Effect of Three Emergent Macrophyte Species on Nutrient Retention in Aquatic Environments under Excess Nutrient Loading.

Emergent macrophyte species selection is critical for the effectiveness of nature-based engineered solutions aiming to address excess nutrient concentrations in freshwater ecosystems. Yet, the mechanisms with which macrophytes enhance nutrient retention need to be further understood. Here, we compared nutrient retention among 12 artificial flumes fed with effluents from a wastewater treatment plant and subjected to four treatments: absence of macrophytes (control) and presence of three different macrophyte species (Iris pseudacorus L., Phragmites australis L., and Schoenoplectus lacustris L.). We estimated the net and gross nutrient uptake based on the longitudinal profiles of ambient concentrations and on pulse injections of ammonium (NH4+) and soluble reactive phosphorus. Further, we investigated the influence of subsurface hydrological retention, attributed to the architectural differences in the roots of these macrophytes, on nutrient retention. Results showed a species-specific effect of macrophytes on nutrient retention and confirmed root-associated subsurface hydrological retention as a driving factor. Schoenoplectus showed both high net and gross NH4+ uptake, thereby being the most effective species to address N loading, compared to Iris and Phragmites. This work contributes to improve our mechanistic understanding of the role of emergent macrophytes on nutrient retention in aquatic environments.

Leachates from Helophyte Leaf-Litter Enhance Nitrogen Removal from Wastewater Treatment Plant Effluents.

Bioengineering techniques are currently used  in a wide variety of wastewater treatment systems. Aquatic plants (i.e., helophytes) used in these techniques reduce excess nitrogen (N) from water column via assimilation. Moreover, leachates from plant leaf-litter can serve as an additional source of labile dissolved organic matter (DOM), which can promote aerobic respiration and N removal via denitrification. We tested the influence of leaf-litter leachates from  Iris pseudacorus and Phragmites australis on the structure and activity of freshwater biofilms grown in flumes fed by effluent from a wastewater treatment plant (WWTP). The responses of the epilithic biofilm to the inputs of leaf-litter leachates were compared to those measured using a brewery byproduct rich in sugars and to the WWTP effluent water (i.e., control). All DOM sources significantly enhanced aerobic respiration and denitrification of the biofilm when compared to the controls, with increases in total microbial abundance but not in denitrifier abundance. The results suggest that metabolic activity of biofilms may be limited by bioavailability of DOM in WWTP effluent; and leaf-litter leachates of helophytes used in bioengineering techniques could alleviate this limitation by enhancing microbial N and C uptake.

The role of helophyte species on nitrogen and phosphorus retention from wastewater treatment plant effluents.

In the Mediterranean region, water scarcity compromises stream water quality particularly downstream of wastewater treatment plants (WWTP). We tested the potential of four helophyte species to reduce dissolved inorganic nitrogen (N) and phosphorus (P) from WWTP effluents. We conducted an 11-month mesocosm experiment to assess differences in N and P content among plant compartments and among species. Moreover, we quantified the relative contribution of above and belowground parts of the plants to N and P retention. The experiment was conducted at the Urban River Laboratory (www.urbanriverlab.com) in artificial channels (12 m long x 0.6 m wide x 0.4 m deep) planted with monospecific stands of Iris pseudoacorus, Typha angustifolia, Phragmites australis and Scirpus lacustris. Channels (three replicates per species) received water from the WWTP effluent, which flowed at a constant rate of 5 L min-1 through the sub-surface. The helophytes were planted in November 2014 and biomass standing stocks of carbon (C), N and P were measured in October 2015 at the time of maximum plant biomass. Differences in the concentration of N and P were larger among plant compartments than among species. The highest N concentration was measured in leaves while rhizomes showed the highest P concentration. The total plant biomass varied greatly among species from 11.4 to 4.6 Kg DW m-2 for Iris and Scirpus, respectively. Iris accumulated the highest amount of N (256 g N m-2) and P (27 g P m-2) in biomass. Plants retained from 8% (Scirpus) to 19% (Iris) of total dissolved inorganic N inputs to the channels (10.4 kg N) during the experiment, and from 6% (Phragmites) to 14% (Iris) of total dissolved inorganic P inputs (1.3 kg P). This study provides quantitative evidence to water managers of the potential role of helophytes to improve water quality in freshwater ecosystems receiving water from WWTP effluents.

Global patterns in the metacommunity structuring of lake macrophytes: regional variations and driving factors.

