Watershed Sources Research Articles

High-Frequency Concurrent Measurements in Watershed and Impaired Estuary Reveal Coupled DOC and Decoupled Nitrate Dynamics

Rapid changes in land use, pollution inputs, and climate are altering the quantity, timing, and form of materials delivered from watersheds to estuaries. To better characterize these alterations simultaneous measurements of biogeochemical conditions in watersheds and estuaries over a range of times scales are needed. We examined the strength of watershed-estuarine biogeochemical coupling using in situ measurements of nitrate, terrestrial dissolved organic carbon (DOC), and chloride collected over a 7-month period in a nitrogen-impaired estuary in northeastern US. The watershed exerted strong control over concentrations of terrestrially derived DOC in the estuary, attributable to relative homogeneity of watershed sources from forested land combined with relatively conservative behavior in estuarine waters. Estuarine nitrate patterns were more complex, suggesting the influence of heterogeneous watershed distribution of non-point and point sources and high reactivity of nitrate in the estuary. Understanding estuarine biogeochemical patterns will be advanced through greater use of simultaneous sub-hourly measurements of inflows, salinity, and water quality in estuaries and their upstream watersheds.

Evaluation of pollutants characteristics and effect of dissolved and particulate contaminants in tributaries of an urban watershed

The importance of tributary management is increasing owing to the increase and diversity of pollutant sources in watersheds. In this study, the characteristics of dissolved and particulate contaminants in tributaries of the Anyang watershed, South Korea, were investigated using various analytical indicators (e.g., organic matter, nutrients, and spectroscopic analysis), heat maps, and multivariate analysis techniques, including cluster analysis and principal component analysis (PCA), to evaluate water quality characteristics of the tributaries and major pollutant sources. The influence of particulate matter from each tributary was also evaluated by investigating changes in sediment composition and re-suspended sediment oxygen demand (RSOD) at the mainstream of Anyang. The heat map patterns of the evaluated tributaries reflect sufficiently the characteristics of the pollutant sources, which enables the differentiation of the pollutant source according to the tributary locations and seasons. PCA results showed that water quality and spectral indicators reflected the different characteristics of the tributary; thus, it enabled a more diverse evaluation of tributary depending on pollutant source and season. The tributary watershed showed a high suspended solids (SS) discharge load depending on the watershed area (r = 0.94, p < 0.05), and the sediment near the tributary with higher SS discharge load revealed a higher content of fine sand (0.002–0.2 mm, r = 0.84, p < 0.05). The finer the particles derived from tributaries are, the higher the increase in RSOD is due to resuspension, most of which are caused by microbial activity (> 80%). Our results demonstrate that comprehensive and visual evaluation—including spectral indicators and heat maps analysis—of the tributary characteristics of pollutant sources is an efficient approach of broadening the knowledge of the distribution and effects of particulate matter from tributaries.

Effects of land-cover and watershed protection futures on sustainable groundwater management in a heavily utilized aquifer in Hawai\u2018i (USA)

Groundwater sustainability initiatives, including sustainable yield and watershed policy protection policies, are growing globally in response to increasing demand for groundwater, coupled with concerns about the effects of climate and land-cover change on groundwater supply. Improved understanding of the impacts of watershed management on groundwater yields and management costs—particularly in the broader context of climate and land-cover change—is critical to inform these initiatives and facilitate integrated land and water management. This study develops a novel, spatially explicit groundwater hydrologic ecosystem services framework, which combines stakeholder-defined land-cover scenarios, sustainable yield estimation using a groundwater simulation optimization approach, and economic valuation, and applies it in the most heavily utilized aquifer Hawai‘i (USA). Sustainable yield estimates and resulting differences in replacement costs are estimated for six land-cover scenarios (with varying levels of urban development and watershed management) crossed with two water demand scenarios in a context of a dry future climate (Representative Concentration Pathway [RCP] 8.5 mid-century). Land-cover change is found to be an important, though less significant drive of changes in groundwater recharge than climate change. The degree of watershed protection, through preventing the spread of high-water-use, invasive plant species, is projected to be a much stronger land-cover signal than urban development. Specifically, full forest protection increases sustainable yield by 7–11% (30–45 million liters per day) and substantially decreases treatment costs compared with no forest protection. Collectively, this study demonstrates the hydrologic and economic value of watershed protection in a context of a dry future climate, providing insights for integrated land and water policy and management in Hawai‘i and other regions, particularly where species invasions threaten source watersheds.

