Oxygen Consumption of Resuspended Sediments of the Upper Elbe Estuary: Process Identification and Prognosis

The resuspension of sediment leads to an increased release of nutrients and organic substances into the overlying water column, which can have a negative effect on the oxygen budget. Especially in the warmer months with a lower oxygen saturation and higher biological activity, the oxygen content can reach critical thresholds in estuaries like the upper Elbe estuary. Many studies have dealt with the nutrient fluxes that occur during a resuspension event. However, the sediment properties that influence the oxygen consumption potential (OCP) and the different biochemical processes have not been examined in detail. To fill this gap, we investigated the biogeochemical composition, texture, and OCP of sediments at 21 locations as well as the temporal variability within one location for a period of 2 years (monthly sampling) in the upper Elbe estuary. The OCP of sediments during a seven-day resuspension event can be described by the processes of sulphate formation, nitrification, and mineralisation. Chlorophyll, total nitrogen (Ntotal), and total organic carbon showed the highest correlations with the OCP. Based on these correlations, we developed a prognosis model to calculate the OCP for the upper Elbe estuary with a single sediment parameter (Ntotal). The model is well suited to calculate the oxygen consumption of resuspended sediments in the Hamburg port area during the relevant warmer months and shows a normalised root mean squared error of < 0.11 ± 0.13. Thus, the effect of maintenance measures such as water injection dredging and ship-induced wave on the oxygen budget of the water can be calculated.

Seasonal Stratification and Biogeochemical Turnover in the Freshwater Reach of a Partially Mixed Dredged Estuary

The Elbe estuary is a substantially engineered tidal water body that receives high loads of organic matter from the eutrophied Elbe river. The organic matter entering the estuary at the tidal weir is dominated by diatom populations that collapse in the deepened freshwater reach. Although the estuary’s freshwater reach is considered to manifest vertically homogenous density distribution (i.e., to be well-mixed), several indicators like trapping of particulate organic matter, near-bottom oxygen depletion and ammonium accumulation suggest that the vertical exchange of organic particles and dissolved oxygen is weakened at least temporarily. To better understand the causal links between the hydrodynamics and the oxygen and nutrient cycling in the deepened freshwater reach of the Elbe estuary, we establish a three-dimensional coupled hydrodynamical-biogeochemical model. The model demonstrates good skill in simulating the variability of the physical and biogeochemical parameters in the focal area. Coupled simulations reveal that this region is a hotspot of the degradation of diatoms and organic matter transported from the shallow productive upper estuary and the tidal weir. In summer, the water column weakly stratifies when at the bathymetric jump warmer water from the shallow upper estuary spreads over the colder water of the deepened mid reaches. Enhanced thermal stratification also occurs also in the narrow port basins and channels. Model results show intensification of the particle trapping due to the thermal gradients. The stratification also reduces the oxygenation of the near-bottom region and sedimentary layer inducing oxygen depletion and accumulation of ammonium. The study highlights that the vertical resolution is important for the understanding and simulation of estuarine ecological processes, because even weak stratification impacts the cycling of nutrients via modulation of the vertical mixing of oxygen, particularly in deepened navigation channels and port areas.

eDNA as a tool for non-invasive monitoring of the fauna of a turbid, well-mixed system, the Elbe estuary in Germany

The Elbe is one of the longest European rivers and features a large, turbid and well-mixed estuary, which runs through the inner city of Hamburg. The Elbe has been closely monitored using classical catch techniques in the past. Here we tested a COI-based eDNA approach for assessing the biodiversity within the Elbe. We sampled three stations in the Elbe, included low and high tide events, as well as two adjoining lakes to compare the recovered faunas. To analyze the data, we employed two different pipelines: the automated mBRAVE pipeline utilizing the BOLD database and one including NCBI BLAST. The number of OTUs with species or higher-level identifications were similar between both approaches with 352 OTUs and 355 OTUs for BLAST and mBRAVE, respectively, however, BLAST searches recovered another 942 unidentified metazoan OTUs. Many taxa were well represented; however, fish species were poorly represented, especially in the Elbe estuary samples. This could be a result of the universal COI primers, which also yielded high read numbers for non-metazoan OTUs, and small-bodies taxa like Rotifera, which might have been sampled together with the eDNA. Our results show a strong tidal influence on the recovered taxa. During low tide, downstream stations resembled sites further upstream, but the former showed a very different OTU composition during high tide and early tide. Such differences might be due to varying impacts of upstream-originating eDNA during tide cycles. Such factors need to be considered when routinely employing eDNA for monitoring programs.