We studied community-environment relationships of lake macrophytes at two metacommunity scales using data from 16 regions across the world. More specifically, we examined (a) whether the lake macrophyte communities respond similar to key local environmental factors, major climate variables and lake spatial locations in each of the regions (i.e., within-region approach) and (b) how well can explained variability in the community-environment relationships across multiple lake macrophyte metacommunities be accounted for by elevation range, spatial extent, latitude, longitude, and age of the oldest lake within each metacommunity (i.e., across-region approach). In the within-region approach, we employed partial redundancy analyses together with variation partitioning to investigate the relative importance of local variables, climate variables, and spatial location on lake macrophytes among the study regions. In the across-region approach, we used adjusted R2 values of the variation partitioning to model the community-environment relationships across multiple metacommunities using linear regression and commonality analysis. We found that niche filtering related to local lake-level environmental conditions was the dominant force structuring macrophytes within metacommunities. However, our results also revealed that elevation range associated with climate (increasing temperature amplitude affecting macrophytes) and spatial location (likely due to dispersal limitation) was important for macrophytes based on the findings of the across-metacommunities analysis. These findings suggest that different determinants influence macrophyte metacommunities within different regions, thus showing context dependency. Moreover, our study emphasized that the use of a single metacommunity scale gives incomplete information on the environmental features explaining variation in macrophyte communities.

Small-scale heterogeneity of microbial N uptake in streams and its implications at the ecosystem level.

Large-scale factors associated with the environmental context of streams can explain a notable amount of variability in patterns of stream N cycling at the reach scale. However, when environmental factors fail to accurately predict stream responses at the reach level, focusing on emergent properties from small-scale heterogeneity in N cycling rates may help understand observed patterns in stream N cycling. To address how small-scale heterogeneity may contribute to shape patterns in whole-reach N uptake, we examined the drivers and variation in microbial N uptake at small spatial scales in two stream reaches with different environmental constraints (i.e., riparian canopy). Our experimental design was based on two ¹5N additions combined with a hierarchical sampling design from reach to microhabitat scales. Regardless of the degree of canopy cover, small-scale heterogeneity of microbial N uptake ranged by three orders of magnitude, and was characterized by a low abundance of highly active microhabitats (i.e., hot spots). The presence of those hot spots of N uptake resulted in a nonlinear spatial distribution of microbial N uptake rates within the streambed, especially in the case of epilithon assemblages. Small-scale heterogeneity in N uptake and turnover rates at the microhabitat scale was primarily driven by power relationships between N cycling rates and stream water velocity. Overall, fine benthic organic matter (FBOM) assemblages responded clearly to changes in the degree of canopy cover, overwhelming small-scale heterogeneity in its N uptake rates, and suggesting that FBOM contribution to whole-reach N uptake was principally imposed by environmental constraints from larger scales. In contrast, N uptake rates by epilithon showed no significant response to different environmental influences, but identical local drivers and spatial variation in each study reach. Therefore, contribution of epilithon assemblages to whole-reach N uptake was mainly associated with emerging properties from small-scale heterogeneity at lower spatial scales.

Temporal variability of nitrogen stable isotopes in primary uptake compartments in four streams differing in human impacts.

Understanding the variability of the natural abundance in nitrogen stable isotopes (expressed as δ(15)N) of primary uptake compartments (PUCs; e.g., epilithon or macrophytes) is important due to the multiple applications of stable isotopes in freshwater research and can give insights into environmental and anthropogenic factors controlling N dynamics in streams. While previous research has shown how δ(15)N of PUCs varies with δ(15)N of dissolved inorganic N (DIN) among streams, less is known about how δ(15)N of PUCs varies over time. Here, we examined monthly variation of δ(15)N of PUCs and of DIN species (nitrate and ammonium) over a year, and compared it among streams with contrasting human impacts and PUC types. Our results showed no evidence of isotopic seasonal patterns. Temporal variability in δ(15)N-PUCs increased with human impact, being the highest in the urban stream, probably influenced by the high variability of δ(15)N-DIN. Among compartments, in-stream PUCs characterized by fast turnover rates, such as filamentous algae, showed the highest temporal variability in δ(15)N values (from -3.6 to 23.2 ‰). Our study elucidates some of the environmental and biological controls of temporal variability of δ(15)N in streams, which should be taken into account when using stable isotopes as an ecological tool.

Nitrogen stable isotopes in primary uptake compartments across streams differing in nutrient availability.

