Abstract The Tapong Bay and the Chiku Lagoon are major lagoons in the south of Taiwan and are ideal sites to study the influence of coastal environment change on the ecosystem. Therefore, this study aims to evaluate the influence of oyster culture racks removal on biochemical processes of carbon, nutrients and mercury in the Tapong Bay, as well as to compare the status of mercury and trace-metal pollution in Tapong Bay and Chiku Lagoon. Before the removal of the oyster culture racks from the Tapong Bay, the annual mean of water exchange time is about 10 days that is longer than that of the present condition (7.1 days). This suggests that the flushing condition of lagoon water is improved after the racks were removed. The annual mean of each nutrient concentration is also lower at present than before, probably due to the enhanced water exchange rate and biological utilization. The annual mean of ∆POC/∆PN is 8.1 that is larger than that of the previous condition (7.3), possibly resulting from the increase of inputs of organic detritus. The Tapong Bay is an autotrophic system (p-r>0) both before and after the removal of oyster culture racks. However, the net ecosystem production (p-r) at present is twice as large as before the removal of oyster racks. After the removal of racks, the annual nitrogen fixation still exceeds the annual denitrification in the Tapong Bay with a magnitude of 5.35 mole N m-2 yr-1. Mercury (Hg) is a highly toxic metal with high affinity to biota. As the lack of Hg distribution data around the coastal zone of Taiwan, the study also aims to develop the analytical methods of Hg species and apply to study Hg biogeochemistry in Tapong Bay and Chiku Lagoon. Distributions of Hg species in the Tapong Bay are spatio-temporally variable, ranging from 6.66 to 12.40 ng/l (ave., 10.01 ng/l) for total Hg (unfilt.), from 1.79 to 3.75 ng/l (ave., 2.56 ng/l) for total dissolved Hg (filt.), from 1.59 to 2.67 ng/l (ave., 1.90 ng/l) for reactive Hg and from 2.51 to 9.45 ng/l (ave., 5.60 ng/l) for particulate Hg. Distributions of Hg species in the Chiku Lagoon are also spatio-temporally variable, ranging from 4.47 to 9.20 ng/l (ave., 6.22 ng/l) for total Hg (unfilt.), from 2.03 to 5.69 ng/l (ave., 4.54 ng/l) for total dissolved Hg (filt.), from 1.70 to 2.87 ng/l (ave., 2.12 ng/l) for reactive Hg and from 2.50 to 7.65 ng/l (ave., 4.79 ng/l) for particulate Hg. The abundance of particulate Hg is positively correlated with chlorophyll a, and total dissolved Hg and reactive Hg are negatively correlated with chlorophyll a. Such relationships imply that distributions of Hg species are primarily controlled by biological uptake and/or adsorption/desorption. Reactive Hg (Hg2+) is also correlated positively with dissolved oxygen concentration suggesting the biological redox effect in modulating the distribution of Hg2+. Particulate Hg also shows positive relationships with total suspended matter and particulate organic carbon, primarily due to biological absorption and particle adsorption/desorption. Enrichment factor (EF) are employed to evaluate trace metal pollution in Tapong Bay and Chiku Lagoon. The results show that the magnitudes of EF are larger in Tapong Bay than in Chiku Lagoon for most metals, particularly for Hg, indicating an thropogenic influence on metal distributions in both lagoons. On the other hand, particulate Hg is poorly correlated with particulate Fe, Mn and Al, strongly indicating relatively little influence of terrestrial detritus in modulating the distributions of particulate Hg.

No description available.