SFS Annual Meeting

6/04/2024  |   13:30 - 13:45   |  Salon 3/4

TOWARD MORE EXPLICIT REPRESENTATION OF HYPORHEIC HYDROLOGY IN ECOSYSTEM PROCESS MODELS Dynamic water exchange between stream channels and hyporheic zones plays a crucial role in ecosystem processing and function. Accurate representation of hyporheic hydrology can be derived from foundational mathematical relationships between the water age distribution of hyporheic discharge, the rate of hyporheic exchange, and the hyporheic storage volume. We demonstrate how hyporheic dynamics within annular flume mesocosms can be represented by such foundational relationships, with implications for modeling near-surface exchange processes in real stream systems. Our approach employs a flexible, user-defined functional form for hyporheic water age distributions, which avoids simplifying assumptions inherent to common models (e.g., well-mixed hyporheic zones, infinite maximum water residence time). Incorporating the hyporheic water age distribution perspective resulted in more accurately modeled tracer dynamics within annular flumes. Our approach is simple enough to be integrated into whole-stream ecosystem process models (e.g., respiration, uptake), which allows for explicit accounting of hydrologic versus biotic effects when exploring relationships in hyporheic zone processes.

Hayley Oakland (Primary Presenter/Author), Montana State University, hayleyoakland@montana.edu;

Samuel F. Fritz (Co-Presenter/Co-Author), Montana State University, samuel.fritz2@student.montana.edu;

Anna C. French (Co-Presenter/Co-Author), Montana State University, annafrench.mn@gmail.com;

Elizabeth Mohr (Co-Presenter/Co-Author), Montana State University, elizabethjmohr@gmail.com;

Lindsey Albertson (Co-Presenter/Co-Author), Montana State University , lindsey.albertson@montana.edu;

Geoffrey C Poole (Co-Presenter/Co-Author), Montana State University, gpoole@montana.edu;

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6/04/2024  |   13:45 - 14:00   |  Salon 3/4

Transit Time Theory approach for modeling long tailed breakthrough curves in streams with Hyporheic Zone The downstream transport of a wide range of contaminants is strongly influenced by advective and turbulent mass exchange between the stream and streambed sediments, broadly referred to as hyporheic exchange. This process can have both positive and negative impacts on the fate and transport of pollutants through catchments. For biogeochemically reactive contaminants, such as ammonium and nitrate, hyporheic exchange can provide an ecosystem service, by facilitating microbially mediated reactions such as nitrification and denitrification. For more conservative pollutants, such as sodium and chloride released from the wash-off of road deicers, hyporheic exchange can lead to legacy pollution, in which the pollutants are temporarily stored in streambed sediments and then slowly released to the stream over time—a process also responsible for the long breakthrough tails in stream pulse experiments. Regardless of the end point of interest, modeling frameworks are clearly needed that accurately capture this first-order feature of pollutant fate and transport in streams. Here we are testing a mean field framework called transit time distribution (TTD) theory with three different forms of the backward transit time distribution (bTTD). Using as a point of comparison previously published measurements of conservative and reactive breakthrough curves from stream pulse experiments, we demonstrate that TTD theory can represent long-tailed breakthrough curves with equal fidelity. Furthermore, TTD theory can be easily generalized to evaluate the influence of unsteady hyporheic exchange—a process which could be particularly important in settings where the pollutant in question is mobilized during storms, such as the deicer wash-off example presented earlier.

