U. H. Brockmann

Cycling of nutrient elements in the North Sea

Abstract The cycling of elements of inorganic and organic nutrients (carbon, nitrogen, phosphorus and silicate) in the North Sea is described. The regional effects on nutrient cycling such as thermal and haline stratification, coastal interaction, river discharges, upwelling and frontal zones are discussed. The horizontal and vertical distribution of the inorganic nutrients (nitrate, phosphate, ammonia and silicate) at the surface is given for the whole North Sea during two situations: spring (1986) and winter (1987). In winter, highest nutrient concentrations were found at the northern boundary in the Atlantic inflow, and in the continental coastal waters. During the winter cruise, nutrient minima were detected in the Dogger Bank area. This is an indication that primary production continues during winter. Generally, the surface concentrations during winter were similar to the bottom concentrations. Except for phosphate, highest concentrations were measured just above the bottom. During late spring 1986 the concentrations of nutrients at the surface and below the densicline were generally significantly lower than during winter. Only at the Atlantic boundary in the north and near the estuaries higher concentrations were detected. In stratified parts of the North Sea, the decomposition of sedimented biomass caused the ammonia concentrations in the bottom layer to be significantly higher in spring than in winter. Incidents of frontal upwelling in the central North Sea introduce nutrient-rich bottom water into the euphotic zone, enhancing phytoplankton growth in the central North Sea during summer. The ratios of nitrogen nutrients to phosphate show that in the central North Sea nitrogen is a limiting factor rather than phosphorus, whereas in the continental coastal water and off England the opposite is true. Riverine input and trapping mechanisms in the estuaries and tidal flats cause the concentrations of organic matter (dissolved and particulate) to be highest in the coastal zones and to decrease seaward. During summer the concentration of dissolved organic carbon increases throughout the North Sea. It is calculated that about 60% of the biomass formed by primary production is converted into dissolved organic carbon, 40% directly goes into the foodweb. The biological impact of the plankton is readily apparent from increased surface concentrations of different dissolved organic substances during spring blooms. Examples of eutrophication and effects of nutrient limitation are given, together with other biological repercussions such as coupling of phytoplankton and nutrient succession. Budget calculations for the different nutrient elements show that in the North Sea the biological turnover greatly exceeds the estimated annual inflow and outflow of nutrient elements. Finally, recommendations are given for future research. They include analysing dissolved organic compounds and micronutrients and following multidisciplinary measurements strategies at one location in order to obtain more information for balancing budgets and for the detailed analysis of nutrient cycling in the North Sea.

North Sea Nutrients and Eutrophication

The term “nutrient” refers to the biologically available inorganic forms of nitrogen, phosphorus, and silica. Unlike carbon, these elements, which are among the major constituents of living organisms, are often present in short supply with respect to the needs of petoplankton in marine ecosystems. Because of this, primary production and phytoplankton standing crop are highly dependent on nutrient concentrations and the processes regenerating nutrients from the organic compounds where they have been incorporated. Input of nutrients from external sources also considerably affects primary producers and the whole ecosystem, sometimes in an undesirable way causing “eutrophication” effects.

Downward flux of particulate fatty acids in the Central Arabian Sea

Abstract Particulate matter collected at 732 and 2914 m during a time-series sediment trap experiment (sampling interval 13 days) in the Central Arabian Sea (14°29′N, 64°46′E; water depth 4016 m) was analysed for its fatty acid and organic carbon contents. The sampling period covered the summer monsoon of 1986. Contents of organic carbon and fatty acid decrease with increasing particle fluxes. At 732 m fatty acids account for 0.6–4% of organic carbon. Particulate organic matter collected during high-productivity and high-flux periods exhibits signs of less intense degradation within the surface layers. Furthermore, loss rates of organic carbon and fatty acids between 732 and 2914 m increase with increasing particle flux, which suggests that water column degradation is intensified during this period. Selective degradation of fatty acids diminishes their contribution to organic carbon in the deeper trap (0.3–0.7%). Despite this, the seasonality in fatty acid flux is maintained down to the deep ocean. Differences in fatty acid composition between the two depths are indicative of the organisms involved in the degradation of particulate organic matter in the water column, especially bacteria. Biological alteration of sinking particulate matter appears to occur mainly on particles.

