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.

The small-scale vertical migration patterns and spatiotemporal variations of copepods in the Tapong Bay, southwestern Taiwan, from 2003 to 2005 were studied. The differences in species composition and abundance of copepods before and after the removal of oyster culture racks in relation to tides, hydrography and other environmental factors were also compared and herein discussed. Tapong Bay has distinct dry (October to April) and wet (June to August) seasons and exhibits apparent inter-annual variation, cold, saline and low Chl a concentration in dry season, and vice versa in wet season. Most copepod species displayed normal diel vertical migration, descending to the deeper water during daytime and ascending to near surface water at night. Copepods were always more abundant at night than during daytime. Higher abundance but smaller species number of copepods were found during ebb than flood tides. In all, 123 copepod species belonging to 18 families and 31 genera were identified. Oithona oculata, Parvocalanus crassirostris, Acartia sinjiensis, Acartia sp., Bestiolina amoyensis and copepod nauplius were predominant and together they composed 74% of the total number of copepods. These dominant species showed apparent seasonal changes, with higher abundance in autumn and winter. Copepod assemblage also showed apparent difference between stations. Warm-saline-coastal species dominated in the outer region, while small-size taxa and common species of estuaries and aquaculture ponds dominated in the inner bay. Results of indicator species analysis could possibly recognize the area affected by tides and water masses, and identified Acrocalanus gracilis to be the indicator species of flood tide and outer region of the bay, and Acartia sinjiensis the indicator species of ebb tide and inner bay. Our results showed that after the removal of oyster culture racks the species number and abundance of copepods were higher but the seasonal and spatial differences in abundance became smaller. Furthermore, the increase in abundance of zooplankton and copepods in the Bay after the removal of oyster culture racks might be due to the absence of oyster population which exerts great filter¡Vfeeding impact on phytoplankton, and decrease of predation pressure from moon-jelly that usually aggregated in the inner Bay and settled down their polyps on the oyster racks but disappeared after the removal.

This study aims to understand the role of the Tapong Bay on carbon biogeochemical cycle in the coastal zone and the influence of terrigenous inputs on ecosystem functioning in the Tapong Bay. The Tapong Bay is a semi-enclosed lagoon, occupied largely by fish farming cages and oyster culture racks. There is only one tidal inlet for exchanging water between the Tapong Bay and Taiwan Strait, which results in a low water exchange rate and oxygen deficient condition in the bottom water of the inner bay. The annual mean of water exchange time is about 10.6 days that is much longer than that in the Chiku Lagoon (5.8 days). Experimental results show that biological activity and variations of hydrochemistry primarily control the distributions of carbon and nutrients. Excess of DIP likely occurred in the Tapong Bay. Seasonal variations of primary productivity are apparently controlled by temperature, solar radiation and turbidity. The regression slope between particulate organic carbon and nitrogen approaches the Redfield ratio, indicating that organic carbon is derived primarily from biological production. The stratification of water column in the Tapong Bay was observed throughout the year. Diffusion from sediment may thus contribute significantly to nutrient distributions in bottom water. Diffusion flux estimated from porewater to bottom water is about 7.6% of annual mean input for DIN and is about 1.0% for DIP. Calcification process was observed in the Tapong Bay indicating that the oyster culture would affect the carbon budget in the bay. The annual mean production rate of organic carbon estimated from the biogeochemical model is about 5.80 mole C m-2 yr-1, implying that the Tapong Bay is an autotrophic system. The net ecosystem production (NEP) derived from diel observation is about 6.29 mmole C m-2 d-1 that is closed to 6.65 mmole C m-2 d-1 estimated from the biogeochemical modeling. The annual nitrogen fixation exceeds the annual denitrification [(nfix-denit)¡×1.30 mole m-2 yr-1] in the Tapong Bay. Carbon biogeochemical fluxes and budgets differ significantly between the Tapong Bay and the Chiku Lagoon, which may be arisen from pronounced difference in terrigenous inputs and seawater exchange rates.