Indian Journal of Marine Science Vol. 39(3), September 2010, pp. 323-333 Bloom of Trichodesmium erythraeum (Ehr.) and its impact on water quality and plankton community structure in the coastal waters of southeast coast of India A K Mohanty 1, K K Satpathy 1, G Sahu 1, K J Hussain 1, M V R Prasad 1 & S K Sarkar 2 1 Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu- 603 102 India 2 Department of Marine Science, University of Calcutta, Kolkata- 700 019 India [Email : [email protected]] Received 14 September 2009; revised 11 January 2010 An intense bloom of Trichodesmium erythraeum was observed in the coastal waters (about 600 m away from the shore) of southeast coast of India during the post-northeast monsoon period. The bloom appeared during a relatively high temperature condition with coastal water salinity > 31 psu. A significant reduction in nitrate concentration was noticed during the bloom period, whereas, relatively high concentration of phosphate and total phosphorous was observed. An abrupt increase in ammonia concentration to the tune of 284.36 µmol l -1 was observed which coincided with the highest Trichodesmium density (2.88 × 10 7 cells l -1). Contribution of Trichodesmium to the total phytoplankton density ranged from 7.79% to 97.01%. A distinct variation in phytoplankton species number and phytoplankton diversity indices was noticed. The lowest diversity indices coincided with the observed highest Trichodesmium density. Concentrations of chlorophyll-a (maximum 42.15 mg m -3) and phaeophytin (maximum 46.23 mg m -3) increased abnormally during the bloom. [Key words : bloom, phytoplankton, tropical, Trichodesmium, oligotrophic, cyanobacteria] Introduction (> 30 km). This appears to be the second report of Trichodesmium erythraeum, a marine Trichodesmium bloom which was sighted near the cyanobacterium, is an important nitrogen-fixer in the coast similar to the last year report from the same sea. It is one of the common bloom-forming species locality 15 . found in tropical and sub-tropical waters, particularly During a regular coastal water monitoring program, in the eastern tropical Pacific and Arabian Sea, a prominent discoloration of the surface water was contributing > 30% of algal blooms of the world 1. noticed in the coastal waters of Kalpakkam (12 o 33' N Estimated global nitrogen fixation by Trichodesmium Lat. and 80 o 11' E Long) (Figure 1) on 19 th February bloom (~ 42 Tg N yr -1) and during non-bloom 2008. The bloom was very dense and created conditions (~ 20 Tg N yr -1) suggests that it is likely to yellowish-green coloured streaks (Figure 2a) of about be the dominant organism in the global ocean 4 to 5m width and 10-20m long patches. The entire nitrogen budget 1, 2 . Trichodesmium normally occurs in bloom extended to several kilometers along the coast. macroscopic bundles or colonies and blooms formed The phytoplankton responsible for discolouration was by it are often extremely patchy. The patchy spatial identified as Trichodesmium erythraeum (Figure 2b). distribution of plankton blooms is usually connected Though, bloom of Noctiluca scintillans 16 , Asterionella to the physical variability of the water body 3. glacialis 17 and Trichodesmium erythraeum 15 in the Reports in literature showed frequent occurrence of coastal waters of the Kalpakkam have been reported, Trichodesmium blooms in Indian waters, however, it the present one has many interesting features. has been reported more frequently in the west Although, the data collected during our regular work coast 4, 5, 6-11 as compared to east coast 12, 13-15 . Equipped were not concerned directly with an investigation into with buoyancy regulating gas vesicles and nitrogen the causes of the bloom, the interest stimulated from fixation enzymes, Trichodesmium is regarded as an the studies of various physicochemical and biological organism well adapted to stratified, oligotrophic characteristics of the coastal water justifies the conditions 2. All the available reports on purpose of this paper. The acumen in investigating Trichodesmium bloom from east and west coast of Trichodesmium bloom appearance and distribution India have been observed far away from the coast stems from the recent report about its harmful nature, 324 INDIAN J. MAR. SCI., VOL. 39, No, 3, SEPTEMBER 2010 Fig. 1 Study area showing the sampling location Fig. 2a & b Discolouration of coastal water of Kalpakkam by Trichodesmium erythraeum bloom patches (a); magnif ied view of bundles formed by trichome (b) sometimes causing damages to coastal fish and shellfish connotations. The impact of bloom on coastal water fauna 18 . Thus, studying the causes that favour the quality and phytoplankton community is reported in this appearance of this bloom has social and economical paper along with the characteristic feature of the bloom. MOHANTY et al .: BLOOM OF TRICHODESMIUM ERYTHRAEUM (Ehr..) AND ITS IMPACT ON WATER QUALITY 325 Materials and Methods Results and discussions Surface water samples