Some changes in the structure of plankton communities are
observed in the near-shore zone of the lake only. The major transformations in
plankton complexes occurred in the mid 1980's in the southern part of the lake.
At present according to characteristics of zooplankton , the southern part of
Lake Ladoga (including Petrokrepost Bay and Volkhov Bay) can be classified as a
mesotrophic region. The near-shore waters of the open Ladoga are considered mesotrophic. The pelagic zone should be characterized as oligo-mesotrophic.
There are several restricted areas, e.g. shallow bays, in which
pollution by effluents from pulp-and-paper mills has led to significant changes
in the benthic communities (Shchuchiy Bay near Priozersk, Hidenselka Bay in Laskela, Pitkaranta Bay).
Presumably due to eutrophication and organic pollution some
oligosaprobic glacial relicts (e.g. Pallasea quadrispinosa and ammaracanthus loricatus lacustris) of the Lake Ladoga invertebrates have decreased in numbers
There is also evidence of general increase of macrobenthos density and biomass
in the deep habitats.
Allien species Gmelinoides fasciatus
Gmelinoides fasciatus, a small amphipod from the Lake
Baikal basin, was discovered in Lake Ladoga in July 1988 (Panov, 1996). Gmelinoides fasciatus
was purposely introduced into Lake Ladoga in the early 1970's. The aim of the
introduction was to enhance fish production in some Karelian Isthmus lakes
located close to Lake Ladoga western shore.
By now the species has firmly established in the littoral
communities along the western and northern shores of Lake Ladoga (Panov, 1996; Kurashov et al., 1996). Gmelinoides
fasciatus is the dominant species in the littoral communities and
contributes over 70% to macrozoobenthos biomass. The species is abundant both
in macrophyte beds and stony littoral areas, both on polluted and undisturbed
sites. The maximum abundance and biomass of Gmelinoides fasciatus are
about 54,000 ind.m2 and 160 g.m2.
It is observed to negatively affect native species (e.g. Gammarus lacustris) as well as original benthic communities in general.
At present Gmelinoides fasciatus is actively expanding its range. According to the most recent observations the species has already colonized the Valaam archipelago and habitats along the southern shore of the lake.
Invader species in the benthic communities of Lake Ladoga are of
special interest, however are left almost totally not assessed. One of examples
of introduced species is Gmelinoides fasciatus that since 1988 has spread over
a large part of the Lake Lake Ladoga, came into conflict with aborigine species
and has influenced negatively on benthic communities.
Pollution of the aquatic environment, eutrophication, hydro-power
engineering and intensive fisheries have caused some transformations in the
fish population of Lake Ladoga. In the early 1990's catches were observed to reduce again, which was due mainly to the general negative phenomena in the economy rather than just a drop of commercial fish stocks. One of the consequences of the former is violations in the recording and reporting system: part of the fish caught is sold by fisheries enterprises or individuals without being recorded in reporting or statistical documentation.
The dramatic situation with the populations of land-locked fish species in Lake Ladoga was caused by the disturbances in the conditions for their reproduction. The main spawning rivers (Svir, Volkhov, Tulema, Vuoksa) are blocked by HPP dams, contaminated by industrial and farm discharge. Not so long ago fish reproduction was harmed by drift timber floating its negative consequences still not overcome completely. Salmon and trout stocks are influenced by disorderly amateur fishing in the spawning rivers. E.g., blocking of the Volkhov and Svir rivers by HPP dams with no fish ladders provided harmed the reproduction of the Ladoga coregonids and put the last atlantic sturgeon stocks surviving in Europe on the verge of extinction.
Contamination of the Lake Ladoga basin and man-made eutrophication of the water body resulted in chronic toxicoses of fish. Coregonids lost many spawning grounds to pollution.
Intensive hunting reduced the abundance of Ladoga seal
from 20,000-30,000 ind. in the beginning of the century to 5,000-10,000 ind. by
the 1960's. Since the 1970's Ladoga seal hunting in Lake Ladoga was controlled
via the introduction of limits on the bag size, in 1975 sport and amateur
hunting was banned. Since the early 1980's Ladoga seal is protected. At present
however its population is not over 5,000 individuals. In view of these estimated
the subspecies was defined as vulnerable in IUCN Red Data Book, and as
rare in the Red Data Book of Eastern Fennoscandia. Ladoga seal is
included also in the Red Data Books of Russia and Republic of Karelia.
It appears that the Ladoga seal was less affected by environmental pollution than the Baltic and Saimaa subspecies. A grave problem for the Ladoga seal is fisheries. Annually 200 to 400 animals die in fishing nets. On the other hand, both fishermen and local inhabitants hold to an idea of seal as a bane ruining the stocks of valuable fish. This contradiction greatly complicates the species protection.
Another adverse effect on the population is the nuisance factor. Human effect is the heaviest in the summer, when seal gather in herds on island shores and shoals. Ships and boats passing the sites of seal aggregations, people coming to the islands (fishermen, hunters, tourists) drive the animal into the water immediately.
