Chapter Twenty-Five

Annual report 2012  of the National Institute of Agricultural Research (INRA)

Physical-Chemical properties

Transparency

Temperature &  Dissolved Oxygen

Turbidity, Stocks of Cations and Anions  &  Conductivity

Nitrogen & Phosphate

Relationship between Nitrogen & Phosphate

Overall conclusion, in particular as to phosphate loading

Microbial composition - phytoplankton

Census of types and quantities of fish in Lake Annecy

Perch, Gardon, Coregone, Omble chevalier

Overall conclusions as to fish population

Microbial composition - zooplankton

Census of types and quantities of fish in Lake Annecy

Perch, Gardon, Coregone, Omble chevalier

Overall conclusions as to fish population

Fish population

Long term evolution of four major types of zooplankton

Seasonal growth of cladoceres, herbivores and predators, and copepods, cyclopides and calanides

Overall conclusions on zooplankton

Transparency

Measured in metres using a Secchi disk this gave annual average, lowest (summer) and highest (winter) readings reflecting the normal variations during the year due to organic growth. As shown in figure l.1 directly below, the results were 6.9, 3.6 and 10.4 metres respectively, which were a little lower than the preceding year but broadly in line with averages over the recent past.  Figure l.3 illustrates these seasonal fluctuations for both big and small parts of the lake during the past 17 years and compared with the standard French 5-level scale for classifying the ecological state of the water (Very good, Good, Average, Mediocre and Poor). This shows that there have been a handful of summers where for a few weeks the transparency has dropped into the “Average” level, but rarely into the "Mediocre" and never the "Poor". However for the majority of years the results have only been “Good” and for the vast majority of weeks in all years the results are well into the “Very Good” category. This is the basis for the claim that Lake Annecy is one of clearest lakes in France.

Figure l.3 shows the fluctuating levels of transparency month by month during the past 17 years.  The higher the graph the deeper the Cecchi reading, meaning the more transparent the water.  The fluctuations during the year are caused by the seasonal changes in the phytoplankton population, which grows rapidly in the spring and summer and declines steeply during winter months.

Temperature

As described in the Physics section of Limnology Chapter 1, temperature is one of the principal factors explaining the biological behavior of the lake. It is responsible for one of the key natural processes which maintain the overall health of the lake, namely the “brassage” or overturning of the whole mass of lake water once a year re-circulating organic debris from the bottom and re-oxygenating the whole lake.  On February 16, the temperature of the whole column of lake water was measured at a low of 4.5 degrees C which indicated a complete “brassage” – a very good thing for the lake’s health. This was a little lower than the previous year. Thereafter the temperature of the uppermost layer, the epilimnion, increased to 25.1 degrees C by 20 August and developed to a depth of two metres. Perfect for swimming. The diagram shows this information with each coloured line showing the temperature readings taken on one particular day on the surface in intervals all the 50 metres down to the bottom of the lake.

Dissolved oxygen

Turbidity

This is another measure of the transparency of the water – why do we need another one? This is because transparency is a vertical measure indicating the clarity of the water looking from top down. Turbidity is a measure taken horizontally at different depths through the lake and give an insight into the level of phytoplanktonic growth at different levels in the lake, not just at the surface. In this case it was found that maximum turbidity occurred on June 26 at a depth of 8 metres, and this was consistent with prior years.

Silicate and stocks of other cations and anions

Conductivity

Conductivity of the water is dependent upon the amount of salts dissolved in the water. Immediately after the brassage in mid February when the whole column of water is at the same temperature and the same level of dissolved oxygen, the conductivity is at its highest 0,301 mS/cm and is uniform throughout the lake. Thereafter conductivity declines as a result of a series of interconnected events. Organic growth takes place, in particular at the surface, and photosynthesis consumes CO2. This leads to an increase in the ph of the water, which in turn leads to a precipation of calcium carbonate. Since calcium is a significant contributor to conductivity, when it is taken out of the water, the level of conductivity drops. So conductivity is another indirect measure of organic growth in the lake and a high level of conductivity, for instance in Feburary, indicates a clear lake.

Nitrogen

Phosphate

Relationship between phosphate and nitrogen

Overall conclusion relative to physical chemical properties of Lake Annecy

As regards to phosphate, the key element which is a limiting factor for the growth of phytoplankton, we note low levels (in line with the range observed over the last 10 years). Lake Annecy is characterized, as already been reported several times in limnological surveys, by a phosphate content below the threshold of 10 μg Per Litre in the upper pelagic zone. If we analyse the historical data available (figure IV.4) concerning phosphate content at the time of the winter brassage this threshold of 10 μg Per Litre is never breached between 1973 and 2012.