Identifying nitrogen sources in intensive livestock farming watershed with swine excreta treatment facility using dual ammonium (δ15NNH4) and nitrate (δ15NNO3) nitrogen isotope ratios axes.

We proposed a novel approach based on dual ammonium and nitrate nitrogen isotope ratios (δ15NNH4 and δ15NNO3, respectively) axes to identify nitrogen sources in intensive livestock farming watersheds, especially those with swine excreta treatment facilities. The δ15NNH4 and δ15NNO3 values in water samples were measured monthly in 2016-2017. Soil and mineral fertilizers, sewage, sewage effluent, manure, and swine effluents were the five sources considered to identify nitrogen sources. The results showed that nitrogen pollution from agricultural activities was well reflected by the seasonal δ15NNH4 and δ15NNO3 patterns in the river, and microbial nitrification was suggested as the dominant nitrogen transformation process in the river. This study revealed that δ15NNH4 and δ15NNO3 axes provided better results than the traditionally used nitrate oxygen (δ18ONO3) and δ15NNO3 axes for identifying nitrogen sources in agricultural watersheds with swine excreta treatment facilities. The mixing model results showed that stream water was severely contaminated with swine effluents (e.g., a mean minimum contribution of 31%), thus affecting the quality of the mainstream (p=0.068<0.10). This study was the first successful application of dual δ15NNH4 and δ15NNO3 axes to better understand nitrogen sources in intensive livestock farming watersheds with swine excreta treatment facilities.

Isolating the AFFF Signature in Coastal Watersheds Using Oxidizable PFAS Precursors and Unexplained Organofluorine.

Water supplies for millions of U.S. individuals exceed maximum contaminant levels for per- and polyfluoroalkyl substances (PFAS). Contemporary and legacy use of aqueous film forming foams (AFFF) is a major contamination source. However, diverse PFAS sources are present within watersheds, making it difficult to isolate their predominant origins. Here we examine PFAS source signatures among six adjacent coastal watersheds on Cape Cod, MA, U.S.A. using multivariate clustering techniques. A distinct signature of AFFF contamination enriched in precursors with six perfluorinated carbons (C6) was identified in watersheds with an AFFF source, while others were enriched in C4 precursors. Principal component analysis of PFAS composition in impacted watersheds showed a decline in precursor composition relative to AFFF stocks and a corresponding increase in terminal perfluoroalkyl sulfonates with < C6 but not those with ≥ C6. Prior work shows that in AFFF stocks, all extractable organofluorine (EOF) can be explained by targeted PFAS and precursors inferred using Bayesian inference on the total oxidizable precursor assay. Using the same techniques for the first time in impacted watersheds, we find that only 24%-63% of the EOF can be explained by targeted PFAS and oxidizable precursors. Our work thus indicates the presence of large non-AFFF organofluorine sources in these coastal watersheds.

Priority watershed management areas for groundwater recharge and drinking water protection: A case study from Hawai‘i Island

Worldwide, water utilities and other water users increasingly seek to finance watershed protection and restoration in order to maintain or enhance water quality and quantity important for drinking water supply and other human use. Hydrologic studies which characterize the relative effectiveness of watershed management activities in terms of metrics important to water users are greatly needed to guide prioritization. To address this need, we worked with a local water utility in Hawaiʻi to develop a novel framework for prioritizing investments in native forest protection and restoration for groundwater recharge and applied it in the utility's priority aquifers and recharge areas. Specifically we combined land cover and water balance modeling to quantify the 50-year cumulative recharge benefits of: 1) protection of native forest from conversion to non-native forest, and 2) restoration of native forest in non-native grasslands. The highest priority areas (80th percentile of benefits) for native forest protection are projected to prevent the loss of over 48,600 m3 per hectare of recharge over 50 years. Incorporating land cover change modeling (versus assuming all areas are equally susceptible to invasion) shifts prioritization towards low to mid-elevation mesic forest areas at the highest risk of invasion by invasive canopy species as well as to high elevation, cloud forest areas at high risk of conversion to non-native grassland or bare ground. We also find that, in the highest priority areas with substantial fog interception, native forest restoration is projected to increase recharge by over 88,900 m3 per hectare over 50 years, but that decreases in recharge occur in areas with low fog interception. This study provides a framework for prioritizing investments in forest protection and restoration for groundwater recharge in a way that incorporates both the threat of conversion as well as changes in hydrologic fluxes. The framework and results can be utilized by current managers and updated as new ecohydrological data become available. The results also provide broad insights on the links between watershed management and groundwater recharge, particularly on islands and in other regions where species invasions threaten source watersheds and where groundwater is a primary water source.

