Nutrient dynamics in the North Sea: Fluxes and budgets in the water column derived from ERSEM

Nutrient dynamics for phosphate, nitrate, ammonium and silicate have been simulated with ERSEM, the European Regional Seas Ecosystem Model. From the model results budgets for the dissolved inorganic nutrients and the corresponding particulate fractions have been calculated. The annual cycles of the nutrients phosphate and silicate compare quite well with the observed ranges of variability. This does not hold for ammonium and nitrate. Biologically mediated transformations such as nutrient uptake and pelagic and benthic mineralization are the dominant processes in changing the nutrient concentrations with the horizontal advective contributions playing a minor role during the productive season.Vertical advection and vertical diffusion have a clear seasonal signal, with a maximum in February. The decay of the advective nutrient transport in summer is caused by the depletion of the upper layer of dissolved inorganic nutrients by algal uptake. The inflow of nutrients in the northwest is almost balanced by the outflow in the northeast, without causing large nutrient transports into the shallower areas from the north. However, from the coastal areas there is a nutrient flow towards the central North Sea, enhancing primary production in the central area.     

Phytoplankton biomass and potential nutrient limitation of phytoplankton development in the southeastern North Sea in spring 1985 and 1986

The vernal phytoplankton bloom was observed during cruises to the southeastern part of the North Sea (east of 6°20' E, south of 56°50'N) in 1985 and 1986. Maximum phytoplankton biomass expressed as phytoplankton carbon was similar in both years (14.5 and 17 g PPCm⁻² respectively). In 1985 the bloom was located in the less saline coastal water close to the North Frisian coast. Phytoplankton was dominated by Coscinodiscus concinnus and Thalassiosira nordenskiöldii. In 1986, highest phytoplankton biomass was observed northwest of the island of Sylt, where Thalassiosira nordenskiöldii was the dominant phytoplankton species. Within the areas of high phytoplankton standing stock, concentrations of the inorganic dissolved nutrients phosphate and silicate had dropped to nearly undetectable concentrations, whereas both in 1985 and 1986 the water was still rich (10 – 20 μM.dm⁻³) in inorganic nitrogen (DIN). This, as well as the high ratios of DIN:PO₄ and DIN:Si(OH)₄ (> 50) point to phosphate and silicate as potential limiting nutrients during the spring phytoplankton bloom. The ratios of total nitrogen (TN) to total phosphorous (TP) (> 30) indicate also that phosphorous might then be in short supply. Phosphate and silicate have to be considered as potentially limiting nutrients during the vernal plankton bloom in the coastal waters of the southeastern North Sea, with nitrogen being present in surplus at that time of the year. However, in the more offshore areas nitrogen may be considered the potentially limiting element at the same time.     

Adventures in holistic ecosystem modelling: the cumberland basin ecosystem model

A holistic ecosystem model has been developed for the Cumberland Basin, a turbid macrotidal estuary at the head of Canada's Bay of Fundy. The model was constructed as a group exercise involving several dozen scientists. Philosophy of approach and methods were patterned after the BOEDE Ems-Dollard modelling project. The model is one-dimensional, has 3 compartments and 3 boundaries, and is composed of 3 separate submodels (physical, pelagic and benthic). The 28 biological state variables cover the complete estuarine ecosystem and represent broad functional groups of organisms based on trophic relationships. Although still under development and not yet validated, the model has been verified and has reached the stage where most state variables provide reasonable output. The modelling process has stimulated interdisciplinary discussion, identified important data gaps and produced a quantitative tool which can be used to examine ecological hypotheses and determine critical environmental processes. As a result, Canadian scientists have a much better understanding of the Cumberland Basin ecosystem and are better able to provide competent advice on environmental management.     

Denitrification and nitrous oxide in the North Sea

In situ sediment denitrification rates were determined in the major areas of deposition of the North Sea, using the acetylene block technique. In addition, nitrous oxide profiles of the water column were determined. Nitrous oxide production generally occurred in the photic zone possibly due to nitrification; and throughout the water column in the German Bight region. Consumption at depth was possibly due to reduction in the anoxic microzones of faecal pellets, concentrated at the thermocline. Saturation of surface waters was 102.2% compared to 130.3% in the German Bight region. Calculated flux of nitrous oxide to the atmosphere was 9.5 × 10⁶ kg yr⁻¹, over half of which was produced in the German Bight.Sediment denitrification rates varied through three orders of magnitude; the highest value of 150 μmol m⁻² d⁻¹ was recorded in the Norwegian Trench. Nitrous oxide production by the sediments was low (1.1 μmol m⁻² d⁻¹ max.), and was undetectable at half of the sites. Sediment nutrient profiles exhibited porewater nitrate concentrations exceeding that of the overlying water suggesting that denitrification was fuelled by nitrification, which, in turn was related to other environmental variables. A significant positive relationship existed between in situ denitrification rate and the nitrate content of the upper sediment. Extrapolation of the rate to the total area of deposition in the North Sea suggests that denitrification is responsible for a minimum loss of 7.5–12% of the total annual nitrogen contaminant input.     

