Abstract:
Small seasonal tanks with the tank cascade systems of Sri Lanka are highly
threatened by anthropogenic disturbances. The aquatic plant diversity within Polaga-
willa seasonal tank during the dry season was estimated. Concentric line transects
extending from the tank bund up to the inundation area during the dry season were
drawn in a digital topographic map. Along these transects, three sub-divisions were
made: lower, middle, and upper, from which quadrat sampling was done to cover 10%
of the total area. The land area that would be inundated during the rainy season was
also sampled following the same method. The Shannon Weiner index (H), and Jaccard
index (SJ) were calculated and compared between the lower, middle, and upper areas.
The overall diversity (H 1.08), the center locale of the tank represented a diversified
aquatic population. The lower and upper sections follow H 0.84 and H 0.71 respec-
tively exhibiting lower diversity. The similarity between the three sections of the tanks
was less than 50%, indicating each of the two sections showed few similarities. The
overall tank encompassed 12 distinct taxa from 10 families, including 25% exotic
species and 67% native species. Salvinia molesta and Ceratophyllum echinatum are
the most abundant species in the three areas of the tank. Endemic Cryptocoryne beck-
ettii was observed in the middle section of the tank. The inundated area during the
rainy season of that tank showed less diversity (H 0.29), of which 70% of the area
was dominated by Paspalum vaginatum, a perennial weed while 15% was covered by
Persicaria attenuata. Severe grazing due to livestock rearing and cultivation within
Thaulla in the catchment area has severely affected the ecosystem.
