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In this study, we investigate the role of sedimentary N cycling in the southern North Sea. We present a budget of ammonification, nitrification and sedimentary NO3- consumption and denitrification in contrasting sediment types of the German Bight (southern North Sea), including novel net ammonification rates. We incubated sediment cores from four representative locations in the German Bight (permeable, semi-permeable and impermeable sediments) with labeled nitrate and ammonium to calculate benthic fluxes of nitrate and ammonium and gross rates of ammonification and nitrification. Ammonium fluxes generally suggest oxic degradation of organic matter, but elevated fluxes at one sampling site point towards the importance of bioirrigation or short-term accumulation of organic matter. Sedimentary fluxes of dissolved inorganic nitrogen are an important source for primary producers in the water column, supporting ∼7 % to 59 % of the average annual primary production, depending on water depth. We find that ammonification and oxygen penetration depth are the main drivers of sedimentary nitrification, but this nitrification is closely linked to denitrification. One-third of freshly produced nitrate in impermeable sediment and two-thirds in permeable sediment were reduced to N2. The semi-permeable and permeable sediments are responsible for ∼68 % of the total benthic N2 production rates, which, based solely on our data, amounts to ∼1030 t N d−1 in the southern North Sea. Thus, we conclude that semi-permeable and permeable sediments are the main sinks of reactive N, counteracting eutrophication in the southern North Sea (German Bight).

Despite decades of research on the physiology and biochemistry of nitrate/nitrite-respiring microorganisms, little is known regarding their metabolic response to temperature, especially under in situ conditions. The temperature regulation of microbial communities that mediate anammox and denitrification was investigated in near shore permeable sediments at polar, temperate, and subtropical sites with annual mean temperatures ranging from −5 to 23 °C. Total N2 production rates were determined using the isotope pairing technique in intact core incubations under diffusive and simulated advection conditions and ranged from 2 to 359 μmol N m−2 d−1. For the majority of sites studied, N2 removal was 2 to 7 times more rapid under advective flow conditions. Anammox comprised 6 to 14% of total N2 production at temperate and polar sites and was not detected at the subtropical site. Potential rates of denitrification and anammox were determined in anaerobic slurries in a temperature gradient block incubator across a temperature range of −1 to 42 °C. The highest optimum temperature (Topt) for denitrification was 36 °C and was observed in subtropical sediments, while the lowest Topt of 21 °C was observed at the polar site. Seasonal variation in the Topt was observed at the temperate site with values of 26 and 34 °C in winter and summer, respectively. The Topt values for anammox were 9 and 26 °C at the polar and temperate sites, respectively. The results demonstrate adaptation of denitrifying communities to in situ temperatures in permeable marine sediments across a wide range of temperatures, whereas marine anammox bacteria may be predominately psychrophilic to psychrotolerant. To our knowledge, we provide the first rates of denitrification and anammox from permeable sediments of a polar permanently cold ecosystem. The adaptation of microbial communities to in situ temperatures suggests that the relationship between temperature and rates of N removal is highly dependent on community structure.

Large‐scale experimental exclusion of lugworms (Arenicola marina) from 400 m2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics, benthic infauna composition, and the dominant mineralization and benthic‐pelagic exchange processes in the sediment. Absence of lugworms resulted in sediment clogging with organic‐rich fine particles, an eightfold decrease in sediment permeability, low oxygen penetration depths, and accumulation of reduced mineralization products in the pore water. The sand flat inhabited by lugworms had low fine‐particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water. The effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to ~20‐cm depth. Sediments with the lugworm shared the characteristic of low‐organic, advection‐driven permeable sand rather than of muddy, diffusion‐dominated sediments in the absence of lugworms. Areal oxygen uptake rates depended strongly on hydrodynamic conditions: under calm conditions, sedimentary oxygen uptake was slightly higher at the exclusion site. Experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions (e.g., storms) due to significantly deeper oxygen penetration permitted by higher sediment permeability. Our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water‐sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems.

Among the increasing number of species introduced to coastal regions by man, only a few are able to establish themselves and spread in their new environments. We will show that the Pacific oyster (Crassostrea gigas) took 17 years before a large population of several million oysters became established on natural mussel beds in the vicinity of an oyster farm near the island of Sylt (northern Wadden Sea, eastern North Sea). The first oyster, which had dispersed as a larva and settled on a mussel bed, was discovered 5 years after oyster farming had commenced. Data on abundance and size-frequency distribution of oysters on intertidal mussel beds around the island indicate that recruitment was patchy and occurred only in 6 out of 18 years. Significant proportions of these cohorts survived for at least 5 years. The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north- and southwards along the coastline. Abundances of more than 300 oysters m−2 on mussel beds were observed in 2003, only after two consecutive spatfalls in 2001 and 2002. Analyses of mean monthly water temperatures indicate that recruitment coincided with above-average temperatures in July and August when spawning and planktonic dispersal occurs. We conclude that the further invasion of C. gigas in the northern Wadden Sea will depend on high late-summer water temperatures.

