lecs 10-14

productivity - ecosystem ecology

• controls on CO2 uptake

• primary production

• carbon sources and sinks

why care about photosynthesis/CO2 uptake via plants?

• rising CO2 concentrations are contributing to global climate change

• keeling curve interannual variability - more photosynthesis in warmer months, more CO2 sequestered

photosynthesis carbon inputs

• single most important chemical process on earth

• carbon enters ecosystems through photosynthesis

  • C drives all biotic processes

  • accounts for half of organic matter on earth, rest is mostly H and O

photosynthesis mechanism

sunlight + CO2 + water → photosynthesis → glucose + O2

photosynthesis constraints

• water and nutrient uptake

  • water - tension-cohesion theory

• light, CO2 and temperature

tension-cohesion theory of water uptake

• water vapor diffuses out of the stomata (open during photosynthesis)

• water evaporates from mesophyll (main site of photosynthesis) cells

• internal leaf spaces contain high concentration of water vapor

  • water vapor level in external air is generally low

  • when stomata open it causes water vapor to diffuse out of the leaf into external air to balance (transpiration)

• tension pulls water column upward and outward in the xylem (transport tissue) of veins in the leaves → stem → roots

  • this is the direction of the pull

• water molecules form cohesive column in xylem

• water movement is from roots → stem → leaves → out

  • water moves into roots via osmosis

stomata

• tiny openings on the underside of leaf

• open/close based on guard cells

  • two guard cells on each stomatal pore

• control water loss by opening/closing stomata

influence of moisture on plant distributions

• too much water - plants can be O2 limited

• too little water - plants can desiccate

O2 limitation - too much water

O2 diffuses 10,000x slower in water than air

• plant doesnt have access to as much O2 as it normally does on land

• plant growth stagnates

• waterlogging

plant responses to waterlogging/O2 limitation

• pneumatophores - adventitious (extrinsic/not inherent) roots

  • mangroves have roots above the water as well to have access to O2

• aerenchyma tissue - allows for greater exchange of gases btwn shoot and root

  • can be found on stem and root of plants

  • phragmites australis aka common reed - higher part of the tidal zone so they can access O2 when the water rises, probably has aerenchyma tissue on higher part of stem

arid environments - too little water

• retention of H2O vs uptake of CO2 from atmosphere !!!

  • both regulated by stomata

  • CO2 flows in but H2O flows out

  • leads to a trade-off btwn carbon (needed for photosynthesis) and water uptake (evaporates when stomata are open)

• reducing heat loading

  • heat focused onto single area

  • increase surface area for dissipation of convective heat

    • example - increased dense hairs/spines (succulents)

• reducing evaporative losses

  • protect surfaces w thick, waxy cuticle so that water cant just accidentally evap out

  • recess stomates in (hair-filled) pits

• orient leaves to minimize solar gain

  • avoid excess evap

• shed leaves and become inactive during periods of stress

  • drop leaves in the winter

  • periodic droughts in mesas

  • dry seasons in tropical forests

water loss vs carbon gain trade-off

graph conditions:

• 1 - no water deficit

• 2 - mild water deficit

• 3 - extreme water deficit

more transpiration with wetter soil, less as soil dries out

under extreme water deficit/stress:

• stomata close

• CO2 uptake stops → photosynthesis stops

net photosynthesis

• balance of two processes

  • photosynthesis - CO2 into plant

  • respiration - CO2 loss from plant (and into environment)

    • plants release CO2 from all cells into atmosphere as a byproduct of breaking down sugars to produce energy

• net photosynthesis depends in part on light availability

  • saturation level

photosynthesis light availability

more light, more photosynthesis until you reach saturation level

• saturation level - where plant currently is irt saturation

• saturation point - point where no matter how much light plant is exposed to it physically cant photosynthesize any more

• compensation point - photosynthesis = respiration, net zero. above - loss, below - gain

  • respiration line at compensation point

• at any given temp plants are either light-limited (more light more co2) or light saturated (no more co2) - this is before/after saturation point

primary production

• production of organic compounds from inorganic material via photosynthesis + chemosynthesis by autotrophs

  • photosynthesis - plants + algae use light to form chemical bonds in carbohydrates

  • chemosynthesis - some bacteria use specific compounds as sources to transform into chemical bonds

    • common in environments w fluctuating redox potential

      • hudson river marsh

      • H2S → SO4 2- → S (sulfate) - OXIDATION, uses H2S for production of organic material

