Aquatic Ecosystems & Hydrology – Key Vocabulary

Vocabulary flashcards covering key hydrology, limnology and river ecology concepts from Courses 1–12, suitable for exam review.

Water Scarcity

A condition where water demand exceeds available supply in a given area or period.

Mean Residence Time

Average time a water molecule spends in a particular reservoir (e.g., 2 500 yr in oceans, 17 yr in lakes).

Water Cycle

The continuous movement of water through evaporation, condensation, precipitation, and runoff, linking all reservoirs.

Ecosystem

Dynamic system of interacting biotic (living) and abiotic (non-living) components tied together by energy flow and nutrient cycling.

Lentic Ecosystem

Standing-water bodies such as lakes and ponds.

Lake Vostok

A 35-million-year-old sub-glacial Antarctic lake lying ~4 000 m under ice.

Lake Baikal

World’s deepest (1 642 m) and oldest (20–25 Ma) lake, containing ~20 % of global freshwater and high endemism.

Caspian Lake

Largest enclosed inland water body (371 000 km²); an endorheic basin with no natural outflow.

Dipole Moment (H₂O)

Separation of charge in a water molecule that enables tetrahedral hydrogen bonding and strong polarity.

Specific Heat Capacity (Water)

4.184 J g⁻¹ °C⁻¹; high value that buffers temperature changes in aquatic systems.

Cohesion

Attraction between identical molecules; in water, due to extensive hydrogen bonding.

Adhesion

Attraction between different substances; causes water to cling to surfaces and form films or foam.

Surface Tension

Energy needed to increase water surface area; ~72.8 mN m⁻¹ at 20 °C, important for neuston habitats.

Solvation Shell (Hydration Sphere)

Layer of water molecules surrounding a dissolved ion or molecule, enabling dissolution and transport.

Fetch

Longest uninterrupted distance wind travels over a lake, influencing mixing and evaporation.

Photosynthetically Active Radiation (PAR)

Wavelength range (≈400–700 nm) usable by photosynthetic organisms.

Light Attenuation

Decrease in light intensity with depth due to reflection, absorption, and scattering.

Turbidity

Water cloudiness caused by suspended particles; influences light penetration and thermal profiles.

Oligotrophic Lake

Clear, nutrient-poor lake with low algal biomass and high dissolved oxygen.

Eutrophic Lake

Nutrient-rich lake with high algal growth, murky water, and potential hypoxia.

Seiche

Standing internal or surface wave in an enclosed water body generated by wind or pressure changes.

Langmuir Cells

Parallel, wind-driven circulation cells producing surface streaks and vertical mixing.

Gaining Lake

Lake segment where groundwater flows into the lake.

Losing Lake

Lake segment where surface water infiltrates into groundwater.

Stratification (Lake)

Thermal layering—typically warm epilimnion, cold hypolimnion—affecting oxygen and nutrient distribution.

Photoinhibition

Reduction of photosynthetic rate at high light intensities, limiting net primary production.

Compensation Point

Light level where photosynthesis equals respiration (NPP = 0).

Euphotic Zone

Upper water layer receiving >1 % surface PAR, net biomass production exceeds loss.

Aphotic Zone

Deep layer where respiration exceeds production; organic matter decomposes.

Riparian Zone

Vegetated corridor along rivers that stabilizes banks, shades water, and supplies organic matter.

Floodplain

Flat land adjacent to a river routinely inundated, fostering biodiversity and nutrient exchange.

Hyporheic Zone

Subsurface interface where surface water mixes with groundwater, hosting specialized biota.

Riffle

Shallow, fast-flowing, coarse-bed section with divergent flow and high oxygenation.

Pool

Deep, slow-flowing section with fine sediments and convergent flow, often scouring.

Step-Pool Sequence

Stair-like channel morphology of alternating steps and pools common in steep streams.

Hjulström Curve

Graph relating flow velocity to grain size, indicating thresholds for erosion, transport, deposition.

Coarse Particulate Organic Matter (CPOM)

Organic particles >1 mm (e.g., leaves, woody debris) entering streams.

Fine Particulate Organic Matter (FPOM)

Organic particles 0.45 µm–1 mm derived from CPOM breakdown or flocculation.

Dissolved Organic Matter (DOM)

Organic molecules <0.45 µm in water; fuel microbial metabolism and color waters.

