The natural quality of inland waters is related to the geomorphology, climate, and land use in the catchment. Thus, size and slope of the catchment, rainfall, temperatures, vegetation, erosion and soil structure play an important role in water quality. However, due to human activities, the high loads of sediments and nutrients transported by rivers have become one of the greatest problems causing the deterioration of the quality of surface waters on a global scale (Bennet et al., 2001; Worrall et al., 2016; Grizzetti et al., 2021; Kelly et al., 2022), and this explains why in recent years, determined efforts have been made both in the field of research and management and restoration.

Although water management and use has been one of the main concerns of the European Water Policy since the 1960s, it was only in the year 2000 that progress was made in the protection of aquatic resources with the publication of the Water Framework Directive (WFD; Directive, 2000) by the European Parliament and the Council of the European Union, in which a community action framework was established in the field of water policy. The WFD focuses on the protection of all water bodies present in the territory of the European Community. Its objectives are to prevent the deterioration of water resources, protect and improve their condition, promote the sustainable use of the resource, guarantee a reduction in pollution, prevent the effects of floods and droughts, and achieve at least good ecological status by the year 2015. Despite the progress made with the scope of improving the environmental quality of the various water sources in Europe, the last report by the European Environment Agency (European Environment Agency, 2018) indicated that 47% of European surface waters continue to fail to meet the minimum objective of good ecological status, with agricultural activities and the urbanisation of rural landscapes being the main threats to fluvial courses (European Environment Agency, 2018). Therefore, the frequent use of agrochemicals to meet production demands means that agricultural systems play an important role as a source of contamination (non-point source) of water bodies (Oenema et al., 2005; Sharpley et al., 2008; Lassaletta et al., 2009). Moreover, over the last few decades, the progressive implementation of modern agriculture and livestock methods, with the introduction of increasingly intensive tillage techniques, an increase in the size of plots, the establishment of monocultures, etc. has promoted runoff and soil erosion (Panagos et al., 2015). In general, chemicals from non-point sources are transported to nearby water bodies either in dissolved form or along with sediment during heavy rainfall and erosion events, even in gently sloping areas (Sharpley et al., 2008; Dupas et al., 2017).

The latest reports from the European Environment Agency noted an improvement in 20% of the stations, directly attributed to the implementation of measures to control diffuse (non-point) contamination and increased efficiency in wastewater treatment (Carvahlo et al., 2019). Thus, based on reported long-term data for nitrates in European waters, there was a falling trend for nitrate concentration in rivers, although this has levelled off since 2009. The reason for nitrate persistence is due to overuse of nitrogen, which can remain in the subsurface flow for decades in catchments with long time response and add to increased nitrate loads long after application practices have changed, or mitigation strategies have been implemented (Fovet et al., 2015; Ehrbardt et al., 2021). These and other provenances of nitrate create a heterogeneous patchwork of source areas across the landscape that can be activated and deactivated in response to changing hydrological conditions (Dupas et al., 2019). For phosphorus, a marked decline was reported due to improved wastewater treatment, reduction of P in detergents, as well as reduction in soil fertilization rates and implementation of measures to reduce soil erosion and connectivity to the receiving waters (Oenema eta l., 2005; Minaudo et al., 2015; Moatar et al., 2017). However, phosphorus may persist through soil enrichment, and therefore it may take a long time before any major water quality improvements can be detected in the recipient water bodies after decreasing fertilisation of agricultural fields (Withers and Lord, 2002; Ekholm et al., 2015).

Correct assessment of the quality of water resources in the catchment is essential to estimate the status of the resources and start management and planning, if required. In fact, the WFD urges members to carry out studies on trends of different contaminants (with trend understood to be the monotonic variation of the concentration in relation to time) and identify those water bodies exceeding established reference nutrient levels. This requires sufficiently frequent monitoring of the water quality over long periods of time, which is an arduous, costly, and time-consuming task. At present, several countries, including Spain, must improve their control programs to guarantee broad and consistent monitoring of water bodies, with appropriate coverage of all relevant quality indicators, since there are still significant deficiencies. However, the number of checkpoints has decreased in recent years as compared to the first planning cycles of the WFD. This includes the Galician region, popularly known as “the country of a thousand rivers.” Despite this, the scientific community did not pay much attention to the health of Galician rivers. To date, they have not undergone extensive monitoring because hydrological and hydrochemical studies based on catchment monitoring are relatively recent in the community. This underlines the need to continue long-term monitoring efforts for water quality assessments and to identify trends.

The objective of this paper was to explore the temporal evolution of nitrate and phosphorus loads and concentrations in a stream draining a typical rural catchment in Galicia (NW Iberian Peninsula), i.e., small population centres and low-intensity agriculture. The study is based on information collected over a period of 14 years in which regular manual sampling was combined with automatic sampling during runoff episodes.

NO₃ ⁻ and P loads were similar or lower compared with other studies carried out in Galicia (Bernárdez et al., 2013; Serrano et al., 2015) and suggest that agricultural activities and forest plantation management in the study area have only a slight impact on surface waters of the Corbeira catchment relative to other mixed land use catchments in Europe (Fovet et al., 2015; Dupas et al., 2017; Bouraoui and Malago, 2020; Ehrhardt et al., 2021). In fact, annual mean concentrations of both elements were well below the recommendations of WFD, the threshold proposed by Camargo and Alonso (2006) for protecting the most sensitive freshwater species and the threshold stablished by Carleton et al. (2009) to prevent nuisance level of periphyton and cyanobacteria (Table 4). When the mean nitrate and phosphorus concentrations obtained in this study are compared to the threshold values reported by Poikane et al. (2021), based on their relationship with macrophytes and phytobenthos, it is observed that mean nutrient concentrations of this study are within the range of predicted nitrogen good-moderate class boundaries for low alkalinity lowland and upland rivers (Table 4). They also are in the boundary values proposed by de Vries et al. (2013) to protect aquatic ecosystems from eutrophication. However, they exceed the threshold reported by Stevenson et al. (2008). In this case, the values reported by this last author coincide with concentrations in the catchment reference site (TP: 15 ± 7 μg L⁻¹; mean and standard deviation from 120 water sampled collected from 2004 to 2016, unpublished data); i.e., in the headwaters of the studied stream, which drains a forest area and is assumed to be free of nutrient inputs from human activities. So, protection of stream with this threshold was deemed impractical. This highlights the need to adapt the thresholds to individual water bodies to protect aquatic ecosystems from eutrophication.

