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Satellite remote sensing of ocean colour properties allows observation of the ocean with high temporal and spatial coverage, facilitating the better assessment of changes in marine primary production. Ocean productivity is often assessed using satellite-derived chlorophyll a concentrations, a commonly used proxy for phytoplankton concentration. We used the Copernicus GlobColour remote sensing chlorophyll a surface concentration to investigate seasonal and non-seasonal variability, temporal trends, and changes in spring bloom chlorophyll a magnitude. Complementary, we analysed the chlorophyll a relationship with sea surface temperature and mixed-layer depth in the German Bight from 1998 to 2020. Empirical orthogonal functions were employed in order to investigate dominant spatial and temporal patterns (modes) related to the main processes of chlorophyll a variability. Multi covariance analysis was used to extract the dominant structures that maximize the covariance between chlorophyll a and sea surface temperature mixed-layer depth fields. High levels of chlorophyll a were found near the coast, showing a decreasing gradient towards offshore waters. A significant chlorophyll a positive trend was observed close to the Elbe estuary and adjacent area, while 55 % of the German Bight was characterized by a significant chlorophyll a negative trend. The chlorophyll a non-seasonal variability showed that the first four modes explained around 45 %, with the first and second modes related to inter- and intra-annual variability, respectively, observed in the temporal principal components spectral analyses. Monthly chlorophyll a concentration anomalies co-varied by 45 % with sea surface temperature anomalies and 23 % with mixed-layer depth anomalies. The monthly averages of chlorophyll a anomaly fields were suitable to investigate long-term trends and variability. The rising water temperature, combined with its indirect effects on other variables, can partially explain the observed trends in chlorophyll a.

The Sylt Roads pelagic time series covers physical and hydrochemical parameters at five neighboring stations in the Sylt–Rømø bight, Wadden Sea, North Sea. Since the beginning of the time series in 1973, sea surface temperature (SST), salinity, ammonium, nitrite, nitrate, and soluble reactive phosphorus (SRP) have been measured twice a week. The other parameters were introduced later (dissolved silicate (Si) since 1974, pH since 1979, dissolved organic nitrogen (DON) since 1996, dissolved organic phosphorus (DOP) since 2001, chlorophyll a since 1979, and suspended particulate matter (SPM) since 1975), and in the case of dissolved oxygen, were already discontinued (1979–1983). In the years 1977, 1978, and 1983, no sampling took place. Since the start of the continuous sampling in 1984, the sea surface temperature in the bight has risen by +1.11 ∘C, with the highest increases during the autumn months, while the pH and salinity decreased by 0.23 and 0.33 units, respectively. Summer and autumn salinities are generally significantly elevated compared to spring and winter conditions. Dissolved nutrients (ammonium, nitrite, nitrate, and SRP) have displayed periods of intense eutrophication (1973–1998) and de-eutrophication since 1999. Silicate has shown significantly higher winter levels since 1999. Interestingly, phytoplankton parameters did not mirror these large changes in nutrient concentrations, as a seasonal comparison of the two eutrophication periods showed no significant differences with regard to chlorophyll a. This phenomenon might be triggered by an important switch in nutrient limitation during the time series. With regard to nutrients, the phytoplankton was probably primarily limited by silicate until 1998, while, since 1999, the SRP limitation has become increasingly important. All data are available in Rick et al. (2017b–e, 2020a–o) from https://doi.org/10.1594/PANGAEA.150032, https://doi.org/10.1594/PANGAEA.873549, https://doi.org/10.1594/PANGAEA.873545, https://doi.org/10.1594/PANGAEA.873547, https://doi.org/10.1594/PANGAEA.918018, https://doi.org/10.1594/PANGAEA.918032, https://doi.org/10.1594/PANGAEA.918027, https://doi.org/10.1594/PANGAEA.918023, https://doi.org/10.1594/PANGAEA.918033, https://doi.org/10.1594/PANGAEA.918028, https://doi.org/10.1594/PANGAEA.918024, https://doi.org/10.1594/PANGAEA.918034, https://doi.org/10.1594/PANGAEA.918029, https://doi.org/10.1594/PANGAEA.918025, https://doi.org/10.1594/PANGAEA.918035, https://doi.org/10.1594/PANGAEA.918030, https://doi.org/10.1594/PANGAEA.918026, https://doi.org/10.1594/PANGAEA.918036, and https://doi.org/10.1594/PANGAEA.918031.

