High Daily and Year-Round Variability in Denitrification and Nitrogen Fixation in a Northern Temperate River

Rates of nitrogen (N) cycling processes like denitrification and dinitrogen (N2) fixation, which together are the primary contributors to N2 flux rates from surface waters, may change at different time scales from seasons to weeks to days. Yet, we know little about the magnitude, mechanisms or drivers of these temporal changes, especially at shorter daily and weekly timescales. Quantifying variation in rates and drivers across temporal scales is essential to understand how nutrient cycling processes operate in aquatic ecosystems and predict how they may respond to shifting seasonal dynamics caused by global change (i.e., earlier snowmelt and extreme weather events). This study quantified denitrification and N2 fixation rates seasonally and daily in a northern temperate river, and explored how environmental conditions such as discharge, light, and N and phosphorus (P) concentrations were related to that variation at different time scales. We measured denitrification and N2 fixation rates on biweekly and daily intervals at a single 20-m long sampling reach in the Pilgrim River in Michigan\u27s Upper Peninsula from May 2017 through May 2019. We found high rates of daily change (difference in rate from one day to the next) for both processes in all seasons (maximum daily change 5,690 μg N/m2/h for denitrification and 38 μg N/m2/h for N2 fixation). No detectable differences in rates among seasons were detected using Multiple Response Permutation Procedure (MRPP). Day-to-day variation did not change before and after elevated discharge events, including a 1,000-year flood that occurred in June 2018. Partial least squares (PLS) regression identified total dissolved N, dissolved organic N, and ammonium as important predictors of denitrification and N2 fixation, but explained only 15–28% of the variation in all measured rates. The unexpectedly high daily variation and lack of seasonal difference in rates found in this study demonstrate the need to use caution when studying these processes and/or extrapolating rates across time scales, as discrete and infrequent measurements may be misleading

Continuing Analysis of Phytoplankton Nutrient Limitation in Farmington Bay and the Great Salt Lake

Farmington Bay is a nutrient-enriched, highly eutrophic embayment of the Great Salt Lake. The highly variable salinity of the bay influences what species of plankton can survive there. Previous analyses suggested that cyanobacteria (blue-green algae) may not be able to survive or fix atmospheric nitrogen at high salinities, thus maintaining the lake in a nitrogen-limited state. To determine the interacting influence of nutrients and salinity on the growth and nitrogen fixation of plankton we performed a 28-day bioassay with water from Farmington and Gilbert Bays in October 2004. We tested the response of the plankton to additions of nitrogen (N) and phosphorus (P) at salinities of 3%, 5%, 70/0, 9% or 11 %. Algal inocula from a variety of salinities were added to provide colonists for the cultures

Analysis of Phytoplankton Nutrient Limitation in Farmington Bay and the Great Salt Lake

The Great Salt Lake is bordered to the south and east by a growing metropolitan area that contributes high nutrients to Farmington Bay. This large bay is eutrophic, and there is concern that continued increases in effluents from the Salt Lake City area could extend to impact the much larger, and currently less productive, Gilbert Bay. This study focused on determining how nutrient supplies might limit, and therefore control, algal populations in Farmington Bay and Gilbert Bay at different salinities. We tested both short and long-term responses of algal growth using laboratory nutrient addition bioassays in the summer and fall of 2003. Because some phytoplankton can alleviate nitrogen deficiency by fixing atmospheric nitrogen, we also determined how nutrients and salinity influenced nitrogen fixation

Hydrogen Sulfide in Farmington Bay and the Great Salt Lake: A Potential Odor-Causing Agent

