Dag Slagstad

Factors controlling the development of phytoplankton blooms in the Antarctic Ocean — a mathematical model

A mathematical model describing the development of phytoplankton blooms as a function of the depth of the wind-mixed layer, spectral distribution of light, passage of atmospheric low-pressure systems, size of the initial phytoplankton stock and loss rates is presented. Model runs represent shade-adapted, large-celled, bloom-forming diatoms. Periodic deep mixing caused by strong winds may severely retard the development of blooms and frequently abort them before macronutrients are completely exhausted. Moderate depths of mixing (40–50 m) in combination with a moderately large total loss rate ( about 0.013 h−1 ) can prevent blooms from developing during the brightest time of the year. Complete exhaustion of macronutrients in the upper waters is likely only if the wind-mixed layer is less than 10 m deep, i.e. in very sheltered waters, and also in the marginal ice zone when ice is meiting. We do not exclude the possibility of control of phytoplankton biomass by iron in ice-free, deep-sea parts of the Antarctic Ocean, but the implied enhancement of export production through addition of iron might be restricted because of limitation by light, i.e. vertical mixing.

Sea ice and wind: Effects on primary productivity in the Barents Sea

Abstract The Barents Sea is divided into a northern and a southern part by the Polar Front (at about 75–76° N) where Atlantic waters descend under Arctic waters. Near to and north of the Polar Front, the spring bloom of phytoplankton is triggered by the stability induced in the upper 20 m by the melting of ice. The pycnocline is too strong to be eroded by wind. Primary productivity after the bloom is therefore small and largely regenerative. Underneath the pycnocline there is a 3–5 m thick layer characterized by dense, slow‐growing algal populations. New productivity north of the Polar Front is no more than 40 g C m−2 a−1. In permanently open waters south of the Polar Front, the spring bloom starts in early May. Rhythmic wind‐induced mixing related to the atmospheric low‐pressure belt reaches an average 40–60 m depth in the growth season, and secondary phytoplankton maxima may arise. As a result, new annual productivity is more than doubled, i.e. 90 g C m−2 a−1, relative to the same system without wind. A...

Ecological investigation on the zooplankton community of Balsfjorden, northern Norway

Abstract Seasonal variation in specific amylase and trypsin (i.e. alkaline proteolytic enzymes) in copepodite stage V and adult male and female Calanus finmarchicus is presented. Adult females had their highest specific amylase and trypsin activity during the spawning period (April-May). Specific amylase and trypsin activities in adult females in this period were respectively 2 and 10 times greater than the levels found in copepodite stage V during the study period. Activity levels of both these enzyme groups in adult males were low when they were present in the population during February-April. The specific amylase activity in copepodite stage V showed a diel variation pattern with maximum activity at night. The seasonal and diel variations in digestive enzymes in C. finmarchicus are discussed in relation to overwintering strategy and variation in abundance and chemical composition of the phytoplankton stock. It is suggested that both internal and external factors regulate the enzyme activity in copepodi...

MODELLED CARBON FLUXES AS VALIDATED BY FIELD DATA ON THE NORTH NORWEGIAN SHELF DURING THE PRODUCTIVE PERIOD IN 1994

Abstract A 3-dimensional coupled hydrodynamic and biological model has been used to study the effect of the physics on the productivity and the carbon export of the ecosystem outside Trams county, northern Norway. The horizontal grid point distance is 4 km. The ecosystem model consists of eight state variabies (nitrate, ammonium, silicate, diatoms, flagellates, microzooplankton, fast sinking detritus, slow sinking detritus) and assumes that nitrogen and silicate are the limiting nutrients. Mseasured mesozooplankton biomass (mainly Calanus finmarchicus) is used to impose the effect of grazing. The parameters used by the model are mostly taken from the literature, but monthly measurements of CTD, nutrients, chlorophyll, micro and mesozooplankton, and sedimentation have been used to validate the model output. The measured data vvere compared and contrasted with two model scenarios (i.e. without and with advection), where the latter run was in best agreement with the field data. This may be due to a more real...