Abstract The German Bight is a shallow area in the southeastern North Sea. Atmospheric forcing, particularly wind stress, plays an essential role in the sea circulation dynamics in the area as a source of momentum and consequent driver of the variability on seasonal to interannual timescales. Westerly and SouthWesterly winds constitute the mean state of this forcing over the North Sea due to the persistent pressure gradients between the Icelandic Low and the Azores High. Consequently, the transport in the North Sea is primarily cyclonic. Nevertheless, the presence of land influences wind stress in the coastal regions (in terms of both intensity and direction). Moreover, distant locations respond differently to the action of the atmospheric pressure centers. Therefore, studies characterizing wind statistics in specific regions are mandatory for understanding and numerically simulating the sea circulation patterns in such areas. We present a detailed analysis of wind patterns in the German Bight, specifically in the Marine Protected Areas and Helgoland Island, using ERA5 reanalysis atmospheric data. We define and catalog area‐specific “events” according to their typical duration and magnitude and analyze their seasonality and interannual variability. We investigate the most recurrent locations and intensities of the high‐ and low‐pressure dipoles causing specific wind patterns over the German Bight during the different periods of the year. We show how, besides Westerly and SouthWesterly winds, NorthWesterly flows are a recurrent pattern in the area; winds from the East are less frequent but can be extremely persistent over the same site in the spring.

We present a test case of river plume spreading to evaluate numerical methods used in coastal ocean modeling. It includes an estuary–shelf system whose dynamics combine nonlinear flow regimes with sharp frontal boundaries and linear regimes with cross-shore geostrophic balance. This system is highly sensitive to physical or numerical dissipation and mixing. The main characteristics of the plume dynamics are predicted analytically but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We document numerical solutions for the Thetis and FESOM-C models on an unstructured triangular mesh, as well as ones for the GETM and FESOM-C models on a quadrilateral mesh. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics. The major result of our comparative study is that accuracy in reproducing the analytical solution depends less on the type of model discretization or computational grid than it does on the type of advection scheme.

This study is dedicated to the tidal dynamics in the Sylt-Rømø Bight with a focus on the non-linear processes. The FESOM-C model was used as the numerical tool, which works with triangular, rectangular or mixed grids and is equipped with a wetting/drying option. As the model's success at resolving currents largely depends on the quality of the bathymetric data, we have created a new bathymetric map for an area based on recent studies of Lister Deep, Lister Ley, Højer Deep and Rømø Deep. This new bathymetric product made it feasible to work with high-resolution grids (up to 2 m in the wetting/drying zone). As a result, we were able to study the tidal energy transformation and the role of higher harmonics in the domain in detail. For the first time, the tidal ellipses, maximum tidally induced velocities, energy fluxes and residual circulation maps were constructed and analysed for the entire bight. Additionally, tidal asymmetry maps were introduced and constructed. The full analysis was performed on two grids with different structures and showed a convergence of the results as well as fulfilment of the energy balance. A great deal of attention has been paid to the selection of open-boundary conditions, model validation against tide gauges and recent in situ current data. The tidal residual circulation and asymmetric tidal cycles largely define the circulation pattern, transport and accumulation of sediment, and the distribution of bedforms in the bight; therefore, the results presented in the article are necessary and useful benchmarks for further studies in the area, including baroclinic and sediment dynamics investigations.

We describe FESOM-C, the coastal branch of the Finite-volumE Sea ice – Ocean Model (FESOM2), which shares with FESOM2 many numerical aspects, in particular its finite-volume cell-vertex discretization. Its dynamical core differs in the implementation of time stepping, the use of a terrain-following vertical coordinate, and the formulation for hybrid meshes composed of triangles and quads. The first two distinctions were critical for coding FESOM-C as an independent branch. The hybrid mesh capability improves numerical efficiency, since quadrilateral cells have fewer edges than triangular cells. They do not suffer from spurious inertial modes of the triangular cell-vertex discretization and need less dissipation. The hybrid mesh capability allows one to use quasi-quadrilateral unstructured meshes, with triangular cells included only to join quadrilateral patches of different resolution or instead of strongly deformed quadrilateral cells. The description of the model numerical part is complemented by test cases illustrating the model performance.

