Dissolved oxygen (DO), total biomass and oxygen demand of nematode communities in the organic shrimp farms located in Tam Giang commune, Nam Can District, Ca Mau province were investigated in three seasons (March-dry, July-transfer and November-wet season) of 2015. The results showed that most of DO values were within permissible limits. However, the frequency distributions of DO values are very compressed at the lower limit of their scale. Total dry biomass varied from 24.77 to 937.04 µgC/10cm2 while oxygen demand ranged from 3467.39 to 64288.50 nlO2/day/10cm2. These values were slightly high when compared to other studies in the world. The following results recorded that the negatively correlation between DO and oxygen demand of nematode communities in the organic shrimp farms. This may well suggest that respiration and metabolic of nematode communities was high and their impact on oxygen dissolved in surface water. Keywords Biomass, dissolved oxygen, Ca Mau, nematode communities, organic shrimp farms, oxgen demand References [1]. P. N. Hong, H. T. San, Mangroves of Vietnam 7 (1993) IUCN.[2]. T. Nga, Hệ thống rừng-tôm trong phát triển bền vững vùng ven biển đồng bằng sông Cửu Long. Tạp chí Khoa học Trường Đại học Cần Thơ 10 (2008) 6.[3]. Thai agricutural standard (TAS), Organic marine shrimp farming, Royal Gazette 124 (2007) Section 78E.[4]. T. T. Thai, N. T. My Yen, N. Tho, N. X. Quang, Meiofauna in the mangrove–shrimp farms ponds, Ca Mau province. Journal of Science and Technology 55(2017) 271.[5]. L. Marte, The Food and Feeding Habit of Penaeus Monodon Fabricius Collected From Makato River, Aklan, Philippines (Decapoda Natantia) 1, Crustaceana 38(1980) 225.[6]. N. Majdi, W. Traunspurger, Free-living nematodes in the freshwater food web: a review, Journal of nematology 47 (2015) 28.[7]. M. C. Austen, Natural nematode communities are useful tools to address ecological and applied questions, Nematology Monographs and Perspectives 2 (2004) 1.[8]. F. Boufahja, H. Beyrem, N. Essid, J. Amorri, E. Mahmoudi, P. Aissa, Morphometry, energetics and diversity of free-living nematodes from coasts of Bizerte lagoon (Tunisia): an ecological meaning, Cahiers de biologie marine 48 (2007) 121.[9]. Ministry of Agriculture and Rural development, 2016. https://tongcucthuysan.gov.vn/en-us/aquaculture/doc-tin/006222/2016-10-28/ca-mau-set-outs-to-become-viet-nams-largest-shrimp-hub. Truy cập ngày 14/8/2017.[10]. M. Vincx, Meiofauna in marine and freshwater sediments, In G. S. Hall (Ed.), Methods for the examination of organismal diversity in soils and sediments Wallinfort, UK, 1996.[11]. A. T. De Grisse, Redescription ou modifications de quelques technique utilis [a] es dan l'etude des n [a] ematodes phytoparasitaires (1969).[12]. R.M. Warwick, H.M. Platt, P.J. Somerfield, Free living marine nematodes. Part III. Monhysterids. The Linnean Society of London and the Estuarine and Coastal Sciences Association, London 1988.[13]. Zullini, The Identification manual for freshwater nematode genera, Lecture book, MSc Nematology Ghent University 2005.[14]. N. V. Thanh, Giun tròn sống tự do Monhysterida, Araeolaimida, Chromadorida, Rhabditida, Enoplida, Mononchida, Dorylaimida. Động vật chí Việt Nam. Hà Nội: Nhà xuât ba̓n khoa học và kỹ thuật, 22, 2007 455. [15]. J. Vanaverbeke, T.N. Bezerra, U. Braeckman, A. De Groote, N. De Meester, T. Deprez, S. Derycke, K. Guilini, F. Hauquier, L. Lins, T. Maria, T. Moens, E. Pape, N. Smol, , M. Taheri, J. Van Campenhout, A. Vanreusel, X. Wu, M. Vincx, (2015)NeMys: World Database of Free-Living Marine Nematodes. Accessed at http://nemys.ugent.be on 2017.[16]. H. M. Platt, R. M. Warwick, Freeliving marine nematodes. Part 1: British enoplids. Pictorial key to world genera and notes for the identification of British species. Cambridge University Press, for the Linnean Society of London and the Estuarine and Brackish-water Sciences Association 1983.[17]. Andrassy I The determination of volume and weight of nematodes, Acta Zoologica 2 (1956) 1.