Physical constrains and productivity in the future Arctic Ocean

Todays physical oceanography and primary and secondary production was investigated for the entire Arctic Ocean with the physical-biologically coupled SINMOD model. To obtain indications on the effect of climate change in the 21th century the magnitude of change, and where and when these may take place SINMOD was forced with down-scaled climate trajectories of the International Panel of Climate Change with the A1B climate scenario which appears to predict an average global atmospheric temperature increase of 3.5 to 4 °C at the end of this century. It is projected that some surface water features of the physical oceanography in the Arctic Ocean and adjacent regions will change considerably. The largest changes will occur along the continuous domains of Pacific and in particular regarding Atlantic Water advection and the inflow shelves. Withdrawal of ice will increase primary production, but stratification will persist or, for the most, get stronger as a function of ice-melt and thermal warming along the inflow shelves. Thus the nutrient dependent new and harvestable production will not increase proportionally with increasing photosynthetic active radiation. The greatest increases in primary production are found along the Eurasian perimeter of the Arctic Ocean (up to 40 g C m-2 y-1) and in particular in the northern Barents and Kara Seas (40-80 g C m-2 y-1) where less ice-cover implies less Arctic Water and thus less stratification. Along the shelf break engirdling the Arctic Ocean upwelling and vertical mixing supplies nutrients to the euphotic zone when ice-cover withdraws northwards. The production of Arctic copepods along the Eurasian perimeter of the Arctic Ocean will increase significantly by the end of this century (2-4 g C m-2 y-1). Primary and secondary production will decrease along the southern sections of the continuous advection domains of Pacific and Atlantic Water due to increasing thermal stratification. In the central Arctic Ocean primary production will not increase much due to stratification-induced nutrient limitation.

A model study of demography and spatial distribution of Calanus finmarchicus at the Norwegian coast

Abstract For calanoid copepods living in an advective environment, any predictive life history model relies on our comprehension of the significance of advection and environmental parameters. To study the propagation of an initial population of Calanus finmarchicus with respect to spatial distribution and stage development throughout time, we have developed a lagrangian particle-tracking model. The advection of the population is driven by a physical component, while a biological model describes vertical migration and stage development. The physical model is composed of large- and small-scale current fields, while the biological model reflects our current understanding of vertical migration and spawning behavior resolved spatially. The results indicate that the population is distributed along the major current branches in the Norwegian Sea, with fractions entering the Barents Sea, West-Spitsbergen and the Norwegian Shelf. A part of the population spends their entire life progression in the oceanic realm, with relatively low rate of advection. The stage progression shows that C. finmarchicus developing in the oceanic region has a late spawning period compared to the fraction of the population residing in the near coastal regions. The life history of Calanus finmarchicus in Norwegian waters is considered to deviate between one and three generations per year. In the shelf region, Lofoten is often referred to as the northernmost area where a two-generation cycle occurs. Our two-generation simulations are compared with field data obtained from the OMEX study site, and the results indicate that the two-generation cycle is a very plausible explanation to the obtained data. The spawning regions of G2 are both oceanic regions in close vicinity to the Norwegian Atlantic Current flowing along the shelf break, and the mid-Norwegian shelf.

Modelling seasonal growth and composition of the kelp Saccharina latissima

A dynamical model for simulating growth of the brown macroalga Saccharina latissima is described. In addition to wet and dry weights, the model simulates carbon and nitrogen reserves, with variable C/N ratio. In effect, the model can be used to emulate seasonal changes in growth and composition of the alga. Simulation results based on published, environmental field data are presented and compared with corresponding data on growth and composition. The model resolves seasonal growth, carbon and nitrogen content well, and may contribute to the understanding of how seasonal growth in S. latissima depends simultaneously on a combination of several environmental factors: light, nutrients, temperature and water motion. The model is applied to aquaculture problems such as estimating the nutrient scavenging potential of S. latissima and estimating the potential of this kelp species as a raw material for bioenergy production.

