Smithsonian Environmental Research Center

Beneath the Earth's surface lies a network of interconnected caves, voids, and systems of fissures forming in rocks of sedimentary, igneous, or metamorphic origin. Although largely inaccessible to humans, this hidden realm supports and regulates services critical to ecological health and human well-being. Subterranean ecosystems are integral to major biogeochemical cycles, sustain diverse surface habitats, and serve as the primary source of irrigation and drinking water. They also offer non-material benefits, including scientific discovery, education, and cultural practices. Yet, these contributions often go unrecognised, partly due to the lack of a unified synthesis of ecosystem services across terrestrial, freshwater, and marine subterranean compartments. This gap limits effective communication of their value to scientists, practitioners, and the public. Through a systematic expert-based review, we show that subterranean ecosystems contribute to up to 75% of classified ecosystem services. Notably, many of these contributions are described only qualitatively, lacking numerical or economic quantification. Next, we list examples of the main ecosystem services provided by subterranean systems to offer a global overview of their multifaceted value and vulnerability to environmental change. We believe this synthesis provides researchers and practitioners with concrete examples to communicate more effectively the importance of subterranean ecosystems to diverse audiences.

Accurate quantification of sediment porewater methylmercury (MeHg) profiles is critical for understanding Hg fate, transport, and informing remediation design, yet direct porewater profiling remains challenging. Equilibrium-based passive samplers (EqPS) offer an alternative, providing fine-scale, time-integrated measurements and convenience. In this study, we deployed an EqPS consisting of activated carbon suspended in agarose gel (ag+AC) in estuarine and freshwater sediment microcosms to obtain porewater MeHg profiles at 0.5 cm vertical resolution. Our objectives were to evaluate whether ag+AC samplers could reproduce porewater MeHg profile shapes and concentrations across redox gradients and contrasting geochemical conditions. Following 4 week deployments across the sediment-water interface ag+AC samplers produced distinct depth profiles that corresponded closely with directly measured porewater profiles and exhibited maxima at redox transition zones where net methylation is most likely. Using a previously derived equilibrium partition coefficient (102·96 L kg-1) ag+AC samplers predicted, on average, directly measured porewater MeHg concentrations to within a factor of about two. Across both sediment types, porewater MeHg alone explained ∼65% of the variability in passive sampler MeHg despite wide ranges in DOC, sulfide, iron, and salinity. These results demonstrate ag+AC EqPS can provide fine-scale porewater MeHg profiles and accurate concentration estimates across diverse sedimentary environments.

Coastal ecosystems rely on microbially mediated biogeochemical processes for ecosystem functioning, particularly carbon (C) and nitrogen (N) cycling across the water-sediment interface, supported by bivalves through bioturbation activity. Here, we conducted an 8-week coastal microcosm experiment using Macoma balthica sampled from the Rhode River (Chesapeake Bay) to assess how bivalve presence and individual traits (burial position and shell length) influence the N cycle (NH3 and NOX) and C cycle (CDOM, measured as aCDOM 250m-1) in the water column and interstitial water. Our results show that bivalve presence significantly affected N cycling: NH3 initially rose in the water column before declining, followed by exponential increases in NOx in both water and interstitial water. Burial position influenced NH3 and NOx concentrations throughout the experiment, and larger individuals were associated with higher NOx levels. Additionally, aCDOM 250m-1 increased over time regardless of bivalve presence, likely due to daily feeding with artificial algae, suggesting that future studies should include additional C parameters such as dissolved and particulate forms of organic and inorganic C. Overall, our findings show that M. balthica's influence on N cycling is shaped by individual traits and temporal dynamics, creating functional heterogeneity with implications for Rhode River nutrient fluxes and ecosystem functioning. While microbiomes might contribute to the nitrification processes observed, future research should clarify the pathways of bivalve-microbiome interactions in M. balthica.

The blue swimming crab ( Portunus pelagicus ) fishery is an essential fishery in the Indo-Pacific region, including Indonesia, but P. pelagicus populations face over-exploitation. This study investigates the sex ratio and reproductive dynamics of P. pelagicus harvested in Pare Pare Bay, South Sulawesi, Indonesia. From July 2023 to June 2024, we conducted monthly surveys at a crab landing site for a local, small-scale fishery. A total of 5777 P. pelagicus were collected and measured to assess sex ratio, spawning season, fecundity, and egg diameter. Environmental data (salinity, water temperature, dissolved oxygen, and pH) were collected monthly at four stations in the Bay. They were compared using correlation analyses to the sex ratio and proportion of ovigerous females. Our results indicate that male crabs were consistently more abundant than females at a ratio of 1.4:1, and month-to-month changes in the sex ratio were positively correlated with warmer water temperatures. For female crabs captured in the Bay, 20 % were ovigerous, peaking at 31 % in March. Larger crabs had significantly higher fecundity and egg diameter that varied from 280 to 328 µm. This study provides baseline information on seasonal variation in sex ratio and reproduction to inform the development of aquaculture and conservation efforts.

