HS10.2

Rivers draining the acid sulphate soils of western Finland are known to deliver large amounts of trace metals (e.g. Al, Cd, Co, Cu, La, Mn, Ni and Zn) with detrimental environmental consequences to the recipient estuaries in the eastern Gulf of Bothnia, northern Baltic Sea. However, the distribution of these metals in the coastal sea area, and the relevant metal transport mechanisms have been less studied.

This study investigates the spatial and temporal distribution of metals in sediments at 9 sites in the Kvarken Archipelago, which is the recipient of Laihianjoki and Sulvanjoki rivers that are among the most acid sulphate soil impacted rivers in Europe. Metal contents increase in the studied cores during the 1960s and 1970s due to the intensive artificial drainage of the acid sulphate soil landscape. The metal deposition has remained at high levels since the 1980s and the metal enrichment in seafloor sediments is currently visible at least 25 km seaward from the river mouths. Comparison to sediment quality guidelines shows that the metal contents are very likely to cause detrimental effects on marine biota more than 12 km out from the river mouths. The dynamic sedimentary environment of the shallow archipelago makes these sediments potential future sources of metals to the ecosystem. Finally, the strong association of metals and nutrients in the same sediment grain size class of 2–6 µm suggests that the transformation of dissolved organic matter and metals to metal-organic aggregates at the river mouths is the key mechanism of seaward trace metal transport, in addition to co-precipitation with Mn-oxyhydroxides identified in previous studies. These findings are important for the estimation of environmental risks and the management of biologically-sensitive coastal sea ecosystems.

This study resulted from the SmartSea project, funded by the Strategic Research Council at the Academy of Finland (grant number 292 985). M.E.Å. additionally acknowledges the Swedish Research Council Formas (grant number 2018-00760). The study has utilized research infrastructure facilities provided by FINMARI (Finnish Marine Research Infrastructure network).

Original publication: Virtasalo, J. J., Österholm, P., Kotilainen, A. T., and Åström, M. E.: Enrichment of trace metals from acid sulfate soils in sediments of the Kvarken Archipelago, eastern Gulf of Bothnia, Baltic Sea. Biogeosciences, 17, 6097–6113, https://doi.org/10.5194/bg-17-6097-2020, 2020.

How to cite: Virtasalo, J., Österholm, P., Kotilainen, A., and Åström, M.: Enrichment of trace metals from acid sulphate soils in sediments of the Kvarken Archipelago, eastern Gulf of Bothnia, Baltic Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8632, https://doi.org/10.5194/egusphere-egu22-8632, 2022.

Land-ocean dissolved organic carbon (DOC) fluxes are a significant and changing component of the global carbon cycle. The current paradigm assumes that these fluxes are dominated by chromophoric or ‘coloured’ material (cDOC). DOC is often characterised and quantified using optical tools which specifically target this fraction. However, multiple studies point towards a potentially sizeable non-coloured or optically ‘invisible’ DOC (iDOC) pool which is not covered by such characterisations. Only a handful of studies have directly investigated iDOC, and so its source, composition, behaviour, and geographic prevalence remain poorly understood.

Here we show that iDOC accounts for 21 % (0.23 Tg C yr^-1) of annual riverine export in Great Britain (GB), with spatial variation in catchment-scale mean annual export depending upon forest cover and mean dairy cattle density. Using > 2,900 samples from across a range of geo-climatic settings across five continents we find a similar result: iDOC accounts for 26 % of the measured DOC flux in freshwaters. Our results indicate that iDOC is more prevalent in systems with a high degree of anthropogenic influence and/or a high residence time. 

We also show that estuarine DOC behaviour is driven by the contributions of cDOC and iDOC, at least within GB estuaries: cDOC almost universally exhibits conservative transport, whilst apparent non-conservative bulk DOC transport is typically caused by fluctuations in the iDOC fraction.

We conclude that iDOC is a globally significant fraction of the land-ocean carbon flux, the broad scale importance of which has been largely overlooked. This has fundamental implications for (1) our understanding of aquatic biogeochemistry and (2) the use and interpretation of optical parameters as they relate to DOC characterisation and quantification.