Localized Water Quality Improvement in the Choptank Estuary, a Tributary of Chesapeake Bay

Chesapeake Bay has a long history of nutrient pollution resulting in degraded water quality. However, we report improvements in chlorophyll a in surface waters and dissolved oxygen in bottom waters at one of three estuarine stations in the Choptank tributary of Chesapeake Bay. We updated a previous nutrient budget for the estuary constructed for reference year 1998 using rates of atmospheric deposition, inputs of watershed diffuse sources (primarily agriculture), and discharges of point sources (primarily human waste) for reference year 2017. Parallel trends suggest that improvements in water quality at the one station were likely due to 20% reductions in direct atmospheric deposition on the estuary’s surface and 78–95% reductions in wastewater N and P due to installation of tertiary treatment. The agricultural sector, the dominant source of N and P, appeared to provide little contribution to improved water quality during this period. Although efforts to reduce nutrient losses from agriculture are common throughout the Choptank basin, widespread reductions from agricultural diffuse sources could make large contributions to improved water quality at all stations in the estuary. The response in the Choptank is similar to those observed elsewhere in the USA, Europe, Australia, and New Zealand due to improved wastewater treatment. Similar to our findings, the upper Potomac River of Chesapeake Bay saw improvements driven by reductions in atmospheric deposition. Unfortunately, few studies elsewhere have shown improvements in water quality due to agricultural management. The data presented here indicate that public and industrial investments in reductions of atmospheric emissions and upgrades to wastewater treatment plants have improved estuarine water quality in the Choptank.

PRELIMINARY ANALYSIS OF E. COLI GENETIC DIVERSITY IN THE GUADALUPE RIVER: INITIAL APPROACH TO FECAL POLLUTION TRACKING

Four sites in the Guadalupe River were sampled for coliform bacteria from the winter 2016 through spring 2017 to assess the microbiological pollution situation in the river and to identify pollution sources. Numbers of putative coliforms detected ranged from below detection limits to 260 CFUs per 100 mL. Over the time period sampled, coliform levels exhibited strong seasonality, probably linked to the amount of rainfall prior to the sampling date. Molecular analysis of confirmed E. coli isolates demonstrated that the spike in coliform numbers 03 December 2016 was dominated by only two unique fingerprints, suggesting the possibility of a watershed source for fecal contamination in the urban area. The diversity of E. coli fingerprints was much greater in the rural portion of the river, suggesting a variety of coliform sources, probably including wildlife. Further studies are needed to identify and manage sources of fecal pollution.

Groundwater from perennial springs provide refuge from wildfire impacts in mountainous semiarid watershed

Warming temperatures, earlier snowmelt, and the elongation of dry seasons are contributing to the propensity for more frequent and severe wildfires in semiarid, mountainous watersheds, which act as source watersheds for communities, especially in the southwestern United States. Research on how vegetation and water sources will respond following wildfires in these watersheds is critical for water resource, land, and forest managers. Little research has been completed on the effect of groundwater discharged from springs on revegetation of burned forests. We test whether springs, as perennial sources of water in semiarid watersheds, have the potential to protect roots and seeds from being damaged and facilitate the regrowth of native vegetation in areas affected by wildfires, thereby creating zones of fire refugia and mitigating the propensity for floods, erosion, and incursion of invasive species. The effect of springs on revegetation is studied in the Asaayi Creek Watershed located in the Chuska Mountains, NM/AZ, and in the Royal Gorge Park located near Cañon City, CO. In the Asaayi Creek watershed, remote sensing imagery and ground surveys show that springs facilitated revegetation following a June 2014 wildfire. Post-fire time lapse photography reveal more abundant vegetation around springs and more diverse vegetation in areas where there are multiple springs. Significantly higher Enhanced Vegetative Index (EVI) values are observed around perennial springs in the Asaayi Creek watershed and also around intermittent springs in the Royal Gorge area, one year after wildfires occurred in each study area. However, the revegetation around the perennial springs in the Asaayi Creek Watershed is widespread compared to Royal Gorge. There are multiple perennial springs that are clustered close together in Asaayi Creek Watershed, and only three intermittent flowing springs are located in the Royal Gorge burn area. Springs in the Asaayi Creek watershed provide a critical source of water that directly impacts soil moisture near the springs and in turn, protects roots and buds allowing native vegetation to regenerate.