Elementary composition of suspended particulate matter in the North Sea

The concentrations of the elements Al, Si, Ti, Fe, Mn, Ca, Mg, K, Cu, Zn, Cr, Pb, Ni, Cd, V and of particulate organic carbon (POC) were determined in 210 samples of suspended matter collected at 119 stations in the North Sea in January-February 1980. The distribution of these 15 elements and POC in the suspended matter in the North Sea was studied, as well as their relation with total suspended matter concentrations and salinity. The interrelationships between these elements were found to differ in different regions of the North Sea, which is related to the origin of the suspended material and the degree of mixing. A cluster analysis including all determined elements at all stations showed that the suspended matter in the North Sea consists of three clearly separated, specific types. One type was found in the northern part of the North Sea, the second in the southern part of the North Sea, while the third type was mainly observed in the Norwegian Channel, Skagerrak and eastern English Channel.The observed distributions are in agreement with what is known on the dispersal and deposition of suspended matter in the North Sea. Material from the southern North Sea hardly reaches the central and northern North Sea and the northern parts of the Norwegian Channel. The distribution of Mn is very strongly related to the accumulation of Mn in the top layers of finegrained bottom sediments and to the resuspension of Mn-enriched particles.     

Phytoplankton primary production and biomass in the western Wadden Sea (The Netherlands); A comparison with an ecosystem model

Dissolved nutrient concentrations (Si, P, N), algal biomass (separated into diatoms and micro-flagellates), and total primary production were determined in 1986 during 7 cruises in both the inner and outer parts of the estuary of the western Wadden Sea. The biomass data, obtained after converting cell biovolume and cell counts into carbon, were compared with those simulated by a two-dimensional multitrophic ecosystem model.The general pattern of the succession of algal species was the same for all compartments: a bloom of diatoms in early spring followed by one of microflagellates, predominantly the colonial alga Phaeocystis pouchetii. Throughout the year the biomass of the two algal groups differed between inner and outer compartments of the estuary. In spring the carbon biomass of the microflagellates in the compartments bordering on the North Sea exceeded that of the inner compartments by a factor of 2. The reverse was found for the diatoms in summer.A reasonable fit between the field data and simulated data was only obtained in spring; large differences were found for the summer period. During that period an extensive diatom population characterized by a low primary production was observed, which did not correspond with the model simulation.Possible factors causing the differences between observed and simulated data, such as nutrient regeneration, were analysed.     

The infauna and epifauna of the northern North Sea

This paper compares the infaunal and epifaunal assemblages from surveys encompassing 121 grab stations and 152 Agassiz trawl samples respectively, collected between 1980 and 1985. The area surveyed is delimited by the Scottish, Norwegian and Danish coasts lying between 56°15'N and 60°45'N. Samples for infauna and environmental parameters were collected by Smith-McIntyre grab and Craib corer.The epifaunal and infaunal assemblages were analysed separately by ordination techniques (DECORANA and TWINSPAN) to detect the major environmental gradients underlying the distribution and abundance of the fauna and to indicate which taxa were characteristic of different zones within the survey area.The major determinant of infaunal community composition was sediment granulometry, with depth being of secondary importance. For the epibenthos, depth was the major factor and the sediment composition seemed less significant. Assemblages identified by TWINSPAN were characterised by particular species, but these ‘community types’ were seen to grade into one another along continuous environmental gradients. These findings are discussed in relation to previous North Sea benthic classification schemes.     

Growth and dynamics in populations of echinocardium cordatum living in the North Sea off the Dutch North coast

Numerical densities and size distributions in populations of the spatangoid Echinocardium cordatum were followed at 19 fixed sampling stations with sandy sediments at depths of 6 to 18 m off the west-Frisian islands of Texel and Terschelling.Recruitment was irregular with some recruitment occurring during half of the years and successful year classes developing during only 3 of the years in the 10 year period of observation (1972–1982). Annual mortality of the strong year class born in 1972 was rather regular in the ages of 1 to 7 years, averaging ∼27% in the 2 areas, but was followed by catastrophic mortality at most stations during the severe winter 1979.Growth was rapid. A mean length of 2 to 2.5 cm was reached after 2 growing seasons and 4 cm after 4 or 5 growing seasons.     