The importance of sediments in the transformation and turnover of nutrients and organic matter in the Wadden Sea and German Bight

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.

Biological effects of triazine herbicide contamination on marine phytoplankton

The effects of the herbicide atrazine on marine phytoplankton typical of the German Bight (North Sea) were investigated by means of four mesocosm (tank) experiments. Three tanks were spiked with atrazine (120, 560, and 5,800 ng/L, respectively). These concentrations cover the range found in the German Bight, in coastal waters, and in the estuary of the river Elbe. The lowest atrazine concentration applied was of the same order of magnitude as the background concentration of triazine herbicides in North Sea water near the island of Heligoland. A relationship between concentration of atrazine (dosis) and concentration of “excreted” dissolved organic phosphorus (DOP) as well as dissolved organic nitrogen (DON) (effect) can be inferred, where high concentrations of triazines lead to high concentrations of DON and DOP. Furthermore, the decrease in the pH-value from 9.2 to about 8.7–8.9 in the three tanks spiked with atrazine during the “stationary growth phase” in comparison with the fourth tank (control, i.e., background levels) implies that photosynthesis is reduced. This effect is accompanied by lower chlorophyll concentrations, reduced primary production, and higher concentrations of amino acids in the water.

Fatty-acid composition of dissolved and particulate matter in surface films

Lipids containing fatty acids, together with other surface-active substances, can form continuous films on the sea surface. Owing to the complex structures of fatty acids, they are suitable for use as tracers in investigations of the origin and fate of surface films. Surface films were sampled west of the island of Sylt from the 20th to 22nd of August, 1974. After filtration, the fatty-acid composition of the chloroform-soluble part was analyzed in the particulate and dissolved fractions, respectively. Thereby different types of surface films could be characterized, which varied in total quantity, as well as in the relative amounts of fatty acids in the filtrate and particulate matter. It is plausible that such fatty acids will accumulate at surfaces which are chemically stable, as e.g. the saturated fatty acids, which are more resistant to photolytic and chemical degradation than their unsaturated counterparts. The fatty-acid patterns changed as soon as the film contained a large amount of particulate lipids, which shows correlations to the dissolved surface-active substances. Thus, correlations between the fatty-acid pattern in the surface film and that of Prorocentrum micans could be determined.

Comparison of heterotrophic bacterial production rates in early spring in the turbid estuaries of the Scheldt and the Elbe

In spring bacterial production rates were estimated by tritiated thymidine incorporation in the turbid estuaries of the rivers Scheldt and Elbe. Bacterial production rates in the Scheldt were 5 times higher than in the Elbe. In the Scheldt bacterial production rates correlated better with the DOC concentration than in the Elbe. Organic matter concentrations in the marine part of the estuaries were the same while in the brackish part concentrations in the Scheldt were much more higher. In the Scheldt, but not in the Elbe, oxygen depletion occurred in the maximum turbidity zone caused by bacterial growth and respiration. The water in the Scheldt was well-mixed while in the turbidity maximum of the Elbe salinity and bacterial production was higher near the bottom than at the surface. Nutrient concentrations in the Scheldt were higher than in the Elbe. Bacterial production rate values in the Scheldt are among the highest reported in the literature. The relatively high bacterial production rates in both estuaries are caused by a high load of waste water. Comparison of bacterial growth rates and water residence time suggests an intensive grazing by probably protozoa. Production rates showed a tidal dynamic. In the Elbe high current velocities caused resuspension of sediment and increased bacterial production rates near the bottom. The high production rates in the turbidity maximum and freshwater part of both estuaries show that a large amount of organic matter is degraded in this region.

