Elevated Nutrient Loads Research Articles

High-frequency monitoring during rainstorm events reveals nitrogen sources and transport in a rural catchment

Rural areas lacking essential sewage treatment facilities and collection systems often experience eutrophication due to elevated nutrient loads. Understanding nitrogen (N) sources and transport mechanisms in rural catchments is crucial for improving water quality and mitigating downstream export loads, particularly during storm events. To further elucidate the sources, pathways, and transport mechanisms of N from a rural catchment with intensive agricultural activities during storm events, we conducted an analysis of 21 events through continuous sampling over two rainy seasons in a small rural catchment from the lower reaches of the Yangtze River. The results revealed that ammonia-N (NH4+-N) and nitrate-N (NO3−-N) exhibited distinct behaviors during rainstorm events, with NO3−-N accounting for the primary nitrogen loss, its load being approximately forty times greater than that of NH4+-N. Through examinations of the concentration-discharge (c-Q) relationships, the findings revealed that, particularly in prolonged rainstorms, NH4+-N exhibited source limited pattern (b = −0.13, P < 0.01), while NO3−-N displayed transport limited pattern (b = −0.21, P < 0.01). The figure-eight hysteresis pattern was prevalent for both NH4+-N and NO3−-N (38.1% and 52.0%, respectively), arising from intricate interactions among diverse sources and pathways. For NO3−-N, the hysteresis pattern shifted from clockwise under short-duration rainstorms to counter-clockwise under long-duration rainstorms, whereas hysteresis remained consistently clockwise for NH4+-N. The hysteresis analysis further suggests that the duration of rainstorms modifies hydrological connectivity, thereby influencing the transport processes of N. These insights provide valuable information for the development of targeted management strategies to reduce storm nutrient export in rural catchments.

The seasonal and spatial variability of ammonium uptake in a hilly watershed

Elevated nutrient loading can cause deleterious impacts on aquatic ecosystems such as eutrophication. Seasonal variability and land use change often lead to varied nutrient uptake from streams. However, the impacts of seasonal and spatial variation on stream nutrient transport within the same watershed haven’t been fully understood. Here, we conducted nutrient addition experiments using the Tracer Additions for Spiraling Curve Characterization (TASCC) approach within the Xiaogang Watershed, Zhejiang Province, China. Six of the experiments were conducted in one stream every other month and eight releases in eight different streams to quantify ammonium uptake kinetics across different seasons and land uses. Our findings suggest that the uptake capability increases with both discharge and ambient concentration (C0): seasonal variability of discharge shows small impacts on uptake metrics, except the ambient uptake length (Sw-amb), which increases with discharge; while the change in C0 could have more significant effects on both ambient areal uptake (Uamb) and maximum areal uptake rate parameter (Umax). Downstream the river network, the increase in discharge led to a significant increase in Uamb, ambient uptake velocity (Vf-amb), and Umax. On the other hand, the change in C0 is less influential than discharge along the river network, which may be correlated with the hydraulic geometry. Additionally, there is an optimal temperature (T) for uptake in our study region around 17°C, which may be explained by the growth of chlorophyll a. The positive correlation between Vf-amb and Q at the spatial scale may be attributed to the increase of dissolved organic carbon (DOC) and river chlorophyll a downstream with stream width. In addition, land use had an important effect on ion concentration in streams, and the proportion of agricultural land was positively correlated with nutrient concentration. Our findings could help provide scientific support for land use management and water quality regulation.

