River systems link terrestrial and marine landscapes through a range of interconnected and complex ecological, hydrologic, and biogeochemical processes along the land-ocean continuum. Rivers are susceptible to perturbations in both terrestrial and aquatic systems, and understanding these changes provides an integrated understanding of impacts of human disturbance on systems. This session aims to explore how rivers are responding to anthropogenically-driven perturbations over time, ideally over decadal timespans. We welcome presentations encompassing a wide variety of river types spanning river order, biome type, seasonality, and ecological condition, as well as exploration of the mechanisms driving such changes. Contributions may include observational and experimental data, as well as empirical and modeling analyses, but should focus on understanding long-term change in river ecosystems. For example, presenters may wish to demonstrate how discharge, nutrients, light availability, or species composition is changing over decadal time spans, as well as potentially explore the mechanisms driving such shifts. We hope to bring together individuals interested in better understanding how and why river systems are changing over time with respect to variety of eco-hydro-biogeochemical processes to improve our ability to forecast and manage future change.
Lead Organizer: Joanna Carey, Babson College (jcarey@babson.edu)
Co-organizers:
Kathi Jo Jankowski, USGS (kjankowski@usgs.gov)
Presentations
04:00 PM
Decadal Shifts in High Latitude River Silicon Stoichiometry across Continents (8095)
Primary Presenter: Joanna Carey, Babson College (jcarey@babson.edu)
High latitude systems are warming at a faster rate that the global average, altering rates of silicon (Si) mobilization from terrestrial to aquatic systems. Aquatic dissolved Si availability and its ratio with other nutrients exert strong control on seasonal algae blooms in freshwater and downstream marine receiving waters, especially in high latitude systems where diatoms are often the dominant autotroph. Here we present an examination of long-term (~20 years) changes of daily river dissolved Si concentrations, loads, and nutrient ratios in >60 rivers spanning high latitude systems in North America, Europe, Asia, and Antarctica. We examined changes in Si on seasonal and annual scales, and with respect to dissolved inorganic nitrogen and phosphorus availability. We find a predominant trend of increasing dissolved Si concentrations, and declining N and P concentrations, ultimately leading to increased loads of Si relative to N and P. We also find that concentrations are changing to a much larger degree than discharge over time. Our analysis demonstrates that trends in high latitude riverine Si varied through space (i.e. across continents) and time (i.e. seasonally) and differed from trends in other nutrients, which has implications for seasonal diatom blooms in high latitude fresh and marine waters.
04:15 PM
Shifts in the seasonality of riverine Si concentrations are driven by changing climate and hydrology (8298)
Primary Presenter: Keira Johnson, Oregon State University (johnkeir@oregonstate.edu)
The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termed DSi regime, mediates the timing of DSi delivery to downstream receiving waters and thus governs river biogeochemical function and the condition of aquatic communities. Previous work identified five distinct regimes identified across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple regimes over time. Several drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, yet large scale spatiotemporal controls on DSi regimes remain unclear. Here, we evaluate the role of several environmental variables on the behavior of average and annual DSi regimes in over 200 rivers across North America. Our models were better able to predict annual DSi regime behavior (correctly classifying > 80% of regimes), compared to average regime behavior (correctly classifying > 55% of regimes). Climate, primary productivity, temperature, evapotranspiration, and discharge were important predictors of regime behavior. This presentation will focus on our investigation of shifting regime behavior over 18 years of record at 159 sites with long-term records. Many sites exhibited shifting regime membership over time, especially sites exibiting spring trough regimes. Together, these results indicate that DSi behavior is more strongly controlled by climate, ecosystem productivity, and hydrology than static watershed characteristics, and demonstrate that fluctuations in climate and hydrology may alter the temporal availability of DSi to rivers.
04:30 PM
Pacific climate modes and landform interact to control multi-decadal patterns of river nutrient concentration and transport in Washington state (8429)
Primary Presenter: Elizabeth Elmstrom, University of Washington (elmstrom@uw.edu)
Understanding the role of climate in driving river nitrogen (N) and phosphorus (P) dynamics is critical to evaluate long-term changes in aquatic ecosystem function from rivers to the coast. Climate variation, however, operates across multiple time scales, including seasonal, multi-annual, and decadal oscillations, which can obscure our understanding of long-term change. Further complicating these different modes of variability is the interaction between climate and heterogenous landscapes, which can generate varied ecosystem responses and make the sensitivity of individual rivers to climate difficult to predict. Here, we apply multivariate time series analyses to assess the role of climate in driving the long-term state (30+ years) of river inorganic N and P concentrations and yields across 15 rivers that drain the Puget Sound Basin, a spatially complex region located in western Washington State. Our findings reveal that multi-decadal changes in river N and P concentrations and yields are strongly influenced by shifts in North Pacific climate modes, particularly by the Pacific Decadal Oscillation (PDO). Moreover, we found that the response of N and P concentrations and yields to PDO shifts varied among rivers, and that a substantial proportion of this variation was explained by a simple topographic feature, mean watershed slope. These results demonstrate the strong influence of climate oscillations on long-term river nutrient patterns, while also highlighting the distinct role of watershed topography in modulating the sensitivity of river N and P dynamics to climatic change.
