“Carbon  cycle  and  multiple  stresses  at  the  land-­‐ocean  interface:  biogeochemical  perspectives”  	  Wei-­‐Jun  CAI  The  University  of  Delaware  School  of  Marine  Science  and  Policy  Newark,  DE    19716  E-­‐mail:   wcai@udel.edu    Organic	   matter	  is	  synthesized	  in	  terrestrial	  biospheres	  and	  is	  transported	  to	  the	  ocean	  along	  a	  leaking	  pathway.	  Subsidized	  by	  land	  carbon	  and	  nutrient	  export,	  the	  coastal	  ocean	  is	  a	  site	  of	  intense	  carbon	  and	  nutrient	  cycling,	  generally	  net	  heterotrophic,	  and	  a	  source	  of	  carbon	  dioxide	  (CO2)	  to	  the	  atmosphere	  during	  preindustrial	  time.	  Human	  actions	  have	  altered	  the	  carbon	  cycle	  and	  fluxes	  in	  coastal	  waters	  and	  turned	  the	  coastal	  ocean	  into	  a	  sink	  of	  CO2	  due	  to	  increased	  atmospheric	  CO2	  concentration	  and	  anthropogenic	  carbon	  and	  nutrient	  export	  from	   the	   land.	   Synthesis	   of	   coastal	   ocean	   CO2	   data	   collected	   in	   the	   past	   three	   decades	  suggests	   that	   the	   air-­‐sea	   CO2	   gradient	   has	   changed	   sign	   (from	   source	   to	   sink)	   and	   is	  increasing,	  making	  the	  coastal	  ocean	  an	  increasingly	  important	  sink	  of	  atmospheric	  CO2	  and	  likely	   a	   source	   of	   CO2	   to	   the	   open	   ocean	   via	   lateral	   transport.	   In	   recent	   decades,	   coastal	  waters	   have	   faced	   multiple	   anthropogenic	   and	   climate-­‐induced	   stressors	   including	   warming,	  sea	   level	   rise,	   eutrophication,	   and	   bottom	   water	   hypoxia.	   More	   recently,	   ocean	   acidification	  (OA)	  has	  emerged	  as	  a	  new	  stressor.	  While	  OA	  has	  been	  primarily	  considered	  an	  open	  ocean	  problem,	   it	   manifests	   more	   dramatically	   in	   coastal	   waters	   as	   it	   interacted	   and	   often	  synergized	  with	  other	  stressors,	  like	  eutrophication.	  This	  lecture	  will	  review	  carbon	  transport	  and	   transformation	   along	   the	   aquatic	   pathway	   from	   the	   land	   to	   the	   ocean,	   in	   particular,	   the	  cycling	   and	   fluxes	   of	   carbon	   in	   coastal	   waters,	   with	   an	   emphasis	   on	   biogeochemical	  understanding	  and	  perspectives.	  I	  will	  explain	  the	  carbon	  cycle	  and	  coastal	  acidification	  from	  first	  principles	  and	  examples	  from	  North	  American,	  European	  and	  Asian	  coastal	  waters.	  	  	  	  Under	   the	   current	   climate	   and	   global	   change	   context,	   rapid	   industrialization	   and	  urbanization	   in	   the	   Asia	   and	   Oceania	   regions	   have	   resulted	   in	   many	   challenges,	   as	   well	   as	  opportunities	   for	   ecosystem	   and	   biogeochemical	   research.	   	   I	   will	   analyze	   some	   of	   these	  new	  challenges	  in	  the	  region	  and	  emphasize	  that	  comparative	  researches	  with	  “old”	  issues	  in	  the	  western	   countries	   will	   be	   highly	   beneficial	   to	   our	   understanding	   of	   carbon	   cycle,	  environmental	   changes,	   and	   ecosystem	   responses.	   One	   example	   is	   the	   more	   recent	  development	   of	   eutrophication	   in	   surface	   water	   and	   hypoxia	   and	   acidification	   in	   bottom	  water	  of	  the	  East	  China	  Sea	  shelf	  off	  the	  Changjiang	  River,	  versus	  a	  similar	  but	  “old”	  problem	  in	   the	   Northern	   Gulf	   of	   Mexico	   off	   the	   Mississippi	   River	   and	   many	   eutrophication	   and	  hypoxia	   locations	   in	   European	   coastal	   waters.	   Another	   good	   example	   comes	   from	   the	  Delaware	   River,	   USA	   and	   the	   Pearl	   River	   in	   China.	   In	   both	   cases,	   deoxygenation	   and	  acidification	  occurred	  as	  a	  result	  of	  human	  activity,	  but	  with	  a	  half	  century	  of	  time	  lag	  (first,	  point	   and	   non-­‐point	   city	   sewage	   discharge	   into	   the	   estuaries,	   and	   then,	   a	   gradual	   water	  quality	   improvement	   through	   better	   sewage	   treatment	   facilities).	   Such	   geographic	   and	  temporally	   contrasting	   comparative	   studies	   will	   provide	   excellent	   opportunities	   for	   us	   to	  understand	  the	  carbon	  cycle	  and	  biogeochemistry	  and	  the	  interaction	  of	  multiple	  stressors	  from	  human	  actions	  and	  climate	  changes.