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Eutrophication has a profound impact on ecosystems worldwide. Grass carp Ctenopharyngodon idella, an herbivorous fish, has been introduced to control aquatic plant overgrowth caused by eutrophication, but could have other, potentially detrimental, effects. We used the Po di Volano basin (south of the Po River delta, northern Italy) as a test case to explore whether grass carp effects on canal aquatic vegetation could be at the root of historical changes in N loads exported from the basin to the Goro Lagoon. We modeled the aquatic vegetation production and standing crop, its denitrification potential, and its consumption by introduced grass carp. We then examined whether changes in historical nitrogen loads matched the modeled losses of the drainage network denitrification function or other changes in agricultural practices. Our results indicate that introduced grass carp could completely remove submerged vegetation in the Po di Volano canal network, which could – in turn – lead to substantial loss of the denitrification function of the system, causing in an increase in downstream nitrogen loads. A corresponding increase, matching both timing and magnitude, was detected in historical nitrogen loads to the Goro Lagoon, which were significantly different before and after the time of modeled collapse of the denitrification function. This increase was not clearly linked to watershed use or agricultural practices, which implies that the loss of the denitrification function through grass carp overgrazing could be a likely explanation of the increase in downstream nitrogen loads. Perhaps for the first time, we provide evidence that a freshwater fish introduction could have caused long-lasting changes in nutrient dynamics that are exported downstream to areas where the fish is not present.
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... The drivers of macroalgal blooms in the Sacca di Goro were studied for nearly three decades and were associated to anthropogenic nutrient loads (Viaroli et al., 2018), nutrient recycling by clams (Bartoli et al., 2001Naldi et al., 2020) and introduction of alien species (Milardi et al., 2020). This study provides evidence of a new possible driver of algal blooms. ...
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Hydrological extremes of unusually high or low river discharge may deeply affect the biogeochemistry of coastal lagoons, but the effects are poorly explored. In this study, microbial nitrogen processes were analyzed through intact core incubations and ¹⁵N-isotope addition at three sites in the eutrophic Sacca di Goro lagoon (Northern Adriatic Sea) both under high discharge (spring) and after prolonged low discharge (late-summer) of the main freshwater inputs. Under high discharge/nitrate load, denitrification was the leading process and there was no internal recycling. The site located at the mouth of the main freshwater input and characterized by low salinity exhibited the highest denitrification rate (up to 1150 ± 81 μmol N m-2 h-1), mostly sustained by nitrification stimulated by burrowing macrofauna. In contrast, we recorded high internal recycling under low discharge, when denitrification dropped at all sites due to low nitrate concentrations, reduced bioturbation and nitrification. The highest recycling was measured at the sites close to the sea entrance and characterized by high salinity and particularly at the clams cultivated area (up to 1003 ± 70 μmol N m-2 h-1). At this site, internal recycling was sustained by ammonification of biodeposits, bivalve excretion and dissimilatory nitrate reduction to ammonium (DNRA), which represented 30% of nitrate reduction. Flash floods and high nitrate loads may overwhelm the denitrification capacity of the lagoon due to the reduced residence time and to the saturation of microbial enzymatic activity, resulting in high transport of nitrate to the sea. Prolonged dry periods favor large internal recycling, due to a combination of high temperatures, low oxygen solubility and low bioturbation, which may prolong the extent of algal blooms with negative effects on lagoon biogeochemical services. We conclude that hydrological extremes, which are expected to become more frequent under climate change scenarios, strongly alter N cycling in coastal sediments.
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Nitrate pollution remains an unsolved issue worldwide, causing serious effects on water quality and eutrophication of freshwater and brackish water environments. Its economic costs are still underestimated. To reduce nitrogen excess, constructed wetlands are usually recognized as a solution but, in recent years, interest has been raised in the role of ditches and canals in nitrogen removal. In this study, we investigated the environmental and economical sustainability of nitrogen removal capacity, using as a model study a lowland agricultural sub-basin of the Po River (Northern Italy), where the role of aquatic vegetation and related microbial processes on the mitigation of nitrate pollution has been extensively studied. Based on the Life Cycle Assessment (LCA) approach and costs and benefits analysis (CBA), the effectiveness of two different scenarios of vegetation management, which differ for the timing of mowing, have been compared concerning the nitrogen removal via denitrification and other terms of environmental sustainability. The results highlighted that postponing the mowing to the end of the vegetative season would contribute to buffering up to 90% of the nitrogen load conveyed by the canal network during the irrigation period and would reduce by an order of magnitude the costs of eutrophication potential.
Article
Although one of the most evident effects of biological invasions is the loss of native taxonomic diversity, contrasting views exist on the consequences of biological invasions on native functional diversity. We investigated this topic using Mediterranean stream, river and canal fish communities as a test case, at 3734 sites in Italy, and distinguishing between exotic and translocated species invasion in three different faunal districts. Our results clearly confirmed that introduced species were widespread and in many cases the invasion was severe (130 communities were completely composed by introduced species). Exotic and translocated fish species had substantially different geographical distribution patterns, perhaps arising from their differences in introduction timing, spread and invasion mechanisms. We also found a clear decreasing trend of functional dispersion along an invasion gradient, confirming our hypothesis that the invasion process can diminish the relative diversity of ecofunctional traits of host fish communities. Furthermore, our results suggested that exotic species might have a greater negative effect than translocated species on the relative diversity of ecofunctional traits of fish communities. This could also be linked to the fact that translocated species are more ecofunctionally similar to native ones, compared to the exotics. Our multivariate analysis of site-specific combinations of ecofunctional traits highlighted some traits characteristic of all invaded communities, while our discriminant analysis underlined how there was a substantial ecofunctional overlap between native, exotic and translocated species groups in most areas.
... These ecosystem shifts might not have been investigated in the past but could be reconstructed through modeling with modern techniques rooted in historical data (see e.g. Milardi et al., 2016;Milardi et al., 2019b), arguably widening and strengthening our understanding of overlooked ecosystem processes. ...
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We investigated whether the introduction of fish (brown trout, Salmo trutta) in previously fishless boreal lakes could alter the aquatic insect subsidy and affect the nesting population of insectivorous migratory birds, with a particular focus on the pied flycatcher, Ficedula hypoleuca, which is the most represented species. According to our paleolimnological data and bioenergetic model, introduced trout increased the adult midge biomass output from 18.6 to 28.7 kg (+54.6%), which was energetically significant and could have theoretically supported an increase in insectivorous birds nesting around the lake. While the long-term series of insectivorous birds nesting around the lake seemed to increase according to the modeled fish effects, we could not establish a clear link with changes in food availability. Our control (pied flycatchers) showed similar patterns as our treatment, suggesting that this species' abundance was affected by factors other than fish presence in the lake. The demographic fluctuations and trends of the pied flycatcher at the regional scale seemed to override cascading effects linking introduced fish, emerging midges, and insectivorous birds. We conclude that further studies will be needed to investigate this topic and propose some areas for future research.
... In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. Increased turbidity would also favor exotic species that thrive in turbid environments (see Milardi and Castaldelli, 2018, Lanzoni et al., 2018, Milardi et al., 2018a, Gavioli et al., 2018, Milardi et al., 2018b. Our bioenergetic model suggested that egestion and excretion could contribute to the increase in N loads, but likely played a minor role (see Supplementary Table 2). ...
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The invasion of exotic species is one of the main threats to worldwide biodiversity and can be aided by changes in environmental conditions. We hypothesized that a temporal trend of decreasing discharge and increasing temperature might have favored the invasion of warm-adapted, lentic exotic fish species in the lower Po River, northern Italy. We used presence/absence data over a long-term period (over 20 years) to investigate the dynamics of exotic fish invasion along water temperature and discharge gradients. Mean annual discharge and temperature did not show a clear trend and did not affect exotic fish species invasion, which progressed with time irrespective of these factors. The total number of species fluctuated without a clear trend, which underlined a progressive substitution of native species with exotic ones. Perhaps surprisingly, the community composition changed over time towards more temperature tolerant but also rheophilic, benthivore and generalist fish species. These results highlight how species interactions could be one of the main factors driving the invasion. Furthermore, our data underlines a continuously rising tide of exotics, which questions the success of past control strategies. Considering the current conservation resources limitations, priority should be given to the development of prevention strategies in order to avoid new species introductions.
... In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. Increased turbidity would also favor exotic species that thrive in turbid environments (see Milardi and Castaldelli, 2018, Lanzoni et al., 2018, Milardi et al., 2018a, Gavioli et al., 2018, Milardi et al., 2018b. Our bioenergetic model suggested that egestion and excretion could contribute to the increase in N loads, but likely played a minor role (see Supplementary Table 2). ...
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The implementation of the European Water Framework Directive, especially regarding the establishment of fish indexes for riverine habitats, has taken different paths in different countries. For example, in Italy previous efforts have been directed towards a taxonomy-based index, contrarily to most other European countries where an ecofunctional approach took place. Taxonomical indexes are particularly hard to apply to Mediterranean countries, where fish taxonomy is often revised causing problems in practical implementation. Alternatively, ecofunctional characteristics of fish communities could be exploited to inform on river habitat quality and to detect anthropogenic impacts, thus reducing the index sensitivity to the taxonomical variability of the fish fauna. We therefore proposed a new, multimetric index based on ecofunctional traits of fish species (EFFI, EcoFunctional Fish Index) and tested it on 208 river sampling stations of the Emilia-Romagna region, northern Italy. Using theoretical reference communities, ecological quality ratios were estimated for the whole area expressing the ecological distance of each site from reference conditions. Perhaps unsurprisingly, this work underlined how fish communities were more degraded at lower altitudes than at higher ones. EFFI scores were remarkably close to two already-established indexes for chemical (LIM) and macrozoobenthos communities (IBE) alteration. Further work should explore the validity of this approach over a wider geographical range as well as investigate the definition of environmental class boundaries and its potential intercalibration with other indexes.
... Among watershed changes, the loss of natural buffers against the accumulation of excessive nitrogen (N) loads could be of paramount importance (Hansen et al., 2018;Hill, 2019). Denitrification, the reduction of nitrate (NO 3 À ) to nitrogen gas (mostly N 2 ) performed by microbial communities under anaerobic conditions, is one of the most important mechanisms of N removal and can take place wherever an anoxic environment, and availability of substrates (NO 3 À and organic carbon) allow it. ...