04:45 PM
Factors driving changes in phosphorus flux since 2011 in 24 U.S. Great Lakes tributaries (7993)
Primary Presenter: Dustin Kincaid, U.S. Geological Survey (dustinkincaid@usgs.gov)
Extensive efforts have been made to reduce phosphorus inputs to the Great Lakes, yet tracking progress remains a challenge. Water quality trend analysis with Weighted Regressions on Time, Discharge, and Season (WRTDS) can remove the influence of discharge variability on concentration and flux estimates through flow-normalization. The results provide a view of underlying trends that can be used to infer watershed management effectiveness. WRTDS was used to evaluate the cause of changes in flow-normalized phosphorus fluxes in 24 U.S. Great Lakes tributaries from 2011-2020. In particular, we evaluated phosphorus concentration-discharge relationships, seasonal patterns and changes in total and dissolved phosphorus flux, and changes in point-source phosphorus inputs. Flow-normalized phosphorus flux decreased in all watersheds with low phosphorus yield (< 50 kg km-2 yr-1) and increasing mean annual discharge over the study period, suggesting the development of supply limitation. Seasonal fluxes of both total and dissolved phosphorus were generally highest in spring, but this pattern was more pronounced for total phosphorus. In all but two tributaries, percent changes in total phosphorus were more negative than percent changes in dissolved phosphorus, indicating potential increases in phosphorus bioavailability. These results may help guide ongoing phosphorus reduction efforts in Great Lakes tributaries.
05:00 PM
TEMPORAL VARIATION IN PH, ALKALINITY, AND TOTAL INORGANIC CARBON IN WESTERN LAKE SUPERIOR TRIBUTARIES (7720)
Primary Presenter: Elizabeth Minor, Large Lakes Observatory (eminor@d.umn.edu)
Inorganic carbon in aquatic systems can be a source or a sink of atmospheric carbon dioxide, affect the pH, and shift the speciation of other chemical species (such as metals). It is also the carbon source for aquatic organic matter production via photosynthesis and influences the quality of life for heterotrophic organisms. However, temporal variation in inorganic carbon cycling in freshwater has not yet been well constrained. This study focuses on sites in three tributaries of different sizes in the Lake Superior watershed to investigate how key inorganic carbon system variables (pH, total alkalinity (TA), and total inorganic carbon (TIC)) are affected by seasonality and to compare this to inter-annual variability. For one tributary site, variations across storm events and a diurnal cycle are also studied. Across seasons, pH, TIC, and TA showed minima in spring and were highest in summer. During the storm events, changes in pH and TA were up to ~60% and ~55% respectively, of the seasonal variation in the tributary. Specific trends during storms appeared related to event duration, total precipitation, and prior precipitation amounts. Over a diurnal cycle measured near the summer solstice, pH increased to a midday maximum and then decreased; TIC and TA increased throughout the day, likely due to the increased influence of groundwater as the stream recovered from a rain event that occurred 4 days prior. Comparing pH data across all four temporal scales, interannual variability is highest, followed by seasonal variability, which is ~40% of interannual variability.
05:15 PM
HARMFUL ALGAL BLOOM HISTORIES: DATA TO SUPPORT TREND ANALYSES IN THE ILLINOIS RIVER BASIN AND BEYOND (7793)
Primary Presenter: Jennifer Murphy, US Geological Survey (jmurphy@usgs.gov)
Harmful algal blooms (HABs) are a growing concern in freshwaters as apparent increases in these events impinge on water availability for humans, animals, ecosystems, and local economies. Efforts have been made to track the occurrence of these events over time in lake and marine environments, but fewer efforts have focused on rivers. Recent high-profile river blooms have led to the question: Are HABs becoming more common in rivers? Algal taxa, cell counts, biovolume, pigment measurements, visual observations, and toxin concentrations acquired through field sampling, in situ florescence sensors, and remote sensing via satellites could be used to answer this question. In the Illinois River Basin, we compiled a database of multisource discrete chlorophyll and algal toxin data from multiple agencies across Illinois, HAB reports from field staff and the public, estimates of chlorophyll and trophic status from remotely sensed spectra, and high frequency pigment measurements. These datasets have various sampling densities, frequencies, and temporal periods and a variety of statistical techniques are used to characterize changes over time. For this presentation, our workflow and “lessons learned” for the acquisition, compilation, and harmonization of this HABs database are summarized. Additionally, we will discuss various methods to integrate these datasets and their trends to better understand how HAB occurrence has varied over time.
SS30A - Exploring Signals and Mechanisms of Long-Term Change In River Dynamics
Description
Time: 4:00 PM
Date: 6/6/2024
Room: Hall of Ideas G