Nutrient Dynamic Processes In The Coral Reef Ecosystem,Nansha Islands,South China Sea

Integrated seawater, sinking particulate, plankton, coral frame and surface sediments in reef lagoon, this study focuses on the biogeochemical cycling of biogenic elements and more than 30 abiogenic elements in coral reefs of the Nansha Islands, South China Sea. We succeed in deploying sediment traps in the seaward slope in 99-4 cruise and firstly reported vertical fluxes of some metal elements such as Mo W Re .~ Os ~ Ni~ Ir both in and out reef lagoon. Some main results are following:1.Physical dynamic process is one of the most important control factors on biogenic cycling in waters. Characteristics of coral reefs and associated depositional environments of Nansha Islands waters reflect that physical factors especially water dynamic process, both oceanic and atmospheric, affect the growth of coral reef. Coral reef ecosystem has the features of storage and rapid uptake nutrients. It is apparent that nutrient distributions such as P04-P are influenced by lagoon water motion manner.2.Nutrient cycling in lagoon has the characteristics of efficiency, rapidness and disequilibrium. Coral reef ecosystem has a high gross productivity while its net productivity is low. It is evident that the rapid cycling and efficient recycling of nutrients are very important in lagoon. At two sites in Zhubi Reef lagoon, 93.55% and 95.83% POC are decomposed respectively before they sink to sediment. In situ regeneration supplies the most nutrients required of reef communities. Release rates of PIN, PON, PTP and POP are 90%, 86%, 5 8.7% and 90% respectively. Vertical fluxes of C, N and P in lagoon are higher than that out of the atoll.The average N:P atomic ratio 8.1 in Zhubi Reef lagoon waters is far less than Redfield Ratio implies N-limit in the lagoon, but the average value 26.7 during 24 hours suggests P-limit exist in the reef flat. Because the recycling rate of P is slower than that of N and P hasn抰 a continuous supply mechanism like biotic fixation nitrogen, P is more probably the limit nutrient in coral reef ecosystems.Experiments of coral culture indicate that corals also have trend to consume exceedingly nutrients when nutrient concentration is far higher than normal. Every group3corals consume nearly all addition N03-N or P04-P in about 24 hours. This strategy makes corals consume most of sudden input nutrients sufficiently in natural seawater. NH4-N is often the limit nutrient for corals at this condition. Another result is that the nutrient consumed rate is more important than its real concentration. In contrast, reef ecosystem can get rid of the nutrient limit by reducing its consumed rate.3.Biogeochemical cycling of abiogenic elements is related to the biologic processes.Two processes, particulate absorption and organism utilization, control the distributions of abiogenic elements in particulates, organisms and sediments. Element analysis result indicate that sinking particulates in lagoon mainly come from biogenic detritus while part of particulates come from resuspend sediments in the seaward slope of reef. But the transfer mechanisms of elements are similar. Hydrodynamics and bio-enrichnient with feeding process are always the key control factors.4.Multiple processes sustain the high biomass and gross productivity in coral reef ecosystem. Nutrient budgets are not always equilibrium in coral reef lagoon, so it is probably that high production in reef communities is supported by the cooperation of several strategies such as topographic choose, 搒imulated drift-net? rapid uptake, sufficient recycling and 搒uspension fishing? etc. Suspension fishing strategy can restore the excessive consumed primary productivity. The organism and its metabolic products play an important role in nutrient continuous input process.