Dynamics of oxygen sources and sinks in the Baltic Sea under different nutrient inputs

The Baltic Sea is one of the marine systems suffering from pronounced man-made hypoxia due to the elevated nutrient loads from land. To mitigate hypoxia expansion and to return the Baltic Sea to a good environmental state, the Baltic Sea Action Plan (BSAP), regulating the waterborne and airborne nutrient input, was adopted by all states surrounding the Baltic Sea. However, at the moment, no significant shrinking of the hypoxic area is observed. In this study, two scenario simulations of the future state of the deep parts of the central Baltic Sea (deeper than 70 meters) were carried out, utilizing a 3-dimensional numerical model. Climate change effects on meteorology, hydrology, and oceanic state were not included. We focused on O2 and H2S sources and sinks under different nutrient input scenarios. We found that under the BSAP scenario, all subbasins in the central Baltic Sea, especially the northern and western Gotland Basin, show significant improvement, namely, oxygenation and oxidation of the deposited reduced material, ceasing its advection to the upper layers and neighboring basins. We found that the nutrient loads are responsible for more than 60% and 80% of the O2 and H2S sources and sinks variability, respectively, at the interannual time scale. We showed that the Baltic Sea could return to the initial state in 1948, but under the more rigorous 0.5 BSAP scenario (nutrient input is halved compared to the BSAP). However, since we observed no hysteresis effect, the system would probably reach the initial state but over a timeframe longer than the 71-year future simulation period.

A national-scale trophic state analysis to prioritize lakes for restoration in Aotearoa New Zealand

ABSTRACTPressure on lakes in Aotearoa New Zealand is increasing because of elevated catchment nutrient loads, establishment of non-native species, and climate change. Current government legislation requires that pressures are managed to avoid eutrophication and degradation of lake health. This approach requires information on the state of lakes at regional and national scales, which is challenging because <5% are currently monitored. In this study, we (1) modelled lake trophic status at a national scale using a highly representative dataset and lake characteristics, land use, and environmental parameters as predictor variables; and (2) identified lakes that should be prioritized for protection to prevent further degradation. Six statistical models were evaluated, with extreme boosting producing the highest predictive power and lowest error. This model indicated that for the 3738 lakes in the dataset, 44% were eutrophic or higher trophic state, 22% mesotrophic, and 34% oligotrophic or lower trophic state. These data provide a benchmark to guide management and support the call for more resourcing to restore lakes in Aotearoa New Zealand. To identify lakes to prioritise for protection, we focused on the ∼800 mesotrophic lakes. We used (1) the portion of lake catchment not within conservation estate, and (2) road access as proxies for the likelihood of land-use intensification and the introduction of non-native species, respectively. We identified >170 lakes with limited catchment protection and easy human access. Immediate attention should be given to protecting these waterbodies to prevent the need for costly and resource-intensive remediation in the future.

Nitrogen from agriculture and temperature as the major drivers of deoxygenation in the central Bohai Sea

Agricultural N losses strongly dominate the N delivery (average 72 % of total N delivery to rivers in the period 1980–2010) in the rivers discharging into the Bohai Sea, a semi-enclosed marginal sea, which has been suffering from eutrophication and deoxygenation since the 1980s. In this paper we investigate the relationship between N loading and deoxygenation in the Bohai Sea, and consequences of future N loading scenarios. Using modeling for the period 1980–2010, the contributions of different oxygen consumption processes were quantified and the main controlling mechanisms of summer bottom dissolved oxygen (DO) evolution in the central Bohai Sea were determined. Model results show that the water column stratification during summer impeded the DO exchange between oxygenated surface water and oxygen-poor bottom water. Water column oxygen consumption (60 % of total oxygen consumption) was strongly correlated with elevated nutrient loading, while nutrient imbalances (increasing N:P ratios) enhanced harmful algal bloom proliferation. Future scenarios show that deoxygenation may be reduced in all scenarios owing to increasing agricultural efficiency, manure recycling and wastewater treatment. However, even in the sustainable development scenario SSP1, nutrient discharges in 2050 will still exceed the 1980 levels, and with further enhancement of water stratification due to climate warming, the risk of summer hypoxia in bottom waters may persist in the coming decades.

