Abstract
Surface waters in many areas of Europe and North America suffered from anthropogenic acidification during the second half of the 20th century. Modern long-term research of acidification and recovery from acidification in the Tatra Mountains started in 1978 when a considerable damage to lake ecosystems was discovered on the Slovak side of the mountain range (i.e., in the Tatra National Park).
Acid atmospheric deposition and a low buffer capacity of the bedrock and soils caused changes in the water chemistry and a decrease in the diversity and numbers of lake biota, including cladocerans. The current cladoceran fauna of Tatra Mts lakes consists of 29 species.
However, many of these species disappeared from high-altitude lakes during the acid rain era, especially between the late 1970s and the early 1990s. Consequently, (ultra)oligotrophic lakes located in the alpine zone were usually inhabited by only a few acid-sensitive members of the family Chydoridae [e.g., Alona (Biapertura) affinis (Leydig, 1860), Acroperus harpae (Baird, 1835), Chydorus sphaericus (O.
F. Müller, 1776); Fig. 1A].
In some cases, only one species occurred, namely C. sphaericus. Moreover, in several small and strongly acidified lakes, C. sphaericus was the only representative of all zooplankton taxa.
This species may be the most abundant cladoceran taxon even where the total zooplankton biomass is predominantly composed of other planktonic invertebrates, which usually do not leave any (sub)fossil remains. These facts suggest that the dominance of C. sphaericus in fossil records indicates some type of extreme conditions, where other species are suppressed due to an unfavorable water chemistry, lack of food, or low water residence time (Fig. 1C), often in the presence of an abundant competitor - e.g., calanoid copepods such as Arctodiaptomus alpinus (Imhof, 1885); Fig. 1D, or a predator - e.g., phantom midges such as Chaoborus obscuripes (van der Wulp, 1859); Fig. 1B).
Therefore, fossil records of C. sphaericus should be interpreted with caution because it is a eurytopic taxon. Its low relative abundance in a fossil assemblage does not usually tell us much about past conditions, but its dominance deserves attention.
Since the 1990s, we observed a slow biological recovery from acidification in Tatra Mts lakes. In a set of 36 selected lakes, the average number of zooplankton species (Cladocera + Copepoda + Rotifera) per lake is currently the same as 110 years ago, before acidification started (ca. 10 species).
On the other hand, this is caused by an increased diversity of rotifer species, which is higher than in the past. The current average number of cladoceran species per lake still does not reach the number from the pre-acidification period (there is a difference of ca. 1 species).
The pre-acidification data used for the comparison come from the period 1909-1913 and were published by a Polish researcher Stanisław Piotr Minkiewicz (1877-1944). To document the first signs of the acidification onset in the Tatra Mts, we studied the history of one of the strongly acidified lakes (Starolesnianske pleso: 1988 m a. s. l., surface area 0.7 ha, maximum depth 4.1 m, rock-meadow watershed of 2.3 ha) using a sediment core covering the period of the last 3500 years and the classical analysis of fossil cladoceran remains.
Despite environmental changes in the last millennia (the Late Antique Little Ice Age, the Mediaeval Warm Period, the Little Ice Age, etc.), the record showed a very stable cladoceran fauna poor in species - Alona quadrangularis (O. F.
Müller, 1776), C. sphaericus, Ceriodaphnia quadrangula (O. F.
Müller, 1785), and Alonella excisa (Fischer, 1854). There was only one statistically significant change at the beginning of the 20th century, characterized by the dominance of C. sphaericus and most probably associated with the acidification onset.
However, we observed also interesting differences between these fossil data and the data from the recent limnological monitoring. Changes in cladoceran assemblages of Starolesnianske pleso were more abrupt in the case of the limnological monitoring data where C. sphaericus was the only cladoceran species during the peaking acidification in the 1980s.
The other three species, in contrast, have never completely disappeared from the fossil record. For the recovery period, the fossil record strongly underestimates the relative abundance of C. quadrangula (for this species, only ephippia were preserved in the sediment core), which became more abundant than C. sphaericus in modern samples since 2005.
Thus, the indicative value of ephippia for relative abundances of species in the past may be different from that of shells, headshields, or postabdominal claws. Comparing results of the (sub)fossil remains analysis with data from zooplankton monitoring at present times can obviously help improve our interpretations of records from a deeper history of lakes.