Roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean

An ocean general circulation model (OGCM) is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. The western boundary reflection is defined as the total Kelvin waves leaving the western boundary, which include the reflection of the equatorial Rossby waves as well as the effects of alongshore winds, off-equatorial Rossby waves, and nonlinear processes near the western boundary. The evaluation of the reflection is based on a wave decomposition of the OGCM results and experiments with linear models. It is found that the alongshore winds along the east coast of Africa and the Rossby waves in the off-equatorial areas contribute significantly to the annual harmonics of the equatorial Kelvin waves at the western boundary. The semiannual harmonics of the Kelvin waves, on the other hand, originate primarily from a linear reflection of the equatorial Rossby waves. The dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial Indian Ocean are investigated. These sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries. Because of the reflections, the second baroclinic mode resonates with the semiannual wind forcing; that is, the semiannual zonal currents carried by the reflected waves enhance the wind-forced currents at the central basin. Because of the different behavior of the zonal current and sea level during the reflections, the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin. In the meantime, the annual harmonic of the sea level remains large, producing a dominant annual oscillation of sea level in the central equatorial Indian Ocean. The linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries. In addition, the weak annual harmonics of sea level in the western basin, resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing, facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period. The Rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the Kelvin waves. The westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the Rossby wave propagation.

Close coupling between phytoplankton growth and microzooplankton grazing in the western South China Sea

We conducted 28 dilution experiments during August-September 2007 to investigate the coupling of growth and microzooplankton grazing rates among ultraphytoplankton populations and the phytoplankton community and their responses to habitat variability (open-ocean oligotrophy, eddy-induced upwelling, and the Mekong River plume) in the western South China Sea. At the community level, standing stocks, growth, and grazing rates were strongly and positively correlated, and were related to the higher abundance of larger phytoplankton cells (diatoms) at stations with elevated chlorophyll concentration. Phytoplankton growth rates were highest (> 2 d(-1)) within an eastward offshore jet at 13 degrees N and at a station influenced by the river plume. Among ultraphytoplankton populations, Prochlorococcus dominated the more oceanic and oligotrophic stations characterized by generally lower biomass and phytoplankton community growth, whereas Synechococcus became more important in mesotrophic areas (eddies, offshore jet, and river plume). The shift to Synechococcus dominance reflected, in part, its higher growth rates (0.87 +/- 0.45 d(-1)) compared to Prochlorococcus (0.65 +/- 0.29 d(-1)) or picophytoeukaryotes (0.54 +/- 0.50 d(-1)). However, close coupling of microbial mortality rates via common predators is seen to play a major role in driving the dominance transition as a replacement of Prochlorococcus, rather than an overprinting of its steady-state standing stock.

Estuarine nutrient loading affects phytoplankton growth and microzooplankton grazing at two contrasting sites in Hong Kong coastal waters

To investigate the effects of enhanced nutrient loading in estuarine waters on phytoplankton growth and microzooplankton grazing, we conducted monthly dilution experiments at 2 stations in Hong Kong coastal waters with contrasting trophic conditions. The western estuarine station (WE) near the Pearl River estuary is strongly influenced by freshwater discharge, while the eastern oceanic station (EO) is mostly affected by the South China Sea. Growth rates of phytoplankton were often limited by nutrients at EO, while nutrient limitation of phytoplankton growth seldom Occurred at WE due to the high level of nutrients delivered by the Pearl River, especially in the summer rainy season. Higher chlorophyll a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at WE than at EO. However, the increase in chlorophyll greatly exceeded the increase in phytoplankton growth rate, reflecting different response relationships to nutrient availability. Strong seasonality was observed at both stations, with temperature being an important factor affecting both phytoplankton growth and microzooplankton grazing rates. Picophytoplankton, especially Synechococcus, also exhibited great seasonality at EO, with summer abundances being 2 or 3 orders of magnitude higher than those during winter, Our results confirm that in eutrophic coastal environments, microzooplankton grazing is a dominant loss pathway for phytoplankton, accounting for the utilization of >50%, of primary production on average.

