A beel thicky infested with water hyacinth

dominant position aAa Chlmuphywe in the cc~lhrl Auram zom. The order of
domination of various cltsaes of phytoplrnktas in central Assun ame is M follows :
Chlorophyceae > Dinophyme > BaciIlariophycene > Myrophymre >
Euglenophyceae.
Phytoplankton diversity in the Iowa Aasam zone of the Brahmaputre valley was
richer. A total of 6 classes represent the phytoplankton group including Chrysophyceae.
AAa Chlorophyceae, Myxophyceae was tbc most dominant group in the lower Assam
zone. Low altitude, higher tcrnpmture and gradual slop provided conducive
conditions for richer plankton growth. Greater amount of nutrient availability favoured
the blooming of the blue green algae (Myxophyceae). The order of domination of
phytoplankton in lower Assam becls was as follows: Chlorophycrae > Myxophyceae >
Dinophyceae > Euglenophyceae > Bacillanophyceae > Chrysophyceae. Another
notable feature in the becls in lower Assam was the high rate abundance of
Euglenophyceae.
Phytoplanktem in the &Is of Barak valley represented by the same four classes
as was observed in the upper Assam beels. The order of domination was Chlorophyceae
> Bac~llanophyceae 1 Myxophyceae> Dinophyceae.
Five groups of zooplankters were present in the beels of Assam viz. Rorifera.
Copepoda. Cladocera, Osfracoda and Protozw. There were 8 genera of Rotifera. 4
genera of Copepoda, 3 genera of Cladocera and 1 each of Osrracoda and Proiozou.
Generally, Copepoda dominated the zooplankton communities in the &Is surveyed,
followed by either Cladocera or Rotifera. Members of Ostracoda were recorded only
from the beels of upper Assam.
Benthos
Benthic communities varied widely in the beels in the range of 0 to 1763
organisms/ml (Table 11). Of the three beels of Tinsukia district, Mota and Rampur
bals arc devoid of any benthic community. Mobpang in the same district has a thin
population of 32 units/m2. Most of the beels of Barak valley districts were poor in
bmthic community while beels of Marigaon and Kamrup districts were rich in bottom
macrofauna. Beels of Golaghat, Nagaon, Darrang and Sonitpur had moderate population
of benthos. The becle of Tinsukia, Dibrugrih and Sibaagnr hod scanty or nq population
of benthos. (hbqods dominated the benUu$ orgnnim followad by bivalves,