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Intensively managed row crop agriculture has fundamentally changed Earth surface processes within the Mississippi River basin through large-scale alterations of land cover, hydrology and reactive nitrogen availability. These changes have created leaky landscapes where excess agriculturally derived nitrate degrades riverine water quality at local, regional and continental scales. Individually, wetlands are known to remove nitrate but the conditions under which multiple wetlands meaningfully reduce riverine nitrate concentration have not been established. Only one region of the Mississippi River basin—the 44,000 km² Minnesota River basin—still contains enough wetland cover within its intensively agriculturally managed watersheds to empirically address this question. Here we combine high-resolution land cover data for the Minnesota River basin with spatially extensive repeat water sampling data. By clearly isolating the effect of wetlands from crop cover, we show that, under moderate–high streamflow, wetlands are five times more efficient per unit area at reducing riverine nitrate concentration than the most effective land-based nitrogen mitigation strategies, which include cover crops and land retirement. Our results suggest that wetland restorations that account for the effects of spatial position in stream networks could provide a much greater benefit to water quality then previously assumed.
... Piccoli and Gerdol, 1981), fish introductions (e.g. Lanzoni et al., 2018) and their effects on biodiversity (e.g. Castaldelli et al., 2013a, Milardi et al., 2019a. ...
... In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. Increased turbidity would also favor exotic species that thrive in turbid environments (see Milardi and Castaldelli, 2018, Lanzoni et al., 2018, Milardi et al., 2018a, Gavioli et al., 2018, Milardi et al., 2018b. Our bioenergetic model suggested that egestion and excretion could contribute to the increase in N loads, but likely played a minor role (see Supplementary Table 2). ...
Article
Full-text available
The aim of the study is to present a complete and updated fish inventory of inland waters of the Emilia-Romagna region, Northern Italy, and to highlight the presence of fully exotic fish communities. Overall, based on 208 sampling locations, the observed fish fauna consisted of 45 species, 22 native and 23 exotics. A significant element of the inventory is the identification of xenodiversity hotspots (spatially clustered sites, one lowland and one upland region), where a complete substitution of native species by exotic species was observed (in total seven sites in the lowland and two sites in the upland with no native species presence). These xenodiversity hotspots were found to host specific combinations of exotic species, which may be able to constitute balanced exotic communities. The hotspots of the lowland region are located in the northeast lowland part of the territory, hosting exotic species combinations mainly composed by wels catfish (Silurus glanis Linnaeus, 1758, a large predator), common carp (Cyprinus carpio Linnaeus, 1758, a large benthivore), crucian carp (Carassius spp., a small-bodied generalist) and other less dominant exotic species. The hotspots in the upland region were located in the southwest part of the territory and were dominated by only one exotic species (rainbow trout, Oncorhynchus mykiss (Walbaum, 1792)). A difference between these xenodiversity hotspots is that in the lowland the introductions were mostly unintentional and are not continued, while in the upland the introduction of rainbow trout is intentional and currently carried out by local fishermen.
... Grass carp Ctenopharyngodon idella (Valenciennes, 1844) is a prime example of such an herbivorous fish; originally from Asia, it has been widely introduced to control excessive or undesirable aquatic vegetation (Kelly et al., 2011;Wittmann et al., 2014). The ''ecosystem engineering" capabilities of grass carp can provoke unwanted effects through altering the abundance of submerged vegetation, decreasing available spawning habitats for native species, increasing turbidity and ultimately favoring other introduced species (Milardi et al., 2018a). Furthermore, an increase of several dissolved N species has been observed following vegetation control with grass carp, partly as a result of sediment resuspension and fecal matter deposition (reviewed in e.g. ...
... In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. Increased turbidity would also favor exotic species that thrive in turbid environments (see Milardi and Castaldelli, 2018, Lanzoni et al., 2018, Milardi et al., 2018a, Gavioli et al., 2018, Milardi et al., 2018b. Our bioenergetic model suggested that egestion and excretion could contribute to the increase in N loads, but likely played a minor role (see Supplementary Table 2). ...
... Denitrification is currently achieved through water treatment plants, which have a major infrastructural and operational cost (Boerema et al., 2014). While fisheries for grass carp have been established, fishing license sales in the region have actually declined by approximately 90% over this period (from 240,000 units in early 1980s to 25,000 units in early 2010s, Emilia-Romagna Region, personal communication), perhaps due to the decline of preferred native fishes (Milardi et al., 2018a;Castaldelli et al., 2013a). Future studies should strive to use modern tools to evaluate costs and benefits in a holistic framework, which could be used by authorities to formulate improved management actions. ...
Article
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While the significance of anthropogenic pressures in shaping species distributions and abundances is undeniable, some ambiguity still remains on their relative magnitude and interplay with natural environmental factors. In our study, we examined 91 late-invasion-stage river locations in Northern Italy using ordination methods and variance partitioning (partial-CCA), as well as an assessment of environmental thresholds (TITAN), to attempt to disentangle the effects of eutrophication and exotic species on native species. We found that exotic species, jointly with water quality (primarily eutrophication) and geomorphology, are the main drivers of the distribution of native species and that native species suffer more joint effects than exotic species. We also found that water temperature clearly separates species distributions and that some native species, like Italian bleak (Alburnus alborella) and Italian rudd (Scardinius hesperidicus), seem to be the most resilient to exotic fish species. We also analyzed the dataset for nestedness (BINMATNEST) to identify priority targets of conservation. As a result, we confirmed that altitude correlated negatively with eutrophication and nestedness of exotic species and positively with native species. Overall, our analysis was able to detect the effects of species invasions even at a late invasion stage, although reciprocal effects seemed comparable at this stage. Exotic species have pushed most native species on the edge of local extinction in several sites and displaced most of them on the rim of their natural distribution. Any potential site- and species-specific conservation action aimed at improving this situation could benefit from a carefully considered prioritization to yield the highest results-per-effort and success rate. However, we encourage future research to update the information available before singling out specific sites for conservation or outlining conservation actions.
... The irrigation season lasts from April to September, when the canals are flooded through a series of siphons that draw water from the Po River, which is a likely trigger for spawning by grass carp (Milardi et al., 2017). The water is delivered from irrigation canals to arable lands through a capillary ditch network; water level in the canals remains relatively stable through a complex management of the drainage system. ...
Article
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Bighead carp (Hypophthalmichthys nobilis) have been introduced throughout Europe, mostly unintentionally, and little attention has been given to their potential for natural reproduction. We investigated the presence of young-of-the-year bighead carp in an irrigation canal network of Northern Italy and the environmental conditions associated with spawning in 2011–2015. The adult bighead carp population of the canal network was composed by large, likely mature, individuals with an average density of 45.2 kg/ha (over 10 fold more than in the main river). The 29 juvenile bighead carp found were 7.4–13.1 cm long (TL) and weighed 9.5–12.7 g. Using otolith-derived spawning dates we estimated that these juveniles were 94–100 days old, placing their fertilization and hatch dates in mid-to-end-June. Using this information in combination with thermal and hydraulic data, we examined the validity of existing models predicting the onset of spawning conditions and the viability of egg pathways to elucidate spawning location of the species. While evidence of reproduction was not found every year, we determined that potentially viable spawning conditions (annual degree-days and temperature thresholds) and pathways of egg drift suitable for hatching are present in short, slow-flowing canals.
... Eutrophication affects freshwater ecosystems more directly, because they are closely connected to agricultural landscapes, but eventually nutrients are carried by rivers to estuarine, coastal and gulf areas, where they can also cause problems (e.g. the Gulf of Mexico, the Baltic Sea, Lake Erie: Rönnberg and Bonsdorff, 2004;Rabalais et al., 2002;Kane et al., 2014). Eutrophication can have a direct effect on algal and aquatic vegetation communities, and cause loss of species and linked ecosystem services (Glibert, 2017). Nutrient loading to freshwaters is strongly influenced by human population within the watershed and by intensity and type of farming practices (Billen et al., 2013). ...
Article
Full-text available
Eutrophication is a complex process and often associated with not only a change in overall algal biomass but also with a change in biodiversity. Common metrics of eutrophication (e.g., chlorophyll a), total nitrogen (TN) and phosphorus (TP) are not adequate for understanding biodiversity changes, especially those associated with harmful algal bloom (HAB) proliferations. Harmful algae can increase disproportionately with eutrophication, depending on which nutrients change and in what proportion. This paper challenges several classic paradigms in our understanding of eutrophication and associated biodiversity changes. The underlying message is that nutrient proportions and forms can alter biodiversity, even when nutrients are at concentrations in excess of those considered limiting. The global HAB problem is on a trajectory for more blooms, more toxins, more often, in more places. Our approach to management of HABs and eutrophication must consider the broader complexity of nutrient effects at scales ranging from physiological to ecological.
... Bare sediments were assigned denitrification rates based on experimental studies performed in the Po di Volano basin (average 53.5 (47-60) mg N per square meter per day; Castaldelli et al. 2015). Sediments colonized by submerged macrophytes were assigned rates measured in freshwater environments of the Po River plain similar to the Po di Volano canals for trophic conditions (in particular water NO 3 À availability) and substrate and accounting for seasonal variations (average 133 (98-168) mg N per square meter per day in April-May and average 259 (140-378) mg N per square meter per day in June-September; Nizzoli et al., 2014, Pinardi et al., 2009, Racchetti et al., 2017. We used this model to estimate the magnitude of the bottom sediment denitrification sink at baseline conditions during the 6-month period when the ditch network is active for irrigation (a period overlapping with the vegetative phase of submerged macrophytes) and for the 1986-1989 period. ...
