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Transcript
high productivity in the mid- to high latitudes. Why? low productivity in the low latitude open ocean; subtropical gyres are “biological deserts” Why? Amazon River coastal upwelling equatorial upwelling so, where would you predict the highest primary productivity? along continental margins, especially in coastal waters Georges Bank “north wall” of the Gulf Stream What relationship, if any, do you see between chlorophyll abundance (top panel) and sea surface temperature (bottom panel)? Northern Hemisphere summer subtropical gyre “Sargasso Sea” cooler sea surface temperatures = higher productivity cooler sea surface temperatures = weaker thermocline 1 Low Latitudes High Latitudes Temperature cold 32 41 50 59 68 77 86 °F 0 5 10 15 20 25 30 °C 0 41 50 59 68 77 86 °F 0 5 10 15 20 25 30 °C 0 EUPHOTIC ZONE summer 100 200 PHOTIC ZONE winter strong thermocline 100 = strong density gradient & barrier to nutrient diffusion 200 the presence or absence of a strong thermocline (including the development of a seasonal thermocline in the mid-latitudes) seasonal changes in solar radiation (angle of incidence) weak thermocline = weak density gradient which does not obstruct nutrient diffusion NUTRIENTS NUTRIENTS (from decomposition of organic matter by bacteria) 300 300 400 (from decomposition of organic matter by bacteria) 400 The thermocline can be an effective barrier to diffusion and vertical advection of nutrients between nutrient-rich deep waters and the sunlit surface waters High Latitudes (Polar) Temperature cold high productivity solar-limited (in meters) solar-limited warm 32 41 50 59 68 77 86 °F 0 5 10 15 20 25 30 °C think of the thermocline as a “density doorway” Mid-Latitudes (Temperate) high 0 (short-lived peak) Water Depth Biomass (abundance of organisms) high seasonal factors that influence primary production across latitude 100 weak thermocline year-round (even in summer) 200 Biomass (in meters) Water Depth EUPHOTIC ZONE Seasonality warm 32 high productivity spring "bloom" solarlimited nutrientlimited (low sun angle) (thermocline) Temperature cold 0 renewed productivity warm 32 41 50 59 68 77 86 °F 0 5 10 15 20 25 30 °C winter (weak thermocline) (in meters) warm (abundance of organisms) cold Water Depth Temperature 100 spring & fall summer (strong thermocline) 200 low low winter spring summer fall 300 productivity is not limited by nutrients “density doorway” is open productivity is limited by available solar radiation angle of solar incidence too low for much of the year short-lived episode of productivity when solar radiation is available 24 hours/day winter spring summer fall 300 winter: nutrients are available (doorway open), but not enough solar radiation spring: nutrients and solar radiation are available = high productivity (“spring bloom”) summer: solar radiation available, but nutrients cut-off by seasonal thermocline (doorway closed) fall: break-down of thermocline = renewed productivity 2 Low Latitudes (Tropics) warm 41 50 59 68 77 86 °F 0 5 10 15 20 25 30 °C Mid-latitude seasonal cycle nutrient-limited year-round (except where there is upwelling) (in meters) Water Depth 0 (abundance of organisms) Biomass high Temperature cold 32 100 200 strong thermocline year-round low 300 winter spring summer fall always enough solar radiation, but nutrients cut-off from below by the strong thermocline (“density doorway” closed tight) except where upwelling occurs (coastal & near equator) coral reefs are an exception – despite very low nutrients in the water, high productivity is driven by highly efficient nutrient recycling Angle of solar radiation controls the development of the seasonal thermocline and the depth of light penetration for photosynthesis. Jan.Jan.-March (1978(1978-1986) Nutrient Cycling organic matter in the ocean: rain of organic matter from the surface waters terrigenous organic matter from the continents AprilApril-June (1978(1978-1986) heterotrophic bacteria break down the organic matter nutrients are recycled back to there useable inorganic form these regenerated nutrients become “trapped” in the deep waters below the pycnocline (thermocline) except where upwelling occurs upwelling provides a mechanism to deliver nutrients back to the sunlit surface waters. 3 Overview of nutrient distribution Continental Margin Open Ocean Surface Waters scarce nutrients = low biomass moderate to abundant nutrients = moderate to high biomass (except where upwelling) (especially where coastal upwelling) warm, nutrient-depleted surface waters Continental Margin moderate to abundant nutrients, moderate to high biomass (especially where/when coastal upwelling occurs) Continental Shelf scarce nutrients, low biomass (except areas of upwelling) (food for benthic organisms) Co shelf edge Open Ocean Surface Waters rain of organic matter oceanic divergence and upwelling: abundant nutrients, high biomass Continental Margin coastal upwelling results in abundant nutrients,high biomass Wa ter Depth (in meters) 0 photic zone aphotic zone (no light below about 200 m) nti lo lS nta ne terrigenous sediments mixed with organic matter (food for benthic organisms) pe Spreading Center Co ntin enta l Pycnocline (Thermocline) cold, nutrient-rich deep waters ("nutrient sink", "storehouse") 100 note change 500 in scale 1000 2000 3000 4000 5000 Ri se Abyssal Pla ins Annual mean sea surface temperature 6000 Marine life is concentrated in sunlit surface waters where photosynthesis occurs; however, vast areas of the surface ocean are depleted in nutrients required for photosynthesis due to the presence of a pycnocline (thermocline) - except where upwelling occurs The subtropical gyres are “biological deserts” very little dissolved nutrients make it out into the open ocean surface waters Annual mean phosphate at the surface Note the scarcity of nutrients where surface waters are warm and well stratified (the thermocline is a barrier to nutrient diffusion to surface waters). Annual mean nitrate at the surface Bay of Fundy, film: “Where the Bay Meets the Sea” 4