High variability in the natural abundance of nitrogen stable isotopes (δ(15)N) has been reported for primary uptake compartments (PUCs; e.g., epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems, but the origin of this variability is not yet well understood. We examined how δ(15)N of different PUC types relate to δ(15)N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km(2)), encompassing a gradient of human pressures from headwaters to the river valley. δ(15)N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ(15)N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel and thus using streamwater as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ(15)N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining the natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ(15)N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool.

Benthic sediment as stores and sources of bacteria and viruses in streams: A comparison of baseflow vs. stormflow longitudinal transport and residence times.

The presence of bacteria and viruses in freshwater represents a global health risk. The substantial spatial and temporal variability of microbes leads to difficulties in quantifying the risks associated with their presence in freshwater. Fine particles, including bacteria and viruses are transported and accumulated into shallow streambed (i.e., benthic) sediment, delaying the downstream transmission during baseflow conditions but contributing to their resuspension and transport downstream during stormflow events. Direct measurements of pathogen accumulation in benthic sediments are rare. Until now, the dynamic role of benthic sediment as both a store and source of microbes, has not been quantified. In this study, we analyze microbial abundance in benthic sediment along a 1 km reach of an intermittent Mediterranean stream receiving inputs from the effluent of a wastewater treatment plant, a known point source of microbes in streams. We sampled benthic sediment during a summer drought when the wastewater effluent constituted 100 % of the stream flow, and thus, large accumulation and persistence of pathogens along the streambed was expected. We measured the abundance of total bacteria, Escherichia coli (as a fecal indicator), and presence of enteric rotavirus (RoV) and norovirus (NoV). The abundance of E. coli, based on qPCR detection, was high (4.99∙102 gc /cm2) along the first 100 m downstream of the wastewater effluent input and in general decreased with distance from the source, with presence of RoV and NoV along the study reach. A particle tracking model was applied, that uses stream water velocity as an input, and accounts for microbial exchange into, immobilization, degradation, and resuspension out of benthic sediment during baseflow and stormflow. Rates of exchange into benthic sediment were 3 orders of magnitude higher during stormflow, but residence times were proportionately lower, resulting in increased longitudinal connectivity from up to downstream during stormflow. Model simulations demonstrated mechanistically how the rates of exchange into and out of the benthic sediment resulted in benthic sediment to act as a store during baseflow and a source during stormflow.

Individual and population-scale carbon and nitrogen isotopic values of Procambarusclarkii in invaded freshwater ecosystems.

Background: Freshwater ecosystems are amongst the most threatened habitats on Earth; nevertheless, they support about 9.5% of the known global biodiversity while covering less than 1% of the globe's surface. A number of anthropogenic pressures are impacting species diversity in inland waters and, amongst them, the spread of invasive alien species is considered one of the main drivers of biodiversity loss and homogenisation in freshwater habitats.Crayfish species are widely distributed freshwater invaders and, while alien species introductions occur mostly accidentally, alien crayfish are often released deliberately into new areas for commercial purposes. After their initial introduction, crayfish species can rapidly establish and reach high-density populations as a result of their adaptive functional traits, such as their generalist diet.The Louisiana crayfish Procambarusclarkii (Girard, 1852) is globally considered one of the worst invaders and its impact on recipient freshwater communities can vary from predation and competition with native species, to modification of food webs and habitat structure and introduction of pathogens. Native to the south United States and north Mexico, P.clarkii has been introduced in Europe, Asia and Africa, determining negative ecological and economic impacts in the majority of invaded habitats where it became dominant within the receiving benthic food webs. Due to its flexible feeding strategy, P.clarkii exerts adverse effects at different trophic levels, ultimately affecting the structure and dynamics of invaded food webs. It is, therefore, paramount to evaluate the ecological consequences of P.clarkii invasion and to quantify its impact in a spatially explicit context. New information: In the past decades, the analysis of stable isotopes of carbon, nitrogen and other elements has become a popular methodology in food web ecology. Notably, stable isotope analysis has emerged as a primary tool for addressing applied issues in biodiversity conservation and management, such as the assessment of the trophic ecology of non-indigenous species in invaded habitats. Here, we built two geo-referenced datasets, resolved respectively at the population and individual scale, by collating information on δ13C and δ15N values of P.clarkii within invaded inland waters. The population-scale dataset consists of 160 carbon and nitrogen isotopic values of the Louisiana crayfish and its potential prey, including living and non-living primary producers and benthic invertebrates. The dataset resolved at individual scale consists of 1,168 isotopic records of P.clarkii. The isotopic values included within the two datasets were gathered from 10 countries located in Europe, Asia, Africa and North America, for a total of 41 studies published between 2005 and 2021. To the best of the authors' knowledge, this effort represents the first attempt to collate in standardised datasets the sparse isotopic information of P.clarkii available in literature. The datasets lend themselves to being used for providing a spatially explicit resolution of the trophic ecology of P.clarkii and to address a variety of ecological questions concerning its ecological impact on recipient aquatic food webs.