      • reverse direction is REDUCTION, uses sulfate for respiration

components of PP

GPP - gross primary production, total carbon assimilated by primary producers

R - respiration, energy consumed by producers for maintenance and biosynthesis

NPP - net primary production, carbon accumulated/stored by primary producers

• NPP = GPP - R

patterns of primary production

• seasonal flux in atmospheric CO2

  • high respiration in winter, high photosynthesis in summer

• PP increases w temp + precip

• PP patterns among ecosystems

  • terrestrial

    • swamp + marsh highest

    • desert lowest

  • aquatic

    • algal beds/reefs highest

    • open ocean lowest (less autotrophs)

measuring terrestrial primary production

• harvest techniques

  • determine dry mass accumulated over specific amt of time

  • experimental procedure - n amount of plots, measured from time t to t+1

• gas exchange techniques

  • determine net CO2 uptake in light (NPP), production of CO2 in dark (R) and their sum is GPP

eddy covariance

used to determine whole ecosystem CO2 exchange = net ecosystem exchange (NEE)

harvard forest + hubbard brook flux towers examples of eddy covariance measuring sites

• two pieces of equipment on the tower

• processing of data ends up with approximation of gas/CO2 exchange

• global models + large timescales

measurement of aquatic primary production

problems aquatic systems pose:

• cant use harvest approach, not practical for small organisms like phytoplankton

  • aquatic eddy covariance exists but we dont cover it

solutons/alternatives:

• light/dark bottles may be used to determing O2 changes (production rates) rather than track CO2

what limits primary production

experimental lakes area study

• canadian lakes have higher P associated with higher algal biomass

• whole lake experiments founded in NW ontario, CA 1968

  • fertilized (P added) vs control (standard P) lakes

  • 1970 - similar initally

  • 1975 - big discrepancy btwn lakes, peak of fertilized lake

  • 1980 - still large discrepancy of 4,600 mg/m³ plankton biomass but fertilized lake getting closer to control. last spike of plankton before drop

  • 1982 - close in phytoplankton biomass again, potentially correlated with lower levels of P in the lakes

• long story short this is if you add stuff that makes plankton grow to place where plankton live they will grow

PP limit ocean

most marine productivity occurs near coasts (more producers close to the surface + more producers overall)

• flask experiments w Baltic sea water

  • reveals that N is the most limiting nutrient in marine water

• correlation + whole-lake experiments

  • P limitation of freshwater ecosystems

essentially:

• freshwater - usually P limited

• open ocean - usually N limited

PP limit terrestrial

• measured diameter growth of canopy trees Metrosideros as productivity indicator

  • young soils - N limited

  • intermediate soils - N + P limited

  • old soils - P limited

• amt of P decreases and N increases over time

natural atmospheric CO2 sinks

• plant sequestration/photosynthesis, CO2

• soil sequestration, C (indirect sink)

• ocean uptake

  • direct - photosynthesis

  • indirect - sedimentation at bottom of ocean

who gaf - carbon uptake by plants offsets fossil fuel emissions by ~30%

eddy covariance study ameriflux sites

• 42 sites used

• eddy covariance techniques

• mean annual net ecosystem exchange (NEE) for 2001-2006

  • most ecosystems are sinks rather than sources with the exception of shrublands (short grasses, very few trees)

  • PNW most effective at offsetting fossil fuel emissions by a margin of 200%, then

human alteration of global sulfur cycle

• S emissions

• S deposition

• impacts on ecosystems

acid rain causes

• SOx and NOx emitted to atmosphere as gases

  • SOx and NOx from manmade sources (gasoline from vehicles, coal and oil from power plants etc)

  • NOx from natural sources (lightning)

• SOx and NOx in atm react with + dissolve in water vapor

  • forms H2SO4 (sulfuric acid) and NHO3 (nitric acid) - -ic acid

• H+, SO4 2- and NO3- get deposited on land in rain/snow/clouds/dust

acid rain history

• mid 1870s - acid rain first observed in england, think river thames and industrial revolution pollution

• 1960s - discovered to be major environmental problem

• 1980 - US congress passed Acid Deposition Act

• 1990 - title IV amendment of Clean Air Act to include mandated control of sulfur dioxide (SO2) and nitrogen oxides (NO and NO2) emissions

many states have seen decrease in S deposition but N deposition is still a major problem

sources of anthropogenic SOx emissions to atm

• electrical power plants (coal, oil)

• automobiles (gasoline)

NA emissions of SO2 peaked in 1970s and have decreased significantly since !!

Asia emissions still very high (37ish mil tonnes/year)

S deposition

gaseous + particulate pollutants in atm

• wet - rain and snow

• dry - gases and particles

• neither - clouds and fog

wet deposition of sulfate

• 1988 - lot of wet sulfate ions (SO4 2-) located in eastern US + puerto rico

• 2025 - very little across US

declining over years

wet deposition of S at hubbard brook, NH

declining from ~63 mu eq/L to ~5 over the course of 1965 - 2020

• measured via rain gauge and wet deposition collector

  • rain gauge - measures volume of rainwater, left

  • wet deposition (rain/snow) collector - collects water to measure concentrations of pollutants, right

steps:

