Role of Trichodestnium spp. in the

Transcription

1 Chapter 4 Role of Trichodestnium spp. in the organic production and nitrogen fixation in the Arabian Sea

2 4.1. Introduction The planktonic marine diazotroph, Trichodesmium spp. is considered to be an important in fixation of both carbon and nitrogen in many tropical seas (Carpenter, 1997). Carpenter and Romans (1991) summarized Trichodesmium biomass measurements for the SW North Atlantic Ocean, through application of relatively conservative (10 day) C and N doubling times and estimated that species in this genus were the most important primary producers and the largest contributor of new N to the oceanic euphotic zone. However, similar estimates in the Arabian Sea are not available and thus need comprehensive studies on C and N fixation rates in the euphotic zone. In this chapter, an attempt has been made to report the standing crop and rate of C fixation by Trichodesmium spp. and a comparison is made with productivity rates for the total phytoplankton community in the Arabian Sea. Thus, while it is known that Trichodesmium is an important entity as a primary producer in the Caribbean Sea, Carpenter and Price (1977) reported that it constituted 60 % of the euphotic zone Chl a and accounted for 20 % of the primary production. There are no comparative data available for the other tropical region. The Arabian Sea show large seasonal variability in primary production influenced by many local physical and chemical forcing suck as coastal upwelling and large inputs of freshwater during monsoons. The seasonal variations in the phytoplankton including Trichodesmium are not established so far in the Arabian Sea. The present study was devoted to detailed evaluation to assess the contribution of Trichodesmium the total phytoplankton, primary production and nitrogen fixation. 85

3 Other recent field studies on C and N fixation provide strong evidence that the biogeochemical role of Trichodesmium spp. in oceanic C production and N2-fixation is considerably greater (Carpenter and Romans, 1991, Capone et al., 1997; Karl, et al., 1997; Gruber and Sarmiento, 1997). As monthly variations in Trichodesmium concentrations are recorded during present study (Chapter 3), further attempt is made to see seasonality in carbon production and nitrogen fixation. The Arabian Sea has unique features with respect to wind direction, speed, solar insulation, currents, water temperature and upwelling between the NE (December to February) and SW (June to September) monsoons (Kumar, et al., 2001). Upwelling during the summer monsoon results in high concentrations of NO3 in surface waters, particularly in the north-west reaches of the basin, thereby precluding N2 fixation. During the oligotrophic intermonsoon periods, surface waters are depleted of combined N and productivity as well as biomass of phytoplankton is considerably lower. Published literature (Lenes, et al., 2001) suggest that along the coast of West Florida shelf, Trichodesmium biomass increased 100 fold due to deposition of nutrients from desert dust. The subsequent increase in DON following a West Florida shelf Trichodesmium bloom has been shown to be the stimulus for harmful algal blooms of the dinoflagellate, Karenia brevis (Walsh and Steidinger, 2001). In the northeastern Gulf of Mexico, the causes of an extensive bloom of Trichodesmium were determined to be the combination of high sea surface temperature, increased light intensity and salinity as well as low nutrients and wind speed (Eleuterius, et al., 1981). The role of Trichodesmium blooms during summer and the resultant increased nutrient load implicate to play an important role in the enhancement of 86

4 heteroptrophic bacterial respiration rates below the euphotic zone (Biddanda, et al., 1997). The bio-geochemical estimates and empirical measurements show that N2 fixation is a major source of new nitrogen supporting primary production in the oligotrophic environment (Capone, et al., 1997). Trichodesmium spp., one of the first known and best studied colonial cyanobacteria (Capone, et al., 1997) is considered as an important source of new nitrogen to the oligotrophic ocean and studies on this cosmopolitan cyanobacterium are still necessary as there are large loopholes in understanding its physiology. Although common in tropical and sub-tropical seas, its distribution is known to be irregular and spatially extensive dense blooms can form when the conditions are appropriate like high sea surface temperature, light intensity, low nutrients, and quiescent seas (Eleuterius, et al., 1981; Capone, et al., 1998). Published reports (Capone, et al., 2005) on measurement of N2-fixation in various parts of the oceanic environment suggest that Trichodesmium is an important source of new nitrogen to nutrient-poor regions of the global ocean. The nitrogen fixation studies are conducted and results are available from Atlantic (Goering, et al., 1966), Pacific (Letelier, et al., 1996; Karl, et al., 1997), and Arabian Sea (Capone, et al., 1998; Lugomela, et al., 2002). The studies are also conducted in the Caribbean (Carpenter and Price, 1977), China (Saino, et al., 1977; Chen, et al., 2003) and Sargasso Sea (Orcutt, et al., 2001). This new nitrogen significantly amplifies primary production and the consequently export carbon to the deep ocean. Trichodesmium sp. has been studied for decades both in natural populations and more recently in laboratory cultures (Ohki, et al., 1986; Chen, et al., 1996, Mulholland, et al., 1999; 87

