Global Ecosystems: Productivity and Biogeochemical Cycles

Trends and Controls of Global Net Primary Productivity (NPP)

• Global Distribution of NPP:

  • Net primary productivity ($g/[m^2 \times year]$) varies significantly across terrestrial and aquatic biomes.

  • Terrestrial Biomes (Highest to Lowest Typical NPP):

    • Tropical forest: ~$2,200$

    • Temperate forest: ~$1,200$

    • Boreal forest: ~$800$

    • Savanna: ~$900$

    • Cultivated land: ~$650$

    • Shrubland: ~$600$

    • Temperate grassland: ~$600$

    • Tundra and alpine: ~$140$

    • Desert scrub: ~$70$

  • Aquatic Biomes (Highest to Lowest Typical NPP):

    • Freshwater swamp and marsh: ~$2,000$

    • Marine algal beds and coral reefs: ~$2,000$

    • Estuaries: ~$1,500$

    • Rivers and lakes: ~$250$

    • Continental shelf: ~$360$

    • Open ocean: ~$125$

• Climatic Limitations in Terrestrial Systems:

  • NPP is primarily limited by temperature and precipitation factors.

  • Temperature Influence: A positive correlation exists between average annual temperature and NPP. As average temperature increases from $-15^{\circ}C$ to $30^{\circ}C$, NPP increases from near $0$ to over $2,500\,g/[m^2 \times year]$.

  • Precipitation Influence: NPP typically increases with annual precipitation up to approximately $200\,cm$. Beyond $200-400\,cm$, NPP may level off or show a slight decline.

  • Latitudinal Correspondence: Terrestrial NPP follows a latitudinal gradient, with the highest carbon production concentrated near the equator ($0^{\circ}$) and decreasing toward the poles ($90^{\circ}N$ and $90^{\circ}S$). Values near the equator often exceed $1,200\,g\,\text{Carbon}/[m^2 \times year]$.

• Latitudinal Species Diversity Gradient:

  • Species diversity reaches its peak near the equator and declines toward the temperate and polar zones.

  • In butterflies, for example, the number of species increases from less than $10$ in polar regions to over $80$ near the equator.

  • Hypotheses for this gradient involve the interaction of latitude, productivity, and species richness.

Marine Ecosystems and Coastal Upwelling

• Controls on Marine NPP:

  • Marine NPP is highest near coastlines rather than the open ocean.

  • Mechanisms for Coastal Productivity:

    • Light: Photosynthesis is limited to shallower regions where sunlight can penetrate.

    • Runoff: Nutrients from terrestrial sources are carried into coastal waters.

    • Upwelling: A process where deeper, cold, nutrient-rich water rises to the surface.

• Coastal Upwelling Process:

  • Surface winds blow warmer surface waters offshore.

  • This movement allows deeper, colder, and nutrient-rich water to rise and replace the displaced surface water.

  • Eastern Boundary Upwelling Systems (EBUS): There are four major systems:

    1. California (San Francisco, Los Angeles).

    2. Peru/Humboldt (Guayaquil, Lima).

    3. NW Africa/Canary (Casablanca, Dakar).

    4. Benguela (Luderitz, Cape Town).

  • Satellite data uses chlorophyll concentration ($mg\,m^{-3}$) as a proxy for phytoplankton biomass to quantify primary production fueled by nitrate supply.

• Climate Variations (El Nio and La Nia):

  • El Nio: Characterized by warmer surface waters and potentially weakened upwelling.

  • La Nia: Characterized by colder surface waters and strengthened upwelling.

The Global Water Cycle

• Biogeochemical Cycling Principles:

  • Materials move between "compartments" or "pools."

  • Pool/Reservoir: The amount of material in a specific compartment.

  • Flux: The amount of material moving between compartments per unit of time (e.g., amount per year).

• Major Water Pools (Units: $10^{18}\,g$):

  • Oceans: $1,338,000$ (The largest pool).

  • Ice and snow: $24,364$

  • Groundwater: $23,416$ (Enters the cycle only when brought to the surface).

  • Surface freshwater: $190$

  • Atmosphere: $13$

  • Soil moisture: $4$

  • Living biomass: $1$

• Major Water Fluxes (Units: $10^{18}\,g/year$):

  • Evaporation from sea: $434$

  • Precipitation over sea: $398$

  • Precipitation over land: $107$

  • Evaporation and transpiration from land: $71$

  • Net transport over land (Atmospheric): $36$

  • Runoff (Land to Sea): $36$

The Global Nitrogen Cycle

• Major Nitrogen Pools (Units: $10^{12}\,g$):

  • Atmospheric $N_2$: $3,900,000,000$

  • Benthic sediments and rocks: $400,000,000$

  • Dissolved nitrogen in ocean waters: $660,000$

  • Soils: $100,000$

  • Plant biomass: $4,000$

  • Marine biomass: $300$

  • Detritus: $1,200-8,000$

• Nitrogen Transformation Processes:

  • Biological Nitrogen Fixation: Conversion of $N_2$ into forms accessible to plants.

