SCENE: Lakes and their Physicochemical Properties

What is salinity in aquatic systems?

Salinity is a measure of the concentration of dissolved ions in water, forming a gradient from freshwater to saline and hypersaline systems.

What is the typical threshold for classifying water as saline?

Approximately 30 parts per thousand (per mil), though this can vary depending on context.

Why is rainwater not salty despite originating from the ocean?

During evaporation and condensation in the water cycle, salt is left behind, resulting in freshwater precipitation.

What is turnover time in aquatic systems?

It is the time it takes for 50% of the water in a system to be replaced or cycled out.

What are lotic and lentic systems?

Lotic systems are flowing waters like rivers; lentic systems are still waters like lakes and ponds.

What human activities use the most water?

Agriculture uses the most, followed by industrial processes like transport and mineral extraction.

How do bacteria play a role in water usage?

Bacteria use large amounts of water in biological processes and play vital ecological roles like nutrient cycling.

Why is freshwater considered more biodiverse than marine systems?

Freshwater systems support more species relative to area due to varied habitats and inputs.

What is the socio-economic importance of inland fishing?

It supports food security, livelihoods, and cultural practices, especially in places like the Nile and Amazon basins.

What is subsistence fishing?

Fishing primarily for personal or community use rather than commercial gain.

What distinguishes lakes from ponds?

There's no strict definition; differences are often based on depth and surface area.

Where are most large lakes found?

In colder and more remote regions of the world.

What is 'fetch' in relation to lakes?

Fetch is the distance wind can travel across water without obstruction, affecting waves and mixing.

What is the euphotic zone in a lake?

The depth where only 1% of sunlight penetrates, limiting photosynthesis below this point.

What role do macrophytes play in lakes?

They provide habitat and food for organisms and structure to the ecosystem.

What are allochthonous materials?

Materials that originate from outside the lake system, transported in via wind, runoff, etc.

How do microbes behave in lakes?

They form mobile communities and migrate vertically with changing light levels.

What is the difference between algae and cyanobacteria?

Cyanobacteria are bacteria that photosynthesize; algae are eukaryotic organisms.

How is detritus classified in aquatic systems?

By particle size, which determines which organisms can consume it.

Why is organism size important in aquatic food webs?

Size affects who eats whom; smaller organisms are usually consumed by larger ones.

What organisms dominate primary consumer roles in lakes?

Zooplankton and other planktonic organisms.

What happens in anoxic zones of lakes?

Anaerobic bacteria dominate and produce gases like methane.

Why does ice float on water?

Ice is less dense due to hydrogen bonding, creating structured space between molecules.

Why is water good at buffering temperature?

It has a high specific heat capacity, resisting quick temperature changes.

What are hydrogen bonds and why are they important?

Weak bonds between water molecules that give water unique thermal and cohesive properties.

What happens to water molecules when frozen or vaporized?

In ice, molecules are structured and spaced out; in gas, they move freely without hydrogen bonds.

How does water dissolve salts?

Water molecules surround and separate ions due to their partial charges.

What is PAR and why is it important?

Photosynthetically Active Radiation is the light range usable by plants for photosynthesis.

What is light attenuation (K) in water?

A measure of how quickly light is absorbed or scattered with depth.

How does fish coloration relate to water depth?

Light wavelengths vary by depth; deeper-dwelling fish are often red due to reduced visibility.

What is lake stratification?

The separation of water into layers based on temperature: epilimnion, metalimnion, and hypolimnion.

What causes lake stratification to break?

Strong winds, storms, or seasonal changes can mix layers.

What happens under ice-covered lakes?

Stratification occurs again, with cold water on top and warmer water beneath.

How does productivity affect stratification curves?

More productive lakes may stratify more or show different oxygen and temperature profiles.

What is 'habitat squeeze' in lakes?

Fish are confined to narrow depth zones due to temperature and oxygen limits.

What are thermal guilds in fish?

Groups of fish with specific temperature preferences for growth and survival.

Why is there less oxygen in deep lake layers?

Lack of mixing prevents oxygen from reaching the hypolimnion.

What happens in anoxic lake zones?

Chemosynthetic bacteria dominate, producing compounds like hydrogen sulfide.