katrinteubner

die geographische lage des sees in den österreichischen alpen

Mondsee, 2010:
Der See wird von vier Gemeinden umgeben: im Norden vom See, wie hier auf dem Foto gezeigt, liegt die Gemeinde Mondsee (Kleinstadt, talwärts liegend) sowie die Gemeinde Tiefgraben (hauptsächlich bäuerliche Höfe mit Milchviehwirtschaft, bergauf liegend). Die beiden weiteren Gemeinden sind Innerschwand und St. Lorenz.Der Mondsee (47°49’41.88''N, 13°22’46.56''E) ist ein Alpensee im Salzkammergut in Oberösterreich und befindet sich etwa 30 Kilometer entfernt von der Stadt Salzburg. Er liegt 481 m über dem Meeresspiegel. Er hat eine Fläche von 14.21 km² und ein Volumen von 510  x  10⁶ m³. Der See hat eine längliche Form, die sich über eine Strecke von etwa 10 km von der kleinen Stadt Mondsee an der nordwestlichen Küste, bis zum Abfluss im Südosten des Sees erstreckt. Der nördliche Teil, nahe der Felswand „Drachenwand“, ist der tiefste Teil des Sees mit einer maximalen Tiefe von 68 m, während das südöstliche Ende in eine flache Bucht ausläuft. Die theoretische Verweilzeit des Wassers im Seebecken Mondsee beträgt 1,7 Jahre (Tabelle 1 in Dokulil et al. 2006 R). Der See wird weitgehend von Wiesen und einigen Waldflächen umgeben. Teilweise gibt es auch felsige Uferbereiche. Einzelne Höfe, kleine traditionelle Dörfer und, wie bereits erwähnt, die kleine, sehenswerte Stadt Mondsee mit ihrem typischen Ambiente, prägen das Ufer des Sees.

Mondsee, 2005:
Die längliche Form des Sees ist vom Schafberg aus gut ersichtlich. Der See ist seit Langem ein beliebtes Ziel für Touristen. Das Bild von einem kleinen Hotel (Pension) am Ufer an der Drachenwand in den späten 40-iger Jahren, wo damals die Weißwäsche am Steg im See gespült wurde, wird beim Zuhören der Erzählungen lebendig. Die Wäsche wurde damals üblicherweise am Ufer, auf der Wiese vor dem Hotel ausgebreitet und so in der Sonne trocknend natürlich gebleicht. Damalige Besucher haben auch gut die damalige Eisenbahnstrecke entlang der Drachenwand oder den Bau der Autobahn bei Mondsee in Erinnerung. Vieles hat seit sich dieser Zeit verändert, anderes ist geblieben. Mehrere Mostschenken, Mostbauern oder Mostheurige, sowie Jausenstationen und kleine individuelle Restaurants um den See, sind bis heute authentisch erhalten und laden mit ihrem alpinen Charme zum Verweilen und Genießen ein. Seit einigen Jahrzehnten ist Mondsee auch ein attraktives Zuzugsgebiet. Ein modern-neutraler Baustil hält seit jüngster Zeit massiv Einzug um den See - selbst im Kernbereich traditioneller Bebauung, wie in der kleinen Stadt Mondsee, und verändert somit sukzessive das Stadtbild bzw. den Landschaftseindruck. Dahingegen freut es, in der ländlichen Region um den Mondsee viele traditionelle, gut erhaltene Bauerhäuser bzw. auch in diesem alpinen Baustil neu gebaute Bauernhäuser der landwirtschatlichen Familienbetriebe zu sehen. Ensprechend der hohen Besiedlungsdichte ist der Verkehr um den See recht belebt. Selbst enge Straßen, die sich unmittelbar am Ufer entlang winden, werden von Lastkraftwagen benützt.

Anders als im Straßenverkehr, wird der Motorlärm auf dem See erfolgreich unterbunden. Naturfreunde können die Stille auf dem See sogar an stark frequentierten Tagen in den Sommerferien genießen. Typischerweise zeigen sich im Sommer unzählige Segelboote auf dem See. Motorboote sind bis auf wenige beruflich begründete Ausnahmen auf dem See nicht erlaubt. Die nachhaltige Nutzung des Mondsee-Einzugsgebiets, wird am Beispiel der Viehwirtschaft im Rahmen anderer alpiner Seen auf der Website über den Attersee S näher beschrieben.

