Optimizing nutrient reduction strategies in the marine environment - a simplified example from the north sea

A coupled three dimensional physical, biological model (NORWECOM) has been used to study the effect of different reduction scenarios of nutrient inputs to the North Sea. Six different simulations where the nutrient loads from the Rhine and the German rivers have been varied, are perfonned. Outside the Belgian and Dutch coast there is a linear relationship between the decreased inputs from the Rhine and primary production with little effects of reduced inputs in the German rivers. In the German BightIDanish west coast effects are almost similar for reductions either in the Rhine or the German rivers. In the German Bight a summer oxygen minimum is also influence by a reduced input of nutnents, but the effects are relatively independent of which reduction scenario that have been chosen

Modelling secondary production in the Norwegian Sea with a fully coupled physical/primary production/individual-based Calanus finmarchicus model system

The copepod Calanus finmarchicus is the dominant species of the meso-zooplankton in the Norwegian Sea, and constitutes an important link between the phytoplankton and the higher trophic levels in the Norwegian Sea food chain. An individual-based model for C. finmarchicus, based on super-individuals and evolving traits for behaviour, stages, etc., is two-way coupled to the NORWegian ECOlogical Model system (NORWECOM). One year of modelled C. finmarchicus spatial distribution, production and biomass are found to represent observations reasonably well. High C. finmarchicus abundance is found along the Norwegian shelf-break in the early summer, while the overwintering population is found along the slope and in the deeper Norwegian Sea basins. The timing of the spring bloom is generally later than in the observations. Annual Norwegian Sea production is found to be 29 million tonnes of carbon and a production to biomass (P/B) ratio of 4.3 emerges. Sensitivity tests show that the modelling system is robust to initial values of behavioural traits and with regards to the number of super-individuals simulated given that this is above about 50,000 individuals. Experiments with the model system indicate that it provides a valuable tool for studies of ecosystem responses to causative forces such as prey density or overwintering population size. For example, introducing C. finmarchicus food limitations reduces the stock dramatically, but on the other hand, a reduced stock may rebuild in one year under normal conditions

A user's guide to NORVECOM V2.0. the norwegian ecological model system

A coupled 3 dimensional Physical-Chemical-Biological model system, NORWECOM, has been developed through cooperation between several Norwegian institutions. The physical module is based on the Princeton Ocean Model already well documented in several reports. This guide is an attempt to give an introduction to the Chemical-Biological module, and will hopefully help new users to understand the flow between and processes inside the various subroutines. The guide will concentrate on how the chemical-biological dynamics is formulated in the module. Why these formulations is preferred to others, is not a part of such a guide

A user's guide to NORVECOM V2.0. the norwegian ecological model system

A coupled 3 dimensional Physical-Chemical-Biological model system, NORWECOM, has been developed through cooperation between several Norwegian institutions. The physical module is based on the Princeton Ocean Model already well documented in several reports. This guide is an attempt to give an introduction to the Chemical-Biological module, and will hopefully help new users to understand the flow between and processes inside the various subroutines. The guide will concentrate on how the chemical-biological dynamics is formulated in the module. Why these formulations is preferred to others, is not a part of such a guide

Eutrophication Scenaria from Reduced Nutrient Loads to the North Sea

The environmental effects of river nutrient loads to the North Sea have been investigated using a numerical biophysical model, NORWECOM, to perform different reduction scenarios. The simulations demonstrate that the river nutrients have a significant contribution on the annual primary production, both in the southern North Sea, in Skagerrak and along the Norwegian west coast. A 50% reduction in the loads of N and P reduces the primary production with 10-30% in the southern North Sea, and 5-10% in Skagerrak and along the Norwegian west coast. Scandinavian rivers only contribute to the 1-2% level in these reductions, thus continental rivers has the major effect on the environment in all downstream areas. However, it should be noted that this reduction, even in the southern North Sea, is less than the natural variability of the production of phytoplankton. A reduction only in the P values, shows that the production regime in the southern North Sea is phosphorous limited, while nitrogen is the limiting nutrient in the northern North Sea. Focusing on the N/P ratio as a possible proxy for eutrophication, a reduction in the N and P loads reduces this ratio by a similar factor, while a reduction in the P loads only, increases it. Based on this it is proposed to use the N/P ratio for eutrophication assessment

Environmental status of the Skagerak and North Sea 2000

An environmental status for year 2000 of the North Sea and Skagerrak has been done based on outputs from a biophysicaf model (NORWECOM). The model results suggests that in year 2000 the mean annua1 primary production in the North Sea was the highest in the period 1985-2000, and that the net inflow through the English Channel, due to an extreme strong influx in the fourth quarter, was the highest on an annua1 basis in the period 1955-2000. Also the oxygen levels and sedimentation rates in the North Sea and Skagerrak have been examined, and a eutrophication assessment conclude that, except for the winter values of nitrate, eutrophication is not a big problem in most of the Skagerrak and Kattegat area. Key words : Skagerrak, North Sea, environmental status NORSK SAMMENDRAG: En koblet fysisk, kjemisk og biologisk havmodell (NORWECOM) er brukt for å simulere år 2000 i Nordsjøen og Skagerrak. Resultatene fra denne modelkjøringen er så brukt for å lage en miljøstatus for dette året. I statusen presenteres blant annet verdier for primærproduksjon, oxygen-nivå, sedimenteringsrate og vanntransport inn til Nordsjøen. Til slutt blir det gitt en vurdering av eutrofieringsgraden i Skage- rak og Kattegat basert på referanseverdier foreslått av svenske miljøstyresmakter

