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Die Charakterisierung und Quantifizierung von Phytoplankton-Gemeinschaften sind ein wesentlicher Aspekt der Gütebeurteilung von Standgewässern. In dieser Arbeit wird eine neue Herangehensweise hergeleitet und demonstriert, welche die klassische, taxonomisch-orientierte Methodik durch einen funktionalen Ansatz ergänzt, der auf den physiologischen und ökologischen Eigenschaften der Taxa (Traits) beruht. Kern des Ansatzes bildet der sogenannte Trait-Schlüssel, mit dem eine taxonomische Information in eine funktionale Information übersetzt werden kann. Die Aussagekraft und Potenziale des Verfahrens werden anhand von Langzeitdaten (50 Jahre) der Phytoplankton-Gemeinschaft der Rappbode-Talsperre exemplarisch demonstriert. Die Trait-basierte Analyse reduziert die Komplexität der Daten und ermöglicht eine quantitative Analyse der beobachteten Veränderungen auf der Basis von statistischen Methoden, die auf den klassischen taxonomischen Ansatz nicht anwendbar sind. Sowohl hinsichtlich der saisonalen Sukzession als auch der langfristigen Trends lassen die funktionalen Eigenschaften Rückschlüsse auf die maßgeblichen Umweltfaktoren zu. Für die wasserwirtschaftliche Praxis erweitert sich damit der Informationsgehalt und die Interpretierbarkeit der vorhandenen Daten. Diese Aufwertung der Ergebnisse kann ohne Mehraufwand erhalten werden. Von besonderem Interesse sind hierbei Kausalanalysen signifikanter Langzeittrends oder von sprunghaften Veränderungen in der Phytoplankton-Entwicklung, die hiermit in einen kausalen und systemanalytischen Zusammenhang gestellt werden können.
Physical determinants of phytoplankton production, algal stoichiometry, and vertical nutrient fluxes
(2010)
Most phytoplankters face opposing vertical gradients in light versus nutrient supplies but have limited capacities for vertical habitat choice. We therefore explored a dynamical model of negatively buoyant algae inhabiting a one-dimensional water column to ask how water column depth and turbulence constrain total (areal) phytoplankton biomass. We show that the population persistence boundaries in water column depth-turbulence space are set by sinking losses and light limitation but that nutrients are most limiting to total biomass in water columns that are neither too shallow or too weakly mixed (where sinking losses prevail) nor too deep and turbulent (where light limitation prevails). In shallow waters, the most strongly limiting process is nutrient influx to the bottom of the water column (e.g., from sediments). In deep waters, the most strongly limiting process is turbulent upward transport of nutrients to the photic zone. Consequently, the highest total biomasses are attained in turbulent waters at intermediate water column depths and in deep waters at intermediate turbulences. These patterns are insensitive to the assumption of fixed versus flexible algal carbon-to-nutrient stoichiometry, and they arise irrespective of whether the water column is a surface layer above a deep water compartment or has direct contact with sediments.
In many lakes, the most conspicuous seasonal events are the phytoplankton spring bloom and the subsequent clear-water phase, a period of low-phytoplankton biomass that is frequently caused by mesozooplankton (Daphnia) grazing. In Central European lakes, the timing of the clear-water phase is linked to large-scale climatic forcing, with warmer winters being followed by an earlier onset of the clear-water phase. Mild winters may favour an early build-up of Daphnia populations, both directly through increased surface temperatures and indirectly by reducing light limitation and enhancing algal production, all being a consequence of earlier thermal stratification. We conducted a field experiment to disentangle the separate impacts of stratification depth (affecting light supply) and temperature on the magnitude and timing of successional events in the plankton. We followed the dynamics of the phytoplankton spring bloom, the clear-water phase and the spring peak in Daphnia abundance in response to our experimental manipulations. Deeper mixing delayed the timing of all spring seasonal events and reduced the magnitudes of the phytoplankton bloom and the subsequent Daphnia peak. Colder temperatures retarded the timing of the clear-water phase and the subsequent Daphnia peak, whereas the timing of the phytoplankton peak was unrelated to temperature. Most effects of mixing depth (light) and temperature manipulations were independent, effects of mixing depth being more prevalent than effects of temperature. Because mixing depth governs both the light climate and the temperature regime in the mixed surface layer, we propose that climate-driven changes in the timing and depth of water column stratification may have far-reaching consequences for plankton dynamics and should receive increased attention.