5 2001) and the importance of new nitrogen input by Trichodesmium to many tropical and sub-tropical gyres has been established (Capone, et al., 2005). However, the reports on N2-fixation rates by Trichodesmium in the Arabian Sea are few although, Trichodesmium frequently occur in this region. Dugadale, et al. (1964) estimated N2- fixation by Trichodesmium in the Arabian Sea. However, these reports have not accounted for Trichodesmium occurrence during various months and resulting intensive blooms those accumulate large biomass. Most of the world oceans are depleted in inorganic nitrogen at the surface. In these extensive areas, it has been traditionally thought that the level of net biological activity is sustained by the sub-surface mixing. This flux of new nitrogen into the euphotic zone supporting primary production is balanced by concomitant losses through sinking particles, vertical migration and mixing of organic nitrogen from the surface waters (Dugdale and Goering, 1967; Eppley and Peterson, 1979; Lewis, et al., 1986; Platt, et al., 1992). Earlier studies (Jenkin, 1988; Lewis, et al., 1986) reported estimates of the nitrogen demand of new production which often exceeded the nitrate flux into the euphotic zone and such estimates must have prompted speculation about unknown or poorly quantified N inputs (Legendre and Gosselin, 1989; Karl, et al., 2002). Above studies indicate that Trichodesmium play an important role in the nutrient cycling of coastal as well as in open ocean, where it can supply new N to vast oligotrophic region. Here, we describe seasonal variation in the occurrence of Trichodesmium in the Arabian Sea. The data generated during the tenure of present study include pigment concentrations, C and N fixation ratio within the bloom to assess the impact 88

6 of this new nitrogen on the planktonic food web and to evaluate the same with nonbloom condition. An attempt has been made in this chapter to elucidate the following aspects Distribution of various pigments in the Arabian Sea Role of Trichodesmium in carbon production and nitrogen fixation in the Arabian Sea 4.2. Results Pigment composition The marker pigments like fucoxanthin, peridinin, zeaxanthin and CM b were studied to monitor phytoplankton at group level as background for carbon production and nitrogen fixation study. At St. A, weekly variations were observed in various pigment composition. Peak in CM a, b and c2 were associated to flagellates and prymnesiophytes population (Fig a). CM a, which represents all types of the phytoplankton, was high during monsoon and post-monsoon season. However, CM b was high during post-monsoon and n-carotene which represents diatom and cyanobacteria was high during monsoon season. CM a was equally high (2.9141: 1) during monsoon and post-monsoon season (Fig b). The monsoon peak was mainly due to fucoxanthin (diatoms), peridinin (dinoflagellates), zeaxanthin (cyanobacteria), CM c2 (prymnesiophytes) and n-carotene (cyanobacteria). However, post-monsoon peak was also supported by green flagellates (CM b) in addition to fucoxanthin and peridinin concentrations (Fig b). 89

7 I g DAD c / vo 4; 4.0 g u. OA j:si AA 1.1 I 1:11'.1';',..11:5.1.q1:45' Wecks V E 65o Fig a. Distribution of different pigments at St. A (Weeks lto17 and 52 to 55 Post-monsoon; 18 to34 Pre-monsoon; 35to 51 Monsoon)

8 5.0 Pre-Monsoon Monsoon Post- Monsoon 0.0 Chl a Chia Chl c3 Chl b p -care Per Fuco Zea Pras Pigments Fig b. Variations in seasonal average composition of pigments at St. A Zea Peri Chl a 47.46% Fig c. Average composition (%) of pigments at St. A

9 Table 4.1a. Pigment distribution (pigl -1) at station B (surface) Months CM a CM b Chl c Fuco Per Zea 13-caro Pras Jan Feb Mar Apr May June July NC Aug Sep Oct Nov Dec Min Max Average SE (NC- not collected; - means not detected)

10 Chl a (ugl-1) cikg Fucoxanthin (i.tg r1) Peridinin (af) fl- carotene (i.tg 1-1 ) IN- Aug Sep A. Oct 2003 Fig a. Distribution of marker pigments in the water column during August, September and October at St. B following upwelling and the development of anoxia

11 Table 4.1b. Pigment distribution (pigl -1) in the coastal water during May Stations Chl a Chl b Chl c3 Fuco Per Zea 13-caro Pras Min Max Average SE (- means not detected)

12 Fuco 0.09% Per 2.59% Zea 11.77% B-caro 8.00% Chl c3 0.06% Chl b 6.84% Chl a 70.56% Fig 4.1.2b. Average composition (%) of pigments during May in the coastal waters of Goa (SASU 45)

13 Table 4.1c. Pigment distribution (pg1: 1) in NE Arabian Sea during December Depth Stations (m) Chi a Chi b Fuco Per Zea p-caro Pras la A

14 Table 4.1c continued----- Depth Stations (m) Chl a Chl b Fuco Per Zea 13-caro Pras Surface (pg1; 1) n=8 Min Max Average SE (- means not detected)

15 Surface Pras Column 32.25% Fig a. Distribution of pigments at the surface (mgm -3) and integrated over the euphotic column (mgni 2) in the NE Arabian Sea during December

16 St 3 St a Pigments (mgm 4) ".." St 4B CI 60-6O - Pigments (mgm 4) St 8B St. 5 St. 9 Pigments (mgm 4) Chl a Other pigments Fig.4.1.3b.1. Distribution of Chl a and other pigments in the NE Arabian Sea during January

17 Conti Fig b.1 St. 6B St. 10B so - Pigments (mgm4) e: I :45 St. 11 Pigments (mgm4) Q St. 12B Pigments (mgm4) S 10-1 ft 15 - GB 20 - In St. 14 St. 15B Pigments (mgm 4) a? 20 e..,.1" 30, g I Pigments (mgm -3) Chla Other pigments