[18]. J. Vanaverbeke, P. M. Arbizu, H. U. Dahms, H. K. Schminke,. The metazoan meiobenthos along a depth gradient in the Arctic Laptev Sea with special attention to nematode communities, Polar Biology 18 (1997) 391.[19]. K. Soetaert, J. Vanaverbeke, C. Heip, P. M. Herman, J. J. Middelburg, A. Sandee, G. Duineveld, Nematode distribution in ocean margin sediments of the Goban Spur (northeast Atlantic) in relation to sediment geochemistry, Deep Sea Research Part I: Oceanographic Research Papers, 44 (1997) 1671.[20]. D.J. Crisp Methods of the study of marine benthos (N.A. Holme & A.D. McIntyre eds), Blackwell Scientific Publications, Oxford, 1971 197. [21]. N. Smol, K. A. Willems, J. C. Govaere, A. J. J. Sandee, Composition, distribution and biomass of meiobenthos in the Oosterschelde estuary (SW Netherlands). In The Oosterschelde Estuary (The Netherlands): a Case-Study of a Changing Ecosystem, Springer Netherlands (1994) 197. [22]. H. Dye, An Ecophysiological Study of the Meiofauna of the Swartkops Estuary, African Zoology 13(1978) 1.[23]. Van Damme, R. Herman, Y. Sharma, M. Holvoet, P. Martens, Benthic studies of the Southern Bight of the North Sea and its adjacent continental estuaries, Progress Report II. Fluctuations of the meiobenthic communities in the Westerschelde estuary. ICES. CM/L, 23 (1980) 131.[24]. Q. X. Ngo, C. Nguyen Ngoc, A. Vanreusel, Nematode morphometry and biomass patterns in relation to community characteristics and environmental variables in the Mekong Delta, Vietnam, Raffles Bulletin of Zoology 62 (2014) 501.[25]. J. M. Whetstone, G. D. Treece, C. L. Browdy, A. D. Stokes, Opportunities and constraints in marine shrimp farming, South Regional Aquaculture Center 2002.

Environmental factors and meiofauna communities in the organic shrimp farms located in Tam Giang commune, Nam Can District, Ca Mau province were investigated in March (dry season), July (transfer season) and November (wet season) of 2015. The results recorded that the environmental factors were not quite optimal for shrimp farming such as the high percentage of TN and TOC and anaerobic condition in sediment. The results were also indicated that DO, TOC and TN showed significant correlation with characteristics of meiofauna communities. Following results were indicated that the meiofauna communities were expressed as the high abundance and slightly biodiversity that is a rich natural food sources for shrimp in the organic shrimp farms ponds. Further more, nematoda dominate numerically in the meiofauna communities. Keywords Biodiversity, Ca Mau, mangroves, meiofauna, organic shrimp farms References [1]. S. Trent, J. Williams, C. Thornton, M. Shanahan, Farming the sea, costing the earth: why we must green the blue revolution (2004).[2]. P. N. Hong, H. T. San, Mangroves of Vietnam 7 (1993) IUCN.[3]. B. T. Nga, Hệ thống rừng-tôm trong phát triển bền vững vùng ven biển đồng bằng sông Cửu Long, Tạp chí Khoa học Trường Đại học Cần Thơ 10 (2008) 6. [4]. Ministry of Agriculture and Rural development, 2016. https://tongcucthuysan.gov.vn/en-us/aquaculture/doc-tin/006222/2016-10-28/ca-mau-set-outs-to-become-viet-nams-largest-shrimp-hub. Truy cập ngày 14/8/2017.[5]. Thai agricutural standard (TAS), Organic marine shrimp farming, Royal Gazette 124 (2007)[6]. T. T. Thai, N. T. My Yen, N. Tho, N. X. Quang, Meiofauna in the mangrove–shrimp farms ponds, ca mau province. Journal of Science and Technology, 55 (2017) 271.[7]. N. Tho, V. N. Ut, R. Merckx, Physico‐chemical characteristics of the improved extensive shrimp farming system in the Mekong Delta of Vietnam, Aquaculture Research 42 (2011) 1600.[8]. R. P. Higgins, H. Thiel, Introduction to the Study of Meiofauna, Smithsonian Institute Press, Washington DC, 1988.[9]. B. C. Coull, Role of meiofauna in estuarine soft‐bottom habitats, Austral Ecology 24 (1999) 327.[10]. N. Majdi, W. Traunspurger, Free-living nematodes in the freshwater food web: a review, Journal of nematology, 47(2015) 28.