1Announcements• Presentation group on Ecol Impacts -see me please• Exam grading• Upcoming - Julie is away Nov 12-17.Ocean BGC and climate trends• How is the ocean N cycle changing now?• Multiple effects of global change onocean carbon uptake• Why is the Southern Ocean soimportant??Thanks to Joellen Russell for this and other figures!Ocean N cycle -coastal issues• Increased nitrate in runofftriggers coastal eutrophication:productivity blooms that lead toanoxia when organic matterdecays• “Dead zones” common in areasoffshore major agriculturaldrainages, e.g. Gulf of Mexico– No bottom-dwellers• Anoxia = denitrifying bacteriaflourish; N2O production• NO3 forms acid - if not utilized,will influence pH alsoNarragansett Bay case study• Decline in PP over past 40 yr– Warming? Clouds?• Reduced food for denitrifyingbacteria– Convert NO3 to inert N– Choosy eaters - consume freshorganic• Replaced by N fixers, which addmore nitrate to system• Excess NO3 not in seds orstimulating PP, so must be flushedout to sea (or stimulatingdenitrification?)• There’s a lot we do not know aboutcoastal N cycle, but the fluxes andvariability are far larger thanexpected.2• NPP declines• Benthic N fixers takeover from denitrifiers• Net result isincreasing NO3production• Loss of couplingbetween pelagic andbenthic N cyclesOcean models of carbon cycleINCLUDE:• Biology• Circulation• Chemistry• Atmosphericexchange and interactionsFuture ocean carbon sink: details (1) (predictions all model-based; how well do these work?)1. Changing ocean geochemistry: CO2 increase• A. reduced CO3= weakens ocean's buffer capacity– CO2 reacts with CO3= in ocean - this draws down CO3=,buffer weakens - this leaves more C as CO2– C sink weakens• B. reduced CO3= and decreased pH increases CaCO3dissolution and decreases calcification rates of organisms– C sink strengthens– (Negative ecosystem consequences, for corals andother calcifiers, and those that depend on them)Future ocean carbon sink: details (2)2. Changing ocean geochemistry: changes to nutrients?– N is added to ocean by human activity, but size of this asa C sink is small (work it out with Redfield ratios)– Fe - Where limiting, can stimulate productivity and Nfixation - May increase or decrease depending on humanactivity and rainfall.– Changes in C sink uncertain, perhaps positive if dustincreases Fe deposition in key areas such as SouthernOcean3. Ocean warming– CO2 solubility is reduced– C sink weakens3Future ocean carbon sink: details (3)4. Changes in ocean mixing• A. Increased stratification generally– Warming, high-latitude precip ==> reduce vertical mixing(surface waters less dense)– Geochemical consequences:• Reduced downward transport of CO2• Weaker C sink– Biological consequences:• Fewer nutrients brought to surface• Overshadowed by increased efficiency in highly productive areaswhere nutrients are not now limiting (Southern Ocean!)• Organisms also receive longer exposure to light• Net effect is a stronger sink!Ocean stratification?• Oceans are characterized by less dense water overlying more dense water– Warm waters are less dense than cold– Fresh waters are less dense than salty• Vertical mixing (by winds, density driven flow, currents, etc.) has to bestrong enough to overcome this density gradient• Global warming is expected to increase density stratification by warmingsurface (and at high latitudes, increased precip) - pink lineOcean productivity and climate• Within grey lines: oceans are stratified (low mixing) and warm• Climate variability within these regions affects stratification– Warming = enhanced strat, less mixing• More stratification = fewer nutrients and lower productivity (inwarm oceans)– Warming trend = stratification trend = decreasing productivity trend• In high-lat oceans, more strat means more time in euphoticzone, and higher productivity– This study does not show, but inferred from others by DoneyBehrenfeld et al. 2006Globaloceanic NPPNPP within warm,stratified zoneOcean productivity and climate• NPP parallels ENSOvariability– Upwelling in E Pac vsother regions?• NPP tracks warming– (post 1999)• Stratification linkdocumented by anindex of stratification(from models + data)NPP within warm,stratified zoneIndex of ENSOMeasure ofstratificationNPP within warm,stratified zoneBehrenfeld et al. 20064Ocean productivity and climateTropics and mid-latitudes (nutrient-limited)High latitudes (light-limited)Future ocean carbon sink: details (4)4. Changes in ocean mixing• B. Thermohaline circulation weakening– Extent very poorly known (2100: from 0 to 40%weaker)– Less C would be taken deep into ocean– Ocean C sink weakensClimate model predictionsof thermohaline circulation:from no change to largechange!Future ocean carbon sink: details (5)• 5. Other biological changes:– Changing Redfield ratios, changing ecology (speciesabundance), changing ratio of organic to inorganicparticles– Direction and magnitude of all of these is uncertain– change in sink strength = ???Future of the ocean carbon sink(more/less refers to change in amount of atmospheric CO2 ocean can absorb;numbers show estimated % change of ocean sink in 2100)Take these with some skepticism, as they are rough!• Geochemistry– Buffering weakens (≥ 30%)– CaCO3 dissolution strengthens (0.1-10%)• Solubility– Temperature weakens (10%)– Salinity negligible• Stratification– Global• Biotic (SO) strengthens (6-40%)• Geochemical weakens (3-20%)– THC weakens (0-30%-more?)(Values mainly from chapters 3 and 13 of Field and Raupach 2004, The Global Carbon Cycle)5Future of ocean carbon sink - summary• General conclusion: ocean C sink will weaken by 2100• Inorganic chemistry changes are well understood and onbalance, will yield weaker sink relative to modern• Biological changes are uncertain, but appear to strengthensink significantly• Circulation changes are highly uncertain (N Atl., S. Oceanare key regions)• Degree of ocean stratification has implications for both bioticand inorganic feedbacks - and is variable from model tomodel• Critical region for all is Southern Ocean (around Antarctica)What’s up with theSouthern Ocean?• Strong winds = intensemixing; strong circumpolarcurrent• Nutrients exist in measurableconcentrations because:– Wind mixes waters intensely,such that nutrient rich watersare mixed below euphotic