Identifying the critical watershed regions creating lake nutrient enrichment (SLRs) based on a watershed-lake integrated perspective – A case study of Chaohu Lake Basin, China

Identifying critical watershed source regions is essential for lake environments protection. However, related studies are mainly focused on assessing watershed Total Nitrogen (TN) and Total Phosphorus (TP) loads, while the impact of watershed on lake nutrients is rarely considered. To fill this gap, the amounts of watershed nutrient loss over the last ten years were assessed, and the corresponding lake TN and TP concentrations were acquired. Finally, the critical watershed regions creating lake nutrient enrichment (SLRs) were identified by analyzing the relationships from the integrated watershed-lake perspective. The results indicated that: i) The watershed TN and TP output amounts showed a rising trend in the last 10 years (mostly between 2012 and 2016); ii) The lake TP and TN in the western lake were much higher than those in the eastern lake; and iii) SLRs were located in the western Chaohu watershed. Specific findings were obtained from the statistical analysis in sub-watershed and intra-annual scales. First, the SLRs for TN were concentrated in the Nanfei (NF), which covered only 32 sub-watersheds and were mainly located in urban construction areas. In contrast, the SLRs for TP were more widely distributed and located contiguously across the major influent streams belonging to the Hangbu (HB) and NF. Second, the intra-annual variance is obvious. For TP, the spring season can be considered the most ideal period for SLRs identification, while the summer season is ideal for TN. These differences are driven by different loss processes, discharge capacities, and occurrence forms for TN and TP. In general, the integrated perspective of lake-watershed is helpful for identifying SLRs, and the temporal analysis at the seasonal scale and spatial analysis at the sub-watershed scale facilitate a deeper understanding of SLRs.

The Effects of Agricultural and Urban Land Use on Drinking Water Treatment Costs: An Analysis of United States Community Water Systems

For community water providers, safeguarding source waters from contamination offers an additional barrier of protection and a potential means of avoiding in-plant treatment costs. Whether source water protection efforts are cost-effective relative to in-plant treatment requires hydrologic, geologic, and climatologic knowledge of source watersheds, as well as an understanding of how changes in source water quality affect treatment costs. Quantitative evidence on the latter relationship is limited. This study estimates separate hedonic cost functions for water systems that primarily use surface water sources and those that primarily use groundwater sources using a database of United States (US) Community Water Systems. Cost functions relate annual variable treatment cost to production, factor input prices, capital stock, and source water quality, as proxied by land use within various ex-ante defined contributing areas (i.e., surrounding land areas affecting source water quality). For surface water systems, a 1% increase in urban land relative to forestland is correlated with a 0.13% increase in annual variable treatment costs. In this analysis, the relationship between costs and agricultural land is not statistically significant. Conversely, for groundwater systems, a 1% increase in agricultural land relative to forestland is correlated with a 0.24% increase in costs, whereas in this analysis the relationship between costs and urban land is not statistically significant. The cost-effectiveness of forestland preservation, based on sample means, varies considerably with the size of the contributing area, with no clear indication as to whether preservation is more likely to be cost-effective for surface water or groundwater systems.

