綠色自然聯盟 Green Nature Union - GNU - L'association d' écologie

Appreciating Seaweed - English Eco - Environment

Something about Seaweed :
Seaweed can be separated into three groups: Green algae (Chlorophyta), Brown algae (Phaeophyta) and Red algae (Rhodophyta). Aside from chlorophyll a, green algae contain chlorophyll b too, which absorb wavelengths of red light. Red algae contain phycoerythrin, which absorbs blue light; while brown algae contain xanthophylls, which absorb the middle part of the solar spectrum. Those wavelengths that are not absorbed but reflected become the main colour of each kind of seaweed. As red light can only reach the surface part of the ocean, green algae are mainly found in shallow waters; blue light can penetrate water to a depth of 100m, so this depth is where red algae flourish. Brown algae are mainly found in water to a depth of about 50m, which is between the two kinds of algae. In Hong Kong, growth zonation of different kinds of algae is not evident, so red algae can sometimes be found in the vicinity of brown algae. Salinity of waters in eastern and southern Hong Kong is not much affected by freshwater runoff from the Pearl River, hence these areas are more suitable for algal growth. Also there are more rocky shores and wavy seas in the eastern and southern parts of Hong Kong, so the algae found there are more diverse. In Hong Kong, algae occur from December through April. Algal growth is seasonal, and different species has its own time. Summer in Hong Kong is too hot for algal growth. The fierce sunshine, rising water temperature and tidal cycle on hot days are all unfavorable. Only Sea lettuce (Ulva lactuca) might occasionally appear at this time. Hence winter is a good time for algae to boom in Hong Kong. Currently some 200 species of algae have been recorded in Hong Kong, a few of which might have been brought into the territory by foreign vessels and water current.

Admiring the Seaweed :
Tung Ping Chau is a good place to spot seaweed. There is a large sea platform around the island, and the shallow water with its rocky bottom is a favorable place for coral and algal growth. Nai Tau Tsui, Pak Lap Wan and Chau Pui are places where algae of diverse species occur. March, as the peak month for algal growth in Hong Kong, is the best time. Select a day with a spring tide, and go watch them by snorkeling when the tide recedes. When the tide retreats, visibility is clearer, causing less stirring up of the sediments on the sea bottom. For your safety, it’s important that you snorkel on a fine day. Water temperature is still low in March. If you can’t stand it, you can just wade along the coast to have a look at the seaweed when the tide is low, or simply watch them from the beach.

You can easily find a particular type of brown algae which is distributed everywhere on your way, just like the holly vine that is used in Christmas decoration. But this vine has branches. What is it then? It is Sargassum. It attaches itself to substratum using plate-like holdfasts, and it has a tough front to overcome strong currents. Many species of Sargassum have gas-filled bladders so that they can float and stand right in water columns and receive an adequate amount of sunlight. In Hong Kong, the commonest Sargassum, the “leaves” of which have a smooth inner margin but a coarsely toothed outer margin; the serrate-lobed Sargassum angustifolium and Sargassum patens, have “leaves” and “branches” that look the same. High season for Sargassum growth is December through April. This is when you can find an “underwater grassland” of Sargassum. All seaweeds are about 3 feet tall, and they all dance with the golden waves under sunlight. You can feel you are flying across a field when you swim above the “grassland”. But in spring the “grassland” will become a “dense forest”, composed of Sargassum that is about 6-7 feet high. At this time you should not risking swimming in it. You may be amazed at the way seaweeds can resurge after they all die in summer. The fact is they produce numerous microscopic spores before they die, which grow when the environmental conditions become favourable again. Seaweed then propagates via this life cycle. Provided that the ocean is not polluted, we can enjoy an annual encounter with the seaweeds.

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