Inshore and offshore diversity of epibenthos dredged in the North Sea

Large epibenthos (>1 cm.) was dredged in the German Wadden Sea and in adjacent offshore areas at 10 to 30 m depth in Helgoland Bight. There were apparent differences in species composition and relative abundances between the 2 regions. The hypothesis of increasing species richness in offshore direction and of increased individual numbers inshore was tested. Neither proposition was found to be generally true.Regional species number was higher offshore (85) than inshore (73). However, a direct comparison of 11 + 11 localities and of 66 + 66 dredge hauls revealed no significant differences. Ratios of species richness for localities versus regions and for hauls versus localities differed significantly, indicating a higher regional heterogeneity offshore and a higher habitat heterogeneity inshore. Thus, causes for an overall similar diversity were found on different spatial dimensions.Few species were encountered on bottoms subject to the riverine waters of Elbe and Weser. Many decapod crustaceans attained stronger populations inshore, while ophiuroids completely dominated the epibenthos offshore and caused higher individual numbers there. Historical sources suggest that the epibenthos was richer in the past than it is now.     

Diversity of the nematode communities in the Southern Bight of the North Sea

The nematode communities from 102 stations in the Southern Bight of the North Sea, sampled between 1972 and 1984, are examined. Overall, 456 species, belonging to 159 genera and 37 families were found.Sediment structure and chlorophyll a content were found to be determining environmental factors in the species composition of the nematode communities.Diversity is determined at different levels of the nematode community; i.e. species diversity of the whole community, species diversity of eight dominant families (Chromadoridae, Comesomatidae, Cyatholaimidae, Desmodoridae, Microlaimidae, Oncholaimidae, Thoracostomopsidae and Xyalidae), species diversity of the four feeding types, family diversity and trophic diversity within the whole community.The open sea stations are characterized by nematode communities which are comparable in terms of species diversity; the number of species S is about 30–35 per sample (sample size is about 100) with H′ between 4.00–4.50 bits/ind. The communities in the coastal stations are less diverse (S=7−22); H′=1.4−3.5 bits/ind.). The stations along the Belgian east coast have communities with low diversity (S=7; H′=1.41 bits/ind.) and with a pronounced dominance of a few species (Simpson index=0.54); the diversity within the eight families is also very low.The relationship between environmental stability (or disturbance) and stability and diversity of the nematode communities can be explained by the following factors; habitat heterogeneity, food availability, productivity, density and population growth rates.     

Nutrient depletion studies in offshore North Sea areas

Nanomolar nitrate and nitrite concentrations are reported for offshore areas of the North Sea during a research cruise in July 1987. The detection system employed was a sea-going chemiluminescence analyser. Surface concentrations during a period of nutrient depletion are reported, not previously described for this sea area.Very low values for the N/P ratio are reported (<0.1), which suggested that phytoplankton primary production was potentially nitrogen limited during the study period for large areas of the North Sea. Some coastal and river influenced regions exhibited a N/P Redfield ratio of >16, suggesting potential phosphate limitation. As well as the horizontal structure for nitrate and nitrite, vertical profiles are reported showing some structure in the nitrate profile above the thermocline. Nitrate levels generally increased towards the surface, from a minimum at the thermocline, and this may represent an important source of nitrogen to the system during nutrient depletion. A nitrite maximum was observed at the base of the thermocline at the deep water sites, this is attributed to either nitrification or nitrate reduction, and indicates that the thermocline region was a site of active nitrogen regeneration. A significant correlation between primary production and the nitrate concentrations below 100 nmoles.dm⁻³ was found, showing that the nitrate pool, as well as the recycling rate, remains an important influence during periods of nutrient depletion.     