Organochlorine micropollutants in the Jiulong River Estuary and Western Xiamen Sea, China

Organochlorine contaminants including 12 polychlorinated biphenyl (PCB) congeners and 18 insecticides were determined in water, pore water and sediments of the Jiulong River Estuary and Western Xiamen Sea, China. The results showed that the levels of the total PCBs ranged from non-detectable to 1500 ngl(-1) in water, from 209 to 3870 ngl(-1) in pore water, and from 2.78 to 14.8 ng g(-1) dry weight in sediments. Total organochlorine insecticide concentrations were from below the limit of detection to 2480 ngl(-1) in water, from 267 to 33400 ngl(-1) in pore water, and from 4.22 to 46.3 ng g(-1) dry weight in sediments. Concentrations of PCBs and insecticides in pore water were significantly higher than those in surface water, due to the high affinity of these hydrophobic compounds for sediment phase. The PCB congeners with the highest concentrations were CB153, CB180 and CB194, which together accounted for 68-87% of total PCBs in water, pore water and sediment. Among the hexachlorocyclohexane (HCH) compounds, beta-HCH was found to be a major isomer. Analysis of 1,1,1-trichloro-2,2-bis-chlorophenyl-ethane (DDT) and its metabolites showed that 1, 1-dichloro-2[o-chlorophenyl]-2[p-chlorophenyl]-ethylene (DDE) was dominant in the group. In comparison to a 1998 study in the Western Xiamen Sea, levels of organochlorines were enhanced due probably to recent inputs and changes in sediments. (c) 2004 Elsevier Ltd. All rights reserved.

The possibility analysis of habitats, origin and reappearance of bloom green alga (Enteromorpha prolifera) on inshore of western Yellow Sea

Combining some information from field investigation of algae along the coastal areas in China and a few pictures materialized from the western Yellow Sea in 2008, authors analyze the necessary conditions and possible water area in China producing a large biomass, some reasons for firestorm, and the possibility of the reappearance of marine bloom green alga Enteromorpha prolifera. The change of habitats and the increase of nutritional levels related to the water area could be considered as direct reasons. It was transferred northward by the combination of the flow of rainwater, wind and alongshore marine current. The original region of large biomass produced is possibly located in the southwestern Yellow sea. It will possibly be appearing again in the coming years or in the future. A summary is also given referring to its reproduction, development and distribution worldwide.

Spatial variations of size-fractionated chlorophyll, cyanobacteria and heterotrophic bacteria in the Central and Western Pacific

Geographic and vertical variations of size-fractionated (0.2-1 mu m, 1-10 mu m, and >10 mu m) Chlorophyll a (Chl.a) concentration, cyanobacteria abundance and heterotrophic bacteria abundance were investigated at 13 stations from 4 degrees S, 160 degrees W to 30 degrees N, 140 degrees E in November 1993. The results indicated a geographic distribution pattern of these parameters with instances of high values occurring in the equatorial region and offshore areas, and with instance of low values occurring in the oligotrophic regions where nutrients were almost undetectable. Cyanobacteria showed the highest geographic variation (ranging from 27x10(3) to 16,582x10(3) cell l(-1)), followed by Chl.a (ranging from 0.048 to 0.178 mu g l(-1)), and heterotrophic bacteria (ranging from 2.84x10(3) to 6.50 x 10(5) cell l(-1)). Positive correlations were observed between nutrients and Chl.a abundance. Correspondences of cyanobacteria and heterotrophic bacteria abundances to nutrients were less significant than that of Chl.a. The total Chl.a was accounted for 1.0-30.9%, 35.9-53.7%, and 28.1-57.3% by the >10 mu m, 1-10 mu m and 0.2-1 mu m fractions respectively. Correlation between size-fractionated Chl.a and nutrients suggest that the larger the cell size, the more nutrient-dependent growth and production of the organism. The ratio of pheophytin to chlorophyll implys that more than half of the > 10 mu m and about one third of the 1-10 mu m pigment-containing particles in the oligotrophic region were non-living fragments, while most of the 1-10 mu m fraction was living cells. In the depth profiles, cyanobacteria were distributed mainly in the surface layer, whereas heterotrophic bacteria were abundant from surface to below the euphotic zone. Chl.a peaked at the surface layer (0-20 m) in the equatorial area and at the nitracline (75-100 m) in the oligotrophic regions. Cyanobacteria were not the principle component of the picoplankton. The carbon biomass ratio of heterotroph to phytoplankton was greater than 1 in the eutrophic area and lower than 1 in oligotrophic waters.