This work was primarily funded by the National Environment Research Council (NERC) through the SPITFIRE Doctoral Training Programme (grant number NE/L002531/1) and the Land Ocean Carbon Transfer Programme (LOCATE; grant number NE/N018087/1). 

How to cite: Felgate, S. L. and the Authors: The importance of 'invisible' dissolved organic carbon along the land-ocean aquatic continuum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13281, https://doi.org/10.5194/egusphere-egu22-13281, 2022.

High levels of Faecal Indicator Organisms (FIOs), such as E Coli and Enterococci, at bathing water sites are linked to disease and public health threats. Hydro-environmental models for coastal areas are important for understanding the transport and fate of FIOs, evaluating effectiveness of environmental management strategies on coastal water quality as well as predicting FIO concentrations in bathing water sites. An important aspect in hydro-environmental models is simulating bacteria decay. Bacteria such as FIOs are generally assumed to undergo a first order degradation, C(t)=C₀exp(-kt),  where C(t) is bacteria concentration at time t; C₀ is initial concentration; k is bacteria decay rate. The bacteria decay rate depends on factors such as temperature, solar irradiation, and suspended solid concentration. A number of bacteria decay models, with various level of complexity, have been developed and applied in different waterbodies such as coastal areas, estuaries, and rivers; there is no consensus regarding to the best model for any given scenario. Generic bacteria decay models have been also attempted but they did not outperform site-specific models. This research evaluates the performance of several bacteria decay models in a data rich test site, namely Swansea Bay, located in South-west of UK. More than 7000 FIO samples were taken at key sources and receptors and analysed over two bathing seasons in two years. Environmental data for stream flows, tide levels, meteorology and water quality are also available. These data are important for hydro-environmental model development, calibration, and validation. This research also provides insights to the key drivers of FIOs at the bathing water sites along Swansea Bay. Hydro-environmental models for the Bay were developed with TELEMAC-2D and -3D hydrodynamic solvers, developed by the Research and Development department of Electricité de France (EDF). TELEMAC-2D solves the two-dimensional Shallow Water Equations (SWE) and TELEMAC-3D solves the three-dimensional Navier Stokes Equations (NSE). The two solvers employ the finite element method on unstructured triangular meshes. The solvers have been used in hydro-environmental studies in coastal areas, lakes, and rivers. Two main decay models were considered in this study; the Stapleton model which considers irradiation and suspended solid effects and the Mancini model which considers irradiation, salinity and temperature effects. King (2019) studied the performance of these bacteria decay models at the case study site and suggested that further improvements might be achieved by combining the two models. In this research, the performance of (i) the Stapleton model, (ii) the Mancini model and (iii) a combination of Stapleton and Mancini model were evaluated against measured FIO concentrations.  It was found that one of the key limitations of the hydro-environmental models is that the hydrodynamics of the wet-dry interface in the swash zone may not be represented accurately. Modelling wet-dry interface remains a numerical challenge; there are different modelling approaches, representing different trade-offs between computational efficiency, numerically stability and scientific accuracy. To compensate for this limitation, sensitivity of FIO concentrations to sampling locations was also evaluated. Reference: (i) King JA (2019). https://orca.cardiff.ac.uk/125923/; (ii) Mancini JL (1978). https://www.jstor.org/stable/pdf/25040179.pdf; (iii) Stapleton CM et al. (2007). https://orca.cardiff.ac.uk/40376/

How to cite: Lam, M.-Y. and Ahmadian, R.: Studying transport and decay models for Faecal Indicator Organisms (FIOs) in nearshore coastal waters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12738, https://doi.org/10.5194/egusphere-egu22-12738, 2022.

