effects of leaf litter quality (e.g., leaf nutrients, lignin and cellulose content), leaf species mixing, and microbial community diversity on in-stream breakdown rates of litter from dominant riparian trees (Melaleuca argentea, M. leucadendra, and Nauclea orientalis) in both a perennial and intermittent river in Australia’s wet-dry tropics. Leaf mass remaining after 82 days of in-stream incubation was negatively correlated (P < 0.05) with initial leaf N and P content while initial lignin and cellulose content had no statistically significant effect. Breakdown rates of incubated leaves of both Melaleuca and Nauclea were significantly higher in mixed litter bags compared with single species litter bags. Although it was expected that leaf N content would decrease from initial levels during decomposition, we found either similar or slightly higher N content following in-stream incubation suggesting microbial colonisation increased overall N content. Stable isotopes of δ13C and δ15N for the major sources and consumers in both rivers provide evidence that leaf litter was an important macroinvertebrate food source in the perennial river where heavy shading may limit algal production. However, in the intermittent river where riparian cover was low, benthic algae were the major organic carbon source for consumers. Our findings suggest that riparian tree species influence rates of in-stream organic matter processing, microbial community composition, and aquatic food web dynamics in tropical wet-dry streams.

Riparian plant litter is a major energy source for forested streams across the world and its decomposition has repercussions on nutrient cycling, food webs and ecosystem functioning. However, we know little about plant litter dynamics in tropical streams, even though the tropics occupy 40% of the Earth’s land surface. Here we investigated spatial and temporal (along a year cycle) patterns of litter inputs and storage in multiple streams of three tropical biomes in Brazil (Atlantic forest, Amazon forest and Cerrado savanna), predicting major differences among biomes in relation to temperature and precipitation regimes. Precipitation explained most of litter inputs and storage, which were generally higher in more humid biomes (litterfall: 384, 422 and 308 g m−2 y−1, storage: 55, 113 and 38 g m−2, on average in Atlantic forest, Amazon and Cerrado, respectively). Temporal dynamics varied across biomes in relation to precipitation and temperature, with uniform litter inputs but seasonal storage in Atlantic forest streams, seasonal inputs in Amazon and Cerrado streams, and aseasonal storage in Amazon streams. Our findings suggest that litter dynamics vary greatly within the tropics, but point to the major role of precipitation, which contrasts with the main influence of temperature in temperate areas.

Pollution abatement through phosphorus and nitrogen retention is a key ecosystem service provided by streams. Human activities have been changing in-stream nutrient concentrations, thereby altering lotic ecosystem functioning, especially in developing countries. We estimated nutrient uptake metrics (ambient uptake length, areal uptake rate, and uptake velocity) for nitrate (NO3–N), ammonium (NH4–N), and soluble reactive phosphorus (SRP) in four tropical Cerrado headwater streams during 2017, through whole-stream nutrient addition experiments. According to multiple regression models, ambient SRP concentration was an important explanatory variable of nutrient uptake. Further, best models included ambient NO3–N and water velocity (for NO3–N uptake metrics), dissolved oxygen (DO) and canopy cover (for NH4–N); and DO, discharge, water velocity, and temperature (for SRP). The best kinetic models describing nutrient uptake were efficiency-loss (R2 from 0.47–0.88) and first-order models (R2 from 0.60–0.85). NO3–N, NH4–N, and SRP uptake in these streams seemed coupled as a result of complex interactions of biotic P limitation, abiotic P cycling processes, and the preferential uptake of NH4–N among N-forms. Global change effects on these tropical streams, such as temperature increase and nutrient enrichment due to urban and agricultural expansion, may have adverse and partially unpredictable impacts on whole-stream nutrient processing.

Dissolved and particulate organic matter are the energy source for secondary production in forested streams. Cycling of or¬ganic matter and stream ecosystem functioning are linked to organic matter input and storage capacity and timing. This study assessed the seasonal variation (dry and rainy seasons) of environmental parameters, organic matter stock and input, and stream metabolism in two first-order tropical streams in the Selva Lacandona, Mexico. We also aimed to identify the drivers of organic matter and stream metabolism seasonality. We found seasonal variation in organic matter stock and input correlated with trop¬ical seasonality. Dissolved organic matter and seston increased in the rainy season, while benthic primary producers and leaf litter stock and input increased in the dry season correlated with lower water discharge. Gross primary production increased in the dry season, while ecosystem respiration did not differ between seasons. Seasonality defined by the rainfall pattern and its effect on stream hydrology is the main driver of organic matter dynamics in tropical streams. However, environmental parameters and organic matter stock and input were not good predictors of stream metabolism.

