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The life cycle of many benthic macro-invertebrates is characterised by a planktonic life stage where larvae are transported by ocean currents and mixing and after some time they reach a stage where they settle onto the ocean bottom and, provided a suitable habitat is available, initiate their benthic life stage as juvenile organisms and complete their life cycle when they mature and spawn as adults. Such age-specific behaviour is in general difficult to include in large scale ocean models unless dispersal is considered on an individual basis, i.e. individual based model coupled with a Lagrangian description of the flow field. However, here we address this issue in an Eulerian framework and develop and apply a new life cycle and population dynamical model of benthic fauna and implement the model in a high-resolution three-dimensional circulation model of the North Sea/Baltic Sea transition zone. The model explicitly describes specific life stages of the population and considers the different processes affecting the organisms during their life cycle, e.g. spawning, dispersion and settling. The model considers different life stages of an idealised marine organism, representing a typical benthic macro-invertebrate species in the area. Populations of juvenile and adult benthic organisms are maintained by spawning, occurring regularly every spring, and subsequent settling of larvae. The pelagic larval stages are simulated by a larval concentration distribution function, i.e. discrete age-classes of the total larval concentration, and age-specific physiological processes, as the onset of their settling behaviour, is explicitly accounted for in the model. Model simulations show, in general, a large connectivity between habitats in the northern and southern part of the area but also that self-recruitment is sufficient to sustain the two populations independently. A sensitivity study were carried out with the spawning rate as a control parameter and two non-trivial quasi-stationary steady states of benthic biomass distributions were identified, characterised by a high and low distribution of organisms in the area, respectively. A stability diagram identifies a bifurcation point when the spawning rate is reduced by 65% and where lower spawning rates implies two different stable equilibria. The existence of multiple quasi-stationary steady states can be explained by the general circulation in the area: when spawning into the surface layer takes place from recruitment areas close to the North Sea where the out-flowing Baltic Sea surface water hinder the southward transport of pelagic larvae. The existence of multiple equilibria support the hypothesis of regime shifts in coupled physical-biological systems where modest changes in critical processes causes rapid and extensive structural changes in the ecosystem. Such changes could occur if a system is close to a bifurcation point such that small changes in critical internal biotic dynamics or in environmental conditions force the system into a new equilibria, for example due to hypoxia or changes in temperature or salinity. Finally, it is shown that model simulations of periods with hypoxic bottom water masses reduces the total benthic biomass distribution in the area significantly.

Hypoxic conditions (O 2 −1 ) are frequently observed in the relatively shallow and stratified North Sea–Baltic Sea transition zone. Inter-annual variability with more extensive hypoxia has been observed in years with calm weather conditions during late summer. A future warmer climate may increase hypoxia in the area due to combined effects from decreased oxygen solubility and increased respiration rates. Feedbacks from climate change can, therefore, amplify negative effects from eutrophication, such as hypoxia. Here we apply a high resolution three-dimensional ocean circulation model with a simple pelagic and benthic oxygen consumption model (OXYCON), based on the seasonal organic carbon budget in the area, and demonstrate that the model is able to simulate the temporal and spatial variability of the observed oxygen concentration in the bottom waters during a three-year period. The potential impact from a warmer climate was analysed in a sensitivity study with a 3 °C warmer climate, and showed a significant increase of hypoxic bottom areas compared to present day conditions. The relative role of increased respiration and decreased oxygen solubility in the inflowing bottom water and at the surface was analysed and it was found that decreased solubility accounted for about 25% of the simulated decrease in bottom water oxygen concentration in the centre of the area in the early fall. A sensitivity study showed that the simulated effect from a 3 °C temperature increase on the bottom water oxygen concentration could be compensated by a 30% reduction in the export production. The model simulations of the North Sea–Baltic Sea transition zone indicate a significant expansion of the hypoxic areas and a lengthening of the hypoxic period under a warmer climate.

Phytoplankton diversity, whether defined on the basis of functional groups or on the basis of numbers of individual species, is known to be heterogeneous throughout the global ocean. The factors regulating this diversity are generally poorly understood, although access to limiting nutrients and light is known to influence distributions for certain groups of phytoplankton. Here, we develop a simple box model of biomasses and a limiting nutrient to describe the composition of phytoplankton communities in the euphotic zone. In addition to analyzing the relative importance of nutrient availability in generating and maintaining diversity, we apply the model to quantify the potential role of zooplankton grazing and ocean transport for the coexistence of competing species and phytoplankton diversity. We analyze the sensitivity of phytoplankton biomass distributions to different types of grazing functional responses and show that preferential grazing on abundant species, for example as formulated by the Holling type III grazing function, is a key factor for maintaining species’ coexistence. Mixing and large-scale advection are shown to potentially have a significant impact on the distribution of phytoplankton species and, in general, enhance phytoplankton diversity. Based on the model solutions, we argue that ocean transports of phytoplankton cells can have a significant influence on species composition and that even transports at horizontal spatial scales of up to at least 102 km and vertical scales of 102 m can be expected to be important for sustaining phytoplankton diversity. The model is applied in a multi-species simulation (n = 200) and model simulations suggest that global patterns of phytoplankton biodiversity are determined by a few dominating species within each group. This finding is shown to be in accordance with ecological observations.