Differentiating between point and non-point source nutrient loadings of wastewater in an agriculturally impacted area using a hybrid statistical model.

Nutrient contamination from point and non-point sources can lead to harmful consequences, such as algal blooms. Point and non-point nutrient loading estimation is determined using modeling approaches and often require an abundance of variables and observations for calibration. Small rural streams that lack water use designations often lack available data to utilize current modeling strategies. This study proposes the use of a 3-phase hybrid stepwise statistical modeling approach using generalized linear mixed models (GLMM) and a reference stream. Two streams in Central Texas were sampled for 13 months between February 2020 and February 2021, one being impacted by a wastewater treatment plant (WWTP). Dissolved phosphorus (PO4-P), ammonia (NH3-N), nitrite/nitrate (NO2 + NO3-N), total nitrogen (TN), and total phosphorus (TP) were sampled in both streams for each month. Non-point sources of contamination, such as land use/land cover and geomorphology composition, were quantified for both sub-basin drainage areas. Phase I models predicted nutrient concentrations in the reference stream using non-point source variables along with discharge and temporal variables. Best fit models were carried forward to phase II and leveraged a point-source variable, which is a naïve estimate of effluent nutrient concentration in the absence of assimilation. Phase II model coefficients highlight the significance of point-source contamination in predicting nutrient concentration, but overall lacked the ability to make future predictions under new hydrologic regimes from WWTP intensification. Phase III models included deterministically calculating an uptake variable using the relationship between discharge and wetted widths, predicting background non-point concentrations by leveraging phase I models, and calculating future nutrient loadings from WWTP intensification. This approach predicted significant increases in nutrient concentrations under planned WWTP intensification scenarios and decreased uptake efficiencies under the new hydrologic regimes.

Compensatory dynamics of lotic algae break down nonlinearly with increasing nutrient enrichment.

One important mechanism governing the temporal maintenance of biodiversity is asynchrony in co-occurring competitors due to fluctuating environments (i.e., compensatory dynamics). Temporal niche partitioning has evolved in response to predictable oscillations in environmental conditions so that species may offset competition, but we do not yet have a clear understanding of how novel anthropogenic stressors alter seasonal patterns of succession. Many primary producers are nutrient limited, and enrichment may decrease the importance of environmental fluctuations that govern which species are effective competitors under naturally low nutrient regimes. Consequently, elevated nutrient concentrations may synchronize species responses to seasonality. By studying benthic algal assemblages over 2 years from 35 streams that spanned a wide gradient of nutrient enrichment, we found that compensatory dynamics characterizing seasonal succession under natural nutrient regimes broke down at relatively low levels of total phosphorus (P) enrichment (~ 25 µg/L). With increasing P more species were able to coexist at any given time, and seasonal variation in assemblage composition was characterized by synchronous swings in species biovolumes. We also observed much higher instability in assemblage biovolumes with declines in compensatory dynamics, which indicates that anthropogenic alteration of nutrient regimes can affect community stability by changing the dominant mode of seasonal succession. Our findings indicate that compensatory fluctuations of stream algae are driven by seasonality and provide insight about how nutrient enrichment alters evolved drivers of species coexistence.

Copper and Gold Nanoparticles Increase Nutrient Excretion Rates of Primary Consumers.

Freshwater ecosystems are exposed to engineered nanoparticles through municipal and industrial wastewater-effluent discharges and agricultural nonpoint source runoff. Because previous work has shown that engineered nanoparticles from these sources can accumulate in freshwater algal assemblages, we hypothesized that nanoparticles may affect the biology of primary consumers by altering the processing of two critical nutrients associated with growth and survivorship, nitrogen and phosphorus. We tested this hypothesis by measuring the excretion rates of nitrogen and phosphorus of Physella acuta, a ubiquitous pulmonate snail that grazes heavily on periphyton, exposed to either copper or gold engineered nanoparticles for 6 months in an outdoor wetland mesocosm experiment. Chronic nanoparticle exposure doubled nutrient excretion when compared to the control. Gold nanoparticles increased nitrogen and phosphorus excretion rates more than copper nanoparticles, but overall, both nanoparticles led to higher consumer excretion, despite contrasting particle stability and physiochemical properties. Snails in mesocosms enriched with nitrogen and phosphorus had overall higher excretion rates than ones in ambient (no nutrients added) mesocosms. Stimulation patterns were different between nitrogen and phosphorus excretion, which could have implications for the resulting nutrient ratio in the water column. These results suggest that low concentrations of engineered nanoparticles could alter the metabolism of consumers and increase consumer-mediated nutrient recycling rates, potentially intensifying eutrophication in aquatic systems, for example, the increased persistence of algal blooms as observed in our mesocosm experiment.

A Metagenome-Based Investigation of Gene Relationships for Non-Substrate-Associated Microbial Phosphorus Cycling in the Water Column of Streams and Rivers.

