维持南沙珊瑚礁生态系统高生产力的新观点——拟流网理论

THE NEW VIEWPOINT OF THE HIGH PRODUCTIVITY SUPPORTING NANSHA CORAL REEF ECOSYSTEM ——SIMULATED DRIFT-NET THEORY

珊瑚礁生态系是海洋中最重要的生态系之一,一般处于热带与亚热带海域。众所周知,热带海洋的上层水由于浅部温跃层的存在,阻碍了营养物质的向上输送,从而使有机物产量和输出通量都较低,即热带海洋的上层水是一层十分贫瘠的寡营养水。但处在热带海域中的珊瑚礁确有异常高的生产力,所以人们称珊瑚礁为热带沙漠海洋中的绿洲,对被包围在贫瘠海水中的珊瑚礁如何能维持高生产力这一问题。

The coral reef ecosystem has high biomass and gross primary productivity, locates in tropical areas noted for very low dissolved nutrient concentrations. The reefs are truely "an oasis in a desert ocean". The high productivity supporting the coral reef ecosystem is a "reef nutrient paradox". Many reasearchers regarded the apparently contradictory aspects of and directed towards resolution of the "reef nutrient paradox". About one quarter of the most recent research reports on "coral reef" concerns problems broardly termed the nutrient paradox. The main conclusion was that the coral reef ecosystems have low net productivity despite their high gross productivity, and are typically autotrophic communities. The main viewpoints were as follows:1. The temperature of coastal upwelling water can drop below the lethal limit of algalcoral ecosystems; nutrient-rich upwelling water should produce a bloom of planktonic and benthic algae which should lead to a decrease in the input of light to the zooxanthellae-coral endosymbiotic system and eventually to eutrophication and coral polyp suffocation; increased destruction of reef frameworks by bioeroders whose abundance is directly related to the increas benthic algal biomass.2. In oligotrophic regions, endo-upwelling can provide new nutrients to sustain the net primary production by zooxanthellae-coral symbiosis; Such primary production in turn supports coral skeletogenesis ,microbial production and internal recycling of detrital organic matter injected into the reef matrix by wave energy;The excess production and recycling of inorganic nutrients and organic carbon (chiefly zooxanthellae, mucus and dissolved organic mater) are exported to the adjacent ocean. Excess zooxanthellae purged in enormous numbers by the coral polyps may be consumed by zooplankton and benthos, the first links of the reef food chain. This food chain centered around atoll and barries reef crests may explain the high biomass and secondary production observed in these oceanic oases. A permanent endo-up-welling can bring new nutrients to the surficial autotrophs, supporting the long-term growth and exportation of the eoosystem.3. In the coral reef lagoons, the nutrient recycling is very rapid, water movement can transport nutrients into the lagoons and accelerate the nutrient assimilation of organisms.Based on a study of the Nansha coral reef ecosystem, South China Sea, this paperpresents a new viewpoint--"simulated drift-net theorlu", which the reef nutrientparodox may be explained satisfactorily. The new viewpoint includes two parts:a. The main reason for the high gross productivity in the coral reef ecosystem is that the fixed tropical organisms there can enrich with a great amount nutrients in the oligotrophic water coming out from the reef. Because the coral reef organisms are relatively fixed and cannot drift with the moving water,they can adsorb much nutrients and become"nutrient enricher" of nutrient-poor waters. As a metaphorical meaning, the fixed organisms like a drift-net. Although fish is relatively little, they come into drift-net and are caught. The result is that fish is very much in drift-net. The "Simulated drift-net theory" is summarized in one sentence: The reason for the high gross productivity supporting the coral reef ecosystem is that the special biochemical reactor made up of relatively fixed tropical organisms enriches with a great amount nutrients the oligotrophic water coming out of the reef.b. The nutrient recycling in coral reef ecosystem is very rapid (about 5 to 25 times a year).