Ahmed Monofy (Primary Presenter/Author), Virginia Tech, monofy@vt.edu;

Stanley Grant (Co-Presenter/Co-Author), Virginia Tech, stanleyg@vt.edu;

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6/04/2024  |   14:00 - 14:15   |  Salon 3/4

SEASONAL CHANGES: AUTUMNAL NITRATE-GPP DYNAMICS IN A SPRING-FED MONTANA STREAM Diel coupling of nitrate [NO3-] uptake and gross primary production [GPP] indicates the influence of autotrophic biota on NO3- uptake; as GPP increases, so does the autotrophic assimilatory demand for NO3-. These cycles can be offset by abiotic factors such as temperature or discharge, causing models that use DO- or GPP-based estimates of NO3- to falter. To avoid this issue, we built a Bayesian timeseries model that estimates NO3- uptake independent of GPP or DO values. Using this model plus high-resolution DO and NO3- sensor data, we quantified the effects of autotrophic uptake on nitrate cycling in Beaver Creek, a spring-fed stream that flows through the alluvial Nyack River floodplain in NW Montana. To identify autumnal NO3-GPP uptake dynamics, data were collected in late August through October 2023. GPP estimates remained at or near August levels throughout the sampling period (450 umol L-1 d-1). Offset NO3-GPP coupling did occur; however, the daytime NO3- uptake (0.3-0.6 umol L-1 d-1) was far lower than the estimated autotrophic demand (~11 umol L-1 d-1, calculated using the standard estimate of autotrophic production = 0.5*GPP). Model uncertainty could explain some but not all of this difference. N limitation is also unlikely to explain the full discrepancy, as daytime NO3- levels remained above 3 umol L-1d-1 at peak uptake. A possible explanation is that, in the late summer and autumn, this standard estimate of autotrophic production does not apply; actual production may be lower. Autumnal biomass production studies would help resolve this question.

Christa L. Torrens (Primary Presenter/Author), University of Colorado at Boulder, christa.torrens@flbs.umt.edu;

Robert O. Hall (Co-Presenter/Co-Author), Flathead Lake Biological Station, University of Montana, bob.hall@flbs.umt.edu;

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6/04/2024  |   14:15 - 14:30   |  Salon 3/4

Effects of Stream Metabolism on Calcium Carbonate Deposition and Nutrient Cycling Patterns Across Neotropical Karst Streams The calcite deposition rates in karst streams are influenced by interactions between biotic factors (e.g., stream metabolism) and abiotic factors (e.g., pH). This study aims to evaluate the impact of stream metabolism on water chemistry and nutrient cycling. Our research, conducted in seven Neotropical Brazilian Savanna (Cerrado) streams, involves an assessment of the biogeochemical characteristics during the dry season. We deployed in situ data loggers in the streams for up to five days to detect diel changes in dissolved oxygen, light, and conductivity at high frequencies. Calcite deposition rates were estimated through mass transfer reactions of water calcium content and from diel calcium carbonate changes estimated from the conductivity curves. The benthic nutrient uptake rates and nutrient limitation were determined using the nutrient pulse release technique. Preliminary results from diel curves indicate strong effects of gross primary production on calcite deposition. The inverse relationship between dissolved oxygen and calcite deposition implies an indirect biological control of water chemistry, as increased GPP raises pH and mineral saturation state. Across the spatial gradient, autotrophic streams exhibited the highest rates of calcite deposition while heterotrophic streams exhibited the lowest rates of calcite deposition. Therefore, we expect that co-precipitation of phosphate during calcite deposition increases nutrient limitation in autotrophic streams compared to heterotrophic. Further analysis should focus on the role of animals in neotropical karst ecosystems. These ecosystems support a high diversity and biomass of consumers that potentially act as biogeochemical hotspots, alleviating nutrient limitations.

Kauan Fonseca (Primary Presenter/Author), State University of Rio de Janeiro , n.kauan@gmail.com;

Rogério Santos (Co-Presenter/Co-Author), Federal Univeristy of Mato Grosso Brazil , rogerclsantos@gmail.com;

Jessica Corman (Co-Presenter/Co-Author), University of Nebraska-Lincoln, jcorman3@unl.edu;

Steven Thomas (Co-Presenter/Co-Author), University of Alabama, sathomas16@ua.edu;

Timothy P. Moulton (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, moulton.timothy@gmail.com;

Vinicius Neres-Lima (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, vinicius.lima.eco@gmail.com;

Eugenia Zandona (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, eugenia.zandona@gmail.com;