Deciphering large-scale spatial pattern and modulators of dissolved greenhouse gases (CO2, CH4, and N2O) along the Yangtze River, China

The Yangtze River, the third largest river around the globe, has been heavily engineered with a series of hydroelectric dams. Meanwhile, it receives elevated organic matter and nutrient loads from its densely populated catchment, subsequently altering dissolved greenhouse gas (GHG) concentrations along the river. However, the large-scale longitudinal patterns and drivers of GHG concentrations in the Yangtze River remain poorly understood. Using longitudinal sampling design in a 2400 km section, we report dissolved carbon dioxide, methane, and nitrous oxide concentrations along the Yangtze River at 145 sites. We observe significant spatial clustering with higher carbon dioxide and nitrous oxide concentrations in the middle reach of the Yangtze River. The results of nonlinear regression reveal that riverine GHGs are high when wetland coverage is high and dissolved oxygen is low. Wetlands and oxygen, not the Three Gorges Dam and tributaries, are the primary correlates of spatial variations of CO2 and CH4 concentrations, respectively. N2O is surprisingly well predicted by CO2, implying their common drivers or sources. We strongly recommend that wetland contribution to GHG budgets and its sensitivity to environmental change be considered when estimating riverine GHGs in the Yangtze River. In light of our study, future control of GHG emissions from large rivers may largely depend on how external inputs and internal metabolism are regulated by decreasing nutrient loading.

Use of biological and water quality indices to evaluate conditions of the Upper uMngeni Catchment, KwaZulu-Natal, South Africa

Urban and agricultural land uses have the potential to severely compromise the quality of impoundments, if ineffectively managed and operated. A case in point is the upper uMngeni Catchment, including Midmar Dam, which is integral to the freshwater supply infrastructure in KwaZulu-Natal, South Africa. Monitoring sites were established in varying land use types in three subcatchments of the upper uMngeni Catchment to assess water quality and ecosystem health impacts of current land uses. Conclusions about water quality were drawn using pairing of SASS5 and spot water quality data. Water quality and ecological condition were highest in commercial plantations and upstream of a high-density settlement where natural land cover and sparse settlement occurred. Although marked declines in water quality and ecological condition were observed under commercial agriculture. The most notable declines in water quality and ecological condition were observed downstream of the settlement with elevated nutrient loads. Shifts in aquatic biota were highly correlated with seasonal shifts in water quality, influenced markedly by land use. The cumulative effects of current land use activities, urban development and agriculture on Midmar Dam’s water quality should be viewed with concern. Additional development in the form of additional social housing projects may exacerbate impacts.

Exposure to elevated nutrient load results in structural and functional changes to microbial communities associated with riparian wetland plants Phalaris arundinaceae and Veronica anagallis-aquatica

The goal of this study was to better understand the resilience of rhizosphere microbial communities located in riparian wetlands subjected to high nutrient loads following effluent or runoff exposure. Specifically, this research monitored functional profiles of mesocosm communities during an artificial high nitrogen load exposure following adaptation to high or low water quality conditions. Based on prior research, two species of wetland plants were chosen, Phalaris arundinaceae and Veronica anagallis-aquatica, both located in local riparian zones, and planted in wetland mesocosms. Rhizosphere bacterial community response was monitored for approximately three weeks after exposure to elevated nutrient loads, following prior adaptation for several months to either high or low (poor) water quality conditions. The impact of prior acclimation conditions was then assessed. Changes to rhizosphere bacterial community structure and function were monitored using PCR-denaturing gel gradient electrophoresis (DGGE) and Biolog™ Ecoplates, respectively. Plants were also sampled to assess any impact on mycorrhizal associations over the exposure period. As an additional indicator of functionality and nitrogen utilization within the mesocosm system, inorganic intermediate N species were monitored at the outflow. Significant differences were observed over time following elevated nutrient exposure in bacterial structural diversity and richness, as well as functional diversity and richness (p < 0.001). The effects of interactions between time, plant species and water quality exposure were also significant for both structural (p = 0.002) and functional (p = 0.05) community diversity. Elevated nutrient exposure resulted in changes to microbial carbon source utilization profiles, specifically, bacterial usage of amino acids (p = 0.032), polymers (p = 0.029) and carboxylic and ketonic acids (p = 0.042). Carboxylic and ketonic acid utilization by bacterial communities was positively correlated with the removal of nitrate and nitrite from experimental mesocosms. Mycorrhizal associations as measured by percent colonization also decreased significantly in P. arundinaceae following elevated nutrient exposure (p < 0.001). Furthermore, abundance of nitrogenous compounds in outflow water differed between plant species treatments, with V. anagallis-aquatica removing more nitrate from the outflow (p = 0.005) while P. arundinaceae removed higher levels of ammonia (p = 0.03). This study provides insight into how microbial communities respond to transient nutrient loads such as that experienced by overland runoff following a high rain event and will assist in the design and rehabilitation of wetlands or riparian buffer strips to manage higher nutrient loads resulting from increasing human activity impacting our freshwater systems.