We used X-ray absorption spectroscopy at the Al K-edge to investigate the atomic structure of biogenic silica and to assess the effect of Al on its crystal chemistry. Our study provides the first direct evidence for a structural association of Al and Si in biogenic silica. In samples of cultured diatoms, Al is present exclusively in fourfold coordination. The location and relative intensity of X-ray absorption near-edge structure (XANES) features suggests the structural insertion of tetrahedral Al inside the silica framework synthesized by the organism. In diatom samples collected in the marine environment, Al is present in mixed six- and fourfold coordination. The relative intensity of XANES structures indicates the coexistence of structural Al with a clay component, which most likely reflects sample contamination by adhering mineral particles. Extended X-ray absorption fine structure spectroscopy has been used to get Al-O distances in biogenic silica of cultured diatoms, confirming a tetrahedral coordination. Because of its effect on solubility and reaction kinetics of biogenic silica, the structural association between Al and biogenic silica at the stage of biosynthesis has consequences for the use of sedimentary biogenic silica as an indicator of past environmental conditions.

The external forcing of the German Bight system is largely due to the atmosphere. Energy fluxes that drive mixing processes and biological productivity, as well as atmospheric nutrient inputs outside the Elbe estuary, are important factors for biomass production. This study is based on the KUSTOS experiments focusing on air-sea exchange with intensive observations of a) radiative fluxes at the surface of the drifting water body; b) atmospheric surface layer parameters determining the mixing conditions in the planetary boundary layer; c) the speciation of atmospheric nitrogen compounds; d) and changes in aerosol and gas composition during transport over sea. These episodic data were complemented by a) synoptic data analysis of water and air temperature, wind, pressure and water vapour pressure over the sea; b) corresponding oceanic data on heat advection and mixed layer depth from an oceanic model driven by observations in the atmosphere; c) computations of the highly variable heat and radiative fluxes with the mesoscale atmospheric model METRAS; d) long-term atmospheric deposition measurements of nutrients in the German Bight; e) investigations of the atmospheric processes responsible for the formation of coarse particulate nitrate by means of a new aerosol submodel in the METRAS transport model. We present detailed seasonal or annual budgets for fluxes of heat, momentum, nitrate, ammonium, persistent organic pollutants. The atmospheric fluxes of heat and chemical matter are compared with load and fluxes in the water column in order to identify when and where the atmospheric impact is relevant and detectable. Spatial and temporal variability is discussed for the fluxes of heat, momentum and nitrogen. From the budgets we identify categories of potential atmospheric impact. Apart from the category “no atmospheric impact≓ valid e.g. for Cr, As, Ni and phosphate, we identify 4 others: 1) “atmosphere driven≓: short term, local dominant impact for Heat and momentum; 2) “episodic atmospheric impact≓: long term, local and dominant impact with large fluxes involved for radiation, PCB, Pb; 3) “persistent atmospheric pollutant≓: long term dominant but regionally indifferent impact for α- and γHCH; 4) “steadily perturbing the marine ecosystem≓: long term, widespread impact superimposed on the dynamic system driven by marine biology for nitrate, ammonium.

A 3-dimensional numerical model calculates the transport of suspended particulate matter (SPM) in the German Bight. The calculations are based on current and wave data computed by other models. The aim is to test the SPM model’s quality by comparing model predicted SPM concentrations (a) with a time series of measured SPM concentrations and (b) with a horizontal distribution of SPM surface concentrations in the German Bight. The comparison shows that the model is not able to reproduce the typical horizontal SPM distribution in the German Bight. The poor performance of the model is primarily due to the fact that its description of currents and waves is too coarse for the hydrodynamically complicated environment of the German Bight.

Based on numerical simulations and high resolution measurements, budgets have been computed for transports of water and heat as well as dissolved and particulate matter between the coastal waters of the German Bight and the North Frisian Wadden Sea. p]During 3 measuring campaigns in summer, spring and winter, current conditions and water transports differed only slightly. In spite of different meteorological situations, inflow and outflow of North Sea water to and from the Wadden Sea were largely balanced. A net heat export from the North Frisian Wadden Sea was determined both in summer 1994 and spring 1995, i. e. the Wadden Sea represented a heat source for the adjacent North Sea waters. To compute transports of dissolved nutrients between the Wadden Sea and adjacent North Sea waters, in situ measurements were linked in different ways with the results of hydrodynamic-numerical models. The results of two independent methods show very good correspondence and definite seasonal differences for nitrate and phosphate. In spring and summer, nitrate input from the adjacent North Sea coastal waters into the Wadden Sea and phosphate export from the Wadden Sea into the German Bight was calculated. The exchange of silicate during spring and summer was within the scatter of silicate transports. In winter, an export of silicate from the Wadden Sea was observed. To compute suspended matter transports, the seston concentrations measured at anchor stations were multiplied by modelled water transports. During summer, input tendencies of particulate organic matter were computed. As a result of their horizontal gradients, some planktonic components (parts of phyto-, zoo-, myco and bacterioplankton) showed transport tendencies counter to the transport directions of total particulate organic matter.

