Distribution,export And Transformation Of Particulate Phosphorus In Daya Bay And Jiaozhou Bay

Phosphorus is a necessary element to sustain marine ecosystem and partly aaffects the ability of oceans to sequestrate CO2.There are many P-limiting areas around the world,leading P to be one of the most concerned elements in marine biogeochemical cycle for the last decades.To date,most studies have focused on the dissolved phosphate concentration and its impacts on primary production,revealing the importance of phosphorus to marine ecosystem.However,there has been a few researches on bioavailable particulate phosphorus.The scenario of the high primary production observed in some marine environments with low phosphate and high particulate phosphorus indicates that particulate P(TPP)might play an important role in supporting primary production activities.To investigate the cycling of TPP and its influence on nutrient structure,a long-term P-limiting bay(Daya Bay)and a seasonally P-limiting bay(Jiaozhou Bay)were selected in this study.Using SEDEX method,five P fractions,i.e.,loosely absorbed phosphorus(Lab-P),ferric-bound phosphorus(Fe-P),authigenic carbonate fluorapatite,biogenic apatite and calcium carbonate associated P(CFA-P),detrital apatite(Detr-P)and refractory organic phosphorus(Org-P),were examined in the spring,summer,autumn and winter.The export(or deposition)and resuspension fluxes of five fractions were calculated based on an optimized 234Th-238U model.Further,we examined the influence of particle dynamics on nutrient structure and the transformation between bioavailable particulate phosphorus(BAPP)and dissolved inorganic phosphorus(DIP).The results revealed the very important role of particulate P in affecting the nutrient structure.On the global scale,both Daya Bay and Jiaozhou Bay are characterized by low phosphate and high particulate phosphorus.In Daya Bay,the highest TPP concentration occurred in the summer.Lab-P and Org-P were the two main fractions of TPP,accounting for 39±16%and 35±12%respectively.Detr-P represented the least fraction.Lab-P was controlled by primary production activities in the spring,summer and winter,while it was closely related with terrigenous inputs in the autumn.Org-P was controlled by river input and sediment resuspension in the autumn and winter respectively.Fe-P was mainly from river and sediment.CFA-P was dominated by biogenic apatite in the spring,riverine calcium carbonate bound phosphorus in the summer,authigenic carbonate fluorapatite in the autumn and sediment input in the winter.In Jiaozhou Bay,there was no obvious seasonal variation in the TPP concentration.Org-P was the main fraction of TPP,contributing 46±8%,and Detr-P only made up 5.1±3.6%of TPP.Phytoplankton activities regulated Lab-P all the year round,while Fe-P,CFA-P and Org-P were mainly from re-suspended fine sediment in the spring,autumn and winter.The averaged TPP export fluxes in the spring,summer,autumn and winter were 412 ?mol m-2 d-1,514 ?mol m-2 d-1,267 ?mol m-2 d-1 and 316 ?mol m-2 d-1 respectively,and the annual average flux was 384 ?mol m-2 d-1.The CFA-P flux in the summer was the highest,and comparable in other seasons.In Jiaozhou Bay,the TPP export fluxes,with little seasonal variation,ranged from 11 ?mol m-2 d-1 to 348?mol m-2 d-1 with the average of 118 ?mol m-2 d-1.The export fluxes of Lab-P,CFA-P and Org-P in Daya Bay showed significant correlations with their stocks in seawater,indicating the regulation of their stocks on removal.Based on these relations,the residence time of particulate P was estimated to be 2-3 days,reflecting that a rapid recycling of P.These results implied that quick recycling might be one of the mechanisms sustaining the high primary production in Daya Bay.The annual TPP export fluxes were 1176±70 t yr-1 and 393±14 t yr-1 in Daya Bay and Jiaozhou Bay respectively.The annual resuspension fluxes of TPP were 936±57 t yr-1 and 167±9 t yr-1.Overall,the total input of P balanced its removal in the two bays,suggesting a nearly steady state given phosphorus.The TPP flux,crossing the water-sediment interface,was 1-2 orders of magnitude higher than other input/removal fluxes,such as the river discharge,atmospheric deposition,submarine ground discharge and interstitial water diffusion,supporting the importance of particulate P for our understanding of P cycling in coastal environments.The BAPP stock was about 2.44 times of DIP in Daya Bay,while they were comparable in Jiaozhou Bay.Thus,the transformation between BAPP and DIP would change the nutrient structure to a large degree.The relations between the solid-liquid partition coefficients of Lab-P,Fe-P and the content of Chl.a in particulate(NChl.a)indicated that Lab-P and Fe-P would release phosphate into seawater when NChl.a was higher than 3.8 mg g-1 in Daya Bay and when NChl.a was higher than 1.0 mg g-1 in Jiaozhou Bay.In Daya Bay,the export flux ratio of particulate nitrogen(PN)to BAPP did not show evident seasonality with the average of(11.5±3.9):1,which was a little lower than Redfield ratio.Except autumn,the deposition flux ratio of PN:BAPP averaged(11.6±4.4):1 in Jiaozhou Bay.In terms of Redfield ratio,particle sinking seemed to remove more P than N.Over a long timescale,this process would result in the P-limiting scenario as observed in Daya Bay.