The influence of legacy contamination on the transport and bioaccumulation of mercury within the Mobile River Basin

Past industrial use and subsequent release of mercury (Hg) into the environment have resulted in severe cases of legacy contamination that still influence contemporary Hg levels in biota. While the bioaccumulation of legacy Hg is commonly assessed via concentration measurements within fish tissue, this practice becomes difficult in regions of high productivity and methylmercury (MeHg) production, like the Mobile River Basin, Alabama in the southeastern United States. This study applied Hg stable isotope tracers to distinguish legacy Hg from regional deposition sources in sediments, waters, and fish within the Mobile River. Sediments and waters displayed differences in δ202Hg between industrial and background sites, which corresponded to drastic differences in Hg concentration. Sites that were affected by legacy Hg, as defined by δ202Hg, produced largemouth bass with lower MeHg content (59–70%) than those captured in the main rivers (>85%). Direct measurements of Hg isotopes and mathematical estimates of MeHg isotope pools in fish displayed similar distinctions between legacy and watershed sources as observed in other matrices. These results indicate that legacy Hg can accumulate directly into fish tissue as the inorganic species and may also be available for methylation within contaminated zones decades after the initial release.

Comparing optical versus chromatographic descriptors of dissolved organic matter (DOM) for tracking the non-point sources in rural watersheds

Identification of the major dissolved organic matter (DOM) sources in watersheds is critical for an efficient management to control the non-point sources associated with DOM and for an understanding of the local carbon cycle. In this study, several DOM indicators, which are derived from fluorescence spectroscopy and size exclusion chromatography, were compared for their applicability to track DOM sources in three forested-agricultural watersheds. Several potential source end-members were collected from upper catchments, which included fallen leaves, riparian reeds, riparian weeds, paddy soils, field soil, riverside soil, cow manure, swine manure, and poultry manure, and the values of their representative DOM descriptors were estimated along with those of the stream samples during high flow periods. The relative source contributions to streams were estimated via an end member mixing analysis using the paired descriptors that were selected based on the criteria in terms of the value ranges and the significant differences in stream samples between the high and low flow periods. Biological index (BIX) and the ratio of the terrestrial humic-like to the humic-like component were found to be optimum optical tracers, while the aromaticity and the C/N ratio of humic substance fraction were an applicable pair of chromatographic tracers. The optical tracers were superior to the latter because of more consistent and less variability in the assigned sources between the different streams in the same watersheds. The optical tracer-based estimations revealed that the major DOM sources to the high flow stream samples were soil (22.3%–58.5%) and manures (12.9%–51.3%). This comparative study provides new insights into the practical feasibility and the overlooked limitations of the common DOM descriptors for source discrimination.

Trihalomethane precursors: Land use hot spots, persistence during transport, and management options

To meet drinking water regulations, rather than investing in costly treatment plant operations, managers can look for ways to improve source water quality; this requires understanding watershed sources and fates of constituents of concern. Trihalomethanes (THMs) are one of the major classes of regulated disinfection byproducts, formed when a specific fraction of the organic carbon pool—referred to as THM precursors—reacts with chorine and/or bromine during treatment. Understanding the source, fate, timing and duration of the organic compounds that react to form THMs will allow identification of targeted and effective management actions. In this study we evaluated THM precursor contributions from multiple land use categories and hydrologic contexts, including novel data for urban land uses that demonstrate strong potential to release water with high THM formation potential (THMFP; median 618 μg L−1): greater than storm runoff integrated across a mixed-use (1/3 natural, 2/3 agricultural) watershed (median 460 μg L−1), irrigation runoff from agricultural systems (357 μg L−1), or runoff from a natural forested (median 123 μg L−1) and shrubland/grassland (median 259 μg L−1) watersheds. While individual storm events released high THM precursor concentrations over short periods, dry season agricultural irrigation as well as urban landscapes have the potential to release water high in THM precursors for several months. Experimental bioassays and sampling along 333 miles of the California Aqueduct confirmed bioavailability and photooxidation potential of less than 10% for THM precursors, suggesting that rivers with residence times of days to weeks may act as THM precursor conduits, shuttling THM precursors from hundreds of miles away to drinking water intakes with minimal degradation. This finding has considerable implications for water managers, who may therefore consider THM precursor management strategies that target even sources located far upstream.