were collected twice daily A. Hydrography The values of pH did not show significant (between 9 to 10 AM and 4 to 5 PM during the bloom variations and ranged from 8.0-8.2 during the study period (19 th to 23 rd February), whereas, during pre- period (Figure-3a). It did not show any correlation and post-bloom periods samples were collected with bloom appearance as it remained almost stable weekly only in the morning hours. Samples were during pre-bloom, bloom and post-bloom periods. drawn by lowering a clean plastic bucket from the The surface water temperature during the study period Jetty of Madras Atomic Power Station (MAPS) and analyzed for various physicochemical parameters. ranged from 27.2-32.6 °C (Figure 3b). Comparatively high temperatures were noticed during the afternoon Temperature was measured by a mercury collections. A general increase in water temperate was thermometer with an accuracy of ±0.1 oC. Winkler’s noticed from January to March, which is a general method 19 was followed for the estimation of DO. phenomenon associated with air temperature in this Salinity was estimated by Knudsen’s method 19 . pH locality during this period of the year. Most of the was measured by a pH meter (CyberScan PCD 5500) marine cyanobacteria exhibit substantial growth in the with an accuracy of ±0.1. Dissolved nutrients such as, temperature ranges 25-35 °C11. The present bloom was nitrite, nitrate, ammonia, silicate and phosphate along noticed during relatively high temperature conditions with total nitrogen (TN) and total phosphorous (TP) were estimated following the methods of Grasshoff (28.4-28.7°C in the morning and 31.2-32.6°C in the et al .19 and Parsons et al .20 . Chlorophyll-a and afternoon). Temperature has long been recognized as Trichodesmium phaeophytin were measured spectrophotometrically an important factor that controls abundance 26-27 . Generally bloom of this filamentous (Parsons et al ., 1984. The phytoplankton density was 13 estimated using Utermohl’s sedimentation technique 21 alga occurred during hot weather season , as and counted using Sedgwick Rafter counting chamber cyanobacteria require relatively high temperature for with the aid of binocular research microscope (Nikon its optimum growth compared to other phytoplankton 28-29 . The present study agreed well with Eclipse-50 i). The identification of phytoplankton 4, 13-15, 30 was done by following standard taxonomic earlier reports , which showed similar monographs such as Desikachary 22 for diatoms; temperature conditions with the appearance of 23,24 25 Trichodesmium bloom during early summer and Subramanian for dinoflagellates and Fristch 31 for green and blue-green algae (Cyanobacteria). spring in the coastal waters of India. As observed, Three diversity indices such as species richness (R), the bloom was more predominant during afternoon species diversity (D) and evenness (J) were computed period when the temperature was relatively high as compared to morning period. to evaluate the variation between phytoplankton community structure and diversity, using standard The observed salinity ranged from 31.58-33.18 psu. formulae of Gleason (1922), Shannon-Weaver (1963) A gradual increase in salinity was noticed during the and Pielou (1966) respectively. study period (Figure 3b). Stable salinity condition close to typical value of 32 psu and above is known to Fig. 3a 326 INDIAN J. MAR. SCI., VOL. 39, No, 3, SEPTEMBER 2010 support the growth and abundance of Trichodesmium . near bloom area is common to post-bloom era and It is well known that the cyanobacterium is a indicative of decayed phase of the bloom. stenohaline form with optimum growth at > 33 psu and can’t survive in low salinities 11-14 . DO B. Nutrients concentration ranged from 6.2-8.1 mg l -1 Nitrate concentrations ranged from 0.17–6.79 (Figure 3a). The lowest and the highest DO µmol l -1, the highest value being observed during the concentration was observed during the post-bloom pre-bloom period and the lowest during the bloom and bloom period respectively. Marginally high DO (Figure 3c). Relatively low nitrate levels, continuous was noticed during the bloom compared to the pre- patches with yellowish green colour and increased and post-bloom period. However, concentrations of primary production (as reflected in chlorophyll-a DO during pre-bloom period were relatively high as values) coinciding with peak bloom period compared to post-bloom period. This could be due to sufficiently indicated that the bloom was in growth photosynthetic release of oxygen by the dense algal phase. A significant reduction in nitrate concentration biomass. Similar increase of DO content during was noticed during the bloom as compared to pre- and Trichodesmium bloom has also been reported post-bloom periods. Similar reduction of nitrate earlier 2,15 . As expected relatively low DO contents concentration during Trichodesmium bloom has also were observed during the post-peak bloom period, been reported by others 12,14-15 .