Phragmites australis as a dual indicator (air and sediment) of trace metal pollution in wetlands - the key case of Flix reservoir (Ebro River).

Evaluation of trace metal pollution in an environmentally complex context may require the use of a suite of indicators. Common reed, Phragmites australis, is a well-known biomonitor of sediment pollution. Here, we show its potential for also assessing air pollution. The plant panicles, holding silky hairs with high surface to volume ratio, are appropriate collectors of atmospheric contaminants, which perform independently from root bioconcentration. We applied the dual value of common reed as an indicator of trace metal pollution to the case of a chlor-alkali plant in the Ebro river bank (Spain). This factory had historically damped waste to the shallow Flix reservoir. Extensive common reed meadows are growing on the top of the waste, in a nearby nature reserve across the reservoir and a meander immediately downriver. Three replicated individuals from a total of 11 sites were sampled, and the trace metal content measured in the main plant compartments (roots, rhizomes, stems, leaves, and panicles). Panicles and roots showed a much larger concentration of trace metals than the other plant compartments. Levels of Hg, Cu, and Ni were markedly higher in panicles at the factory and nearby points of the reserve and lowered at the meander. In contrast, Cd, Zn, and Mn in roots increased from the factory to the meander downriver. We conclude that panicles show recent (less than a year) airborne pollution, whereas roots indicate the long-term transport of pollutants from the waste in the shoreline of the factory to downriver sedimentation hotspots, where they become more bioavailable than in the factory waste. The Hg spatial pattern in panicles agree with air measurements in later years, therefore, confirming the panicles suitability for assessing airborne pollution and, consequently, Phragmites as a potential dual biomonitor of air and sediments.

Wastewater treatment plant effluent inputs induce large biogeochemical changes during low flows in an intermittent stream but small changes in day-night patterns.

Wastewater treatment plant (WWTP) effluents alter stream water chemistry and metabolic activity. Yet, essential aspects influencing the biogeochemical response of receiving streams such as hydrology and diel oscillations of light and temperature remain largely unexplored. We measured day vs night water chemistry and in-stream net nutrient uptake velocity (Vf) in an intermittent forested stream, upstream and downstream of a WWTP effluent under contrasting hydrological conditions. The WWTP effluent negatively influenced stream water chemistry, especially during the dry period. Despite large diel oscillations in light inputs, day-night differences in nutrient and oxygen concentrations were small, suggesting that heterotrophic respiration drove stream metabolism with a minor contribution of gross primary production. The magnitude of Vf was similar between day and night at the two reaches. Yet, at the downstream reach, in-stream net DIN uptake occurred more often at night, and values of Vf for ammonia and nitrite indicated enhanced in-stream nitrification. The two reaches showed a small capacity to retain DIN and soluble reactive phosphorus from the water column. Positive values of in-stream net nutrient uptake (i.e. uptake > release) occurred mostly during the dry period, highlighting that in-stream biogeochemical processing can contribute to improve water quality in streams receiving point-sources effluents in regions with low water availability.

Non-native Minnows Threaten Quillwort Populations in High Mountain Shallow Lakes.

Submersed aquatic plants are a key component of shallow, clear water lakes contributing to primary production and water quality. High mountain lakes are naturally fishless although invasive trout and most recently minnows have been introduced causing a major impact on fauna richness. The Pyrenean high mountain range has preserved soft-water oligotrophic boreal isoetids in their southern limit of distribution but the recent fish introduction is a potential factor of stress that needs to be addressed. We here work under the hypothesis that due to contrasting ecological features, trout will not be heavily affecting quillwort populations while minnows will have a stronger effect on zooplankton and zoobenthos that will promote algal growth and reduce light availability for the underwater meadows. Ten Pyrenean shallow lakes representative of three scenarios -fishless, with trout and with minnows-, were sampled for meadow structure, water column and benthic environment characterization in mid-summer 2015 and 2016. Quillwort biomass allocation (above vs. belowground), epiphytic load, and composition of the algal community (abundant cyanobacteria) differed in the presence of minnows. In trout lakes biomass allocation and epiphytic load were average and the algal community composed by chlorophytes and diatoms as in fishless lakes. Biomass ratio was close to thresholds of negative buoyancy in minnow lakes indicating that meadows were at risk of uprooting and consequent de-vegetation. Total and soluble carbohydrates were lower and the sporangia contained significantly less reserves to constrain growth and expansion in the presence of minnows. Lake scenarios were coupled to physicochemical differences with low light, high phosphorus and Chl-a (mesotrophia) in minnow lakes, while trout and fishless lakes remained oligotrophic. This is the first study assessing the impact of non-native fish on soft-water isoetids from mountain lakes and shows that minnows are a major threat to quillworts. The impaired light environment (from epiphytic algal overgrow and water column Chl-a) entails consequent regression (i.e., no recruitment) and de-vegetation (uprooting) of the meadows. Since soft-water oligotrophic mountain lakes are protected under the Habitats Directive, some action needs to be urgently implemented not only to preserve quillworts but to the overall ecological integrity of the lakes.

