Alexander J. Horne

Denitrification in constructed free-water surface wetlands: II. Effects of vegetation and temperature.

Abstract Constructed wetlands are increasingly being used for treating nitrogen-rich wastewaters. Of the 115 treatment wetlands listed in the North American Treatment Wetland Database which record nitrogen data, a large portion are used for treating secondary treated or lower quality (e.g. primary, agricultural runoff, stormwater) wastewater. Twenty-five percent treat agricultural and stormwater runoff, and only seven are used for either advanced secondary or tertiary treatment. Yet constructed wetlands may provide an attractive and economical alternative to conventional treatment plants for denitrifying high quality, nitrified wastewater. In populated areas where this is most needed, high land costs will increase the capital costs of this technology. Moreover, in semi-arid regions like the western and southwestern USA, high evaporation and evapotranspiration rates may hinder this technology by concentrating total dissolved solids (TDS) and dissolved organic carbon (DOC) concentrations. Implementation of management and design practices for denitrification may be one method to increase efficiencies, reduce costs and increase reliability. One relatively unknown variable in denitrification is the role of different plant species. If one plant provides substantially better conditions for denitrification, wetlands designed for denitrification could be smaller and less expensive. Three commonly used free-surface marsh vegetation treatments (bulrush Scirpus spp., cattail Typha spp., and a mixed stand of macrophytes and grasses) were used in replicated macrocosms to determine nitrate removal rates. Nitrate removal rates between vegetation types were large and differed significantly ( P −2 day −1 , bulrush=261 mg N m −2 day −1 , and mixed=835 mg N m −2 day −1 ). Mass balance calculations demonstrated that bacterial denitrification rather than plant uptake was the main mechanism for nitrate removal. Both water temperature (temperature–activity coefficient θ =1.15–1.22) and organic carbon availability affected denitrification rates whereas surface water dissolved oxygen (DO) and nitrogen concentrations did not. This experiment could not distinguish why the different vegetation types resulted in different denitrification rates. Plant productivity differed between treatments. Plant physical structure, waterfowl grazing pressures and wind disturbance affected the rate litter entered the water column. The literature reports that plant decomposition rates depend upon the plants C:N litter ratio and the plant fiber content. All these factors likely affected the rate bioavailable organic carbon was made available to microbial denitrifiers. Based on our study and a literature review, in organic carbon–limited free-surface wetlands, a mixture of labile (submergent, floating) and more recalcitrant (emergent, grasses) are recommended for improving denitrification rates.

Estimating denitrification in a large constructed wetland using stable nitrogen isotope ratios

Abstract Nitrate losses and their relationships to nitrogen isotope fractionation were evaluated in a wetlands environment in southern California. This paper reports the first study to follow isotope ratio changes in a large natural system with extensive macrophyte growth. As NO 3 concentrations decreased during flow through the wetlands, progressive enrichment in 15 NO 3 was found. This increase corresponded to an enrichment value of −2.5‰, a value much lower that those reported for laboratory studies of denitrification (−17 to −29‰), but closer to the range of enrichment factors attributed to denitrification in studies of groundwaters (−3.5 to −16‰). By considering a laboratory derived value of −17‰ as the enrichment factor strictly due to denitrification, 10–23% of the Prado losses could be attributed through isotope enrichment to denitrification. Other studies at Prado, using a mass balance approach which considered macrophyte growth, concluded that bacterial denitrification was the major loss mechanism (89–95%). A similar, but somewhat smaller discrepancy between estimates of denitrification was found in nearby wetlands with large shallow water bodies with dense phytoplankton growth but negligible macrophytes. These discrepancies suggest that enrichment or fractionation factors derived under laboratory conditions where all nitrogen losses are attributable to denitrification cannot be used directly as the denitrification end-member in field situations where organic matter decomposition and nitrogen recycling are occurring simultaneously or sequentially with denitrification. Further studies which consider entire growing and decomposition cycles in macrophyte-rich wetlands are needed to resolve the discrepancy in the various methods of estimating dentrification in natural wetlands.

