Lynn A. Bartsch

Nitrification in the Upper Mississippi River: patterns, controls, and contribution to the NO3− budget

Abstract We measured nitrification rates in sediment samples collected from a variety of aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7 times between May 2000 and October 2001. We also conducted nutrient-enrichment experiments and analyzed vertical profiles of sediment to determine factors regulating nitrification. Nitrification rates were relatively high compared to other ecosystems (ranging from 0–8.25 μg N cm−2 h−1) and exhibited significant temporal and spatial patterns. Nitrification rates were greatest during the summer and spring compared to autumn and winter (ANOVA, p < 0.05) and were greater in contiguous backwater and impounded habitats compared to main and side-channel habitats (p < 0.05). Regression analysis indicated that nitrification rates were weakly (r2 = 0.18, p < 0.0001) related to temperature and exchangeable NH4+ of the sediment. However, nutrient-enrichment experiments showed that NH4+ availability did not limit nitrification in 3 sediment types with variable organic matter. Vertical profiles of sediment cores demonstrated that oxygen concentration and nitrification had similar patterns suggesting that nitrification may be limited by oxygen penetration into sediments. We conclude that temperature and sediment NH4+ can be useful for predicting broad-scale temporal and spatial nitrification patterns, respectively, but oxygen penetration into the sediments likely regulates nitrification rates in much of the UMR. Overall, we estimated that nitrification produces 6982 mt N/y of NO3− or 7% of the total annual NO3− budget.

Strong effects of predation by fishes on an invasive macroinvertebrate in a large floodplain river

Abstract We assessed the effects of fish predation on zebra mussels (Dreissena polymorpha) in Navigation Pool 8 of the Upper Mississippi River from 13 May to 5 October, 1998. Concrete-block samplers were deployed at 18 randomly chosen sites in the main-channel border, with 6 sites in the upper, middle, and lower segments of the pool. Two blocks, 1 of which was enclosed in a cage to exclude large predatory fishes, were deployed at each site. After 145 d, blocks were retrieved from 12 of the 18 sites, and zebra mussels were found on all blocks. Densities of zebra mussels were higher on caged blocks than uncaged blocks, and the magnitudes of the differences varied spatially. Mean mussel densities on uncaged blocks were reduced by 66%, 86%, and 20% compared to caged blocks in the upper, middle, and lower pool segments, respectively, over the 145-d interval. Mean daily instantaneous zebra mussel mortality rates from large predators ranged from 0.0016 to 0.0138. Similarly, biomass of zebra mussels was higher on caged than uncaged blocks. Mean mussel biomass on uncaged blocks was reduced by 64% pool-wide, relative to biomass on caged blocks. Zebra mussels were consumed by at least 6 fish taxa including redhorse suckers (Moxostoma spp.), common carp (Cyprinus carpio), bluegill (Lepomis macrochirus), quillback carpsucker (Carpiodes cyprinus), flathead catfish (Pylodictis olivaris), and freshwater drum (Aplodinotus grunniens). Fish predation had an important moderating effect on zebra mussel demography in Pool 8.

Variability and regulation of denitrification in an Upper Mississippi River backwater

Abstract Sediments in the backwaters of the Upper Mississippi River (UMR) are highly organic and provide an optimal environment for N removal. We monitored an 8.6-ha UMR backwater site near La Crosse, Wisconsin, for nearly 3 y to assess temporal variability, seasonal trends, and the factors regulating denitrification. We measured rates of unamended denitrification (DEN) and denitrification enzyme activity (DEA) rates at ambient temperature and DEA at 30°C (DEA30). Seasonal mean (±1 SE) DEN rates ranged from 0.041 ± 0.015 to 0.47 ± 0.23 μg N cm−2 h−1 and were highest in winter and lowest in autumn. Seasonal rates of DEA exhibited a different pattern with the highest rates in summer (25.6 ± 3.4 μg N cm−2 h−1) and the lowest rates in winter (10.6 ± 2.1 μg N cm−2 h−1). The overall mean DEA30 rate was 31.0 ± 1.9 μg N cm−2 h−1 but showed no significant seasonal pattern. Short-term (weekly) and seasonal variability exhibited by rates of DEN and DEA were best explained by water-column NO3− concentration and temperature, respectively. No environmental variables explained a significant amount of variability in DEA30. Our results suggest that nutrient (i.e., NO3−) availability and temperature are both regulators of denitrification, with NO3− concentration being the most important limiting factor in this system. The high DEN rates during winter were in response to elevated NO3− concentrations resulting from a chain reaction beginning with algal blooms creating oxic conditions that stimulated nitrification. Increasing hydrological connectivity in large rivers as a river management tool to reduce N flux to downstream areas may be beneficial.