18 Although various organisms like diatoms, dinoflagellates and green flagellates were observed at St. A, zeaxanthin (3.83 %) and n-carotene (2.48 %) were low which is known to originate from Trichodesmium spp (Fig c). As seen in taxonomic study (Chapter 3), fucoxanthin was high (16.35 %), followed by peridinin (8.12 %) and Chl b (7.68 %) as well as Chl c2 (12.67 %) in total pigment composition (Fig c). The data obtained in the present study at St. A, indicate that it was influenced by tidal flow thus suggesting that physiographic conditions at this station influence its non suitability to support filamentous cyanobacteria. Pigment composition at St. B, which is a typical coastal station showed large variation in Chl a ( p,gl-1), fucoxanthin ( E 1) and zeaxanthin ( E 1) as well as n-carotene i.e E 1 (Table 4.1a). Similarly, zeaxanthin and n-carotene variation were due to Trichodesmium bloom observed at St. B (Table 3.3a). The zeaxanthin concentration increased during February ( E 1) and showed an increasing trend upto May ( lagi., -1) due to growth of Trichodesmium (Table 4.1a). It was observed that the highest concentration of zeaxanthin (originated from Trichodesmium) was during May. Further, detailed investigations were carried out during SASU 45 cruise in the coastal water and the results are presented in Table 4.1b. During SASU 45 (May) the Chl a varied from to p,gl -1 a was found at stations 1 and 7 respectively (n=9). The maximum and minimum Chl (Table 4.1b). The average Chl a content was ± (n=9). Zeaxanthin and 13 -carotene, marker pigment of Trichodesmium varied from to (Table 4.1a). Incidentally, the fucoxanthin variation coincided with diatom population 90

19 Table 4.1c.1. Integrated column pigment distribution (mgm -2) in the NE Arabian sea during December Stations Chl a Chl b Fuco Per Zea ll-caro Pras la n= 8 Min Max Average SE Table 4.1d. Pigment distribution in the NE Arabian Sea during January Chl a Chl b Chl c2 Chl i3 Fuco Per Zea fl-caro Pras Surface (ItgL-1) n =13 Min Max Average SE Integrate column (mgm.2) n = 13 Min Max Average SE

20 Fig b.2. Average surface composition of pigments (p.gl -1) in the NE Arabian Sea during January Pras 13.81% Column Chl a 11.76% 0.33% Fig b.3. Average column composition of pigments (mgm -2) in the NE Arabian Sea during January

21 and from to respectively. The average zeaxanthin was ± (n=9) and 13-carotene was ± ; ri= 9 (Table 4.1b). CM a was the most dominant (70.56 %) followed by zeaxanthin (11.77 %) and 13 -carotene (8 %) (Fig b). The pattern of pigment distribution in the NE Arabian Sea was much different than the coastal water (St. B). The December data from the NE Arabian Sea is presented in the Table 4.1c and Fig a. Sub-surface values of pigments are higher than surface values. In the surface water CM a varied form to tigl -1. The CM a at St. 16 was high and at Sts.la and 9 was low (Table 4.1c). The average CM a during December was found to be ± tigl -1 (n=8). At the surface, peridinin content was %, followed by CM a (20.54 %), zeaxanthin (8.31 %), 13-carotene (7.65 %) and fucoxanthin (6.94 %) of total production (Fig a). In the water column peridinin was most dominant pigment during December. In the surface water peridinin varied from to ggl -1, whereas in the columnar water it varied from mgm -2. The integrated values also showed same pattern as that of surface (Table 4.1c.1). Although, prasinoxanthin has been detected in water column (21.62 %), it was completely absent in the surface water (Fig a). Pigment distribution during January has been depicted in Fig b.1. The average CM a was around 30 % of total pigment at surface and column (Fig b.2 and 4.1.3b.3). The CM a at surface varied from to tgL -1. The high CM a was found at St. 12b and low at St. 4b (Fig b.1). Peridinin was second most dominant pigment followed by CM a (0.44 to tgL -1), fucoxanthin (0.046 to ggl-1), CM b, CM c, prasinoxanthin, zeaxanthin and 13-carotene (Fig b.1). 91

22 In the water column similar pattern was seen as that observed for surface. In the integrated water column the average CM a was dominant pigment averaging 30.8 % in total pigment. This was followed by peridinin (21.40 %), prasinoxanthin (13.81 %), fucoxanthin (11.76 %), CM b (8.81 %) and R - carotene (7.02 %) in the total pigment (Fig b.3). The high zeaxanthin was found at St. 12b, due to the presence of Trichodesmium bloom (Chapter 3). The distribution patterns of pigment (January) at the surface and in the column are similar. The CM a (- 30 %) was followed by Peridinin (- 25 %) and fucoxanthin (- 12 %) of total pigment (Fig b.3). The pigments variations during February are presented in Table 4.1e and Fig.4.1.3c. The CM a varied from to gge l. The high CM a was found at St. 2 and low at St. 8 in the NE Arabian Sea. The average CM a was µe.,-1 (n=3) which was reasonably high. The CM c2 which originates from prymensiophytes varied from to and CM b from to E 1. The average CM c2 was ± : 1 (n=3) and CM b was ± 0.03 µg1: 1 (n=3) however, the average zeaxanthin and [3-carotene content was ± ligl-1 (n=3) and ± E 1 (n=3) respectively. The CM a was - 35 % followed by CM c2 and b (-20 %), prasinoxanthin (5 %), zeaxanthin and P-carotene (4.60 and 7.32 %) of total pigments respectively (Fig c). The pigment pattern during March in NE Arabian Sea was similar to that of February. These two months are part of winter monsoon season in NE Arabian Sea and diversity and composition of phytoplankton was of similar nature (Table 4.1f). During March, CM c2 and CM b were extremely high in proportion to total pigment 92