Overall growth of phytoplankton of Lake Annecy

For the Big Lake, the seasonal dynamics can be summarized this way:
- A winter phase (January 18 to February 16) largely dominated by pinnate diatoms and to a lesser extent centric diatoms.
- A spring phase (March 13-May 16), which sees the phytoplankton biomass increase until 15 May. Diatoms and Chrysophyceae represent most of the biomass. We also note a non-negligible biomass of several species of Chlorophyceae (Scenedesmus spp.) usually indicative of more eutrophic conditions. Their presence can also be linked to favorable thermal conditions.
- At the end of May, the grazing pressure of zooplankton (Daphnia in particular) contributes to the decline in phytoplankton biomass.
- A summer (August 16-June 20), dominated by diatoms and Chrysophyceae, and a relatively large biomass of Chlorophyceae compared to previous years.
- An autumn and winter phase (September 17 to December 10) marked by a decrease in biomass and the dominant presence of diatoms and Chrysophytes.

The inter-annual dynamics can be summarized in this way: the year 2012 has a higher biomass than previous years (2008-2011). The proportion of Chlorophyceae is also more significant; this is generally considered to be a group indicative of more eutrophic conditions. The presence of this group explains the drop in value of the Brettum index in 2012 compared to the three previous years. Their growth may simply be attributable to random interannual variability (similar to one-off variations in growth in cyanobacteria in some previous yearss). Indeed, as with other groups typically associated with oligotrophic conditions, during these years cyanobacteria or Chlorophyceae have probably benefitted from favourable environmental conditions, especially thermal. Some changes seem to fit a clear trend, for example the increasing proportion of microphytoplankton (this year, for example a significant increase in Fragilaria crotonensis which is considered to be thermophilic). Again, changes in the intensity and duration of the thermal stratification of the lake could be the cause. Structural changes in size (micro versus nano phytoplankton) are taking place concurrently with the lowering of the thermocline (the result of a more pronounced and stable stratification). The apparent links between cause and effect for this kind of data in the medium term should however, be handled with caution, as many other factors are at work influencing the development of phytoplankton.

Evolution of groups of phytoplankton that are key indicators of the health of the lake

Evolution of overall indicator of quality of phytoplankton of the years

Micro and Nano Plankton

Overall conclusion as to phytoplankton of Lake Annecy

As established during previous monitoring reports, the chemical composition and transparency of the water place Lake Annecy very clearly in the category 'very good'.

Low levels of nutrient supply naturally limit the production of phytoplankton. However, although in 2011 we noted that, for the 4th consecutive year, the values of phytoplankton biomass were among the lowest observed, in 2012 we have seen an increase in phytoplankton biomass, approaching the levels of 2002-2003 (and also approaching levels of biomass measured at Lake Bourget in 2011).

Lake Annecy, however, remains characterized by relatively low phytoplankton biomass values consistent with the classification 'oligotrophic'.

Diatoms and chrysophytes are the dominant types of phytoplankton. The mixotrophic group (e.g. Dinobryon) which tend to feature in environments with low supply of nutrients continue to have high relative biomass. These mixotrophic groups use either osmotrophy (absorption of tiny nutrients particles by osmosis) or phagotrophy (engulfing and then digesting much larger food particles from organic sources) to get nutritious food in conditions of limited supply. In oligotrophic environments or periods of depletion of phosphorus, mixotrophy thus offers a significant competitive advantage to such photosynthetic microalgae with dual food processing capabilities. (Stickney et al 2000. Domaizon et al. 2003). If the presence of these mixotrophs has been a constant in the data from limnological studies of Lake Annecy, there has been from time to time exceptional according features such as the sudden appearance of a relatively large biomass of Chlorophyceae in 2012 (rarely seen in recent times).

The presence of this group of phytoplankton (which is usually found in waters much richer in nutrients that those of Lake Annecy) also influenced the Brettum index score, which nevertheless still remains characteristic of water of a “very good” classification.

Among trends appearing in recent years, we note a change in the relative amounts of nano- and micro- phytoplankton. Specifically, over the past five years the relative share of microphytoplankton has increased, whereas since the 1990s, lake Annecy was characterized by the dominance of nanoplankton.

The two species mainly responsible for this change over these last five years, are mixotrophic Chlorophyceae and non-toxic cyanobacteria (two groups with growth rates more efficient in the warm waters. For these one can hypothesize that the very stable stratification (of a layer of warm water within the lake) brought about by an unusually warm spring and summer had a rather beneficial effect). Other species responsible for the change were such as centric diatoms with a size slightly greater than 20 μm (size limit for distinguishing between nano- and micro-plankton).

It is however important to note that conventional counts made in the survey did not quantify the proportion of species of very small plankton (picoplankton, size <3 microns).

Picocyanobacteria, well represented amongst all the picoplankton compartment, have competitive advantages over larger cells (resource use and light). They are often prevalent in ecosystems with poor nutrient supply and they participate significantly, at least at certain times of the year in the epilimnion in total primary production and therefore support secondary production of the zooplankton.

Recent studies by the INRA Thonon-les-Bains, have shown that this group is very well represented in the main body of water of Lake Annecy. During the period 2003-12 picocyanobacteria have annual concentrations on average two times higher than those measured at Lake Bourget; in winter and spring periods these densities can be 7 to 10 times higher than those recorded in Lake Bourget (Personnic et al 2009).