Trends in water quality in a subtropical Australian river-estuary system: Responses to damming, climate variability and wastewater discharges

The Logan-Albert estuary in southeast Queensland, Australia, has high biodiversity and supports multiple economic and recreational services. Elevated nutrient and sediment loads have been a longstanding management issue for the estuary. We investigated the spatial and seasonal patterns of nutrients and turbidity along the Logan-Albert estuary and assessed the effects of a recently constructed upstream dam. Nutrient concentrations and turbidity levels were analysed using 15 years of monitoring data from 19 water quality sites throughout the estuary. We hypothesised that the construction of Wyaralong Dam would act as a nutrient and sediment sink which may have positive effects on downstream water quality. Long-term trends of water quality constituents were evaluated using a non-parametric seasonal Mann-Kendall test and the effect of upstream impoundment was assessed with a Before-After Control-Impact (BACI) test. Nutrient concentrations and turbidity levels declined significantly with time in the upper Logan estuary and, to a lesser extent, in the lower Albert estuary. The general improvement of water quality in the upper Logan estuary was attributed to construction of the Wyaralong Dam. Significant decreases in concentrations of total phosphorus (TP) and oxidised nitrogen (NOx-N) along the lower Albert were principally attributed to wetter conditions over the 15-year dataset, which diluted point-source loads from a nearby wastewater treatment plant (WWTP). Our results show that estuarine water quality changes can be highly dynamic with interactions amongst climate and management practices that necessitate long-term monitoring programs with good spatial coverage.

Elevated nutrient inputs to marshes differentially impact carbon and nitrogen cycling in two northern Gulf of Mexico saltmarsh plants

Salt marsh biogeochemical processes are regulated by ecosystem structure (e.g. plant community composition). However, plant-specific responses to stressors such as elevated nutrient inputs can have differing impacts on nitrogen (N) removal and carbon (C) sequestration. We conducted a field manipulation to investigate the impact of elevated nutrient loading on ecosystem C dynamics and nitrate reduction pathways (denitrification and dissimilatory nitrate reduction to ammonium (DNRA)) in plots dominated by either Juncus roemerianus or Spartina alterniflora that were collocated in a northern Gulf of Mexico salt marsh. We increased N and phosphorus (P) inputs by two- and three-times current levels in the region. Nutrient enrichment had no effect on net ecosystem exchange. However, a three-fold increase in nutrient input resulted in nearly one-third increases in gross primary productivity ($$GPP$$) and ecosystem respiration in S. alterniflora plots, whereas there was no impact in J. roemerianus plots. Denitrification increased in S. alterniflora plots tenfold at both treatment levels relative to controls, but as with $$GPP$$, there was no response in J. roemerianus plots to higher nutrient inputs. In contrast, a three-fold increase in nutrients reduced DNRA by half in J. roemerianus plots. This work demonstrates that plant species-specific responses in marshes need to be considered for determining the impact of higher nutrient inputs on plant productivity and N-removal and retention.

Landscape Drivers of Dynamic Change in Water Quality of U.S. Rivers.

