A large number of semi-artificial, groundwater-dependent ecosystems (GDEs), locally known as “fontanili”, occur in the Po and Venetian plains (Northern Italy) along the alluvial fans and terrace deposits of watercourses in the transition zone from higher to lower plain. Across this so-called spring belt, due to changes in both terrain slope and sediment grain size [1
], groundwater outflows through aquifers under a natural hydraulic gradient occur, which are usually further facilitated by driving perforated pipes and soil excavation. Most of the fontanili can be classified as rheo-limnocrenic springs [5
]. The prevalent morphology consists of a spring area, known as “head”, which is relatively deep and where the water is almost still, and a drainage channel through which water flows downstream. In order to maintain suitable hydrological conditions, these systems require periodic management to remove the aquatic vegetation and the accumulation of organic sediment.
Lowland springs are often described as stable habitats, with modest changes in hydrological, thermal, chemical, and biological characteristics, as they are fed by aquifers that guarantee relatively constant conditions at seasonal and interannual timescales [2
The fontanili were exploited over centuries as micro-climate regulators in the lowland grassland of Lombardy. Here, until the late 1960s, in the cold season, stable meadows (locally called ”marcite”) were submerged in water from lowland springs at nearly 10–12 °C, a temperature much greater than the atmospheric one and sufficient to allow a slight warming and to avoid water freezing. Therefore, the productivity of such meadows was higher compared to traditional lawns.
Lowland springs are components of interconnected systems of lentic waters, streams and channels, where nutrients can be removed by both aquatic vegetation and microbial processes [7
]. They host relict (palaeo-) endemics, i.e., cold-stenotherm plants and animal taxa which underwent altitudinal displacements from mountain areas during the Würm glacial expansion, whose persistence is strictly linked to the conservation of these peculiar “ecological islands” surrounded by human-dominated landscapes [9
]. Overall, lowland springs are refuge areas for species suffering strong declines in the study area, e.g., pike (Esox lucius
Despite their great potential value as ecosystem services providers, poor or inappropriate management practices, excessive water abstraction, land reclamation due to agricultural activities and the expansion of grey infrastructures, the spreading of alien species and the impacts of climate change on hydrology are threatening the conservation status of lowland springs [13
]. Moreover, agriculture affects surface and groundwater quality due to the application of fertilizers that are largely in excess compared with crop uptake, resulting in a sensible increase in nutrient concentrations in most of these GDEs [17
The hydrochemical and ecological features of the fontanili are relatively well studied [2
], especially in the central area of the Po river plain [17
]. In particular, two studies have investigated the relationships between water chemistry and ostracod assemblages [10
]. Ostracods (class Ostracoda) are small bivalved crustaceans occurring in almost all aquatic ecosystems, mostly in benthic and periphytic habitats. According to Meisch et al. [25
], there are presently 2330 subjective species of non-marine ostracods in 270 genera, all belonging to the order Pocodocopida. The Italian non-marine ostracod fauna consists of about 160 species, and it is currently the most diverse in Europe [26
Accumulating evidence indicates that non-marine ostracods differently respond to environmental conditions, and may be used as bioindicators [27
]. Their potential as environmental indicators has long been recognized by palaeolimnologists, who infer temporal changes in the local environment from changes in the assemblages of calcified ostracod valves [36
The present research aimed at comparing the current hydrochemical status and the composition of the ostracod communities of 50 lowland springs of the Po river plain with those reported in previous studies carried out in the period 2001–2004 [10
]. The magnitude of changes was statistically estimated using univariate methods, both between and within different sub-catchments to identify possible spatial patterns. Particular attention was paid to the trends of the stoichiometric ratios of the major dissolved nutrients. In addition, the distribution patterns and the compositional turnover of the ostracod communities over the considered time period were investigated, and the relationships between ostracod occurrence and environmental variables were evaluated by multivariate analysis. The results of this study can be used to assess the often-presumed stability of lowland springs, in terms of weak or delayed responses to anthropic disturbance in their hydrochemical and biological characteristics, and to provide insights into the potential future trajectories of these threatened ecosystems.
The results of this study indicate a rapid worsening of environmental conditions in a number of lowland springs, as can be inferred from changes in water quality and, to a lesser extent, also in their hydrological and biological features. These aspects are discussed in detail, taking into account the context of growing anthropogenic pressures on agricultural soils and water resources, and the effects resulting from their mutual interactions and feedbacks.
4.1. Effects of Hydrological Factors and Agricultural Practice on Water Quality
This study, contrarily to what was previously described, highlights the marked variability of the environmental conditions of lowland springs, which are mainly linked to hydrological factors and multiple stressors operating at local scales and which, in turn, have repercussions for the composition and stability of the ostracod communities. In addition, the comparison with previous data shows that these ecosystems are subject to rapid changes, with a trend towards losses of functionality and reduced efficiency in negative feedbacks.