Von all den Seen im Salzkammergut ist der Mondsee wohl der am besten untersuchte See. Die Temperatur und wesentliche Nährstoffkomponenten wurden an der Oberfläche bzw. im Tiefenprofil seit den späten sechziger Jahren gemessen, dh. seit mehr als 40 Jahren. Weiters sind die Abundanz und das Biovolumen von photosynthetischen Mikroorganismen, wie den Algen und Cyanobakterien, ebenfalls über vier Jahrzehnte zum Teil sehr detailliert analysiert worden (Dokulil & Teubner 2012 R, 2023 R).
Die nachfolgende ökologische Beschreibung konzentriert sich auf die primären Produzenten des Sees, vor allem auf die photosynthetischen Mikroorganismen im Wasserkörper und zu einem gewissen Grad auch auf die Wasserpflanzen. Informationen über andere Studien vom Mondsee, sind auf den Websites vom Forschungsinstitut für Limnologie (in Mondsee) und dem Bundesamt für Wasserwirtschaft (BAW) (in Scharfling) mit limnologischen Themenschwerpunkten wie Gewässerökologie, Fischereibiologie und Seenkunde gegeben.

a lake turns up side down:
the importance of deep living Planktothrix rubescens in mondsee

Planktothrix rubescens im Mondsee, am Nordwest-Ufer, 2004:
Den Ausbruch der metalimnetischen Planktothrix rubescens kann man am besten an einem windberuhigten Tag beobachten. Dieses Cyanobakterium zeigt sich dann unverwechselbar als bordeauxrot gefärbter Schaum, der an der Seeoberfläche aufrahmend, gut sichtbar wird. Solche Schaumauflagen werden üblicherweise nicht über das ganze Jahr im Mondsee gefunden, sondern treten in manchen Jahren für einige Tage bis Wochen in der Sommerzeit auf. Vorbedingung für einen „Ausbruch“ ist, dass dieses Cyanobakterium in tiefen Seenschichten, nämlich im Metalimnion, bereits massive Biomassen entwickeln konnte. Tritt ein Ausbruch bei stürmischem Wetter auf, so rahmt diese „rot-aussehende Blaualge“ nicht als Schaumauflage auf, sondern färbt den Wasserkörper bordeauxrot durch.
Massive Blüten von P. rubescens in den 70-iger Jahren gefährdeten den Tourismus in Mondsee und veranlassten daher die damalige grundlegende Sanierung des Sees. Nach einem fast völligen Rückgang im gesamten Wasserkörper, ergab sich in den letzten Jahren ein recht erhebliches und damit unerwünschtes Neuaufkommen von Planktothrix rubescens im Mondsee (Dokulil & Teubner 2012 R, 2023 R) .
Es wird im Text darauf verwiesen, dass selbst bei einer sehr stark reduzierten und nur sporadischen Probenahme, das Beproben vom Metalimnion in solchen Seen nicht völlig ausgelassen werden sollte. Es muss bei der Probenahme in solchen Planktothrix-Seen darüber hinaus darauf Wert gelegt werden, genau in der Tiefe die Probe zu entnehmen, wo das Biomassemaximum dieses Cyanobaktriums liegt. Dieses Maximum dehnt sich oft nur über eine Schicht von etwa ein bis zwei Metern aus (siehe Tiefenmaximum von dem Pigment Chlorophyll-a (Chl-a) bei etwa 12 bis 14m in Abb. 4b in Dokulil & Teubner 2012 R; in dem Tiefenmaximum wird das Chl-a hauptsächlich von dem Cyanobakterium Planktothrix rubescens gebildet, wohingegen der Chl-a Gehalt an der epilimnetischen Oberfläche von einem sehr gemischt zusammengesetzten Algenplankton herrührt). Es kommt für eine sinnvolle Probenahme erschwerend hinzu, dass diese schmale Schicht, im Jahresverlauf vertikal wandert. Im Mondsee, zum Beispiel, variierte die Lage dieser Tiefenschicht im Verlaufe vieler Jahre zwischen 8 und 15 m (siehe braun-rot gefärbte Fläche zur Darstellung der Tiefenverteilung von Planktothrix rubescens für die drei Beispieljahre 1982-1984 in Abb. 3b in Dokulil & Teubner 2012 R). Im späten Sommer lag das Biomassemaximum im Mondsee häufig bei 12,5 m. Eine Probenahme in limnologisch sinnvollen Abständen, wie es z.B. über viele Jahre in dem gut untersuchten Mondsee üblich war, verhindert das versehentliche Übersehen der sich im Verborgenen erneut entwickelnden Planktothrix rubescens. Auch scheint es so, dass die Ursache des neuen Massenaufkommens nicht unbedingt nur über eine unmittelbare „Nährstoff-Biomasse-Relation“, wie etwa in den 70-iger Jahren, erklärt werden kann. So bedingt eine im Jahr um so frühzeitigere thermische Sommerschichtung infolge des Klimawandels die Zunahme von Planktothrix rubescens im Frühsommer (s. Abbn. 9b und 10 in Dokulil & Teubner 2012 R). Der günstige Klimaeinfluß auf die Entwicklung von Planktothrix rubescens, oder genauer gesagt der positive Einfluss des Trendes einer Erwärmung auf die Biomasseentwicklung von Planktothrix rubescens, lässt sich auch in Abb. 9a R ablesen (zum "NAO"-Klima-Index siehe auch AMMERSEE S). Neben dem Klimawandel begünstigen auch andere Habitatbedingungen die derzeitige Biomassezunahme von Planktothrix rubescens im Mondsee. Dies spricht wiederum dafür, auch heute eine detaillierte limnologische Untersuchung solcher Seen durchzuführen.
