Hydrological responses to rainfall events including the extratropical cyclone Gloria in two contrasting Mediterranean headwaters in Spain; the perennial font del Regàs and the intermittent Fuirosos

Catchment hydrological responses to precipitation inputs, particularly during exceptionally large storms, are complex and variable, and our understanding of the associated runoff generation processes during those events is limited. Hydrological monitoring of climatically and hydrologically distinct catchments can help to improve this understanding by shedding light on the interplay between antecedent soil moisture conditions, hydrological connectivity, and rainfall event characteristics. This knowledge is urgently needed considering that both the frequency and magnitude of extreme precipitation events are increasing worldwide as a consequence of climate change. In autumn 2018, we installed water level sensors to monitor stream water and near-stream groundwater levels at two Mediterranean forest headwater catchments with contrasting hydrological regimes: Font del Regàs (sub-humid climate, perennial flow regime) and Fuirosos (semi-arid climate, intermittent flow regime). Both catchments are located in northeastern Spain, where the extratropical cyclone Gloria hit in January 2020 and left in ca. 65 h outstanding accumulated rainfalls of 424 mm in Font del Regàs and 230 mm in Fuirosos. During rainfall events of low mean intensity, hydrological responses to precipitation inputs at the semi-arid Fuirosos were more delayed and more variable than at the sub-humid Font del Regàs. We explain these divergences by differences in antecedent soil moisture conditions and associated differences in catchment hydrological connectivity between the two catchments, which in this case are likely driven by differences in local climate rather than by differences in local topography. In contrast, during events of moderate and high mean rainfall intensities, including the storm Gloria, precipitation inputs and hydrological responses correlated similarly in the two catchments. We explain this convergence by rapid development of hydrological connectivity independently of antecedent soil moisture conditions. The data set presented here is unique and contributes to our mechanistic understanding on how streams respond to rainfall events and exceptionally large storms in catchments with contrasting flow regimes

Biogeochemical processes in forest riparian zones

The area of interaction between terrestrial and aquatic environments, i.e. the riparian zone (RZ), has long been recognized as an important landscape feature from both scientific and management perspectives. Surface water quality is to a great extent regulated by substances exported from RZs. Boreal forest RZs are characterized by high organic matter content, which drives important biogeochemical processes. The overall objective of this thesis was to develop the understanding of RZ biogeochemical processes so as to provide a sound scientific basis for protection of surface water quality in boreal forests. The work was based on measurements from stream, riparian, and upslope monitoring sites in the 68 km² Krycklan catchment in northern Sweden. Specifically, the study of dissolved organic carbon (DOC), base cations (BC), silicon (Si), and sulfate (SO₄²⁻) was included. Opposite SO₄²⁻ and DOC trends were observed in both stream and riparian water, whereas no trends were apparent in the upslope area. This suggests that RZ processes control stream SO₄²⁻ and DOC independently of upslope soils. On the other hand, the upslope signal of BC and Si was maintained through the RZ and subsequently exported to the streams. RZs were found to have a long-term potential to export DOC from both new and old carbon pools. This, in combination with predicted future climate and exhaustion of residual S pools accumulated during the acidification period, is likely to increase DOC concentrations affecting surface water quality and the global carbon cycle. The hydrological connectivity between RZs and streams can be conceptualized as a ‘dominant source layer’, the narrow depth range with the highest contribution to solute and water fluxes. Local hydromorphology drives spatial heterogeneity in the distribution of RZs and in the intensity of their biogeochemical functions including retention via interaction with organic matter and transformation via redox reactions. Overall, this thesis provides new insights into the role of the RZ in catchment biogeochemistry and proposes a conceptualization of its function and heterogeneity. These outcomes are an essential prerequisite for ecologically sustainable and economically effective land management that preserves surface water quality

Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO3_{3} –^{–} stoichiometry along a headwater Mediterranean stream