23 Table 4.1e. Pigment distribution (igl -1 ) during February in the NE Arabian Sea Stations Chl a Chl b Chl c2 Fuco Per Zea II-car Pras Min Max Average SE Table 4.1f. Pigment distribution (igl -1) in the NE Arabian Sea during March Stations Chl a Chl b Chl c2 Chl c3 Fuco Per Zea 13 -caro Pras Min Max Average SE

24 Zea Per 3.33% Fuco 2.82% p-caro 7.32% Chl a 34,89% Chl c3 0.00% Chl c % Fig c. Average surface composition of pigments (lgu l) in the NE Arabian Sea during February (n=3) Per 5.59% Zea 2.92% Chl a Fuco 3.45% 6-caro 2.87% Chl c3 2.71% Chl b 22.98Y Chl c % Fig d. Average surface composition of pigments (iagl -1) in the NE Arabian Sea during March (n=3)

25 composition (Fig d). The observations made indicate the average values of the Chl a (1.555 f ggl -1), Chl c2 (1.837 ± }.1,0: 1), Chl b (1.783 ± ggl" 1) at surface respectively. The other pigments comprised of fucoxanthin (0.177 ± gge 1), peridinin (0.314 f }rgl -1) zeaxanthin (0.167 f }rgl -1) and f -carotene (0.117 ± ggl -1) contributed marginally to the surface productivity during March (Table A case study was designed and executed in the Lakshadweep Sea in April (Fig e.1) coinciding with the occurrence of Trichodesmium bloom. It was found that zeaxanthin (13.23 %) and 0-carotene (11.92 %) were the most dominant pigments other than CM a (66.64 % ; Fig e.2). The Chl a varied from 0.47 to }re whereas, zeaxanthin ranged from 0.11 to 3.50 }rge l and 0-carotene from 0.07 to 3.07 }.1,gL -1 (Fig e.1). The high Chl a, zeaxanthin and 0-carotene at St. 2 was due to intense bloom of Trichodesmium (Fig e.1). During this study, blooms of the both the species of Trichodesmium (T. erythraeum and T. thiebautii) were observed besides samples taken from other associated forms of the phytoplankton. Further, the primary population due to Trichodesmium and primary production fixed was also estimated in the Lakshadweep waters. Thus, large variations were observed in Trichodesmium concentrations from February to May at various stations collected during SASU-45 and Lakshadweep Sea in the April. This spatial and temporal variability (biomass) was further studied for carbon production and nitrogen fixation by Trichodesmium. 93

26 St. 1 St. 2 St. 3 St , Chl b Fuco Zea Pras P caz 0 Pigments Fig e.1. Distribution of pigments in the Lakshadweep Sea during April Fuco 0.33% Chl b 1.55% Fig e.2. Average composition of pigments (.1,g1: 1) in the Lakshadweep Sea during April

27 Carbon production in the coastal waters of Goa Coastal water of Goa (St. B, stations covered during SASU 16 and SASU 45) have shown the development of Trichodesmium bloom during February to May period (Table 4.2a.1). The rate of carbon production by Trichodesmium erythraeum and Trichodesmium thiebautii studied during February to May period and is presented in Table 4.2.a. The high rates of carbon production by Trichodesmium were found during March (2582 mgcred -1) and May (21150 mgcm-3d-1) compared to February (0.86 to mgcm3d-1) and April (740 mgcm -3d1). In all these months Trichodesmium erythraeum was dominant species contributing to primary productivity. Thus, the present study reveals that in the coastal waters of the Goa, Trichodesmium erythraeum was more important species compared to Trichodesmium thiebautii Carbon production in the NE Arabian Sea November Primary productivity and Chl a data of various stations studied during November is presented in Fig. 4.2a. The surface maxima of Chl a at Sts. 12 and 15 was due to Trichodesmium bloom. Similarly, at same stations the primary productivity was also high at surface and was attributed to Trichodesmium bloom. Although, at St. 13 Chl a was high at 40 m due to Trichodesmium bloom at bottom of euphotic zone, productivity was negligible due to non availability of light (Fig. 4.2a). At other stations, it was observed that the sub-surface Chl a maxima due to other phytoplankton rarely coincided with primary productivity maxima. The result 94

28 Table 4.2a. Variability in carbon (mgcm -3d-1) production during Trichodesmium bloom at St. B Months Depth (m) 7'. erythraeum 7'. thiebautii Jan Feb Mar Apr May June July NC Aug Sep Oct Nov Dec (NC- not collected ; - means not detected)

29 Chlorophyll ),,g L -1 ) Chlorophyll (mil L -1 ) Chlorophyll a 61 6' 1 ) t eo - 80 a " i 40 a Primary productivity imgcm3d.1 I Primary productivity (mpcm 3d -1 ) Primary productivity (m9cm -3 c1-1 ) 80 Fig. 4.2a. Vertical distribution of CM a and Primary productivity in the NE Arabian Sea during November

30 Tri. sp. St % St. 5 Other 92.37% St. 9 Tri. sr 41.63c. Other 58.37% III 6.51% Tri. sp. St. 20 Other 93.49% Other 97.45% Fig. 4.2a.1. Surface carbon production (%) by Trichodesmium thiebautii in the NE Arabian Sea during November

