Mesozooplankton Abundance and Community Structure in Vembanad-Kol Wetland Ecosystem, Kerala, India
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Indian Journal of Geo-Marine Sciences Vol. 45(4), April 2016, pp. 533-545 Mesozooplankton abundance and community structure in vembanad-kol wetland ecosystem, kerala, India Retina I Cleetus, Asha C V, Suson P S & S Bijoy Nandan* Department of Marine Biology, Microbiology & Biochemistry, School of Marine Sciences, Cochin University of Science and Technology, Fine Arts Avenue, Kochi-682016, India *[ E,Mail: [email protected] ] Received 01 June 2015; revised 16 July 2015 Abundance and community structure of mesozooplankton were studied (March 2012-February 2013) in the context of prevailing environmental parameters in the Vembanad estuarine system, a Ramsar site on the southwest coast of India. Of the sixteen taxa of mesozooplankton, 16 calanoid copepods and 26 rotifers were identified upto species level. Copepods was the most dominant taxon where calanoids (63%), followed by rotifers (23%), copepodites (2 %), amphipodes (2%), crusteacean naupli (1.5 %) and others contributing in minor proportions. Freshwater diaptomid species, including Heliodiaptomus cintus and Allodiaptomus mirabilipes contributed around 52 % of calanoids in the southern stations (1, 2 and 3) and Bestiolinas imilis (19%) and Acartia southwelli (12.3%) formed the major share of calanoid copepods in northern stations (8, 9 & 10). Seasonal distribution of zooplankton showed bimodality, with a primary peak in premonsoon (av. 25073 ind. m-3) followed by secondary peak in post monsoon (av. 21093ind. m-3) coinciding with maximum of copepod species. Multi-Dimensional Scaling analysis reveal 80% similarity in abundance of plankton between southern (1 to 7) and northern stations (8, 9 and 10). Phosphate, nitrite, temperature, dissolved oxygen, and salinity best determined the abundance and distribution of mesozooplankton in Vembanad estuary based on multivariate BIOENV analysis. [Key words: Meszooplankton, abundance, distribution, Vembanad estuary, copepods, rotifers] Introduction structure in Vembanad estuary extending from Mesozooplankton contribute significantly to south (Alappuzha) to (Cochin) north is very biological production and are important scarce and are limited to the prebarrage phase of contributors and modifiers of vertical flux of Thanneermukkom barrage (TMB). Construction organic matter to depth and play a crucial role in of TMB in 1976, to prevent the saline intrusion modeling the carbon dioxide dynamics of the during summer months, so as to make possible system1. The relationship between the paddy cultivation in the agrarian zone of composition and abundance of zooplankton and Kuttanad, interfered the natural cleansing the trophic state of lakes has been studied in mechanism of the estuary along with accelerated both temperate 2,3,4&5 and tropical ecosystems6&7. loss of habitats and biodiversity15. Even though In most subtropical and tropical part of the some information on the zooplankton ecology world’s oceans copepods dominate the and abundance pattern are reported from the mesozooplankton communities8, where they south of the estuary during the post barrage comprise about 75% of total abundances9&10 phase, a comprehensive study on with strong seasonality in accordance with the mesozooplankton involving both southern and changes in salinity11&12. Subtropical and tropical northern parts of the estuarine system is severely regions are also known to be colonized by lacking. Hence the present study has been diverse groups of species13. As zooplankton is at undertaken in the Vembanad estuarine system to the base of the aquatic ecosystem, understanding understand the spatio-temporal distribution, their distribution and abundance in space and abundance and community structure of time with prevailing hydrographic parameters is mesozooplankton (MZ; 20-200 µm) with special inevitable for understanding and modelling reference to calanoid copepods and rotifers of community dynamics, and is essential to the estuary. understand physical and biological processes Investigations on distribution and diversity of that structure marine ecosystems14. Recent plankton in Vembanad backwater were made on information relating to mesozoopankton south of the barrage16,17&18. Zooplankton abundance, distribution and community composition and abundance in Cochin estuary 534 INDIAN J. MAR. SCI., VOL. 45, NO.4 APRIL 2016 and associated environs has been investigated by estuary is enriched with the rivers Manimala, several authors19,20,21,22&23. The distribution of Meenachil, Pamba, Achenkovil on the south of zooplankton in Indian estuaries has been TMB and Muvattupuzha river on the north. extensively studied by several researchers Stations south of TMB (St. 1, 2 and 3) are 24,25,26,27,28&29. Bhattacharya