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Liu K.-K.,National Central University | Kao S.-J.,Academia Sinica, Taiwan | Chiang K.-P.,National Taiwan Ocean University | Gong G.-C.,Institute of Marine Environmental Chemistry and Ecology | And 3 more authors.
Marine Chemistry | Year: 2013

In July 2009 an intense algal bloom with maximum Chl-a concentration reaching 166μgL-1 occurred in the highly eutrophic Danshuei River estuary, which receives waste discharges from the densely populated Taipei metropolitan area in northern Taiwan. The estuary is often burdened with very high concentration of ammonium (up to ~550μM), which dominates the dissolved inorganic nitrogen species in the estuary. The observed δ15N values of particulate nitrogen ranged from -8.6‰ to 0.2‰, and the δ15N values of coexisting ammonium ranged from 4.6 to 11.9‰. Notably the offset between δ15NPN and δ15NNH4 (δδ15N) showed significant correlation with ammonium concentration. The ε-values were calculated to be between -4.7 and -16.4‰. The range overlaps with that of previous estimates (-6.5 to -18.1‰) based on field observations. We plotted all field observed ε-values vs. corresponding ammonium concentrations and found a trend similar to that previously observed for marine bacterium, Vibrio harveyi, in laboratory cultures. Thus, we constructed a concentration dependent curve of the ε-value for ammonium uptake by phytoplankton in natural waters. The curve shows the maximum magnitude of ε-value (-20‰) at ammonium concentration around 100μM with decreasing isotope effect on both sides; at lower concentrations, the ε-value diminishes to zero; at higher concentrations, it slopes gradually towards an asymptotic value around -2‰. More than half of the ε-values derived from laboratory cultures of diatoms also fall on this curve. However, a few culture-based ε-values fall on another curve with similar pattern but considerably larger maximum magnitude. The maximum isotope effect is probably attributed to the cumulative isotope effects from ammonium deprotonation and the subsequent membrane diffusion of ammonia. This study provides the first field observed evidence of concentration dependent nitrogen isotope fractionation during ammonium uptake by phytoplankton and reconciles partially the disparity between estimates from field observations and from laboratory cultures. © 2013 Elsevier B.V.

Tsai A.-Y.,Institute of Marine Environmental Chemistry and Ecology | Gong G.-C.,Institute of Marine Environmental Chemistry and Ecology | Sanders R.W.,Temple University | Chen W.-H.,National Taiwan Ocean University | And 3 more authors.
Aquatic Microbial Ecology | Year: 2011

We investigated temporal variations in the effects of bacterivory by different sizes of heterotrophic nanoflagellates (HNF) and pigmented nanoflagellates (PNF) during a warm period (May to September) in oligotrophic coastal waters of the subtropical western Pacific. Short-term experiments with fluorescently labeled bacteria (FLB) demonstrated ingestion rates of 0.3 to 5.8 bacteria HNF-1 h-1 by HNF in the size range 3-6 mu;m-rates that were higher than observed for other sizes of HNF. Rates of ingestion by PNF ranged between 0.9 and 15.5 cells PNF-1 h-1, and, as for HNF, were greatest for PNF in the 3-6 mu;m size group. Nanoflagellates of size lt;6 mu;m removed about 98% of the total amount of bacteria consumed. The 3-6 mu;m PNF, 2-3 mu;m HNF, and 3-6 mu;m HNF were major consumers in the nanoflagellate community and were responsible for an average of 52, 28 and 16% of the total consumption of bacteria, respectively. The smallest PNF (2-3 mu;m) consumed only about 2% of the total and were considered to be primarily autotrophic. Despite ingestion rates in the range of those reported elsewhere, the low abundance of nanoflagellates observed resulted in relatively low grazing impacts (lt;10% of bacterial standing stock). We found a significant negative correlation between PO4 concentrations and ingestion rates of the 3-6 mu;m PNF, suggesting that the PNF ingestion rate increased under nutrient-deficient conditions. © Inter-Research 2011.

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