31 obtained during November for primary production by Trichodesmium is presented in Table 4.2d. The carbon produced by Trichodesmium varied from 0 to mgcm - 3d-1. The rate of carbon production was high at St, 21. The average carbon produced during cruise was ± mgcni 3d-1. During this period only 12.8 % of carbon was produced by Trichodesmium and 87.2 % was produced by other phytoplankton (Fig.4.2a.1). The high (41.63 %) carbon was produced at St. 9 and low (2.55%) at St. 12 of the total phytoplankton. The highest value ( mgcni 3d-1) was recorded at St 21 at the surface (Table 4.2d). December Chl a and primary productivity in the water column is presented in Fig. 4.2b. The high Chl a at surface (Sts. 4, 5, 9, 10, 12 and 14) was mainly due to Trichodesmium bloom. The primary productivity maxima at 40 m (St. 5) was attributed due to Trichodesmium bloom. Similarly, at St. 14, high productivity at 40 m and below was also due to Trichodesmium (Fig. 4.2b). The primary production by Trichodesmium during December (NE monsoon) is presented in the Table 4.2e. The average carbon produced by Trichodesmium spp. was ± mgcni 3e. At the surface, the carbon production varied from 0 to 1.08 mgcni 3e. The high rate of carbon production was observed at St. 12, at which Trichodesmium bloom was observed in high densities. The carbon production by Trichodesmium thiebautii and Trichodesmium erythraeum was of the same order i.e and mgcni 3d-1 T erythraeum primary production was high respectively. However, in the column ( mgcm-2d-1) compared to the primary production by T. thiebautii (

32 ea I Primary productivity (mocm 4d4 ) Primary productivity (mpcp-3,1) Primary productivity imp0m 4d 1 1 Fig. 4.2b. Vertical distribution of Chl a and primary productivity during December

33 Tri. spp. 0.10% St. 1 St % Tri % Tri. spp. 0.08% St % Fig. 4.2b.1. Surface carbon production (%) by Trichodesmium spp. in the NE Arabian Sea during December

34 Table 4.2b. Carbon (mgcni3d-1) production during Trichodesmium bloom in the coastal waters of Goa during March St. No. Depth (m) T. erythraeum T. thiebautii S S S S S S S Table 4.2c. Carbon (mgcm -3d-1) production during Trichodesmium bloom in the coastal waters of Goa during May St. No. Depth (m) T. erythraeum T. thiebautii

35 Table 4.2d. Carbon (mgcm -3d1) production during Trichodesmium bloom in the NE Arabian Sea during November St. no Depth (m) T. eryffursesins Z thiebassai St. no Depth (m)11 mammon T. tkiebasstil

36 Table 4.2e. Carbon (mgcrn -3d-1) production during Trichodesmium bloom in the NE Arabian Sea during December St. No Depth (in) T. etyllorsouts T. Ildebtoutti St. No Depth (m) T. oryllsmeauts T. Atli la _

37 mgcm-2d-1). The percentage of Trichodesmium productivity in the total productivity was around 0.11 % (Fig. 4.2b.1). January During January, Chl a and primary productivity was low at the surface and high at deeper depths. At St. 2a, highest primary productivity and Chl a was at 50 m depth due to Trichodesmium bloom (Fig. 4.2c). The study of carbon fixed during January by Trichodesmium is presented in Table 4.2f. The high carbon production was observed at St. 17b at 60 m coinciding with the bloom of Trichodesmium thiebautii. In most of the stations carbon was produced by Trichodesmium in the water column however, at St. 12b it was observed that, Trichodesmium was active in carbon production only at the surface (Table 4.2f). February During February, Trichodesmium bloom was found to be associated with surface water (Fig. 4.2d). Overall primary productivity and diversity was low due to Noctiluca bloom in this month. The amount of primary production due to Trichodesmium during this month varied from 1.61 to 8.25 mgcm -3d-1 (Table 4.2g). The high carbon was produced at St. 4. The average carbon production was 7 mgcm" 3d-1. The observations made in the present study indicate that Trichodesmium contributed to a major portion (60 %) of carbon produced at this time. Further, it was found that there was no occurrence of diatoms and dinoflagellates, thus implying that 96

38 Table 4.2f. Carbon (mgcm-3d-1) production during Trichodesmium bloom in the NE Arabian Sea during January St. No. Depth (m) T. evtluvressan T. thiebasstli 2B A 12B 15B 17B

39 , Productivity pne33-3, 1,,, uc30vky (533C014,01 P Fig.4.2c. Vertical distribution of Chl a and primary productivity in the NE Arabian Sea during January

40 Table 4.2g. Carbon (mgcm -3d-1) production during Trichodesmium bloom in the NE Arabian Sea during February St. No. Depth (in) T. evytkreseum T. elsiebasstii

41 Chlotophyll / o p I_ 11 ) Chlorophyll a (691_ -, ) C3 02 CA Primary productivity (m9crsr 7d 1 ) Primary productivity (mi.cm4d 1 ) Primary productivity (m9cm 4 c1., ) Fig. 4.2d. Vertical distribution of Chl a and Primary productivity in the NE Arabian Sea during February

