Nutrient and carbon dynamics along the river-estuary-ocean continuum on Central European scale

Nutrient and carbon dynamics within the river-estuary-coastal water systems are key processes to understand the matter fluxes from the terrestrial environment to the ocean. In a large-scale study we analysed those dynamics with the focus of the prevailing low water conditions by following a sampling approach based on the travel time of water. We started with a nearly Lagrangian sampling along the River Elbe (German part; 580 km within 8 days travel time). After a subsequent investigation of the estuary, the plume of the river was followed by raster sampling the German Bight (North Sea) using three ships simultaneously. In the river, intensive growth of phytoplankton was determined connected with high oxygen saturation and pH values as well as under-saturation of CO2, whereas concentrations of dissolved nutrients declined. In the estuary, the Elbe shifted from an autotrophic to a heterotrophic system: Phytoplankton died off upstream of the salinity gradient causing minima in oxygen saturation and pH, supersaturation of CO2, and a release of nutrients. In the coastal region, phytoplankton and nutrient concentrations were low, oxygen close to saturation, and pH in a typical marine range. We detected a positive relationship between pH values and oxygen saturation and a negative one between pCO2 and oxygen saturation. Corresponding to the significant particulate nutrient flux via phytoplankton, flux rates of dissolved nutrients from the river into the estuary were low and determined by depleted concentrations. In contrast, fluxes from the estuary to the coastal waters were higher and the pattern was determined by tidal currents. Overall, the presented observation approach is appropriate to better understand land-ocean fluxes, particularly if it is performed under different hydrological conditions including extremes and seems to be suitable to investigate the impact of such events in freshwater on coastal systems in future. The study was conducted within the frame of the Helmholtz MOSES initiative (Modular Observation Solutions for Earth Systems) targeting processes and impacts of hydrological extremes

Elbe 2020 – investigating a river-sea system from upstream into the North Sea

Understanding river-sea-systems requires a thorough understanding of processes that span different Earth system compartments. To overcome issues related to the interaction of different scientific disciplines and compartments, such as different measurement and calibration standards, quality control approaches and data formats for specific environmental parameters, joint measurement campaigns have been initiated within the Helmholtz Association’s MOSES (Modular Observation Solutions for Earth Systems) project. Following multiple senor comparison and intercalibration campaigns in 2019, MOSES’ Hydrological Extremes event chain working group initiated joint field campaigns in summer 2020 covering the Elbe river from the Czech-German border to the tidal Elbe and further on into the estuary and the German Bight. The fundamental objective was to establish scientifically sound and resilient multi-ship applicable sampling procedures and to create reference data for the main environmental parameters for future investigation of extreme events such as flooding and drought and their overall impact on the catchment region and the adjacent estuarine area of a large European fresh water / marine system. The campaign involved four research vessels, four research centers and spanned nearly two months. Measurements included standard hydrological and oceanographic parameters, as well as quantities relevant to the nutrient and carbonate system. Furthermore, selected water quality indicators and atmospheric measurements were performed. In the fresh water section of the Elbe river measurements were taken while drifting with the water mass. In the tidal section of the river sampling was done against the ebb current while in the North Sea a grid covering a large part of the German exclusive economic zone (EEZ) was sampled. We detected a longitudinal increase of phytoplankton biomass along the 585 km freshwater part of the river towards the tidal system. In contrast, concentrations of dissolved nitrate and phosphate decreased to low values due the uptake by planktonic algae. The concentration of dissolved CO2 decreased caused by increasing photosynthesis while the concentration of methane increased along the river stretch, particularly in the most downstream part when sedimentation of phytoplankton increased the organic load of sediments. The tidal part of the transect showed a strong influence of Hamburg harbor on almost all quantities, while downstream towards the estuary, the effects of the tidal cycle dominated variabilities. In the marine area, elevated chlorophyll concentrations were mainly found near the west coast of Schleswig-Holstein, probably mostly influenced by the Eider river outflow or the adjacent tidal flats. While most of the measured parameters showed an expected behavior relative to their individual compartments, the transfer of quantities between the compartments revealed rather complex and sometimes difficult to understand behaviors and patterns, especially when considering a functional quantitative analysis. The first results of this trans-compartment campaign showed that a quantitative understanding of the fate and dynamics of water constituents across compartments from the spring to the sea needs enhanced scientific collaboration and awareness to finally come to a better integrated understanding of physical, biogeochemical and biological processes from the local to the global scale

Recent inorganic carbon increase in a temperate estuary driven by water quality improvement and enhanced by droughts

Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer, and transformation between land and the coastal ocean. Estuaries are susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant mid-estuary increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg−1 yr−1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary) was driven by an increase in upper-estuary particulate organic carbon (POC) content of 8–14 µmol kg−1 yr−1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is effectively remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of this significant long-term DIC increase, a recent extended drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s−1) compared to the long-term mean (690 ± 441 m3 s−1, 1960–2020), while the late spring internal DIC load in the estuary doubled. The drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a more complete remineralization period of POC. Annually, 77 %–94 % of the total DIC export was laterally transported to the coastal waters, reaching 89 ± 4.8 Gmol C yr−1, and thus, between 1997 and 2020, only an estimated maximum of 23 % (10 Gmol C yr−1) was released via carbon dioxide (CO2) evasion. Export of DIC to coastal waters decreased significantly during the drought, on average by 24 % (2014–2020: 38 ± 5.4 Gmol C yr−1), compared to the non-drought period. In contrast, there was no change in the water–air CO2 flux during the drought. We have identified that seasonal changes in DIC processing in an estuary require consideration when estimating both the long-term and future changes in water–air CO2 flux and DIC export to coastal waters. Regional and global carbon budgets should therefore take into account carbon cycling estimates in estuaries, as well as their changes over time in relation to impacts of water quality changes and extreme hydrological events

Significant shifts in inorganic carbon and ecosystem state in a temperate estuary (1985–2018)

Estuaries regulate carbon cycling along the land-ocean continuum and thus influence carbon export to the ocean, and global carbon budgets. The Elbe Estuary in Germany has been altered by large anthropogenic perturbations, such as widespread heavy metal pollution, minimally treated wastewater before the 1980s, establishment of wastewater treatment plants after the 1990s, and an overall nutrient and pollutant load reduction in the last three decades. Based on an extensive evaluation of key ecosystem variables, and an analysis of the available inorganic and organic carbon records, this study has identified three ecosystem states in recent history: the polluted (1985–1990), transitional (1991–1996), and recovery (1997–2018) states. The polluted state was characterized by very high dissolved inorganic carbon (DIC) and ammonium concentrations, toxic heavy metal levels, dissolved oxygen undersaturation, and low pH. During the transitional state, heavy metal pollution decreased by > 50%, and primary production re-established in spring to summer, with weak seasonality in DIC. Since 1997, during the recovery state, DIC seasonality was driven by primary production, and DIC significantly increased by > 23 μmol L−1 yr−1 in the mid to lower estuary, indicating that, along with the improvement in water quality the ecosystem state is still changing. Large anthropogenic perturbations can therefore alter estuarine ecosystems (on the order of decades), as well as induce large and complex biogeochemical shifts and significant changes to carbon cycling

Large-scale nutrient and carbon dynamics along the river-estuary-ocean continuum

Highlights • Nutrient and carbon fluxes are key processes in land-ocean interactions. • We sampled along the river-estuary-ocean system according to travel time of water. • The river was autotrophic with phytoplankton growth, high pH and oxygen concentration, and CO2 undersaturation. • Phytoplankton died off in the estuary causing low pH and oxygen concentration, CO2 supersaturation, and nutrient release. • The approach is suitable to investigate single events such as hydrological extremes. Nutrient and carbon dynamics within the river-estuary-coastal water systems are key processes in understanding the flux of matter from the terrestrial environment to the ocean. Here, we analysed those dynamics by following a sampling approach based on the travel time of water and an advanced calculation of nutrient fluxes in the tidal part. We started with a nearly Lagrangian sampling of the river (River Elbe, Germany; 580 km within 8 days). After a subsequent investigation of the estuary, we followed the plume of the river by raster sampling the German Bight (North Sea) using three ships simultaneously. In the river, we detected intensive longitudinal growth of phytoplankton connected with high oxygen saturation and pH values and an undersaturation of CO2, whereas concentrations of dissolved nutrients declined. In the estuary, the Elbe shifted from an autotrophic to a heterotrophic system: Phytoplankton died off upstream of the salinity gradient, causing minima in oxygen saturation and pH, supersaturation of CO2, and a release of nutrients. In the shelf region, phytoplankton and nutrient concentrations were low, oxygen was close to saturation, and pH was within a typical marine range. Over all sections, oxygen saturation was positively related to pH and negatively to pCO2. Corresponding to the significant particulated nutrient flux via phytoplankton, flux rates of dissolved nutrients from river into estuary were low and determined by depleted concentrations. In contrast, fluxes from the estuary to the coastal waters were higher and the pattern was determined by tidal current. Overall, the approach is appropriate to better understand land-ocean fluxes, particularly to illuminate the importance of these fluxes under different seasonal and hydrological conditions, including flood and drought events