A beel thicky infested with water hyacinth

to upper zone. Density of plankton in the beel of uppa zone varied bctwem nil to 84
dl. PhytoplPnkton pup WM genaally nprescnted by Chlorophyeae,
Baclllariophyceae and Myxophyceae.
Zooplanliters were rcpwmtad by Osrracoda, Copepoda, Cladocera and
Rorrjcra. Generic repmentation under various gmq~ also found to be very poor. The
Omracode were found only in Mota beel under Tinsukia district as a unique occurrence.
Domination of one group over the other cannot be generalised, however, in many cases
phytoplankton dominated over zooplankton. In some stray cases, poor representation of
phytoplankton community was observed indicating low productivity. The possible
reasons are low temperature causing poor nutrient load as indicated by poor
conductivity and nitrate nitrogen and phosphate in water phase.
Beels in the central Assam zone showed higher plankton count. Phytoplankton
density in this zone varied between nil to 948 dl. Phytoplankton groups were well
represented by good number of genera under Chloroph.vceae. Bacillarioph~~ceue,
M!:rophvceoe, Dlnoph,vceae and Euglenophyceae. The striking feature of this zone IS
the appearance of Euglenophyceae which was not found in the beels of upper Assam
7me. Zoo plankters were represented by Rotifera. Copepoda. Cladocera and Prorozocr.
In the central zone phytoplankters dominated over zooplankters as general trend.
Plankton density was relatively richer in the beds of lower Assam zone too, the density
ranging between nil to 8180 dl. Generic representation was found to be similar to that
of central Assam beels. Beels in the Barak valley, with low productive water and soil
quallty, had plankton in moderate numben, ranging between 9 and 406 uil.
In general, productivity in terns of phytoplankton density was poor in all the beels
of Assam. Although low pH, transparency and conductivity are attributable to th~s, low
plankton concentration. the predominant factor is the competition with macrophytes In
garnering sunlight and nutrients. Moderate to high level of infestation of various
categories (emergent, submerged, floating, ere.) of macrovegetation were always
associated with low plankton count. The relatively high plankton count in Central and
lower Assam bcels is attributable to the low altitude with sloppy gradient of the
Brahmaputra. In these regions, riverine flow slows down leading to the settling of
nutrients. Relatively high temperature also favoured production of plankton.
Distribution of plankton in diffmt beds of Assam is presented in Table 10.