42 about 40 % carbon produced was mainly by other algal population (Noctiluca miliaris, silicoflagellates etc.) The high carbon produced (65.85 %) by Trichodesmium was at St. 7 and low at St. 6 (0.14 %; Fig.4.2d.1). March The Chl a and primary productivity of total phytoplankton during March is presented in Fig. 4.2e. In this month, at St. 11 and 12 the Trichodesmium was found to be aggregated at the surface. The primary production due to Trichodesmium during this month varied from 0.69 to 0.88 mgcni 3d-1 (Table 4.2h). The average was 0.79 ± mgcni3d-1 (n=3). About 29 % carbon was produced by Trichodesmium of total carbon production (Fig. 4.2e.1). April The variations in the Chl a and primary productivity of total phytoplankton at various stations is presented in Fig. 4.2f. At St. 18 and 35, the high Chl a and primary productivity was due to Trichodesmium bloom throughout the water column. The primary production by Trichodesmium during this month is presented in Table 4.2.i. Trichodesmium erythraeum was the only species found during this month. So the amount of carbon produced was only by of T. erythraeum. The average carbon production was ± mgcm -3d-1 (n=29). At the surface it varied from 0 to mgcn 3d-1. The high carbon production throughout the water column was recoded at Sts. 12, 18, 34 and 35. During this season Trichodesmium played an 97

43 Table 4.2h. Carbon (mgcm -3d-1) production during Trichodesmium bloom in the NE Arabian Sea during March St. No. Depth (m) T. nythraeum T. thiebantii

44 Chlorophyll a 130 L.1 ) Chlorophyll a ( 91_ I 120 ) Primary predwahypy (mpcarld.1 ) Primary produotiviry (rnpem 4e1 ) Fig. 4.2e. Vertical distribution of Chl a and Primary productivity in the NE Arabian Sea during March

45 Other St. 6 St % Tri. spp % Other 11, 34.15% Tri spp % Other 84.56% St. 7 Tri spp % Fig. 4.2d.1. Surface carbon production (%) by Trichodesmium spp. in the NE Arabian Sea during February Tri sp. St. 11 St. 12 Other 55.26% Tri sp % Other 86.61% Fig. 4.2e.1. Surface carbon production (%) by Trichodesmium spp. in the NE Arabian Sea during March

46 Table 4.2i. Carbon (mgcni 3d-1) production during Trichodesmium bloom in the NE Arabian Sea during April St. No Depth (n) T. nyvireasun T. Osiebassili St.No. Depth hn) T. dyikreasum T. fisiebassili

47 Conti---- Table 4.2i St. No Depth (m) T. nythreasim T. thiebastlii St-No. Depth (m) T. egyilsreamm T. ddebtustii

48 E ,.' ' Primary productivity (mgc ) Chlorophyll (,.g L -1 ) Chlorophyll alp ) GI C6 G P10. 0stt'4( l 'ty , / 600 P4, 81'0410 pfotr ucth;1180,01, Fig.4.2f. Vertical distribution of Chl a and primary productivity in the NE Arabian Sea during April

49 Tri sp. 13% St. 5 Tri sp. St % Others St. 13 Tri sp. 8.06% St.15 Tri sp St. 19 Fig. 4.2f.1. Surface carbon production (%) by Trichodesmium erythraeum in the NE Arabian Sea during April

50 Conti----Fig. 4.2f.1 Other St 20 27% St. 21 Other 43% Tri sp. 57% Tri sp. 73% Tri sp. 3.25% St. 25 Tri sp. St % Ok Tri sp. 7% St. 31 Other 93%

51 important role contributing 30 % of total production by other phytoplankton (Fig. 4.2f.1). In another study conducted in the Lakshadweep Sea during April (Table 4.2j) showed that the average higher carbon production by the T. erythraeum ( ± mgcm-3d-1 ; n=4) than Trichodesmium thiebautii (0.988 ± mgcm -3d-1 ; n=4). During this time the carbon production by Trichodesmium spp. ranged from 2.4 to 66.5 mgcm-3d Nitrogen fixation in the coastal waters of Goa Trichodesmium being a diazotrophic in nature can fix nitrogen in marine environment. The study was conducted on the amount of N2 fixed by Trichodesmium in the coastal water of Goa and in the NE Arabian Sea. The data is presented in Table 4.3a. At St. B (coastal) the N2-fixation rate varied from 2.4 to InnolNin -3d-1. The high nitrogen fixation was found during month of May (Table 4.3a; Fig. 4.4a) and low in February. The average N2-fixation rates for coastal water was ± InnolNin-3d-1 (n=4). In the coastal waters of Goa, the amount of nitrogen fixed by Trichodesmium erythraeum was pnolnm-3d-1 which was found to be on higher side as compared to InnolNin -3d-1 fixed by Trichodesmium thiebautii (Table 4.3) Nitrogen fixation in the NE Arabian Sea The data on N2-fixation during November in NE Arabian Sea is presented in 98

52 Table 4.3a. Rate of nitrogen fixation (nnolntn -3d-1) by Trichodesmium spp. at St. B Months Depth (in) T. etvilsraeunt T. fitiebasstli Jan 0 Feb March April May June Aug Sep Oct Nov Dec

53 N, fixation OlmoiNm -3d -' ) 25 0 N2lidcatiogo(moIN in-3) i 1 i Mar [1 Apr I s F. a. G T. erythraeum III-- T. thiebautii Fig. 4.4a. Variability in nitrogen fixation rates (.1.molNm -3d-1) at St. B

