Biology Rivers and Lakes Exam #1

What is water?

An inorganic, transparent, odorless, nearly colorless substance made of 2 H atoms covalently bonded to 1 O atom in a bent shape (104.5°), forming the basis of the hydrosphere and all living fluids.

Why is water a unique molecule?

Its polarity (O is slightly negative, H slightly positive) allows each molecule to form up to 4 hydrogen bonds with neighbors, keeping it liquid at normal temps, and giving it high solvency, high specific heat, and strong surface tension.

Density anomaly

Water is densest at 3.98°C; ice is less dense than liquid water and floats, insulating water bodies in winter and driving stratification/turnover.

Excellent solvent

Polar water dissolves ions/minerals via hydration shells, driving weathering and nutrient release; gas solubility (O2) decreases as temp increases, causing summer oxygen stress.

Property 3: High specific heat/heat of fusion & vaporization

Breaking H-bonds to change temp or state takes lots of energy, buffering aquatic habitats from rapid temperature swings and enabling evaporative cooling.

High surface tension/cohesion-adhesion

Strong H-bonding creates a "skin" at the surface, supporting neuston/pleuston communities (e.g., water striders) and driving capillary action in soils/plants.

What is Reynolds Number (Re)?

A dimensionless ratio of inertial forces to viscous forces (Re = ρUl/μ) describing whether flow is laminar or turbulent.

Low Reynolds Number

Viscous forces dominate (microorganisms); flow is laminar, water feels thick/syrupy, streamlining is ineffective, no coasting/inertia.

High Reynolds Number

Inertial forces dominate (larger organisms/fast flow); flow becomes turbulent with eddies, streamlining reduces drag, organisms coast when they stop moving.

How do chemicals move in water?

Via molecular diffusion (slow, driven by concentration gradients, dominant at micro-scales/boundary layers) and advective/eddy diffusion (fast, driven by bulk currents/turbulence, dominant in open water).

How does heat move in water?

Enters via solar radiation/conduction, mixes rapidly in surface layers via turbulent advection, but diffuses very slowly across stratified thermoclines.

What is a watershed?

All land area that drains into and feeds a specific stream channel above a given point (also called a catchment or drainage basin).

Why classify streams by watershed area?

Predicts discharge, channel width/depth, morphology, substrate size, and sediment transport.

Why classify streams by stream order?

1st-order = no tributaries; order increases only when two equal-order streams merge; predicts channel size, energy source, and macroinvertebrate communities.

Why classify streams by discharge over time?

Hydrographs classify streams as perennial (year-round flow), intermittent (seasonal + groundwater), or ephemeral (rare flow, no groundwater); discharge variability shapes disturbance and biota adaptations.

Why classify streams by vegetation?

Determines light/shading and energy input: forested streams get allochthonous CPOM (P/R

What is the Valley Channel Classification system?

A hierarchical system classifying stream channels from valley/hillslope scale down to reaches/microhabitats based on sediment transport and geomorphology.

Cascade reach

Steep slope, disorganized bed of large boulders/cobbles that rarely move.

Step-pool reach

Large particles form channel-spanning steps alternating with small pools.

Plane-bed reach

Intermediate gradient; long, straight, featureless armored gravel-cobble bed; low habitat diversity.

Pool-riffle reach

Moderate/low gradient; alternating shallow fast riffles and deep slow pools connected by runs.

Braided reach

Broad, steep, high sediment load; water flows in shifting sheets/channels across sand/gravel bars.

Dune-ripple reach

Low gradient; fine sand/silt forming shifting ripples and dunes.

What is the River Continuum Concept (RCC)?

Vannote et al. (1980) concept viewing rivers as a continuous predictable gradient of physical, chemical, and biological traits from headwaters to mouth.

RCC: Headwaters (orders 1-3)

Narrow, shaded, cold, rocky; allochthonous CPOM energy; P/R<1 (heterotrophic); dominated by shredders & collectors.

RCC: Mid-reaches (orders 4-6)

Wider, open canopy, warmer; autochthonous production (periphyton/macrophytes); P/R>1 (autotrophic); dominated by scrapers & collectors.

RCC: Large rivers (orders >6)

Wide, deep, turbid, light-limited; imported FPOM + phytoplankton; P/R<1 (heterotrophic); dominated by collectors/filter feeders.

Pond vs Lake

small/shallow, light reaches bottom everywhere, macrophytes throughout. Lake: naturally formed, deep enough to have distinct pelagic/profundal zones.

Large lake vs Reservoir

Huge natural with seiches/rotational currents (e.g. Great Lakes). Man-made dammed river, dendritic shape, deepest water near dam, riverine-to-lacustrine gradient.

How are lakes formed? (list types)

Glacial (scouring/kettle lakes), tectonic (faulting), fluvial (oxbow lakes), volcanic (caldera lakes), solution/karst (dissolved limestone), and biogenic/anthropogenic (beaver dams, reservoirs).

Lake morphometry basics

Described by surface area, max/mean depth, volume, shoreline development index, and hydraulic retention time; shallower mean depth = generally more productive.

Littoral zone

Shallow nearshore area where light reaches the bottom, supporting rooted macrophytes and periphyton.

Pelagic (limnetic) zone

Open water column with enough light for photosynthesis; home to phytoplankton, zooplankton, and nekton.

Profundal zone

Deep zone below light penetration where P < R; dominated by decomposers and benthos.

Plankton vs Nekton vs Benthos

weakly swimming/floating (phyto/zooplankton). Nekton: strong swimmers (fish). Benthos bottom-dwelling organisms.

Periphyton & Neuston/Pleuston

Algae/biofilm attached to submerged surfaces. Neuston/Pleuston: organisms living on/at the surface tension film.

What drives lake stratification?

Temperature-dependent density differences, with water density maximum at 3.98°C.

Epilimnion, Metalimnion, Hypolimnion

Epilimnion: warm, mixed surface layer. Metalimnion/thermocline: middle layer with rapid temp/density change. Hypolimnion: cold, dense, dark bottom layer.

Spring & Fall overturn

Water reaches isothermal 3.98°C, allowing wind to mix the whole water column top to bottom.

Winter stratification

Surface water cools below 3.98°C toward 0°C and floats above denser 3.98°C water; ice forms on top and blocks wind mixing.

Dimictic lakes

Mix twice a year (spring and fall); typical of cold-temperate regions.

Monomictic lakes (cold vs warm)

Mix once a year. Cold monomictic: high latitude/altitude, mixes only in brief ice-free summer. Warm monomictic: never freezes, mixes in winter, stratifies in summer.

Polymictic lakes

Mix frequently/continuously; common in shallow or tropical lakes.

Amictic lakes

Perennially ice-covered; never mix.

Meromictic lakes

Permanently stratified due to high-salinity bottom layer (monimolimnion) too dense for wind to mix.

Vannote et al. 1980 - main findings

Proposed the River Continuum Concept: physical gradients downstream (width, depth, velocity, temp) predictably shape P/R ratios, organic matter type (CPOM→FPOM), and macroinvertebrate functional feeding groups (shredders→scrapers→collectors) along stream order.