Die Ökophysiologie von Planktothrix rubescens wird darüber hinaus im Text dieser Webseite besprochen. Es wird dabei erörtert, dass dieses Cyanobakterium ein natürlicher Vertreter der belebten metalimnetischen Schicht in alpinen mesotrophen Seen ist. Dabei widerspricht ein geringes Mischaufkommen von Planktothrix rubescens NICHT der guten ökologischen Referenz eines solchen tiefen alpinen Sees. Lediglich eine Massenentwicklung dieses Cyanobakteriums ist problematisch.
Planktothrix rubescens im Mondsee, 2004:
Wie das Foto links, aber als Detailansicht. High concentrations of nutrients from sewage effluents were accumulated in lake Mondsee from 1969 to 1984. During this period, the peak concentration of the nutrient element phosphorus calculated as an annual mean of total phosphorus of a water sample was 35 µg L^-1 in 1978 (1.13 µmol L^-1, results at p.32 in Dokulil & Teubner 2012 R; further Dokulil & Teubner 2005 R). In many lakes nutrient enrichment is stimulating the growth of photosynthetic microorganisms near the water surface. The community of such primary producer organisms that is floating in the water is called phytoplankton and relates to various algae and cyanobacteria in a lake. These microorganisms contain green pigments as chlorophylls and other photosynthetic pigments and accordingly the watercolour changes to green. Not so far for lake Mondsee, at least in the period of remarkable nutrient input, the watercolour changed to bordeaux-red (Greisberger & Teubner 2007 R, Dokulil & Teubner 2012 R). The reason was the predominant development of the red coloured photosynthetic microorganism, the cyanobacterium Planktothrix rubescens, which were at that time the dominant form among other algae and cyanobacteria in summer in Mondsee. The absorption properties of a specific photosynthetic pigment, the red coloured phycoerythrin, allow this organism to survive even at deeper strata of 9-12 m (Dokulil & Teubner 2000 R, Fig.6 in Greisberger & Teubner 2007 R, Figs.3-4 in Dokulil & Teubner 2012 R). At this depth dim light only is available for photosynthesis. These low-light intensities relate to about 0.1-1% of light intensity penetrating from the surface through the water column. The biomass of Planktothrix rubescens ,however, is often estimated by another specific pigment occurring in Planktothrix and only a few other organisms. This marker pigment is oscillaxanthin (Table 2 in Greisberger & Teubner 2007 R, Fig.7 in Dokulil & Teubner 2012 R).
For a period lasting even over years, Planktothrix rubescens may build up their main biomass in deeper depths, while closer to the surface this cyanobacterium is absolutely rarely found by observing samples under a microscope. For example, in July 2002, the biovolume of this red coloured cyanobacterial plankton organism was at 12.5 to 14.5 m depth about 1 mm³ L^-1 while less than 0.1 mm³L^-1 was measured in the surface layer. Other photosynthetic organisms contribute a biovolume of 0.14 mm³L^-1 in deep strata (mainly algae as Cryptomonas, Rhodomonas, some diatoms) and 0.06 mm³L^-1 in the surface layer only. In this case Planktothrix rubescens is dominating the lake phytoplankton composition, and hence this cyanobacterium is the main contributor to phytoplankton productivity of the lake. In lakes usually the majority of photosynthetic plankton occurs in the surface layer due to the availability of sufficient light for photosynthesis. In Planktothrix-lakes, however, the water body seems turned upside down as the phytoplankton life is focused not at surface but at the deep layer (see also Ammersee, Teubner et al. 2007 R). This lake phenomenon is called to build up a deep chlorophyll maximum. Chlorophyll-a is here used as a rough estimator for phytoplankton biomass, as all photosynthetic plankton organisms from eukaryotic algae to prokaryotic photosynthetic (cyano)bacteria have in common the photosynthetic pigment chlorophyll-a among other more specific pigments in their photosynthetic apparatus. Living in the deep layer, Planktothrix rubescens can undergo a remarkable development over periods of eutrophication not visible in the surface water. A trend of increasing biomass of Planktothrix rubescens can be easily overlooked unless samples are collected with extreme care and attention from deeper layers in Mondsee (Teubner et al. 2022 R), see also discussion about useful sampling intervals on the website about Bergknappweiher S).