54 Table 4.3b. Rate of nitrogen fixation (punolnm -3d1) by Trichodesmium spp. in the coastal water during March St. No. Depth (m) T. etythmeam T. fidebalitti S S S S S S S Table 4.3c. Rate of nitrogen fixation (i.nnolnni 3d-1 ) by Trichodesmium spp. in the coastal water during May St. No. Depth (m) T. eotbraeum T. lidebantli

55 Table 4.3d and Fig. 4.4b. The average N2 fixed was ± 114 i.unol Nn-f3d-1. It varied from 0.6 to imnolnin -3d-1. The high nitrogen was fixed at St. 21 (Table 4.3d) due to Trichodesmium bloom. A comparative evaluation of amount of N2- fixation rate revealed that in the November month the rate was high as compared to December in the NE Arabian Sea. The N2- fixation by Trichodesmium in December varied from 0 to 3 imnolnm -3d-1 (Table 4.3e). The high and low N2 fixed at St. 10 and 12 respectively was largely due to intensity of the bloom. The average N2 fixed was 1.1 Imo' Nm-3d-1 (Table 4.3e and Fig. 4.4c). The present study revealed that, N2- fixation rate varied both among species. The amount of nitrogen fixed by T erythraeum varied from 0.5 to 2.9 limo' Nm -3d-1. In the column, the nitrogen fixed by T erythraeum was high (27. 6 ± 'Arno' Nm 2d1) than T. thiebautii ( 16.9 ± Imo' Nm-2d-1). The nitrogen fixed by Trichodesmium during January is presented in the Table 4.3f and Fig. 4.4d. At the surface, nitrogen fixation was nil except at St. 12B (0.8 i.unolnm3d-1). The species level variations was marked wherein, it indicated that the nitrogen fixed by T Thiebautii was high (average ± gmolnin -3d-1) than T. erythraeum (6.785 ± 'Arno' Nni 3d-1 ). The amount of nitrogen fixed in the February is presented in the Table 4.3g and Fig. 4.4e. The total nitrogen fixed by the Trichodesmium varied from 0 to 84.5 gmolnin-3d1 (Table 4.3g) with an average of 19.2 ± imnolnm -3d-1 (n=7). The high nitrogen was fixed at St. 4 and low at St. 1. The amount of N2 fixed by T erythraeum varied from 1.5 to 45.3 i.unolnm -3d-1 which was much higher as compared to T thiebautii (0 to 39.2 i.unolnm-3d-1 ). 99

56 Table 4.3d. Rate of nitrogen fixation (iimolnin -3d-1) by Trichodesmium thiebautii in the NE Arabian Sea during November St. no Depth (m) T. eryllwassias T. klebasdit St. no Depth (m) T. eolltranue T. thiebasstil

57 Slaw i Fig. 4.4b. Variability in nitrogen fixation rates (iimolnm -3d-1) in the NE Arabian Sea during November

58 Table 43e. Rate of nitrogen fixation (p.molnm-3d-1) by Trichodesmium spp. in the NE Arabian Sea during December St. No Depth (m) T. erytkraetan T. thieltautii St. No Depth (m) T. etytkrtseu t T. thieboatii la

59 1 2A Stations Fig. 4.4c. Variability in nitrogen fixation rates (umolnm -3d-1) in the NE Arabian Sea during December

60 Table 4.3f. Rate of nitrogen fixation (p.rnolnm -3d-1) by Trichodesmium spp. in the NE Arabian Sea during January St. No. Depth (m) L merman,' T. Odebasdfi 2B A B B

61 Stations Fig. 4.4d. Variability in nitrogen fixation rates (pnolnni 3d-1) in the NE Arabian during January

62 As compared to February, the amount of N2 fixed by Trichodesmium was low during March in the NE Arabian Sea. The nitrogen fixed by Trichodesmium varied from 1.9 to 2.4 limolnm 3 d1 (Table 4.3h; Fig. 4.4f). The average amount of nitrogen fixed was 2.2 InnolNm -3d-1 (n=3). The average nitrogen fixed by T. thiebautii was high (2.1± 0.304) as compared to T. erythraeum. At Sts 12 and 13, the high nitrogen was fixed by T thiebautii whereas, at St. 11 the major contribution to N2-fixation was T erythraeum (Table 4.3h). During April, T erythraeum was the dominant species of phytoplankton in the NE Arabian Sea. The detail study related to nitrogen fixation by Trichodesmium in the April is presented in Table 4.3i and Fig. 4.4g. The average N 2-fixation was ± gmolnin3d1 (n=29). At the surface it varied from 0 to p,molnm -3d-1 (Table 4.3i ). The peak value was reported at St. 21 throughout the water column. If integrated to column the N2-fixation varied from to pmol Nm -3d-1. In the Lakshadweep Sea, during April the N2-fixation varied from 23.6 to pmolnm-3d-1 (Table 4.3j and Fig. 4.4h). The variation in the rate was found to be according to the species. The average N2 fixed by the T. erythraeum was high (76 ± 34.9 gmolnm3d-1) as compared to T. thiebautii (2.8 ± 0.85 grnolnrn-3d-1 ) Detection by OCM images OCM based Chl a is used as a tool for detecting the high patches of the Trichodesmium blooms in the NE Arabian Sea. Total seven images are processed during November ( Fig ). Chl a retrieved from OCM ranged from to i.tgl-1. During December, four images processed indicated much lower Chl a 100

63 Table 4.3g. Rate of nitrogen fixation (nnolnm -3d-1) by Trichodesmium spp. in the NE Arabian Sea during February St. No. Depth (m) T. esyttansensn T. thieboutii