Phosphorus (P) is a nutrient of primary importance in all living systems, and it is especially important in streams and rivers which are sensitive to anthropogenic P inputs and eutrophication. Microbes are accepted as the primary mineralizers and solubilizers of P improving bioavailability for organisms at all trophic levels. Here, we use a genomics approach with metagenome sequencing of 24 temperate streams and rivers representing a total P (TP) gradient to identify relationships between functional genes, functional gene groupings, P, and organisms within the P biogeochemical cycle. Combining information from network analyses, functional groupings, and system P levels, we have constructed a System Relational Overview of Gene Groupings (SROGG) which is a cohesive system level representation of P cycle gene and nutrient relationships. Using SROGG analysis in concert with other statistical approaches, we found that the compositional makeup of P cycle genes is strongly correlated to environmental P whereas absolute abundance of P genes shows no significant correlation to environmental P. We also found orthophosphate (PO43-) to be the dominant factor correlating with system P cycle gene composition with little evidence of a strong organic phosphorous correlation present even in more oligotrophic streams.

Microbial Community Structure and Function Decoupling Across a Phosphorus Gradient in Streams.

Phosphorus (P) is a key biological element with important and unique biogeochemical cycling in natural ecosystems. Anthropogenic phosphorus inputs have been shown to greatly affect natural ecosystems, and this has been shown to be especially true of freshwater systems. While the importance of microbial communities in the P cycle is widely accepted, the role, composition, and relationship to P of these communities in freshwater systems still hold many secrets. Here, we investigated combined bacterial and archaeal communities utilizing 16S ribosomal RNA (rRNA) gene sequencing and computationally predicted functional metagenomes (PFMs) in 25 streams representing a strong P gradient. We discovered that 16S rRNA community structure and PFMs demonstrate a degree of decoupling between structure and function in the system. While we found that total phosphorus (TP) was correlated to the structure and functional capability of bacterial and archaeal communities in the system, turbidity had a stronger, but largely independent, correlation. At TP levels of approximately 55 µg/L, we see sharp differences in the abundance of numerous ecologically important taxa related to vegetation, agriculture, sediment, and other ecosystem inhabitants.

Freshwater eutrophication drives sharp reductions in temporal beta diversity.

Eutrophication has become one of the most widespread anthropogenic forces impacting freshwater biological diversity. One potentially important mechanism driving biodiversity changes in response to eutrophication is the alteration of seasonal patterns of succession, particularly among species with short, synchronous, life cycles. We tested the hypothesis that eutrophication reduces seasonally driven variation in species assemblages by focusing on an understudied aspect of biodiversity: temporal beta diversity (ßt ). We estimated the effect of eutrophication on ßt by sampling benthic macroinvertebrate assemblages bimonthly for two years across 35 streams spanning a steep gradient of total phosphorus (P) and benthic algal biomass (as chlorophyll a [chl a]). Two widely used metrics of ß diversity both declined sharply in response to increasing P and chl a, regardless of covariates. The most parsimonious explanatory model for ßt included an interaction between P and macroinvertebrate biomass, which revealed that ßt was lower when macroinvertebrate biomass was relatively high. Macroinvertebrate biomass explained a greater amount of deviance in ßt at lower to moderate concentrations of P, providing additional explanatory power where P concentration alone was unable to fully explain declines in ßt . Chl a explained similar amounts of deviance in ßt in comparison to the best P model, but only when temperature variability, which was positively related to ßt , also was included in the model. Declines in ßt suggest that nutrient enrichment decreases the competitive advantage that specialists gain by occupying particular temporal niches, which leads to assemblages dominated by generalists that exhibit little seasonal turnover. The collapse of seasonal variation in assemblage composition we observed in our study suggests that treating dynamic communities as static assemblages is a simplification that may fail to detect the full impact of anthropogenic stressors. Our results show that eutrophication leads to more temporally homogenous communities and therefore degrades a fundamental facet of biodiversity.

Influence of drought and total phosphorus on diel pH in wadeable streams: implications for ecological risk assessment of ionizable contaminants.

Climatological influences on site-specific ecohydrology are particularly germane in semiarid regions where instream flows are strongly influenced by effluent discharges. Because many traditional and emerging aquatic contaminants, such as pharmaceuticals, are ionizable, we examined diel surface water pH patterns (i.e., change in pH over a 24-h period) at 23 wadeable streams in central Texas, USA, representing a gradient of nutrient enrichment during consecutive summers of 2006 and 2007. The years of our study were characterized by decidedly different instream flows, which likely affected production:respiration dynamics and led to distinctions in diel pH patterns between 2006 and 2007. Site-specific ambient water quality criteria for NH(3) and the aquatic toxicity of the model weak base pharmaceutical sertraline were predicted using continuous water quality monitoring data from the sites. Drought conditions of 2006 significantly increased (p<0.05) diel pH changes compared to high instream flows of 2007,and the magnitude of diel pH variability was most pronounced at nutrient-enriched sites in 2006. Differences in diel pH change patterns between 2006 and 2007 affected predictions of the environmental fate and effects for model weak base pharmaceuticals and NH(3). Overall, site-specific diel pH was more variable at some sites than the difference in mean surface water pH between the 2 summers. Diel pH variability affected regulatory criteria, because 20% of the study sites in 2006 experienced greater than 5-fold differences in National Ambient Water Quality Criteria for NH(3) over 24-h periods. Our study emphasizes the potential uncertainty that diel pH variability may introduce in site-specific assessments and provides recommendations for environmental assessment of ionizable contaminants.

