Table 1.

Description of the lakes.

	Zeilu	Mazais Kivdalova	Ivusku	Dunakla	Audzelu
Average depth of lake water layer (m)	1.1	1.3	1.0	2.2	2.0
Sapropel layer depth (m)	4.0–9.5	1.7–11.2	2.2–10.4	0.9–9.5	2.65–11.4
Lake surface area (h)	44.8	14.7	1.9	82.7	64.9
Lake bottom structure	Muddy	Muddy	Muddy	Muddy Gravelly	Muddy Sandy Rocky
Proximity of access roads (km)	0.5	0.2	0.2	0.5	0.2
Surroundings	Surrounded by reed beds, marshy forest; rural houses and service buildings; farmland; cemetery	Rural village Nuksi; rural houses with the adjacent area; field; forest	Farmland; cemetery	Close to town Ludza; one island in the lake	Populated area Vecsloboda; forest
Hydrological regime	Streams: 2 inflowing brooks and 6 ditches, 1 outflowing brook	Streams: 1 inflowing river and 3 ditches	Streams	Streams: 1 inflowing brook and 2 ditches, 1 outflowing river	Streams: 1 inflowing river, 1 brook and several ditches, 1 outflowing brook,
Sedimentological description of a section	Brownish grey to dark brown, flowing in upper layers, getting jelly-like in the lower layers, with plant residues, some H2S odour	Grey to dark brown, flowing, jelly-like with plant residues, top layers have a bit of rough sand, lower layers become denser	Predominantly yellowish-green, sometimes light brown, flowing, jelly-like, contains poorly decomposed peat admixture	Fark brown, Flowing in upper layers, getting jelly-like in deeper layers, with plant residues and well-decomposed peat impurities	Dark brown, Moderately flowing in the upper layer, the lower layer becomes extremely dense, mainly jelly-like, in deeper layers with admixture of sand and gravel
Additional data	Water transparency exceeding maximum depth (> 1.6 m); overgrown	Brown water lake	Overgrown	Very intense and regular water blooming; fish thirsting is observed in harsh winters; there was a Ludza bird integrated plant, later a meat processing plant nearby (water intake for economic needs (1000 m3/day)	Regular water blooming