Suppression of nitrogen fixation by blue-green algae in a eutrophic lake with trace additions of copper.

Nitrogen fixation by blue-green algae in highly eutrophic Clear Lake, California, was severely inhibited by trace amounts of copper. The chelation capacity of the lake is probably saturated by indigenous copper. Additions were only 1/200 of those normally used in algal control. Since nitrogen fixation provides half of the lakes annual nitrogen budget, economical eutrophication control appears possible.

Copper cycles and CuSO4 algicidal capacity in two California lakes

A new reservoir in southern California and a large eutrophic lake in northern California were the sites for an investigation of copper (Cu) in aquatic systems, with attention focused upon toxicity of Cu to algae. Results of bioassay experiments showed significant copper depression of chlorophyll a levels, photosynthesis, and nitrogen fixation at concentrations of 5–10μg//. Blue-green algae are especially susceptible to copper toxicity, primarily because of the inhibition of nitrogen fixation. Theoretical considerations show that Cu is likely to be strongly complexed in natural fresh waters, but not chemically precipitated. Field measurements following a Cu algicide treatment at the reservoir confirmed the expected stability of dissolved copper, showing elevated concentrations persisting for several weeks. Our present information about copper in aquatic systems permits descriptions of various pathways that eventually lead to the sediments. The algicidal effectiveness of copper is quite variable but it is likely to be greatest in lakes where nitrogen-fixing blue-green algae are abundant.

Plant carbohydrate limitation on nitrate reduction in wetland microcosms.

Although nitrate limitation in most natural wetlands results in pseudo-first-order reductions, large site-to-site variations in apparent denitrification rates cannot be easily explained by water quality (e.g., pH, Temp, DOC) or plant productivity. Our microcosm results show increasing nitrate removal efficiencies at higher ratios of total applied plant carbon to nitrate reduced, suggesting that denitrification rates may be limited by the rates of supply of both electron donor or acceptor, described by an applied carbon to nitrate (C(App): N(Red)) ratio. However, the observed first-order rate constants varied more strongly (r2 = 0.77, p <0.0001) with the acid-soluble carbohydrates to nitrate (CH2O(App): N(Red)) ratio than the total C(App): N(Red) ratio. Although observed rate constants for bulrush (Scirpus sp.) were significantly lower (0.01 <p<0.06) than for other plant sources (Hydrocotyle sp., Lemna sp., Typha sp.), there were no significant differences in the plant-specific rate constants when compared at the same CH2O(App): N(Red) ratio. In full-scale wetlands, this suggests that either high plant productivity or low NO3- loading (high CH2O(App): N(Red)) may contribute to a high effective denitrification rate. In contrast, low productivity or a high NO3- loading (low CH2O(App): N(Red)) would promote a lower denitrification rate. This co-limitation between plant carbon and nitrate may confound first-order rate comparisons in full scale denitrification wetlands since highly N-loaded systems may become carbon limited, requiring higher order reaction kinetics to better describe performance variations. In addition to hydraulic residence time. temperature and nitrate removal data, extending these results to compare large wetlands will require estimation of the CH2O(App): N(Red) ratio from an inventory of plant species, productivity estimates and carbon quality.

Macronutrient controls on nitrogen fixation in planktonic cyanobacterial populations

Abstract Experiments described in the literature have demonstrated that additions of N, Fe, and occasionally P, influence planktonic cyanobacterial nitrogen fixation in lakes and estuaries throughout the world. Increase in abundance of cyanobacteria which can fix nitrogen did not necessarily indicate that N2 fixation had occurred. In natural plankton assemblages, N2 fixation was normally stimulated by low total inorganic nitrogen (TIN) and depressed by additions of TIN. Nitrogenase was often stimulated by addition of Fe but soluble reactive phosphate (SRP) alone only stimulated nitrogenase activity occasionally. Luxury consumption and storage of P, but not N, explains the lack of P stimulation in nature. Nitrogenase activity was usually repressed at TIN oncentrations of >50–100 μg l‐1. Additions of N + P had variable effects which may depend on the balance between nitrogenase inhibition by N and general growth stimulation by N + P (which reduces ambient N).

