comparison

Greg Dutton

4/18/03

Dr. Hays Cummins

A comparison of Terrestrial and Marine Primary Productivity



Over the semester I committed a great deal of time studying primary production.



During the time several key differences became apparent between marine and terrestrial



primary production. I also realized the lack of comprehensive research comparing the



two. To illustrate this, I tried to look up information to compare them on Google. The



first two sites that came up were a lecture, which I used extensively in my paper, and a



site I had created only several months earlier. In reaction to the interest I had in the topic,



and the lack of knowledge available I decided to set out and try to compare the two on



my own. What follows is that comparison.



The “amount of plant tissue built up by (the) process of photosynthesis over time



is primary production”. Gross primary production (GPP) is the total amount of organic



material produced by photosynthesis. However, net primary production (NPP) represents



the amount of organic material available to support consumers, nonphotosynthetic



protists, and decomposers (Primary). There are two differentiated locations where



primary productivity occurs: marine and terrestrial ecosystems. Due to the multitude of



differences between these two systems, primary productivity in each varies almost as



much as the two systems themselves.



Plant leaves contain tiny pores called stomata, which obtain Carbon Dioxide



(CO2) from the atmosphere through diffusion. Now at the site of photosynthesis, the



CO2 reacts chemically with water and sunlight in a process where chlorophyll acts



together with other pigment, protein, sugar, lipid, and nucleic acid molecules. The



products of photosynthesis include oxygen that is released into the atmosphere, and a

carbohydrate such as sugar glucose. These sugars can be converted to starch for storage



or combined with other sugars to form cellulose, used in the construction of cell walls, or



other specialized carbohydrates. The sugars can also be combined with nutrients such as



phosphorous, sulfur, and nitrogen to create complex molecules needed like proteins and



nucleic acids. In this way, these nutrients can be a limiting factor in plant growth (IPCC).



Several factors influence and limit the primary production in marine and



terrestrial environments. One such factor that limits primary productivity in marine



environments is light. Photosynthesis is only possible when light reaching the production



cell is above a certain intensity. Photosynthesis increases with light intensity up to a



maximum value known as Pmax, which is specific to each species of producer. Beyond



this value, the rate of photosynthesis declines due to lack of light. Light is influenced by



a number of meteorological features such as clouds, and dust, that reduce the amount of



available light. It is also influenced by reflection, scattering, and absorption of various



wavelengths due to the water itself. The point or depth at which productivity exactly



equals respiration is the compensation point. Past this depth there is no net primary



productivity (Primary).



The amount of light also varies with latitude, decreasing from the equator toward



the poles. Several factors such as this affect Marine productivity in different manners



than would influence terrestrial productivity. In example, at polar regions a pulse of



phytoplankton abundance – or primary production- occurs during the summer when light



becomes sufficient for a net increase in primary productivity. At temperate latitudes



primary productivity is usually highest in spring and fall when both sufficient light and



high nutrient concentrations allow plankton blooms to occur. In the tropics, intense

surface heating produces ideal light conditions so that phytoplankton are nutrient-limited



year round, and hence experience only small fluctuations in primary productivity



(Primary).



Nutrients can also be a major limiting factor in marine environments. Primary



inorganic nutrients that are required by phytoplankton are nitrogen and phosphorus.



These nutrients occur in small amounts and are thus limiting factors for primary



productivity. Each species of phytoplankton has a particular response to different



concentrations of limiting nutrients as well as a maximum growth rate. The three main



nutrient level environments are: oligotrophic with low concentrations of essential



nutrients, eutrophic waters contain high nutrients and support high numbers of



phytoplankton, and mesotrophic waters’ nutrient levels are between those of the two



extremes (Primary).



Possibly one of the most crucial and variable factors affecting marine primary



productivity is that of hydrographic conditions. These are factors that move water masses



around in the oceans such as currents, upwelling, and diffusion. Upwelling occurs when



nutrient-rich deep water rises to the surface and replaces surface waters moving away



from shore. Divergence of currents also brings up deeper nutrient-rich water while



vertical mixing brings up nutrients and pushes down phytoplankton. Wind mixing, which



brings nutrients up to the surface, increases from the tropics to the polar regions. Thus



forming an inverse relationship involving the abundance of light and nutrients, which



determines the pattern of production in different latitudes (Primary).



The factors which limit terrestrial primary productivity are even more numerous



and complex than those affecting marine primary productivity. There are many different

types of terrestrial ecosystems and all must be looked at individually with varying levels



of available water and nutrients, differing atmospheric C02, and temperature gradients.



Just varying levels of elevation will change CO2 concentrations, temperature, and



precipitation, all of which affect NPP. Temperature and moisture indirectly influence



NPP through controls over decomposing organic matter, which directly affects nutrient



availability (Perry, 1994). As can be easily seen, relationships in terrestrial ecosystems as



they affect NPP are quite complex.



