Carbon metabolism in marine ecosystems C.M. Duarte and S. Agusti (1998) J.-P. Gattuso, M. Frankignoulle and S.V. Smith (1999) J.-P.Gattuso, M. Frankignoulle and R. Wollast (1998) P.A. Raymond, J.E. Bauer and J.J. Cole (2000)

Overview Introduction Estuaries Macrophyte-based ecosystems Mangroves Coral reefs

Introduction: main processes Primary prod : CO 2 + H 2 O + light = CH 2 O + O 2 Respiration : CH 2 O + O 2 = CO 2 + H 2 O Calcification : Ca HCO 3 - = CaCO 3 + CO 2 +H 2 O R>GPP : Heterotrophic source of CO 2 GPP>R : Autotrophic sink of CO 2 R R = R P 2/3 : in aquatic ecosystems

Introduction: coastal zone < 200 m, 7% of total ocean Great diversity of primary producers Contributes for: 14-30% to oceanic pp 80% to OM burial 90% to sedimentary mineralization 75-90% to oceanic sink of suspended river load 50% to deposition of CaCO 3

Marine ecosystem: Estuaries 0.38% of total ocean Nutrient input  eutrophication  anoxic parts Nutrient input  turbidity rises  light limitation Benthic contribution to R T 25-50%

Marine ecosystem: Estuaries CO 2 exchange Flux = k([CO 2 ]water – [CO 2 ] air ) k is based on wind speed Fluxes DIC Input flux (mmol s -1 ) = Q(C s – C o ) C o = concentration at zero salinity C s = concentration at seawater end-member

Marine ecosystem: Macrophyte-based ecosystems Macrophyten: sea grasses and macro algae 0.56% of total ocean Biomass of Macrophyten 400x higher than phytoplankton Mostly appear in nutrient-poor and non-turbid areas Seasonal variation C:N:P ratio is ~274:24:1 Human activities are damaging the sea grasses

Marine ecosystem: Macrophyte-based ecosystems Decomposition within the system is an important process (>40% of Pn) Herbivore pressure is significant for macro algae Export is an important process (24-43%) Storage in the sediment is only significant for seagrasses (>15%)

Marine ecosystem: Mangroves Intertidal forests which cover 0.06% of total ocean Environmental forcing and forest type causes variation in metabolism Benthic heterotroph use C from litter fall and deposited phytoplankton Benthic micro-algae production is low, but benthic bacterial productivity is high Nowadays ~50% is destroyed by human

Marine ecosystem: Coral reefs Carbonate structures located near sea level 0.17% of total ocean Dominated by benthic processes Algal dominated R Corg  R Calcification  (compared to coral/algae dominated reef flats) C:N:P ratio is 550:30:1

GPP vs R in different coastal ecosystems

P/R Ratios of different systems