studied the influenced by freshwater inflow from the rivers community Achencovil, Pamba and Manimala through structure of mesozooplankton in coastal waters Kuttanad (in the western side of Vembanad of Sundarban mangrove wetland30. Plankton estuary) and agricultural runoff from production in the Vembanad Lake was studied padashekarams (paddy fields) which together with respect to environmental factors31. plays a significant role in the nutrient dynamics Copepods32,33,34 &35 on the east coast of India was of the aquatic ecosystem. Intense and studied briefly by Sewell and Ummerkutty. unregulated backwater tourism (house boat) Ecological investigations on cyclopoid activities also play a great impact on the ecology copepods36 of Cochin estuary and availability of the southern stations. Pathiramanal Island (St. and abundance of rotifers in certain brackish 5) which is a treasure trove of biodiversity, water regions of southern Kerala were provide habitat for resident and seasonal documented by several researches37,38&39 while migratory water fowl and other aquatic studying the general plankton communities in organisms and is also famous for its tourism these areas. potential. Stations 7 to 10 are influenced by the tides and currents from the Arabian Sea (Cochin Materials and Methods barmouth). Station 8) Varanadu is influenced by Study Area discharge from distilleries on the western side of The Vembanad estuarine system (09˚00' -10˚40' the water body and regular dredging for lime N and 76˚00' - 77˚30' E) spreads across three beds. Land filling and illegal constructional districts of Kerala and has a surficial area of ~ activities is a marked feature of Perumbalam (St. 36,500 ha (13224 ha on south and 23500 ha on 9). The map showing the study stations is given north). It extends 96 Km in a NW - SE direction in Figure 1. from Munambam in the north to Alappuzha in the south, with an average depth of <1m to 9m. The southwest monsoon and northeast monsoon forms the two rainy seasons in Kerala of which summer monsoon (June-September) contributes more than 60% of the total rainfall. The tides in the estuary are of a mixed, semidiurnal type with a maximum range of about 1 m40. Construction of saltwater barrage (1400 m) in 1976, has grossly affected the hydrobiology of the estuary to a great extent41, separating the estuarine system into a freshwater dominant southern zone and a salt water dominant northern zone. Ten different stations of ecological importance were selected, with six stations on southern side and four on northern side of TMB (9° 30' - 9° 53' N and 76° 21' - 76° 18' E) extending from Punnamada in south to Aroor in north. The location of each study sites was statistically tested and the geographic positions were fixed using GPS (Global Fig. 1 Map of Vembanad estuarine system indicating the Positioning System). study stations The study stations where, St. 1 Punnamada, St. 2 Pallathuruthy, St. 3 Rani, St. 4 Aryad, St. 5 Sampling strategy and analyses Pathiramanal, St. 6 Thaneermukkom South, St. Water and biotic samples were collected on 7 Thaneermukkom North, St. 8 Varanadu, St. 9 monthly cruises onboard the vessel, King Fisher Perumbalam and St. 10, Aroor. The Vembanad from March 2012 to February 2013. Based on prevailing meteorological conditions, three CLEETUS et al., MESOZOOPLANKTON VEMBANAD KOL-WETLAND ECOSYSTEM 535 seasons were distinguished, the premonsoon study area. Diversity indices (Table. 3) such as (February – May), monsoon (June – September) Shannon-Wiener diversity (H'), Margalefs and post monsoon (October –January)40. Richness index (d) and Pielou’s eveness index Surface water samples were collected using a (J') were computed using PRIMER Vs. 6.0 59. standard Niskin sampler (General Oceanics, 5L capacity). pH and temperature was measured Results and Discussion onboard by Systronics water analyzer (model Physico - chemical characteristics No.371). Dissolved oxygen (DO) was fixed A distinct spatial and temporal heterogeneity in onboard and was measured by modified different water quality parameters was observed Winkler’s method42 and salinity by Mohr- in the Vembanad estuary during the study. The Knudsen titrimetric method43. Water samples depth of the estuary ranged between 1.4 to 8.5m were analyzed for inorganic nutrients: with maximum of 8.5 m in Station 10 (Aroor) phosphate-phosphorus, nitrite-nitrogen and and minimum of 1.4 m in Station 8 (Varanadu). silicate-silicon44. ANOVA of depth in the estuary showed Mesozooplankton samples was collected in the significant seasonal variation (p<0.05) and also forenoon hours using a plankton net (mesh size influenced the distribution of species in the 200 μm) with a mouth area of 0.28m2. The net estuary. Depth in tropical waters forms a was towed horizontally just below the surface (1 significant factor in shaping