In forest headwater streams, metabolic processes are predominately heterotrophic and depend on both the availability of carbon (C) and nitrogen (N) and a favourable C:N stoichiometry. In this context, hydrological conditions and the presence of riparian forests adjacent to streams can play an important, yet understudied role in determining dissolved organic carbon (DOC) and nitrate (NO$_{3}$ $^{–}$) concentrations and DOC : NO$_{3}$ $^{–}$ molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO$_{3}$ $^{–}$ supply and DOC : NO$_{3}$ $^{–}$ stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO$_{3}$ $^{–}$ concentrations and DOC : NO$_{3}$ $^{–}$ molar ratios during both base flow and storm flow conditions at three stream locations along a longitudinal gradient of increased riparian forest coverage. Further, we performed an event analysis to examine the hydroclimatic conditions that favour the transfer of DOC and NO$_{3}$ $^{–}$ from riparian soils to the stream during storms. Stream DOC and NO$_{3}$ $^{–}$ concentrations were generally low (overall averages ± SD were 1.0 ± 0.6 mg C L $^{–1}$ and 0.20 ± 0.09 mg N L $^{–1}$), although significantly higher during storm flow compared to base flow conditions in all three stream sites. Optimal DOC : NO$_{3}$ $^{–}$ stoichiometry for stream heterotrophic microorganisms (corresponding to DOC : NO$_{3}$ $^{–}$ molar ratios between 4.8 and 11.7) was prevalent at the midstream and downstream sites under both flow conditions, whereas C-limited conditions were prevalent at the upstream site, which had no surrounding riparian forest. The hydroclimatic analysis of storms suggested that large and medium storm events display a distinct mechanism of DOC and NO$_{3}$ $^{–}$ mobilization. In comparison to large storms, medium storm events showed limited hydrological responses that led to significantly lower increases in stream DOC and NO$_{3}$ $^{–}$ concentrations. During large storm events, different patterns of DOC and NO$_{3}$ $^{–}$ mobilization arise, depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of the riparian groundwater table, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO$_{3}$ $^{–}$ concentrations compared to large storm events preceded by wet conditions. Our results suggest that (i) increased supply of limited resources during storms can potentially sustain in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were overcome downstream, likely due to higher C inputs from riparian forests present at lower elevations. The contrasting spatiotemporal patterns in DOC and NO$_{3}$ $^{–}$ availability and DOC : NO$_{3}$ $^{–}$ stoichiometry observed at the studied stream suggest that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments

Stream metabolism sources a large fraction of carbon dioxide to the atmosphere in two hydrologically contrasting headwater streams

Headwater streams are control points for carbon dioxide (CO$_{2}$) emissions to the atmosphere, with relative contributions to CO$_{2}$ emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in-stream metabolic processes. We analyzed continuous measurements of stream dissolved CO$_{2}$ and oxygen (CO$_{2}$) concentrations during spring and early summer in two Mediterranean headwater streams from which we evaluated the contribution of in-stream net ecosystem production (NEP) to CO$_{2}$ emission. The two streams exhibited contrasting hydrological regimes: one was non-perennial with relatively small groundwater inflows, while the other was perennial and received significant lateral groundwater inputs. The non-perennial stream exhibited strong inverse coupling between instantaneous and daily CO$_{2}$ and CO$_{2}$ concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO$_{2}$–O$_{2}$ relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. Mean NEP contribution to CO$_{2}$ emission was 51% and 57% at the non-perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO$_{2}$ concentration, in-stream CO$_{2}$ production consistently and substantially contributed to total atmospheric CO$_{2}$ flux in both streams. We conclude that in-stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed

Boreal forest riparian zones regulate stream sulfate and dissolved organic carbon

In boreal forest catchments, solute transfer to streams is controlled by hydrological and biogeochemical processes occurring in the riparian zone (RZ). However, RZs are spatially heterogeneous and information about solute chemistry is typically limited. This is problematic when making inferences about stream chemistry. Hypothetically, the strength of links between riparian and stream chemistry is time-scale dependent. Using a ten-year (2003 − 2012) dataset from a northern Swedish catchment, we evaluated the suitability of RZ data to infer stream dynamics at different time scales. We focus on the role of the RZ versus upslope soils in controlling sulfate (SO42−) and dissolved organic carbon (DOC). A priori, declines in acid deposition and redox-mediated SO42− pulses control sulfur (S) fluxes and pool dynamics, which in turn affect dissolved organic carbon (DOC). We found that the catchment is currently a net source of S, presumably due to release of the S pool accumulated during the acidification period. In both, RZ and stream, SO42 − concentrations are declining over time, whereas DOC is increasing. No temporal trends in SO42 − and DOC were observed in upslope mineral soils. SO42 − explained the variation of DOC in stream and RZ, but not in upslope mineral soil. Moreover, as SO42 − decreased with time, temporal variability of DOC increased. These observations indicate that: (1) SO42 − is still an important driver of DOC trends in boreal catchments and (2) RZ processes control stream SO42 − and subsequently DOC independently of upslope soils. These phenomena are likely occurring in many regions recovering from acidification. Because water flows through a heterogeneous mosaic of RZs before entering the stream, upscaling information from limited RZ data to the catchment level is problematic at short-time scales. However, for long-term trends and annual dynamics, the same data can provide reasonable representations of riparian processes and support meaningful inferences about stream chemistry

Retention efficiency for microplastic in a landscape estimated from empirically validated dynamic model predictions