Even on fairly uniform terrain, soils can vary widely. Depending on specific



effects and resource limitations at a particular site, productivity may either decrease or



increase with disturbances such as landslides, windthrow, fire, or floods. In example,



areas that have experienced frequent or hot fires may have lower nutrient stocks, than an



area with a milder burn pattern. In another situation, an area that experiences windthrow



exposing fresh rock may enhance NPP by increasing nutrient levels through weathering



(Perry, 1994).



A major factor influencing primary productivity in terrestrial environments is that



of community- level effects of silviculture versus monoculture. Silviculture is an



ecosystem in which several species of plants are present. Productivity will be higher in



mixture than monoculture if one or both of two factors is true. First, more resources are



available to the mixture. This can be true due to different species using the resources



more fully than a single species, or from species in the mixture enhancing a limiting



nutrient such as nitrogen- fixing plants. Given that nitrogen is a major limiting factor,



ecosystems with nitrogen fixing plants enhance growth of associated trees, or possibly



even increase total ecosystem net primary productivity (Perry, 1994).

Mixtures may be more productive if different adaptations within the community



allow resources to be used more completely than by a single species. This is the case in



which shade- tolerant and shade- intolerant species are together. In explanation, one can



consider the following formula:



NPP=GPP-R



Where R is respiration. In shade- intolerant species that receive less light, the ability to



maintain positive net photosynthesis is dependant on the respiratory cost of the associated



individual. This renews the idea of compensation point. Shade- tolerant species rely on



their low respiration rate, hence low compensation point, to continue net primary



productivity. At the same time, shade- intolerant species, such as those of a canopy



species in forests rely more heavily on high availability of light (Perry, 1994).



Phytoplankton are microscopic, single-celled aquatic plants that provide the



primary source of food for marine life. Like terrestrial plants, phytoplankton contain



chlorophyll-a and other pigments that absorb sunlight and are the main primary producers



of marine ecosystems. During photosynthesis, they remove dissolved carbon dioxide



from seawater to produce sugars and other simple organic molecules while releasing



oxygen as a by-product. The increasing atmospheric concentration of carbon dioxide,



which may produce a global warming, underscores the additional importance of



phytoplankton to the carbon cycle and the Earth's climate. Only satellite observations can



provide the necessary rapid, global coverage required for worldwide ocean productivity



studies. Due to this, magnitude and variability of primary production are poorly known



on a global scale (Ocean).

Unlike marine ecosystems where the main primary producers are microscopic,



terrestrial ecosystems main primary producers are trees (Primary). The NPP for a forest



consists of four parts: tree tissues including branches, leaves, roots, etc; litter from the



trees; tree tissues consumed by heterotrophs or decomposers; and trees that die (Perry,



1994). Additionally, unlike marine environments where other animals eat much of the



production, in terrestrial ecosystems much of the matter is consumed by decomposers



(Primary). Unfortunately, terrestrial primary productivity resembles marine primary



productivity in that it is difficult to get accurate measurements. Above ground a plot scale



can be used to measure biomass over a certain time period such as a year. It can also be



used to measure harvest vegetation by calculating annual growth of wood, mass of



foliage at peak of annual season, and litter- fall to estimate seasonal loss of aboveground



tissue. One a global scale satellite remote sensing must once again be used as in marine



NPP measurements. Below ground primary productivity is a different story that presents



many more problems. Far fewer studies have been done on below ground NPP than



above ground NPP due to these problems. Sequential coring measures root mass in cores



over a short time interval to minimize underestimates due to turnover. Problems with this



method include destruction to plants measured, and the variability of results. Ingrowth



cores can be used to measure growth of new roots in "empty" or controlled soils over



short time intervals. Problems with this method arise because root behavior is different



in somewhat sterilized conditions than if in natural soils (Global).



Nutrient regeneration is important for both terrestrial and marine ecosystems.



Many different nutrients are necessary to continue net primary productivity. Nutrients in



marine ecosystems move in cycles such that most of the biomass produced by marine

photosynthesis is eventually consumed by herbivores to be converted to more bodies like



carnivores or forms into fecal waste. These bodies/wastes become particles that sink to



depths below the photic zone where they are decomposed by bacterial action thus



releasing phosphates, nitrates, and other nutrients for reuse by the primary producers.



This determines the most ecologically important aspect for the sea community: the rate at



which growth- limiting nutrients are recycled (Primary).



In marine environments, carbon dioxide from the atmosphere is dissolved in the



ocean where it is bound in bicarbonate and carbonate ions that act as a reservoir of CO2.



When CO2 is taken up by photosynthesizers it is converted to organic compounds, which



release more CO2. Respiration by consumers, decomposers, and the producers



themselves break down the organic compounds and make carbon dioxide. It is then made



available to be bound in the reservoir ions and eventually available once again to the



photosynthesizers. Carbon dioxide is lost from the cycle during calcification through



manufacturing skeletons. After death this skeletal material sinks and either becomes



buried in the sediments where is removed from the cycle, or it dissolves and becomes



available for uptake yet again (Primary).