Selenium detoxification in wetlands by permanent flooding: I. Effects on a macroalga, an epiphytic herbivore, and an invertebrate predator in the long-term mesocosm experiment at Kesterson Reservoir, California

Storage of Se-rich agricultural drainwater in an inland saline marsh (Kesterson Reservoir) resulted in heavily contaminated biota (100 to 300 ppm Se as dw). A hypothesis was proposed that permanent flooding of the marsh with low-Se water (to create anoxic sediments) would immobilize Se in an insoluble fraction unavailable to aquatic biota. This was tested in a 0.4 ha mesocosm at Kesterson Reservoir over 2.3 yr by measuring the decline in Se in plants and animals. The study was part of a larger project, only data for soluble and particulate Se, in the submerged macroalga Chara, and in the aquatic and aerial forms of typical epifloral invertebrates — a herbivore (chironomid), and one of its predators (damselfly) are reported here. Permanent flooding resulted in biologically immobile Se. A rapid initial decline in Se lasting a few months was followed by a slower, irregular but persistent decrease. Soluble Se declined most quickly, followed by that in damselfly and chironomid populations. Declines in the vegetation were slowest but Se fell to a lower level (3 to 4 ppm) than in the animals (14 to 15 ppm), although Se levels were still dropping in all biota at the end of the experiment. Between 85 and 93% of initial Se was lost by the 3 types of organisms over 2.3 yr. There was little difference in loss rates between predator and herbivore. Because Chara was the most common submerged vegetation, its loss of 94% of initial Se to concentrations considered safe for wildlife, is most important to detoxification of the entire wetland.

A Review of Managed Nitrate Addition to Enhance Surface Water Quality

Abstract Nitrate is a significant water pollutant with potential environmental impacts ranging from eutrophication to health risks to infants. But under certain circumstances nitrate may enhance water quality through a number of mechanisms, including enhancing oxidant capacity, regulation of redox potential, and suppression of nitrogen-fixing cyanobacteria. In this review the authors explore a range of case studies in which nitrate addition enhanced surface water quality including: purposeful addition of nitrate salts to lakes to repress internal phosphorus (P) loading, enhance organic matter oxidation, or impede bottom-water accumulation of methylmercury; purposeful and incidental addition of nitrate from point and nonpoint discharges to reservoirs and lakes; nitrogen (N) addition to lakes to affect phytoplankton and zooplankton composition; and nitrate addition to estuary sediment to repress hydrogen sulfide production. Nitrate addition decreased internal P and methylmercury loading, repressed sulfide production, and enhanced surface water quality by lowering total P, chlorophyll content, and phytoplankton dominance by cyanobacteria. No case study reported a worsening of eutrophic conditions due to nitrate addition, and a number of studies reported near complete loss of nitrate from the systems to which it was added. When purposely adding nitrate to anoxic surface waters, protocols should be used to maximize nitrate loss via biological denitrification but minimize enhancement of phytoplankton productivity. These protocols should include adding nitrate close to the sediment-water interface to promote nitrate loss via denitrification, managing the timing and magnitude of nitrate addition so that nitrate is depleted prior lake overturn in the fall, and not adding nitrate to N-limited systems. Elimination of existing N discharges to receiving waters should be implemented on a case-by-case basis with the awareness that nitrate in discharges may enhance surface water quality, particularly by suppressing internal P loading and associated phytoplankton productivity. In addition, managers and regulators should look to couple existing nitrate discharges with hypoxic water bodies in an effort to sustainably enhance water quality while removing nitrate from aquatic ecosystems via biological denitrification.