Primary Productivity

The flow of energy through any ecosystem starts with the fixation of sunlight by plants and other autotrophic organisms. In this way the plant accumulates energy and this energy is called primary production. The rate at which this energy accumulates is called primary productivity. The total energy accumulated is gross primary production, however, since plants use some of this energy themselves, it is not all available for the food web. The difference between what is accumulated and what is available for the food web is called net primary production expressed in

kilocalories or grams m-2 y-1 or kcal or g/m2/y. This is measured by sequentially measuring growth of the biomass over time by marking the plants somehow, or measuring a total at the end of the growing season. Alternatively you can measure oxygen production or CO2 consumption both of which equal grams C produced. (experiment possiblities!)

In general, swamps and marshes have the highest primary production of all the world's ecosystems. Primary production of all wetland types varies from 600-2000 gC/m2/y.

To review a little:

In general, the "openness" of a wetland to hydrological fluxes is probably one of the most important determinants of primary productivity. So wetlands that are stagnant are less productive than those that flow or are open to flooding rivers. This makes sense because a flow-through system constantly gets more nutrients. This isn't 100% though because wetlands get most of their nutrients from recycling rather than from the outside. This is what allows them all to be fairly productive.

Salt Marshes:

These tend to be the most productive ecosystems in the world. Estimates of the southern coastal plain of the U.S. have topped 8000 gC/m2/y through the combined efforts of marsh grass, mud algae, and phytoplankton in the tidal creeks. The southern marshes do better than the northern ones partly because of the greater influx of solar energy and longer growing season, and partly because of the nutrient rich sediments carried by rivers in that region.

Low or intertidal marshes are more productive than high marshes because of the increased exposure to tidal flow.

Belowground production is high. Under unfavorable soil conditions, plants seem to put more energy into root production. (experiment possibility!)

Generally, plant production depends on light, water, nutrients, and toxins. If you look at a salt marsh it has full sun, limitless water, and the sedimentary soil is generally rich in nutrients so you'd expect uniformly high production. That doesn't happen:

1. Productivity declines northward as the growing season shortens.

2. Water looks plentiful but because it's salty the plants tend to lose water and so must expend energy in order to take it up. It's been shown that plant growth is progressively inhibited by salt (experiment possibility!)

3. The less oxygen the soil contains, the worse the plants do even if they're adapted to survive that, and salt marsh sediments tend to be low in oxygen.

Tidal Freshwater Marshes

Productivity is generally high here (1000-3000 g/m2/y) but variable, depending on:

1. Types of plants present. Unlike salt marshes, freshwater tidal marshes have a wide diversity of plants and so productivity depends partly on how well the particular species of plant grows.

2. Tidal energy. Moving water generally supports production.

3. Soil nutrients, grazing, and toxins all have an effect.

Mangroves

Generally productivity is highest in riverine mangroves and lowest for dwarf mangroves (1100-5400 g/m2/y) Here again, the key seems to be the increased nutrients supplied by the tide.

Freshwater Marshes

Productivity in these is high, upward of 1000 g/m2/y. This is lower than what we've looked at so far, but still higher than that of intensely cultivated farm crops. It's variable, again because of the variety of plants that might be involved. There's a strong relationship between above ground biomass and summer temperatures so southern marshes are more productive than those in the north.

Bogs

In these, much of the production is below ground and mosses, especially Sphagnum , account for 1/3-1/2 the total production.

These wetlands are far less productive and other wetlands and are generally less productive than terrestrial ecosystems in the same regions (250-500 g/m2/y)

Cypress Swamps

As shown in the figure, the highest productivity in cypress swamps seems to occur when conditions are neither too wet nor too dry and there is seasonal pulsing. The more water that flows through, the better. Even so, these are less productive than marshes (500-1800 g/m2/y).

(from Mitch and Gosselink, 1993)

Riparian Wetlands

Remember that these are wetlands occurring along the edge of lakes and rivers. They are generally more productive (600-1400 g/m2/y) than their adjacent uplands due to:

1. Flooding provides adequate water supplies

2. Nutrients are supplied and favorable alteration of soil chemistry results from the periodic overbank flooding (nitrification, sulfate reduction, nutrient mineralization--all these make the nutrients more available to the plants)

3. Flowing water offers a more oxygenated root zone than if the water were stagnant. The "flushing" carries away waste products.

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