Their contribution in terms of phytoplankton biomass is very significant and it is likely that this microbial resource plays an important role in determining the trophic state of the lake. It should also be noted that picocyanobacteria are able to react positively to the summer warming of the mass of water (Domaizon et al., 2013).

Long term evolution of major types of zooplankton

Overall conclusions on zooplankton

In 2012, the average volume of crustacea was less than at any year during the period 1995 – 2011: at the same time there is no clear tendency towards a reduction in zooplankton biomass in line with the oligotrophic state of the lake. These average values are slightly less than those measured in neighbouring lakes (Leman and Bourget, respectively mesotrophic and oligo-mesotrophic. ) Note that the dominant groups of crustaceans are however different in each of these three lakes: the calanoid and water fleas dominate respectively Lake Geneva and Lake Bourget, while cyclopids represent the group dominating lake of Annecy.  A notable development of the past 10 years in Lake Annecy was in respect of changes in the proportion of calanoid.  There was for calanoid, whose abundance increased between 2001 and 2008, a significant decrease in 2010 which was confirmed in 2011 and appears to be stabilizing in 2012. The other two groups (cyclopids and water fleas that represent over 75% of the total mass of Crustaceans) benefit from this reduction in the abundance of calanoid. These structural changes of the zooplankton community are clearly the result of several different factors (food resource, temperature, pressure predation etc).

The planktonic biomass, although not representing a standard indicator in the evaluation of water quality, is an integral part of the ecological state of the lake; it is therefore important to study it in order to understand the changes in the functioning of the whole ecosystem. The structure of zooplankton is subject to complex regulatory pressures (Quantity and quality of phytoplankton resources, predation pressure, zooplanktivores, thermal conditions with direct or indirect effects). The annual average quantity of microcrustaceans estimated on Lake Annecy, fluctuated moderately over the period 1995-2012, with no clear trend. If we put things in context, it is interesting to note that, despite the trophic level difference in the neighbouring lakes (Annecy oligotrophe; Bourget: oligo-mesotrophic; Geneva: mesotrophic) the overall average presence of the three main groups of microcrustaceans in these large peri-alpine lakes is relatively close in terms of density (individuals / m2). The proportions of various crustacean groups are however different. The cyclopids represent the dominant group in Lake Annecy. The calanoid increased their role from 2004 (amounts rising to 34-36% microcrustaceans in 2008-2009) but that peak volume has been reduced since 2010. The amount, size and nutritional quality of phytoplankton were identified as factors explaining these changes, however, other factors come clearly to play on the dynamics of zooplankton development.
The importance of the microbial sphere may be one of the possible explanations of the lack of direct relationship between phytoplankton and zooplankton in Lake Annecy (Perga et al. 2010).

Overall conclusion on fish population

The overall picture of the fish population of Lake Annecy confirms the good quality of the water confirmed by other indicators. Driving factors such as increasing temperature increments (Beniston, 2006), the impact of fishing, availability of food and other environmental parameters (Millennium Assessment, 2005) impact fish populations in multifaceted ways, which is why there is a need to continue to acquire data over the long term. The methods used in the context of scientific inventories (acoustic and fishing CEN), independent from the data collected by amateur and professional fishery, permit the establishment of sometimes difficult to quantify parameters (CPUE rejection depending on the size, not fish species) and thus to obtain a repeatable and reliable picture of the stock of fish in Lake Annecy. Already established by the analysis of the physical-chemical constituents of the water and composition of planktonic life in the annual monitoring, the lake’s oligotrophic status is further reinforced by the results of the survey of the fish population, with overall levels and particularly those of whitefish char relatively abundant. It is necessary to continue to sample at an annual rate in order to identify development trends in the population.

Data for estimating fish stocks has been incorporated in 2012 in the scientific monitoring of Lake Annecy. The structure of the fish community is stable compared to the previous years. Perch and perch juveniles,  and secondarily roach, dominate the fish community in the epilimnion.
The total stock of perch is at a high level, but not at the record level as it has been by the past. The age structure of this species serves to emphasize that while birth rates are very high f birth strong regulation by predation takes place during the first year.
In the deeper layers, whitefish is predominant. It is interesting to note that the levels of populations of whitefish and omble chevalier were estimated in 2012. The size of 0+ and 1+ Whitefish (from the few samples obtained) is similar to that obtained in 2007 and 2010. It is still necessary to balance this very positive data with the fact that these fish (perch, roach, whitefish) appear to have lower individual sizes in Lake Annecy by comparison with those measured (of the same age) on Lake Bourget. The available data for comparison is sometimes scare (in particular whitefish) and therefore these comparisons should be double checked and they are provided here for reference only.

The stock of fish from Lake Annecy confirms the quality of water observed through the other biological or chemical indicators. Lake Annecy is considered a reference model in terms of oligotrophy and good water quality. The long-term monitoring of its physical, chemical and biological characteristics, as well as more specific research programs, contribute to the understanding of its ecological functioning and intelligent management of the lake as a whole.

Continue Reading   Chapter Twenty-Six