    • Symbiotic Fixation: Bacteria (e.g., Rhizobiaceae in legumes; Frankia in woody plants) form mutualistic relationships with plant roots. Plants provide carbohydrates; bacteria provide fixed nitrogen ($NH_4^+$, $NO_3^-$).

    • Aquatic Fixation: Conducted by certain archaea and cyanobacteria.

  • Nitrification: Conversion of ammonium ($NH_4^+$) to nitrites ($NO_2^-$) and then to nitrates ($NO_3^-$) by bacteria.

  • Denitrification: Conversion of nitrates ($NO_3^-$) back into atmospheric nitrogen gas ($N_2$).

  • Ammonification: Decomposers (bacteria and fungi) convert organic nitrogen into ammonium.

• Human Alterations of the Nitrogen Cycle:

  • Haber-Bosch Process: Industrial nitrogen fixation using high temperature, pressure, and an iron catalyst.

    • $3CH_4 + 6H_2O \rightarrow 3CO_2 + 12H_2$

    • $4N_2 + 12H_2 \rightarrow 8NH_3$

  • Impact of Fertilizers: Humans have doubled the natural rate of nitrogen fixation. ~$40\%$ of the world\u2019s population depends on nitrogen fertilizers for food production.

  • Contribution Sources:

    • Fertilizer production: ~$1/2$

    • Increased biological nitrogen fixation (cultivated crops): ~$1/4$

    • Fossil fuel combustion: ~$1/4$

  • Consequences of Over-fertilization:

    • Nitrogen Deposition: Volatilization from fields and increased soil nitrification/denitrification leading to transport via rain or snow.

    • Eutrophication: Excess nutrients in ground and surface water.

The Global Phosphorus Cycle

• Characteristics and Reservoirs:

  • Phosphorus is primarily found in mineral form, not the atmosphere.

  • Major Reservoirs: Underground phosphate reserves, rock formations.

  • Cycles: Released via weathering of rocks, absorbed by plants, incorporated by animals, and returned to the soil/water via decomposition and leaching.

  • Geological Lifespan: Geological uplift eventually forms new rock, but this occurs over long timescales.

• Human Impacts on Phosphorus:

  • Phosphate Mining: Major industrial activity, particularly in Florida (e.g., Mosaic mining and gypsum stacks).

  • Non-Renewable Nature: Phosphorus is a finite resource. Projections suggest a "peak phosphorus" production period early in the 21st century followed by a decline.

Eutrophication and Hypoxic Dead Zones

• The Eutrophication Process Step-by-Step:

  1. Nutrient runoff (Nitrogen and Phosphorus) from land enters a water body.

  2. Surge in algal productivity (algal bloom).

  3. Algae begin to die off.

  4. Aerobic bacteria decompose the dead algae.

  5. The decomposition process depletes dissolved oxygen in the water.

  6. Hypoxia occurs, leading to the death of fish and other aquatic organisms.

• Case Studies in Hypoxia:

  • Gulf of Mexico Dead Zone: A massive hypoxic zone (<2\,mg/L dissolved oxygen) caused largely by nutrient runoff from the Mississippi River watershed, which covers over $40\%$ of the continental U.S.

    • In 2017, the zone reached a record size of $8,776$ square miles.

  • Florida Water Bodies: Algae blooms in Lake Okeechobee, the St. Lucie Canal, and the Caloosahatchee River have led to massive environmental degradation and public health warnings.

  • Global Trends: The number of observed dead zones has roughly doubled each decade since the 1960s, with over $400$ existing globally.

• Mitigation Strategies to Reduce Nutrient Runoff:

  • Reducing Application: Identifying optimal nutrient needs, using cover crops, and practicing crop rotation (alternating N-fixer and non-N-fixer crops).

  • Controlling Runoff:

    • Adjusting the timing of fertilizer applications.

    • Using organic or slow-release fertilizers.

    • Establishing Buffer Zones (strips of grass or natural vegetation between fields and water).

    • Recapturing, recycling, or treating runoff (e.g., Sweetwater Wetlands Park).

Questions & Discussion

• Question: What would happen if the surface winds slow in an upwelling system?

  • Answer: The availability of nutrients near the ocean's surface would decrease, and the water would be warmer than usual at the ocean's surface because the cold, nutrient-rich deep water is no longer being pulled up.

• Question: What is the net flux of Nitrogen ($N$) from the atmosphere to land and water?

  • Answer: Calculated by comparing natural fixation ($128\times 10^{12}\,g$ on land and $120\times 10^{12}\,g$ in oceans) against the total denitrification returning to the atmosphere ($158\times 10^{12}\,g$ from land and $110\times 10^{12}\,g$ from oceans), combined with human-driven increases.

• Question: Eutrophication in water bodies can lead to the formation of dead zones due to a depletion of which element?

  • Answer: Oxygen ($O_2$).