Abstract

Media Summary

Key Words

Introduction

Harmful Algal Blooms has received considerable and interactions among these organisms are the subject of many studies (Rice, 1984; Anderson and Garrison, 1997). However, the role of allelopathy on those interactions is not well understood. To investigate the role of allelopathy in phytoplankton interactions, it is important to study the variation of plankton population for longer term. As Inderjit and Dakshini (1994) pointed out to get an in-depth insight into algal allelochemical interactions, it is important to generate data on bloom formation, algal succession and their physical and chemical effects on the substratum. Our present work deals with the study of variation of marine plankton community of Bay of Bengal at Digha, Talsari and Sankarpur region throughout two years (2002-2003) to gain a clear understanding about interactions between different marine plankton species. Harmful phytoplankton plays a role in changing/moulding the plankton community. In this regard, an attempt has been made to identify the effect of such harmful algal species on succession and blooms of other phytoplankton species. Our emphasis was on the Harmful Phytoplankton Species, Noctiluca scintillans belonging to the group Dinoflagellates of the Division Dinophyta. The other two phytoplanktons we have chosen are Coscinodiscus excentricus and Coscinodiscus oculus irridis belonging to the group Diatom of the Division Bacillariophyta. This two dominant diatom species occur throughout the year and produces the base of the aquatic food web.

Methods

The study area extends from Talsari (Orissa, India) to Digha Mohana (West Bengal). Geographically the area is situated between 21°37′ Northern Latitude and 87°25′ Eastern Longitude to 21°42′ Northern Latitude to 87°31′ Eastern Longitude (Figure 1).

Figure 1. Map of Coastal Region of West Bengal and part of Orissa, India. (Source : CIFRI, Barrackpore, India)

The study was carried out from January 2002 to December 2003 and frequency of sampling was every fortnight. Samplings were done aboard a 10-meter fishing vessel. Plankton samples were collected both from the surface and subsurface water (1-2 meter depth) by horizontal plankton tow with 20-micron mesh net and 0.3m in diameters. The collected samples were preserved in 3% formaldehyde in seawater. Counting phytoplankton was made under microscope using Sedgewick-Rafter counting cell and are expressed in nos./liter. Identification of plankton community was done following Davis (1955), Newell and Newell (1979) and Tomas (1997). Surface water samples were collected from each station and physio-chemical properties (viz. Water temperature, pH, salinity, dissolved oxygen and conductivity), recorded. Mettler-Toledo MX300 X-matepro of Switzerland measured water temperature, pH and conductivity of surface water. Salinity was measured by using Salinity Refractometer. The dissolved Oxygen content of the sample water was estimated by standard Winklers method as given by Strickland and Parsons (1969).

Results

A total of 216 samples were collected in 36 sample days. The present study shows that the presence of 88 phytoplanktonic species, including 53 diatoms, and 7 blue green algae (Cyanophyceae), 9 green algae (Chlorophyceae) and 19 Dinoflagellates in the coastal water chosen (Table 1)

Table 1. List of total phytoplankton species found in Bay of Bengal at Digha-Talsari-Sankarpur Region during 2002 to 2003.

Noctiluca scintillans shows five peaks (i.e. occurring in larger number) over the year (Figure 2). First smaller peak (30896 no./m³) was recorded on 28.02.02, second slightly larger peak (69909 no./m³), on 27.06.02, third very smaller peak (8114 no./m³) on 13.08.02, fourth large peak (226754 no./m³) on 23.10.02 and last peak (79786 no./m³) on 19.12.02. From our field data, it is indicated that Noctiluca scintillans might produce some toxin and that have a direct effect on Coscinodiscus excentricus and Coscinodiscus oculus iridis. C. excentricus is highly sensitive to Noctiluca toxins but C. oculus iridis shows concentration-dependent activity. At first time peak of Noctiluca (30896 no./m³), the blooms of C. excentricus completely disappear but C.oculus iridis can withstand that toxin level. At the time of fourth large peak of Noctiluca (226754 no./m³), C. oculus iridis are unable to resist at level and both species completely disappear at that time.

The interactions of these three species do not match well with the hydrological parameters. C. excentricus shows inverse relation with salinity but C.oculus iridis shows parallel relation with salinity(Figure3).

We are unable to describe these three species interactions by hydrological parameters but these interactions obtained from our data shows that harmful phytoplankton may play a role in succession and blooms of other phytoplankton species.

Discussion

Harmful phytoplankton certainly plays an important role in the blooms and succession. Probably the main reason behind population succession and blooms are due to the toxin produced by harmful phytoplankton. When blooms of a particular harmful phytoplankton occur, the cumulative effect of the entire toxin released may affect the other organisms, causing mass mortality. Bloom of toxic phytoplankton depends on the effects observed and not necessarily because of a large biomass.

From our field data, it is indicated that Noctiluca scintillans might produce some toxin and that have a direct effect on Coscinodiscus excentricus and Coscinodiscus oculus iridis. Coscinodiscus excentricus is highly sensitive to Noctiluca toxins but Coscinodiscus oculus iridis shows concentration-dependent activity. The hydrological parameter that we have studied does not play any direct role in these three species interactions but it may help in species succession. Harmful phytoplankton controls the community structure and planktonic blooms.To establish the fact firmly, it is necessary to identify the toxic chemical and find out the amount of doses is required for the termination of planktonic blooms in a specified area. Our future work will be in this direction.

References

Anderson, D.M. and Garrison, D.J. 1997. The ecology and oceanography of harmful algal blooms. Limnology and Oceanography 42: 1009-1035.

Davis, C.C. (1955). The marine and fresh water plankton. Michigan State Univ. Press.

Inderjit and Dakshini, K.M.M. (1994). Algal allelopathy. Botanical Review. 60, 182,

Newell, G.E. and Newell, R.C. Marine plankton. A practical guide. 5^th Edition. Hatchinson and Co.

Rice, E.L. (1984). Allelopathy 2^nd Edition, Academic Press, London.

Tomas, C.R. (1997). Identifying marine diatoms and dinoflagellates. A P San Diego, C.A.

Strickland, J.D.H. and Parsons, T.R. (1969). A practical handbook of sea water analysis Bull. 167. Fisheries Research Board of Canada.