Inga Hense

Generation of subsurface cyclonic eddies in the southeast Baltic Sea: Observations and numerical experiments

[1] Closely spaced CTD transects performed in the summertime reveal simultaneous downward/upward bendings of temperature/salinity contours in the seasonal thermocline/ permanent halocline of the Stolpe Channel and the Gulf of Gdansk, which may be interpreted as geostrophically balanced cyclonic eddies in the intermediate layer. To examine processes capable of forming the observed cyclonic eddies, a numerical simulation based on the Princeton Ocean Model (POM) has been initiated. The subsurface cyclones in the Stolpe Channel were satisfactory simulated in model runs under easterly/ northerly wind conditions. Their formation was shown to result from the adjustment of the high potential vorticity (PV) outflow (from the Bornholm Basin via the Stolpe Sill) to low potential vorticity environment by vortex stretching (so-called the PV outflow hypothesis by Spall and Price [1998]). In accordance with the real wind conditions, a cyclonic eddy observed in the intermediate layer of the Gulf of Gdansk was satisfactorily reproduced in a model run with the westerly wind shutdown, which implies westward transport throughout the Stolpe Channel and thereby excludes the PV outflow hypothesis. The subsurface cyclone simulated in the Gulf of Gdansk was traced to form in the course of relaxation of the coastal downwelling baroclinic jet.

Projected climate change impact on Baltic Sea cyanobacteria

Compared to other phytoplankton groups, nitrogen-fixing cyanobacteria generally prefer high water temperatures for growth and are therefore expected to benefit from global warming. We use a coupled biological-physical model with an advanced cyanobacteria life cycle model to compare the abundance of cyanobacteria in the Baltic Sea during two different time periods (1969–1998; 2069–2098). For the latter, we find prolonged growth and a more than twofold increase in the climatologically (30 years) averaged cyanobacteria biomass and nitrogen fixation. Additional sensitivity experiments indicate that the biological-physical feedback mechanism through light absorption becomes more important with global warming. In general, we find a nonlinear response of cyanobacteria to changes in the atmospheric forcing fields as a result of life-cycle related feedback mechanisms. Overall, the sensitivity of the cyanobacteria-driven system suggests that biological-physical and life-cycle related feedback mechanisms are important and must therefore be included in future projection studies.

Plankton dynamics in frontal systems of the Southern Ocean

A Biological Model of the Antarctic Polar Front (BIMAP) has been developed. The model comprises two biochemical cycles, silica and nitrogen, and five to seven compartments. Model runs are initialized using the WOCE data set and forced by an annual cycle of solar radiation and mixed layer depth. Sensitivity experiments indicate that disregarding remineralization and dissolution of silica does not affect phyto- and zooplankton biomass significantly. Experiments with different half saturation constants of silicate uptake indicate that values between 4 and 8 are reasonable for the plankton community at the Antarctic Polar Front. The role of iron limitation is investigated using different Si:N uptake ratios and reduced growth rates. While reducing the maximum growth rate leads only to slightly lower phyto- and zooplankton biomasses, different Si:N uptake ratios affect the development and maximum of plankton biomass significantly. Specifically, primary production and plankton biomass are strongly reduced by increasing the silica to nitrogen uptake ratio to values greater than 2. An Si:N uptake ratio between 2 and 4 appears to be reasonable for the region of the Polar Front.

Regional ecosystem dynamics in the ACC: simulations with a three-dimensional ocean-plankton model

Abstract Within the high nutrient–low chlorophyll regime of the Antarctic Circumpolar Current (ACC), high phytoplankton concentrations are frequently observed in the vicinity of the Antarctic Polar Front (APF). As is typical for frontal systems, hydrography in this region is characterized by meanders and eddies as well as up- and downwelling cells which redistribute nutrients and influence the depth of the euphotic zone. To study the processes leading to the observed phytoplankton distribution, a coupled ocean-plankton model for ecosystem studies in the ACC has been developed. The ocean component is an eddy-resolving version of the s-Coordinate Primitive Equation Model (SPEM). The model has a horizontal resolution of 1/12° and a vertical resolution increased near the surface. The biological model (BIMAP) comprises two biogeochemical cycles—silica and nitrogen—and a prognostic iron compartment to include possible effects of micronutrient limitation. Model results indicate that part of the ecosystems regional variability can be attributed to the effect of vertical and horizontal advection. However, frontal dynamics alone cannot explain the observed enhanced concentrations of phytoplankton biomass near the APF and the minima in the northern and southern ACC. Only when iron limitation is taken into account, the model simulates plankton concentrations in close agreement with observations during the SO-JGOFS cruises. While in the northern ACC phytoplankton growth is limited by silicate, primary production is limited by iron south of the APF. Near the APF, mesoscale iron upwelling enhances primary production, leading to increased phyto- and zooplankton biomass. The meridional structure with two plankton maxima is closely linked to the cross-front overturning circulation. This double-cell circulation with two upwelling branches is caused by the northward sloping large-scale bottom topography.