River and stream metabolism have been proposed as an important tool to determine ecological status, as it encompasses most of the environmental interactions. However, some of the factors influencing it have not been studied with enough depth, which is essential to define its utility as a monitoring and diagnostic tool, especially under the variable conditions of the current global changes. One of these understudied factors is temperature, which may become problematic considering the increasing temperatures and heatwaves occasioned by climate change. For instance, increasing temperatures due to climate change or extreme events may favor the proliferation of algal species resistant to high-temperature variability occasioning blooms and altering ecosystem metabolism. Thus, there is a need to understand how temperature affects ecosystem metabolism and its components, to be able to propose better and more integrative measures to counteract negative changes and make predictions of possible scenarios. This work presents a meta-analysis of the current information that is available on the response of ecosystem metabolism to temperature and highlights some of its implications and perspectives. With this information, scientists, managers, and stakeholders might be able to have a wider perspective and propose more adequate measurements in terms of ecosystem metabolism and ecological status.

How to cite: Reyes Aldana, H. E., Graber, D., Weitere, M., Cohen, M., and Risse-Buhl, U.: The warmer the better?: the relationship between ecosystem metabolism and temperature, possible implications under climate change – a meta-analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10786, https://doi.org/10.5194/egusphere-egu22-10786, 2022.

Globally, flooding is one of the most commonly occurring natural disasters and their frequency of occurrence and intensity is predicted to increase as a result of climate change and associated influences on rainfall intensity, duration and timing. The impact of floods can be exacerbated by associated damage to transport infrastructure, which can impede disaster relief activities, often where needed most. Thailand, and especially Southern Thailand suffers greatly every year and sometimes multiple times a year from flooding causing dramatic human and economic losses. In 2020 for example, after six days of heavy rains, 351 villages were affected by flooding representing a total of 16,709 households and almost 50,000 people.

Flood risk assessments are increasingly considered vital for societies across the world and as a result, flood modelling has considerably improved in recent decades with new formulations, the acquisition of extremely accurate geodesic data and powerful computers able to handle data processing.

This study used a bespoke software Flowroute for the flood risk assessment and flood modelling. This modelling software uses meteorological data and detailed GIS data to produce flood maps with return periods of 20, 50 and 100 years within the six largest catchments of the Krabi and Nakhon Si Thammarat provinces in Southern Thailand. Flood forecast models were run using downscaled regional (3km resolution) predictions under the AR6 RCP6.0 scenario, based on 20 year, 50 year and 100 year return period events.

Results showed a 16-17% increase in flooded area by 2100 compared with 2020 for the 100 year return period events in the Krabi province and a 22-38% increase in flooded area for the 100 year return period events in Nakhon Si Thammarat over the same time period.

The greatest impacts are likely to occur in the middle and lower parts of the catchments. These areas are flatter with a low angled slope in comparison to the higher parts of the catchments running into the valleys of the mountain chains. The sudden topographical changes between the upper part of the catchments and their lower parts means that during heavy rainfall, large amounts of water are very quickly drained towards a main stream that is not able to cope with it, hence water spreading over the river banks and settling more easily on those flat coastal plains. These areas are generally densely populated, used for industrial purposes and farming representing valuable assets for the economy of both provinces and the country. . Anthropic activities such as dam/weir construction or channel realignment are common in these areas and those changes exacerbate the stress on the river system created by the natural setting of these areas.

Based on the information provided by these models, authorities and managers can undertake flood mitigation measures by adapting, improving or creating new flood defences within the catchments. A variety of methodologies have been used in the UK from re-establishing the natural flow of the rivers and streams to developing retention basins along the streams.

How to cite: Ward, R., Curoy, J., Martin, D., Puch, E., Tenedor, J., Wang, Y., Almirew, N., Dudhia, J., Barlow, J., Moses, C., and Nakhapakorn, K.: Climate change driven flood modelling predictions within Southern Thailand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4459, https://doi.org/10.5194/egusphere-egu22-4459, 2022.

