katrinteubner

die geographische lage des sees in den bayerischen alpen

Ammeree, 2012:
Blick vom Westufer, von Schondorf aus, in die südliche Richtung des Seebeckens.
Der Ammersee (48°00’34.5''N, 11°7’2.4''E) ist ein voralpiner See. Er liegt 524 m über dem Meeresspiegel in Oberbayern (Deutschland), etwa 50 km entfernt von München.
Der See zählt zu den sechs größten Seen in Deutschland, hat ein Wasservolumen von 1750 x 10⁶ m³, eine Fläche von 46.61 km² und eine maximale Tiefe von 81 m. Die Verweildauer des Wassers beträgt 2.7 Jahre (Tabelle 1 in Dokulil et al. 2006 R).
Der Ammersee wird als mesotroph eingestuft. Dies bedeutet, dass die Nährstoffkonzentrationen im See, vor allem von dem Nährelement Phosphor, leicht über dem natürlich niedrigen Nährstoffniveau der voralpinen Seen liegen. Das Uferröhricht wird über weite Strecken von Schilf (Phragmites australis) gebildet. Solche breiten Schilfgürtel sind am Ammersee noch weit verbreitet ausgebildet, wodurch die natürliche Wasserreinhaltung des Sees unterstützt wird. Hier unterscheidet sich der Ammersee wesentlich vom Mondsee S im Salzkammergut. Neben urbanen Bereichen ist das bestimmende Bild der Ufer am Ammersee durch intakte Schilfbestände gekennzeichnet. Am Mondsee dagegen entsprechen sowohl die Schilfbestände als auch die Uferbereiche in weiten Bereichen nicht mehr dem ursprünglich-natürlichen Zustand.

Ammersee, 2012:
Das Schilf (Phragmites australis) bildet breite Röhrichtgürtel am Westufer. Ammersee, 2012:
Blick auf den Röhrichtgürtel am Westufer. Im seichten Uferwasser (litorale Zone des Sees) breitet sich das Schilf aus.

Wie die Seen im Salzkammergut in Österreich (siehe Website zum alpinen Attersee S), profitiert auch der Ammersee von seinem alpinen Einzugsgebiet. Dies wird im Wesentlichen durch eine nachhaltige Landwirtschaft und einen kontrollierten Umgang mit Abwässern über Kläranlagenbehandlung in den bayerischen Alpen gewährleistet.

Einzugsgebiet vom Ammersee, 2002:
Nachhaltige Milchkuh-Viehhaltung in den Dörfern um den Ammersee. Das Foto wurde in Haunshofen aufgenommen (Landkreis Weilheim - Schongau). Einzugsgebiet vom Ammersee, 2002:
Traditionelle Tierhaltung in der Ebene während der Vegetationsperiode.

Im nachfolgenden Text wird die Erwärmung des Tiefenwassers durch Klimaeffekte für den Ammersee beschrieben. Der Gleichklang der Klimaeffekte auf zugleich andere Seen in Europa wird dargestellt. Darüber hinaus wird der Ammersee detaillierter mit dem Mondsee in dem Abschnitt "Ammersee und Mondsee: Zwei ähnliche Seen" verglichen (ammersee and mondsee: two lakes but one story). Einen breiteren Raum nimmt dabei die Beschreibung des Phytoplanktons ein. Beide Seen sind von dem metalimnetischen Cyanobakterium Planktothrix rubescens geprägt (sommerliches Vorkommen in der Tiefenwasserschicht 7-12 m). Darüber hinaus wird die Phytoplanktonzusammensetzung für die verschiedenen Oberflächen- und Tiefenwasserschichten im Ammersee im Detail beschrieben. Eine Vielfalt von begeißelten Phytoplanktonarten wird entlang der Tiefenprofile im Ammersee gefunden. Mehr als 40% der 83 gefundenen Phytoplanktontaxa sind hier solche bewegliche Flagellaten.

deepwater warming and climate response of lake ammersee shows a particular close regional synchrony with lakes of the western alps

Ammersee, 2012:
Blick vom nördlichen Westufer über den See in Richtung Süden. The coherence, i.e. the synchrony, between lakes can be analysed by the correlation between time-series of lake-data over a same certain time period. As climate is global and may affect regional weather conditions in many places, an unusual warm or cold winter-spring period will certainly not only affect a single lake but many lakes over longer distances across Europe at the same time. As mentioned for Mondsee, coherence is best seen for the over-all lake physics as e.g. water temperature than for lake-specific issues, as e.g. catchment dependent chemistry, human impacted nutrient availability or biology. A synchrony of deepwater temperature was indeed found between 12 studied lakes across Europe, from Sweden, United Kingdom, Germany, Austria, Switzerland and France (Fig.2 in Dokulil et al. 2006 R).

The deepwater temperatures of the hypolimnion varied from year to year but increased in all these lakes by about 0.1–0.2°C per decade (Figs.2&4, Table 4 in Dokulil et al. 2006 R). The most consistent predictor of hypolimnetic temperatures was seen in the climate signal of the North Atlantic Oscillation (specifically the mean NAO index for January–May), which explained 22-63 % of the interannual variation of lake temperatures in 10 of 12 the lakes (exceptions were two remote lakes in less wind-exposed alpine valleys; lake Walensee and lake Hallstätter See). Lake Ammersee shown in this study has the closest deepwater synchrony to four lakes in the western Alps, i.e. to Lake Geneva, Zürichsee, Walensee and lake Constance, than to lakes elsewhere.

ammersee and mondsee: two lakes but one story

Ammersee, 2012:
Blick vom nördlichen Teil des Westufers in Richtung Süden über den See.Lake Ammersee is about 200 km away from lake , with a catchment area that is about four times larger than of Mondsee S. The lake basin of Ammersee is about 10m deeper and has a three times larger water volume and water surface, respectively, than Mondsee. The water retention time is accordingly more extended than for Mondsee, namely 1 year longer (2.7 years). Both lakes are mesotrophic.