An outburst of this deep living cyanobacterium even in the surface layer may also occur from time to time. At least in years of heavy biovolume development, as was common during the nutrient-rich summer periods in the seventies, the shoreline of lake Mondsee was coloured bordeaux-red. The peak biovolume of Planktothrix of 4.5 mm³L^-1 for a depth-integrated sample was recorded in summer 1978. The biovolume for a separate deep-water sample was even higher at that time. Holidaymakers avoided going swimming in lake Mondsee at that time. After construction of a sewage drain system (‘Ringkanalysation’) in the 70ies to 80ies, which carries sewage from housing around the lake to sewage treatment facilities, the water quality of lake Mondsee has been improved. The trophic state of Lake Mondsee in recent years is described as oligo- to mesotrophic. This means that the lake ecosystem was largely recovered from nutrient rich conditions and that water quality is now close to the reference status of an alpine lake (see reference conditions described by ultra-oligotrophic status of the alpine lake Attersee S).

the transparency of water:
what does it mean for photosynthetic microorganisms in Mondsee

Mondsee, 2002:
Kristallklares Wasser auf dem offenen See, an dessen tiefster Stelle von 68 Metern. Deep living Planktothrix rubescens is an exception to the rule of planktonic primary producers in a lake (Teubner et al. 2022 R). Different from Planktothrix rubescens most photosynthetic organisms need much higher light intensities than dim light for photosynthesis. Hence the primary producers are growing in the surface layer in lake Mondsee. This lake surface layer inhabiting commonly photosynthetic organisms extends from the top 100% light intensity to deeper strata where at least 1% of surface light intensity is penetrating. This surface layer is called the euphotic zone. Phytoplankton organisms, however, are not necessary evenly distributed in the euphotic zone along the vertical profile. The near-surface maximum of phytoplankton biomass most days a year or at least on calm days without mixing by wind, is found at the shallow depth with 11-12% of surface light intensity. Such light intensities are typically found at 2-5m water depth in an alpine lake like Mondsee. At the very near surface, at 0-0.5m, usually not that many phytoplankton organisms are occurring in Mondsee as stress of high light intensity and UV radiation are inhibiting photosynthesis and hence reduce phytoplankton growth rates on the top surface of the lake. 

The depth of the euphotic zone (euphotic depth) may vary among seasons and years (Teubner I., 2005 R). According to measurements of underwater light climate, the euphotic depth was on average at 11.3m in 1999 and at 11.1m in 2000 for lake Mondsee. In the case of only a few floating organisms, as in winter or in years of low nutrient concentrations, light is less absorbed or scattered by particles and can hence penetrate into deeper strata than at higher particle densities as in summer or nutrient-rich years. For practical reasons, the euphotic depth is usually not directly measured by a light sensor but is visually estimated by Secchi disc transparency (Secchi depth). Absorption and scattering properties by living particles containing even pigments are distinct from those of non-living particles and hence the conversion factor between both methods change with the season. During the growing season, when water transparency is mainly reduced due to living algal particles, i.e. from May to September, the Secchi depth might be multiplied by the factor 3.42 to calculate the euphotic depth in Mondsee. For the remaining period each year, from October to April, the Secchi depth multiplied by 1.8 is a good proxy to estimate the euphotic depth in this alpine lake (the factors relate to monthly to biweekly measurements by both methods in lake Mondsee for 2000-2001). Light, or better to say the (photosynthetically active radiation of wavelength from 400 to 700 nm (PAR), is utilised via photosynthesis by primary producers as aquatic plants, algae and photosynthetic bacteria. The number of pigments that are associated with photosynthesis found in a water sample, for example, depends largely on the diversity of composition of phytoplankton. Some pigments are ubiquitous as they are common in all or many algal classes, others are occurring in only a few algae. In lake Mondsee, exemplified for the period 2000-2001, about 60 phytoplankton genera were found by microscopy (the number of species is even higher as in many cases several species of very similar pigment composition are assigned to one genus). This phytoplankton composition relates to a variety of abundant pigments as four chlorophylls, three phycobiliproteins and 23 carotenoids (Greisberger & Teubner 2007 R). All these pigments were analysed by acetone extraction and chromatography; and were found in at least 20% of the phytoplankton samples taken in 2000-2001 (the number of analysed samples is 584). The chlorophylls, phycobilines (e.g. phycoerythrin in Planktothrix rubescens see above) and a few carotenoids are involved in light harvesting of photosynthesis. Most of the carotenoids, however, have the function to protect the photosynthetic apparatus against too high intensities of light (PAR) and ultraviolet radiation (UV). Shifts in the proportion between light-harvesting and light-protective pigments enable phytoplankton organisms to cope well with the varying weather situation, as they adjust rapidly to low and high light intensities on a clouded or sunny day (Teubner et al. 2001 R). Other species are in general adapted to a specific light intensity range and are found typically in winter-spring or in summer-autumn. Such species are described as common taxa for a specific seasonal period. 