Soils are recipients of microplastic that can be subsequently transferred to the sea. Land sources dominate inputs to the ocean, but knowledge gaps about microplastic retention by land hinder assessments of input rates. Here we present the first empirical evaluation of a dynamic microplastic fate model operating at landscape level. This mechanistic model accounts for hydrology, soil and sediment erosion, particle characteristics and behavior. We predict microplastic concentrations in water and sediments of the Henares river (Spain) within the measurement uncertainty boundaries (error factors below 2 and 10, respectively). Microplastic export from land and discharge by river fluctuates in a non-linear manner with precipitation and runoff variability. This indicates the need of accurate dynamic descriptions of soil and stream hydrology even when modeling microplastic fate and transport in generic scenarios and at low spatio-temporal resolution. A time-averaged landscape retention efficiency was calculated showing 20–50% of the microplastics added to the catchment over a multiannual period were retained. While the analysis reveals persistent uncertainties and knowledge gaps on microplastic sources to the catchment, these results contribute to the quantitative understanding of the role of terrestrial environments in accumulating microplastics, delaying their transport to the sea

Dissolved organic carbon bioreactivity and DOC:DIN stoichiometry control ammonium uptake in an intermittent Mediterranean stream

1. Heterotrophic organisms in streams use dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) from the water column to meet their growth and energy requirements. However, the role of DOC availability in driving DIN uptake in headwater streams is still poorly understood. In this study, we focus on how DOC:DIN stoichiometry and DOC bioreactivity control ammonium (NH$_4$$^+$) uptake and heterotrophic aerobic respiration, and how this influence varies among seasons in a forested Mediterranean headwater stream. 2. We estimated in-stream NH$_4$$^+$ uptake rates seasonally by conducting whole-reach constant-rate additions of NH$_4$$^+$ with and without amendments of either lignin (recalcitrant DOC) or acetate (labile DOC). During each addition, we characterised microbial community composition by molecular analyses, stream metabolism with the single-station method, and heterotrophic aerobic respiration by adding a metabolic tracer (resazurin). 3. The stream was heterotrophic (net ecosystem production 800% higher during the co-additions of acetate than when adding NH$_4$$^+$ either alone or with lignin. 4. Our results indicate that in-stream NH$_4$$^+$ uptake was largely controlled by heterotrophic bacteria, and that the stoichiometric balance between organic resources and nutrients was key to explaining the variability of in-stream NH$_4$$^+$ uptake and heterotrophic aerobic respiration. Moreover, the observed increase in NH$_4$$^+$ uptake during acetate additions suggests that heterotrophic activity was limited by labile DOC availability. 5. Our study highlights that both DOC:DIN stoichiometry and DOC bioreactivity are relevant factors driving the seasonal pattern of in-stream N processing in this forested Mediterranean headwater stream

Evolution from Late Miocene to present-day of sandy deposits on mixed depositional systems in the Gulf of Cadiz

Póster presentado en Mixed / hybrid systems (Turbidite, MTDs and Contourites) on continental margin. Lisbon (Portugal), 21-22 June, 2023This work contributes to improve the knowledge of deep marine deposits, specifically those generated by the interaction of gravitational sedimentary processes and bottom currents (mixed processes). The study of these mixed deposits is of great relevance both in the academic world, and in the industry, due to their potential as energy resources and for carbon dioxide (CO2) storage. However, there is still a great lack of knowledge about their diagnostic criteria, how they are generated, how they evolve, and their socio-economic implications. Offshore seismic reflection and logging data analyses from the continental slope of the Gulf of Cadiz reveal Late Miocene and Pliocene-Quaternary contourite and mixed deposits that host sandy bodies of special interest as potential geological storages. In general, it has been observed that the sandier deposits, exhibiting high-amplitude reflections (HARs), are located mainly on the erosional elements of these depositional systems or in the transition between the erosional and the depositional features: a) contourite channels (or moats and furrows); b) in the proximal setting of contourite terraces; and c) at the exits of the Gibraltar Strait. The sandier deposits in these systems are brought into the channels/moats and terraces by gravitational processes; and once inside they are reworked by the bottom currents, being laterally transported and deposited by a higher velocity core of the current along it. The contourite and mixed deposits generated before the full opening of the Gibraltar Strait, during the Late Miocene (~8.2-5.33 Ma), display different depositional and erosional features related to the paleo-Mediterranean Outflow Water (MOW) circulating through the Betic and Rifian corridors prior to the restriction of the Mediterranean-Atlantic gateway. Natural gamma-ray logs from exploration wells in this area show sand deposits up to 50 m thick, except in wells close to the Miocene paleo-shore where alternating deposits of sand and clay are identified. After the opening of the Gibraltar Strait, the Mediterranean Outflow Water (MOW) has generated a complex Pliocene-Quaternary (5.33 Ma-present) contourite depositional system. Sampling of sandy contourites associated with seismic features also suggests the extensive distribution of mature, well-sorted Pliocene¿Quaternary sand about 600 m thick, and showing the following characteristics: a) Early Pliocene deposits (~5.3-3.2 Ma), correspond to sheeted drifts developed mixed with gravitational sedimentary processes and low acoustic response; b) Late Pliocene-early Quaternary deposits (~3.2-2 Ma) displaying sheeted contourite drifts and enhanced acoustic response towards the top, especially in areas adjacent to highs and banks. Borehole logs for these deposits show cyclic swings in amplitude that are generally lower than those observed for the overlying Quaternary deposits; c) Natural gamma-ray logs along the Quaternary sequence show medium-amplitude cyclic swings, varying on decimetre to sub-meter scale, with no major steps in base levels. The main differences of LM sandy deposits respect to the recent succession, consist of higher thicknesses, better-sorted and higher grain size of the sands, and increasing medium-high amplitude on seismic reflections. The Miocene and Pliocene deposits also exhibit cyclic swings in gamma ray logs, but with lower amplitude and at lower frequencies than those observed for the Quaternary ones