Recent sediment deposition rates in the oyster ground,North Sea

Recent sediment accumulation rates were calculated from downcore ²¹⁰Pb-excess activity profiles obtained from box cores taken in the Oyster Ground, North Sea. The rates ranged from 0.04 to 0.94 cm·yr⁻¹ with an average of 0.39 cm·yr⁻¹. In the top sediment layer the activities were rather low, the highest being only 1.15 dpm·g⁻¹, and the average value 0.92 dpm·g⁻¹.As shown by X-ray radiographs, downcore variations in texture were small: most samples being homogeneous and lacking primary sedimentary structures. Burrowing was evident in all cores, and most of them had a surface mixed layer of at least 5 cm, below which the ²¹⁰Pb activity rapidly dropped to background value.Downcore deviations from the logarithmic decrease in ²¹⁰Pb with depth could generally be attributed to biological disturbance or physical processes causing reworking of the sediment. Only in one core did the ²¹⁰Pb activity decrease regularly with depth. The activities of ¹³⁷Cs in this core and another demonstrated recent accumulation rates of 0.44 and 0.88 cm·yr⁻¹, respectively, which are in good agreement with the rates obtained from the ²¹⁰Pb method in the same cores (viz. 0.37 and 0.94 cm·yr⁻¹). From these sedimentation rates the total amount of mud yearly deposited in the muddy part of the Oyster Ground is estimated to be in the order of 2×10⁹ kg·yr⁻¹, which is 4 to 5% of the total amount of mud yearly deposited in the North Sea.     

An experimental approach to the distribution of mud in the Southern North Sea

Fine grained material was collected from an extensive North Sea mud field north of 53°30′N. Of one sample deposition and erosion velocities were measured in a series of laboratory experiments using a circular tack in which currents with different speeds could be generated. A series of consolidation periods were applied.The results are compared with tidal current velocity patterns and wave information for the North Sea obtained from published data. It is shown that the boundary between the mud field and the sandy Souther Bight is principally determined by spring tide current velocities. In the mud field net deposition of mud is possible because the increase of erosion velocity by consolidation proceeds faster than the increase of current velocity after slack tide.     

The benthic biological submodel in the European regional seas ecosystem model

The submodel describing benthic biology including a bioturbation module as incorporated in the European Regional Seas Ecosystem Model (ERSEM) is discussed. It is linked to a nutrient dynamic model. The structure of the benthic model food web is presented. There are four macrobenthic functional groups, meiobenthos and aerobic and anaerobic bacteria. The modelling uses ‘standard organisms’ as basic building blocks. The choice of parameter values is discussed. The results demonstrate the dependence of the benthic system on the pelagic system. The importance of features such as predation within functional groups for stability of the system is investigated. Detritus input from the pelagic system and detritus recycling is most important in the benthic food web. The web of carbon and nutrient fluxes through the system is analysed. On the basis of the food web analysis, the trophic positions of the functional groups are calculated. Besides the benthic biology, the mathematical formulation of the bioturbation and diffusion enhancement is discussed. Macrobenthic presence and activity enhance diffusion in the sediment and contribute essentially to vertical transport of particulate matter. This is of great importance for the vertical distribution of detritus, and as a consequence, for microbial activity in the sediment layers.     

Factors controlling the summer development of copepod populations in the southern bight of the North Sea

In two consecutive years an intensive sampling programmed was implemented at a fixed station in Dutch coastal waters to obtain a detailed record of the summer development of copepod populations in relation to phytoplankton and macroplankton abundance. The central question was whether densities of copepods are controlled by predation, in particular by invertebrate pelagic carnivores, or by food limitation. Methods applied to estimate daily predation by observed stocks of carnivores included analysis of gut contents and digestion rate, extrapolation of experimental feeding rates and of literature data on daily rations and maintenance needs. Chlorophyll-a and cell concentrations served as a rough measure for algal food supply.Since a decline in copepod densities manifested itself most clearly in decreasing naupliar numbers in both years, populations were assumed to be regulated mainly by recruitment or survival of these early life stages. Naupliar declines coincided with maximum densities of the hydromedusa Phialidium hemisphaericum, which dominated the macroplankton both in abundance and in biomass and reached a maximum density of 467 specimens·m⁻³ or 7 mg C·m⁻³. Copepod eggs appeared by far the most frequent prey item in their guts. However, these eggs are digested very slowly, if at all, and may often be ejected without any visible damage. The effect of egg predation on naupliar recruitment seems therefore relatively unimportant. Predation on swimming copepod stages was generally low. There was no evidence of selective feeding on nauplii. The maximum values of calculated predation pressure exerted by Phialidium populations matched daily copepod production only by way of exception. Impact of other invertebrate carnivores was negligible. As predation did not play a significant role, food availability seems the key factor underlying copepod population dynamics. The consequences of food limitation (reduced egg production, production of diapause eggs and enhanced cannibalism) are discussed. The observed coincidence of maximum predator abundance and minimum chlorophyll-a and diatom concentrations does not support the hypothesis that carnivores are able to indirectly benefit phytoplankton growth by reducing grazing pressure of herbivores.     

