| As one of the most carbon-rich ecosystems,mangroves have high carbon sequestration efficiency.However,carbon buried in mangroves can be exported to coastal waters through submarine groundwater discharge?SGD?.This subsurface pathway is significant but often overlooked.Therefore,this dissertation took two typical mangrove bays,the Zhenzhu Bay and Maowei Sea,of Beibu Gulf as study areas.Using 222Rn tracer,we estimated the SGD rate and its associated carbon fluxes.Then,the carbon budget in each area was obatined.Combining global literatures'data,the influences of rainfall and tide on SGD in mangrove systems were discussed.In addition,based on ultraviolet absorption and three-dimensional fluorescence technology,the spatiotemporal characteristics and biogeochemical behaviors of chromophoric dissolved organic matter?CDOM?in submarine groundwater were also discussed.The main conclusions are as follows:1.Through 27-hour continuous field observation and collection of coastal submarine groundwater?well water and pore water?and river water in January 2019 in the Zhenzhu Bay,we obtained the concentrations of 222Rn,dissolved inorganic carbon?DIC?and dissolved organic carbon?DOC?in various water bodies,established the sources and sinks of 222Rn,and caclutaed the SGD rate and the budgets of DIC and DOC in the bay.The results showed that the 222Rn activities in the submarine groundwater ranged from 1.06×103 Bq/m3 to 8.05×103 Bq/m3,with a mean value of?3.23±2.89?×103 Bq/m3?n=5?,which was significantly higher than that in river water?640±120 Bq/m3,n=1?and surface seawater?151±84 Bq/m3,n=55?.The average concentrations of DIC?1.39±0.60 mmol/L,n=5?and DOC?0.11±0.06 mmol/L,n=5?in submarine groundwater were also higher than those in river water?DIC:0.13 mmol/L,DOC:0.08 mmol/L?.In the Zhenzhu Bay,the inputs through SGD and flood tide were the main sources of 222Rn,while the outputs through mixing loss and ebb tide were the main sinks of 222Rn.Using the 222Rn mass balance model,the SGD rate was estimated to be 0.36±0.36 m/d,which was at a high level in the global mangrove systems probably due to the high tidal range in this bay.Combined with the concentrations of DIC and DOC in the submarine groundwater end-members,the SGD-derived DIC and DOC fluxes were estimated to be?2.41±2.63?×107 mol/d and?1.96±2.20?×106 mol/d respectively,which were much higher than local river inputs?DIC:1.10×105 mol/d,DOC:6.98×104 mol/d?.In addition,carbon budgets confirmed that SGD-derived carbon was the most important carbon source in this bay,accounting for 91%?DIC?and 89%?DOC?of the total inputs,respectively.The carbon exported by SGD was an important component of the carbon budget,and played an important role in biogeochemical cycle in the coastal zone.2.In January 2019,the 222Rn activities in the submarine groundwater ranged from 1.47×103 Bq/m3 to 6.37×104 Bq/m3 in the Maowei Sea,with a mean value of?1.42±1.81?×104 Bq/m3?n=14?.It was significantly higher than that in river water?330±35 Bq/m3,n=2?and surface seawater?104±19 Bq/m3,n=40?.The average concentrations of DIC?1.11±0.91 mmol/L,n=14?and DOC?0.09±0.08 mmol/L,n=14?in submarine groundwater were also higher than those in river water?DIC:0.28±0.18mmol/L,DOC:0.07±0.01 mmol/L,n=2?.In the Maowei Sea,the main sources and sinks of 222Rn were the same as those of Zhenzhu Bay.Based on the 222Rn mass balance model,the SGD rate of Maowei Sea was estimated to be 0.16±0.15 m/d during January2019.Comparing the previous studies of different seasons in this region,it was found that there was no obvious seasonal variation of the 222Rn activities in submarine groundwater,but the SGD rate in flood season?0.36 m/d?was2 times higher than that in dry season?0.16 m/d,0.20 m/d?,and it was significantly positively correlated with rainfall.Combining the SGD rates in global mangrove ecosystems with related rainfall and tidal range data,it was found that SGD rates were jointly controlled by rainfall and tidal range.In the same study area,tidal range changed little in defferent seasons?<1m?,the SGD rates were mainly controlled by seasonal rainfall,and the ratio of SGD rate in flood season to dry season was generally between 1.6-2.3;In different study areas,the SGD rates tended to be higher where the tidal range was larger.In the Maowei Sea,the SGD-derived DIC and DOC fluxes were estimated to be?2.85±2.97?×107mol/d and?2.6±3.08?×106 mol/d,which were at least 12 times and 1.3 times higher than river input,respectively.Carbon budget confirmed that SGD-derived carbon is an important carbon source in the Maowei Sea,with 79%?DIC?and 29%?DOC?of the total input,respectively.However,the proportion of SGD-derived DOC flux was less than that in the Zhenzhu Bay.3.Through the analysis of ultraviolet absorption and three-dimensional fluorescence in submarine groundwater?well water and pore water?,river water,surface seawater samples in the Zhenzhu Bay?May-June 2018 and January 2019?and the Maowei Sea?January 2019?,the spatiotemporal characteristics and biogeochemical behavior of CDOM were discussed.The results showed that the spatial and temporal discrepancies in DOC and the absorption characteristics of CDOM in submarine groundwater were siginificant.Firstly,due to soil leaching,pore water in mangroves had higher concentration of DOC?234±77?mol/L,n=10?,higher content of CDOM(reflected by higher a280,10.6±6.6 m-1),higher molecular weight(reflected by lower S275-295,0.0170±0.003 nm-1)and higher degree of aromatization(reflected by higher SUVA254,4.93±1.50 m2/g-C).And owing to larger runoff,higher temperature and more active microbial activities in flood season,these absorption characteristics were higher than those in dry season.Most well waters had low concentration of DOC?54±45?mol/L,n=13?and low content of CDOM(a280=3.30±3.99 m-1),but characterized by higher molecular weight(S275-295=0.0164±0.003 nm-1)and aromatization degree(SUVA254=5.31±3.37 m2/g-C).Besides,well waters showed obvious spatial differences,which may be due to the different effect of residence time,river runoff,well depth and geological conditions.As for pore water in the Zhenzhu bay beach,the accumulation of DOM with high molecular weight and high degree of aromatization in the bottom layer were observed in different seasons.Through parallel factor analysis?PARAFAC?,we found the fluorescent components C1 and C2 were two humic-like peaks with similar behaviors in the two study areas.In the Zhenzhu Bay,where the SGD-derived DOCcontributed significantly?89%of the total input?,C1 and C2 in submarine groundwater were main sources of these two components in adjacent sea.However,this phenomenon was not obvious in the Maowei Sea,which may because of the less proportion of SGD-derived DOC in the Maowei Sea.C3 was tyrosine-like component mainly derived from terrestrial organic matter and/or in situ production of DOM.Its behavior showed no obvious seasonal variations while large spatial difference existed.C4 may be a low-excited tyrosine-like component related to microbial activities and aquaculture activities.C4 derived by SGD had a weak effect on the offshore water in the two study areas.Therefore,the refractory carbon pool transported by the mangrove ecosystems through SGD was one of the carbon sink in adjacent sea. |
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