The Lena Delta in Northern Siberia is one of the largest river deltas in the world. During peak discharge, after the ice melt in spring, it delivers between 60–8000 m3 s−1 of water and sediment into the Arctic Ocean. The Lena Delta and the Laptev Sea coast also constitute a continuous permafrost region. Ongoing climate change, which is particularly pronounced in the Arctic, is leading to increased rates of permafrost thaw. This has already profoundly altered the discharge rates of the Lena River. But the chemistry of the river waters which are discharged into the coastal Laptev Sea have also been hypothesized to undergo considerable compositional changes, e.g. by increasing concentrations of inorganic nutrients such as dissolved organic carbon (DOC) and methane. These physical and chemical changes will also affect the composition of the phytoplankton communities. However, before potential consequences of climate change for coastal arctic phytoplankton communities can be judged, the inherent status of the diversity and food web interactions within the delta have to be established. In 2010, as part of the AWI Lena Delta programme, the phyto- and microzooplankton community in three river channels of the delta (Trofimov, Bykov and Olenek) as well as four coastal transects were investigated to capture the typical river phytoplankton communities and the transitional zone of brackish/marine conditions. Most CTD profiles from 23 coastal stations showed very strong stratification. The only exception to this was a small, shallow and mixed area running from the outflow of Bykov channel in a northerly direction parallel to the shore. Of the five stations in this area, three had a salinity of close to zero. Two further stations had salinities of around 2 and 5 throughout the water column. In the remaining transects, on the other hand, salinities varied between 5 and 30 with depth. Phytoplankton counts from the outflow from the Lena were dominated by diatoms (Aulacoseira species) cyanobacteria (Aphanizomenon, Pseudanabaena) and chlorophytes. In contrast, in the stratified stations the plankton was mostly dominated by dinoflagellates, ciliates and nanoflagellates, with only an insignificant diatom component from the genera Chaetoceros and Thalassiosira (brackish as opposed to freshwater species). Ciliate abundance was significantly coupled with the abundance of total flagellates. A pronounced partitioning in the phytoplankton community was also discernible with depth, with a different community composition and abundance above and below the thermocline in the stratified sites. This work is a first analysis of the phytoplankton community structure in the region where Lena River discharge enters the Laptev Sea.

Bio-optical measurements and sampling were carried out in the delta of the Lena River (northern Siberia, Russia) between 26 June and 4 July 2011. The aim of this study was to determine the inherent optical properties of the Lena water, i.e., absorption, attenuation, and scattering coefficients, during the period of maximum runoff. This aimed to contribute to the development of a bio-optical model for use as the basis for optical remote sensing of coastal water of the Arctic. In this context the absorption by CDOM (colored dissolved organic matter) and particles, and the concentrations of total suspended matter, phytoplankton-pigments, and carbon were measured. CDOM was found to be the most dominant parameter affecting the optical properties of the river, with an absorption coefficient of 4.5–5 m−1 at 442 nm, which was almost four times higher than total particle absorption values at visible wavelength range. The wavelenght-dependence of absorption of the different water constituents was chracterized by determining the semi logarithmic spectral slope. Mean CDOM, and detritus slopes were 0.0149 nm−1(standard deviation (stdev) = 0.0003, n = 18), and 0.0057 nm−1 (stdev = 0.0017, n = 19), respectively, values which are typical for water bodies with high concentrations of dissolved and particulate carbon. Mean chlorophyll a and total suspended matter were 1.8 mg m−3 (stdev = 0.734 n = 18) and 31.9 g m−3 (stdev = 19.94, n = 27), respectively. DOC (dissolved organic carbon) was in the range 8–10 g m−3 and the total particulate carbon (PC) in the range 0.25–1.5 g m−3. The light penetration depth (Secchi disc depth) was in the range 30–90 cm and was highly correlated with the suspended matter concentration. The period of maximum river runoff in June was chosen to obtain bio-optical data when maximum water constituents are transported into the Laptev Sea. However, we are aware that more data from other seasons and other years need to be collected to establish a general bio-optical model of the Lena water and conclusively characterize the light climate with respect to primary production.