Observing Water Surface Temperature from Two Different Airborne Platforms over Temporarily Flooded Wadden Areas at the Elbe Estuary—Methods for Corrections and Analysis

Over the Hahnöfer Nebenelbe, a part of the Elbe estuary near Hamburg, Germany, a combined aerial survey with an unmanned aerial system (UAV) and a gyrocopter was conducted to acquire information about the water surface temperatures. The water temperature in the estuary is important for biological processes and living conditions of riverine organisms. This study aimed to develop a workflow that allows for comparing and analysing surface temperatures acquired by two different remote sensing systems. The thermal infrared (TIR) datasets were compared with in situ measurements gathered during the data acquisition, where both TIR datasets showed a varying bias. Potential error sources regarding the absolute and relative accuracy were investigated and modelled based on the available measurements, including emissivity, atmosphere, skin effect at the water surface, camera flat field correction and calibration. The largest effects on the observed TIR water temperature had the camera calibration and the modelled atmospheric effects. After the correction steps, both datasets could be combined to create a multitemporal representation of the temperature pattern and profiles over the survey area’s wadden flats.

Tidal Oscillation and Resonance in Semi-Closed Estuaries—Empirical Analyses from the Elbe Estuary, North Sea

Many tidal influenced estuaries and coastal basins feature tidal amplification because of, e.g., convergence and reflection. Increasing amplification rates were observed in the Elbe estuary, with consequences for construction measures, nautical manoeuvring, flood protection, riverbed morphology and ecosystems. Although many studies were conducted investigating the tidal wave transformation in estuaries, studies based on spatially well-distributed empirical data covering periods over more than a year are rare. To fill this gap, a self-developed adapted harmonic analysis method of least squares was applied to hydrographs from 25 gauges, distributed over the tidal influenced estuary from the river mouth to the tidal border which is given by the weir 160 km upstream of the river mouth. The investigation period for the harmonic analyses covers a whole nodal cycle of 18.613 a beginning in the year 2000. The tidal constituents’ oscillatory behaviour including the appearance of compound tides, generated by nonlinear shallow water processes, and the formation of reflection induced partially standing waves are determined. The tidal constituents show shared frequency-group specific partial clapotis, but also have significant differences in amplification within those groups. The latter fact contributes to the detected inverse proportionality of tidal range amplification inside the estuary to incoming tidal wave height. As reflection can cause resonance in tidal influenced rivers, tests are developed to analyse whether criteria for resonance are met. To determine the system’s specific resonance frequency, a new method was introduced with the three-parameter Lorentzian curve-fitting. As the detected resonance frequency is not close to tidal frequencies, full-established resonance of the tidal wave and of the tidal constituents is not observed in the Elbe estuary. Migrating nodes of the partially standing tidal wave hint at increasing latent resonance.

Metabolic alkalinity release from the Elbe estuary to the North Sea

&lt;p&gt;The Elbe is the largest river entering the German Bight. Its estuary is a heavily used waterway connecting the sea to Germany&amp;#8217;s biggest port in Hamburg. The Elbe navigation channel is continuously dredged, and agricultural fertilizer input from the catchment ensuing large phytoplankton blooms in the river Elbe exerts additional anthropogenic pressure. Biogeochemistry in the estuary is additionally governed by the North Sea and its strong tidal cycles, which ensure an exchange of fresh and marine waters.&lt;/p&gt;&lt;p&gt;The aims were to quantify the release of the carbon species total alkalinity (TA) and dissolved inorganic carbon (DIC) along the Elbe estuary, and to estimate the contribution of aerobe and anaerobe metabolic processes. Therefore, we used water samples collected continuously during a cruise in June 2019, to measure TA and DIC, and the stable isotopes of nitrate. We applied mass balances, to characterize the metabolic activity and detect their effect on the carbon species&lt;/p&gt;&lt;p&gt;The Elbe estuary could be subdivided into two parts: 1) an outer marine driven part, which is dominated by conservative mixing, also visible in higher TA than DIC values, and 2) an inner fresh water part in which metabolic processes play an important role.&lt;/p&gt;&lt;p&gt;We found a strong increase in TA and DIC (several hundred &amp;#181;mol kg&lt;sup&gt;-1&lt;/sup&gt;) in the Hamburg port area, with higher DIC than TA values. We unraveled the water column impacts of nitrification and denitrification on TA and DIC by analyzing the stable isotopes &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N-NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O-NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, and identified water column nitrification as a dominant pelagic process in the port of Hamburg and in the fresh water part further downstream. Because nitrification cannot explain the significant increase of TA and DIC in the port region, anaerobic processes such as denitrification in the sediment also appear to play an important role.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Pelagic oxygen consumption rates in the Elbe estuary: Proxies and spatial patterns.