Water security is a top concern for social well-being, and dramatic changes in the availability of freshwater have occurred as a result of human uses and landscape management. Elevated nutrient loading and perturbations to major ion composition have resulted from human activities and have degraded freshwater resources. This study addresses the emerging nature of streamwater quality in the 21st century through analysis of concentrations and trends in a wide variety of constituents in streams and rivers of the U.S. Concentrations of 15 water quality constituents including nutrients, major ions, sediment, and specific conductance were analyzed over the period 1982-2012 and a targeted trend analysis was performed from 1992 to 2012. Although environmental policy is geared toward addressing the long-standing problem of nutrient overenrichment, these efforts have had uneven success, with decreasing nutrient concentrations at urbanized sites and little to no change at agricultural sites. Additionally, freshwaters are being salinized rapidly in all human-dominated land use types. While efforts to control nutrients are ongoing, rapid salinity increases are ushering in a new set of poorly defined issues. Increasing salinity negatively affects biodiversity, mobilizes sediment-bound contaminants, and increases lead contamination of drinking water, but its effects are not well integrated into current paradigms of water management.

Coagulation Process for Removing Algae and Algal Organic Matter—An Overview

In drinking water sources, seasonal algal blooms have augmented greatly dur-ing the last decades following the elevated temperature and nutrient loading in surface water because of agricultural and surface runoff. More than 95% of algal cells may be retained via coagulation/flocculation techniques. Nevertheless, algal organic matter (AOM) stays not eliminated well throughout coagulation, therefore it provokes many working dares in potable water treatment. This work aims to discuss the performance of coagulation on AOM reduction. The main pathway of algae and AOM reduction stays charge neutralization (CN) at an optimum pH of about 6.0. More research has to follow the reduction of low-molecular weight AOM, reluctant to coagulate, with additional treatment methods to diminish its negative influence on water safety. As dissolved microcystins (MCs) are efficaciously eliminated via CN, enhanced coagulation (EC) would be more suitable for their elimination. On the other hand, some precautions must be followed to guarantee that the acid injection has not a secondary impact in the form of algaecide treatment to avert the dissolved MCs concentration augmentation. Consequently, both algae and dissolved MCs may be efficiently removed by EC when appropriate rapid mixing and acid/coagulant dosage are guaranteed.

Tracing nitrate sources with a combined isotope approach (δ15NNO3, δ18ONO3 and δ11B) in a large mixed-use watershed in southern Alberta, Canada

Rapid population growth and land-use intensification over the last century have resulted in a substantial increase in nutrient loads degrading marine and freshwater ecosystems worldwide. In mixed-use watersheds, elevated nitrogen loads from wastewater treatment plant (WWTP) effluent or agricultural runoff often drive the eutrophication of waterways. Accordingly, the objective of this research was to identify sources of riverine nitrate (NO3), a deleterious dissolved species of nitrogen, with a combined isotopic tracing technique in the Bow River and the Oldman River in Alberta, Canada. Riverine NO3 and boron (B) concentrations, mean daily flux and δ15NNO3, δ18ONO3, and δ11B values were determined at 17 mainstem sites during high and low discharge periods in 2014 and 2015. The data for mainstem sites were then compared to results for effluent from seven WWTPs, eight synthetic fertilizers, cow manure, and three predominantly agricultural tributary sites to estimate point and non-point NO3 sources. The NO3 flux, δ15NNO3 and δ18ONO3 values indicated the city of Calgary’s Bonnybrook WWTP effluent accounts for the majority of the NO3 flux in the Bow River downstream of Calgary. δ15NNO3 and δ11B values in the Bow River highlighted an increase in agricultural NO3 loading downstream of irrigation return-flows. A three-fold decrease in the NO3:B flux ratio indicated NO3-removal processes are active in the lower reaches of the Bow River. For the Oldman River, δ11B values revealed elevated nutrient loading from the Lethbridge WWTP effluent (10% of downstream B flux). Furthermore, the agricultural tributaries contributed 25% of the local B flux to the Oldman River. Overall, δ11B was proven to be an effective co-tracer for discriminating between urban and agricultural sources of NO3 in these large mixed-use watersheds. This combined isotope tracing approach has significant potential to identify point and non-point NO3 sources driving eutrophication around the world.