Odors from Farmington Bay and/or the Great Salt Lake frequently impact residents of Salt Lake and Davis counties, but the agent causing the problem and the origin of the odor is uncertain. Hydrogen sulfide (H2S) gas is produced in the deeper layers of water in Farmington Bay and Gilbert Bay in the Great Salt Lake, but these deeper waters are generally part of high salinity deep-brine layers that are resistant to wind mixing. Hydrogen sulfide has a rotten-egg odor and is a likely component contributing to the lake stink. The goals of this study were to determine (1) whether wind driven mixing events drove mixing of the deep-brine layers in Farmington and Gilbert Bays, (2) determine the amount of hydrogen sulfide present in each of these bays, and (3) determine the potential of each area of the lake to release hydrogen sulfide from those deep-brine layers and cause odor events. We found that in Farmington Bay, which was \u3c1.5 m (5 ft) deep during the study, the deep-brine layer was entrained (mixed) four times in three months during high-wind events. By contrast, in Gilbert Bay, which has a deeper water column and more stable deep-brine layer, the brine layer was never completely entrained. However, the top 0.9 m (3 ft) of the deep brine layer eroded between July and October. Hydrogen sulfide concentrations in the surface mixed layers of both bays were insignificant on each sampling date, but concentrations in the deep-brine layers were significant. In Farmington Bay H2S concentrations reached 8 mg/L in the deep-brine layer. In Gilbert Bay H2S concentrations in the deep-brine layer ranged from 11 mg/L in late July to 4 mg/L in November 2003. The higher concentrations in Gilbert Bay are likely due to decreased mixing and therefore increased time intervals of hydrogen sulfide accumulation in Gilbert Bay. Both bays may release H2S into the airshed, and thus contribute to odor problems. Large releases of H2S into the water columns could result in rapid deoxygenation and toxicity to aquatic organisms. Detailed whole water column monitoring of oxygen, salinity, and H2S concentrations in both bays should be undertaken to assess these potential threats. It will necessary to do these studies at a variety of lake levels in order to fully understand the driving mechanisms creating H2S and allowing it to be released from the lake

Ecological Analysis of Nutrient, Plankton and Benthic Communities in Farmington Bay and the Great Salt Lake, Utah (2004)

In Fall 2004, the Aquatic Ecology Practicum class at Utah State University finished a third year of research on limnological and ecological characteristics of Farmington Bay and Gilbert Bays of the Great Salt Lake. Our previous research has produced interesting findings in Farmington Bay, including hypereutrophy (Marcarelli et a!. 2001), high phosphorus loading into the Bay, overnight water column anoxia linked to high winds (Wurtsbaugh et a!. 2002), potential predator control of brine shrimp, and high levels of hydrogen sulfide in the sediment and deep brine layer (Marcarelli et a!. 2003). These class findings have lead to increased interest in Farmington and Gilbert Bays. Because of the breadth of research now occurring in Farmington Bay, the topics studied by the students this fall encompassed a wider range of research than ever before. The reports ranged from an expanded analysis of nutrients entering Great Salt Lake, including external loading and biological nitrogen fixation, benthic ecology of Gilbert Bay including analyses of stromatolites and brine shrimp cysts in sediments, and more focused experiments on brine shrimp survival and predation by corixids in Farmington Bay. Key findings of the students are identified below

Nutrient limitation of algae and macrophytes in streams: Integrating laboratory bioassays, field experiments, and field data

Successful eutrophication control strategies need to address the limiting nutrient. We conducted a battery of laboratory and in situ nutrient-limitation tests with waters collected from 9 streams in an agricultural region of the upper Snake River basin, Idaho, USA. Laboratory tests used the green alga Raphidocelis subcapitata, the macrophyte Lemna minor (duckweed) with native epiphytes, and in situ nutrient-limitation tests of periphyton were conducted with nutrient-diffusing substrates (NDS). In the duckweed/epiphyte test, P saturation occurred when concentrations reached about 100 μg/L. Chlorophyll a in epiphytic periphyton was stimulated at low P additions and by about 100 μg/L P, epiphytic periphyton chlorophyll a appeared to be P saturated. Both duckweed and epiphyte response patterns with total N were weaker but suggested a growth stimulation threshold for duckweed when total N concentrations exceeded about 300 μg/L and approached saturation at the highest N concentration tested, 1300 μg/L. Nutrient uptake by epiphytes and macrophytes removed up to 70 and 90% of the N and P, respectively. The green algae and the NDS nutrient-limitation test results were mostly congruent; N and P co-limitation was the most frequent result for both test series. Across all tests, when N:P molar ratios \u3e30 (mass ratios \u3e14), algae or macrophyte growth was P limited; N limitation was observed at N:P molar ratios up to 23 (mass ratios up to 10). A comparison of ambient periphyton chlorophyll a concentrations with chlorophyll a accrued on control artificial substrates in N-limited streams, suggests that total N concentrations associated with a periphyton chlorophyll a benchmark for desirable or undesirable conditions for recreation would be about 600 to 1000 μg/L total N, respectively. For P-limited streams, the corresponding benchmark concentrations were about 50 to 90 μg/L total P, respectively. Our approach of integrating controlled experiments and matched biomonitoring field surveys was cost effective and more informative than either approach alone

Eutrophication and Metal Concentrations in Three Bays of the Great Salt Lake (USA)

