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Effects of habitat conditions on the diversity and abundance of molluscs in floodplain water bodies of different permanence of flooding

Jurkiewicz-Karnkowska E.

Polish Journal of Ecology

2011

Vol. 59, nr 1

165-178

Riverine floodplains are regarded as one of the most heterogenous and dynamic ecosystems. In natural state they encompass a variety of wetland sites like pools, lakes, channels etc. related and linked with the flood pulses; being the hot spots of high and specific biodiversity they are often under protection. The diversity and abundance of molluscs in the floodplain water bodies were investigated in order to find the shaping impact of chemical factors of water and sediments. The water bodies were located within 140 km section of the lower Bug River valley (eastern Poland, 190 to 50 km of the river course). The investigations were carried out in the years 2007-2009 in 25 permanently flooded, 25 semi-permanent sites (i.e. habitats partly dried-up because the water volume has decreased significantly during low river discharge) and 24 temporary water bodies holding water for at least few months (up to 8-9 months). The first group of sites consisted of large water bodies (the area above 1000 m2, depth mostly exceeded 2 m) representing earlier and middle successional stages. The second group contained relatively shallow (1-1.5 m) water bodies of medium or large size (from below 500 to above 1000 m2) representing more advanced successional stages. Temporary water bodies were very diverse in size (from about 100 to 1000 m2) their depth ranged from below 0.5 m to about 1 m and they mostly represented less advanced successional stages. Molluscs were collected from the bottom (using a hand net) and from macrophytes (using a frame). Concentrations of oxygen, nitrate and ammonia nitrogen, phosphates, calcium, chlorides, as well as BOD[5], pH and conductivity were measured in water. Organic matter, nitrogen and phosphorus content in bottom sediments were determined, as well as C/N and N/P ratio.Permanent water bodies supported the richest malacofauna - 52 species (over 91% of all species found within the study area), whereas in semipermanent sites and temporary ones 29 and 36 mollusc species were found. Mean number of species per site amounted to 18.7 [plus or minus] 4.0, 7.7 [plus or minus]3.2 and 9.6 [plus or minus] 5.1 in permanent, semi-permanent and temporary water bodies respectively. Mean values of Shannon-Weaver index (H.) in three site groups mentioned amounted to 3.18 [plus or minus] 0.55, 2.18 [plus or minus] 0.72 and 2.07 [plus or minus] 0.83, respectively. Mean density of molluscs significantly differed among site groups (from 80 indiv. m[^-2] in semi-permanent sites to 292 indiv. m[^-2] in temporary ones) being the highest in temporary habitats (the range of values from 34 to 1840 indiv. m[^-2]). Phosphates, ammonia nitrogen and pH of water significantly influenced the abundance of molluscs, whereas oxygen concentration and BOD[5] affected species richness, diversity and abundance of molluscs within the permanent water bodies. In the other site groups chemical parameters of water did not affect significantly the mollusc communities. The influence of organic matter, nitrogen and phosphorus content in bottom sediments on malacocoenoses was not significant. The quality of organic matter contained in bottom sediments expressed as C/N (the range of values from 8 to over 40) showed structuring influence on malacocoenoses. Low food quality (C/N> 17), especially in many semi-permanent habitats, may constrain the development of molluscan communities.