A Comparison of Terrestrial and Marine Primary Productivity

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

/

Sea

Majority of primary productivity on land comes from larger plants like grasses or trees / Majority of primary producers are microscopic
Much primary productivity is inedible or indigestible: thus, most enters the decomposer cycle / A major fraction of primary productivity is consumed, digested, and assimilated: little enters the decomposer cycle
Productivity ranges from 0 to 3500 g C/m2/yr; terrestrial productivity is in general always higher / Productivity ranges from 50-600 g C/m2/yr
Plants have a greater biomass, slower growing and use more of their production for respiration; thus they have lower P/B ratios--about 0.5-2.0 / Production to biomass ratio (P/B) is higher in the marine environment--about 100-300
Animals are not the dominant group on land; plants are; thus while the terrestrial realm contributes more than 50% of the primary production, it contributes less than 50% of the secondary production / Secondary production in the sea is higher; animals are the dominant group in the sea

Terrestrial and Sea Comparison of Primary Productivity

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

IPCC, Climate Change 2001: Scientific Basis, 4

Ocean Color from Space

Global Productivity and Carbon Cycles

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