From 1994 through 1996 transformation processes in the water column of the German Bight and the adjacent Wadden Sea were investigated in the projects TRANSWATT and KUSTOS. On the basis of a review of carbon and nutrient budgets we examine the role of processes in the sediment for overall carbon and nutrient cycling in the Wadden Sea and adjacent German Bight. We distinguish two aspects: the sediment as the site where organic matter is rapidly turned over and the sediment as the site where organic matter and nutrients are immobilized. The relative importance of the sediment for the remineralisation of organic matter depends on the water depth: The review of carbon budgets suggests that in the Wadden Sea (2 - 3 m) about 50% of the remineralisation occurs in the sediment. In the German Bight (20 m), 10 - 20% of the primary production is remineralised in the sediment. The budgets further show that the Wadden Sea is heterotrophic. About 100 gC m-2 y1 is imported from the coastal zone. This implies a net autotrophy of the coastal zone, which is in line with the results from the projects TRANSWATT and KUSTOS. Within the Wadden Sea, organic matter has to be turned over two to three times and in the German Bight three to four times to explain the annual primary production. This is lower than in the offshore North Sea where annual turnover rates up to five have been found. Several processes remove nutrients on longer time-scales from the biogeochemical cycle. The importance of the local formation of phosphorus containing minerals like apatite as a phosphorus sink is shown. A discussion of several denitrification estimates concludes that in the German Bight and adjacent Wadden Sea on average about 8 -16% of the total nitrogen influx (from the coastal zone, from rivers and via the atmosphere) is lost to the atmosphere.

Between February 1992 and June 1993 the distribution of particulate and dissolved phosphorus in the Ems estuary was investigated in approximately monthly intervals. Furthermore, the phosphorus content of sediments was assessed. These data as well as literature data were used to construct a phosphorus budget of the Ems estuary. Fluxes were dominated by the particulate fraction representing 77% of the total phosphorus input. 40% of the phosphorus input is lost to the sediment. In suspended matter from the coastal zone and from rivers, organic and iron-bound phosphorus dominate (> 75%). Within the sediments phosphorus is mainly associated with calcium minerals like apatites (65%). The presence of authigenic apatites was confirmed with a sequential extraction method. Combination of the phosphorus budget with the data on the composition of particulate phosphorus shows that the organic and iron bound phosphorus is not stable on longer time scales: about 80% is either transformed into dissolved phosphorus and exported from the estuary or transformed to apatites and buried. It is suggested that formation and retention of authigenic apatites is the major process withdrawing phosphorus from the coastal biogeochemical cycle.

The resuspension of microphytobenthos (mainly benthic diatoms) and mud (

The Wadden Sea is a shallow tidal area along the North Sea coast of The Netherlands, Germany and Denmark. The area is strongly influenced by rivers, the most important of which are the rivers Rhine, Meuse and Elbe. Due to the increased nutrient load into the coastal zone the primary production in the Wadden Sea almost tripled during the past few decades. A conceptual model is presented that links nitrogen input (mainly nitrate) via Rhine and Meuse with the annual nitrogen cycle within the Wadden Sea. Three essential steps in the model are: (1) nitrogen limits the primary production in the coastal zone, (2) a proportional part of the primary produced organic matter is transported into the Wadden Sea and (3) the imported organic matter is remineralized within the Wadden Sea and supports the local productivity by nitrogen turn-over. The conceptual model predicts that during years with a high nutrient load more organic matter is produced in the coastal zone and more organic matter is transported into and remineralized within the Wadden Sea than during years with low nutrient loads. As a proxy for the remineralisation intensity ammonium plus nitrite concentrations in autumn were used. Based on monitoring data from the Dutch Wadden Sea (1977–1997) the above mentioned model was statistically tested. In autumn, however, a significant correlation was found between autumn values of ammonium and nitrite and river input of nitrogen during the previous winter, spring and summer. The analysis supports that in years with a high riverine nitrogen load more organic matter is remineralized within the Wadden Sea than in years with a low nitrogen load. A comparison with older data from 1960 to 1961 suggests that the remineralisation intensity in the Wadden Sea has increased by a factor of two to three. This is not reflected by a two to three-fold increase in riverine nitrogen load from 1960 to present. It is suggested that the increased remineralisation rates in the Dutch Wadden Sea between the 1960s and the 1980s/1990s are largely caused by an increased nitrogen flux through the Channel and the Strait of Dover and by an increased atmospheric nitrogen input.