Article
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Riverine ecosystems receive nitrogen loads from point and diffuse sources that are transferred downstream. Such loads may undergo poorly explored retention and dissipation processes, varying along gradients of nitrogen availability due to different interactions among primary producers and microbial communities. We measured carbon uptake and nitrogen fluxes in microcosms containing riverine sediments with benthic algae and submerged macrophytes (Vallisneria spiralis L.). Coupled nitrification–denitrification rates were determined via 15N–NH4+ injection in the pore water and quantification of the produced 29N2 and 30N2. Two sites with different N–NO3− concentration and sediment organic content were investigated. We hypothesized that: (1) N–NO3− availability promotes water column N uptake and attenuates primary producers-bacteria competition; (2) coupled nitrification–denitrification is stimulated by radial oxygen loss from roots; (3) macrophyte meadows favour both temporary nitrogen retention and permanent removal. Sediments with V. spiralis were mostly inorganic C and N sinks and always displayed higher coupled nitrification–denitrification rates compared to sediments with microphytobenthos. Highest rates, up to 100 µmol N m−2 h−1, were measured at the more eutrophic site and in the light. This is likely due to a shift from root to leaf uptake at the N–NO3− rich site, attenuating plant-bacteria competition, and to increased radial oxygen loss in organic-rich sediments, stimulating nitrification. High rates of N uptake and loss in lotic sediments represent natural buffers preventing N transport downstream and stress the need to preserve aquatic vegetation and its ecosystem services.
... The Maumee and Sandusky Rivers are primary nutrient sources for this basin and their associated estuaries host large vegetated areas. Grass carp spawning (Embke et al., 2016) and recruitment (Chapman et al., 2013) have recently been identified in those rivers. Grass carp grazing in these rivers might exacerbate eutrophication through denitrification function loss. ...
Article
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Grass carp (Ctenopharyngodon idella), an invasive species of Asian carp, has been stocked for many decades in the United States for vegetation control. Adult individuals have been found in all of the Great Lakes except Lake Superior, but no self-sustaining populations have yet been identified in Great Lakes tributaries. In 2012, a commercial fisherman caught four juvenile diploid grass carp in the Sandusky River, a major tributary to Lake Erie. Otolith microchemistry and the capture location of these fish permitted the conclusion that they were most likely produced in the Sandusky River. Due to this finding, we sampled ichthyoplankton using paired bongo net tows and larval light traps during June–August of 2014 and 2015 to determine if grass carp are spawning in the Sandusky River. From the samples collected in 2015, we identified and staged eight eggs that were morphologically consistent with grass carp. Five eggs were confirmed as grass carp using quantitative Polymerase Chain Reaction for a grass carp-specific marker, while the remaining three were retained for future analysis. Our finding confirms that grass carp are naturally spawning in this Great Lakes tributary. All eggs were collected during high-flow events, either on the day of peak flow or 1–2 days following peak flow, supporting an earlier suggestion that high flow conditions favor grass carp spawning. The next principal goal is to identify the spawning and hatch location(s) for the Sandusky River. Predicting locations and conditions where grass carp spawning is most probable may aid targeted management efforts.
... These ecosystem shifts might not have been investigated in the past but could be reconstructed through modeling with modern techniques rooted in historical data (see e.g. Milardi et al., 2016;Milardi et al., 2019b), arguably widening and strengthening our understanding of overlooked ecosystem processes. ...
Article
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The additional input and enhanced cycling of nutrients derived from introduced fish can be a significant factor altering nutrient dynamics in oligotrophic lakes. To test this, we used a bioenergetic model to estimate the fish-derived nutrient load in Lake Kuutsjärvi, a historically fishless boreal lake of northern Fennoscandia. The lake was selected because of the absence of other anthropogenic stressors, a known stocking history and the possibility of quantitatively estimating the size-structure and biomass of the fish population through a mass removal. Subsequently, we used a mass balance model to compare fish-derived nutrients with other nutrient load pathways. For comparison over longer timescales, we used lake sediment records of diatoms, chlorophyll and carotenoid pigments, C:N ratios and stable isotopes to infer whether fish introduction produced detectable changes in the lake trophic state, primary productivity and terrestrial nutrient input. Based on the nutrient mass balance model, we found that phosphorus and nitrogen derived from fish were 0.46% and 2.2%, respectively, of the total load to the lake, suggesting that fish introduction could not markedly increase the nutrient load. Accordingly, the palaeolimnological record indicated little increase in primary production but instead a shift from pelagic to benthic production after fish introduction.
... We provide evidence that a freshwater fish introduction could have caused cascading and long-lasting changes in nutrient dynamics, with detrimental ecological consequences exported downstream, far from where grass carp were present. Grass carp is a long-lived species (over 25 years, Clemens et al. (2016)) and our model indicates that effects on submerged macrophytes would persist, as individuals grow larger and increase consumption, even accounting for mortality losses (Osborne and Riddle, 1999), with a minimal effect on temporary N storage. Further grass carp stocking took place in the early 1990s (Melotti et al., 1987;Milardi, unpublished data) and could not be estimated during this study, but might have further contributed to these effects until modern times. ...
Article
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Grass carp Ctenopharyngodon idella are sometimes used as a biological tool for managing aquatic vegetation in reservoirs. Sterile, triploid fish were stocked in Devils Lake, Oregon, during 1986, 1987, and 1993 to control aquatic vegetation. We present a case study for using multiple measures on the same fish to determine whether illegal stocking of fertile, diploid grass carp occurred. An investigation into the estimated age of a dead grass carp found in Devils Lake suggested that it was significantly younger than would otherwise be expected, given the only stocking events occurred during 1986, 1987, and 1993. To determine whether illegal stocking or reproduction by presumed sterile grass carp had occurred in Devils Lake, we conducted a study that balanced the needs of lethally sampling grass carp for biological measures with the socially and politically sensitive sentiment of the pro–grass carp citizenry of Devils Lake. These considerations, in combination with a low catch per-unit effort, resulted in a modest sample size for grass carp. We sampled grass carp and recorded multiple measures for each fish. Ploidy testing of blood samples indicated the grass carp were all triploid. Based on gonadal histopathology, six fish were male, two were female, and two were sex-indeterminate with severe gonadal dysgenesis. Age estimates from lapillus otoliths were consistent with fish originating from the legal stocking events in Devils Lake. The grass carp were 21–30 y old, and we were unable to find published reports of grass carp anywhere else in the world that are older. The grass carp were significantly smaller than much younger fish from other regions. The small size of these grass carp relative to their age in Devils Lake suggests food limitations that stunted growth. The dead grass carp that was the impetus for this study was aged by anatomical structures that we have since found to be unreliable. This suggests that the dead grass carp was probably in fact older and originated from the legal stockings. The use of multiple biological measurements on a modest sample size of grass carp, combined with the knowledge that no juvenile grass carp have been observed since legal stocking occurred, lead us to conclude that the grass carp in Devils Lake are sterile fish that originated from legal stocking events.
... Grass carp was first introduced in Western Europe in the 1980s (FAO, 2016), as a mean of aquatic weed control. In Italy, the first introduction to the wild was at the end of the 1980s (Melotti et al., 1987), in the Po di Volano basin, in the southern part of the Po River delta, where it has recently been found to reproduce naturally (Milardi et al., 2015). Large-scale grass carp introduction at the end of the 1980s coincided with the onset of well-studied massive algal blooms and anoxia in the Goro Lagoon, downstream of the Po di Volano (Pugnetti et al., 1992;Viaroli et al., 1995Viaroli et al., , 1996. ...
... We thus evaluated whether grass carp introduction could be at the root of historical changes in N loads exported from the Po di Volano drainage basin, a network of artificial and semi-artificial canals in the Ferrara province (Northern Italy), to the Goro Lagoon. We focused on this area because grass carp were recently found to recruit naturally there after their introduction (Milardi et al., 2015), and because of the abundant historical information available on agricultural practices, nutrient loads (e.g. Castaldelli et al., 2013b), aquatic vegetation (e.g. ...
... When first introduced, grass carp were believed to be unable to reproduce in this area and thus were deemed an ideal candidate for biological control of vegetation, because densities could be controlled through a balance between stocking and mortality (Melotti et al., 1987). However, grass carp were recently found to recruit in this area (Milardi et al., 2015), which implies that population control might be already lost and that grass carp grazing pressure would not necessarily decrease through natural mortality. ...
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). ...
Article
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Introductions of grass carp, well known for their potentially negative ecosystem effects, have been performed in several countries around the world. As the species was considered unable to reproduce naturally under non-native environmental conditions, little attention was initially given to its invasive potential. We studied an area in northern-Italy where, contrary to expectations, introductions that were performed in the early 80s still exert a considerable pressure on aquatic macrophytes. In order to reveal whether the observed population dynamics are the result of natural events or stocking we analysed the density, age-and size-structure of the grass carp population and the migration pathways available to it. Telephone surveys were also used to check for fish transport from national and international suppliers. We also sampled potential spawning and nursery areas for young individuals and, when some were captured, we applied stable isotope analyses to discriminate their origin. We found that the population of large individuals likely originated solely from early stocking. We also documented the first analytical evidence of grass carp recruitment in the study area and, to our knowledge, in Western Europe. Therefore the species has the potential to become invasive in these areas and more detailed studies are needed to assess this potential. Further management should account at least for natural recruitment and potential negative environmental effects, controlling the species where needed.
... Nutrient loading to freshwaters is strongly influenced by human population within the watershed and by intensity and type of farming practices (Billen et al., 2013). However, eutrophication could arise not only from increases in nutrient loading, but also from changes within the watersheds, when these alter the watersheds' capacity to metabolize nutrient loads (Beaulieu et al., 2015;Pinay et al., 2002). ...
Article
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Nitrogen (N) uptake in streams is an important ecosystem service that reduces nutrient loading to downstream ecosystems. Here we synthesize studies that investigated the effects of urban stream burial on N-uptake in two metropolitan areas and use simulation modeling to scale our measurements to the broader watershed scale. We report that nitrate travels on average 18 times farther downstream in buried than in open streams before being removed from the water column, indicating that burial substantially reduces N uptake in streams. Simulation modeling suggests that as burial expands throughout a river network, N uptake rates increase in the remaining open reaches which somewhat offsets reduced N uptake in buried reaches. This is particularly true at low levels of stream burial. At higher levels of stream burial, however, open reaches become rare and cumulative N uptake across all open reaches in the watershed rapidly declines. As a result, watershed-scale N export increases slowly at low levels of stream burial, after which increases in export become more pronounced. Stream burial in the lower, more urbanized portions of the watershed had a greater effect on N export than an equivalent amount of stream burial in the upper watershed. We suggest that stream daylighting (i.e., uncovering buried streams) can increase watershed-scale N retention.