Abstract. High-mountain ecosystems are experiencing the acute effects of climate change, most visibly through glacier recession and the greening of the terrestrial environment. The streams draining these landscapes are affected by these shifts, integrating hydrologic, geologic, and biological signals across the catchment. We examined the organic and inorganic carbon dynamics of streams in four Alpine catchments in Switzerland to assess how glacier loss and vegetation expansion are affecting the carbon cycle of these high-mountain ecosystems. We find that the organic carbon concentration and fluorescence properties associated with humic-like compounds increase with vegetation cover within a catchment, demonstrating the increasing importance of allochthonous dissolved organic carbon sources following glacier retreat. Meanwhile, streams transitioned from carbon dioxide sinks to sources with decreasing glacier coverage and increased vegetation coverage, with chemical weathering and soil respiration likely determining the balance. Periods of sink behavior were also observed in non-glaciated streams, possibly indicating that the chemical consumption of carbon dioxide could be more common in high-mountain, minimally vegetated catchments than previously known. Together, these results demonstrate the dramatic shifts in carbon dynamics of high-mountain streams following glacier recession, with significant changes to both the organic and inorganic carbon cycles. The clear link between the terrestrial and aquatic zones further emphasizes the coupled dynamics with which all hydrologic and biogeochemical changes in these ecosystems should be considered, including the carbon sink or source potential of montane ecosystems.

Dissolved inorganic carbon export by mountainous tropical rivers of the Western Ghats, India

São Paulo state is the largest sugarcane cultivation area in Brazil and the largest sugarcane producer in the world, yet the impact of sugarcane cultivation on carbon dynamics in tropical stream ecosystems remains poorly understood. We investigated CO2 emissions and concentrations in streams draining sugarcane fields and native vegetation catchments to elucidate the influence of sugarcane cultivation on CO2 dynamics in streams. Contrary to our hypothesis, streams from native vegetation catchments exhibited greater CO2 emissions and concentrations than those from draining sugarcane fields. This result can be attributed to the soil respiration, which is higher in native vegetation catchments because of higher organic matter inputs. Our findings emphasize the significant role of tropical vegetation dynamics in shaping carbon dynamics in freshwater ecosystems and the connections between terrestrial and aquatic ecosystems in headwaters. Additionally, we observed higher CO2 emissions during the summer, attributable to increased temperatures, streamflow, and terrestrial organic matter inputs in soils and streams. The variables influencing CO2 concentrations were pH, conductivity, season, and methane concentration, highlighting the complex interplay of environmental factors. Future research should address critical gaps, including the effects of soil texture and liming on CO2 dynamics, and the quantification of the contributions of methane oxidation to CO2 emissions. Understanding these factors is vital for assessing the impact of sugarcane cultivation on freshwater carbon cycles, particularly in regions such as Brazil, which is a major contributor to global sugarcane production.

Soil organic carbon (SOC) is known to vary among different ecosystems and soilscapes, yet the degree of variation remains uncertain. Comparing SOC levels in undisturbed ecosystems like forests with those in gradually altered ecosystems can provide valuable insights into the impact of land use on carbon dynamics. This study aimed to evaluate the effects of different land uses on soil fertility parameters in the tropical region of Kerala, focusing on forests as well as cultivated agricultural landscape such as coconut, pepper, tapioca, acacia plantations, and mixed home garden cropping systems. Significant variations were observed among different crops and land use systems in terms of soil fertility. Forests exhibited the highest SOC content at 3.78 g kg-1,while acacia plantations showed the lowest at 0.76 g kg-1. Additionally, various soil properties such as different carbon fractions (e.g., humic acid, fulvic acid), total nitrogen, carbon, available nutrients, physical properties, aggregate size fractions, microbial biomass carbon, and spectral signatures differed significantly across the different land uses. These findings suggest a decline in soil fertility in altered ecosystems compared to adjacent forest soils, highlighting the vital role of forests in conserving natural resources and maintaining soil health. In addition, among the different landscapes studied, mixed cropping systems of home gardens sustained soil fertility better than monocropping systems. The observed variations in soil physicochemical properties among different land use types indicate a threat to sustainable crop production. Effective management practices aimed at improving soil fertility and sustaining crop production in these altered ecosystems are essential. This study highlights the importance of adopting appropriate management strategies to conserve soil health and ensure sustainable crop production in tropical landscapes like Kerala. The holistic approach adopted in this study, encompassing a wide range of soil fertility parameters across various land uses, along with its implications for sustainable land management, adds significant novelty and relevance to the existing literature on soil dynamics in tropical regions like Kerala.

Background Tropical forests play a critical role in global biodiversity conservation and carbon storage. In human-modified landscapes, secondary forests are becoming increasingly common, yet their ecological functioning remains underexplored. Comparing the forest dynamics in mature and regenerating forests offers insights into forest recovery and carbon dynamics. Aims To compare forest structure, floristic composition, stand dynamics and carbon of a mature and secondary tropical rainforest. Methods We assessed tree community and carbon dynamics over 5 years in two 1-ha long-term ecosystem monitoring plots, one each in mature tropical rainforests (MR) and 10-year post-agroforestry secondary rainforests (SR) in India’s Western Ghats mountains. Both plots were established in 2017 and monitored annually. We expected (1) higher tree diversity, differences in species composition, and greater carbon stock in MR; (2) higher carbon sequestration rates in SR; and (3) carbon dynamics shaped by growth and mortality in SR and MR, respectively. Results The SR plot had fewer species (67 vs. 84), stored substantially less carbon (76 vs. 193 Mg), and comprised a distinct community with fewer late-successional species than MR. SR gained 5.8 Mg carbon, due to tree growth exceeding losses from mortality, while in MR mortality exceeded growth and recruitment resulting in a 3.3 Mg carbon decline over five years. Conclusion While MR had higher tree diversity, carbon stocks and relatively intact composition, the high rates of biodiversity and carbon accrual in SR highlight the conservation and climate significance of post-agroforestry secondary forests. Moderate carbon losses noted here in MR, as in other mature South Asian tropical forests, is a cause for concern under ongoing climate change.

