2.1 Selection and description of the study site and study period Dekar haor was selected for the present study because it is one of the largest and most important haor in Bangladesh. It lies between latitude 24°46´N to 24°57´N and longitude 91°20´E to 91°31´E. It is a seasonal-perennial haor. Data were collected from four sites namely Joykalash, Sunamganj Dakshin, Purbo pagla and Paschim pagla under Daksin Sunamganj upazila. The primary criterion for the selection of the study area was a suitable geographical coverage of haor as far as possible. At first, primary information was collected from Dakshin Sunamganj Upazila Fisheries Officer regarding concentration of aquatic weeds in the study area. On the basis of preliminary survey the final decision was taken for the study of this locality. The study was conducted for a period of 9 months covering three seasons namely winter, summer and monsoon from December 2016 to August 2017.
2.2 Collection of data Several sampling methods are used for studying the structure of sociological order in any plant community viz quadrate method, transect method, loop method and point method. For the present study, randomized quadrate method for sampling was used. The quadrate is a square sample plot or unit for a detailed analysis of vegetation. A plastic frame quadrate (1×1 m2) was laid at the specific sampling spots with minimum three replications and average abundance was expressed as No./quadrat. During data collection, both primary and secondary sources were strongly considered.
2.3 Sample Collection Aquatic weed samples were collected from the haor on each sampling day. Samples were collected from various sites of the haor. Monthly sampling was done and data were recorded. Weeds were manually collected from the sampling spots and then brought to the laboratory immediately and preserved in 7-10% formalin for further classification and identification.
2.4 Identification Samples of weeds were placed on a table for the easy contrast of vision for the identification. Weeds were identified using standard available literature. Identification was done up to genus and species level. Data of research were finally tabulated.
2.6 Determination of indices of species diversity In the present study, aquatic weeds diversity evaluated by dint of Shannon-Wiener index (H′ ), Species richness by Margalef index (d) and Evenness by Pielou’s index (J′ ). Shannon-Weiner diversity index (H′ ):
H′ = – Σ pi log pi
Where, H′ = Shannon Weiner index, Pi = ni/N, ni = no. of individuals of a species, N = Total number of individuals
Margalef species richness (d): d = (S - 1) / ln N Where, S = Total species, N = Total individuals
Pielou's evenness index (J' ): Evenness or equitability of the species in the study sites was calculated using Margalef’s equation: (J' ) = H(s) / H(max.)
Where, H(s) = the Shannon-Weiner information function H (max.) = theoretical maximum value for H(s) if all species in the sample were equally abundant