Assessing the importance of zooplankton sampling patterns with an ecosystem model

The copepod Calanus finmarchicus is the dominant species of mesozooplankton in the Norwegian Sea and an important food source for multiple commercially exploited pelagic fish stocks. In addition to the patchy distribution of species, the vast size of the Norwegian Sea makes synoptic zooplankton monitoring challenging. Monitoring includes relatively few sampling stations, and the number as well as the geographical location of these vary in time and space among years. In the present study, we explored the sampling patterns in 2 existing datasets: (1) for the period 1994-2004: size-fractionated zooplankton biomass, which allows for estimation of C. finmarchicus fractions, at irregularly spaced locations, and (2) for the period 1995-2017: non-size-fractionated zooplankton biomass data, gridded by objective analysis. We first assessed the C. finmarchicus data set by virtual sampling in C. finmarchicus spatial fields from the end-to-end ecosystem model NORWECOM.E2E. We found that non-consistent sampling patterns during the month of May caused the biomass estimate to be highly dependent on the chosen sampling strategy: sampling patterns from the first part of the period generally produced the highest biomass estimates. We then assessed the gridded zooplankton dataset by applying the 1995-2004 sampling patterns as well as a recent (2020) sampling pattern, which included regular and more numerous sampling locations, and found systematic differences. We conclude that the present May sampling pattern is much more robust and thereby also more likely to provide a good estimate of the interannual variability of the total biomass in the area. This study is an example of how models can be used to mechanistically interpret experimental datasets, and more specifically, how models can be used to assess sampling patterns and reveal their limitations.submittedVersio

DRIFT PATTERNS OF ANCHOVY ENGRAULIS CAPENSIS LARVAE IN THE SOUTHERN BENGUELA, AND THEIR POSSIBLE IMPORTANCE FOR RECRUITMENT

In the southern Benguela, successful recruitment of Cape anchovy Engraulis capensis is highly variable and seems to be dependent on the spawning biomass only to a small extent. This paper investigates how the variations in the drift patterns of larvae from the spawning areas on the Agulhas Bank to the upwelling-dominated regions on the west and south-west coasts of South Africa can explain these observed variations. Through the coupling of a three-dimensional circulation model, a Lagrangian particle-tracking model and spawning data, the number of particles ending up in different areas are used to investigate observed variability in recruitment. The model was able to explain 95&#37 of the observed variability in recruitment for the period 1987–1993 (excluding 1989) using a simple linear regression formula. The model also suggests that, of the interannual variability in currents and egg distribution, the latter is the most important cause of the changes observed in the recruitment of anchovy.Afr. J. mar. Sci. 25: 37–4

Modelling the variability of the drift of blue whiting larvae and its possible importance for recruitment

The first 8 months of the drift of blue whiting larvae has been simulated and studied for the 20 year period 1976-1994, using the 3-dimensional numerical model system NORWECOM (the NORWegian ECOlogical Model system). The model particles assumed to represent larvae are released identically in space and time each year according to best "average" knowledge. Large variations in the drift pattems are seen from year to year, but generally much larger amounts of larvae than expected seems to drift southward from the Porcupine Bank area west of Ireland to the Bay of Biscay. Many of these larvae drift into deep water where chances of survival are assumed to be small. Typically larvae hatched on the northem side of the Porcupine Bank drift northwards through the Faeroe-Shetland Channel, and most of these larvae are located on the shelf. Several distribution parameters are selected and compared with ICES time-series of recruitment

Transportation and predation control structures the distribution of a key calanoid in the Nordic Seas

The largest Calanus species in the Nordic Seas is also the copepod for which we have the poorest knowledge. Recent studies have shown that C. hyperboreus is more likely of sub-Arctic rather than Arctic origins, and the Nordic Seas are part of its core distribution areas worldwide. Large size and high fat content makes C. hyperboreus important prey for planktivores, and the Nordic Seas serve as main feeding grounds for a considerable biomass of planktivorous fish. We develop an individual-based model (IBM) based on existing knowledge of growth and life history of C. hyperboreus, and couple the IBM to an ecosystem model system encompassing physics, a nutrients–phytoplankton–zooplankton–detritus (NPZD) model and an IBM of the Atlantic congener C. finmarchicus. Given the main circulation routes in the region, a key question we address in this study is why C. hyperboreus is measured in low abundances in the (presumably) more favourable Norwegian Sea environment. We show that a core population of C. hyperboreus in the Greenland Sea supplies individuals to both the Iceland and Norwegian Seas, and that most copepods will visit more than one of the three regions during their life time. Advective pathways through environmental gradients creates intraspecific variation in development rates as reported by in situ observational studies. Furthermore, our results suggest that low abundances in the Norwegian Sea are more likely controlled by top-down processes (predation) rather than environmental limitations on growth or resource competition with C. finmarchicus.publishedVersio