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Chapter 19

Chapter 19

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Biological Sciences
Marc Cadotte

Chapter 19  Originally thought that physical environment at depths of the ocean were like desert because it is completely dark and extremely high pressure o When hydrothermal vents were discovered, found that areas around the hydrothermal vents were teeming with life  Areas with vents are only decades old so how did organisms get nutrients for food?  Water from vents is rich in sulfides as well as heavy metals which inhibit metabolic activity in most organisms  Two mysteries from hydrothermal vents: o What is source of energy that sustains organisms o How do the organisms tolerate high concentrations of potentially toxic sulfides in water INTRODUCTION  Lindeman grouped organisms in a lake into categories based primarily on how they obtained their energy o First in area of ecosystem science  Term ecosystem first coined by A.G. Tansley to refer to all components of an ecological system PRIMARY PRODUCTION  Refers to energy derived from fixation of carbon during photosynthesis and chemosynthesis  Represents important energy transition for light energy from sun into chemical energy that can be used by autotrophs and consumed by heterotrophs  Primary productivity: rate of primary production Gross primary production is total ecosystem photosynthesis  Gross primary production (GPP): amount of carbon fixed by autotrophs in ecosystem o In terrestrial ecosystems I equivalent to total of all plant photosynthesis  GPP controlled by rate of photosynthesis and leaf area index (leaf are of plants per unit of ground area)  Incremental gain in photosynthesis for each added leaf layer decreases because lower leaf layers are shaded by the higher ones  Plant uses approximately half of the carbon it fixes by photosynthesis in cellular respiration to support biosynthesis and cellular maintenance o All living plant tissues lose carbon via respiration but not all acquire carbon via photosynthesis o Plants that have large proportion of nonphotosynthetic stem tissue tend to have higher overall respiratory carbon losses than herbaceous plants  Plant respiration rates increase with increasing temperatures Net primary production is the energy remaining after energy loss  Net primary production (NPP)= GPP – respiration o Amount of energy captured by autotrophs that result in increase in biomass (living plant matter) o Also represents total input of carbon into ecosystems  Carbon not used in respiration can be allocated to growth and reproduction, storage and defense against herbivory o Plants respond to varying environmental conditions by allocating carbon to growth of different tissues  Plants’ allocation of NPP to growth of leaves, stems and roots is balanced to maintain supplies of water, nutrients and carbon to match plant’s requirements o Tend to allocate most NPP to tissues that acquire resources that limit their growth  Allocation of NPP to storage compounds provides insurance against losses of tissue to herbivores and disturbances NPP changes during ecosystem development  Most ecosystems have highest NPP at mid-successional stages o Proportion of photosynthetic tissues, plant diversity and nutrient supply tend to be highest at this stage  NPP decreases in late successional stages but old-growth ecosystems have large pools of stored carbon and nutrients NPP can be estimated by number of methods  Measuring NPP for ecosystem is importance because changes in primary productivity can be symptomatic of stresses Terrestrial ecosystems:  Methods for estimating NPP in forest and grassland ecosystems are best developed due to economic importance for wood and forage production o Traditional techniques measure increase in plant biomass during growing season by harvesting plant tissues in experimental plots, measuring biomass and scaling results up to ecosystem level  Corrections must be made for tissue loss due to herbivory and mortality  Measuring allocation of NPP to growth belowground is more difficult because root growth is more dynamic than growth of leaves and stems o In some plants, roots grow faster than stems or transfer carbon into mycorrhizal or bacterial symbionts o Harvests for measuring root biomass must be more frequent o Use of minirhizotrons (underground viewing tubes outfitted with video cameras) led to advance in understanding belowground production processes  Labor-intensive and destructive, harvest techniques are impractical for estimating NPP over large areas or biologically diverse ecosystems  Alternatives to harvesting but with less precision: o Remote sensing o Atmospheric CO2 measurements o Combination of data collection, modeling of plant physical and climatic processes to accurately estimate actual fluxes of carbon  Concentration of chlorophyll in plant canopy provides proxy for photosynthetic biomass that can be used to estimate GPP and NPP o Measured via remote sensing that rely on reflection of solar radiation o Can overestimate NPP if vegetation is no physiologically active like boreal forests in winter  NPP can be measured via GPP and plant respiration o Involves measuring change in CO2 concentration in closed system  Net ecosystem production/NEE: net exchange of CO2 between GPP and total respiratory release o More refined estimate of ecosystem carbon storage than NPP  Another method for estimating NEE uses measurements of CO2 and microclimate at various heights throughout plant canopy and into open air above canopy o Known was eddy covariance Aquatic ecosystems  Dominant autotrophs in both freshwater and marine ecosystems are phytoplankton o Shorter lifespan than plants so biomass at any given time is very low compared with NPP  Rates of photosynthesis and respiration measured in water samples collected in bottles and incubated at collection site with light and without light o Difference between two rates is equal to NPP  Remote sensing of chlorophyll concentration in oceans using satellite-based instruments provides good estimates of marine NPP REMOTE SENSING  Technique that takes advantage of light reflection and absorption to estimate density of composition of objects on Earth’s surface  Ecologists use remote sensing to estimate NPP by taking advantage of unique reflectance pattern of chlorophyll-containing plants, algae and bacteria  Normalized difference vegetation index: uses differences between visible-light and near- infrared reflectance to estimate density of chlorophyll o NDVI=(NIR-red)/(NIR + red) where NIR is near-infrared wavelength and red is red wavelength band o Coupled with estimates of efficiency of light absorption to estimate photosynthetic CO2 uptake  Remote sensing of light reflectance from Earth’s surface and atmosphere can be done at large spatial scales using satellites ENVIRONMENTAL CONTROLS ON NPP NPP in terrestrial ecosystems is controlled by climatic factors  NPP increases as average annual precipitation increases up to a maximum of 2, 400 mm per years after which it decreases because: o Cloud cover over long periods lower available sunlight o High amounts of precipitation leach nutrients from soils o High soil water content results in hypoxic conditions that cause stress for both plants and decomposers  NPP increases with average annual temperature o However loss of carbon via respiration also increases at warmer temperature so NEE may potentially decrease  Carbon sinks: greater GPP than carbon loss due to respiration
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