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6/04/2024  |   14:30 - 14:45   |  Salon 3/4

ECOSYSTEM ENGINEERING EFFECTS ON MICROBIAL PROCESSES IN STREAMS Ecosystem engineers have profound effects on habitat and resource availability. Previous research on ecosystem engineering interactions has focused less on microbes than other types of beneficiaries, despite microbes exhibiting a similar magnitude of response as other groups. Microbes play a significant role in nutrient cycling in streams; therefore, ecosystem engineering activity that affects microbial metabolism may have an indirect yet outsized effect on whole-stream processes. Using observations from the field and laboratory, we studied how different engineering behaviors (stabilizing vs. destabilizing forces) affect microbes and nutrient cycling. We found that microbes associated with caddisfly (Hydropsychidae) structures, which stabilize sediments and slow down flows, were enriched in denitrifying genes compared to nearby rock biofilm controls, suggesting higher rates of denitrification in these habitats. Conversely, destabilizing crayfish (Pacifastacus leniusculus) burrowing activities revealed no substantiative change in nitrate concentrations in riverbank sediments or water. These experimental findings may be reflective of the methodological challenges associated with measuring complex microbial processes. We use these findings to propose a conceptual model that links microbes with ecosystem engineering: activities that stabilize the streambed (such as the creation of bio-structures by caddisflies) decrease oxygenation and result in increased denitrification rates. Alternatively, activities that destabilize the streambed (such as the creation of burrows by signal crayfish) increase oxygenation and result in reduced rates of denitrification. Simultaneous interactions between various ecosystem engineers and microbes highlight the need for future research to better understand dynamic, often counteracting, forces to evaluate the net effect of ecosystem engineering activity in rivers.

Anna C. French (Primary Presenter/Author), Montana State University, annafrench.mn@gmail.com;

Samuel F. Fritz (Co-Presenter/Co-Author), Montana State University, samuel.fritz2@student.montana.edu;

Hayley Oakland (Co-Presenter/Co-Author), Montana State University, hayleyoakland@montana.edu;

Geoffrey Poole (Co-Presenter/Co-Author), Montana State University, Montana Institute on Ecosystems, gpoole@montana.edu ;

Lindsey Albertson (Co-Presenter/Co-Author), Montana State University , lindsey.albertson@montana.edu;

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6/04/2024  |   14:45 - 15:00   |  Salon 3/4

HYPOTHESES AND CAUSALITY IN STREAM ECOSYSTEM RESEARCH: PURGING THE EDUCATED GUESS Hypotheses are central elements of scientific research, and yet their form and function remain misunderstood. For decades, scientists have inaccurately taught that a hypothesis is an ‘educated guess’. Review of materials used in secondary and higher education, as well as most research proposals submitted for funding, indicate that the error persists. Besides being inaccurate, the definition hampers the formulation of new ideas and execution of the research designed to address them. Here, I propose use of ‘proposed explanation’ as a replacement for the recalcitrant educated guess. Accordingly, hypotheses make sense only in the context of a clearly defined research question, must deal with causality, and are formed at the level of ecological theory. As such, the best hypotheses include the word ‘because’. Here, I provide a conceptual model for the form of research hypotheses, distinguish them from predictions, and emphasize the need for a revised perspective on how hypotheses are used. Drawing from over 30 years of my own research on stream biogeochemistry, I illustrate use of explanatory hypotheses as key components in the scientific method, and link the findings to contemporary ideas of extended insight. Examples include hypotheses addressing disturbance and recovery in desert streams, nutrient processing in the hyporheic zone, forest succession and retention in streams, and landscape scale assessment of mass-balance in stream reaches. Finally, I argue that explanatory hypotheses addressing how research programs are sustained as productive and creative entities may be of equal or greater value than any single hypothesis directed at ecological relationships.

H. Maurice Valett (Primary Presenter/Author), University of Montana, Division of Biological Sciences, maury.valett@umontana.edu;

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