Seasonal differences in concentrations of particulate lipids,proteins and chitin in the North Sea

The concentrations of particulate lipids, proteins and chitin were determined in the North Sea in January 1992, October 1992 and March 1993, with additional measurements of total suspended matter concentrations and organic matter contents in the suspended matter (POC) by ashing and wet-oxidation. During each of the three cruises, the proteins were by far the most important of the three components measured, followed by the lipids. The concentrations of chitin were always very low. The concentrations of these three components were highest in March and lowest in January; there was also a variation in the distribution over the North Sea between the three cruises. In January 1992, the suspended matter contained more organic matter in the northern North Sea than in the southern North Sea, but the higher concentrations of total suspended matter in the southern North Sea resulted in an inverse distribution for the concentrations of organic matter per volume of water. In March 1993, highest concentrations of the three organic compounds were observed in the Skagerrak and along the eastern side of the North Sea, decreasing towards the west. The suspended matter was richest in organic compounds in the Skagerrak and in the Norwegian Channel. In October 1992, the situation was very similar to that observed in January 1992, but the concentrations of organic compounds in the suspended matter and per volume of water were higher. The ratio of proteins to lipids was similar to this ratio in phytoplankton. The composition of the particulate organic matter became very close to that of phytoplankton if we assume that we missed a large amount of carbohydrates; the small differences persisting were probably due to the contributions of zooplankton, bacteria and terrestrial sources. The total contents of particulate lipids + proteins + chitin were low, but when estimated amounts of carbohydrates were added, the total particulate organic-matter concentrations found were very similar to the total particulate organic carbon concentrations found by wet-oxidation. During the period of sampling, no large concentrations of phytoplankton occured.     

Simulations of the north sea circulation,its variability,and its implementation as hydrodynamical forcing in ERSEM

The rationale is given of how the gross physical features of the circulation and the stratification of the North Sea have been aggregated for inclusion in the ecosystem box model ERSEM. As the ecosystem dynamics are to a large extent determined by small-scale physical events, the ecosystem model is forced with the circulation of a specific year rather than using the long-term mean circulation field. Especially the vertical exchange processes have been explicitly included, because the primary production strongly depends on them. Simulations with a general circulation model (GCM), forced by three-hourly meteorological fields, have been utilized to derive daily horizontal transport values driving ERSEM on boxes of sizes of a few 100 km. The daily vertical transports across a fixed 30-m interface provide the necessary short-term event character of the vertical exchange.For the years 1988 and 1989 the properties of the hydrodynamic flow fields are presented in terms of trajectories of the flow, thermocline depths, of water budgets, flushing times and diffusion rates. The results of the standard simulation with ERSEM show that the daily variability of the circulation, being smoothed by the box integration procedure, is transferred to the chemical and biological state variables to a very limited degree only.     

Distribution and abundance of the zooplankton of the Ems estuary (North Sea)

From 1974 to 1977 a study was made of the abundance and the distribution of the zooplankton species of the Ems estuary (The Netherlands and the Federal Republic of Germany), an area of about 500 km² with extensive tidal flats.The most important component of the zooplankton consisted of holoplanktonic calanoid copepods with, during the summer, a significant contribution of meroplankton, mainly consisting of polychaete and cirripede larvae. Zooplankton abundance showed a marked seasonality, with a pronounced spring peak and a smaller late summer/autumn peak. In the low salinity area the spring bloom was dominated by Eurytemora affinis, which persisted in the salinities below S=5 upriver during the whole year. In the polyhaline area Acartia bifilosa was the main component of the spring bloom. The late summer maximum from August to October was dominated by Acartia tonsa in the meso- and polyhaline area and by A. discaudata and Centropages hamatus in the euhaline part of the estuary bordering on the Wadden Sea proper. The number of abundant (> 1000 ind·m⁻³) species increased from the inner, fresher part of the estuary towards the Wadden Sea but, except for April, average zooplankton density in salinities <18 was similar (2768 ind·m⁻³) to the density in salinities >18 (2817 ind·m⁻³).The distribution patterns show that there are only a few autochthonous species and that the majority of species is allochthonous and penetrates more or less deeply into the estuary from the Wadden Sea and North Sea, varying with species and season.