The hydrography and plankton community structure was investigated in the Andaman Sea off Phuket, Thailand. Two cruises were conducted in 1996, one representing the calm dry NE monsoon season (March) and the other representing the stormy and rainy SW monsoon season (August). Sampling was performed along 3 transects perpendicular to the shelf break, from the coast across the shelf into deep water. The water column at the nearshore stations was vertically mixed, while the water column at off shore stations was strongly stratified, hence a frontal zone was established at the mid shelf. A prominent feature of the area was the pronounced internal wave centred around the pycnocline. The wave was observed from the outermost stations to the mid-shelf front. The height of the wave reached peak values of approximately 60 m in areas of approximately 300 m bottom depth. At all stations in stratified waters the vertical distribution of the phytoplankton showed a pronounced subsurface chl a peak in association with the pycnocline. The highest chl a values and primary production was observed at the front established at the mid shelf where the pycnocline meets the bottom, and salt nutrient-rich water is mixed up in the surface layer. We did not find any relationships between hydrography and the other key components of the microbial food web. No difference in productivity or food web structure was observed between the 2 seasons despite a significant difference in climatic forcing. Pico-and nanoplankton dominated the biomass in both seasons and Synechococcus contributed 72 to 74% of the biomass. Analysis of the microbial food web and establishment of carbon-flow budgets illustrates the importance of the microbial food web for making the primary producers available to the higher trophic levels.
We studied the cross-shelf variation in hydrography and plankton dynamics off west Thailand, focusing on physical-biological linkages. The overall research programme investigated linkages between physics, chemistry and plankton biology; in the present paper we consider the findings based on the sampling of fish larvae and mesozooplankton. Surveys were carried out during 2 monsoon periods in March and August 1996, using 3 cross-bathymetric transects extending to the deeper part of the shelf slope of the Andaman Sea. Station distances were either 5 or 10 n miles apart, and at each station a series of net tows were carried out, targeting different size ranges of organisms. Plankton were identified to order (invertebrates) or family (fish larvae), and their abundances and biomass estimated. The abundance of both mesozooplankton and fish larvae peaked mid-shelf (50 to 65 m bottom depth) coinciding with a hydrographic front generated where the pycnocline meets the sea-bottom. An internal wave of pronounced amplitude interacts with the shelf slope at ca. 300 m bottom depth, and findings indicated another zone of enhanced abundance in this area. Analysis of the relative abundances of fish larvae within families revealed a marked cross-shelf structuring into a number of larval assemblages. Distinct assemblages were identified in nearshore areas, at midshelf in the area of the hydrographic front, and off the shelf break in oceanic water. Less pronounced variation was seen in the along-shelf direction and between monsoon periods. KEY WORDS: Fish larval assemblages · Cross-shelf plankton distribution · Frontal hydrography · Monsoon periods Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 274: [87][88][89][90][91][92][93][94][95][96][97] 2004 tion, while fewer studies have been carried out in tropical waters, where hydrographical conditions are different. One characteristic feature of the tropical seas is the strong, permanent temperature stratification of the water column. This is based on the permanent, conspicuous heating of the surface water, and the inflow of a cold subsurface water mass formed outside the tropic region, principally by sinking of water near the subtropical convergence. Wind-induced turbulence mixes heat down from the surface to a specific depth, and a sharp thermocline is established in the depth range of 30 to 50 m. Another characteristic of the tropical seas (from west Africa to SE Asia) is the monsoon seasonality, driven by the twice-yearly passage of the atmospheric inter-tropical convergence zone across the equator. This passage leads to wet/dry seasonality at the coast, dry when the wind blows from the continent, and wet when strong winds blow oceanic air towards the coast. The monsoon periods influence water circulation, and can lead to flow reversal and upward transport of nutrients (Longhurst & Pauly 1987).The monsoon seasonality is likely to exert a strong influence on the tropical shelf ecosystem. However, it has proven difficult to ...
The status and seasonal variation of heavy metals in surface sediment were investigated at Don Hoi Lot, located in the Mae Klong estuary, Thailand. Results revealed that all the measured heavy metals, except Zn, in the sediments had lower concentrations than in other nearby estuaries. Only Zn may be of concern for potential negative effects on estuarine biota in the study area. With the exception of Fe, all the studied heavy metals showed seasonal variation, but the patterns were diverse. Organic matter and the clay fraction in sediments were good sinks for heavy metals, excluding Zn, while Fe and Mn were good catchers. Principal component analysis suggested that Zn might have different origins and/or mechanisms of transport, accumulation and circulation, compared with the other heavy metals studied. A better understanding of sources and the behavior of Zn would enhance the efficiency of the estuary management plan in this study area.
Nematode communities were investigated in January and June 2008 at the western Thachin River mouth, Samut Sakhon, Thailand. The sampling site is comprised of two different habitats, i.e. mangrove forest characterized by Avicennia marina and mudflat adjacent to the mangrove forest. Mean densities of the nematode ranged from 110 -995 ind.10 cm -2 while taxonomic classification showed 32 and 50 genera in the mudflat and mangrove forest, respectively. The high diversity indices of nematode assemblages were recorded in both habitats with values ranged between 0.49 -2.32 in mudflat and 2.02 -2.92 in mangrove. The predominant genera were Terschellingia sp. Daptonema sp. Sabatieria sp. and Hopperia sp. Two distinct groups of nematode communities between the two habitats were detected by multidimensional scaling, analyses of similarity percentages. The number of species and density were significantly different in the two different habitats (p < 0.05). The mangrove forest provided wide variety of microhabitat and physically stable environment while the mudflat area was exposed to water turbulence and prone to anthropogenic activities. The environmental differences of the two communities may have resulted in higher diversity and density of nematodes in the mangrove forest.
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