... Knowing canal surface areas, we used this value to estimate the total biomass of submerged aquatic vegetation in the basin at baseline conditions and the N pool of the standing stock by means of the vegetation N content, based on literature (i.e. a maximum 0.25% of wet biomass, see e.g. Pinardi et al., 2009, Nizzoli et al., 2014. A detailed map of the canal and ditch network was created in QGIS 2.18 by merging vector data obtained from the Emilia-Romagna Region (http://geoportale.regione.emiliaromagna.it/it) ...
... We also used the reported density to model the growth of submerged macrophytes through the irrigation season, by fitting a Gompertz curve to the existing information (i.e. a daily average vegetation growth rate of around 2%; Supplementary Fig. 1; Larson, 2007, Nizzoli et al., 2014Saunkaew et al., 2011). We estimated that, during winter when little water is present in the canals and temperatures are low, a 90% reduction to the standing crop of submerged vegetation would occur due to senescence and drying out of ample portions of the canals (Westlake, 1973). ...
... Bare sediments were assigned denitrification rates based on experimental studies performed in the Po di Volano basin (average 53.5 (47-60) mg N per square meter per day; Castaldelli et al. 2015). Sediments colonized by submerged macrophytes were assigned rates measured in freshwater environments of the Po River plain similar to the Po di Volano canals for trophic conditions (in particular water NO 3 À availability) and substrate and accounting for seasonal variations (average 133 (98-168) mg N per square meter per day in April-May and average 259 (140-378) mg N per square meter per day in June-September; Nizzoli et al., 2014, Pinardi et al., 2009, Racchetti et al., 2017. We used this model to estimate the magnitude of the bottom sediment denitrification sink at baseline conditions during the 6-month period when the ditch network is active for irrigation (a period overlapping with the vegetative phase of submerged macrophytes) and for the 1986-1989 period. ...
Article
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The influence of Potamogeton pectinatus colonisation on benthic nitrogen dynamics was studied in the littoral zone of a lowland pit lake with high nitrate concentration (~200 μM). Our hypothesis was that in aquatic environments where nitrogen availability is not limiting, colonisation by rooted macrophytes changes the dynamics of the benthic nitrogen cycle, stimulating N assimilation and denitrification and increasing the system capacity to take up external nitrogen loads. To test this hypothesis, we quantified and compared seasonal variations of light and dark benthic metabolism, dissolved inorganic nitrogen (DIN) fluxes, denitrification and N assimilation rates in an area colonised by P. pectinatus and a reference site colonised by microphytobenthos. In both areas, the benthic system was net autotrophic and a sink for DIN (2,241–2,644 mmol m−2 y−1). Plant colonisation increased nitrogen losses via denitrification by 30% compared to the unvegetated area. In contrast to what is generally observed in coastal marine systems, where the presence of rooted macrophytes limits denitrification rates, under the very high nitrate concentrations in the studied lake, both denitrification (1,237–1,570 mmol m−2 y−1) and N assimilation (1,039–1,095 mmol m−2 y−1) played important and comparable roles in the removal of DIN from the water column.
... Lanzoni et al., 2018) and their effects on biodiversity (e.g. Castaldelli et al., 2013a, Milardi et al., 2019a. We used this information to model aquatic vegetation production and standing crop, its denitrification potential, and its consumption by grass carp in the drainage network, with the aim to verify whether historical N loads were affected by grass carp or other changes in agricultural practices. ...
... N loads were calculated by interpolating linearly the N species concentrations between measurements (Kronvang and Bruhn, 1996;Letcher et al., 2002). This method, previously applied to several watercourses of this basin (Castaldelli et al., 2013a), was the most sensitive for small lowland waterways characterised by sudden changes in hydrological regime. ...
... Denitrification is currently achieved through water treatment plants, which have a major infrastructural and operational cost (Boerema et al., 2014). While fisheries for grass carp have been established, fishing license sales in the region have actually declined by approximately 90% over this period (from 240,000 units in early 1980s to 25,000 units in early 2010s, Emilia-Romagna Region, personal communication), perhaps due to the decline of preferred native fishes (Milardi et al., 2018a;Castaldelli et al., 2013a). Future studies should strive to use modern tools to evaluate costs and benefits in a holistic framework, which could be used by authorities to formulate improved management actions. ...
Article
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Freshwater ecosystems worldwide are experiencing native fish losses with severe threats to the conservation of freshwater biodiversity and ecosystem functioning, and the debate on whether the cause is biotic or abiotic disturbance is still open. Temporal variation in fish assemblages was analysed over an 18 year period in 14 waterways of the lowland backwaters of the Po River in north-eastern Italy, which are important feeding, spawning and nursery sites for native fish. In 1991, 14 native and eight exotic species were collected. In less than 20 years 10 native species underwent local extinction, three of which – Rutilus pigus, Rutilus aula, and Chondrostoma soetta – were endemic to the Padano-Veneto District in northern Italy. Ordination of the data (MDS, CLUSTER, ANOSIM, SIMPER) showed a clear temporal gradient in fish community structure. After the establishment of the exotic predator Silurus glanis, some native species significantly declined in abundance and biomass (i.e. Alburnus arborella and Scardinius erythrophthalmus) or disappeared (i.e. Rutilus aula and Tinca tinca). Moreover, exotic species Cyprinus carpio, Ameiurus melas, and Carassius auratus from previous introductions, underwent significant changes in their abundance and biomass. No correlation was found between fish community structure and water quality parameters (BIOENV). The success of exotic species, particularly S. glanis which thrived in this degraded habitat, seems to have led to the decline of native fish fauna in the canals of the lower portion of the Po River basin. Conservation strategies focusing on the containment of exotic species and habitat restoration are recommended.
... The human-mediated nutrient enrichment of natural ecosystems, commonly known as eutrophication, has a profound impact on ecosystems worldwide (Garnier et al., 2010;Paerl, 2009). Eutrophication affects freshwater ecosystems more directly, because they are closely connected to agricultural landscapes, but eventually nutrients are carried by rivers to estuarine, coastal and gulf areas, where they can also cause problems (e.g. the Gulf of Mexico, the Baltic Sea, Lake Erie: Rönnberg and Bonsdorff, 2004;Rabalais et al., 2002;Kane et al., 2014). Eutrophication can have a direct effect on algal and aquatic vegetation communities, and cause loss of species and linked ecosystem services (Glibert, 2017). ...
... Clayton and Wells, 1999;Cassani et al., 2008;Cudmore et al., 2017) have not considered the potential effect of denitrification function loss. This effect could be of substantial concern in North America's Laurentian Great Lakes, where hazardous algal blooms are becoming more frequent in Lake Erie's western basin (Kane et al., 2014). The Maumee and Sandusky Rivers are primary nutrient sources for this basin and their associated estuaries host large vegetated areas. ...
Article
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Both abiotic and biotic explanations have been proposed to explain recent recurrent nuisance/harmful algal blooms in the western basin and central basin of Lake Erie. We used two long-term (> 10 years) datasets to test (1) whether Lake Erie total phytoplankton biomass and cyanobacterial biomass changed over time and (2) whether phytoplankton abundance was influenced by soluble reactive phosphorus or nitrate loading from agriculturally-dominated tributaries (Maumee and Sandusky rivers). We found that whereas total phytoplankton biomass decreased in Lake Erie's western basin from 1970 to 1987, it increased starting in the mid-1990s. Total phytoplankton and cyanobacterial seasonal (May–October) arithmetic mean wet-weight biomasses each significantly increased with increased water-year total soluble reactive phosphorus load from the Maumee River and the sum of soluble reactive phosphorus load from the Maumee and Sandusky rivers, but not for the Sandusky River alone during 1996–2006. During this same time period, neither total phytoplankton nor cyanobacterial biomass was correlated with nitrate load. Consequently, recently increased tributary soluble reactive phosphorus loads from the Maumee River likely contributed greatly to increased western basin and (central basin) cyanobacterial biomass and more frequent occurrence of harmful algal blooms. Managers thus must incorporate the form of and source location from which nutrients are delivered to lakes into their management plans, rather than solely considering total (both in terms of form and amount) nutrient load to the whole lake. Further, future studies need to address the relative contributions of not only external loads, but also sources of internal loading.
... However, instream vegetation could also be lost through biological control (i.e. the stocking of an herbivorous species to control vegetation growth, Lodge (1991)). Grass carp Ctenopharyngodon idella (Valenciennes, 1844) is a prime example of such an herbivorous fish; originally from Asia, it has been widely introduced to control excessive or undesirable aquatic vegetation (Kelly et al., 2011;Wittmann et al., 2014). The ''ecosystem engineering" capabilities of grass carp can provoke unwanted effects through altering the abundance of submerged vegetation, decreasing available spawning habitats for native species, increasing turbidity and ultimately favoring other introduced species (Milardi et al., 2018a). ...
Article
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Intentional introductions of nonindigenous fishes are increasing globally. While benefits of these introductions are easily quantified, assessments to understand the negative impacts to ecosystems are often difficult, incomplete, or absent. Grass carp (Ctenopharyngodon idella) was originally introduced to the United States as a biocontrol agent, and recent observations of wild, diploid individuals in the Great Lakes basin have spurred interest in re-evaluating its ecological risk. Here, we evaluate the ecological impact of grass carp using expert opinion and a suite of the most up-to-date analytical tools and data (ploidy assessment, eDNA surveillance, species distribution models (SDMs), and meta-analysis). The perceived ecological impact of grass carp by fisheries experts was variable, ranging from unknown to very high. Wild-caught triploid and diploid individuals occurred in multiple Great Lakes waterways, and eDNA surveillance suggests that grass carp are abundant in a major tributary of Lake Michigan. SDMs predicted suitable grass carp climate occurs in all Great Lakes. Meta-analysis showed that grass carp introductions impact both water quality and biota. Novel findings based on updated ecological impact assessment tools indicate that iterative risk assessment of introduced fishes may be warranted.
... We used non-parametric tests (the Mann-Whitney U test and the Kolmogorov-Smirnov test on the mean and shape of the distributions, respectively) to verify changes in the N load before and after the modeled introduction effects (pre/post periods, 1980-19 89/1990-1999). Statistical analyses were run with the PAST 3.06 software (Hammer et al., 2001). ...
