Inorganic nutrients budget and its effects on organic carbon cycling in the tropical lagoon of the Dapeng Bay

[[abstract]]缺氧為大鵬灣潟湖生態系常見之現象，為探討並瞭解其產生之可能原因，本研究主要分二部份進行；第一部份利用盒子模式估算大鵬灣潟湖內之無機營養鹽收支平衡，第二部份主要將探討此無機營養鹽對潟湖生態系有機物質循環之貢獻與影響。第一部研究於大鵬灣內佈設16個採樣點進行各水質因子之分析，再經由盒子模式來計算大鵬灣之營養鹽收支。模式估算整個潮汐週期時段內營養鹽進出潟湖及營養鹽進入沉積物之淨通量。模式計算結果顯示，硝酸鹽、亞硝酸鹽、氨氮於潟湖內屬於累積狀態，而磷酸鹽及懸浮固體則由潟湖經由潮口向外輸出，每個潮汐週期約有13至15噸之懸浮固體由潟湖向外輸出至臨近海域。為進一步探討輸入至潟湖內無機營養鹽對此生態系有機碳循環之影響，第二部份將瞭解大鵬灣內浮游生物活動於碳循環所扮演之角色，尤其是在營養鹽突然注入增加後之情況。結果顯示，相當大量的溶解性無機營養鹽進入潟湖內，其主要來源是經由逕流或是溝渠輸入，亦或是由沉積物產生之再生性無機營養鹽。在空間分布上，無機營養鹽濃度則由外海向潟湖內部方向漸增。雖然各觀測因子之變化非常大，但各因子相互間皆存在顯著之線性相關。有機碳在此水柱中最重要之消費者為浮游植物及細菌群落，計算結果此生態系統中它們約佔87%之總有機碳消費。此生態系統中基礎生產率與族群呼吸率比值平均值高達1 33，顯示潟湖環境為自營性生態系，此結果亦顯示此生態系統為碳匯，而殘存碳會流至近岸海域或是進入潟湖之沉積物內保存，因此造成底部水體形成缺氧狀況。[[abstract]]Hypoxia has been frequently observed in the Dapeng Bay In order to understand the potential causes of this phenomenon two approaches have been applied in this study The first one was to estimate the net nutrient fluxes into and out of Dapeng Bay through the inlet and some drainage creeks by using the box-model budget method Secondly the role biological activity plays in carbon cycling is also examined in Dapeng Bay especially after a natural nutrient pulse For the first objective water samples in Dapeng Bay were obtained from 16 sites Sampled data were used to construct a box model for the nutrient budget in the Bay We have estimated the net nutrient fluxes into and out of the lagoon and also calculated the total amount of nutrient deposition to the sediment in a full tidal cycle The results show that for both January and April nutrients (including nitrate nitrite and ammonia) accumulate in the lagoon but phosphate total phosphate and suspended solids were leaving the lagoon from the tidal inlet In addition a huge amount of suspended solids (13-15 tons per tidal cycle) flows from the lagoon to the adjacent ocean through the entrance In the second objective the role biological activity plays in carbon cycling in Dapeng Bay was examined especially after a natural nutrient pulse The results indicated that significant amounts of dissolved inorganic nutrients were either run off into inner lagoon through drainage or regenerated from the bottom sediments of the inner lagoon Spatially the lowest values were found in the seaward opening and the values increased toward the inner lagoon Even though there were orders of variation observed in the variables linear relationships were significantly evident between variables As to organic carbon consumption in the water column both phytoplankton and bacterial communities are considered the most important components; they accounted for more than 87% of total organic carbon consumption in this ecosystem A large ratio of primary production to community respiration was observed with a mean of 1 33 which suggests that this lagoon is autotrophic It also indicates that there is a carbon sink in this ecosystem and the residual carbon is either exported into the coastal sea or preserved in lagoon sediments resulting in the hypoxic condition of the bottom wate