64 Table 43h. Rate of nitrogen fixation (p.molnni 3d-1) by Trichodesmium spp. in the NE Arabian Sea during March St. No. Depth (m) T. etytitmeum 7'. tldebantii

65 T. erythmeum T. thiebautii Stations 9 10 Fig. 4.4e. Variability in nitrogen fixation rates in the NE Arabian Sea by two different species during February 2 T. erythmeum 7'. thiehautii Stations Fig. 4.4f. Variability in nitrogen fixation rates in the NE Arabian Sea by two different species during March

66 Table 4.3i. Rate of nitrogen fixation (µmolnni3d-1) by Trichodesmium erythraeum in the NE Arabian Sea during April St. No. Depth (m) T. eryouraum T. Odebasitii StNo. Depth (m) T. eolkwannt T. Ildebandi

67 Cont----Table 43 i St. No. Depth (m) T. erstlartaini T. tkiebtartii St.No. Depth (m) T. erythreannt T. tkiebantii

68 ' 'e ) Stations Fig. 4.4g. Variability in nitrogen fixation rates (ptmolnni 3d4) in the NE Arabian during April

69 Table 4.3j. Rate of nitrogen fixation (µmolnni3d-1) by Trichodesmium spp. in the Lakshadweep Sea during April St. No. T. eryiltraeum T. ildebatitii

70 ; M III I Stations Fig. 4.4h. Nitrogen fixation rates (pinolnm -3d-1) in Lakshadweep Sea during April

71 IF I November 4; , 22,1 14q 12 16,1 64, OWE E 72 E ) Eon swo 1i 66 E 46 E 70 E 72 E November If ,4 22,1 20,1 24,4 22, ChbrophyU -71 '1E E err 411 E Eon 725E I' m.. Ami USE 66 E EWE 70 1 (IMMO) 72 E 111 N :11.1 Ira E 70`t 721E 000.0A Fig Chl a from OCM in the NE Arabian Sea during November

72 04 E OO.E GEE 70 E 7 E WE WE Jr. 7 E Conti-- Fig NY N 22, N rN E 7 E rE 44% OWE 1pE 72, Ona m41 4.0

73 Fig Chl a from OCM in the NE Arabian Sea during December

74 , ' ,0", 22 OVN , E IRS P4 -OCM Ch orophyll Image IRS P4 -OCM Chlorophyll Image rui /0.001 /2. 00, / "E 'OWE 74 'ME 20.00N ,^N 24' ,^N ', irup in ^N N j6n 2003 TIMM I ii,11111 '1 NI ha-ft E Grove 70.00E 72. TOT ,N IL wi Ill TtSt [ T '00-0 Op/a , N 22 VON 20170N IRS P4 -OCM Chlorophyll Image E 26.0'0, " ON , 'N 20" , E "E 70'0.0.2 al Fig Chl a from OCM in the NE Arabian Sea during January

75 En IRS Yo4 C1PCWI teltlam Image 64E 66E 6BE 70E 72E 64E ME ESE 70E 72E 64E 66E 86E 70E 72E 24N 27N 20N 18N if 711 IMO _. A IFT,7,7,.,_,...,. I. ',...ept! imilimalicir II RAF 6BE 70E 72E 64E 66E &BE 70E 72E N 20N 18N me nt -3 Fig Chl a from OCM in the NE Arabian Sea during February

76 IRS 1=114 01C1VI Chlorophyll Image 84E 88E E 72E 84r oar eor 7or 72r 64E BBE 68E 70E 7/E 24N , Ill 24N 22N 20N 1111Mor. If 20N 19N 74N 24N N N 18N 849 aek E E 72E = I3N 84 40'06E 00 40X PE E E INS :11 ' " ICI 24u 'N gal % % "N % 18"0'0" ' "N "N 2060VN "N 11*-1a I', N E E E E 010/ YN N Fig Chl a from OCM in the NE Arabian Sea during March

77 00' WOW S7'001 Sr WOW OWN 05`0,201 04'0, ,7N '0011 IS. Oterg ,71 75, P : ,5' IF ,12014 WOW 111 4!Titill ,0014,11;NICIIIIII lir l7,s;;%.- -A 77. Prl 2 ''''' ib, ,...,:,',-.,,., liiii:: ::. 2;1 WOW MI 17-00, i IktilirfelillEll _ N WOW III WOW WAYS UMW aa11et ItrucrE 2/ ' , ` `11 111, Fig Chl a from OCM in the NE Arabian Sea during April

78 01, Conti Fig WOVE ,01 WOVE 807 3P E 3/ OPE EN,04 EV 'VP ":

79 6200E 144' WOVE E 74.00E IIMILIMs11 *, Mir ' =IMO. l r 4..0:i 41'1E1E1E1 IMP ' It.V t' 44 ( If um V'Se,1101 It?' -.:..,,a2r k. I SlYKrE W.< 8111t 70.00e '0V. 78' E `004 WOVE E Emmy -;,--4,61umsami.is I.144 I I V*: IL.:1 1!o WEE, '-' Irxf / I 1/: m NAMED 1f= -111M,t4h F t I / MOM WOVE 84'004 IIVO.!WIVE rpoon I nt-3 Fig Chl a from OCM in the Lakshadweep Sea during April

80 Varladont in New production Ibr year 2000,2001 and 2002 disingbiooro Fig New Production in the NE Arabian Sea from OCM during February, and March,