• measure volume of rainwater RG

• measure concentration of S in lab from wet deposition sample from WDC

• convert concentration of sulfur to flux of sulfur using

  • concentration of S from WDC * vol of RG precipitation

hubbard brook sulfur decreasing in deposition via precipitation (wet deposition) and stream water !!!

v-notched weirs

measuring streamwater S

v-notch - weirs (stainless steel plates) with a v shape cut into them placed into part of a stream where all the water has to flow through it

• measures volume of stream water = the flow

• from here you measure S concentration from stream water sample

• convert concentration of S into flux of S using stream flow/volume

  • S concentration = mass S/volume streamwater = mu mol S/L

  • S flux = mass/unit area/time = mu mol S/ha/yr

acid rain solved?

no lol. made huge headway/covered most of our bases but some challenges remain (surface acidic waters mobilizes inorganic aluminumm, acidifies waterways, accumulates mercury in fish + sulfate aerosols leading to diminished visibilty and global warming offset)

impacts of acid/sulfur deposition

• acidifies waters

  • depletes soil of base cations (Ca, K, Mg)

  • mobilizes/moves around dissolved inorganic Al

  • acidifies waterways

• surface water acidification → mercury accumulation in fish

• sulfate aerosols

  • diminish visibility/views (think fog)

  • bright side - may offset global warming in the short term

    • reflect sunlight, exist in atmosphere for a brief period (3-5 days) but produced constantly, so overall cooling effect

cation exchange capacity - acid rain

trait of soils around the globe

• clay minerals have (-) surface charge

• hold onto hydronium (H+) that fall in rain/snow or released from natural plant/microbial processes

  • acid rain (wet deposition)

  • respiration (plant/microbial process)

• presence of H+ blocks other (+) ions from binding (K+, Mg+2, Ca+2)

• K+, Mg+2, Ca+2 cant bind to soil so they’re released into the soil solution

• can be taken up by plants later

problem: increased acidity from acid rain makes enhances this process to a level higher than standard bg levels

• cations can be leached out with precipitation and lead to nutrient imbalances in trees aka rain drives these cations off of the soil and the trees end up lacking these nutrients → impedes development

sources of H+ in soil water

• plant roots

  • cation exchange

• nitrification

• acid rain

  • both nfication and acid rain can be increased via atmospheric deposition/acid rain

cation exchange - H+ in soil water through plant roots

same thought process as acid rain cation exchange

• plant roots respire and release CO2

• CO2 dissolves in soil water and converts to H2CO3

  • H2CO3 → HCO3- + H+

cation exchange - nutrient runoff effect on trees

red spruce

• calcium leached from needle membranes (lack of Ca+2 taken up from leaves/needles)

• leads to decreased cold tolerance + increased freezing injury

sugar maple

• calcium and magnesium leached from soil (lack of Ca+2 or Mg+2 taken up from soil)

• aluminum moved up and taken up by tree

• leads to impaired root function + nutrition

multiple stress syndrome

sugar maple decline

• forests in ridgeway township, pennsylvania

• both forests have same level of insect defoliation

• bottom site (represented w yellow dot) has higher concentrations of cations in soil

• found that cation depletion makes trees more susceptible to harm caused by insect defoliation - aka the more cations you have in soil the more hardy the trees are to insects chewing on leaves/needles/etc

acidity on fish community diversity

• adirondack region of NY

• n = # lakes in each pH class (4 to >8 in increments of 0.5)

• long story short higher acidity = lower fish diversity

  • likely due to limiting range of tolerance like how less species can live in extreme environments

ecosystem recovery from acid rain

function of/relies on

• how much S and N deposition in the past

• rate and magnitude of decreases in acid deposition

• how many base cations have been depleted from the soil

• extent of S and N accumulation in soil and the rate at which they’re released through decomposition of SOM

• weathering rate of soil/underlying rock and base cations associated w that weathering rate

• rate of atmospheric deposition of base cations

clean air act accomplishments since 1990 summary

• national electric utilities have met/surpasses emission targets set by 1990 CAA title IV amendments

• HOWEVER !!! targets are still not sufficient for full recovery of sensitive ecosystems

  • need lower SOx and NOx emissions bc of poor buffering capacity of acid sensitive/low base cation available ecosystems

general ecosystem nutrient model for N cycle

atmosphere

• solar energy

plants

• solar flows into autotrophs/plants

soils

• autotrophs flow into heterotrophs - consumption

• heterotrophs flow into detritus/SOM - mortality/death

• SOM flows into inorganic nutrients (N, P) - decomposition

• inorganic nutrients flow into autotrophs - nutrient uptake through soil

N cycle inputs

atmosphere → plant

• CO2 enters plant via photosynthesis

atmosphere → soil

• N from atm fixed through S or N deposition, results in leaf/root literfall and animal/microbial mortality

plant → soil

• leaf/root literfall results in soil organic matter (SOM) accumulation

soil processes

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N cycle outputs