Production II

Learning goals:
* Understand the sources of production for aquatic and marine ecosystems, and what factors limit their rates
* Learn the general patterns of production across the globe at the biome/ marine biological zone scale
* Be able to describe what constitutes secondary production

Limitations of NPP in aquatic ecosystems
Experiments in lake and stream ecosystems suggest strong nutrient limitations of NPP, usually by P, sometimes by N
Amount of NPP is low in streams and rivers, with most energy coming in from terrestrial ecosystems
Autochthonous energy is produced within the ecosystem
Allochthonous energy comes from outside of the ecosystem
Autochthonous energy in rivers generally increases in a downstream direction
Sediment can limit NPP in lower reaches of rivers

Limitations of NPP in marine ecosystems
Nutrient inputs (particularly N) into estuaries correlate with variation in NPP
Large amounts of N from agriculture will trigger blooms of NPP in estuaries, followed by decomposition and consumption of oxygen by decomposers, leading to large “dead zones”
In coastal areas, kelp forests may have leaf area indices and rates of NPP as high as those of tropical forests.
“Meadows” of seagrasses such as eelgrass (genus Zostera) are also important nearshore zones.
Nutrient inputs (mainly N) increase NPP, and cause changes in species composition
In the open ocean, NPP is mainly from phytoplankton.
Picoplankton (cells < 1 μm) contribute as much as 50% of the total marine NPP.  Highest in zones of upwelling
Floating seaweeds such as Sargassum also contribute to NPP.

In the open ocean, NPP is mostly limited by nitrogen.  But NPP in the equatorial Pacific Ocean appears to be limited by iron.  Windblown dust from Asia is a source of iron, and influences the global climate system through its influence on marine NPP, and thus on atmospheric CO2 concentrations.   During glacial periods, large parts of the earth could have contributed dust (and iron) that fertilized the oceans.

Global patterns of NPP
Global patterns of net primary production reflect climatic controls and biome types
Total terrestrial and oceanic NPP are almost equal (54 % terrestrial, 46% marine), with higher rates in terrestrial ecosystems (3x marine) making up for lower areal coverage (30% of Earth surface)
NPP varies among biomes: Tropical forests and savannas contribute about 60% of terrestrial NPP (30% of global NPP).
Variation in NPP among terrestrial biomes is associated mostly with differences in leaf area index and length of growing season.
Coastal zones account for 20% of oceanic NPP, or about 10% of total global NPP.
 The open ocean accounts for the majority of oceanic NPP, and about 40% of total global NPP.
 Variation in NPP among marine ecosystems is primarily related to variation in inputs of nutrients.

Secondary Production
Secondary production = energy derived from consumption of organic compounds that were produced by other organisms; heterotrophic production
Heterotrophs are classified according to the type of food they eat:
herbivores consume plants and algae;
carnivores consume other live animals;
detritivores consume dead organic matter (detritus),
omnivores consume both plants and animals, and sometimes detritus
Net secondary production is heterotrophic growth:

Net secondary production = ingestion – respiration – egestion (urine and feces)
Net secondary production depends on the “quality” of the heterotroph’s food (digestibility and nutrient content), and physiology
Animals with high respiration rates (e.g., endotherms) have less energy left over to allocate to growth.

Net secondary production in most ecosystems is a small fraction of NPP. The fraction is greater in aquatic ecosystems than terrestrial.
Most is associated with detritivores, primarily bacterial and fungi.

One strategy for determining what a consumer is eating involves measuring the stable natural abundance isotope composition of elements found in their food: C, N, S; these elements have rare, non radioactive forms that vary in abundance in different food sources