The section before described the synchrony of deep water warming trends in various lakes across Europe. The temporal coherence of water surface temperature trends between neighbouring lakes, however, can be even stronger. Therefore, despite the rather weak coherence of deepwater warming in Ammersee and Mondsee, an indeed significant strong synchrony in particular for water surface temperature, but also for some parameters describing the whole water body seasonality was found. Very similar to Mondsee S , the time lag between the annual peak of total incoming radiation and air temperature is 32 days, the surface water temperature 42 days, the thermal stability of water column calculated as Schmidt stability 45 days and the heat content of the whole water body of lake Mondsee 54 days, respectively. As in Mondsee, the timing of the annual maximum of these temperature related parameters and the spring peak of phytoplankton tend to pass earlier the year (study period 1985-2001).

Phytoplankton composition in Ammersee is similar to that of Mondsee as both alpine lakes passed years of eutrophication before restoration, mainly achieved by sewage treatment in the catchment, which turned the lake to a reoligotrophication period. As in Mondsee, the cyanobacterium Planktothrix rubescens contributed mostly to phytoplankton biovolume and formed a pronounced deep chlorophyll maximum (DCM) at the layer from 7-12 m (Fig.2i in Teubner et al. 2003 R, Figs.11&28 in Teubner et al. 2004 R, Fig.1 in Teubner et al. 2006 R). The chlorophyll-a concentration of the deep layer was at that study period 1998-2001 significantly higher than of the epilimnion in Ammersee, which was a situation typically also found in Mondsee and hence already discussed there in greater detail.

P. rubescens stratified most frequently at the meta- to hypolimnetic depth layer of 0.91 %surface light intensity in Ammersee, while the epilimnetic phytoplankton formed preferentially the main biomass peak at 11.9 % light intensity (study period 1998-2001, Figs. 28 & 30 in Teubner et al. 2004 R, Fig.1 in Teubner et al. 2006 R). Other algae, which also built their biomass mainly in the deep layer and were hence associated with P. rubescens were diatoms as Asterionella formosa and flagellates as Katablepharis ovalis, Gymnodinium helveticum and Woloszinskia/Peridiniopsis (detailed study in 2001, see microphotographs & Fig.2 in Teubner et al. 2003 R ). This vertical pattern also became evident from the deep maxima of marker pigments, specifically for the cyanobacterium P. rubescens (oscillaxanthin) and for cyanobacteria in general (myxoxanthophyll, zeaxanthin and xanthaxanthin), further for diatoms (fucoxanthin) and cryptophyceans (photosynthetic alpha-carotene; depth distribution of pigments in Fig.3 in Teubner et al. 2003 R). In total more than 40% of 83 observed taxa were flagellates in alpine Ammersee (Teubner et al. 2003 R). Besides the afore mentioned deep-living flagellates, some flagellates were found to be more abundant in the euphotic layer 0-7 m than in deeper strata such as Chrysochromulina parva, Cryptomonas erosa, Pseudokephyrion entzii, Rhodomonas lens, R. minuta and Ceratium hirundinella (Fig.2 in Teubner et al. 2003 R). Such pattern of vertical separation of phytoplankton species and their marker pigments described here for Ammersee was, in principle, also found for lake Mondsee (Teubner & Greisberger 2007 R, Dokulil & Teubner 2012 R).

Identifying steady-state phytoplankton assemblages in Ammersee, three spatially heterogeneous environments for vertical niche separation were compared within the top 12 m: the euphotic epilimnion (2 and 5 m), the euphotic metalimnion (7 m) and the metalimnion below the euphotic zone with dim-light less than 1% light from the water surface (10 and 12 m) in Ammersee. The deep living phytoplankton assemblage at the dim-light level below 1 % light intensity can be described as a rather persistent community (Fig.5 in Teubner et al. 2003 R). The species composition reached here more than 80 % similarity between successive monthly samples, associated with almost zero net-change rates of phytoplankton biovolume (Fig.7 in Teubner et al. 2003 R). The deep-layer phytoplankton data points cover a hump-shaped curve, with net-change rates plotted on the x-axes and similarity on the f(x)-axis. The similarity of phytoplankton composition between successive monthly samples in the euphotic layers (2, 5 and 7 m) never reached more than 60 % and was usually significantly lower, even if biovolume net change was around zero. From this observation, it could be concluded that only during stratification and only in the metalimnion below the euphotic zone steady state assemblages can be expected in the deep mesotrophic Ammersee. Such a detailed analysis of species shifts along depths was not conducted for lake Mondsee, but an overwhelming biomass of P. rubescens persistent in deep strata, and a more frequent phytoplankton species shift in near surface strata was also found in Mondsee (Dokulil & Teubner 2012 R).

To sum up here, the two alpine lakes 200 km apart, Mondsee and Ammersee, are in view of general lake phenology, eutrophication history and phytoplankton composition rather similar than different.