New insights into the role of lake sediments in understanding watershed non-point source phosphorus pollution

A deep understanding of the relationship between watershed non-point source (NPS) pollution and lake total phosphorus (TP) accumulation is the basis of watershed–lake environmental management and ecological reconstruction, in which the acquisition of long-term NPS P and lake TP data over corresponding temporal and spatial scales seems to be the key issue. In this study, three typical sub-basins within Hongze Lake were selected as study sites. The sediment TP concentrations were designated as the lake indicator to relate with the NPS P loads, and the role of sediment in representing watershed NPS pollution was quantitatively analyzed. The results showed that, first, a notable fluctuation tendency in NPS P loads, which was largely influenced by policy, was found for all three sub-basins. Second, the deposition rate for three sediment cores was approximately 0.40 cm per year, with 13 cm cores reflecting the period since 1990. Third, the sediment is a faithful recorder of watershed NPS P pollution and lake nutrient enrichment. The correlation coefficient (r) between watershed NPS P loads and lake sediment TP concentrations showed notable differences when each watershed was considered separately. In particular, an ideal relationship was found in Basins 1 and 2, especially in the bottom 7 and 8 cm of the sediment cores, in which r exceeded 0.8. This showed that sediment can be an effective indicator of NPS P pollution during certain periods, such as 1990 – 2002, and diverse roles can be attributed to land-use/cover change and lake resuspension conditions. In addition, the weakest relationship existed in the No. 3 watershed, largely due to higher rainfall amounts compared with other two watersheds. For all the watersheds, lake sediment is a suitable indicator of watershed NPS P pollution over certain temporal and spatial scales.

ASLO 2020 Award Winners

<scp>ASLO</scp> 2020 Award Winners

Riverine nitrate source apportionment using dual stable isotopes in a drinking water source watershed of southeast China

It is crucial to quantitatively track riverine nitrate (NO3−) sources and transformations in drinking water source watersheds for preventing current and future NO3− pollution, and ensuring a safe drinking water supply. This study identified the significant contributors to riverine NO3− in Zhaoshandu reservoir watershed of Zhejiang province, southeast China. To achieve this goal, we used hydrochemistry parameters and stable isotopes of NO3− (δ15N-NO3− and δ18O-NO3−) accompanied with a Markov Chain Monte Carlo mixing model to estimate the proportional contributions of riverine NO3− inputs from atmospheric deposition (AD), chemical nitrogen fertilizer (NF), soil nitrogen (SN), and manure and sewage (M&S). Results indicated that the main form of riverine nitrogen in this region was NO3−, constituting ~60% of the total nitrogen mass on average (total organic nitrogen ~37% & ammonium ~3%). Variations in the isotopic signatures of NO3− demonstrated that microbial nitrification of NF, SN and M&S was the primary nitrogen transformation process within the Zhaoshandu reservoir watershed, whereas denitrification was minimal. A classical dual isotope bi-plot incorporating chloride concentrations suggested NF, SN and M&S were the major contributors of NO3− to the river. Riverine NO3− source apportionment results were further refined using the Markov Chain Monte Carlo mixing model, which revealed that AD, NF, SN and M&S contributed 7.6 ± 4.1%, 22.5 ± 12.8%, 27.4 ± 14.5% and 42.5 ± 11.3% of riverine NO3− at the watershed outlet, respectively. Finally, uncertainties associated with NO3− source apportionment were quantitatively characterized as: SN > NF > M&S > AD. This work provides a comprehensive approach to distinguish riverine NO3− sources in drinking water source watersheds, which helps guide implementation of management strategies to effectively control NO3− contamination and protect drinking water quality. Summary of the main finding from this works (Capsule)We utilized NO3− stable isotope analysis and a Markov Chain Monte Carlo mixing model to quantify riverine nitrate pollution sources in a drinking water source watershed in Zhejiang province, southeast China. Markov Chain Monte Carlo mixing model output showed that NF, SN and M&S were the dominant sources of riverine NO3− during the sampling period in Zhaoshandu watershed. Uncertainty analysis characterized the variation strength associated with contributions of individual nitrate sources and indicated the greatest uncertainty for SN, followed by NF, M&S and AD.