The distribution of dissolved and particle-bound phosphorus (P) was investigated in the Elbe estuary during March 1995. The forms of particulate P were studied with a sequential extraction technique. Organic P dominated particle-bound P in the outer reaches of the estuary (52%), decreased to a minimum of 21% in the turbidity zone, and increased to 33% further upstream. Fe-bound P was the second most important P species in the outer reaches (27%) and dominated in the turbidity zone (up to 57%) and upstream of the turbidity zone (up to 48%). The P:Fe ratio increased with decreasing salinity, from 0.11 in the outer reaches to about 0.22 at zero salinity. Dissolved inorganic P release from reverine suspended matter was about two to three times larger than release, from marine suspended matter and was dominated by release of Fe-bound P. Dissolved inorganic P release from marine and from riverine organic matter were of equal importance. Because marine suspended matter dominates in the estuary, this suggests riverine organic matter is remineralized much faster than marine organic matter. This is in line with the refractory nature of marine organic matter (no phytoplankton bloom) and the easily degradable character of the riverine suspended matter (phytoplankton bloom) in the Elbe estuary during March 1995.

From February 1992 until June 1993, the distribution of dissolved and particulate phosphorus and nitrogen was investigated in the Ems estuary at approximately monthly intervals. Nutrient import was quantified from the river load. Nutrient export to sea was quantified from river discharge and from the salinity-nutrient gradient in the outer estuary. In addition, sediment cores were taken from four sites along the main axis of the estuary in October 1992. On the basis of these data a nitrogen and phosphorus budget was made. On an annual basis, 45 × 106 mol P and 2,360 × 106 mol N are imported into the Ems estuary. Freshwater runoff is the main source of input, accounting for about 92% of the nitrogen input and 71% of the phosphorus input. Import of particulate phosphorus from the sea is important in the phosphorus budget (27%). Seventy-five percent of the nitrogen input is transported to the North Sea. Denitrification is the major loss factor (19% of the nitrogen input), and burial explains 6%. Of the phosphorus input, 60% is transported to the North Sea and 40% accumulates in the sediment. Nitrogen import during summer explains about one third of the annual primary production, indicating that nitrogen turn over is about three times. Phosphorus import during summer explains less than 16% of the annual primary production. We suggest that trapping of particulate P and adsorption onto Fe(oxy)hydroxides during the entire year and the release of Fe-bound P during summer after reduction of Fe(oxy)hydroxides is instrumental in sustaining high primary production, which could not be sustained if it depended only on P imported during the growing season.

Temporal dynamics and vertical patterns in bacterial abundances and activities were studied in a shallow subtidal sand flat in the Sylt-Rømø Basin (North Frisian Wadden Sea, Germany). Extracellular enzymatic activities, bacterial carbon production and community respiration showed strong (factor of 4–5) temporal variations that were mostly related to seasonal temperature change and to changes in substrate availability. These temporal patterns in enzymatic activity were barely reflected in bacterial (200–400 mmol C m−2) and microphytobenthic biomass (800–1500 mmol C m−2) or the sedimentary carbohydrate inventory (1300–2900 mmol C m−2), suggesting that grazing controls the standing stocks of the microphytobenthic and bacterial assemblages. Despite their exposure to strong hydrodynamic forces such as tidal currents and wind-induced wave surge, the subtidal sandy sediments showed persistent vertical gradients in bacterial abundances, carbon production and extracellular enzymatic activities at all times. The vertical distribution of these parameters was tightly coupled to that of the microphytobenthos, dominated by diatoms. Despite the low organic carbon content typical for surge-exposed sandy sediments, high extracellular enzymatic activities and bacterial carbon production rates indicate a very active heterotrophic bacterial community, with a gross secondary productivity of 30–180 mmol C m−2, and a biomass turnover time of 2–18 days. Our data suggest that this high activity is supported by the rapid flux of carbohydrates from microphytobenthic primary productivity. Accordingly, the potential activities of enzymes hydrolyzing carbohydrates cover most of the total bacterial carbon demand during all seasons.

Experimental observations showed that the presence of relatively dense populations of benthic diatoms in the water can be caused by water currents of relatively low velocity. However, field observations showed that wave action can strongly increase the whirling up of these populations. Observations done in the laboratory and in the field showed that benthic diatoms reached the water column together with resuspended sediment. The field observations indicated that, in contrast with the fine sediment fraction, bare parts of sand grain particles are relatively insignificant as a substrate for benthic diatoms. It is suggested that in turbid areas with extensive flats the benthic diatom populations are an important additional food source for the pelagic fauna.