In addition to the adjustment to light by changing pigment composition, many phytoplankton organisms are able to move to a certain water depth layer that provides ‘optimal’ light intensities for their physiology. This phenomenon has been mentioned before, saying that the near-surface maximum biomass of phytoplankton is usually found at a depth of 11-12% of surface light intensity in Mondsee. Some algae and in particular, photosynthetic cyanobacteria (e.g. Planktothrix rubescens is most abundant at a depth of 0.1-1% of light intensity, see above) are known to move vertically by buoyancy regulation. Many algae even can swim, e.g. by using flagella. On a calm day, these algae are able to move from centimetres to metres along the vertical water column within some minutes. Some of the flagellated forms even move regularly during day and night. Such pronounced diurnal migration was e.g. recorded for algae as Ceratium and some Cryptomonas species in Mondsee July 2000 and describes a common summer situation for some flagellated phytoplankton species living in deep lakes. A very detailed taxonomic analysis of phytoplankton in the alpine mesotrophic lake Ammersee has shown, that a considerable number of phytoplankton taxa, namely 40% of a total number of 83 phytoplankton taxa, belong to flagellated forms (Teubner et al. 2003 R). Even if not analysed in detail, the number of flagellates might also hold for Mondsee as both lakes are of similar trophic status and phytoplankton composition. The key species of the aquatic food webs in Mondsee and other deep temperate lakes are in detail decribed in Qu et al. 2021 R.

annual cycles by heating and cooling of the water body
affect the growth of photosynthetic microorganisms in mondsee

Mondsee, Dezember, 2001:
Wärmeverlust durch Oberflächenkühlung – See-Nebel (’lake fog’) bedeckt den See im Spätherbst bzw. mit Winterbeginn. In addition to the primarily effect of light on photosynthesis, sunshine can also affect growth of primary producers via temperature change by heating or cooling the water body. In Mondsee, the annual minimum of light and temperature occurs in winter, the maximum in summer, as given by a temperate climate zone. With increasing sunshine duration from winter to summer the temperature increases, air and water successively warm up. The summer peak of air temperature lags 28 days behind the annual peak of total incoming radiation. It needs, however, considerably longer to warm up the water to its summer maximum. The time lag between the annual peak of total incoming radiation and surface water temperature is 37 days, the thermal stability of the water column calculated as Schmidt stability is 46 days and the heat content of the whole water body of lake Mondsee is 53 days, respectively. In other words, the summer peaks are passing successively, on average on 24.July for air temperature (on Julian day 205, i.e. the 205th day in the year), on 5.August for water surface temperature (Julian day 217), on 12.August for thermal stability of the water column (Julian day 224) and on 20.August for the heat content of the whole water body of Mondsee (Julian day 232), respectively. From those days onward in the year, the air and water are becoming cooler. Heat loss by surface cooling due to evaporation can be seen on a calm day in winter, when lake fog covers Mondsee. The seasonal development of phytoplankton measured by chlorophyll-a - the pigment that is common in all these planktonic primary producers - shows twice a year pronounced phytoplankton biomass peaks. The main phytoplankton peak appears on average in spring on 25.April (Julian day 115), the second peak is in autumn on 11.October (Julian day 284), respectively. The timing of these two phytoplankton peaks is not by chance but relates to growth periods of sufficient nutrient and light availability during the two mixing periods, namely in spring after ice-cover and winter stagnation and in autumn after summer stratification (spring turnover and autumn turnover). The complete circulation from surface water to the bottom layer at sediment two times a year, in other words, the twice mixing of the whole water body even at the deepest site of 68m, characterize Mondsee as a dimictic lake. Still in ice-free winters, the lake Mondsee the bottom temperature is higher than the water surface relating to inverse stratification. Hence the lake has tended to maintain a dimictic mixing regime even in years with mild winters, at least for the observation period from 1978 to 2004.

