Biogeography

Date: January 1999
Course: Marine Science (MSC 111)
Institution: MIT
Model Reference: ECCO2 and MITgcm (MIT General Circulation Model)
- Focus on oceanic life distribution and nutrient cycling.

Key Species: Prochlorococcus, Synechococcus, Diatoms.
Nutrient Measurements:
- Presented in mmol P m^-2 across various depths (from Page 1).

Linking Processes:
- Integrates the physical processes (e.g., gyre circulation, upwelling, thermohaline circulation).
- Links these processes with chemical distributions (nutrients, products, energy).
- Objective: Determine and describe marine life distribution and the reasons behind this distribution.
Simulation Model:
- Developed by the Follows group at MIT.
- Further details at: Darwin Project and publication references from Cambridge University Press.

Parameters for Life Zones:
- Depth: Varies in influence on light, temperature, and nutrients.
- Latitude: Influences ecological zones and climatic conditions.
- Distance from Shore: Affects light penetration and oceanic conditions.

Energy Sources:
- Sunlight or organic carbon for carbon fixation by organisms (e.g., chemoautotrophs).
Nutrient Sources:
- Major Nutrients: Nitrogen (nitrate), Phosphorus (phosphate).
- Minor Nutrients: Silica, iron, magnesium, cobalt, zinc, copper.
Temperature: Species adapted to vary optimum temperatures across different environments.

Critical Environmental Conditions:
- Nutrients, light, and temperature are paramount in defining marine life zones.
Depth Variations:
- Depth influences temperature and nutrient availability, with major patterns observed at varying depths.

Euphotic Zone: Top layer of the ocean where photosynthesis occurs due to sufficient light penetration.
- Light intensity decreases exponentially with depth.
- Average depth of euphotic zone: ~100 m, with most of the ocean being dark below this zone.
- Depth profile emphasizes three zones:
1. Euphotic (photonic)
2. Twilight Zone (mesopelagic)
3. Aphotic Zone (dark ocean).

Northern Hemisphere Seasonal Effects:
- Hours of daylight vary significantly between summer and winter.
Equatorial Regions:
- Sunny throughout the year, averaging about 12 hours of daylight/day.
Polar Regions:
- Exhibit extremes with dark winters and continuous daylight during summer solstice.

Based on Backus (1986):
- Major definitional factors: temperature and nutrients.
Pelagic Environment Zones:
- Examples include Antarctic polar, subtropical, tropical, and temperate zones.
Nutrient Distribution:
- High nutrients typically found in subpolar zones; gyres known for lower nutrient concentrations.

Thermohaline Circulation:
- Described as a simplified conveyor belt; deep water formation replenishes nutrients over time through aging processes.
Oxygen and Nutrients Relation:
- Aging surface water travels depleting nutrients, while deep waters accumulate higher nutrient concentrations.

Phosphate Levels:
- Profile across N-S Atlantic indicates surface levels high in phosphate from photosynthesis but decreases with depth due to respiration processes.
Dissolved Oxygen Levels:
- Similar profile to phosphates; production at the surface contrasts with consumption at depth.

Influence on Nutrient Availability:
- Upwelling brings nutrient-rich deep water to the surface.
- Examples of areas with upwelling: California, Peru, and Benguela.
Chemical Compounds:
- Upwelled water contributes essential nutrients to support primary production.

Primary Production:
- Upwelling leads to enhanced production, positively influencing zooplankton and fish biomass.
Biomass Ratios:
- C:N:P ratio of organic matter: 106:16:1, with visible biomass expressed in terms of chlorophyll.
Ecosystem Interactions: High productivity in upwelling regions often linked to oxygen minimum zones (OMZ).

Definition: Excessive nutrient enrichment resulting in oxygen depletion or dead zones.
- Typical Eutrophication Cycle: Nutrients → Production → Respiration → Oxygen Depletion.
Affected Regions:
- Significant issues in estuaries like Chesapeake Bay, Biscayne Bay, and Long Island Sound.

Zones of the Ocean:
- Pelagic, benthic, and intertidal environments categorized based on depth and distance from shore.
- Epipelagic Zone: Up to 200 m depth; light penetrates.
- Benthic Zone: Involves interactions between aquatic organisms and the seabed.

Plankton:
- Drift with currents, including phytoplankton and zooplankton.
Nekton:
- Actively swim and move through water column (e.g., fish, squid).
Benthos:
- Bottom dwellers, many existing in deep-sea environments without sunlight.

Techniques:
- Surveys through net tows, video monitoring, genomic testing for species diversity.
- Patterns examined through data analysis and mathematical modeling to predict climate impact on ecosystems.

Sampling Methods:
- Use of specific nets targeting different planktonic species.
- Emphasis on using genomic assessments to study community diversity.
- Analysis of environmental DNA (eDNA) to assess microbial and species presence.

Importance of Continuous Study:
- Acknowledgment of how changing climates and ocean conditions influence marine life distribution and biogeography.
- Future insights coming from technological advances in sampling and genome analysis.

General literature references as noted in lecture materials (Backus 1986, Sarmiento & Gruber, and others).