Oceanography and fluorescence at the shelf break off the north Norwegian coast (69°′N-70°30′N) during the main productive period in 1994

Abstract Data on hydrography, fluorescence, wind and currents from shelf studies on the Norwegian shelf between 69°30′N and 70°30′N are presented. The sampling was performed along five transects, covering a narrow shelfbreak site, a trench, and a bank. The sampling programme was conducted during cruises of 4-5 days duration each month from March until October 1994. Three distinct water masses were identified: i) Coastal Water (S 12 oq above the shelf and the shelfbreak (0-300 m depth), 2) Atlantic Water (S > 35; 5 10 oq off shelf (0-400 m depth) and below the Coastal Water in the trenches (300-400 m depth), and 3) Norwegian Sea Deep Water (S < 35; T < 0 °C) off the shelf break, below 700 m depth. Wind direction between southeast and northwest prevailed during all cruises, except in May, when northeasterly wind was most frequent. Typical wind speeds were in the order of 7-l2 m s-1, but the wind exceeded l 5 m s-1 frequently during the whole investigation period. Current regime above the...

Microzooplankton and mesozooplankton in an upwelling filament off Galicia: modelling and sensitivity analysis of the linkages and their impact on the carbon dynamics

Abstract This paper reports estimates of trophic flows of carbon off the Galician coast from a 1D ecological model, which are compared with field data from a two week Lagrangian drift experiment. The model consists of 9 biological components: nitrate, ammonium, >5μm phytoplankton, 20 μm), ciliates, fast sinking detritus and slow sinking detritus. Calculations were made for the fluxes of carbon between biological components within the upper 45m of the water column. The temporal development of primary production during the simulation period of two weeks was in good agreement with field estimates, which varied between 248 and 436mgC.m −2 .d −1 . Heterotrophic nanoflagellates had the greatest impact on carbon flux, with a grazing rate of 168mgC.m −2 .d −1 . Herbivorous grazing by microzooplankton amounted to 215mgC.m −2 .d −1 , whereas grazing by copepods on phytoplankton was 35mgC.m −2 d −1 . Copepods grazing on microzooplankton was minor (0.47mgC.m −2 .d −1 ) and the export flux from the upper 45m was 302mgC.m −2 .d −1 . Sensitivity analyses, in which the grazing parameters (i.e the functional relationship between ingestion and food concentration) were changed, were carried out on the heterotrophic dinoflagellate, ciliate and heterotrophic nanoflagellates/dinoflagellate components of the model. These changes did not alter the temporal development of heterotrophic nanoflagellates/dinoflagellates biomass significantly, but ciliates and heterotrophic dinoflagellates were more sensitive to variations in the grazing parameters. The overall conclusion from this modelling study is that the coupling between small phytoplankton and heterotrophic nanoflagellates was the quantitatively most important process controlling carbon flow in this region.

Modelling the 3-D carbon flux across the Iberian margin during the upwelling season in 1998

Abstract A 3-dimensional (3-D) primitive equation model coupled to a biological model was used to study the effects of physics on the productivity and carbon export along the Iberian coast during the upwelling season (May–September). Open boundaries for this model were generated with a basin scale model covering major parts of the North Atlantic and nested in two steps into models with grid resolutions of 10 and 3.3km, respectively. The ecosystem model consists of eight state variables (nitrate, ammonium, silicate, diatoms, flagellates, meso- and microzooplankton, fast and slow sinking detritus) and assumes that nitrogen and silicate are the potential limiting nutrients. The primitive equation models produced coastal upwelling that injects nutrients into the euphotic zone in response to northerly wind forcing. Filaments were formed when the upwelling favourable wind peaked in July/August. High primary production was found on the shelf and in the core of the upwelling filaments. The simulations used atmospheric input from 1998 allowing comparison with a Lagrangian experiment in August 1998. The ecological model, having the same structure as a model been applied on boreal shelves, gave good simulations of the carbon flux on and off the Iberian margin. However, some stocks and rates of carbon were not well validated with the data from the field investigation. The simulated net export of carbon from May to September across the 400km long, 200m isobath is estimated to be 4.4×10 11 gC or 6.3mgC m −2 .s −1 .