Because of the more powerful aquifers that are recharged by water infiltration from Alpine watersheds compared to those in the Apennine area, the lowland springs in Lombardy have higher discharges and hydrological stability than the ones in Emilia-Romagna [46
]. These latter, in fact, may undergo wide seasonal fluctuations in water level, and some may occasionally dry out due to water table lowering related to periods of low precipitation, intense water withdrawal for sprinkler irrigation and a prevalently intermittent hydrological regime of watercourses [48
]. We therefore hypothesized that poor maintenance, and not water scarcity, is the main cause of drying in the seven springs in Lombardy, as reported in this survey.
Water scarcity in the right side of the Po river basin may cause an increase in solute concentration when water demand is the highest and supply is the lowest, typically in summer, when typical local crops, like tomatoes, are grown. On the Alpine side of the Po basin, water is much more readily available thanks to the higher and more continuous flows of rivers, which are in some cases further guaranteed by water released from regulated subalpine lakes, as in the Adda river [48
]. Therefore, in Lombardy, soil submersion and flood irrigation are very common. These techniques favor the recharge of the aquifers on which the springs depend, and thus also a gradual increase in their discharge rate. If soils are fertilized, irrigation also leads to a delivery of nutrients to groundwater; on the other hand, large irrigation water volumes can also increase solute dilution [49
Different agricultural practices and crop types have varying effects on groundwater quality. In Emilia-Romagna, the autumn increase in DIN is compatible with nitrogen infiltration from soils fertilized in late summer for winter crop sowing (e.g., winter wheat and barley). Lower values in groundwater are likely due to either higher nitrogen uptakes by growing crops or possible denitrification. Higher DRSi values in summer can be related to a greater Si concentration due to the decrease in stored groundwater volumes, but also to infiltration from fields cultivated with cereals. Conversely, lower values in autumn may depend on a higher dilution; the reduction in the quantities of manure which returned to fields and the lack of cereal straw burial can further exacerbate such a decrease in DRSi concentration [51
In Lombardy, permanent meadows and corn (mostly harvested at a waxy maturation stage) prevail; DIN does not increase in autumn because there is no tillage for the sowing of winter cereals, and the low concentrations in DRSi may be due to dilution, the water that infiltrates into aquifers being low in Si [48
4.2. Trends in Physico-Chemical Characteristics of Spring Waters
The comparison of the physico-chemical water variables between both spring areas and surveys conducted in different years (Table 1
; Figure 2
and Figure 3
) must be considered with some caution due to the aforementioned differences in hydrological conditions, irrigation practices, crop types and, for Emilia-Romagna springs, site sampling periods. Nevertheless, some trends are detectable.
Water temperature generally exhibited low variability, apart some evident deviations, both positive and negative, from average values in some springs of Emilia-Romagna (Figure 2
), where the groundwater is more influenced by meteoclimatic conditions due to small stored water volumes and/or reductions in aquifer recharge due to excessive withdrawal. A reduction in the outflows has the effect of transforming the springs from rheo-limnocrenic to more markedly lentic systems, with a consequent greater variability in the physical and chemical characteristics of the water.
The positive linear correlation between TA and EC emphasizes how changes in solute concentrations are comparable in both the Apennine and Alpine sides of the Po river. They likely depend upon groundwater budgets, the water abstraction being responsible of water shortages and concentration increases, while the infiltration of soft water, e.g., from perialpine lakes in the Alpine side of the Po plain, may induce dilution. The other variables usually show marked changes (Table 1
). In particular, our results revealed a decline in water quality, more evident in the springs located in Emilia-Romagna, related to the “nitrogen legacy” from the past half century, when high nitrogen amounts were delivered to the farmland through massive applications of fertilizers and manure, resulting in a feedback loop between soil and water use [48
]. In addition, in the whole study area a marked decrease in dissolved reactive silica is supposed to occur, related to changes in both cultivation and breeding practices and crop typologies in the last decades [51
]. SRP concentrations were always very low, and often undetectable, because of an effective geochemical control of P availability due to the soils being rich in Ca and carbonates. In the Po river basin, N and P surpluses increased until the 1990s, then P markedly decreased, whilst N remained rather high [53
]. The nutrient imbalance towards an N excess is further likely aggravated by the Si shortage, with potential impacts on aquatic vegetation/primary producers [51
4.3. Structure, Distribution Patterns and Evolution of Ostracod Assemblages
The ostracod communities of the studied lowland springs are generally composed of species that are commonly found in Northern Italy [26
], and that are characterized by wide ecological tolerance. A notable exception is Ilyocypris salebrosa
, which, however, was not found in the PR39 spring where it was present in 2001, and so far this is the only record of this species for Italy [26
Some of the collected ostracod species (e.g., Herpetocypris brevicaudata
, Ilyocypris bradyi
, Ilyocypris inermis
, Prionocypris zenkeri
, Scottia pseudobrowniana
) can be considered crenophilous, being usually associated to springs or waters connected to springs [44
]. Scottia pseudobrowniana
, recovered from mosses in S21, is restricted to semi-terrestrial conditions [44
]. No stygobiont or non-native species were found in the sampled sites. Of the two species that are clearly separated from the central cluster of the CCA (Figure 6
), Heterocypris salina
does indeed reflect its actual preference for salty waters, while the isolated position of the euryecious Candona candida
simply depends on the physico-chemical characteristics of the only spring in which it occurs (S05).