Mondsee, März 2005:
Der zugefrorene See - Stockschießen auf dem Eis im Winter („Eisstockschießen“) erfreut sich immer wieder großer Beliebtheit. The period between spring and autumn, which relates mainly to the growing season in the field, is characterized by thermal summer stratification in the lake. This stratification subdivides horizontally the water body in three layers. The top water layer is heated up by solar radiation. This warmed up surface layer of low density, the epilimnetic layer (epilimnion), stratifies on top of cool and hence denser water of deeper strata, the hypolimnetic layer (hypolimnion). In early summer, the epilimnion is very shallow but extends with time during a calm weather situation. The temperature difference between epilimnetic and hypolimnetic layer increases and culminates in a maximum difference in early August in lake Mondsee. On 12.August (Julian day 224), the day of annually peaking water column stability, for example, the temperature is 20.13°C for the epilimnion of the top 6m and 5.07°C for hypolimnion from 30-68m in Mondsee (Julian day and temperature calculated as 26-year-average for Mondsee period 1978-2003). While the epilimnion is turbulent and hence plankton organisms are circulating by vertical mixing within this layer, the hypolimnion is rather stagnant. The third layer during thermal stratification, namely that located between epi- and hypolimnion, is the metalimnetic layer (metalimnion) and extends during stable stratification for some metres. This layer is characterized by an enhanced density gradient that creates a strong buoyancy force suppressing vertical motion and hence avoids entrainment of particles e.g. filaments of deep living Planktothrix rubescens as mentioned above. Within the metalimnion the thermocline can be defined as narrow depth, identified by the maximum value of relative thermal resistance against mixing, and relates to the mixing depth. In Mondsee, the euphotic depth usually exceeds the mixing depth throughout the period of thermal stratification in summer. The euphotic depth of 1% of surface light intensity was exemplified for 1999 and 2000 above. During thermal stratification in summer 1999 and 2000, the euphotic zone exceeds the mixing zone on average by about 1.4m, which relates to the ratio of mixing zone to euphotic zone of 0.9 during thermal stratification. In other words, phytoplankton organisms, which are circulated by vertical mixing in the epilimnion, move within the euphotic zone, i.e. move within a zone of sufficient light for photosynthesis. Nevertheless, other factors than light may limit the growth of phytoplankton. Nutrient limitation of phytoplankton growth occurs at various periods in lake Mondsee, not outlined in detail here. It is noteworthy that phytoplankton living at the epilimnion, or even some species in the metalimnion, undergo a growth of nutrient limitation during thermal summer stratification. This is as the metalimnion acts as the barrier where nutrient elements released from sediment cannot freely pass to the upper layer. Hence the replenishment of nutrients goes much more slowly than rapidly demanded by epilimnetic phytoplankton growth. During stratification, epilimnetic phytoplankton growth is reliant on small portions of fast re-cycled, exudated or excreted phosphorus by other organisms in the nearby environment. Sufficient nutrients are replenished in the epilimnion by autumn and spring turnover as nutrient rich hypolimnetic water is circulated to the surface layer of the lake (hypolimnetic nutrient enrichment is due to release of nutrients from the sediment during stratification). In this view, solar ration ‘controls’ the growth of primary producers not only via the light-photosynthesis relationship but also via seasonal temperature dynamics.