The Great Salt Lake, bordered by several major population centers in the State of Utah, has received heavy loading of various pollutants that until recently have received little attention. However, during the last decade management agencies and environmental groups have become concerned that these contaminants might limit the beneficial uses of the lake. Recent work by the Utah Department of Environmental Quality, The Utah Division of Forestry, Fire and State Lands, universities, and federal agencies have focused on how selenium, mercury, eutrophication and salinity may influence recreational use of the lake and the migratory bird populations that rely on the lake for feeding and nesting

Nitrogen fixation: A poorly understood process along the freshwater-marine continuum

N2 fixation is a major component of the global N cycle and has been extensively studied in open-ocean and terrestrial ecosystems. Yet rates and ecological dynamics remain virtually unknown for the inland and coastal aquatic ecosystems (lakes, wetlands, rivers, streams, and estuaries) that connect terrestrial and marine biomes. This is due to the diversity of these habitats as well as the traditional paradigm that N2 fixation rates were low to nonexistent, and therefore not important, in these ecosystems. We identify three major research themes to advance understanding of aquatic N2 fixation: (1) the biological diversity of diazotrophs and variability of N2 fixation rates, (2) the ecological stoichiometry of N2 fixation, and (3) the upscaling of N2 fixation rates from genes to ecosystems. Coordinating research across these areas will advance limnology and oceanography by fully integrating N2 fixation into ecological dynamics of aquatic ecosystems from local to global scales

An invasive riparian tree reduces stream ecosystem efficiency via a recalcitrant organic matter subsidy

A disturbance, such as species invasion, can alter the exchange of materials and organisms between ecosystems, with potential consequences for the function of both ecosystems. Russian olive (Elaeagnus angustifolia) is an exotic tree invading riparian corridors in the western United States, and may alter stream organic matter budgets by increasing allochthonous litter and by reducing light via shading, in turn decreasing in-stream primary production. We used a before-after invasion comparison spanning 35 years to show that Russian olive invasion increased allochthonous litter nearly 25-fold to an invaded vs. a control reach of a stream, and we found that this litter decayed more slowly than native willow. Despite a mean 50% increase in canopy cover by Russian olive and associated shading, there were no significant changes in gross primary production. Benthic organic matter storage increased fourfold after Russian olive invasion compared to pre-invasion conditions, but there were no associated changes in stream ecosystem respiration or organic matter export. Thus, estimated stream ecosystem efficiency (ratio of ecosystem respiration to organic matter input) decreased 14%. These findings show that invasions of nonnative plant species in terrestrial habitats can alter resource fluxes to streams with consequences for whole-ecosystem functions

Climate, snowmelt dynamics and atmospheric deposition interact to control dissolved organic carbon export from a northern forest stream over 26 years

Increasing concentrations of dissolved organic carbon (DOC) have been identified in many freshwater systems over the last three decades. Studies have generally nominated atmospheric deposition as the key driver of this trend, with changes in climatic factors also contributing. However, there is still much uncertainty concerning net effects of these drivers on DOC concentrations and export dynamics. Changes in climate and climate mediated snowfall dynamics in northern latitudes have not been widely considered as causal factors of changes in long-term DOC trends, despite their disproportionate role in annual DOC export. We leveraged long-term datasets (1988–2013) from a first-order forested tributary of Lake Superior to understand causal factors of changes in DOC concentrations and exports from the watershed, by simultaneously evaluating atmospheric deposition, temperature, snowmelt timing, and runoff. We observed increases in DOC concentrations of approximately 0.14 mg C l−1 yr−1 (mean = 8.12 mg C l−1) that were related with declines in sulfate deposition (0.03 mg SO24− l−1 yr−1). Path analysis revealed that DOC exports were driven by runoff related to snowmelt, with peak snow water equivalences generally being lower and less variable in the 21st century, compared with the 1980s and 1990s. Mean temperatures were negatively related (direct effects) to maximum snow water equivalences (−0.71), and in turn had negative effects on DOC concentrations (−0.58), the timing of maximum discharge (−0.89) and DOC exports (indirect effect, −0.41). Based on these trends, any future changes in climate that lessen the dominance of snowmelt on annual runoff dynamics—including an earlier peak discharge—would decrease annual DOC export in snowmelt dominated systems. Together, these findings further illustrate complex interactions between climate and atmospheric deposition in carbon cycle processes, and highlight the importance of long-term monitoring efforts for understanding the consequences of a changing climate