Spatial variation of bacterial and fungal communities of estuarine seagrass leaf microbiomes

The health of seagrass plants, and thereby the ecosystems they form, is linked to their associated microbial communities. However, the role of the microbiome in holobiont function and health remains poorly understood for most seagrass species and environmental pressures, and there is, therefore, a need to better understand the drivers behind the formation of and external influences on the seagrass microbiome. Using a core microbiome framework, we characterised the leaf microbiomes of 6 estuarine seagrass populations after a precipitation event to explore how the microbiomes vary across different sites and salinities over a regional spatial scale. We found that each estuary had distinct core bacterial community structures (beta-diversity), but shared a more similar fungal core community structure. We hypothesise that the differences in the bacterial members of the microbiomes among estuaries are generally the result of each estuary being influenced by unique watersheds and sources of prokaryotes. In contrast, the similarity in the core fungal communities suggests that the eukaryotic components of the microbiomes are likely under selection or result from similar colonisation pathway(s). We also found that the bacterial taxa driving the differences among estuaries were linked to the salinity of the estuary, likely due to (1) the general epibiotic nature of colonisation (i.e. watershed source and exposure) and (2) members or functional groups within the leaf microbiome assisting seagrasses in coping with the extreme salinities. These results are valuable for linking microbiomes to the resilience of seagrasses living within dynamic estuaries experiencing a range of physicochemical pressures.

Tracking nitrogen pollution sources in plain watersheds by combining high-frequency water quality monitoring with tracing dual nitrate isotopes

High-frequency water quality measurements at daily resolutions combined with tracing of nitrate δ15N and δ18O isotopes was proposed as a method to quantify nitrogen (N) export from point and nonpoint sources, which is usually difficult to achieve at a watershed scale. The method was applied to two agricultural watersheds located in two nearby plains along the Xifeihe and Nihe Rivers in China. The two watersheds had different crops and planting patterns, in addition to their different urbanization percentages of 25.3% and 14.1%, respectively. Analysis showed that these two watersheds exported approximately 7.8 and 11.2 kg N ha−1y−1 to their receiving rivers. Isotope analysis identified that point sources of sewage/manure contributed majority (53%) of nitrate load to the Xifeihe River, whereas nonpoint sources from soils, fertilizer and atmospheric deposition contributed majority (60%) of nitrate load to the Nihe River. Crop patterns reduced the N export through soybean planting or denitrification in rice paddies despite 200–300 kg N ha−1 fertilizer application to the fields per year. Annual nitrate export patterns of the two watersheds did not show correlations with their corresponding daily flow rates, which had important implications to watershed monitoring design. Nitrogen (N) concentrations were neither chemeostatic nor discharge-correlated except for during high flows in the soybean-wheat agriculture dominant Xifeihe watershed. This indicated that the traditional monitoring protocol for pollution source identification with measurements only conducted during few rainfall events was likely to be insufficient to estimate the total loads of nutrients (e.g., N) from watersheds.

Detroit River phosphorus loads: Anatomy of a binational watershed

As a result of increased harmful algal blooms and hypoxia in Lake Erie, the US and Canada revised their phosphorus loading targets under the 2012 Great Lakes Water Quality Agreement. The focus of this paper is the Detroit River and its watershed, a source of 25% of the total phosphorus (TP) load to Lake Erie. Its load declined 37% since 1998, due chiefly to improvements at the regional Great Lakes Water Authority Water Resource Recovery Facility (WRRF) in Detroit and phosphorus sequestered by zebra and quagga mussels in Lake Huron. In addition to the 54% of the load from Lake Huron, nonpoint sources contribute 57% of the TP load and 50% of the dissolved reactive phosphorus load, with the remaining balance from point sources. After Lake Huron, the largest source is the WRRF, which has already reduced its load by over 40%. Currently, loads from Lake Huron and further reductions from the WRRF are not part of the reduction strategy, therefore remaining watershed sources will need to decline by 72% to meet the Water Quality Agreement target - a daunting challenge. Because other urban sources are very small, most of the reduction would have to come from agriculturally-dominated lands. The most effective way to reduce those loads is to apply combinations of practices like cover crops, buffer strips, wetlands, and applying fertilizer below the soil surface on the lands with the highest phosphorus losses. However, our simulations suggest even extensive conservation on those lands may not be enough.