<strong class="journal-contentHeaderColor">Abstract.</strong> Here we present a comprehensive dataset of hydrologic, biogeochemical, microbial, and macroinvertebrate community measurements from a set of multi-year, co-occurring, watershed studies in non-perennial stream networks that dynamically expand and contract over space and time. The data were collected over the 2022&ndash;2024 water years across three stream networks draining watersheds with a similar humid, subtropical climate but distinct physiographies (i.e., Piedmont, Appalachian Plateau, Coastal Plain) in Alabama, USA. Our goal was to characterize the spatiotemporal patterns and drivers of how non-perennial stream networks expand and contract, as well as the biogeochemical, microbial, and macroinvertebrate dynamics associated with changes in network connectivity and water availability. We used a combination of spatial, temporal, and spatiotemporal sampling and sensor-based monitoring approaches to capture hydrologic, biogeochemical, and ecological responses to network expansion and contraction in each watershed. This manuscript describes the overall study design, monitoring network and sampling approaches, data and sample collection and analysis, and specific datasets generated. All data products are publicly available through the Hydroshare data repository for hydrologic, biogeochemical, and macroinvertebrate data (<a href="https://www.hydroshare.org/group/247" target="_blank" rel="noopener">https://www.hydroshare.org/group/247</a>) and through the NCBI data repository for microbial data. All data product-specific DOIs and repository-specific unique IDs are cited in Appendix A (Table A1, Table A3).

The lack of practical knowledge and effective engagement with gender in blue carbon initiatives poses a threat to their success and sustainability and risks exacerbating existing gender inequalities. Developed by an interdisciplinary team of experts, this paper proposes a research agenda, comprised of seven research areas as well as priority actions, designed to catalyze a shift towards more just, equitable and effective blue carbon initiatives. This paper is a call-to-action to increase commitment, funding, and collaborative, interdisciplinary science to close the gender gap in the conservation and restoration of blue carbon ecosystems and enhance their social-enovironmental impacts.

Plants often form partnerships with symbiotic microbes that improve plant performance. Increasing environmental stress is altering the dynamics of interactions between plants and their below-ground symbiotic partners, which can have cascading effects on above-ground trophic interactions. However, little work has examined how microbes protect plants against multiple stressors acting in concert, or the extent to which microbial genetic variation shapes context-dependent outcomes of tripartite interactions between microbes, plants, and herbivores. We experimentally manipulated the genetics of symbiotic microbial partners by individually inoculating a single legume species (Glycine max) with twenty-four genetically distinct strains of rhizobial bacteria. We quantified insect herbivore feeding and growth rates, as well as plant ecophysiological and performance traits under different combinations of drought and herbivore exposure. Rhizobial strains differentially influenced the effect of drought on legume ecophysiological traits. Certain strains increased photosynthesis and enhanced photoprotection under drought. Moreover, legumes under drought and herbivore exposure also exhibited increased photoprotection. Some strains were observed to limit herbivore growth rates under drought more effectively than others. A multivariate trait analysis revealed that strains differentially influenced legume trait syndromes and plasticity in response to stress in certain environments. Rhizobial strain variation was a key driver of tripartite interactions through changes in plant trait expression and plasticity under simultaneous abiotic and biotic stress, which had cascading effects on insect herbivore growth. A community approach incorporating multiple beneficial microbial strains may be effective for managing legume-rhizobia symbioses and introducing functional trait diversity and resistance traits into ecosystems.

ABSTRACTThe Janzen‐Connell Hypothesis posits that plant species diversity is maintained by a reduction in seedling survival near living conspecific trees relative to heterospecifics–known as negative conspecific density dependence (CDD). CDD facilitates coexistence if stronger than heterospecific density dependence (HDD). However, whether and how long CDD persists after trees die is unknown. In a three‐year study across three forests, we monitored seedling survival near living and dead trees, both conspecific and heterospecific, across a seven‐year chrono‐sequence since tree death. CDD persisted for at least 5 years after tree death (‘legacy CDD’), and most species showed stronger CDD relative to HDD through time. We used our empirical findings to parametrize a theoretical community dynamics model. Our model suggests that both stabilising niche differences and fitness differences persist after tree death. While legacy CDD can facilitate coexistence, fitness differences often overwhelmed niche differences, making competitive exclusion the most likely outcome.

While studies have demonstrated that higher tree species richness can increase forest productivity, the relationships between tree species richness, tree growth and herbivore damage remain insufficiently explored. Here we investigate these linkages using data from 8,790 trees across 80 species in 9 biodiversity experiments, spanning temperate and subtropical biomes. Despite considerable geographic variation, we reveal an overall positive relationship between tree species richness and insect herbivory, as well as between tree growth and herbivory, at individual, species and community levels. The tree growth-herbivory relationship is further influenced by leaf functional traits. In particular, we show that tree species with a higher carbon to nitrogen ratio and, to a lesser extent, tougher leaves, experienced higher herbivory when their growth rate increased. The associations between tree growth and herbivory are further modulated by climatic and soil variation among the sites. Our study highlights the role of functional traits in shaping the relationship between tree growth and herbivory, supporting the resource availability and plant vigour hypotheses.