[11]. C. L. Marte, The Food and Feeding Habit of Penaeus Monodon Fabricius Collected From Makato River, Aklan, Philippines (Decapoda Natantia) 1, Crustaceana 38 (1980) 225.[12]. M. Vincx, Meiofauna in marine and freshwater sediments, In G. S. Hall (Ed.), Methods for the examination of organismal diversity in soils and sediments Wallinfort, UK, 1996[13]. S. Porrello, P. Tomassetti, L. Manzueto, M. G. Finoia, E. Persia, I. Mercatali, P. Stipa, The influence of marine cages on the sediment chemistry in the Western Mediterranean Sea, Aquaculture, 249 (2005) 145.[14]. P. Munsiri, C. E. Boyd, D. Teichert-Coddington, B. F. Hajek, Texture and chemical composition of soils from shrimp ponds near Choluteca, Honduras, Aquaculture International 4 (1996) 157.[15]. C.E. Boyd, Best management practices for water and soil management in shrimp farming. Workshop (2003) in MazatlaŁn, Mexico.[16]. X. N. Quang, A. Vanreusel, N. V. Thanh, N. Smol, Biodiversity of meiofauna in the intertidal Khe Nhan mudflat, Can Gio mangrove forest, Vietnam with special emphasis on free living nematodes, Ocean Science Journal 42 (2007) 135.[17]. X. N. Quang , A. Vanreusel, N. Smol, N. N. Chau, Meiobenthos assemblages in the mekong estuarine system with special focus on free-living marine nematodes, Ocean Science Journal 45 (2010) 213.[18]. S. Vanhove, M. Vincx, D.V. Gansbeke, W. Gijselinck, D. Schram, The meiobenthos of five mangrove vegetation types in Gazi Bay, Kenya, Hydrobiologia 247 (1992) 99.[19]. B. Kondalarao, Distribution of meiobenthic harpacticoid copepods in Gautami-Godavari estuarine system, Indian Journal of Marine Sciences 13 (1984) 80.[20]. A.M.A. Sultan, K. Krishnamurthy, M.J.P. Jeyaseelan,. Energy flows through the benthic ecosystem of the mangroves with special reference to nematodes. Mahasagar Bull. Nat. Inst. Oceanogr., 16 (1983) 317.[21]. A.H. Dye, Vertical and horizontal distribution of meiofauna in mangrove sediments in Transkei, Southern Africa, Estuarine, Coastal and Shelf Science 16 (1983) 591.[22]. D.M. Alongi, Intertidal zonation and seasonality of meiobenthos in tropical mangrove estuaries, Marine Biology 95 (1987) 447.[23]. N. K. Panikkar, Possibilities of further expansion of fish and prawn cultural practices in India, Current Science 21 (1952) 29.[24]. V. C. Chong, A. Sasekumar, Food and feeding habits of the white prawn Penaeus merguiensis, Marine ecology progress series 5 (1981) 185.[25]. Z. A. Ansari, B. S. Ingole, A. H. Abidi, Organic enrichment and benthic fauna–Some ecological consideration, Indian Journal of Geo-Marine Sciences 43 (2014) 554.

Nematode communities were used as a tool to assess the environmental quality status of sediment of the water bodies in Ben Tre city. Eight locations in the main canals and river in the city were surveyed during the rainy season (September). The study recorded 51 genera belonging to 33 families, 10 orders (Araeolaimida, Chromadorida, Desmodorida, Dorylaimida, Enoplida, Monhysterida, Mononchida, Plectida, Rhabditida, and Triplonchida), 2 classes (Chromadorea and Enoplia). The density of nematode communities at most survey locations is quite high, ranging from 29.88 +/- 38.01 to 1172.08 +/- 659.74 individuals/10 cm2. However, the biodiversity is quite low, species richness index (S) ranged from 5.33 1.15 to 18.33 4.72, and Shannon diversity index (H') from 1.28 +/- 0.12 to 3.19 +/- 0.50 and Pielou's evenness index (J') from 0.47 +/- 0.04 to 0.93 +/- 0.04. The Maturity Index (MI) of nematode communities was applied to assess the environmental quality status of sediment. The results showed that the environmental quality status of sediment recorded disturbances, classified as bad to moderate. The colonizer-persister (c-p) combined with the MI is a potential tool in biological monitoring of environmental quality status. However, to increase the reliability of evaluation conclusions, the combination of MI and biological indicators as well as physical-chemical parameters is necessary.