Monsoon-driven hydrological variability and anthropogenic pressures in tropical critical zones pose significant threats to water quality and public health. This study investigates the spatial and temporal distribution, sources, and human health risks of trace and heavy metals at the Munnar Critical Zone Observatory in the Western Ghats, India. Through integration of hydrogeochemical analyses, land use-land cover data, and health risk frameworks, revealed pronounced seasonal contrasts: monsoon rains drive two-threefold increases in Al, Fe, Pb, and Zn in surface waters due to intensified erosion of hornblende-biotite gneiss and agrochemical runoff from tea and eucalyptus plantations. Groundwater chemistry reflects aquifer lithology, while Cd and Pb enrichment during monsoon recharge is significantly influenced by anthropogenic sources. Multivariate statistical analyses attribute (PCA, HCA) 60-75% of metal variability to geogenic weathering (PC1: Al, Fe, Ti) and 15-25% to anthropogenic sources (PC3: Cd, Pb from agrochemicals). Pollution indices (WPI, HPI, HEI) classify contamination as low-to-moderate overall, yet Fe exceeds thresholds in rivulets during monsoon (notably at S1 and S6). Health risk assessments prioritise Co, Mn, and Pb as non-carcinogenic threats (HI < 1), while Cr and Cd pose significant carcinogenic risks, with children's ingestion risk 3-4 times higher than adults. Concentration-discharge relationships indicate consistent chemodynamic metal transport in river systems, contrasting with seasonally-variable behaviour in rivulets, where chemostasis occurs for specific elements during pre-monsoon. These findings underscore the need for regulating agrochemical use in erosion-prone catchments and prioritising Cr and Cd monitoring in vulnerable sites. This provides a holistic framework for managing water security in climate-sensitive, human-altered landscapes globally.

<strong class="journal-contentHeaderColor">Abstract.</strong> High-mountain ecosystems are experiencing acute effects of climate change, most visibly through glacier recession and the greening of the terrestrial environment. The streams draining these landscapes are affected by these shifts, integrating hydrologic, geologic, and biological signals across the catchment. We examined the organic and inorganic carbon dynamics of streams in four Alpine catchments in Switzerland to assess how glacier loss and vegetation expansion are affecting the carbon cycle of these high mountain ecosystems. We find that organic carbon concentration and fluorescence properties associated with humic-like compounds increase with vegetation cover within a catchment, demonstrating the increasing importance of allochthonous carbon sources following glacier retreat. Meanwhile, streams transitioned from carbon dioxide sinks to sources with decreasing glacier coverage and increased vegetation coverage, with chemical weathering and soil respiration likely determining the balance. Periods of sink behavior were also observed in non-glaciated streams, indicating geochemical consumption of carbon dioxide may be more common in high-mountain, minimally vegetated catchments than previously described. Together, these results demonstrate the dramatic shifts in carbon dynamics of alpine streams following glacier recession, with significant changes to both the organic and inorganic carbon cycles. The clear link between the terrestrial and aquatic zones further emphasizes the coupled dynamics with which all hydrologic and biogeochemical changes in these ecosystems should be considered, including the role of mountain streams in the global carbon cycle.

<strong class="journal-contentHeaderColor">Abstract.</strong> High-mountain ecosystems are experiencing acute effects of climate change, most visibly through glacier recession and the greening of the terrestrial environment. The streams draining these landscapes are affected by these shifts, integrating hydrologic, geologic, and biological signals across the catchment. We examined the organic and inorganic carbon dynamics of streams in four Alpine catchments in Switzerland to assess how glacier loss and vegetation expansion are affecting the carbon cycle of these high mountain ecosystems. We find that organic carbon concentration and fluorescence properties associated with humic-like compounds increase with vegetation cover within a catchment, demonstrating the increasing importance of allochthonous carbon sources following glacier retreat. Meanwhile, streams transitioned from carbon dioxide sinks to sources with decreasing glacier coverage and increased vegetation coverage, with chemical weathering and soil respiration likely determining the balance. Periods of sink behavior were also observed in non-glaciated streams, indicating geochemical consumption of carbon dioxide may be more common in high-mountain, minimally vegetated catchments than previously described. Together, these results demonstrate the dramatic shifts in carbon dynamics of alpine streams following glacier recession, with significant changes to both the organic and inorganic carbon cycles. The clear link between the terrestrial and aquatic zones further emphasizes the coupled dynamics with which all hydrologic and biogeochemical changes in these ecosystems should be considered, including the role of mountain streams in the global carbon cycle.