20 years of the German Small-Scale Bottom Trawl Survey (GSBTS): A review

AbstractThe German Small-scale Bottom Trawl Survey (GSBTS) was initiated in 1987 in order to provide complementary investigations to the International Bottom Trawl Survey (IBTS) in the North Sea, using the same methodology but focussing high-intensity sampling on selected survey areas. Over the last 20 years, the initial number of 4 survey areas (10 × 10 nautical miles; “Boxes”) has been increased to 12, which are distributed over the entire North Sea. This paper describes the survey methods of the GSBTS, summarizes the scientific outcome of the first 20 years, and suggests that international fisheries research institutions would join the GSBTS.The major outcomes of the survey include to date:— Documentation changes in the distribution of fish species and in species assemblages (e.g. changes in species richness, shifts in the southern species component).— Geostatistical evaluation of GSBTS data.— Analysis of spatial scale effects: the relevance of GSBTS survey results for interpreting large-scaled abundance and distribution data from the IBTS.— Description of benthic habitats, composition of invertebrate fauna and its variability.— Process studies, especially investigation of predator-prey interactions between fish through analyses of stomach contents.— Characterization of the typical hydrographic conditions in the survey areas and their variability, and description of the nutrient supply.— Observations of seabirds and their feeding habits.— Analysis of the effects of different parameters on catch rates for bottom fish and on the estimates of abundance indices (e.g. vessel and gear effects, towing time, hydrographic conditions, time of day, number of hauls per area). In continuing this interdisciplinary survey with simultaneous sampling of all faunal and environmental compartments and especially in making it an international effort, we see the possibility of contributing data for the implementation of the ecosystems approach to fisheries management. Particularly, the following aspects can be addressed and would further increase the scientific value of the GSBTS:— Combining the survey data with highly resolved data from the commercial fishery to separate the effects of fishing from natural variability.— Further interdisciplinary analyses of the entire data set. Main aspects include benthos-fish-bird-community changes over time and their relation to historic fisheries impacts, and the coupling of biological and physical habitat characterisation.— Collection of accompanying data (phyto-, zoo- and ichthyoplankton data) in order to make the GSBTS a true ecosystem survey in detecting temporal changes in nearly all major levels of the food web.

Parallel plastic tank experiments with cultures of marine diatoms

The reproductibility of tank experiments concerning unicellular marine algal development was analyzed by means of parallel experiments with cultures ofThalassiosira rotula andSkeletonema costatum, using large flexible plastic tanks under semi-natural conditions. The tanks (3–4 m3, 4–5 m deep) were exposed in the German Bight at a station in the outer harbour of helgoland. The water was obtained from the open North Sea in towable tanks; it was filtered (plate filter), enriched with nitrate (20–30 μgat dm−3), phosphate (1.3–2.3 μgat dm−3) and silicate (15–23 μgat dm−3)-nearly natural springtime concentrations in this area-and inoculated with 103–105 cells dm−3. The water was mixed with non-metal stirring equipment. Within 5 days, concentrations of 106–107 cells dm−3 in an exponential growth phase were obtained. In experiments withT. rotula a parallel development was achieved in spite of some contamination by surrounding water. This is the case for nearly all parameters analyzed (nutrient salts, phytoplankton, bacteria, C, N and particulate carbohydrates). The heterotrophic bacteria, which were determined by means of the plate method, reached concentrations of up to 106 (T. rotula) and 105 (S. costatum) CFU cm−3, respectively. They showed a consistent retrograde development at diatom concentrations above a certain level. The crop did not increase again until the diatoms had reached the stationary phase. During exponential growth ofT. rotula (G=8.9–11.7 h) a partially synchronous cell division was observed. There were also rhythms with respect to cell size (pervalvar axes) and chain length (number of cells). For the experiments withS. costatum (G=10–11.4 h) diurnal variations of cell size and chain length occurred. The present results indicate acceptable reproducibility of algal development and related phenomena in enclosed water bodies.

Natürliche Oberflächenfilme im Seegebiet vor Sylt

The physical and chemical properties of the sea surface can be altered considerably by the formation of slicks and the accumulation of particles (including plankton). Investigations of natural slicks near the island of Sylt consisted of analyses of the dissolved and particulate surface-active substances, as well as the identification of plankton and bacteria collected by a surface-film sampler. The analyzed surface material consisted primarily of phytoplankton (Prorocentrum micans in particular), bacteria, detritus and dissolved compounds of fatty acids. These fatty acid compounds alone would not account for the slick. The slick appeared to be caused by the calm weather, the active accumulation of P. micans at the surface, and the increase in bacteria associated with this accumulation. Here a decrease was observed in the dissolved surface-active substances compared with the concentration usually found in normal surface waters, of which the total fatty acid content in a sample was taken as indicator. In some cases a correlation was found between fatty acids and particulate matter in naturally occurring slicks. Particularly large variations were found in the fatty acid patterns in the filtrate and filter residue at a time when no slick was present and no particulate matter had accumulated at the surface.