Dissolved Organic Carbon (DOC)

Carbon component of DOM, often measured to assess organic content.

Humic Substances

Recalcitrant, brown-colored DOM fraction (humic acids, fulvic acids, humin) from plant decay.

Van Krevelen Diagram

Plot of H:C vs O:C ratios used to classify DOM molecular composition.

Black Carbon

Pyrogenic carbon from wildfires; comprises ~10 % of riverine DOC, subject to photo- and biodegradation.

Browning

Increase in water color and light attenuation due to rising DOM/DOC concentrations.

Soluble Reactive Phosphorus (SRP)

Bioavailable inorganic phosphate fraction in water.

Dissolved Inorganic Nitrogen (DIN)

Sum of nitrate, nitrite, and ammonium readily available to biota.

Nutrient Spiraling

Concept describing repeated uptake, transformation, and downstream transport of nutrients in streams.

Uptake Length (Sₚ)

Average distance a nutrient molecule travels in water before biological uptake.

Gross Primary Production (GPP)

Total carbon fixation via photosynthesis per unit area and time.

Respiration (R)

Total metabolic CO₂ release by organisms in an ecosystem.

Net Primary Production (NPP)

GPP minus respiration by primary producers; net biomass increase.

Net Ecosystem Production (NEP)

GPP minus total ecosystem respiration (ER); positive values indicate autotrophy.

Baseflow

Groundwater contribution sustaining streamflow during dry periods.

Flood Pulse Concept

Ecological model emphasizing the role of predictable flooding in nutrient delivery and biodiversity of floodplains.

Free-Flowing River (FFR)

River with natural connectivity, unimpeded by dams or major alterations.

Connectivity Status Index (CSI)

Metric quantifying river fragmentation from barriers and floodplain disconnection.

Strahler Stream Order

Hierarchical classification of stream size based on tributary confluences.

Riverscape

Three-dimensional mosaic of river channels, floodplains, and adjacent landforms.

Hyporheos

Assemblage of organisms inhabiting the hyporheic zone.

Hydraulic Geometry

Relations among channel width, depth, velocity, and discharge at a given cross-section.

Hypoxia (Aquatic)

Dissolved oxygen concentration below ~2 mg L⁻¹, stressing aerobic organisms.

Trophic Cascade

Change in abundance or behavior at one trophic level indirectly affecting multiple other levels.

Top-Down Control

Regulation of trophic structure by predators suppressing prey populations.

Bottom-Up Control

Regulation of ecosystem by resource (nutrient, light) availability affecting producers.

Ecosystem Engineering

Process by which organisms modify habitats, e.g., salmon bioturbation resuspending sediments.

Anadromous Species

Fish that migrate from the ocean to freshwater to spawn (e.g., salmon).

Methanogenesis

Anaerobic microbial production of CH₄ from CO₂ or acetate.

Ebullition

Release of gas bubbles (e.g., CH₄) from sediments into water and atmosphere.

Drawdown Area

Exposed reservoir margin during low water, prone to CO₂ emissions.

Particulate Organic Carbon (POC)

Organic carbon in particles >0.45 µm transported in water columns.

Microbial Loop

Pathway where bacteria recycle DOM into biomass, transferring energy to higher trophic levels.

Allochthonous Input

Organic matter or energy originating outside an ecosystem (e.g., leaf litter in streams).

Autochthonous Production

Organic matter produced within the aquatic ecosystem (e.g., algal biomass).

Lake Kivu

East African rift lake storing ~300 km³ CO₂ and ~65 km³ CH₄, exploited for power generation.

Lake Edward

Rift lake north of Kivu with organic-rich sediments serving as hydrocarbon sources.

Marine-Derived Nitrogen (MDN)

Nitrogen transported from oceans to freshwater ecosystems by migrating fish carcasses/excretions.

Spiraling Length (S)

Sum of uptake length (Sₐ) plus turnover distance (Sₚ) for a nutrient cycle in streams.

Intermittent Stream

Watercourse that ceases flow for part of the year; becoming more common under climate change.

Brown Food Chain

Detritus-based trophic pathway relying on decomposers and heterotrophs.

Green Food Chain

Grazing-based trophic pathway originating with live primary producers.

Gas Exchange Velocity (k)

Coefficient describing rate of gas transfer across the air-water interface, influenced by turbulence.