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

Abstract

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.

Metabolic alkalinity release from large port facilities (Hamburg, Germany) and impact on coastal carbon storage

“…where FInput is the input value, FOutput the output value, FBaseline the salinity corrected baseline value, FASF (O2, CO2) the air-sea flux of O2 and CO2, respectively, and FMetabolic the metabolic gain as the closing term. The uncertainty of the closing term has been estimated by using the analytical measurement precision of 2 µmol kg -1 for DIC and TA, 0.5 µmol L -1 for NO3 -, and 5 % for O2 (Petersen et al, 2011). The gas exchange was considered with an uncertainty of 20 % (Wanninkhof, 2014;Watson et al, 2009).…”

“…and may hold oxygen deficits (Mucci et al, 2011) leading to high N retention (De Jonge et al, 1994). Compared to deep estuaries, shallow ones like the Elbe Estuary, are usually well ventilated (Pein et al, 2021;Abril et al, 2002), have a strong benthic-pelagic coupling due to vertical exchange, and respond directly to seasonal forcing.…”

“…Just upstream of the port of Hamburg, dredging activities have increased the depth from around 5 m to about 20 m to guarantee accessibility of large seagoing vessels. Here, a hotspot of organic matter turnover exists (Pein et al, 2021;Sanders et al, 2018;Schöl et al, 2014). Increasing depths favor seasonal stratification, and sedimentary structures in the port area favor the formation of oxygen-poor or even hypoxic zones (Pein et al, 2021;Kerner, 2007), which are essential for anaerobic metabolic processes that use terminal electron acceptors other than O2 (e.g.…”

“…Here, a hotspot of organic matter turnover exists (Pein et al, 2021;Sanders et al, 2018;Schöl et al, 2014). Increasing depths favor seasonal stratification, and sedimentary structures in the port area favor the formation of oxygen-poor or even hypoxic zones (Pein et al, 2021;Kerner, 2007), which are essential for anaerobic metabolic processes that use terminal electron acceptors other than O2 (e.g. NO3 -, Fe3 + , Mn4 + , SO4 2-) to respire organic matter.…”

“…1). We took surface water (1.2 m depth) samples every 20 minutes out of the flow-through FerryBox system (Petersen et al, 2011) that concurrently provides the required physical parameters such as salinity, temperature and oxygen. Water depth in the Elbe Estuary was around 15-20 m but sharply decreased to 5 m upstream Elbe stream km 620.…”