Molly S. Costanza-Robinson

All organisms contain carbon, nitrogen, and phosphorus in widely ranging amounts and proportions. Integrating existing datasets enables quantification of this variation at global scales. Such efforts could leverage ecological stoichiometry theory, the study of elemental supply and imbalances in ecological interactions, to connect ecological drivers and taxonomic constraints to ecosystem structure and function. Towards this goal, we developed the Limnology Stoichiometric Traits of Organisms In their Chemical Habitats (Limno‐STOICH) database. The Limno‐STOICH database includes 51,576 observations of organismal elemental stoichiometry from >3100 rivers, lakes, wetlands, and other aquatic ecosystem sites on seven continents, derived from 190+ sources. It also includes extensive spatial and temporal metadata to link elemental stoichiometry with ecosystem type, trophic status, etc., and information on organismal data (body size, taxonomic classifications, stable isotope composition) and water physicochemical parameters. The Limno‐STOICH database sets the stage for significant applications across food web ecology, evolutionary ecology, biogeochemistry, and other disciplines.

Molybdenum blue colorimetry (MBC) is the dominant, well-established method used for determining total P in environmental media, including in organismal tissues. However, other elemental methods for P determination are available, including inductively coupled plasma mass spectrometry (ICP-MS). Given the extensive literature using MBC to determine P in organismal samples, it is important to assess P analyses by ICP-MS and MBC to ensure that the two methods produce comparable data. In this work, we compared ICP-MS and MBC for total P determination in freshwater invertebrates, including the potential for analytical interferences, by applying both methods to three standard reference materials (SRMs) and 106 freshwater invertebrate samples. Average total P recoveries for SRMs were slightly higher for ICP-MS (99.8 ± 5.2%) than MBC (96.5 ± 5.4%), but both methods indicated good accuracy. Total P in invertebrates determined using the two methods was strongly linearly correlated (r = 0.96) with a slope of 1.01. On the whole, total P measured using ICP-MS exceeded that measured by MBC, but average pair-wise differences in %P were biologically negligible (0.044 ± 0.054). %P for SRMs and invertebrate samples run on ICP-MS in kinetic energy discrimination and standard modes compared favorably (e.g., SRM P recovery of 102% by both methods), indicating negligible influence of polyatomic ions on ICP-MS analysis. Similarly, analysis of P spike recoveries by ICP-MS (100.2 ± 3.4%) and MBC (107.0 ± 2.8%) were both considered acceptable. We conclude that ICP-MS represents a reliable and comparable alternative to MBC for determining total P in freshwater invertebrates while also offering the opportunity to measure additional biologically relevant elements in a single analysis.

Surfactant miscible-displacement experiments represent a conventional means of estimating air–water interfacial area (A_I) in unsaturated porous media. However, changes in surface tension during the experiment can potentially induce unsaturated flow, thereby altering interfacial areas and violating several fundamental method assumptions, including that of steady-state flow. In this work, the magnitude of surfactant-induced flow was quantified by monitoring moisture content and perturbations to effluent flow rate during miscible-displacement experiments conducted using a range of surfactant concentrations. For systems initially at 83% moisture saturation (S_W), decreases of 18–43% S_W occurred following surfactant introduction, with the magnitude and rate of drainage inversely related to the surface tension of the surfactant solution. Drainage induced by 0.1 mM sodium dodecyl benzene sulfonate, commonly used for A_I estimation, resulted in effluent flow rate increases of up to 27% above steady-state conditions and is estimated to more than double the interfacial area over the course of the experiment. Depending on the surfactant concentration and the moisture content used to describe the system, A_I estimates varied more than 3-fold. The magnitude of surfactant-induced flow is considerably larger than previously recognized and casts doubt on the reliability of A_I estimation by surfactant miscible-displacement.

Achieving a representative elementary volume (REV) has become a de facto criterion for demonstrating the quality of CT measurements in porous media systems. However, the data quality implications of an REV requirement have not been previously examined. In this work, deterministic REVs for porosity, moisture saturation (S_W), and air‐water interfacial area (A_I) were estimated using a set of 49 CT images of eight unsaturated homogeneous porous media with heterogeneity in moisture distributions present in varying degrees. Estimated porosity REVs were 3 for all cases, smaller than typical CT image sizes (∼100 mm³). Estimated S_W and A_I REVs were 3 for cases with homogeneous moisture distributions but could not be estimated for cases with heterogeneous moisture distributions, due to the absence of a distinct “REV plateau” within the maximum imaged volume. Conventionally, S_W and A_I data from such non‐REV cases would be excluded. The implications of excluding data on the basis of REV were examined by comparing A_I‐S_W data measured on image windows of increasing size against the expected linear A_I‐S_W relationship. At measurement scales exceeding porosity REV, random fluctuations in A_I‐S_W data were excluded, even for cases containing heterogeneous moisture distributions. In contrast, requiring measurement scales to exceed S_W and A_I REV appeared overly restrictive and resulted in visible loss of reliable A_I‐S_W data. We attribute these findings to overestimation of REVs due to inherently problematic estimation of deterministic REVs in real systems. Implications of these findings for ensuring CT data quality and the efficient use of CT data are discussed.

In this work, total smooth air–water interfacial areas were measured for a series of nine natural and model sandy porous media as a function of water saturation using synchrotron X-ray microtomography. Interfacial areas decreased linearly with water saturation, while the estimated maximum interfacial area compared favorably to the media geometric surface areas. Importantly, relative interfacial area (i.e., normalized by geometric surface area) versus water saturation plots for all media collapsed into a single linear cluster (r² = 0.93), suggesting that geometric surface area is an important, and perhaps sufficient, descriptor of sandy media that governs total smooth interfacial area−water saturation relationships. Measured relationships were used to develop an empirical model for estimating interfacial area−water saturation relationships for sandy porous media. Model-based interfacial area estimates for independent media were generally slightly higher than interfacial areas measured using aqueous-phase interfacial tracer methods, which may indicate that microtomography captures regions of the air–water interface that are not accessible to aqueous-phase interfacial tracers. The empirical model presented here requires only average particle diameter and porosity as input parameters and can be used to readily estimate air–water interfacial area−water saturation relationships for sandy porous media.