Energy Flow & Nutrient Cycling in Marine Ecosystems

Vocabulary flashcards summarizing key terms on energy flow, primary productivity, trophic interactions, and biogeochemical cycles in marine ecosystems.

Energy Equilibrium (Earth)

The state in which ~100 units of electromagnetic energy enter the atmosphere and the same amount leave, maintaining a stable global temperature.

Carbon Fixation

The conversion of inorganic carbon (CO₂) into organic molecules, forming the base of the marine food web.

Photosynthesizers

Organisms that use sunlight to transform inorganic matter into organic matter, providing the primary energy input for ecosystems.

Organic Molecule

A compound that contains carbon–hydrogen (C–H) bonds, e.g., glucose (C₆H₁₂O₆).

Inorganic Molecule

A compound lacking C–H bonds, such as H₂O or CO₂.

Atmospheric Energy Balance

The atmosphere receives and emits ~156 energy units, preventing long-term warming or cooling.

Average Global Temperature

Earth’s mean surface temperature of ~15 °C (59 °F), sustained by balanced incoming and outgoing energy.

Phytoplankton

Microscopic, free-floating autotrophs that dominate primary production in the ocean.

Autotroph

An organism that produces its own organic matter, mainly through photosynthesis or chemosynthesis.

Heterotroph

An organism that obtains energy by consuming other organisms or organic matter.

10 Percent Rule

Only ~10 % of energy in one trophic level is transferred to the next; ~90 % is lost as heat.

Respiration (Consumers)

The metabolic process by which organisms release energy from glucose for growth and survival.

Chemosynthetic (Deep-Ocean) Autotroph

A primary producer that fixes carbon using chemical energy from hydrothermal vent fluids instead of sunlight.

Primary Productivity

The rate at which autotrophs fix carbon into organic molecules; controlled by light and nutrient availability.

Thermocline

A steep temperature gradient in ocean water that hinders vertical mixing and nutrient replenishment.

Spring Bloom

Rapid phytoplankton growth in temperate oceans when light and nutrients become plentiful after winter mixing.

Food Web

A complex network of feeding relationships that illustrates multiple energy pathways in an ecosystem.

Trophic Pyramid

A representation of energy or biomass distribution across trophic levels, widest at producers.

Decomposer

Primarily bacteria (and fungi) that break down dead organic matter, releasing nutrients back into the ecosystem.

Upwelling

The rise of deep, nutrient-rich water to the surface, boosting primary productivity.

Biomass Pyramid (Aquatic)

An inverted pyramid where carnivores hold more biomass than primary producers due to rapid turnover of phytoplankton.

Detritus

Dead organic matter, including decaying organisms, fecal pellets, and marine snow.

Dissolved Organic Matter (DOM)

Organic molecules <0.7 µm released from damaged or lysed cells, fueling microbial activity.

Particulate Organic Matter (POM)

Organic particles >0.7 µm, such as marine snow, consumed by filter feeders.

Filter Feeder

An organism (e.g., sponges, bivalves) that strains plankton and POM from the water.

Nitrogen Cycle

Processes that convert atmospheric N₂ into biologically usable forms (e.g., nitrate) and back, sustaining marine productivity.

Nitrification

Bacterial conversion of ammonia (NH₄⁺) to nitrate (NO₃⁻), a form usable by primary producers.

Denitrification

Bacterial reduction of nitrate back to N₂ gas, returning nitrogen to the atmosphere.

Phosphorus Cycle

Movement of phosphate from rivers to the ocean, through food webs, and into sediments, then eventual recycling.

Carbon Cycle (Marine)

Exchange of CO₂ between atmosphere and ocean, its fixation by photosynthesis, and deposition in sediments as detritus or CaCO₃.

Benthic Community

Organisms living on or in seafloor sediments that consume settling organic matter.

Marine Snow

Continuous shower of organic detritus falling from the upper water column to deeper layers.

Remineralization

Microbial breakdown of organic matter into inorganic nutrients (e.g., nitrate, phosphate) and CO₂.

Anoxic Environment

Sedimentary or water conditions devoid of oxygen, favoring anaerobic microbial processes.

Anaerobic Bacteria

Microbes that decompose organic matter without oxygen, often using sulfate or metals as electron acceptors.

Hydrothermal Vent

Seafloor opening emitting mineral-rich, hot fluids that support chemosynthetic communities.

Scavenger

An animal (e.g., crabs, shrimp, sea cucumbers) that feeds on dead organisms and detritus.

Microbial Loop

Pathway in which DOM is taken up by bacteria, which are then consumed by micro-grazers, recycling energy and nutrients.

Virally Mediated Lysis

Destruction of microbial cells by viruses, releasing DOM back into the environment.