Estuarine flooding is driven by extreme sea-levels and river discharge, either occurring independently or at the same time, or in close succession to exacerbate the hazard, known as compound events. There is a need to identify site-specific thresholds for flooding in estuaries, which represent the magnitude of key drivers over which flooding occurs. Site-specific thresholds for flooding can be used to support forecasts and warnings, emergency response and long-term management plans. This research uses historic records of flooding in estuaries around the UK combined with 40 years of historical 15-minute frequency sea-level and river discharge data to establish the magnitude and relative timing of the drivers of flooding in 11 estuaries. The results identify estuaries which are more likely to experience flooding due to extreme compound events, e.g. Conwy, N-Wales, or independent extreme events e.g. Humber, E-England. The key limitation of using historic records of flooding is that not all flooding events have been documented, and there are gaps in the record. Therefore, this research also identified the top 50 extreme sea-level and river discharge events in the historic gauge measurements at each estuary, and cross-checked these against online sources (news reports and academic literature), to establish if these events also led to flooding. A more comprehensive historic record of flooding allows more accurate thresholds for flooding to set in each estuary. Future work will utilise numerical modelling tools in 4 estuaries to simulate flooding under different sea-level and river discharge conditions to further isolate accurate thresholds.

How to cite: Robins, P., Lyddon, C., Lewis, M., Barkwith, A., Vasilopoulos, G., and Coulthard, T.: Using historic records of compound flood events to identify site-specific thresholds for flooding in UK estuaries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12938, https://doi.org/10.5194/egusphere-egu22-12938, 2022.

River and sea ecosystem services contribute to the rapid and intensive development of delta social-ecological systems. This development, alongside other global change drivers, threatens the biodiversity of these deltas and in turn the ecosystem services that this biodiversity supports. However, biodiversity can itself mediate anthropogenic impacts by increasing ecosystem resilience. Linkages between biodiversity and ecosystem services are increasingly established, but we lack understanding of whether the mediating effects of biodiversity are global and ubiquitous, and whether they mediate global change drivers in deltas.

Here, we examine the potential for biodiversity to mediate the relationships between five anthropogenic indicators and global change drivers (population, infrastructure, land use change, climate change in temperature and precipitation) and 19 ecosystem properties and services. We assess these relationships across a global dataset of 235 large deltas. We find that in 89% of cases, greater biodiversity (species richness and the intactness of biodiversity) is connected to a weakened or reversed association between anthropogenic drivers and ecosystem services. Such weaker or reversed associations were found across different ecosystem services (e.g. food production, carbon sequestration, soil regulation), most commonly with climate change and population.

We then investigated the contribution of biodiversity and abiotic and anthropogenic drivers to delta ecosystem service supply. Ecosystem service supply was most strongly and consistently associated with abiotic drivers (mostly climatic), but biodiversity and anthropogenic drivers were also important to individual services (productivity and crop-related services respectively). Deltas showed fewer than expected associations between biotic, abiotic and anthropogenic indicators and ecosystem services, yet weakened or reversed associations were more frequent than in other social-ecological systems. Our results empirically show how biodiversity can both act as a resource and mediate social-ecological relationships, but that both of these roles could be compromised as deltas become more modified.

How to cite: Reader, M. O., Eppinga, M. B., de Boer, H. J., Petchey, O., Damm, A., and Santos, M. J.: Biodiversity mediates human-environment interactions in deltas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11383, https://doi.org/10.5194/egusphere-egu22-11383, 2022.

Connectivity describes the efficiency of material transfer between the components of a system. The definition of these components varies among different disciplines and in relation to the material under consideration.

River systems are complex and dynamic environments where multiple and highly inter-correlated processes occur at various spatial and temporal scales. Because of this reason, in-situ traditional techniques for inland waters monitoring are often inadequate to the full understanding of river processes, making the evaluation of river system and inland-marine water connectivity a challenging task.

In this study, we use high-resolution multispectral satellite data acquired by the Sentinel-2 Earth observation mission of the EU Copernicus Programme to investigate the connectivity of the lower Tiber River basin (Italy) from a sedimentological and biogeochemical point of view. To this end, Level-1C satellite imagery, collected on the study area for the period 2017-2020, were processed through the ACOLITE software to perform image atmospheric correction and to obtain water turbidity (WT) and chlorophyll-a (Chl) concentration values on multiple regions of interest along the river course up to the river mouth and the adjacent coastal area. WT and Chl are indeed key parameters for both sediment transport and water quality monitoring of inland and coastal waters. River connectivity was then evaluated by analyzing the spatio-temporal variability of seasonal climatologies of the satellite-derived parameters.