the response to climate change in lake Mondsee

Mondsee am Mondsee, 2012:
Am Nordufer vom See Mondsee grenzt direkt die kleine Stadt Mondsee an.
Frühzeitiger Schnee im Herbst. The yield of phytoplankton biomass is often discussed by its relationship to nutrient concentrations in a lake. A questioner is typically asking for quantities, as e.g. ‘How much phytoplankton biomass is built in spring in Mondsee?’ A different view comes up with questions starting with the word when: When is the spring peak of phytoplankton biomass achieved in Mondsee? Asking for the timing of particular events or the duration of periods is close to climate lake research, the lake phenology. ‘Global’ climate acts through ‘local’ weather in the Salzkammergut district. The geographically most relevant climate signal for lake Mondsee is the North Atlantic Oscillation index (Dokulil et al. 2006 R, Blenckner et al. 2007 R, Dokulil et al. 2010 R) . This index relates to the atmospheric pressure differences between Iceland and the Azores and shows positive and negative anomalies, as the values for certain months or seasons or years are above or below the respective long-term average. For Mondsee, the weather conditions are significantly linked to this large-scale climate index. In years when the NAO value is above the long-term NAO average, the so-called NAO-positive years or years with positive NAO signal, the ‘local’ weather situation is described by high solar radiation, high air temperature and low precipitation in the Salzkammergut district. In turn, NAO negative years refer mainly to cold and rainy years in the Mondsee region. As the climate signal is closely linked to solar radiation and temperature currencies, not surprisingly, the climate effects on the lake are primarily seen by changes in lake physics. Alterations of lake physical conditions, as e.g. the timing of ice-off or the duration of thermal summer stratification might affect further other compartments as e.g. the timing of nutrient availability for phytoplankton growth or the timing of grazer offspring (zooplankton). In this way, the changing climate conditions have been cascading effects through the ecosystem, from physics to chemical or even biological features in Mondsee. The effects of climate signal, however, become weaker the less general rules of lake physics, and the more individual circumstances of lake biology are concerned. The temporal coherence among lakes in the Salzkammergut district is for physical parameters, as the seasonal development of water temperature, much closer than for lake chemistry and sometimes even lost on the side of the lake biology (Fig.1 in Dokulil & Teubner 2002 R, Fig.3 in Blenckner et al. 2007 R, Fig.6 in Dokulil et al. 2010 R) . 

Mondsee, 2006:
Fischerboote im Frühjahr – das zeitige Frühjahr ist die Jahreszeit, wo Änderungen infolge der globalen Erwärmung am deutlichsten sichtbar werden, u.a. durch Verschiebungen der zeitlichen Abläufe von Prozessen im See (Seen-Phänologie). The climate phenomenon of global warming in recent decades is often discussed for ecosystems worldwide, including alpine lakes from Austria in such large data set (O’Reilly CM et al., 2015 R, Sharma et al., 2015 R, Woolway et al., 2015 R) A coherent trend of increasing deep-water temperature of about 0.1 to 0.2 degree Celsius per decade was found for lake Mondsee and other lakes across Europe (hypolimnetic water temperature, Fig.2 & Table 2 in Dokulil et al. 2006 R). The response to global warming is usually best seen early in the year, in spring. The example here relates again to thermal stratification in summer as described before. The time series of the onset of stratification of recent decades is depicted by the Julian day each year, when a critical threshold value of thermal resistance against mixing is passed over. In other words, it is the date each year when a larger extent of surface layer is heated enough by solar radiation that thermal stratification holds on against vertical mixing by epilimnetic turbulence even induced by moderate wind. This onset of thermal stratification varied from year to year in lake Mondsee in the past decades. Extreme early stratification was found on 19.April-2.May in warm years (Julian day 109-122), i.e. in NOA-positive years (Fig.10 in Dokulil et al. 2010 R). In cold years, i.e. in NAO negative years, the on-set was about one month delayed, occurring around 20.-28.May (Julian day 140-148). In moderate years, not too warm and not too cold, the on-set of thermal stratification usually was passed from early to mid May. The off-set of thermal stratification differed also from year to year in lake Mondsee, depending on a warm or cold year, but dates were less fluctuating with time than the on-set. 

The change with time, as climate change, is seen by temporal trends. In recent decades, exemplified here for the period 1982-2003, the on-set of stratification became progressively much earlier in the year(Fig.10 in Dokulil et al. 2010 R), the off-set slightly later. In other words, today spring events in lake Mondsee, have the tendency to pass usually earlier than about 20 years ago. The time-span between onset and offset of thermal stratification is the measure for the duration of thermal stratification. This time-span, the duration of ‘lake summer’ in Mondsee, became also progressively longer, namely 6.5 days per decade. Such temporal trends were responded to by biological trends, as the spring peak of phytoplankton today also tends to appear earlier in the year (Fig.8 in Dokulil et al. 2010 R). Warming, however, might not be necessarily beneficial for photosynthetic microorganisms. Planktothrix rubescens provides an example of a microorganism to benefit from climate warming early in the year only, namely during late spring overturn and early summer. Longer periods of summer stratification did not favour biovolume development of this cyanobacterium in lake Mondsee (Figs.9&10 in Dokulil & Teubner 2012 R).