Wastewater treatment plant effluent inputs influence the temporal variability of nutrient uptake in an intermittent stream

Wastewater treatment plant (WWTP) effluents alter water chemistry and in-stream nutrient uptake rates of receiving freshwaters, thus changing the magnitude and fate of the nutrients exported. In Mediterranean regions, the dilution capacity of receiving streams can vary strongly over time due to the seasonal occurrence of floods and droughts, causing temporal variability of nutrient uptake. We assessed the temporal patterns and the controlling factors of net nutrient uptake in an intermittent Mediterranean stream receiving WWTP effluent inputs. We compiled the longitudinal concentration profiles of ambient dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) along a 800 m reach on 47 sampling dates between 2001 and 2017, encompassing a wide range of hydrological conditions. We estimated net nutrient uptake in the receiving stream. In 72% of the dates, high rates of net ammonium uptake co-occurred with net releases of either nitrate or nitrite. This pattern suggests that the receiving stream has a high nitrification capacity. Conversely, 75% of the dates did not show any longitudinal pattern in SRP concentration, suggesting that uptake and release processes for this element were either counterbalanced or both occurred at very low rates. Finally, net ammonium uptake was low when the stream had a low dilution capacity (< 40%) and ammonium concentration was high. Overall, we demonstrate that consideration of the receiving stream’s dilution capacity is imperative to the management of freshwaters to guarantee an adequate dilution of WWTP effluent inputs and avoid saturation of in-stream nutrient uptake capacity under low flow conditions in urban landscapes

Late Miocene to present-day Sandy deposits in the Gulf of Cadiz associated to the Mediterranean Outflow Water

Póster presentado en 4th Deep-Water Circulation Research Conference (4DWC) , Edinburgh, 24-26 May 2023This work contributes to improve the knowledge of deep marine deposits, specifically those generated by the interaction of gravitational sedimentary processes and bottom currents (mixed processes). The study of these deposits is of great relevance both in the academic world, and in the industry, due to their potential as energy resources and for carbon dioxide (CO2) storage. However, there is still a great lack of knowledge about their diagnostic criteria, how they are generated, how they evolve, and their socio-economic implications. Offshore seismic reflection and logging data analyses from the continental slope of the Gulf of Cadiz reveal Late Miocene and Pliocene-Quaternary contourite and mixed deposits that host sandy bodies of special interest as potential geological storages. In general, it has been observed that the sandier deposits, exhibiting high-amplitude reflections (HARs), are located mainly on the erosional elements of these depositional systems or in the transition between the erosional and the depositional features: a) contourite channels (or moats and furrows); b) in the proximal setting of contourite terraces; and c) at the exits of the Gibraltar Strait. The sandier deposits in these systems are brought into the channels/moats and terraces by gravitational processes; and once inside they are reworked by the bottom currents, being laterally transported and deposited by a higher velocity core of the current along it. The contourite and mixed deposits generated before the full opening of the Gibraltar Strait, during the Late Miocene (~8.2-5.33 Ma), display different geometries and correspond to depositional features such as sheeted, plastered, confined, mounded and mixed drifts, as well as levees, and erosional features such as furrows, contourite and turbidite channels. These features are related to the paleo-Mediterranean Outflow Water (MOW) circulating through the Betic and Rifian corridors prior to the restriction of the Mediterranean-Atlantic gateway. Natural gamma-ray logs from exploration wells in this area show sand deposits up to 50 m thick, except in wells close to the Miocene paleo-shore where alternating deposits of sand and clay are identified. After the opening of the Gibraltar Strait, the Mediterranean Outflow Water (MOW) has generated a complex Pliocene-Quaternary (5.33 Ma-present) contourite depositional system