The analysis showed a significant dependence of suspended sediment transport and chlorophyll concentration on hydrological conditions; however, complex dynamics arises. From a sedimentological point of view, as expected, connectivity seems to be positively correlated with the magnitude of the hydrological events, with the highest and lowest degrees of connectivity of WT during the highest and lowest discharge events respectively (winter and summer). From a biogeochemical point of view, there seems to be an optimum window during moderate hydrological conditions (spring) that, on one hand, allow for sediment resuspension and, therefore, nutrients transport along the river course, but on the other, prevent to reach critical resuspension values that would reduce and/or hinder Chl concentration along the river course.

How to cite: Belloni, R., Pitarch, J., Adduce, C., Tarpanelli, A., and Falcini, F.: A Satellite-based analysis of Tiber River inland-marine water connectivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6445, https://doi.org/10.5194/egusphere-egu22-6445, 2022.

Estuarine management requires fundamental system understanding on drivers and effects of flow and transport dynamics. Among other system descriptors, tidal asymmetry is a fundamental prop-erty, used in many ways, e.g. to define the dominant direction of sediment transport in estuaries. There are several different parametrizations of tidal asymmetry, and the number of methods of their derivation has increased in recent years. We present an attempt to discuss comparability of descriptors for tidal asymmetry. We computed descriptors from one-year measured monitoring data of the Ems estuary. Using conformal mapping we scaled each of these for comparison. A sen-sitivity analysis shows the pronounced influence of freshwater discharge on descriptors derived from velocity data and, on the other hand, the influence of wind on quantities based on duration of tidal phases. The impact of spring neap variability changes over the estuary. Our results show that observations of short periods (e.g. two tides) are not robust compared to the average of a spring neap cycle. Finally, we conclude that the classification of the estuary in terms of flood or ebb dominant sediment transport is critically dependent on location and period of the input data. Further, we discuss how to interpret hydrodynamic parameters derived from point measure-ments. The actual characterization of an estuary requires more comprehensive data, such as var-iability over cross sections, data of suspended sediment concentration and a consideration of the entire density-driven circulation.

How to cite: Wünsche, A., Becker, M., Jürges, J., Kelln, J., and Winter, C.: Comparison of tidal asymmetry descriptors – a sensitivity study based on one-year monitoring data of the Ems estuary, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13455, https://doi.org/10.5194/egusphere-egu22-13455, 2022.

Compared to deltas, lakes and estuaries, rivers generally are characterized by their natural downstream flow that can often be dealt with adequately by having recourse to 1D models. The cross-section integrated Saint-Venant equations are widely used in river modeling and engineering applications. In order to ensure the mass conservation the conservative form of the equations is preferred. In this case, the flux and source terms may be formulated in several ways. It is seen, however, that not all of them lead to stable and accurate numerical results. The choice of the convenient unknown and intermediate variables allows getting an optimal stability with fewer numerical adjustments. Furthermore, in a realistic domain, two different issues should be carefully dealt with, namely the relative paucity of geometric data points and the connection to larger water bodies ( delta, lakes ...). Regarding the data interpolation, the reference level for data definition and interpolation is generalized along the river instead of associating a local reference frame to each cross-section, allowing to obtain a smooth, stable source term. As for the connection to a 2D model, a boundary-connected coupling based on flux continuity is adopted. The aforementioned modules are implemented in the framework of a discontinuous Galerkin finite-element model, i.e., the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM, www.slim-ocean.be). Validation is performed by running the model in idealized configurations. Then, the river-lakes-delta continuum of the Mahakam River (Borneo, Indonesia) is modeled and validation is based on measured water level.

How to cite: Draoui, I., Lambrechts, J., Legat, V., and Deleersnijder, E.: The discontinuous Galerkin method for coupling a 1D river model to a 2D shallow water one, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1084, https://doi.org/10.5194/egusphere-egu22-1084, 2022.