Biological Control on Acid Generation at the Conduit-Bedrock Boundary in Submerged Caves: Quantification through Geochemical Modeling

No-mount Cave, located in Wekiwa Springs State Park in central Florida, USA, is an aphotic, submerged, freshwater cave in which large colonies of sulfur-oxidizing bacteria live in filamentous microbial mats. Upwardly discharging groundwater enters the cave from the Upper Floridan aquifer, specifically the Eocene-aged Ocala Limestone. We undertook a combined field, laboratory, and modeling study in which we sought to determine the amount of calcite dissolution attributable to the generation of protons by microbially mediated sulfide oxidation. The chemical compositions of groundwater within the limestone formation collected through a newly designed sampling device and of water in the cave conduit were used in geochemical modeling. We used the reaction-path model PHREEQCI to quantify the amount of calcite dissolution expected under various plausible scenarios for mixing of formation water with conduit water and extent of bacterial sulfide oxidation. Laboratory experiments were conducted using flow-through columns packed with crushed limestone from the study site. Replicate columns were eluted with artificial groundwater containing dissolved HS- in the absence of microbial growth. Without biologically mediated sulfide oxidation, no measurable calcite dissolution occurred in laboratory experiments and no additional amount of speleogenesis is expected as formation water mixes with conduit water in the field. In contrast, significant calcite dissolution is driven by the protons released in the biological transformation of the aqueous sulfur species. Although a range of results were calculated, a plausible amount of 158 mg Ca2+ released to conduit water per liter of groundwater crossing the formation-conduit boundary and mixing with an equal volume of conduit water was predicted. Our modeling results indicate that significant cave development can be driven by microbially mediated sulfide oxidation under these hydrogeochemical conditions.žKeywords: calcite dissolution, microbial sulfide oxidation, geochemical model.DOI: 10.3986/ac.v42i2-3.66

Preparing aquatic research for an extreme future: call for improved definitions and responsive, multidisciplinary approaches

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Aoki, L. R., Brisbin, M. M., Hounshell, A. G., Kincaid, D. W., Larson, E., Sansom, B. J., Shogren, A. J., Smith, R. S., & Sullivan-Stack, J. Preparing aquatic research for an extreme future: call for improved definitions and responsive, multidisciplinary approaches. Bioscience, 72(6), (2022): 508-520, https://doi.org/10.1093/biosci/biac020.Extreme events have increased in frequency globally, with a simultaneous surge in scientific interest about their ecological responses, particularly in sensitive freshwater, coastal, and marine ecosystems. We synthesized observational studies of extreme events in these aquatic ecosystems, finding that many studies do not use consistent definitions of extreme events. Furthermore, many studies do not capture ecological responses across the full spatial scale of the events. In contrast, sampling often extends across longer temporal scales than the event itself, highlighting the usefulness of long-term monitoring. Many ecological studies of extreme events measure biological responses but exclude chemical and physical responses, underscoring the need for integrative and multidisciplinary approaches. To advance extreme event research, we suggest prioritizing pre- and postevent data collection, including leveraging long-term monitoring; making intersite and cross-scale comparisons; adopting novel empirical and statistical approaches; and developing funding streams to support flexible and responsive data collection

Biological Control on Acid Generation at the Conduit-Bedrock Boundary in Submerged Caves: Quantification through Geochemical Modeling

No-mount Cave, located in Wekiwa Springs State Park in central Florida, USA, is an aphotic, submerged, freshwater cave in which large colonies of sulfur-oxidizing bacteria live in filamentous microbial mats. Upwardly discharging groundwater enters the cave from the Upper Floridan aquifer, specifically the Eocene-aged Ocala Limestone. We undertook a combined field, laboratory, and modeling study in which we sought to determine the amount of calcite dissolution attributable to the generation of protons by microbially mediated sulfide oxidation. The chemical compositions of groundwater within the limestone formation collected through a newly designed sampling device and of water in the cave conduit were used in geochemical modeling. We used the reaction-path model PHREEQCI to quantify the amount of calcite dissolution expected under various plausible scenarios for mixing of formation water with conduit water and extent of bacterial sulfide oxidation. Laboratory experiments were conducted using flow-through columns packed with crushed limestone from the study site. Replicate columns were eluted with artificial groundwater containing dissolved HS- in the absence of microbial growth. Without biologically mediated sulfide oxidation, no measurable calcite dissolution occurred in laboratory experiments and no additional amount of speleogenesis is expected as formation water mixes with conduit water in the field. In contrast, significant calcite dissolution is driven by the protons released in the biological transformation of the aqueous sulfur species. Although a range of results were calculated, a plausible amount of 158 mg Ca2+ released to conduit water per liter of groundwater crossing the formation-conduit boundary and mixing with an equal volume of conduit water was predicted. Our modeling results indicate that significant cave development can be driven by microbially mediated sulfide oxidation under these hydrogeochemical conditions

Stimulation of Phytoplankton Production by Anthropogenic Dissolved Organic Nitrogen in a Coastal Plain Estuary

There is increased focus on nitrogen (N)-containing dissolved organic matter (DOM) as a nutrient source supporting eutrophication in N-sensitive estuarine ecosystems. This is particularly relevant in watersheds undergoing urban and agricultural development, leading to increased dissolved organic N (DON) loading. To understand how this shift in N-loading influences estuarine phytoplankton production, nutrient addition bioassays were conducted in the N-limited Neuse River Estuary, North Carolina from 2014 to 2015. Additions included N-rich DOM sources characteristic of urban and agricultural development, including chicken and turkey litter leachate, wastewater treatment facility effluent, and concentrated river DOM (used as a reference). Each DOM addition was coupled with an inorganic nutrient treatment to account for inorganic nutrient concentrations (NO_2/3, NH₄, PO₄) in each respective DOM addition. Repeated measures analysis of variance (RM-ANOVA) showed that chicken litter leachate stimulated phytoplankton growth greater than its coupled inorganic nutrient treatment. Wastewater treatment facility effluent, turkey litter leachate, and concentrated river DOM did not stimulate phytoplankton growth greater than their respective inorganic nutrient controls. DOM fluorescence (EEM-PARAFAC) indicated the chicken litter contained a biologically reactive fluorescent DOM component, identified as the nonhumic, biologically labile, “N-peak”, which may be responsible for stimulating the observed phytoplankton growth in the chicken litter leachate treatments