欧亚国际

    Nayar, S. and Bott, K等发表了Uptake and translocations of ammonium and nitrate by temperate seagrass Zostera nigricaulis in Port Phillip Bay. South Australian Research and Development Institute (Aquatic Sciences)的论文Report Series No. 819. 51pp.该研究在Port  Port Phillip Bay进行，具体研究内容详见以下摘要。在该研究中用到了Aquation公司的代表水准的水下原位光合-呼吸自动测量系统。

  This study was conducted in Port Phillip Bay (PPB; 38S, 148E), a large, shallow, semi-enclosed marine embayment with a water spread and catchment area of 1,950 km2 and 9,790 km2, respectively. The bay has a largely urbanised watershed with the twin cities of Melbourne and Geelong fringing it. Plant growth is thought to be nitrogen-limited, and with annual nitrogen loads ranging between 6,000 and 8,000 t, there is a pressing need to understand nitrogen budgets in the bay to help manage the ecosystem better. This study quantified specific uptake rates by above-ground seagrass biomass (leaves), below-ground seagrass biomass (roots and rhizomes), epiphytic algae and phytoplankton using stable isotope (15N) labelled ammonium and nitrate in situ using diver-deployed benthic chambers. 

    This study was undertaken to obtain environmentally realistic data on specific uptake rates and resource allocations to develop nitrogen budgets and to test the hypothesis that the above-ground and below-ground compartments of Zostera nigricaulis are able to re-mobilise ammonium and nitrate through uptake, translocations and assimilation from the above-ground compartment to the below-ground compartment and vice-versa as a mechanism to adapt to nitrogen availability in the environment. The study is unique in that the quantification of specific uptake rates of inorganic nitrogen by the above-ground biomass, below-ground biomass, epiphytic algae and phytoplankton were undertaken simultaneously with studies to quantify assimilation and translocations rates between the above- and below-ground compartments at each of three sites in Port Phillip Bay. As the specific uptake rates and translocations studies for ammonium and nitrate were undertaken at the same time, it allows comparison between the two nitrogen sources. The preferential uptake of ammonium over nitrate by Zostera nigricaulis was observed, where the specific uptake rate for nitrate was about one sixth of that of ammonium. This increased affinity for ammonium over nitrate was also observed in other biotic components associated with the seagrasses, such as epiphytes and phytoplankton. 

    Overall the mean specific uptake rates of ammonium and nitrate by leaves (ammonium: 18.8 –¬ 58.0 g N g-1 DW h-1; nitrate: 4.6 – 12.2 g N g-1 DW h-1) was highest followed by epiphytes (ammonium: 13.3 – 20.6 g N g-1 DW h-1; nitrate: 2.8 – 17.6 g N g-1 DW h-1), roots (ammonium: 0.3 – 7.8 g N g-1 DW h-1; nitrate: 0.5 – 1.7 g N g-1 DW h-1), and finally phytoplankton (ammonium: 0.05 – 0.53 g N g-1 DW h-1; nitrate: 0.01 – 0.08 g N g-1 DW h-1). Leaves dominated the resource allocations of ammonium, accounting for between 71 – 93% of the total resource, followed by epiphytes (9 –17%) and roots (1 – 12%). Similarly, leaves dominated the assimilation of nitrate, accounting for 72 – 83% of the total resource in contrast to epiphytes (11 – 13%) and roots (5 – 15%). The assimilation of ammonium and nitrate by phytoplankton was negligible (<0.5%). The translocations experiments demonstrated the uptake of nitrogen by both the above-ground biomass and the below-ground biomass, respectively from the water column and pore water, and subsequent translocations to the opposite compartment. The uptake by leaves of both ammonium and nitrate from the water column (97 – 100% ammonium and 94 – 97% nitrate) dominated the uptake by the root tissue from the pore water (68 – 82% ammonium and 8 – 89% nitrate). The quantum of nutrients taken up and subsequently translocated from the root tissue to the leaf tissue (18 – 32% for ammonium and 11 – 19% for nitrate) significantly surpassed the translocations from leaf tissue to the root tissue (0 – 3% for ammonium and 3 – 6% for nitrate). Therefore, the study clearly demonstrated acropetal translocations (translocations from below-ground biomass to above-ground biomass) dominating basipetal translocations (translocations from above-ground biomass to below-ground biomass) in Zostera nigricaulis under the given conditions at all three study sites. This is a unique outcome, as basipetal translocations has been widely reported for Zostera by other researchers.