M Arunachalam

Ecology of Indian estuaries—V: Primary productivity of the Ashtamudi estuary, south-west coast of India

Primary productivity at four representative stations in the Ashtamudi estuary has been presented. The average gross and net productivities in the surface water was maximum at Neendakara, the bar mouth zone and minimum at Kadapuzha, the riverine zone consistently exposed to pollution from paper mill effluents. Ashtamudi, the station directly on the gradient line and Kanjirakode, the station equidistant from Kadapuzha and Ashtamudi presented a transition phase with regard to the primary productivity. In the bottom water also productivity was minimum at the polluted zone. Generally productivity at sub-surface levels was higher than that at the surface. A seasonal pattern, independent of various environmental parameters, was discernible both in the gross and net productivity changes at surface and sub-surface levels. High concentrations of dissolved oxygen, silicate and nitrite helped to maintain higher productivities at Neendakara, Ashtamudi and Kanjirakode on several occasions during the year. The fact that productivity at the polluted Kadapuzha station was the lowest, indicates that the paper mill effluent has considerably disturbed the productivity mechanism of this otherwise healthy ecosystem.

Growth rates ofSalvinia molesta Mitchell with special reference to salinity

InSalvinia molesta Mitchell growth rate was uniform among the various size groups except in the proliferating floating stage in which it was faster. An average increase of about 25% weight occurred during a period of 5 days under laboratory condition. In general fresh set of leaves developed at an average interval of 4·5 days but this period may vary under different environmental conditions. In low salinities the growth of the fern showed retardation while in higher salinities it underwent complete destruction. Salinities above 7‰ was harmful for the weed while it completely withered in salinities 11‰ and above. Minimum period required for the total withering of the weed in salinities 34‰ and 11‰ were 30 min and 20 h respectively. These results are of value in the effective control of the weed by flushing them into brackish water lakes during monsoon.

Ecology of biofouling on Crassostrea madrasensis (Preston) (Mollusca:Bivalvia) in a tropical backwater

Ecology of biofouling on the edible estuarine oysterCrassostrea madrasensis (Preston) has been investigated in the Ashtamudi Backwater of the southwest coast of India. Fouling was highly conspicuous throughout the year and dominant groups included barnacles, serpulids, bryozoans and modiolids. Intensity of fouling varied from 73 to 179 % on living oyster valves and 65 to 172 % on dead valves with respective annual averages of 118 and 127%. Substrate selection and settlement of the different groups were mostly opportunistic. Barnacles were the most dominant, living and dead ones collectively contributing to about 26 % of fouling on living valves and 32 % on dead valves. Serpulid fouling was 22 % both on living and dead oyster valves, bryozoans 15 and 12%, modiolids 11 and 12% and the miscellaneous groups formed 27 and 20 % respectively. Availability of free settling space and fouling in relation to substrate size were also investigated. Total fouling was very intense on oysters of 25–35 cm2 size group. Impact of biofouling on oysters and certain earlier studies on the topic are discussed.

Harpacticoid copepods associated with the seagrass Halophila ovalis in the Ashtamudi Estuary, south-west coast of India

A temporal study of harpacticoid copepod populations associated with the seagrass Halophila ovalis was undertaken in the Ashtamudi Estuary, south-west coast of India. A total of 19 species representing 8 families was recorded in this assemblage. Harpacticoids formed 7.52% of the total phytal meiofauna. At the species level, harpacticoids exhibit parallel assemblages with phytal zones, found in other localities.

Ecology of Indian estuaries: studies on the zooplankton ecology of kadinamkulam backwater

Ecology of the zooplankton of Kadinamkulam Backwater, a brackish water lake along the south-west coast of India, has been studied from February 1980-January 1981. Seasonal distribution of the zooplankton followed an uniform pattern throughout the backwater. Zooplankton in the Kadinamkulam Backwater is composed of foraminifers, coelenterates, nematodes, rotifers, chaetognaths, polychaetes, cladocerans, ostracods, amphipods, copepods, decapod larvae, insect larvae, bivalves, tunicates, and fish eggs and larvae. Among these, rotifers, copepods and copepod nauplii are the major components which form the bulk of the zooplankton (nearly 98 %). A distinct regional variation is discernible in the dominance of zooplankton components. Copepods constitute the dominant group near the barmouth and middle portion of the backwater, while rotifers are dominant in the upper reaches, where freshwater influx was relatively high. Factors influencing seasonal variation and distribution have also been discussed.

Studies on the ecology ofSalvinia molesta Mitchell: B. Faunal associates of lentic and lotie habitats

Seasonal fluctuations of the fauna associated with the water fern,Salvinia molesta growing in a lotic habitat and the fluctuations during the three stages of the fern’s growth in a lentic habitat were studied. As many as 66 species belonging to six phyla were recorded, the major components being crustaceans, insect larvae and gastropods. The density of the fauna of the lotic habitat varied between 1267–7663/kg ofSalvinia, and thatof Salvinia of floating stage, intermediate stage and grown up stage (overcrowded) of the lentic habitat were 9610, 8860, 108 respectively/kg. Prolonged overcrowding ofSalvinia resulted in a sharp decline of the associated fauna.

Studies on the life cycle and ecology ofSalvinia molesta Mitchell

Salvinia molesta Mitchell propagates vegetatively by proliferation. Spores are nonviable. Mechanical disturbance enhances the rate of proliferation. The rates of proliferation within 30 days are 1: 8 in agitated media and 1: 1 · 6 in intact media under laboratory conditions. While the frequency of addition of fresh set of leaves ranged between 3–18 days in the natural habitat it was between 3–9 days under the laboratory condition. Crowding ofSalvinia is a prerequisite for the cessation of proliferation and initiation of the final ‘grown up stage’.Salvinia of ‘floating stage’, ‘intermediate stage’ and ‘grown up stage’ have different density ranges such as 765–986, 942–1280, and 65–345/m2 and biomass ranges of 18–25, 50–70 and 340–375 g dry wt./m2 respectively under natural conditions.