New lakes, old systems.
This small lake, which has since then been known as the 'New Pool' (Estanyol Nou), formed in exactly the same way as the other comparable lakes in the Banyoles system. The pools are located in an area of sulphaterich limestone through which water filters, rapidly causing erosion as it dissolves the surface and the underground structure. The gypsum contained in the marls of the upper layers, the result of a rising water table, dissolve, forming large cavities that eventually collapse like the Estanyol Nou. The basins formed in this way, in the form of inverted cones, produce lakes of a type that are uncommon elsewhere.
The Estanyol Nou is not, however, the most remarkable of the lakes forming the Banyoles system. The most unusual is the pool at Can Ciso, an almost hemispherical small basin, 82 ft (25 m) wide and 26 (8 m) deep, that shows spectacular seasonal changes in the color of the surface layers of water. In winter, in fact, the Ciso pool turns an intense wine red. In summer, the surface water is warmer than that at the bottom and the lake is stratified, whereas in winter the waters mix owing to the effect of the wind, and the water column is uniform in structure. Why does this affect the color of the water? The organisms responsible are the purple sulphur bacteria that live in the waters.
So much organic matter is available in the Ciso pool that the decomposer organisms, by oxidizing it, completely exhaust the oxygen in their environment, while the sulphate-reducing organisms living in the sediment produce large amounts of sulphides throughout the entire year. In summer, these processes are restricted to the bottom, but in winter they occur throughout the water column and the pool becomes totally anaerobic. The lack of oxygen is the reason why the dominant organisms (phototrophic bacterioplankton and some ciliates) are anaerobic, although some aerobic organisms that tolerate low concentrations of sulphide ions are present in the shallow aerobic layer at the top. This is why there are no fish in the Ciso pool. Light and temperature are very important factors determining the composition of the planktonic microbial communities. There is a correlation between the photosynthetic activity of its components and the depth at which they occur.
Bacterioplankton accounts for most of the biomass present in the Ciso pool. Most of the organisms belong to one of two families, the Chromatiaceae (purple sulphur bacteria) and Chlorobiaceae (green sulphur bacteria). They are anaerobic microorganisms with a phototrophic metabolism. In other words, they can photosynthesize in the absence of oxygen, as did the first photosynthetic organisms on the primitive earth. These sulphur bacteria use hydrogen sulphide (as aerobic photosynthetic organisms use water) in the presence of light, converting it first to sulphur and then to sulphate. The components of the microbial community in the Ciso pool may also include phototrophic protoctists (unicellular algae and some ciliates and flagellates) that also eat bacteria, bacterial primary producers, bacteria that attack and devour other bacteria, and organisms of different groups that live together, forming symbiotic, commensal, mutualist, or parasitic associations.
The vertical stratification of the populations of phototrophic bacteria found in Ciso corresponds to the norms of general ecological theory. Light plays a very important role in vertical organization: The photic zone contains the primary producers; below this are the heterotrophic organisms. The vertical distribution of biological, physical, and chemical variables (which in the Ciso pool are mainly oxygen and hydrogen sulphide) gives rise to a redox (reduction-oxidation) gradient. The pool's stratification parallels the vertical distribution of organisms in tropical forests and in microbial carpets. The differences between these ecosystems are mainly due to the size of the habitat; in forests, for example, the photosynthetic layer is several meters thick, while in stratified planktonic communities, such as the one in the Ciso pool, this layer is only a few centimeters thick, and in microbial carpets it is only a few millimeters thick.
The different types of bacterial interactions in the Ciso pool give rise to an anaerobic microbial ecosystem, a reminder of what aquatic ecosystems must have been like in the Archaean Age (between 3,800 and 2,500 million years ago), when the earth's atmosphere contained no free oxygen and the only organisms were prokaryotic bacteria. The main primary producers were anaerobic phototrophic bacteria and, as happens in the Ciso pool, during much of the year, the sulphate-reducing bacteria closed the sulphur cycle. This pool is thus an excellent model to interpret the ecological relationships that arose between different microorganisms in the earth's aquatic ecosystems 3,000 million years ago. It is also a model for the study of sulphated karstic lakes in the light in which oxygen is lacking and hydrogen sulphide is abundant. The Ciso pool is a relic of ecosystems of the remote past and a biological jewel. This was not, however, enough to convince the sole inhabitant of the land under the Estanyol Nou to return home!