Effects of zebra mussels on food webs: interactions with juvenile bluegill and water residence time

We evaluated how water residence time mediated the impact of zebramussels Dreissena polymorpha and bluegill sunfish Lepomis macrochirus on experimental food webs established in1100-l outdoor mesocosms. Water residence time was manipulated asa surrogate for seston resupply – a critical variable affectinggrowth and survival of suspension-feeding invertebrates. We used a2×2×2 factorial experimental design witheight treatment combinations (3 replicates/treatment) including thepresence or absence of Dreissena (2000 per m2), juvenilebluegill (40 per mesocosm), and short (1100 l per d) or long (220 lper d) water residence time. Measures of seston concentration(chlorophyll a, turbidity and suspended solids) were greaterin the short- compared to long water-residence mesocosms, butintermediate in short water-residence mesocosms containing Dreissena. Abundance of rotifers (Keratella and Polyarthra) was reduced in Dreissena mesocosms and elevatedin short residence time mesocosms. Cladocera abundance, in general,was unaffected by the presence of Dreissena; densities werehigher in short-residence time mesocosms, and reduced in thepresence of Lepomis. The growth of juvenile Lepomiswere unaffected by Dreissena because of abundant benthicfood. The final total mass of Dreissena was significantlygreater in short- than long-residence mesocosms. Impacts of Dreissena on planktonic food webs may not only depend on thedensity of zebra mussels but also on the residence time of thesurrounding water and the resupply of seston.

Particulate organic matter quality influences nitrate retention and denitrification in stream sediments: evidence from a carbon burial experiment

Organic carbon supply is linked to nitrogen transformation in ecosystems. However, the role of organic carbon quality in nitrogen processing is not as well understood. We determined how the quality of particulate organic carbon (POC) influenced nitrogen transformation in stream sediments by burying identical quantities of varying quality POC (northern red oak (Quercus rubra) leaves, red maple (Acer rubrum) leaves, red maple wood) in stream mesocosms and measuring the effects on nitrogen retention and denitrification compared to a control of combusted sand. We also determined how POC quality affected the quantity and quality of dissolved organic carbon (DOC) and dissolved oxygen concentration in groundwater. Nitrate and total dissolved nitrogen (TDN) retention were assessed by comparing solute concentrations and fluxes along groundwater flow paths in the mesocosms. Denitrification was measured by in situ changes in N2 concentrations (using MIMS) and by acetylene block incubations. POC quality was measured by C:N and lignin:N ratios and DOC quality was assessed by fluorescence excitation emission matrix spectroscopy. POC quality had strong effects on nitrogen processing. Leaf treatments had much higher nitrate retention, TDN retention and denitrification rates than the wood and control treatments and red maple leaf burial resulted in higher nitrate and TDN retention rates than burial of red oak leaves. Leaf, but not wood, burial drove pore water to severe hypoxia and leaf treatments had higher DOC production and different DOC chemical composition than the wood and control treatments. We think that POC quality affected nitrogen processing in the sediments by influencing the quantity and quality of DOC and redox conditions. Our results suggest that the type of organic carbon inputs can affect the rates of nitrogen transformation in stream ecosystems.

Zebra mussels (Dreissena polymorpha) limit food for larval fish (Pimephales promelas) in turbulent systems: a bioenergetics analysis*

We conducted a factorial experiment, in outdoor mesocosms, on the effects of zebra mussels and water column mixing (i.e., turbulence) on the diet, growth, and survival of larval fathead minnows (Pimephales promelas). Significant (P< 0.05) larval mortality occurred by the end of the experiment with the highest mortality (90%) occurring in the presence of both turbulence and zebra mussels, whereas mortality was 37% in treatment with turbulence and 17% and 18% in the zebra mussels treatment, and the control, respectively. The size of individual fish was significantly different among treatments at the end of the experiment and was inversely related to survival. Levels of trophic resources (i.e., phyto and zooplankton) varied among treatments and were treatment specific. Turbulent mixing facilitated removal of phytoplankton by zebra mussels by making the entire water column of the tanks available to these benthic filter feeders. Early in the experiment (Day = 0 to 14) the physical process of turbulent mixing likely caused a reduction in standing stocks of zooplankton. The interactive effect of turbulence and mussels reduced copepod and rotifer stocks, through physical processes and through filtration by zebra mussels, relative to the turbulence treatment. The reductions in the number of total zooplankton in the turbulent mixing mesocosms and the further reduction of rotifer and copepod in the turbulence and mussels treatment coincided with a period of increased reliance of larval fathead minnows on these prey. Estimates of consumption from bioenergetics modeling and measured prey standing stocks indicated caloric resources of suitable prey in turbulence treatments during the early weeks of the experiment were insufficient to prevent starvation. Early mortality in the turbulence and mussels treatment likely released surviving fish from intense intraspecific competition and resulted in higher individual growth rates. A combination of high abundance of zebra mussels in an environment with a well-mixed water column can have significant effects on larval fish survival and growth.

Wetland management reduces sediment and nutrient loading to the upper Mississippi river.

