Food Webs

 

1. Food webs and food chains are the feeding relationships among organisms in an ecosystem.

Food webs show more complex, but complete feeding relationships, for example see class notes or notes in the library.

The trophic level is the feeding level in a food web, e.g.,:

3^rd level carnivore (top predator)

2^nd level carnivore

1^st level carnivore

herbivore

primary producer (plant)

Trophic efficiency is the efficiency of energy transfer from one trophic level to the next.

E = (growth + reproduction)

food consumption

Productivity is the rate of production of biomass. Biomass is the weight of living organisms.

In the oceans (but not on land), the size of organisms tends to increase with increasing trophic level. The number of individuals and their total productivity decreases with increasing trophic level.

Organism	Relative Size	Relative Number	Productivity
Small Fish	10 cm	0.0005	1
Zooplankton	0.01-1 cm	5	10
Phytoplankton	0.001-0.01 cm	100	100

2. Controls on Primary Productivity

Since primary productivity is the basis of the entire food web, primary productivity is one important factor determining the productivity of higher trophic levels.

The two major factors controlling primary productivity (plant productivity) are the availability of:

Light and Nutrients

Nutrients are the main factor controlling the geographic differences in the amount of primary production in ocean surface waters.

• Nutrients are the chemical substances that a plant needs to grow in addition to carbon dioxide and water.
• They include phosphate and fixed nitrogen (usually ammonium or nitrate). They also include silica (for siliceous organisms only) and iron.

Light (or, rather, lack of light) limits primary production in waters beneath the photic zone (depths greater than 100-200 m) and in temperate and polar regions in winter.

The Nutrient Cycle

The nutrient cycle refers to the recycling of nitrogen, phosphorus, silica, and other substances that phytoplankton need in order to grow.

• Dead organic matter (dissolved or particulate) is broken down by bacteria, producing carbon dioxide and the nutrients ammonium, nitrate, and phosphate.
• If the nutrients are recycled in the photic zone, they are rapidly used again by the phytoplankton.
• Some organic matter sinks out of the photic zone and decomposes deeper in the ocean. This strips nutrients out of surface waters, and results in high nutrient concentrations in the aphotic zone.
• Nutrient resupply to the photic zone occurs when surface and deeper waters mix or when there is upwelling.

Geographic variations in nutrient availability:

• Oligotrophic ocean areas have low primary productivity. They are found in tropical ocean areas, away from land. This corresponds to the central regions of the major ocean gyres.
• At these latitudes (about 10^°-35^°) thermal stratification is fairly strong year-round. (Surface waters are warm year-round).
• This means that there is little vertical mixing of the water, so the supply of nutrients (fixed nitrogen and phosphate) from deep waters to surface waters is small.
• Fixed nitrogen and phosphate concentrations in the photic zone are extremely small.
• Since fixed nitrogen is usually depleted before phosphate, fixed nitrogen has been called the limiting nutrient for ocean primary productivity.
• However, nitrogen fixation by blue green algae can allow primary productivity where fixed nitrogen concentrations are very low.
• Iron concentrations are extremely low in seawater, less than 5 nanograms of iron per liter of water (1 nanogram = 0.000000001 g), and seem to limit the productivity of diatoms.
• Temperate and high-latitude ocean areas have medium levels of primary productivity.
• Seasonal cooling of the surface water leads to absence of stratification in winter.
• Vertical mixing of the water is strong and supplies nutrients to the surface water. But, phytoplankton are mixed too deeply to get enough light to grow.
• In spring, surface water warms and the water becomes stratified. Vertical mixing decreases, and phytoplankton get enough light to grow.
• Phytoplankton grow rapidly until the nutrients supplied by winter mixing are used up, a spring phytoplankton bloom.
• Primary productivity continues at a lower rate through the summer, largely using nutrients recycled from dead or eaten phytoplankton.
• Fall mixing, when the surface water cools, can cause another bloom until vertical mixing extends too deeply and light limitation prevents plant growth.
• The seasonal changes are similar in temperate and polar oceans, except that the productive season becomes shorter with increasing latitude.
• Areas of the ocean that have permanent ice cover (mainly the central Arctic) have very low productivity due to lack of light under the ice.

 

• Upwelling areas at tropical and temperate latitudes have very high primary productivity.
• Winds cause upwelling (a slow, upward flow) of water), which brings a continuous supply of nutrients to the ocean surface. In most upwelling areas, the winds very seasonally, and upwelling is strong only part of the year.
• At temperate and tropical latitudes, there is sufficient stratification of surface waters most or all of the year to keep phytoplankton in the photic zone.
• Since phytoplankton have both light and nutrients, they grow rapidly.
• Coastal areas at tropical and temperate latitudes often have high productivity.
• Nutrients are supplied by rivers and streams. Freshwaters usually have higher nutrient concentrations than ocean waters.
• Nutrients are also supplied from subsurface waters, because particles can’t sink to great depths before decomposing, as they can in the deep ocean areas

3. Fish Production

	Tons/year	% of plant production
Open ocean	1,600,000	0.01
Coastal ocean	120,000,000	3
Upwelling areas	20,000,000	20

Fishery productivity is controlled by primary productivity and food web structure.

• Primary productivity is the basic food source for the food web.
• Primary productivity can be affected by weather and climate. For example, usually warm, calm summers can lead to lower productivity because of lower nutrient supply.
• The coupling of primary productivity to higher trophic levels is also important. Sometimes, spring phytoplankton blooms do not occur at the right time or place to supply food to newly-hatched zooplankton and meroplankton.
• Food web structure is important to fishery productivity in several ways, including:
• The trophic level of the fish, since there is about a 90% decrease in productivity with each trophic level.
• The competition for food at each trophic level. Only the fraction of the available food that leads directly to the fish will benefit fishery productivity.

• Benthic and demersal productivity correlates negatively with depth, because the greater the water depth, the more animals compete for the food on its way to the bottom. Nearly all commercial fisheries for benthos are in water less than 200 m deep.
• Humans also affect fishery productivity:
• Excessive removal of adults, leading directly to reduced populations or to reductions in spawners and thus reproduction.
• Commerical harvests, susbsistence harvests. By-catch (catch of non-target species, often discarded.
• Destruction or damage to habitat, e.g., by trawling or damming of spawning streams (for freshwater spawners)

• Pollution (eutrophication, toxins)
• Note that fishing does not necessarily lead to decreased fish populations, although, there are many examples of overfishing destroying fisheries.
• If food supply is limiting juvenile survival or adult growth, then removal of a limited number of adults is possible without harming the fishery.
• The quantity that can be removed without decreasing the resource is called “Maximum Sustainable Yield” or MSY.

The problem for fishery managers is that they have only the foggiest idea what the MSY is.

4. Summary

Primary productivity is controlled by the availablilty of light and nutrients.

Primary productivity is lowest in the central ocean gyres (equivalent to deserts on land in yield per acre), medium in temperate and subpolar oceans (comparable to grasslands or savanna on land), high in many coastal areas (comparable to wheat fields), and very high in upwelling areas (comparable to the most productive croplands, e.g., sugar cane or rice).

Fishery productivity is affected by primary productivity, food web structure, and human exploitation of fisheries.