Article
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A comprehensive, but simple-to-use software package for executing a range of standard numerical analysis and operations used in quantitative paleontology has been developed. The program, called PAST (PAleontological STatistics), runs on standard Windows computers and is available free of charge. PAST integrates spreadsheettype data entry with univariate and multivariate statistics, curve fitting, time-series analysis, data plotting, and simple phylogenetic analysis. Many of the functions are specific to paleontology and ecology, and these functions are not found in standard, more extensive, statistical packages. PAST also includes fourteen case studies (data files and exercises) illustrating use of the program for paleontological problems, making it a complete educational package for courses in quantitative methods.
... The Maumee and Sandusky Rivers are primary nutrient sources for this basin and their associated estuaries host large vegetated areas. Grass carp spawning (Embke et al., 2016) and recruitment (Chapman et al., 2013) have recently been identified in those rivers. Grass carp grazing in these rivers might exacerbate eutrophication through denitrification function loss. ...
Article
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We use aging techniques, ploidy analysis, and otolith microchemistry to assess whether four grass carp Ctenopharyngodon idella captured from the Sandusky River, Ohio were the result of natural reproduction within the Lake Erie Basin. All four fish were of age 1+. Multiple lines of evidence indicate that these fish were not aquaculture-reared and that they were most likely the result of successful reproduction in the Sandusky River. First, at least two of the fish were diploid; diploid grass carp cannot legally be released in the Great Lakes Basin. Second, strontium:calcium (Sr:Ca) ratios were elevated in all four grass carp from the Sandusky River, with elevated Sr:Ca ratios throughout the otolith transect, compared to grass carp from Missouri and Arkansas ponds. This reflects the high Sr:Ca ratio of the Sandusky River, and indicates that these fish lived in a high strontium environment throughout their entire lives. Third, Sandusky River fish were higher in Sr:Ca ratio variability than fish from ponds, reflecting the high but spatially and temporally variable strontium concentrations of southwestern Lake Erie tributaries, and not the stable environment of pond aquaculture. Fourth, Sr:Ca ratios in the grass carp from the Sandusky River were lower in their 2011 growth increment (a high water year) than the 2012 growth increment (a low water year), reflecting the observed inverse relationship between discharge and strontiumconcentration in these rivers. We conclude that these four grass carp captured from the Sandusky River are most likely the result of natural reproduction within the Lake Erie Basin
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). ...
Article
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Grass carp (Ctenopharygodon idella) reduced the areal coverage of submersed macrophytes from 44% to 0% in a 8,100-hectare reservoir. This resulted in increased nutrient concentrations and abundance of all major phytoplankton divisions. During this seven-year study, conducted between 1979 and 1987, average annual chlorophyll a levels increased from 12–13 mg/m to 19–22 mg/m. Water clarity declined, and this was attributable to higher algal biomass and not to increases in abiotic turbidity. Summer blue-green algal density and relative abundance to the phytoplankton increased over time coincident with macrophyte elimination. Densities of rotifers, cladocerans, and total zooplankton decreased approximately 1.5 years after macrophyte removal. This decline was likely due to increased abundance of planktivorous fishes.
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. ...
... Furthermore, an increase of several dissolved N species has been observed following vegetation control with grass carp, partly as a result of sediment resuspension and fecal matter deposition (reviewed in e.g. Dibble and Kovalenko, 2009). These effects not only cascade through the food web, but as long as the population survives (through long-life, continued stocking or natural recruitment) and spreads (through natural or aided dispersion) these effects can be magnified and have basinwide consequences (Rabalais et al., 2002;Rönnberg and Bonsdorff, 2004). ...
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. ...
Article
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The exotic grass carp ( Ctenopharyngodon idella ) has been used for almost a half a century in the United States as a bi- ological agent to control and manage aquatic plants. This long-lived generalist herbivore consumes large amounts of vegetation and can considerably alter habitat and impact aquatic communities. We conducted a literature review to determine whether previous studies adequately addressed ecological impacts of grass carp and their underlying mech- anisms. Our goal was to identify strengths and limitations of ecological assessment in the literature and suggest a trajec- tory of future research. The review yielded 1,924 citations on grass carp; however, data on ecological interactions were limited, and most research emphasized the biology of grass carp or eradication of aquatic plants rather than ecological mechanisms responsible for ecosystem-wide impacts. Very few studies addressed effects on habitat complexity or com- munity-structuring processes. We provide a comprehensive tabulated overview of feeding preferences and environmen- tal impacts of grass carp. We argue that ecology is para- mount to evaluating grass carp impacts and thorough understanding of these impacts is essential for the appro- priate management of aquatic communities. Current knowledge is not sufficient to accurately predict long-term effects of grass carp on freshwater ecosystems. We advise a more cautious approach to developing guidelines for grass
... Eutrophication can have a direct effect on algal and aquatic vegetation communities, and cause loss of species and linked ecosystem services (Glibert, 2017). Nutrient loading to freshwaters is strongly influenced by human population within the watershed and by intensity and type of farming practices (Billen et al., 2013). However, eutrophication could arise not only from increases in nutrient loading, but also from changes within the watersheds, when these alter the watersheds' capacity to metabolize nutrient loads (Beaulieu et al., 2015;Pinay et al., 2002). ...
Article
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The nitrogen cycle of pre-industrial ecosystems has long been remarkably closed, in spite of the high mobility of this element in the atmosphere and hydrosphere. Inter-regional and international commercial exchanges of agricultural goods, which considerably increased after the generalization of the use of synthetic nitrogen fertilizers, introduced an additional type of nitrogen mobility, which nowadays rivals the atmospheric and hydrological fluxes in intensity, and causes their enhancement at the local, regional and global scales. Eighty-five per cent of the net anthropogenic input of reactive nitrogen occurs on only 43 per cent of the land area. Modern agriculture based on the use of synthetic fertilizers and the decoupling of crop and animal production is responsible for the largest part of anthropogenic losses of reactive nitrogen to the environment. In terms of levers for better managing the nitrogen cascade, beyond technical improvement of agricultural practices tending to increase nitrogen use efficiency, or environmental engineering management measures to increase nitrogen sinks in the landscape, the need to better localize crop production and livestock breeding, on the one hand, and agriculture and food demand on the other hand, is put forward as a condition to being able to supply food to human populations while preserving environmental resources.
... We focused on this area because grass carp were recently found to recruit naturally there after their introduction (Milardi et al., 2015), and because of the abundant historical information available on agricultural practices, nutrient loads (e.g. Castaldelli et al., 2013b), aquatic vegetation (e.g. Piccoli and Gerdol, 1981), fish introductions (e.g. ...
... The Po di Volano basin (~713 km 2 ) constitutes the terminal part of the Po River floodplain, the largest Italian alluvial plain ( Fig. 1a and b). The Po River plain and the Po di Volano basin are heavily cultivated, with >70% of the Po di Volano basin area classified as utilized agricultural land (mainly cereals and industrial crops) and livestock farming is a minor component (Castaldelli et al. (2013b) ; Fig. 1c). Surface water movement in this basin is artificially controlled, regulated by a capillary network of open-earth canals and ditches (~1300 km of artificial waterways, Fig. 1d) serving for drainage and irrigation. ...
... Currently, submerged aquatic vegetation is almost nonexistent in the canals, and emergent vegetation has been actively managed since the mid-1990s (e.g. mechanical removal; Castaldelli et al., 2013b, Pierobon et al., 2013 and is now present only in stretches of the canal network at low flood risk, representing less than 5% of the total length (Soana et al., 2019). ...
Article
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Detailed studies on pollutants genesis, path and transformation are needed in agricultural catchments facing coastal areas. Here, loss of nutrients should be minimized in order to protect valuable aquatic ecosystems from eutrophication phenomena. A soil system N budget was calculated for a lowland coastal area, the Po di Volano basin (Po River Delta, Northern Italy), characterized by extremely flat topography and fine soil texture and bordering a network of lagoon ecosystems. Main features of this area are the scarce relevance of livestock farming, the intense agriculture, mainly sustained by chemical fertilizers, and the developed network of artificial canals with long water residence time. Average nitrogen input exceeds output terms by ~60 kg N ha(-1) year(-1), a relatively small amount if compared to sub-basins of the same hydrological system. Analysis of dissolved inorganic nitrogen in groundwater suggests limited vertical loss and no accumulation of this element, while a nitrogen mass balance in surface waters indicates a net and significant removal within the watershed. Our data provide multiple evidences of efficient control of the nitrogen excess in this geographical area and we speculate that denitrification in soil and in the secondary drainage system performs this ecosystemic function. Additionally, the significant difference between nitrogen input and nitrogen output loads associated to the irrigation system, which is fed by the N-rich Po River, suggests that this basin metabolizes part of the nitrogen excess produced upstream. The traditionally absent livestock farming practices and consequent low use of manure as fertilizer pose the risk of excess soil mineralization and progressive loss of denitrification capacity in this area.
... Lanzoni et al., 2018) and their effects on biodiversity (e.g. Castaldelli et al., 2013a, Milardi et al., 2019a. We used this information to model aquatic vegetation production and standing crop, its denitrification potential, and its consumption by grass carp in the drainage network, with the aim to verify whether historical N loads were affected by grass carp or other changes in agricultural practices. ...
Article
Although one of the most evident effects of biological invasions is the loss of native taxonomic diversity, contrasting views exist on the consequences of biological invasions on native functional diversity. We investigated this topic using Mediterranean stream, river and canal fish communities as a test case, at 3734 sites in Italy, and distinguishing between exotic and translocated species invasion in three different faunal districts. Our results clearly confirmed that introduced species were widespread and in many cases the invasion was severe (130 communities were completely composed by introduced species). Exotic and translocated fish species had substantially different geographical distribution patterns, perhaps arising from their differences in introduction timing, spread and invasion mechanisms. We also found a clear decreasing trend of functional dispersion along an invasion gradient, confirming our hypothesis that the invasion process can diminish the relative diversity of ecofunctional traits of host fish communities. Furthermore, our results suggested that exotic species might have a greater negative effect than translocated species on the relative diversity of ecofunctional traits of fish communities. This could also be linked to the fact that translocated species are more ecofunctionally similar to native ones, compared to the exotics. Our multivariate analysis of site-specific combinations of ecofunctional traits highlighted some traits characteristic of all invaded communities, while our discriminant analysis underlined how there was a substantial ecofunctional overlap between native, exotic and translocated species groups in most areas.