The effects of land use disturbance vary with trophic position in littoral cichlid fish communities from Lake Tanganyika

AbstractImpacts of anthropogenic disturbance are especially severe in freshwater ecosystems. In particular, land use disturbance can lead to increased levels of pollution, including elevated nutrient and sediment loads whose negative impacts range from the community to the individual level. However, few studies have investigated if these impacts are uniform across species represented by multiple trophic levels. To address this knowledge gap, we focused on Lake Tanganyika cichlid fishes, which comprise hundreds of species representing a wide range of feeding strategies. Cichlids are at their most diverse within the near‐shore environment; however, land use disturbance of this environment has led to decreasing diversity, particularly in herbivores. We therefore tested if there is a uniform effect of pollution across species and trophic groups within the hyper‐diverse rocky shore cichlid fish community.We selected three sites with differing levels of human impact along the Tanzanian coastline and 10 cichlid species, comprising varying taxonomic and trophic groups, common to these sites. Nitrogen and carbon stable isotope values for 528 samples were generated and analysed using generalised linear mixed models. We also estimated stomach contents including sediment proportions.Our study highlights that multiple sources of pollution are having differing effects across species within a diverse fish community. We found that nitrogen stable isotope values were significantly higher at the most disturbed (urbanised) site for benthic feeding species, whereas there was no difference in these isotopes between sites for the water column feeding trophic group. Stomach contents revealed that the elevated δ15N values were unlikely to have been caused by differences in diet between sites. However, at the most disturbed site, higher proportions of sediment were present in most herbivores, irrespective of foraging behaviour.It is likely that anthropogenic nitrogen loading is the cause of higher nitrogen stable isotope values since there was no evidence of species shifting trophic levels between sites. Results support our previous study showing herbivore species to be most affected by human disturbance and make the link to pollution much more explicit. As lower diversity of consumers can negatively affect ecosystem processes such as stability, alleviating environmental impact through sewage treatment and afforestation programmes should continue to be a global priority for the conservation of aquatic ecosystems, as well as human health.

Nitrogen cycling in large temperate floodplain rivers of contrasting nutrient regimes and management

AbstractHydraulic connection between channels and floodplains (“connectivity”) is a fundamental determinant of ecosystem function in large floodplain rivers. Factors controlling material processing in these rivers depend not only on the degree of connectivity but also on the sediment conditions, nutrient loads, and source. Nutrient cycling in the nutrient‐rich upper Mississippi River (MISS) is relatively well studied, whereas that of less eutrophic tributaries is not (e.g., St Croix River; SACN). We examined components of nitrogen cycling in 2 floodplain rivers of contrasting nutrient enrichment and catchment land use to test the hypothesis that N‐cycling rates will be greater in the MISS with elevated nutrient loads and productivity in contrast to the relatively nutrient‐poor SACN. Nitrate (NO3−‐N) concentrations were greatest in flowing habitats in the MISS and often undetectable in isolated backwaters except where groundwater inputs occurred. In the SACN, NO3−‐N concentrations were greatest in the flowing backwater where groundwater inputs were high. Ambient nitrification in the MISS was twice that in the SACN and tended to be lowest in the main channel. Denitrification was 3× greater in the MISS than that in the SACN, N‐limited in both rivers. Community production/respiration was &gt;1 in the MISS and likely provisioned labile C to fuel microbial metabolism and dissimilatory NO3−‐N reduction, whereas the heterotrophic (production/respiration &lt; 1) nature of the SACN likely limited microbial metabolism and NO3−‐N dissimilation. It appears that N‐cycling in the SACN was driven by groundwater, whereas that in the MISS was supported mainly by water column N‐sources.

