Piet Ruardij

Impact of interannual variability in hydrodynamic circulation on egg and larval transport of plaice Pleuronectes platessa L. in the southern North Sea

Abstract A realistic 2D circulation model of the southern North Sea was applied to simulate the interannual variability in dispersal (advection and diffusion) of plaice Pleuronectes platessa L. eggs and larvae from the spawning area in the Southern Bight of the North Sea towards the Dutch coastal nursery areas. The model is driven with daily varying current fields, which are the result of driving a general hydrodynamical model with realistic tidal and meteorological (wind) forcing for the period 1974–1981. Model simulations of plaice egg and larval transport are compared with observed larval concentrations near coastal nursery areas (Delta area and Wadden Sea). The model simulations show that: (1) the interannual variability in transport is large and of the same order as the year-to-year variations observed in larval concentrations near the nursery areas; (2) the interannual variability increases with increasing distance from the spawning area; (3) for the years in which larval estimates were available, the model predictions were positively correlated with observed estimates for the Marsdiep tidal inlet. This suggests that the variability in circulation patterns during the early pelagic stages in the open sea might be a key factor in determining year-class strength of plaice.

The effects of river input on the ecosystem dynamics in the continental coastal zone of the North Sea using ERSEM

Abstract The general characteristics of the continental coastal zone, with nutrient concentrations, primary production and biomass high near the coast but decreasing with distance from the coast, are simulated by a box-refined version of the ecosystem model ERSEM. Aggregated model results compared to the literature as well as to two different three-dimensional models show a good agreement in the coastal region. The dynamical interactions as simulated by the ecosystem model are presented in the form of N P ratios, the limitation by various nutrients and changes in the pathways of the flow of matter in the boxes; e.g. while the silicate limitation stops the spring bloom offshore, near the coast it is terminated by zooplankton grazing. When the river load was reduced by 50%, the largest effect was observed in the coastal boxes with 15% reduction of the net primary production. The discharges of the major rivers hardly affect the central North Sea, but lead to significant changes in nutrient limitations and mass flows in the coastal area. The realistic forcing, which was adopted for this setup, allows a higher net primary production in the southern North Sea in 1989 than in 1988, even though the nutrient river loads in 1989 were lower. The reason appears to be a higher solar energy input in 1989, by about 10 W m −2 d- −1 , compared to 1988.

The impact of thermal stratification on phytoplankton and nutrient dynamics in shelf seas: a model study

Abstract Local heating rate within the water column depends on energy input by solar radiation and on heat exchange across the surface as controlled by wind and convection. The heating results in thermal stratification of the water column, which in turn affects the vertical transport of, for example, nutrients. In this paper the implications of the stratification on the biota by focusing on the time of its onset and its variability in time and (vertical) space are evaluated. Especially the consequences of the stratification on the phytoplankton dynamics and the trophic interactions are shown. For this purpose, an integrated ecosystem model is developed which includes a physical submodel and an ecological model. The model is calibrated with data from a mooring project, during which a large number of physical and biological parameters have been measured at a site on the Oyster Grounds in the North Sea over a 15-month period. The physical model is a one-dimensional entrainment/detrainment model. The ecological model consists of submodels which are part of the ERSEM ecological model and which describe the biological and chemical processes in the water column and in the benthos. Results show that stratification has a major impact on the biota. Especially the timing of the onset of the stratification has major consequences for the production and succession of phytoplankton and the structure of the food web during the entire growing season.

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.

Influence of variability in water transport on phytoplankton biomass and composition in the southern North Sea: a modelling approach (FYFY)

Abstract A model for phytoplankton composition and succession coupled to a transport model for the southern North Sea is presented. This model is used to examine the time and spatial variability in phytoplankton biomass and succession. Long term time variability due to the variability in horizontal water transport is studied by using daily varying transport fields for the period 1970–1981. These transport fields result from simulations with a circulation model driven by realistic wind fields for this period. Selective factors for phytoplankton are resource competition and zooplankton grazing. This leads to a general abundance of edible phytoplankton groups in the whole southern North Sea, while poorly edible groups mainly occur in the eutrophicated coastal areas. Apart from this, phytoplankton groups which are specialized in growth under nitrogen-limited conditions are selected in open sea while, near the Dutch coast and the German Bight, phosphate-specialized groups are selected. From a comparison of simulations with yearly averaged and daily varying transport fields, it is concluded that differences with respect to the annual mean phytoplankton biomass are negligible. However, large differences are found for the distribution and abundance of specific phytoplankton groups. A simulation for the period 1970–1981 shows that part of the observed variability in spring biomass as well as the variability in the duration of dominance and abundance of species near the Dutch coast can be attributed to the variability in the horizontal water transport.

PRIMARY PRODUCTION IN A TURBID ESTUARY, THE EMS-DOLLARD - FIELD AND MODEL STUDIES

Abstract The Ems-Dollard Estuary has a steep axial gradient in turbidity which strongly regulates primary production of phytoplankton and microphytobenthos. A combination of the turbidity and nutrient gradients causes a high pelagic primary production in the outer, seaward part of the estuary, and a high benthic production on high-level mudflats (with long effective photoperiods) in the inner part. Model simulations were used to study phytoplankton succession and the factors influencing this succession. Light limitation was evaluated in a sensitivity run which used a lower compensation light intensity.

The Integrated North Sea Programme (INP)

We present detailed (hourly sampled) bio-optical and physical data from an extensive (27 months in four years) mooring programme in the central North Sea (Oyster Grounds). The purpose of the programme was a detailed study on the variability of phytoplankton abundance and the, possible associated, impact of vertical exchange by atmospheric disturbances across the (seasonal) thermocline. After this campaign, we conclude that sophisticated moorings are not yet adequate for long-term, routine oceanographic monitoring purposes, as the instruments generally need too much attendance by regular in-situ calibration and servicing (at least once a month). The data anlaysis showed that no observational evidence has been found for the spring bloom to enhance the (onset of) stratification or vice versa for the stratification to favour the spring bloom. Instead, a spring bloom is found before the stratification becomes well established and, prior to that, a subtle dependance of the evolution of the spring bloom has been found on the turbulence intensity in the water column. From a numerical model, in which we used the observations for initiaition and verification, it became clear that the timing of the onset of stratification is critical for the entire growth season. In summer no sub-surface maximum in chlorophyll was observed at our mooring site, because sufficient irradiation reached to the bottom. Despite the strong stratification, a bloom developed after a strong (convective) mixing event in late summer, and another one also prior to that, probably after internal mixing events induced by strong current shear across the pycnocline, although the role of horizontal advection could not be ruled out entirely.

Numerical modeling of physical processes in the North Sea and Wadden Sea with GETM/GOTM

At the Royal Netherlands Institute for Sea Research (NIOZ), several research projects are carried out on the concentration and transport of Suspended Particulate Matter (SPM) in the Southern North Sea and Wadden Sea. So far the focus has been on field data collection and analysis, but in recent years a numerical modeling capacity has been set up, using the GETM/GOTM model. The underlying purpose is twofold. Firstly, a numerical model can provide insight into the hydrodynamics and SPM transport, complementing field observations, helping to interpret and identify the key physical mechanisms. Secondly, it provides a much-needed tool in ecological studies, forming the basic physical core on which the transport of for instance nutrients and larvae depends; thus, these kinds of numerical models provide a valuable bridge in interdisciplinary studies. First steps in the use of the model are reported here, offering a synopsis of its potential.