... Among watershed changes, the loss of natural buffers against the accumulation of excessive nitrogen (N) loads could be of paramount importance (Hansen et al., 2018;Hill, 2019). Denitrification, the reduction of nitrate (NO 3 À ) to nitrogen gas (mostly N 2 ) performed by microbial communities under anaerobic conditions, is one of the most important mechanisms of N removal and can take place wherever an anoxic environment, and availability of substrates (NO 3 À and organic carbon) allow it. ...
Article
This review evaluates research in the past 20 years focusing on groundwater nitrate removal in the riparian zones of agricultural watersheds. Studies have reported a large range in the magnitude of groundwater and nitrate fluxes to buffers in different hydrogeologic settings. An earlier focus on buffers with shallow subsurface flow has expanded to include sites with deep flow paths and groundwater-fed overland flow. Nitrate removal efficiency and the width required for removal have been linked to riparian sediment texture and depth to an impervious layer. Denitrification has been identified as the dominant mechanism of nitrate removal based on evidence that this process occurs at depth in many buffers which contain buried organic-rich deposits. Several studies have assessed the cumulative effect of riparian buffers on nitrate removal at the watershed scale. Despite considerable research progress areas of uncertainty still remain. Buffers with coarse-textured sediments located in landscapes with upslope sand aquifers have received most attention. In contrast, few sites have been analysed in weathered bedrock and glacial till landscapes. Many studies have reported nitrate removal efficiency based on nitrate concentrations rather than measuring groundwater fluxes which assess the magnitude of nitrate removal. More information is needed on interactions between riparian hydrological flow paths and biogeochemical processes. Further research is recommended on the effect of riparian zone nitrate removal at the watershed scale and long-term monitoring with respect to buffer restoration, the ability to sustain nitrate removal and responses to land use and climate change.
... Currently, submerged aquatic vegetation is almost nonexistent in the canals, and emergent vegetation has been actively managed since the mid-1990s (e.g. mechanical removal; Castaldelli et al., 2013b, Pierobon et al., 2013 and is now present only in stretches of the canal network at low flood risk, representing less than 5% of the total length (Soana et al., 2019). ...
Article
Although ubiquitous elements of agricultural landscapes, the interest on ditches and canals as effective filters to buffer nitrate pollution has been raised only recently. The aim of the present study was to investigate the importance of in-ditch denitrification supported by emergent aquatic vegetation in the context of N budget in agricultural lands of a worldwide hotspot of nitrate contamination and eutrophication, i.e. the lowlands of the Po River basin (Northern Italy). The effectiveness of N abatement in the ditch network (>18,500 km) was evaluated by extrapolating up to the watershed reach-scale denitrification rates measured in a wide range of environmental conditions. Scenarios of variable extents of vegetation maintenance were simulated (25%, 50% and 90%), and compared to the current situation when the natural development occurs in 5% of the ditch network length, subjected to mechanical mowing in summer. Along the typical range of nitrate availability in the Po River lowlands waterways (0.5–8 mg N L−1), the current N removal performed by the ditch network was estimated in 3300–4900 t N yr−1, accounting for at most 11% of the N excess from agriculture. The predicted nitrate mitigation potential would increase up to 4000–33,600 t N yr−1 in case of vegetation maintenance in 90% of the total ditch length. Moreover, a further significant enhancement (57% on average) of this key ecosystem function would be achieved by postponing vegetation mowing at the end of the growing season. The simulated outcomes suggest that vegetated ditches may offer new agricultural landscape management opportunities for effectively decreasing nitrate loads in surface waters, with potential improved water quality at the watershed level and in the coastal zones. In conclusion, ditches and canals may act as metabolic regulators and providers of ecosystem services if conservative management practices of in-stream vegetation are properly implemented and coupled to hydraulic needs.
... In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. Increased turbidity would also favor exotic species that thrive in turbid environments (see Milardi and Castaldelli, 2018, Lanzoni et al., 2018, Milardi et al., 2018a, Gavioli et al., 2018, Milardi et al., 2018b. Our bioenergetic model suggested that egestion and excretion could contribute to the increase in N loads, but likely played a minor role (see Supplementary Table 2). ...
Article
Rivers worldwide are impacted by human activities such as habitat degradation, habitat fragmentation, waterway flow regulation, and introduction of exotic species, which are responsible for the reduction or the disappearance of native species in many parts of the world. The Oglio River, a tributary of the Po River in Northern Italy, is a good example of a river with a long history of human alteration and where exotic invasions are present. We used data on water parameters and fish communities along the watercourse to investigate whether low flow conditions, degraded water quality, abundant exotic species, and the presence of migration barriers could be a disadvantage for native species. We used ordination methods (redundancy analysis), variance partitioning analysis, and the threshold indicator taxa analysis to explore changes in community composition and ecofunctional traits along an altitude gradient. We found that exotic species affected native ones more than water quality and hydromorphological parameters. Native species were most abundant in the upper reach of the Oglio River, despite low flow and shallow depth. Moreover, rheophilic and clear water native fish decreased rapidly in the lower reach of the river, where exotic species increased. This distribution could be explained by the presence of barriers in the middle reach, which block exotic species migrating upstream from the highly invaded Po River, and by a lower suitability of the upper reach for some exotic species. Our results provide a general description of the fish fauna of a strongly regulated river and can contribute to develop more effective fish and water management practices.
... Pinardi et al., 2009;Alldred and Baines, 2016). Aquatic vegetation is thus a key component of the buffer capacity of wetlands (Choudhury et al., 2018;Bastviken et al., 2009) or agricultural ditches (Vymazal and Březinová, 2018;Castaldelli et al., 2015). Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. ...
Article
Agricultural drainage is significant source of nutrients, which contributes to eutrophication of reservoirs and coastal areas. Constructed wetlands and vegetated ditches are promising techniques used for elimination of nutrients and suspended solids from agricultural drainage. While constructed wetlands have been successfully used for several decades vegetated drainage ditches have been used only recently. The current study presents the results from a two-year monitoring (2015–2016) of a naturally vegetated drainage ditch in the Czech Republic. The drainage ditch was 200 m long with Phragmites australis, Typha latifolia and Glyceria maxima being the dominant macrophytes growing in the ditch. Removal of nitrogen averaged 1070 kg ha⁻¹ yr⁻¹ with 804 kg ha⁻¹ yr⁻¹ being removed through denitrification of nitrate nitrogen. Plant uptake was responsible for 26.3% of the removed nitrogen. The removal of TP averaged 142 kg ha⁻¹ yr⁻¹ with plant uptake being responsible for 14% of the removed load. Mean removal of suspended solids amounted 20 437 kg TSS ha⁻¹ yr⁻¹. Removal of BOD5 and COD averaged 1500 kg ha⁻¹ yr⁻¹ and 7000 kg ha⁻¹ yr⁻¹, respectively. The removal of nitrogen and organics was strongly dependent on water temperature while removal of phosphorus and suspended solids were temperature-independent. The results of this study revealed that the naturally vegetated drainage ditch has comparable treatment efficiency with constructed wetlands in terms of nutrients, suspended solids and organics.
... Pinardi et al., 2009;Alldred and Baines, 2016). Aquatic vegetation is thus a key component of the buffer capacity of wetlands (Choudhury et al., 2018;Bastviken et al., 2009) or agricultural ditches (Vymazal and Březinová, 2018;Castaldelli et al., 2015). Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. ...
... The water is delivered from irrigation canals to arable lands through a capillary ditch network; water level in the canals remains relatively stable through a complex management of the drainage system. The irrigation period overlaps with the vegetative phase and the warm season, when high water temperatures enhance N dissipation, supported by the action of macrophytes and microbial communities , Choudhury et al., 2018. In October, at the end of the irrigation season, little water is let through the siphons and large portions of the canal network are dry until the next irrigation season, occasionally serving as flood controls in case of intense rainfall. ...
... Because no historical information was available on individual growth rates, we assumed that the individuals would grow at 89% of their maximum growth potential, which is a value commonly found for immature grass carp (e.g. van der Lee et al., 2017). ...
Article
Grass carp (Ctenopharyngodon idella), native to eastern Asia, have established populations throughout the Missis-sippi River basin and are now reproducing naturally in the Great Lakes basin. As a large herbivorous fish, there is concern that an established grass carp population in the Great Lakes may threaten nearshore vegetated areas and wetlands. We parameterized a bioenergetics model for grass carp from the primary literature to quantify individual consumption levels and estimate the impacts of an established population on macrophytes in representative areas of Lakes Erie and Ontario. Individual life time consumption was estimated under average, cool and warm temperature conditions. Under average temperature conditions, a population of grass carp could consume up to 27.6 kg of vegetation per kg of fish per year, depending on energy density of the vegetation. When consumption was estimated for populations of various grass carp biomass densities, most simulated scenarios resulted in b 50% of vegetation remaining in an invaded wetland after one year, with the majority of consumption from pre-adult stages. Direct impacts will likely exceed these effects due to losses to vegetation production potential from grass carp feeding early in the growing season and grass carp foraging activity resulting in plant damage or uprooting.
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). ...
... Grass carp is considered to pose a substantial risk of invasion and detrimental effects elsewhere in its introduced range (Maceina et al., 1992, Scarnecchia, 2000, Cudmore and Mandrak, 2011, but the extent to which this risk applies to Italy is not yet clear (Milardi et al., 2015). In other invaded areas, detrimental effects on the environment have been reported through the overgrazing of submerged vegetation or bioturbation and nutrient resuspension (Cudmore and Mandrak, 2004, Cudmore and Mandrak, 2011, Dibble and Kovalenko, 2009). In Italy, nutrient and sediment resuspension and reduction of aquatic vegetation could have adversely influenced native species, which are mostly adapted to clearer water conditions and require a plant substrate for spawning. ...
