以海洋光學測繪南灣底質分布

Abstract

珊瑚白化為全球氣候變遷的重要指標之一，例如 1998年的珊瑚白化受到聖嬰事件 (El Niño) 影響最為嚴重(戴，1998)，根據國際珊瑚礁學會的統計，全世界至少有五十個國家的珊瑚礁發生大量白化的現象，其範圍遍及太平洋、印度洋及大西洋的主要珊瑚礁區。當海域環境變遷時，可能會影響底質的組成分布，進而改變珊瑚礁生態系的物種分布，為迅速獲取海域中珊瑚生長狀態，本篇以南灣海域為例，由福衛二號多光譜衛星影像來測繪海底的物質分布情況，藉以評估珊瑚受到環境變遷的影響。 南灣海域有黑潮流經，水溫非常適合珊瑚生長，珊瑚群聚密度很高，海灣中分布二、三百種珊瑚。本篇以衛星接收到的資訊計算出藍綠波段的反射率，再依據珊瑚、大型藻類、白沙在藍綠波段的反射率特性不同，可推得底質覆蓋率，其中利用水體幾乎不反射近紅外光的特性進行大氣和波浪校正。 為了檢驗衛星估測底質覆蓋率的方法是否準確，採用拖曳式水下攝影拍攝南灣海域的底質照片做驗證。此方式是在水下載具 (Vfin) 內裝載攝影機，經由船隻拖曳，對南灣海域進行海域水下攝影。由2009 年水下攝影實測資料對同年衛星影像估測不同底質覆蓋率分布結果做驗證，得知其相關係數都大於0.7，表示衛星估測和水下攝影實測底質覆蓋率分布的相關性高。再計算覆蓋率差異的平均值，珊瑚和大型藻類覆蓋率的平均值的差異都接近 0.1，白沙更小於0.1，表示本文由衛星推估底質覆蓋率的方法是可信的。由兩張拍攝時間不同的衛星影像來推估南灣海域底質變化，發現2009年後壁湖某塊區域(120.749~120.753 oE, 21.94 ~ 21.944 oN) 的珊瑚明顯比2007年多，藻類比2007年少。本文使用的衛星遙測底質覆蓋率方法，可由不同時間的影像對海域進行長期監測底質的變化，其優點為可估測船隻無法到達海域或者水深較淺區域的水質，在下次環境變遷現象發生時能快速的推估海域中底質的變化，估測珊瑚受損的程度與範圍概況。

Coral bleaching is one of the important indicators of global change. According to International Society for Reef Studies, there were over fifty countries coral reef bleaching when El Niño happened in 1998 and the incidence range from Pacific, Indian oceans, to Atlantic ocean. (Tai, 1998). Bottom type may change with environmental change. Formosat-2 images may be used to map the bottom types over large area and monitor coral bleaching events. Because Kurshio flows into Nanwan Bay, the sea temperature is suitable for coral growth. There are two to three hundreds kinds of coral species in this area and the density of coral is abundant. Firstly, we use the Formosat-2 image to get the substrate reflectance from blue and green band, and near-infrared band to do atmospheric correction and surface wave correction. Secondly, we can classify the bottom substrates to three types (coral, macro algae, sand) by using different optical property to identify different bottom substrates. The advantages of using satellite images are we can map coral habitat from shallow region where ship can’t navigate and can estimate coral area in large region, but it’s restricted to weather condition and water quality. Another way to investigate sea bottom substrates is the ship towing underwater vehicle (Vfin), we can use it to examine the satellite estimated bottom substrates distribution result.The DV in Vfin records video images of sea bottom in Nanwan Bay. After examing the 2009 satellite-derived bottom coverage types, and 2009 Vfin in situ data, the correlation of coverage rate of bottom substrates are over 0.7. It shows high correlation between estimated data and in situ data. The difference of coral and macro algae coverage rates are close to 0.1, and sand is even smaller. We can say our method of estimating coverage rate of bottom substrates is reliable. From comparison of 2007/07/20 and 2009/01/29 satellite images, we found one area from Hobihu (120.749~120.753oE, 21.94 ~ 21.944oN) in 2009 having more corals but lower macro algae concentration than in 2007. When next El Niño happens, consecutive satellite images may be used to estimat bottom substrates distribution, and their change of distribution instantly, over larger area than human diving investigation.