the macrophytes of lake Mondsee:
a tender green belt covers the narrow shore area

Mondsee, 2012:
Litorale Uferzone mit Phragmites australis - punktuelle Schilfinseln sind hier das Überbleibsel eines ehemaligen Schilfgürtels, bedingt durch die Zerstörung der Land-Wasser-Grenzzone über viele Jahrzehnte. Je schwächer die Bestände eines Schilfgürtels werden, desto verwundbarer wird das Seen-Ökosystem; je weniger intakt die Land-Wasser-Grenzzone ist, desto mehr verliert das Seen-Ökosystem seine Fähigkeit, eine gute Wasser-Qualität zu garantieren. Aquatic plants, called macrophytes, grow well on a wetland close to the shoreline or under-water (littoral) until at water-depth where sufficient light is still available for plant growth. The morphology of the water basin of deep alpine lake Mondsee refers to steep shores and hence, the shallow zone of the shore, the littoral zone of the lake, is narrow for macrophyte stands living in the water. In addition, a coarse grained sometimes even rocky underground at the bottom in the littoral zone, is not beneficial for macrophyte rooting and sprouting. Expanded macrophyte stands cannot be expected in lake Mondsee as found, for example, from riverine shallow lakes in North Europe or from mouths of large rivers as e.g. the Danube delta (see on this website Danube S). Nevertheless, some macrophytes inhabit naturally the lake Mondsee. The macrophyte Phragmites australis for example, reed, occurs in lake Mondsee. This species is common in the world. Littoral reed belts are known to contribute to ecosystem health in many ways. They provide microhabitats for attached living algae (benthic algae, here mainly diatoms), which compete well for phosphorus and hence can be seen to control to some extent the growth of other algae as phytoplankton species. Further, non-living particles such as, for example, dead cells of phytoplankton, are settling in littoral reed stands. This organic material mineralised by microorganisms is fertilizing the sprouts of Phragmites. Underground rhizomes of Phragmites are known to contribute to an aeration of sediment. And it shouldn’t be forgotten that littoral reed belts serve as valuable habitats for many animals as molluscs, crustaceans, insects, fish and water birds.

Mondsee, 2012:
Dies ist die einzig erhaltene nasse Wiese am nordwestlichen Ufer. Die letzten hier wachsenden Relikte von „Land-Schilf -Beständen“ (Phragmites australis ) gleichen bereits isolierten Inseln. Die weitläufigen nassen Wiesen am Nordwest-Ufer vom See existieren heute nicht mehr. Sie mussten innerhalb eines kurzen Zeitraumes, nämlich im Verlauf der letzten 25 Jahre, massiv den Bebauungen durch Straßen, neuen Siedlungen von Wohnhäusern und Einkaufszentren weichen. In general, Phragmites can grow on both habitats, near and in the water and is therefore, a dominant land-water ecotone species in the world. Phragmites spreads from the land into the lake as underground rhizomes grow horizontally from the wetland into the littoral zone of a lake. Sprouts from land stands hence can continuously re-generate reed stands in the lake. In case the land-habitat of Phragmites is destroyed, e.g. by changed land-use, sprouts in the littoral zone of the lake cannot be generated by wetland stands. Hence it is often just a question of time before remaining stands in the littoral zone also disappear. Land-water ecotones of lake Mondsee, became more and more rare in the recent 30 years. Limited littoral reed belts are still occurring in the north-west shore. Wetland and meadows on this shore, however, were drained and have now been replaced by housing and shopping areas in the last decade. Meadows on the west-shore also act not as refuge for land stands of Phragmites. Here, at the shallow littoral zone in Mondsee, which might be covered by Phragmites, reed is in some years partly replaced by filamentous green algae competing well for phosphorus among plants. Some reed occurs in the shallow south bay of Mondsee. Even here, Phragmites appears spot wise only, built rather littoral reed islands than littoral reed belts. Due to the road narrow the shoreline of lake Mondsee, the connectivity between wetland or meadows and littoral zone of the lake is almost completely lost. Phragmites cannot pass the road and hence regeneration of littoral Phragmites stands by land stands is today much more rarely found than a decade ago. 

Other macrophytes, as e.g. submerged Nuphar and Potamogeton also occur in lake Mondsee and built littoral stands between reed belt and the open water of Mondsee.