Water quality assessment in the Cherry Creek watershed: Patterns of nutrient runoff in an agricultural watershed

Access to safe, high quality water for consumption, agriculture, industry, and recreation is critically important. Continuous agricultural and mining activities have impaired the waters of the Grand Lake watershed in the central Great Plains region of the United States. The Grand Lake watershed encompasses portions of southeast Kansas, southwest Missouri, northwest Arkansas, and northeast Oklahoma, and drains into Grand Lake in northeast Oklahoma. The Cherry Creek watershed drains approximately 882.2 km2 (218,000 ac) of land in southeast Kansas and is a contributor of water to the Grand Lake watershed. This paper presents a water quality assessment in the Cherry Creek watershed, with an end toward mitigation of nonpoint source pollutants that are a major contributor to sediment and nutrient contaminants in Grand Lake. A hydrological model was developed using the Hydrological Simulation Program Fortran code and was updated, calibrated, and verified with measured data reported by US Geological Survey (USGS) and the Kansas Department of Health and Environment (KDHE). The model was extended to simulate water quality within the study area. Nitrate (NO3−-N), total ammonia (TAM), total phosphorus (TP), and orthophosphate (PO43−-P) concentrations measured at the USGS stations were used to calibrate the model, and concentrations reported by KDHE at downstream locations were used to verify the model. Results indicate good performance of the hydrological model as tests of fitness were within levels established in previous studies (root-mean-squared-error to standard deviation ratio [RSR] 0.6; NS > 0.5). Nutrient measurements below the minimum quantifiable limit (MQL) hampered precise simulation of nutrient changes, though simulated values were acceptable in terms of ranges of contaminant concentration values and seasonal trends. The calibrated model was used to estimate the probability that nutrient levels would exceed established water quality criteria for rivers and streams. Concentration values of NO3−-N and TAM are shown to be low for an agricultural watershed: 75% of the NO3−-N and TAM concentrations are lower than 0.41 mg L−1. The probability of NO3−-N and TAM concentrations being toxic for aquatic communities is lower than 9%. Although model-estimated PO43−-P concentrations were low in numerical value (ranging from 0 to 2.60 mg L−1), they could still promote eutrophication according to the accepted 0.05 mg L−1 criteria for maximum PO43−-P concentrations in streams. Concentrations of PO43−-P in Cherry Creek have a 30.4% probability of exceeding that threshold. Extensive use of animal manure (primarily poultry) or manure from cattle grazing on pasture may account for the elevated levels of PO43−-P observed in the watershed. Although nutrient runoff from agricultural watersheds is anticipated to be a major contributor to elevated stream nutrient loads, these results indicate minimal contamination from Cherry Creek.

Hyporheic Passive Flux Meters Reveal Inverse Vertical Zonation and High Seasonality of Nitrogen Processing in an Anthropogenically Modified Stream (Holtemme, Germany)

AbstractTransformation and retention of nitrogen and other biologically reactive solutes in the hyporheic zones of running water contribute to an essential ecosystem service. However, the synoptic impact of intense agricultural or urban land‐uses, elevated nutrient loading, flow alterations, riparian clear‐cutting, and channelization on the source‐sink behavior of solutes in hyporheic zones remains largely uncharacterized and unquantified. Therefore, we studied nutrient dynamics in a hydromorphologically and chemically modified stream reach using a new monitoring approach allowing the simultaneous measurement of nutrient and water flux through a screened area in the subsurface of rivers (hyporheic passive flux meter, HPFM). With HPFMs we directly assessed time‐integrated lateral hyporheic nitrate fluxes during early spring and midsummer covering different temperature and discharge regimes. Contrary to our expectations, higher stream discharge coincided with substantially lower hyporheic exchange rates. While in streams featuring a natural morphology, bed form induced exchange commonly increases with surface flow, the influence of groundwater level was dominant in this reach. Furthermore, in contrast to less impacted environments, where progressive substrate depletion with depths reduces metabolic rates in the subsurface, we identified not the upper, but the intermediate layer of the hyporheic zone as hot spot of nutrient turnover. Overall, the hyporheic zone at the study site functioned partly as nitrate source, partly as a sink. Neither of the commonly used determinants redox state and residence time could explain this source or sink function. Our results give clear evidence to carefully transfer the knowledge of hyporheic zone processes from “natural” systems to anthropologically modified streams.