&lt;div&gt; &lt;div&gt;&lt;span&gt;The Elbe estuary is strongly impacted by human activities including dredging, land reclamation and eutrophication. Since about 30 years, water quality improved leading to major phytoplankton blooms in the Elbe river. When these blooms enter the upper estuary including the Hamburg port area, they collapse leading &amp;#160;to low oxygen conditions. During a cruise in September 2020 we measured oxygen consumption rates in water samples of the Elbe Estuary between the coastal North Sea (Wadden Sea) and &amp;#160;the weir in Geesthacht including a large freshwater part of the Elbe estuary. In addition, suspended matter samples were taken and analysed for chlorophyll, particulate C (PC) and particulate N (PN). Oxygen consumption rates reached maximum values at both the marine side (~0.3 &amp;#181;mol/(l*h)) and the freshwater &amp;#160;side of the estuary (~0.65 &amp;#181;mol/(l*h)) and a distinct minimum near the Estuarine Turbidity Maximum near the onset of the salinity gradient. Based on specific weights of phytoplankton and PC content, we estimated the contribution of newly formed organic matter. This estimate correlated significantly with the observed oxygen consumption rates. We suggest that most of the riverine organic matter is degraded within the freshwater part of the Elbe estuary before reaching the salinity gradient. This is in line with significant amounts of nitrate being released within the freshwater part of the estuary.&lt;/span&gt;&lt;/div&gt; &lt;/div&gt;

Long-term changes in inorganic carbon in the Elbe estuary

&lt;p&gt;Overall, estuaries are net CO&lt;sub&gt;2&lt;/sub&gt; sources to the atmosphere, releasing an estimated 0.25 Pg C yr&lt;sup&gt;-1&lt;/sup&gt;, which could counterbalance the shelf uptake of approximately 0.25 Pg C yr&lt;sup&gt;-1&lt;/sup&gt;. River discharge can influence both, the CO&lt;sub&gt;2&lt;/sub&gt; flux from estuary to the atmosphere, as well as the magnitude of dissolved inorganic carbon (DIC) exported to coastal waters. In Europe, climate change is expected to cause an increased precipitation in winter and longer periods of drought in summer. The goal of this study is to elucidate the influence of climate-change-induced hydrological changes on an estuarine carbonate system.&lt;/p&gt;&lt;p&gt;The Elbe River is one largest river basins in central Europe, where over 24 million people live in the catchment area. Since 2014, annual Elbe river discharge has been relatively low at 492.95 m&lt;sup&gt;3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, compared to the mean river discharge from 2008 to 2018 at 652.95 m&lt;sup&gt;3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;. 2018 was especially dry, with a discharge of 441 m&lt;sup&gt;3 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, the lowest annual mean river discharge since 1992. The Elbe estuary has been extensively sampled by the Flussgebietsgemeinschaft (FGG) Elbe (Elbe River Basin Community), qualifying the region as a suitable site to study the natural and anthropogenic impacts on estuarine systems.&lt;/p&gt;&lt;p&gt;Preliminary results of the 1985-2018 FGG dataset indicate a major shift in the carbonate system dynamics in the Elbe estuary. From assessing the behaviour of DIC and other ecosystem parameters along the estuary over time, the region can be separated into three ecosystem states. During the time of high pollution, from 1985 to 1990, the estuary exhibited high levels of DIC. Between 1991 and 1996 is the transitional period. After 1997, the ecosystem parameters appear to be exhibiting similar patterns throughout each year with similar levels and therefore this period can be classified as the current ecosystem state. Since 1997, DIC exhibits a drawdown in spring and summer months in the upper region, coinciding with the increase in dissolved oxygen saturation and pH, which can indicate that this region is net autotrophic. Further downstream, DIC then increases along the estuary, and often peaks in the maximum turbidity zone.&lt;/p&gt;&lt;p&gt;For this study, we apply multiple linear regression to determine the relative importance of ecosystem variables that contribute to annual and monthly DIC variability in the recent ecosystem state. Key ecosystem variables include particulate and dissolved organic carbon, pH, dissolved oxygen and river discharge.&lt;/p&gt;

Effects of mean sea level rise and tidal flat growth on tides and storm surge events in the Elbe estuary

&lt;p&gt;Future mean sea level rise will influence tidal dynamics and storm surge events in estuaries. The bathymetry in estuaries and coastal areas will also be affected by mean sea level rise, since it is in a morphodynamic equilibrium with hydrodynamic forces. Tidal flats, which are an important component of coastal protection, will grow to a certain extent with mean sea level rise in case of sufficient sediment availability.&lt;/p&gt;&lt;p&gt;With the help of a highly resolved hydrodynamic-numerical model of the German Bight (North Sea), we analyse the potential influence of mean sea level rise and vertical growth of tidal flats on tidal dynamics and storm surge events in the Elbe estuary.&lt;/p&gt;&lt;p&gt;The results show an increase of tidal amplitude and storm surge water levels due to mean sea level rise. A bathymetric rise of tidal flats in the German Bight and the mouth of the Elbe estuary leads to a decrease in storm surge water level and tidal amplitude compared to the scenario with sole mean sea level rise without a change in bathymetry. Further analyses show, how geometric parameters of the Elbe estuary are changing due to mean sea level rise and tidal flat growth. These changes in geometry influence tidal dynamics and can therefore be an explanation for the observed changes in tidal amplitude and storm surge water levels.&lt;/p&gt;&lt;p&gt;These findings enable a better understanding of future changes in the Elbe estuary and support coastal managers in decision making processes concerning adaptation options to reduce the impacts of climate change.&lt;/p&gt;