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AbstractA simple, but general, simulation model is specified according to the state-of-the-art within phytoplankton modelling: Process representations are based upon prevailing theoretical and empirical representations given in the literature, and a set of earlier published values of model coefficients that have demonstrated good fit to reliable observations was selected. The emerging phytoplankton model was then validated against data obtained from enclosure experiments with light-, N-, P- and Si-limitations. We applied no tuning of the coefficients as the purpose of this test was to estimate the predictive power of the proposed model. The general standard deviations between model predictions and observations were on the range 0.04–0.36 and 0.13–0.42 for the nutrient and phytoplankton state variables respectively. Not surprisingly, these values are higher than those obtained in tuned simulations. Nevertheless, several characteristics, such as the balance between diatoms and flagellates, were predicted by the model. The phytoplankton model was set up and driven by a 3-dimensional physical model for the North Sea. The period February-June 1988 was simulated and forced with realistic topography, meteorological data, riverine freshwater and nutrient input. Simulated developments in nutrients, diatoms and flagellates are presented with references to actual observations and the Chrysochromulina polylepisbloom in 1988. Several important characteristics, such as the timing of two diatom blooms in March and April and one flagellate bloom in May together with vertical and horizontal distributions of nutrients, were simulated without tuning of the model to the actual observations. The present simulations support the general idea that flagellates in the coastal areas of the North Sea are stimulated by anthropogenic nutrients, but more specifically that a strong flagellate bloom in the Kattegat-Skagerrak area, corresponding to the C. polylepisbloom, was stimulated by such nutrients in May 1988. Although the model should be improved before it is applied in a management context, the great potential of using such models in environmental management is demonstrated.

AbstractThe literature on zooplankton population dynamics provides more estimation techniques than reliable estimates of population parameters. In this review we show how different techniques relate to each other in terms of underlying models and assumptions. There are two main routes to parameter estimates. The vertical approaches utilize the stage-structure of samples taken at the same point in time. They require assumptions about the constancy in the parameters, but relax assumptions concerning advective influence. The horizontal approaches utilize information provided by the stage structure within samples, as well as information on temporal changes in abundance. They relax assumptions about the constancy of parameters, but advective influences may be introduced. Of the horizontal methods, variants of what are commonly termed cohort methods have been widely used. These provide mathematical simplicity, but are based on more restrictive assumptions than methods fitting predescribed models (delay differential, partial differential equation, and matrix models of population dynamics). Because the quality of estimates, such as of mortality rates, depends upon the quality of the data, some sampling considerations in regard to advection and biases in estimating stage composition are discussed. It is important to reduce the number of parameters to be estimated from census data, thus we review some techniques for independent measurement of stage durations and birth rates. Finally, we provide some general recommendations for future studies.

AbstractEmiliania huxleyi(Lohmann) Hayet Mohleris represented as a state variable in a general phytoplankton model that also includes one diatom group and one phytoplankton group encompassing the ‘other flagellates’ than E. huxleyi. Furthermore, three nutrient variables (nitrogen-, phosphorus-, and silicon-nutrients) are included. The main features of the model are that E. huxleyihas been given a higher growth affinity for orthophosphate than the two other groups, and that diatoms, besides being dependent on silicon, have been given a higher maximal growth rate than the two flagellate groups. The output from the simulation model is compared with observations made in mesocosm experiments loaded with different amounts of nitrate and orthophosphate. The simulated E. huxleyiis not able to grow as well as the real E. huxleyi, and this applies especially to the experiments low in orthophosphate. It is difficult to account for the high observed numbers of E. huxleyiin terms of the available inorganic othophosphate in these experiments. The literature, however, gives some evidence that E. huxleyiis able to utilise organic phosphorous sources. By assuming an organic phosphorus source that adds phosphorus to the orthophosphate pool, however, the fit between the model and the observations is improved. Furthermore, simulations indicate that the phosphorus originating from organic substances should be more available for the E. huxleyithan for the other groups. In simulation models this is achieved by defining a higher E. huxleyiuptake affinity for orthophosphate, but it may also be assumed that the orthophosphate originating by enzymatic activity on organic substances are spatially (at the scale of the uptake process) more close to the E. huxleyicell than to the other phytoplankters present in the water body.

AbstractThe distribution, biomass, and predator-prey relationships of the pelagic assemblage in Masfjorden, western Norway, was studied in January 1989. The pelagic biomass was dominated by particulate organic matter. Biomasses of copepods, macroplankton, and mesopelagic fishes were of the same order of magnitude, while the biomass of larger pelagic fishes were one order less. Predator-prey relationships seemed most important at intermediate and higher trophic levels. Two sound-scattering layers, consisting of adult Maurolicus muelleri(lower layer) and juvenile M. muelleri(upper layer) performed instantaneous lightdependent vertical migration. Vertical distributions are explained in terms of balancing food demands against predation risk.

AbstractModels of visual range and location distances are crucial for quantification of vision based feeding opportunities and predation risk in the pelagic habitat. We compare an earlier published model with measurements of the reactive distance of Gobiusculus flavescensrelative to two species of copepods. Although this model gave reasonable predictions at low light intensities, the measurements of reactive distance at higher intensities were much lower than those predicted by the model. We modified the model to account for saturation at high light intensities. With this additional feature, the correspondence with the G.flavescensobservations was significantly improved. Furthermore, the revised model is consistent with earlier published data on fish contrast thresholds obtained over a wide range of target sizes and irradiance levels. Given the values of only two parameters, one sensitivity threshold and one saturation parameter, the model is capable of predicting visual ranges for relatively large intervals of light intensity, prey size and turbidity. Other published visual range models are briefly reviewed and compared with our model.