The ostracod assemblages of the lowland springs are generally simple, as they are expected to be in crenal habitats [10
]. However, comparing the specific richness with that reported in previous studies in the same springs, it can be seen that the average number of species goes from 3.6 in 2004 to 2.5 in 2015 in Lombardy, and remains substantially stable in Emilia-Romagna (from 2.0 in 2001 to 2.2 in 2015–2016); in both cases, these are lower values than those reported for the rheo-limnocrenic springs of the Palearctic region (4.7 ± 2.7) [55
]. The observed ostracod species diversity may be influenced by the sampling period. Pieri et al. [15
] reported that out of the 16 ostracod taxa collected in lowland springs from Lombardy, 9 were present in both summer and autumn samples, 5 only in summer and 2 only in autumn. This and, for some species, the presence of larval stages only, likely indicates the different phenological behaviors for ostracods even in habitats where water temperature varies within a narrow range.
Some differences in the ostracod distribution exist between springs in Emilia-Romagna and Lombardy, possibly related to different concentrations of the dissolved inorganic fraction, in particular nitrogen, as evidenced by the results of the CCA (Figure 4
). Nevertheless, no relationships between ostracod community similarity and spatial distance were found. The low similarity in species composition between geographically proximate springs does not seem to be attributable to a low dispersal capacity of ostracods. Frequent disturbances may lead to local extinctions followed by colonizations from nearby sites in spring ostracods communities, according to source–sink dynamics; under such conditions, stochastic processes might be more influential than environmental characteristics in shaping ostracod assemblages during early recolonization [56
]. In addition, over a longer time scale, differences in environmental conditions occurring in adjacent springs may also determine the occurrence of species which, though generally widespread in Northern Italy, in our study have been shown to belong to a restricted number of genera compared to the full regional pool [26
]. The above-stated hypotheses are supported by the high species turnover observed in the study area (Table 2
The results on ostracod assemblages only partially agree with those found for other fontanili-dwelling communities. Despite the progressive worsening of their environmental conditions, the lowland springs of the Po river plain still act as important refuge habitats for both plant and animal stenothermal and/or rare species [3
]. However, swift changes in species and functionality were recently reported for fish communities in response to temperature increases, invasions of exotic fish, and habitat quality degradations [61
The lowland springs of the Po river plain can be regarded as important sentinels of environmental changes because the monitoring of their hydrological, hydrochemical and biological conditions provides useful information on the present status and trends of an area subjected to multiple interacting pressures.
The alleged stability of the lowland springs is usually observed in the case of water temperature, while hydrochemical variables show more or less wide variations over a period of 10–15 years. Hydrological characteristics and agriculture practices cause clear differences in water quality and in ostracod composition, both between catchments and frequently also within the same sub-catchment, indicating a strong dependence of springs on local impacts. In general, the trends of the stoichiometric ratios of the major nutrients indicate breakdowns of the biogeochemical cycles due to multiple impacts. Ostracod communities are generally characterized by a low alpha diversity and a high species turnover, which seem to be indicative of severe disturbances due the variability of environmental conditions.
The future climate is likely to further emphasize the fragility of these vulnerable ecosystems, as a result of a greater demand for water. Actions must be put in place to counter the deterioration or even the disappearance of these lowland springs, and more generally the GDEs, which constitute essential elements of the landscape of the Po valley, as well as aquatic biodiversity hotspots in a heavily modified agricultural area. An effective implementation of good agricultural practices to minimize the nitrate contamination, as well as contamination from the emerging contaminants of groundwater, is imperative. In addition, more effective guidelines for the correct maintenance of these ecosystems are needed to preserve or improve their hydraulic and ecological functionality. In this regard, a better integration between ecological and hydrogeological approaches in studying GDEs is essential so as to better understand the dynamics of these systems and their responses to climate change and human pressures.