The marine nitrogen cycle is equally if not more complex than the carbon cycle



because nitrogen occurs in several forms that are not easily converted from one to



another. A dominant form is nitrate, which originates from nitrogen in the atmosphere



and is taken up by bacteria and cyanobacteria, or nitrogen fixers, who then convert it to



nitrate. Photosynthesizers then take the nitrate back up. Iron is required in marine



environments to form enzymes used in the conversion of nitrite, and nitrate into



ammonium. Ammonium is then used to make amino acids. Production will not increase

if iron is limited even in the presence of abundant nitrates. Nitrogen-containing urea and



ammonia wastes can regenerate nitrogen, which can then be taken up directly by



phytoplankton. Bacteria carry out the oxidation of ammonia to nitrite, known as



nitrification. Nitrogen also regenerates when the decaying bodies of dead phytoplankton



form reduced nitrogen compounds by decomposers though a process known as



denitrification (Primary).



In most terrestrial ecosystems, nutrient input is far below growth requirements,



thus productivity is limited by the nutrient cycling. Plants take up and release nutrients to



solution, while the decomposers take up nutrients in the liter and eventually release them



again for reuse. Some nutrients may enter the soil directly from the atmosphere just as



they may also be released into the atmosphere directly form the soil. Some nutrients such



as iron can from insoluble combinations and precipitate out of solution only reentering



again very slowly. Nutrients can be lost to streams and runoff, but this flux is minimal in



undisturbed ecosystems (Perry, 1994).



Some plants such as trees have internal nutrient cycling. In this case nutrients



drawn from aging and dead tissues within the tree are then used to support new growth.



This is comparable to the practice of invertebrates to eat their own feces, or decomposers



decomposing other decomposers. Use of this cycle, known as the biochemical cycle,



may provide from 5 to 90 percent of a plants nitrogen, phosphorus, potassium, and



magnesium needed for new growth. The extent to which a nutrient is internally cycled



depends on several factors. First, a nutrients solubility, or ease with which it could be



turned soluble will factor on its frequency of cycling. Next, it’s relative abundance to



other nutrients in the soil, and time of nutrient availability corresponding to periods of

tree growth. In essence, a nutrients timing of availability, and ease of transportation have



major impacts on it’s place in a plants biochemistry (Perry, 1994).



Carbon dioxide can many times be the limiting factor in plants. In some cases



where an excess of CO2 exists, plants will allocate more carbon to roots, thus keeping



below ground resources in equilibrium with the CO2 supply. A high level of atmospheric



CO2 requires less stomata opening to take in the same amount of CO2 thus more efficient



water use since less water is lost through the stomata’s. In high elevations however,



productivity may be limited by CO2. Although CO2 is constant everywhere, the thinner



air in higher elevations creates lower absolute amount of CO2. Even so, productivity



may not actually decline in higher elevation. As elevations increase, water availability



usually does as well, partly due to clouds. In this case stomata’s may be kept fully open



for longer times, thus collecting more CO2. Through these methods and ecological



interactions, plants are able to compensate for changing environmental factors such as



CO2 levels, and elevation (Perry, 1994).



Overall, I feel more extensive research is necessary for both terrestrial and marine



primary productivity. In terms of a comparison between the two, more professional



research needs to be done on the topic, as well as more obtainable resources made



available. Through my own research, I believe terrestrial NPP to be more complicated



than marine NPP due to such factors as elevation, latitude effects, and the internal or



biochemical cycle. Below is a summary of the key differences between marine and



terrestrial Primary Productivity.

Terrestrial and Sea Comparison of Primary Productivity





Terrestrial Sea

Majority of primary productivity on land Majority of primary producers are

comes from larger plants like grasses or microscopic

trees

Much primary productivity is inedible or A major fraction of primary productivity is

indigestible: thus, most enters the consumed, digested, and assimilated: little

decomposer cycle enters the decomposer cycle

Productivity ranges from 0 to 3500 g Productivity ranges from 50-600 g C/m2/yr

C/m2/yr; terrestrial productivity is in

general always higher

Plants have a greater biomass, slower Production to biomass ratio (P/B) is higher

growing and use more of their production in the marine environment--about 100-300

for respiration; thus they have lower P/B

ratios--about 0.5-2.0

Animals are not the dominant group on Secondary production in the sea is higher;

land; plants are; thus while the terrestrial animals are the dominant group in the sea

realm contributes more than 50% of the

primary production, it contributes less than

50% of the secondary production

Table 1: Terrestrial vs. Sea Primary Production. (Source: Primary Production, Website)



Bibliography

Perry, David A. 1994 Forest Ecosystems John Hopkins University Press, Baltimore and

London

Primary Productivity

http://maritime.haifa.ac.il/departm/lessons/ocean/lect26.htm

IPCC, Climate Change 2001: Scientific Basis, 4

Ocean Color from Space

http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/ocdst_history_and_other_bac

kground_info.html

Global Productivity and Carbon Cycles

jan.ucc.nau.edu/~bah/BIO479/Lecture8.pdf