Ecosystem Ecology and Primary Productivity

Ecology: Organisms and Their Environment

Ecology is the scientific study of the relationships between organisms and their environment.

Factors Influencing Species Distribution

Species presence varies across space and time due to:
- Interactions with other species.
- Impact on community structure.
- Ecosystem function affected by energy transfer, matter cycling, and climate.

Course Units

Unit 1: Organisms & their environment
Unit 2: Populations across space and time
Unit 3: Species interactions & communities
Unit 4: Ecosystems, Biodiversity, & Conservation

Scaling Up: From Communities to Ecosystems

Transition from viewing communities and ecosystems as distinct biological entities to understanding them as energy machines and nutrient processors.

Learning Outcomes

Understand ecological systems through the lens of energy and nutrient flow.
Describe the relationship between energy captured during photosynthesis and net/gross primary productivity.
Relate energy cycling to food webs in terrestrial and aquatic systems.

Ecosystem Ecology

Ecosystem defined (Tansley 1935): Combination of abiotic and biotic components influencing energy and element flow.
Ecosystem ecology focuses on aggregate processes like energy, water, and nutrient flows.

Ecosystem Ecology: Energy and Nutrient Flows

Ecosystem ecology studies the flows of energy, water, and nutrients, considering biological, physical, and chemical processes.
Fundamental areas of interest:
- Primary production (plant/algae conversion of energy into biomass).
- Secondary production (conversion of energy into biomass by consumers).
- Nutrient cycling.

Primary Productivity

Primary productivity provides energy to the ecosystem and is limited by photosynthesis, nutrient availability, water availability, and temperature.

Photosynthesis and Ecosystems

Photosynthesis at the ecosystem level:
- Task: Absorb $CO<em>2$ , use light energy and water, convert to sugar, and release $O</em>2$ .
- Photosynthetic machinery uses solar energy to synthesize sugars.

Primary Production

Primary production: Production of biomass by autotrophs (plants, phytoplankton) in an ecosystem over time.
Phytoplankton and plants fix carbon needed by other organisms for food.

Primary Productivity: Rate and Biomass

Primary productivity: Rate at which solar (or chemical) energy is converted into chemical bonds.
- Photosynthesis, chemosynthesis.
- Units: Energy per unit space per unit time (e.g., Joules (J) per square meter per year).
Standing stock (or standing crop): Biomass present in the ecosystem at a specific time.
- Units: Mass per area (e.g., Grams per square meter).

Gross Primary Production (GPP) vs. Net Primary Production (NPP)

Gross Primary Production (GPP): Rate at which energy is captured and assimilated by producers.
- Energy per unit time fixed via photosynthesis.
Net Primary Production (NPP): Rate at which energy is assimilated by producers and converted into biomass.
- Energy per unit time fixed by photosynthesis minus energy per unit time used by respiration.
- $NPP = GPP - Respiration$

GPP vs. NPP Differences

GPP: Rate of biomass/organic matter production by producers during photosynthesis.
NPP: Biomass/organic matter available for consumption by heterotrophs.
Both measured in kCal (or joules) per square meter per year or in g carbon per square meter per year.

Gross vs. Net Primary Productivity

Gross: Total rate of energy capture & assimilation (carbon producers are fixing).
Net: Rate of energy assimilation into biomass. Primary productivity remaining after accounting for respiration by producers.
$NPP = GPP - R$
- Respiration (energy used for metabolism) = Net primary productivity (energy used to build biomass, available for growth & reproduction).
- Gross primary productivity (energy fixed during photosynthesis).

Energy Capture Inefficiency

99% of solar energy is reflected or passes through producers without being absorbed.
1% of solar energy striking producers is captured by photosynthesis (GPP).
60% of GPP is lost to respiration.
40% of GPP supports producer growth and reproduction (NPP).

Quantifying Primary Productivity

Measure changes in producer biomass over time.
- How much biomass is produced during a growing season?
- Limitations of this approach?
- Measures net primary productivity.

Quantifying Primary Productivity: CO2 Uptake and Release

Measure changes in $CO_2$ over time (uptake and release) in light and dark environments.
Estimate GPP based on these measurements.
In aquatic ecosystems, track changes in $O<em>2$ instead of $CO</em>2$ .

Net Primary Productivity (NPP) Variation

Net primary productivity varies with latitude.

Global NPP Discussion Points

Why might NPP increase in higher latitudes during certain seasons?
How do changes in solar radiation across seasons influence NPP at different latitudes?
Role of temperature and precipitation in NPP patterns?
How does seasonality affect tropical, temperate, and polar ecosystems in terms of NPP?

Global NPP Trends

Atmospheric carbon removed by plants varies (millions of grams per square kilometer per day).

Global NPP- Climate Change Impacts

Impact of climate change on latitudinal NPP patterns?
How could shifts in NPP affect food webs or carbon cycling across biomes?
If NPP is increasing in some regions but decreasing in others, what might that mean for global carbon balance?

NPP of Different Ecosystems

Terrestrial ecosystems: Tropical rainforest, tropical seasonal forest, temperate rainforest, temperate seasonal forest, boreal forest, savanna, cultivated land, woodland/shrubland, temperate grassland, tundra, desert scrub, extreme desert.
Aquatic ecosystems: Swamps and marshes, coral reef, salt marsh, upwelling zone, lakes and streams, continental shelf, open ocean.
NPP measured in grams of carbon per square meter per year.

Factors Limiting Primary Production in Terrestrial Ecosystems

Temperature and precipitation are major drivers of NPP.

Terrestrial Primary Productivity

Temperature and precipitation are major drivers.
Graphs illustrating relationship between annual precipitation, average annual temperature, and net primary productivity.

NPP and Environmental Factors

Temperature, water availability, and nutrients impact NPP.
Water availability limits photosynthesis rate and leaf area.

Nutrient Limitations on Primary Productivity

Besides temperature and moisture, nutrients limit primary productivity. (Nitrogen or phosphorus).
In grasslands and tundra, adding both N and P increases NPP more than either alone, indicating co-limitation.

Grassland Diversity and NPP

Increasing grassland diversity from 4 to 16 species increased NPP as much as adding 54 kg $ha^{-1}$ $y^{-1}$ of nitrogen fertilizer.
Important for evaluating ecosystem function, understanding consequences of global change, and understanding foundational structure of ecological systems.

Aquatic Primary Productivity

Temperature, light, and nutrients are important.
Summer chlorophyll $a$ (mg/ $m^3$ ) related to total phosphorus (mg/ $m^3$ ).

Nutrient Limitation in Aquatic Ecosystems

Biomass of phytoplankton in the epilimnion affected by nutrient additions (carbon, nitrogen, phosphorus).

Aquatic System Characteristics

Clear Lake (eutrophic), Castle Lake, Lake Tahoe (oligotrophic).
Depth of light penetration determines the zone of primary production.

Ocean Primary Production

Limited by nutrients and light.
Most fish are caught where nutrients and light are abundant.

Upwelling and Coastal NPP

Upwelling brings deeper, colder, nutrient-rich water to the surface.
Shallow coastal waters show highest NPP due to nutrient transport from bottom sediments and terrestrial inputs.