Benthic Nutrient Fluxes At Typical Areas In Chinese Coast, With Emphasis On Bioturbation

Benthic nutrient fluxes can modify the nutrient concentrations of water column in shallow coast markedly. But the influence of bioturbation on benthic nutrient fluxes must be assessed while a high abundance of macrofauna emerged in the sediment.The effects of bioturbation by dominant macrofauna on benthic nutrient fluxes were investigated by sediment-water incubation at three typical areas in Chinese coast: Qinlan mangroves （tropical intertidal zone）, Eastern intertidal zone of Chongming （wetland at large estuary） and Jiaozhou Bay （semi-close bay）.The bioturbation of Metaplax elegans, dominant species of macrofauna at Qinlan mangroves, was separated into two pathways: enhancing benthic molecular diffusion by bioirrigation and changing the biochemical processes in sediment. The contributions of the two pathways were evaluated by incubation experiments. The results showed that M. elegans bioirrigation promoted molecular diffusive fluxes considerably at sediment-water interface, though apparent area of burrow was neglectable comparing to total sediment-water interface area. The biogeochemical processes in M. elegans burrow were significantly different to surrounding or surface sediment. Biochemical processes in M. elegans burrow enhance nutrients release from sediment, whereas biochemical processes in the surface sediment accelerates the DIN （dissolved inorganic nitrogen） and Si（OH）4 uptake from water column in mangrove forest of Qinglan. Our results highlight that bioturbation may play an important role to modify the function of mangrove system, but the contribution of bioturbation as a whole may be obscure due to that the effects by different pathways may be offset with each other.Seasonal difference of bioturbation was evident by measuring the effects of Ilyrplax deschampsi, dominant benthos at Eastern intertidal zone of Chongming, on benthic nutrient fluxes. Bioturbation accelerated the DIN and Si（OH）4 uptake to sediment in winter, whereas cut down the DIN and PO₄^3- uptake in autumn. The sediments of Yangtze estuary intertidal zone are net sinks of DIN and PO₄^3- in winter and autumn, though the differences among areas were significant. Based on benthic nutrient fluxes in Eastern intertidal zone of Chongming, we estimate estuary intertidal zone can remove 0.70% （DIN） and 3.7% (PO₄^3-) of the nutrient transport fluxes from Yangtze river in dry season, 0.064% （DIN） and 2.2% (PO₄^3-) in wet season. Comparing to transport fluxes from Yangtze river, benthic flux of Si（OH）4 is neglectable.Nutrient excretion rates and the impact of Ruditapes philippinarum, dominant aquaculture species in Jiaozhou bay, on benthic nutrient fluxes were measured by incubation experiments, and these results were used to assess the effect of R. philippinarum aquaculture on nutrient recycling in Jiaozhou bay. Our research indicates that the bioturbation of R. philippinarum would modify biogeochemical progresses in sediment, and restrain nutrients excreted by R. philippinarum releasing to overlying water. Only 37% DIN and 34% PO₄^3- excreted by R. philippinarum were released to water column across the sediment-water interface due to bioturbation of R. philippinarum. According to the reduction of nutrient exchange across the sediment-water interface by bioturbation, only 19% （N） and 17% （P） of biogenic elements fed by R. philippinarum can be recycled and return to water column, while the others would be removed from water column in Jiaozhou Bay. The ratios of Si:N and Si:P of nutrients released from sediment to water column with bioturbation of R. philippinarum were about 5:18 and 5:1, which were far less than Redfield ratios. Therefore, aquaculture of R. philippinarum should promote BSi biodeposition to sediment and reduce the nutrient ratios of Si:N and Si:P in seawater of Jiaozhou bay.