Restored riparian wetlands in the Upper Mississippi River basin have potential to remove sediment and nutrients from tributaries before they flow into the Mississippi River. For 3 yr we calculated retention efficiencies of a marsh complex, which consisted of a restored marsh and an adjacent natural marsh that were connected to Halfway Creek, a small tributary of the Mississippi. We measured sediment, N, and P removal through a mass balance budget approach, N removal through denitrification, and N and P removal through mechanical soil excavation. The marsh complex had average retention rates of approximately 30 Mg sediment ha yr, 26 kg total N ha yr, and 20 kg total P ha yr. Water flowed into the restored marsh only during high-discharge events. Although the majority of retention occurred in the natural marsh, portions of the natural marsh were hydrologically disconnected at low discharge due to historical over-bank sedimentation. The natural marsh removed >60% of sediment, >10% of P, and >5% of N loads (except the first year, when it was a N source). The marsh complex was a source of NH and soluble reactive P. The average denitrification rate for the marsh complex was 2.88 mg N m h. Soil excavation removed 3600 Mg of sediment, 5.6 Mg of N, and 2.7 Mg of P from the restored marsh. The marsh complex was effective in removing sediment and nutrients from storm flows; however, retention could be increased if more water was diverted into both restored and natural marshes before entering the river.

Fatty Acid Composition at the Base of Aquatic Food Webs Is Influenced by Habitat Type and Watershed Land Use

Spatial variation in food resources strongly influences many aspects of aquatic consumer ecology. Although large-scale controls over spatial variation in many aspects of food resources are well known, others have received little study. Here we investigated variation in the fatty acid (FA) composition of seston and primary consumers within (i.e., among habitats) and among tributary systems of Lake Michigan, USA. FA composition of food is important because all metazoans require certain FAs for proper growth and development that cannot be produced de novo, including many polyunsaturated fatty acids (PUFAs). Here we sampled three habitat types (river, rivermouth and nearshore zone) in 11 tributaries of Lake Michigan to assess the amount of FA in seston and primary consumers of seston. We hypothesize that among-system and among-habitat variation in FAs at the base of food webs would be related to algal production, which in turn is influenced by three land cover characteristics: 1) combined agriculture and urban lands (an indication of anthropogenic nutrient inputs that fuel algal production), 2) the proportion of surface waters (an indication of water residence times that allow algal producers to accumulate) and 3) the extent of riparian forested buffers (an indication of stream shading that reduces algal production). Of these three land cover characteristics, only intense land use appeared to strongly related to seston and consumer FA and this effect was only strong in rivermouth and nearshore lake sites. River seston and consumer FA composition was highly variable, but that variation does not appear to be driven by the watershed land cover characteristics investigated here. Whether the spatial variation in FA content at the base of these food webs significantly influences the production of economically important species higher in the food web should be a focus of future research.

Effect of habitat type on in-stream nitrogen loss in the Mississippi River

Eutrophic and hypoxic coastal waters are often associated with high nutrient inputs from riverine systems. For example, nitrogen (N) export from the Mississippi River into the Gulf of Mexico has been identified as an important factor causing eutrophication and seasonal hypoxia. Modelling studies of N flux in large rivers, including the Mississippi River, suggest that much of the N that enters rivers remains in solution and is exported downstream. However, patterns of N cycling in the Mississippi River are complex and vary according to habitat type and season. Here we use spatial habitat data and empirically derived denitrification rates to extrapolate N loss to various reaches in 2,400 km of the Mississippi River from Minneapolis, Minnesota to the Atchafalaya diversion. Our results indicate that 9.5 % of the total N load is lost through denitrification in the river and that reaches containing large areas of impoundments and backwater lakes exhibit elevated rates of N loss. The northern 1,041 km reach of the river contains significant areas of impoundments and backwater lakes and yielded a total N loss from denitrification of 89,172 t N y–1. In comparison, total N loss from the southern 1,352 km open river was 69,872 t N y–1. Our results are consistent with high throughput of N in large rivers, but specify that habitat diversity, channel complexity, and retention time are important factors affecting nitrogen loss in rivers.

Buried particulate organic carbon stimulates denitrification and nitrate retention in stream sediments at the groundwater–surface water interface

The interface between ground water and surface water in streams is a hotspot for N processing. However, the role of buried organic C in N transformation at this interface is not well understood, and inferences have been based largely on descriptive studies. Our main objective was to determine how buried particulate organic C (POC) affected denitrification and NO3− retention in the sediments of an upwelling reach in a sand-plains stream in Wisconsin. We manipulated POC in mesocosms inserted in the sediments. Treatments included low and high quantities of conditioned red maple leaves (buried beneath combusted sand), ambient sediment (sand containing background levels of POC), and a control (combusted sand). We measured denitrification rates in sediments by acetylene-block assays in the laboratory and by changes in N2 concentrations in the field using membrane inlet mass spectrometry. We measured NO3−, NH4+, and dissolved organic N (DON) retention as changes in concentrations and fluxes along groundwater flow paths in the mesocosms. POC addition drove oxic ground water to severe hypoxia, led to large increases in dissolved organic C (DOC), and strongly increased denitrification rates and N (NO3− and total dissolved N) retention relative to the control. In situ denitrification accounted for 30 to 60% of NO3− retention. Our results suggest that buried POC stimulated denitrification and NO3− retention by producing DOC and by creating favorable redox conditions for denitrification.