Enhancement of carbon and nitrogen removal by helophytes along subsurface water flowpaths receiving treated wastewater.

Wastewater treatment plant (WWTP) effluents are sources of dissolved organic carbon (DOC) and inorganic nitrogen (DIN) to receiving streams, which can eventually become saturated by excess of DIN. Aquatic plants (i.e., helophytes) can modify subsurface water flowpaths as well as assimilate nutrients and enhance microbial activity in the rhizosphere, yet their ability to increase DIN transformation and removal in WWTP-influenced streams is poorly understood. We examined the influence of helophytes on DIN removal along subsurface water flowpaths and how this was associated with DOC removal and labile C availability. To do so, we used a set of 12 flow-through flumes fed with water from a WWTP effluent. The flumes contained solely sediments or sediments with helophytes. Presence of helophytes in the flumes enhanced both DIN and DOC removal. Experimental addition of a labile C source into the flumes resulted in a high removal of the added C within the first meter of the flumes. Yet, no concomitant increases in DIN removal were observed. Moreover, results from laboratory assays showed significant increases in the potential denitrifying enzyme activity of sediment biofilms from the flumes when labile C was added; suggesting denitrification was limited by C quality. Together these results suggest that lack of DIN removal response to the labile C addition in flumes was likely because potential increases in denitrification by biofilms from sediments were counterbalanced by high rates of mineralization of dissolved organic matter. Our results highlight that helophytes can enhance DIN removal in streams receiving inputs from WWTP effluents; and thus, they can become a relevant bioremediation tool in WWTP-influenced streams. However, results also suggest that the quality of DOC from the WWTP effluent can influence the N removal capacity of these systems.

Land plants of amphibious Littorella uniflora (L.) Aschers. maintain utilization of CO2 from the sediment.

Submerged macrophytes of the isoetid life form derive the majority of their CO2 for photosynthesis from the sediment. The experiments described here were designed to test the hypothesis that root uptake of CO2 is important also in the terrestrial form of Littorella uniflora. The results of 14CO2 experiments showed that sediment CO2 contributed 56% of the total fixation at 0.1MM CO2 in the rhizosphere, 83% at 0.5MM and 96% at 2.5MM. Sediment CO2 in emergent Littorella stands ranged from 0.1 to 1.0MM and averaged 0.5MM. Measurements of the net CO2 exchange over the leaves showed an even higher dependence of the sediment as CO2 source. Littorella leaves had no stomata at the base and densities (ca. 100 mm-2) typical of terrestrial plants at the tip, allowing sediment-derived CO2 to be supplied along the length of the leaf. The stomata permit supply of CO2 from the air during periods of reduced sediment CO2 concentrations (e.g. if the sediment dries up) and regulate transpiration.

Differential accumulation of mercury and other trace metals in the food web components of a reservoir impacted by a chlor-alkali plant (Flix, Ebro River, Spain): Implications for biomonitoring.

Comparative studies of biomonitors of trace metal contamination are relatively scarce. We took advantage of a point source pollution in a reservoir (Flix, Spain) to compare trace metal (Hg, Pb, Cd, Se, As, Zn, Cu, Cr) bioaccumulation patterns among 16 food web components. Our results indicate that most organisms are suitable for Hg biomonitoring, whereas other metals are better monitored by only some of them. Biofilms and zebra mussel were the organisms with larger and more diverse biomonitoring capacity. However, we show that using groups of biomonitors increase the scope and strengths of the conclusions and specific goals can be better addressed. We conclude providing an overview of the strengths and weaknesses of the main organisms considered for biomonitoring trace metals in rivers and reservoirs.