Nitrate concentration exceeds the concentration collected by Macías et al. (1991) as reference values in slightly polluted rivers (1.19 mg L⁻¹), the background concentration proposed by Meybeck (1982) for the major unpolluted rivers (0.44 mg L⁻¹), as well as the levels suggested for uncontaminated natural rivers in Galicia (1–4 mg L⁻¹, Antelo and Arce, 1996) and the mean concentration in the catchment reference site (1.99 ± 0.54 mg L⁻¹; mean and standard deviation from 120 water sampled collected from 2004 to 2016, unpublished data). Naves et al. (2019) observed nitrate contamination in the groundwaters (concentrations between 2.54 and 47.1 mg L⁻¹) of the study area linked to an inadequate management of the manure in the field and even to an occasional discharge of slurry from local farms. However, the results of this study are far from what was observed by these authors, since nitrate concentrations in summer (mean = 4.13 mg L⁻¹) -when discharge is dominated by groundwater-are well below the values reported by Naves et al. (2019) as a reference in the study area (1.2–9.9 mg L⁻¹).

Loads and the concentrations of both nutrients showed a marked seasonal pattern related to the hydrological behaviour of the catchment, that is, the highest values were recorded in winter and autumn, coinciding with the highest discharge and runoff events (Rodríguez-Blanco et al., 2019), while the lowest were obtained in summer, coinciding with the period of low waters and the greatest influence from the baseflow. This behaviour suggests a greater nutrient mobilization during the wet period (October-March), which is consistent with a predominant diffuse source of nitrate and phosphorus (Abbott et al., 2018; Dupas et al., 2019; Garzon-Vidueira et al., 2020). Thus, wet periods increase the connectivity between nutrient sources and the stream network (Rodríguez-Blanco et al., 2013; 2015), promoting the transfer of nutrients. Corbeira is a transport-limited catchment, where the nutrient fluxes are controlled by the hydrology, as the flow is the main driver of nutrient transport, since it controls the connectivity between the source (e.g., soils) and the stream. Nitrate is mainly transported in subsurface flow (Rodríguez-Blanco et al., 2015) while P (mainly associated with sediment) in surface runoff (Rodríguez-Blanco et al., 2013).

The results showed the lack of consistent trends of N and P over time, although a decreasing trend appears for N loads, mainly linked to the reduction of nitrate transport during autumn and spring (Table 2), although it might also be linked to a decline in agricultural activities in the catchment, which leads to a decrease in the application of manure and fertilisers—ideally in the autumn and spring. Forest plantation cutting cycles should also be considered since forest cut decrease nutrient uptake, making more nutrients available for leaching into water courses (Thompson et al., 2009). Forest roads, especially when intercepting the stream can be an important source of nutrients associated (mainly P) with sediments in plantation areas. Phosphorus points to an upward trend, mainly in winter, which could be related to the positive flow trend in the study area (Rodríguez-Blanco et al., 2020). An increase in discharge levels translates into a greater flow velocity and movement of sediments, which is essential for the transport of P in the catchment since it is intrinsically related to the particulate fraction. A similar pattern was observed in another mixed land use catchment (Minaudo et al., 2015; Bol et al., 2018), demonstrating the significance of suspended sediment in the dynamics of P. The results also show an increase in the nutrient concentrations, despite the fact that discharge and nutrient load decrease in the summer, meaning that the flow is being enriched owing to lesser dilution. As has been reported in several other catchments with forestry and agricultural uses (e.g., Abbott et al., 2018), this could be indicative of surplus nutrients applied to the soil being available for export to water bodies by drainage or runoff.

Despite the decline in population and reduced agricultural activities in the catchment, only a limited improvement in nitrate concentrations was observed. This may be due to the potentially long transit time in the catchment, so reducing non-point source nutrients may take decades to affect the water chemistry of rivers (Fovet et al., 2015; Dupas et al., 2017). The increase in winter flow could also counteract the effect of the reduction in the number of people, so that no trend at annual scale will be detected.

The concentrations of N and P measured in the catchment are relatively low, which shows that agricultural practices and the forest plantation management in the catchment do not jeopardise (for the moment) the water quality, although the catchment nutrient reference levels were widely exceeded. The variability of export fluxes is high, despite their “natural nature,” due to the fact that connectivity between the nutrient source area and the stream is dependent on flow conditions. A lack of consistent trends of N and P over time was observed, but the nutrients analysed behaved differently. Thus, N appeared to show a downward trend, mainly associated with declining transport in autumn and spring. Phosphorus showed an upward trend in winter linked to a greater flow and sediment transfer. As a result, any increase in erosion and runoff will almost certainly result in an increase phosphorus load in the study area.

Despite the decrease in population and abandonment of agricultural land in the catchment, the concentrations of nitrate do not show a sizable reduction, which could be indicative of surplus nutrients provided to the soil being available for export to water bodies via drainage or runoff. As a result, a decrease in inputs from various sources does not result in rapid improvement in riverine water quality.