Influence of nitrogen and phosphorus concentrations and ratios on Lemna gibba growth responses to triclosan in laboratory and stream mesocosm experiments.

The effects of co-occurring nutrient and contaminant stressors are very likely to interact in aquatic systems, particularly at the level of primary producers. Site-specific nitrogen (N) and phosphorus (P) concentrations are often much lower and differ in relative availability than those used in nutrient-saturated laboratory assays for aquatic plants, which can introduce uncertainty in prospective ecological hazard and risk assessments. Because triclosan, an antimicrobial agent included in personal care products, potentially presents high relative risk among antimicrobial agents to aquatic plants and algae, we performed laboratory experiments with the model aquatic macrophyte Lemna gibba across a gradient of environmentally relevant N:P levels with and without triclosan co-exposure. Frond numbers (7 d) were significantly higher in N:P treatments of 16 and 23 but were lower in N:P of 937 and 2,500 treatments relative to standardized control media (N:P=3). When triclosan co-exposure occurred at high nutrient concentrations, frond number median effective concentration values at N:P 0.75, 3, and 16 were more than twofold lower than triclosan median effective concentration values in low nutrient media N:P ratios. However, a triclosan median effective concentration for frond number was twofold lower at N:P of 2,500 than at other N:P ratios in low concentration media. Influences of P enrichment on triclosan toxicity to L. gibba were further explored during a 14-d outdoor experimental stream mesocosm study. Effects of 2.6 and 20.8 microg L(-1) triclosan on L. gibba growth rates were more pronounced with increasing P treatment levels, which was generally consistent with our laboratory observations. Findings from these laboratory and field studies indicate that site-specific nutrient concentrations and ratios should be considered during assessments of primary producer responses to chemical stressors.

Estimating ecological thresholds for phosphorus in the Everglades.

The Florida Everglades, a wetland of international importance, has been undergoing a significant shift in its native flora and fauna due to excessive total phosphorus (TP) loadings (an average of 147 t per annum from 1995to 2004) and an elevated mean TP concentration (69 microg L(-1) of TP in 2004) from agricultural runoff and Lake Okeechobee outflow despite the use of 16000 ha of stormwater treatment areas. Here, we present a Bayesian changepoint analysis of long-term experimental research and show that exceeding a surface water geometric mean TP threshold concentration of 15 microg L(-1) causes an ecological imbalance in algal, macrophyte, and macroinvertebrate assemblages as well as slough community structure. A phosphorus threshold for all trophic levels may be more realistic and protective when presented as a threshold zone (12-15 microg L(-1)) because estimates of uncertainty must be utilized to accurately define TP thresholds, which change with seasons and water depths. Most interior areas of the Everglades are currently at or below this threshold zone, but the exterior areas near inflow structures (except for the Everglades National Park) are presently receiving double or triple the proposed threshold. Our Bayesian approach, used hereto address ecological imbalance along nutrient gradients, is applicable to determining thresholds and stable states in other aquatic ecosystems.

Integrating bioassessment and ecological risk assessment: an approach to developing numerical water-quality criteria.

Ioassessment is used worldwide to monitor aquatic health but is infrequently used with risk-assessment objectives, such as supporting the development of defensible, numerical water-quality criteria. To this end, we present a generalized approach for detecting potential ecological thresholds using assemblage-level attributes and a multimetric index (Index of Biological Integrity-IBI) as endpoints in response to numerical changes in water quality. To illustrate the approach, we used existing macroinvertebrate and surface-water total phosphorus (TP) datasets from an observed P gradient and a P-dosing experiment in wetlands of the south Florida coastal plain nutrient ecoregion. Ten assemblage attributes were identified as potential metrics using the observational data, and five were validated in the experiment. These five core metrics were subjected individually and as an aggregated Nutrient-IBI to nonparametric changepoint analysis (nCPA) to estimate cumulative probabilities of a threshold response to TP. Threshold responses were evident for all metrics and the IBI, and were repeatable through time. Results from the observed gradient indicated that a threshold was > or = 50% probable between 12.6 and 19.4 microg/L TP for individual metrics and 14.8 microg/L TP for the IBI. Results from the P-dosing experiment revealed > or = 50% probability of a response between 11.2 and 13.0 microg/L TP for the metrics and 12.3 microg/L TP for the IBI. Uncertainty analysis indicated a low (typically > or = 5%) probability that an IBI threshold occurred at < or = 10 microg/L TP, while there was > or = 95% certainty that the threshold was < or = 17 microg/L TP. The weight-of-evidence produced from these analyses implies that a TP concentration > 12-15 microg/L is likely to cause degradation of macroinvertebrate assemblage structure and function, a reflection of biological integrity, in the study area. This finding may assist in the development of a numerical water-quality criterion for TP in this ecoregion, and illustrates the utility of bioassessment to environmental decision-making.