... Grass carp is a specialized herbivorous fish, that can survive on alternative prey such as aquatic invertebrates or small fish (Shireman and Smith, 1983). We used the Wisconsin model (Hanson et al., 1997), a well-known and relatively simple bioenergetic model, to model grass carp consumption of submerged vegetation. ...
... The presence of aquatic vegetation boosts N processing by providing carbon and creating oxic-anoxic interfaces in the rhizosphere, thus increasing the suitable habitats available to the denitrifying microbiota and resulting in higher denitrification rates compared to bare sediment (2-4 folds higher for sediments covered by submerged vegetation; see e.g. Pinardi et al., 2009;Alldred and Baines, 2016). Aquatic vegetation is thus a key component of the buffer capacity of wetlands (Choudhury et al., 2018;Bastviken et al., 2009) or agricultural ditches (Vymazal and Březinová, 2018;Castaldelli et al., 2015). ...
Article
Human activity is accelerating changes in biotic communities worldwide. Predicting impacts of these changes on ecosystem services such as denitrification, a process that mitigates the consequences of nitrogen pollution, remains one of the most important challenges facing ecologists. Wetlands especially are valued as important sites of denitrification, and wetland plants are expected to have differing effects on denitrification. We present the results of a meta-analysis, conducted on 419 published estimates of denitrification in wetlands dominated by different plant species. Plants increased denitrification rates by 55% on average. This effect varied significantly among communities as defned by the dominant plant species, but surprisingly did not differ substantially among methods for measuring denitrification or among types of wetlands. We conclude that mechanistically linking functional plant traits to denitrification will be key to predicting the role of wetlands in nitrogen mitigation in a changing world.
... In Italy, the first introduction to the wild was at the end of the 1980s (Melotti et al., 1987), in the Po di Volano basin, in the southern part of the Po River delta, where it has recently been found to reproduce naturally (Milardi et al., 2015). Large-scale grass carp introduction at the end of the 1980s coincided with the onset of well-studied massive algal blooms and anoxia in the Goro Lagoon, downstream of the Po di Volano (Pugnetti et al., 1992;Viaroli et al., 1995Viaroli et al., , 1996. However, the potential contribution of grass carp to those blooms has never been investigated. ...
... In this case, the increase in N load to the Goro Lagoon in the early 1990s caused dystrophic crises, which spurred research interest on eutrophication (see e.g. Pugnetti et al., 1992, Viaroli et al., 1995. The macroscopic effect of these dystrophic crises were massive algal blooms and, due to long water retention time in the Goro Lagoon, these effects persisted through the 1990s and continue today. ...
Chapter
The present study deals with the processes leading to summer dystrophic crises in a Po River Delta lagoon — the Sacca di Goro. Plankton population distribution was studied in relation to the variability of hydrological indicators of trophic state. Attention was then paid to problems concering growth and decomposition of Ulva rigida beds: the elemental composition of the macroalga was analyzed and experiments on the thalli decomposition and the related nutrient regeneration processes were carried out.
... We also used the reported density to model the growth of submerged macrophytes through the irrigation season, by fitting a Gompertz curve to the existing information (i.e. a daily average vegetation growth rate of around 2%; Supplementary Fig. 1; Larson, 2007, Nizzoli et al., 2014Saunkaew et al., 2011). We estimated that, during winter when little water is present in the canals and temperatures are low, a 90% reduction to the standing crop of submerged vegetation would occur due to senescence and drying out of ample portions of the canals (Westlake, 1973). We also estimated that, when grass carp were present in the system, submerged macrophytes would also be grazed outside of the irrigation season, causing an additional 5% winter reduction. ...
... However, instream vegetation could also be lost through biological control (i.e. the stocking of an herbivorous species to control vegetation growth, Lodge (1991)). Grass carp Ctenopharyngodon idella (Valenciennes, 1844) is a prime example of such an herbivorous fish; originally from Asia, it has been widely introduced to control excessive or undesirable aquatic vegetation (Kelly et al., 2011;Wittmann et al., 2014). The ''ecosystem engineering" capabilities of grass carp can provoke unwanted effects through altering the abundance of submerged vegetation, decreasing available spawning habitats for native species, increasing turbidity and ultimately favoring other introduced species (Milardi et al., 2018a). ...
... Pinardi et al., 2009;Alldred and Baines, 2016). Aquatic vegetation is thus a key component of the buffer capacity of wetlands (Choudhury et al., 2018;Bastviken et al., 2009) or agricultural ditches (Vymazal and Březinová, 2018;Castaldelli et al., 2015). Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. ...
... We then modeled the denitrification function in the Po di Volano basin, taking into account the total surface of the canals and the seasonal evolution of denitrification rates for bare sediments and for sediments colonized by submerged macrophytes. Bare sediments were assigned denitrification rates based on experimental studies performed in the Po di Volano basin (average 53.5 (47-60) mg N per square meter per day; Castaldelli et al. 2015). Sediments colonized by submerged macrophytes were assigned rates measured in freshwater environments of the Po River plain similar to the Po di Volano canals for trophic conditions (in particular water NO 3 À availability) and substrate and accounting for seasonal variations (average 133 (98-168) mg N per square meter per day in April-May and average 259 (140-378) mg N per square meter per day in June-September; Nizzoli et al., 2014, Pinardi et al., 2009, Racchetti et al., 2017. ...
... The water is delivered from irrigation canals to arable lands through a capillary ditch network; water level in the canals remains relatively stable through a complex management of the drainage system. The irrigation period overlaps with the vegetative phase and the warm season, when high water temperatures enhance N dissipation, supported by the action of macrophytes and microbial communities ( Soana et al., 2015, Choudhury et al., 2018. In October, at the end of the irrigation season, little water is let through the siphons and large portions of the canal network are dry until the next irrigation season, occasionally serving as flood controls in case of intense rainfall. ...
Article
Organic enrichment may deeply affect benthic nitrogen (N) cycling in macrophyte meadows, either promoting N loss or its recycling. This depends upon the plasticity of plants and of the associated microbial communities, as those surrounding the rhizosphere. Rates of denitrification, dissolved inorganic N fluxes and N uptake were measured in sediments vegetated by the submerged macrophyte Vallisneria spiralis L. under increasing organic matter loads. The aim was to investigate how the combined N assimilation and denitrification, which subtract N via temporary retention and permanent removal, respectively, do vary along the gradient. Results showed that V. spiralis meadows act as regulators of benthic N cycling even in organic enriched sediments, with negative feedbacks for eutrophication. A moderate organic load stimulates N uptake and denitrification coupled to nitrification in the rhizosphere. This is due to a combination of weakened competition between macrophytes and N cycling bacteria and enhanced radial oxygen loss by roots. An elevated organic enrichment affects N uptake due to hostile conditions in pore water and plant stress and impairs N mineralisation and its removal via denitrification coupled to nitrification. However, the loss of plant performance is almost completely compensated by increased denitrification of water column nitrate, resulting in a shift between the relative relevance of temporary and permanent N removal processes.
... Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. For example, in-stream vegetation can be mechanically removed to increase the flow of water through rivers and canals (Levavasseur et al., 2014;Pierobon et al., 2013). However, instream vegetation could also be lost through biological control (i.e. the stocking of an herbivorous species to control vegetation growth, Lodge (1991)). ...
... Currently, submerged aquatic vegetation is almost nonexistent in the canals, and emergent vegetation has been actively managed since the mid-1990s (e.g. mechanical removal; Castaldelli et al., 2013b, Pierobon et al., 2013 and is now present only in stretches of the canal network at low flood risk, representing less than 5% of the total length (Soana et al., 2019). ...
Article
Secondary drainage canals have the potential to effectively mitigate excess nitrogen loads from diffuse and point sources. In vegetated (Phragmites australis and Typha latifolia) and in unvegetated canals subjected to diffuse and point pollution, nitrogen removal was evaluated by means of simple in–out mass balance and potential uptake by macrophytes was estimated from biomass data. Results suggest an elevated control of nitrogen in vegetated ditches receiving point source of pollution (average abatement of 50% of the total N load per linear km), whereas removal processes are much less effective in unvegetated ditches. The comparison between net abatement and plant uptake, highlights the presence of other unaccounted for processes responsible for a relevant percentage of total N removal. Overall, results from this study suggest the importance of actions aiming at the appropriate management of emergent vegetation, in order to improve its direct and indirect metabolic functions and maximize nitrogen removal in impacted watersheds.
... Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. For example, in-stream vegetation can be mechanically removed to increase the flow of water through rivers and canals (Levavasseur et al., 2014;Pierobon et al., 2013). However, instream vegetation could also be lost through biological control (i.e. the stocking of an herbivorous species to control vegetation growth, Lodge (1991)). ...
Article
Man-made drainage networks consisting primarily of ditches have been constructed by farmers for centuries to remove excess water from fields. In addition to removing water, ditch networks are involved in other hydrological processes, such as redistributing soil or pesticides. To maintain the hydraulic capacity of ditches, farmers apply various management regimes to regulate ditch vegetation cover that can impact the aforementioned hydrological processes. The objective of this study was to characterise the time-varying impacts of ditch management regimes on vegetation cover and to discuss the resulting hydrological impacts. The vegetation associated with the surveyed ditches was observed and used to define the different ditch management regimes and to quantify their impacts on ditch vegetation cover. The management regimes were heterogeneous and involved a combination of dredging, chemical weeding, mowing and burning. The variety of management practices and the frequency with which they were applied to the ditches resulted in strong spatial variability in the vegetation cover and in strong temporal variability for a given ditch. Finally, it is probable that the surveyed management regimes had time-varying and contrasting impacts on the hydraulic capacity of ditches, the erosion of ditch banks, sedimentation and pesticide retention in the ditches, as indicated by the extensive literature on these topics.
... However, a more holistic perspective, considering loss of denitrification potential, might shift this perspective. Denitrification is currently achieved through water treatment plants, which have a major infrastructural and operational cost (Boerema et al., 2014). While fisheries for grass carp have been established, fishing license sales in the region have actually declined by approximately 90% over this period (from 240,000 units in early 1980s to 25,000 units in early 2010s, Emilia-Romagna Region, personal communication), perhaps due to the decline of preferred native fishes (Milardi et al., 2018a;Castaldelli et al., 2013a). ...
