Aquatic Ecosystem Impacts of Land Sharing Versus Sparing: Nutrient Loading to Southeast Asian Rivers

Cascade damming alleviates imbalanced particulate C: N: P stoichiometry to regulate methane accumulation in the upper Yangtze River, China

Cascade damming and changes in land use are significant human activities that alter natural flow patterns and the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P). However, there is limited understanding of how damming and other terrestrial human activities affect stoichiometric imbalances, contributing to uncertainties in the processes and mechanisms of methane (CH4) emissions from river-reservoir systems. Here, we discuss the spatiotemporal patterns of particulate C: N: P mole ratios and dissolved CH4 in the upper Yangtze River, which covers 11 large dams along its main stem. The particulate C: N ratio increased with particle size, but C: P and N: P ratios were synchronous and showed adverse trends. Autochthonous (A-POC) and terrigenous (T-POC) particulate organic carbon to the bulk POC accounted for 44 % and 54 %. The POC/PON close to the Redfield ratio was predominantly influenced by coupling effects between damming and terrestrial anthropogenic activities. An increase in terrestrial anthropogenic activities tended to balance POC/PON but unbalance POC/PP and PON/PP. The cascade damming effect can alleviate stoichiometric imbalances and contribute about 15 %, 61 %, and 98 % for POC/PON, POC/PP, and PON/PP close to balance. The imbalances of POC/PP and PON/PP would be gradually amplified downstream in the cascade reservoir. A-POC can lead to short-term CH4 accumulation, but T-POC with higher POC/PON, lower POC/PP, and PON/PP could potentially regulate long-term CH4 accumulation. Particulate C: N: P close to balance and CH4 oxidation before the dams reduced CH4 accumulation in the upper Yangtze River. These insights have substantial potential implications for the adaptive management of cascade reservoir systems.

A network of grassroots reserves protects tropical river fish diversity

Intensive fisheries have reduced fish biodiversity and abundance in aquatic ecosystems worldwide^1-3. 'No-take' marine reserves have become a cornerstone of marine ecosystem-based fisheries management^4-6, and their benefits for adjacent fisheries are maximized when reserve design fosters synergies among nearby reserves^7,8. The applicability of this marine reserve network paradigm to riverine biodiversity and inland fisheries remains largely untested. Here we show that reserves created by 23 separate communities in Thailand's Salween basin have markedly increased fish richness, density, and biomass relative to adjacent areas. Moreover, key correlates of the success of protected areas in marine ecosystems-particularly reserve size and enforcement-predict differences in ecological benefits among riverine reserves. Occupying a central position in the network confers additional gains, underscoring the importance of connectivity within dendritic river systems. The emergence of network-based benefits is remarkable given that these reserves are young (less than 25 years old) and arose without formal coordination. Freshwater ecosystems are under-represented among the world's protected areas⁹, and our findings suggest that networks of small, community-based reserves offer a generalizable model for protecting biodiversity and augmenting fisheries as the world's rivers face unprecedented pressures^10,11.

The Landscape Ecology of Rivers: from Patch-Based to Spatial Network Analyses

Purpose of Review

We synthesize recent methodological and conceptual advances in the field of riverscape ecology, emphasizing areas of synergy with current research in landscape ecology.

Recent Findings

Recent advances in riverscape ecology highlight the need for spatially explicit examinations of how network structure influences ecological pattern and process, instead of the simple linear (upstream-downstream) view. Developments in GIS, remote sensing, and computer technologies already offer powerful tools for the application of patch- and gradient-based models for characterizing abiotic and biotic heterogeneity across a range of spatial and temporal scales. Along with graph-based analyses and spatial statistical stream network models (i.e., geostatistical modelling), these approaches offer improved capabilities for quantifying spatial and temporal heterogeneity and connectivity relationships, thereby allowing for rigorous and high-resolution analyses of pattern, process, and scale relationships.

Summary

Spatially explicit network approaches are able to quantify and predict biogeochemical, hydromorphological, and ecological patterns and processes more precisely than models based on longitudinal or lateral riverine gradients alone. Currently, local habitat characteristics appear to be more important than spatial effects in determining population and community dynamics, but this conclusion may change with direct quantification of the movement of materials, energy, and organisms along channels and across ecosystem boundaries—a key to improving riverscape ecology. Coupling spatially explicit riverscape models with optimization approaches will improve land protection and water management efforts, and help to resolve the land sharing vs. land sparing debate.