Räumlich-zeitlliche Verteilungsmuster von Plankton in Seen: Abiotische Störungen und organische Interkationen mit dem besonderem Fokus auf Planktothrix rubescens
Zusammenfassung der Projektergebnisse
The investigations have shown that physical forcing, resource availability, and organismic interactions are important features that affect the spatial and temporal distribution patterns of plankton in lakes. The harmful and buoyant cyanobacterium Planktothrix rubescens increased its abundance in many pre-alpine and alpine lakes during the last decades concurrent with the process of re-oligotrophication. In Lake Ammer, P. rubescens dominates the annual-mean phytoplankton biomass and forms a deep chlorophyll maximum (DCM) in the metalimnion. However, the abundance of P. rubescens in Lake Ammer underlies high inter-annual variability, but the decisive causes and their interplay (e.g., vertical mixing intensity, mixed layer depth, Secchidepth, and nutrient pulses by river inflow) that explain the inter-variability are still not disentangled. At the beginning of the project, a new in-situ technique that combines information from optic and acoustic instruments was developed, which allows qualitative and quantitative observation of P. rubescens, but also distinction between P. rubescens, other phytoplankton, and zooplankton at once. As the measuring devices can sample in situ at high rates they enable assessment of plankton distributions at high temporal (minutes) and spatial (decimeters) resolution or covering large temporal (seasonal) and spatial (basin-scale) scales. Internal wave motions are a prominent feature in Lake Ammer that cause periodic (e.g., ≈23 hours - basin-scale Kelvin wave) vertical displacements of P. rubescens and other plankton between 5 and 10 m at the lake ends. These vertical displacements cause fluctuations in the light intensity available at the depth of the P. rubescens layer, which in turn leads to differences in the daily mean available light intensity between lake ends. As a consequence, the daily mean specific production rate of P. rubescens can differ by up to one order of magnitude between lake ends, which may lead to patchiness. Vertical displacements caused by internal waves can be distinguished from other factors influencing the distribution of P. rubescens (e.g. active buoyancy control, production, vertical mixing) by a temperature-based data transformation. This technique may be of general use for separating wave-induced transport from other processes (e.g. sedimentation, vertical mixing) that affect the distributions of dissolved substances and suspended particles. Plankton and in specific P. rubescens are heterogeneously distributed and is highly affected by internal waves and river inflow. In Lake Ammer, systematically higher concentrations of P. rubescens were observed at the southern end of the lake, which can be explained by local growth due to higher nutrient supply by the River Ammer and/or by the widening of the thermocline due to internal waves and river inflow. The typical length-scales of P. rubescens patches range between 0.7 and 1.8 km, independent of the abundance of P rubescens. The diel vertical migration (DVM) pattern of zooplankton and the vertical displacement of P. rubescens and especially their spatial interplay, are affected or dominated by internal wave motions. The spatial interplay depend on the phase relation between the DVM of zooplankton and the internal wave. The following interactions or changes in the behavior of zooplankton under the presences of P. rubescens were identified: the magnitude and the pattern of the DVM of Daphnia is not affected by the abundance and vertical position of the P. rubescens layer; the predacious zooplankter Cyclopoida avoid the layer of P. rubescens, and Calanoids were found to co-exist with P. rubescens (strategy of active niching), which may reduce the pressure of predators (e.g., fishes) that avoid high concentrations of P. rubescens. The measured microcystin (MYC) concentration is highly correlated with the concentration of P. rubescens. Apparently, the amount of toxin per biomass remains constant independent of the light climate or the density of P. rubescens. MYC are rather stable compounds. Hence, zooplankton that actively or passively feeds on P. rubescens or feeds on other zooplankton that grazed on P. rubescens incorporates and accumulates MYC within its own biomass. Hence, the MYC produced by P. rubescens can be transferred to higher trophic levels and thus may affect the entire food chain of aquatic ecosystems. Application of a 3D hydrodynamic model to Lake Ammer provided numerical simulations of temperature distribution, water currents, and internal wave motion that agreed well with observations. In the case of Lake Ammer, internal wave activity in combination with a shallow bay were identified as potential cause for the generation of patchiness at spatial scales around 1-2 km. Project results were communicated to the lake management authorities and to the broad public (e.g., stake holders, civil servants, and sailing club). This supported the visibility of the DFG-funded PlanktoPat-project and helps to transfer the main results of the project to the public.
Projektbezogene Publikationen (Auswahl)
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2013. In-situ optical and acoustical measurements of the buoyant cyanobacterium P. rubescens: Spatial and temporal distribution patterns. PLoS ONE 8(11): e80913
Hofmann H. and F. Peeters
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2014. The consequences of internal waves for phytoplankton focusing on the distribution and production of Planktothrix rubescens. PLoS ONE. 9, e104359
Hingsamer P., F. Peeters, and H. Hofmann