Modelling Nutrient Load Changes from Fertilizer Application Scenarios in Six Catchments around the Baltic Sea

The main environmental stressor of the Baltic Sea is elevated riverine nutrient loads, mainly originating from diffuse agricultural sources. Agricultural practices, intensities, and nutrient losses vary across the Baltic Sea drainage basin (1.75 × 106 km2, 14 countries and 85 million inhabitants). Six “Soil and Water Assessment Tool” (SWAT) models were set up for catchments representing the major agricultural systems, and covering the different climate gradients in the Baltic Sea drainage basin. Four fertilizer application scenarios were run for each catchment to evaluate the sensitivity of changed fertilizer applications. Increasing sensitivity was found for catchments with an increasing proportion of agricultural land use and increased amounts of applied fertilizers. A change in chemical fertilizer use of ±20% was found to affect watershed NO3-N loads between zero effect and ±13%, while a change in manure application of ±20% affected watershed NO3-N loads between zero effect and −6% to +7%.

Nutrient Dynamics of the Delta: Effects on Primary Producers

doi: https://doi.org/10.15447/sfews.2016v14iss4art4 Increasing clarity of Delta waters, the emergence of harmful algal blooms, the proliferation of aquatic water weeds, and the altered food web of the Delta have brought nutrient dynamics to the forefront. This paper focuses on the sources of nutrients, the transformation and uptake of nutrients, and the links of nutrients to primary producers. The largest loads of nutrients to the Delta come from the Sacramento River with the San Joaquin River seasonally important, especially in the summer. Nutrient concentrations reflect riverine inputs in winter and internal biological processes during periods of lower flow with internal nitrogen losses within the Delta estimated at approximately 30% annually. Light regime, grazing pressure, and nutrient availability influence rates of primary production at different times and locations within the Delta. The roles of the chemical form of dissolved inorganic nitrogen in growth rates of primary producers in the Delta and the structure of the open-water algal community are currently topics of much interest and considerable debate. Harmful algal blooms have been noted since the late 1990s, and the extent of invasive aquatic macrophytes (both submerged and free-floating forms) has increased especially during years of drought. Elevated nutrient loads must be considered in terms of their ability to support this excess biomass. Modern sensor technology and networks are now deployed that make high-frequency measurements of nitrate, ammonium, and phosphate. Data from such instruments allow a much more detailed assessment of the spatial and temporal dynamics of nutrients. Four fruitful directions for future research include utilizing continuous sensor data to estimate rates of primary production and ecosystem respiration, linking hydrodynamic models of the Delta with the transport and fate of dissolved nutrients, studying nutrient dynamics in various habitat types, and exploring the use of stable isotopes to trace the movement and fate of effluent-derived nutrients.

Watershed Land Use and Seasonal Variation Constrain the Influence of Riparian Canopy Cover on Stream Ecosystem Metabolism.

Ecosystem metabolism is an important determinant of trophic structure, nutrient cycling, and other critical ecosystem processes in streams. Whereas watershed- and local-scale controls on stream metabolism have been independently investigated, little is known about how controls exerted at different scales interact to determine stream metabolic rates, particularly in urban streams and across seasons. To address this knowledge gap, we measured ecosystem metabolism in four urban and four reference streams in northern Kentucky, USA, with paired closed and open riparian canopies, during each of the four seasons. Gross primary production (GPP), ecosystem respiration, and net ecosystem production (NEP) were all best predicted by models with season as a main effect, but interactions between season, canopy, and watershed varied for each response. Urban streams exhibited higher GPP during most seasons, likely due to elevated nutrient loads. Open canopy reaches in both urban and forested streams, supported higher rates of GPP than the closed canopy which reaches during the summer and fall, when the overhead vegetation shaded the closed reaches. The effect of canopy cover on GPP was similar among urban and forested streams. The combination of watershed and local-scale controls resulted in urban streams that alternated between net heterotrophy (NEP <0) and net autotrophy (NEP >0) at the reach-scale during seasons with dense canopy cover. This finding has management relevance because net production can lead to accumulation of algal biomass and associated issues like nighttime hypoxia. Our study suggests that although watershed urbanization fundamentally alters ecosystem function, the preservation and restoration of canopied riparian zones can provide an important management tool at the local scale, with the strongest impacts on stream metabolism during summer.