Modeling the Role of pH on Baltic Sea Cyanobacteria.

We simulate pH-dependent growth of cyanobacteria with an ecosystem model for the central Baltic Sea. Four model components—a life cycle model of cyanobacteria, a biogeochemical model, a carbonate chemistry model and a water column model—are coupled via the framework for aquatic biogeochemical models. The coupled model is forced by the output of a regional climate model, based on the A1B emission scenario. With this coupled model, we perform simulations for the period 1968–2098. Our simulation experiments suggest that in the future, cyanobacteria growth is hardly affected by the projected pH decrease. However, in the simulation phase prior to 1980, cyanobacteria growth and N2-fixation are limited by the relatively high pH. The observed absence of cyanobacteria before the 1960s may thus be explained not only by lower eutrophication levels, but also by a higher alkalinity.

Evolution in temperature-dependent phytoplankton traits revealed from a sediment archive: do reaction norms tell the whole story?

The high evolutionary potential of phytoplankton species allows them to rapidly adapt to global warming. Adaptations may occur in temperature-dependent traits, such as growth rate, cell size and life cycle processes. Using resurrection experiments with resting stages from living sediment archives, it is possible to investigate whether adaptation occurred. For this study, we revived resting cysts of the spring bloom dinoflagellate Apocalathium malmogiense from recent and 100-year-old sediment layers from the Gulf of Finland, and compared temperature-dependent traits of recent and historic strains along a temperature gradient. We detected no changes in growth rates and cell sizes but a significant difference between recent and historic strains regarding resting cyst formation. The encystment rate of recent strains was significantly lower compared with historic strains which we interpret as an indication of adaptation to higher and more rapidly increasing spring temperatures. Low encystment rates may allow for bloom formation even if the threshold temperature inducing a loss of actively growing cells through resting cyst formation is exceeded. Our findings reveal that phenotypic responses of phytoplankton to changing temperature conditions may include hidden traits such as life cycle processes and their regulation mechanisms. This study emphasizes the potential of living sediment archives to investigate plankton responses and adaptation to global warming.

The Role of Life Cycle Characteristics in Harmful Algal Bloom Dynamics

Life cycle-based adaptations are integral to the ecology of most organisms. For the toxic microalgal species Pyrodinium bahamense, Alexandrium fundyense, Pseudo-nitzschia spp., and Nodularia spumigena, the properties and behaviours of their life cycle stages enable them to thrive in diverse marine environments. Planktonic blooms of these species are associated with a range of negative impacts including fisheries closures and animal die-offs. As a result, their bloom dynamics have been studied extensively, illustrating the ways that each organism’s life cycle is adaptive to recurring biotic and abiotic stressors. Both P. bahamense and A. fundyense form thick-walled resting cysts that play a major role in the dynamics of episodic blooms and can lie dormant for extended intervals in bottom sediments. These cysts function effectively in both tropical and temperate habitats. Nodularia spumigena uses a related but different strategy by producing akinetes that help in its recurrence. Pseudo-nitzschia species do not form resting or benthic stages, but undergo sexuality to counteract the progressive decrease in cell size due to cell division with a rigid siliceous frustule. These life cycles are clearly adaptable to a broad range of environments as shown by their widespread distribution and abundance. Continued investigation of these life cycles, especially stage-specific interactions with biotic and abiotic conditions, is likely to provide further insights into algal species ecology broadly, including responses to global climate change, ocean acidification, and coastal nutrient enrichment.