... Grass carp have been introduced nearly worldwide, yet existing risk assessments (see e.g. Clayton and Wells, 1999;Cassani et al., 2008;Cudmore et al., 2017) have not considered the potential effect of denitrification function loss. This effect could be of substantial concern in North America's Laurentian Great Lakes, where hazardous algal blooms are becoming more frequent in Lake Erie's western basin (Kane et al., 2014). ...
... Our model shows that the submerged macrophyte standing stock would be progressively diminished and ultimately eliminated, which is what was actually reported in this area (water management authority, personal communication) but also elsewhere (see e.g. Bonar et al., 2002), leading to a substantially diminished denitrification function of vegetated substrates and leaving only that of bare sediments. Stocking in the early 1990s would have sustained, or heightened, this effect. ...
Article
We investigated effects of triploid grass carp Ctenopharyngodon idella on aquatic macrophyte communities, water quality, and public satisfaction for 98 lakes and ponds in Washington State stocked with grass carp between 1990 and 1995. Grass carp had few noticeable effects on macrophyte communities until 19 months following stocking. After 19 months, submersed macrophytes were either completely eradicated (39% of the lakes) or not controlled (42% of the lakes) in most lakes. Intermediate control of submersed macrophytes occurred in 18% of lakes at a median stocking rate of 24 fish per vegetated surface acre. Most of the landowners interviewed (83%) were satisfied with the results of introducing grass carp. For sites where all submersed macrophytes were eradicated, average turbidity was higher (11 nephelometric turbidity units, NTU) than at sites where macrophytes were controlled to intermediate levels (4 NTU) or unaffected by grass carp grazing (5 NTU). Chlorophyll a was not significantly different between levels of macrophyte control; therefore, we concluded that most of this turbidity was abiotic and not algal. Triploid grass carp were a popular control option and effectively grazed most submersed macrophytes in Washington State. However, calculating stocking rates based on landowner estimates of aquatic plant coverage rarely resulted in intermediate levels of aquatic plant control. Additionally, the effects of particular stocking rates varied considerably. We recommend against using grass carp in Washington lakes where eradication of submersed vegetation cannot be tolerated.
... After the modeled submerged macrophyte collapse, the grass carp population of the Po di Volano could have survived on terrestrial vegetation and aquatic invertebrates (Terrell and Fox, 1974), which has also been observed in the study area (Mattia Lanzoni, personal communication) as well as for other carp species (Miller and Crowl, 2006). In narrow canals such as these, high ratios of water edge to surface area maximize terrestrial vegetation and terrestrial invertebrate resources, but terrestrial vegetation feeding could potentially cause bank degradation through the burrowing action of the carp (Kilgen and Smitherman, 1971). Large-scale environmental changes can have multiple effects on components of the food web and on ecosystem functions (e.g. on organic material decomposition), making it hard to have a truly holistic view of the ultimate consequences of environmental disruptions. ...
Article
that biological methods be used for controlling undesirable aquatic plants in both artificial impoundments and natural waters, since many of our existing chemical nntrol methods are potentially hazardous to the ecological balance of a pond, lake, or river, as well as to man himself. A number of organisms exist which will effectively control or eliminate certain aquatic plants. Several fish species have shown promise fs weed control agents, one of these being the white amur (Ctenopharjngodonidellua Va, lenciennes) also known as grass carp (Avault, Smitherman, and Shell, 1968). This fish has been in the United States for a decade, but is an exotic species indigenous to large rivers in China. One of the major reasons that white amur are not being widely used in this country is the fear that they may become established in our natural waters, as did the common carp, and compete with native fish for food organisms. Hora and Pillky (1962) reported that white amur do not spawn in ponds--they spawn in large rivers. If this is so, and it seems to be, it is possible that they can be used in ponds which are not connected to natural waters. Hora and Pillay (1962) classified white amur as an omnivore, and stated that it has a
... Castaldelli et al., 2013b), aquatic vegetation (e.g. Piccoli and Gerdol, 1981), fish introductions (e.g. Lanzoni et al., 2018) and their effects on biodiversity (e.g. ...
... In the 1980s, submerged macrophyte communities in the Po di Volano basin were mostly represented by Myriophyllum spp., Ceratophyllum spp., Potamogeton spp., Elodea spp., and Vallisneria spiralis, which were reported to cover the entire area of the basin during the irrigation period (Piccoli and Gerdol, 1981, Piccoli and Gerdol, 1983, Melotti et al., 1987. The average density of submerged vegetation, prior to grass carp introduction, was 1 kg (wet weight) per square meter (0.2-1.95 kg per square meter; Melotti et al., 1987). ...
Article
Weed vegetation in rice-fields and the associated irrigation system in Ferrara Province (northern Italy) is described. The main weed communities are: Oryzeto-Cyperetum difformis, Spirodelo-Lemnetum minoris, Najadetum minoris, Potamogetono-Najadetum marinae and Ceratophylletum demersi. Fragmentary halophyte communities also occur on the canal sides.Data on climate and the environment are also reported.
... For example, in-stream vegetation can be mechanically removed to increase the flow of water through rivers and canals (Levavasseur et al., 2014;Pierobon et al., 2013). However, instream vegetation could also be lost through biological control (i.e. the stocking of an herbivorous species to control vegetation growth, Lodge (1991)). Grass carp Ctenopharyngodon idella (Valenciennes, 1844) is a prime example of such an herbivorous fish; originally from Asia, it has been widely introduced to control excessive or undesirable aquatic vegetation (Kelly et al., 2011;Wittmann et al., 2014). ...
Article
Conventional wisdom holds that live macrophytes are rarely consumed and are functionally unimportant in aquatic food webs. With a review of the literature, I first demonstrate that macrophyte biomass, productivity, and species composition is often influenced by a variety of vertebrate and invertebrate grazers. Many grazers destroy much more macrophyte tissue than they eat. Contrary to conventional wisdom, live macrophytes are engaged in aquatic food webs, but the functional importance of grazing remains largely untested. Second, I evaluate the hypothesis that macrophytes are a poor quality food (low in protein). Nitrogen content (as a percentage of dry weight), as summarized from published literature, differs little among algae, emergent macrophytes, floating macrophytes, submersed macrophytes, trees, terrestrial forbs, and terrestrial grasses. Thus, nitrogen content could not be a reason to expect low herbivory on macrophytes. Third, I present previously unpublished data on the selectivity of crayfish grazing. A correlational analysis of the grazing hierarchy of crayfish and published hierarchies of other grazers (moose, carp, snails, and crayfish) suggest that herbivores have apparently similar selectivities among macrophyte species. Previously unpublished (for crayfish) and published proximate and mineral analyses of macrophytes eaten by grazers suggest no basis for selectivity by crayfish and other grazers, with the exception of a preference by moose for high sodium and protein. However, a correlational analysis of independently published grazer preferences and plant tissue phenolic and alkaloid concentrations suggests that phenolic, but not alkaloid, content is negatively related to grazing preference. Finally, I point out the need for unifying approaches in the study of freshwater herbivory. To understand the influence of herbivory (relative to other biotic and abiotic factors) on macrophyte populations and assemblages, extensive comparisons of grazing damage across environmental gradients and across macrophyte and grazer species must be made. Susceptibility to grazers must be evaluated in light of the contrasting life history strategies evident in different macrophytes. Reasonable starting points for general approaches to macrophyte-herbivore interactions may include the apparency and resource availability models developed for terrestrial plant-herbivore interactions. Given the apparently negative relationship between grazing preference and phenolic content of macrophytes, more investigation of the role of secondary compounds is necessary.
... Pinardi et al., 2009;Alldred and Baines, 2016). Aquatic vegetation is thus a key component of the buffer capacity of wetlands (Choudhury et al., 2018;Bastviken et al., 2009) or agricultural ditches (Vymazal and Březinová, 2018;Castaldelli et al., 2015). Therefore, human actions that disrupt aquatic vegetation could reduce denitrification buffers and further contribute to eutrophication. ...
Article
Optimising nitrate removal and identifying critical factors for nitrate removal in wetlands is an important environmental task in the effort to achieve better surface water quality. In this study, eighteen free water surface wetlands with similar shape and size (22m2 each) received groundwater with a high nitrate-N concentration (about 11mgl−1). The effects of two hydraulic loads, 0.13md−1 and 0.39md−1, and three vegetation types – emergent, submersed and freely developing vegetation – on the nitrate-N removal were investigated through mass inflow and outflow measurements.No significant difference in nitrate removal between the different hydraulic loads could be detected. Significantly higher area-specific nitrate removal and first-order area-based rate coefficients were found in the basins with emergent vegetation, with no difference between the basins with submersed and freely developing vegetation. The nitrate-N removal increased as the wetlands matured and the vegetation grew denser, emphasizing the role of dense emergent vegetation for nitrate removal at high nitrate concentrations.
... We also used the reported density to model the growth of submerged macrophytes through the irrigation season, by fitting a Gompertz curve to the existing information (i.e. a daily average vegetation growth rate of around 2%; Supplementary Fig. 1; Larson, 2007, Nizzoli et al., 2014Saunkaew et al., 2011). We estimated that, during winter when little water is present in the canals and temperatures are low, a 90% reduction to the standing crop of submerged vegetation would occur due to senescence and drying out of ample portions of the canals (Westlake, 1973). ...
Article
We evaluated one-sided competition from the floating-leaved plant Nymphoides peltata (non-indigenous in Sweden) on three submerged plant species, Ceratophyllum demersum, Elodea canadensis and Ranunculus circinatus, in a controlled experiment. The three submerged species were allowed to grow for 21 days in the absence of N. peltata and with the species present at densities of approximately 33, 66 and 100% cover. All species retained a positive relative growth rate (RGR) based on length at all N. peltata densities, but responded with negative growth based on weight for several treatments. C. demersum achieved RGR of 0.03day−1 in the absence of N. peltata, RGR of 0.02day−1 in the lowest N. peltata density but negative RGR in the two denser treatments. E. canadensis responded similarly with RGR of 0.04day−1 in the absence of N. peltata, RGR of 0.01day−1 in the lowest N. peltata density and negative RGR in the two denser treatments. R. circinatus, on the other hand, never achieved positive RGR based on weight. These results suggest that one-sided competition from floating-leaved plants has a profound effect on the submerged plant community.