Physical properties Physical properties of aquatic ecosystems are determined by a combination of heat, currents, waves and other seasonal distributions of environmental conditions. The
morphometry of a body of water depends on the type of feature (such as a lake, river, stream, wetland, estuary etc.) and the structure of the earth surrounding the body of water.
Lakes, for instance, are classified by their formation, and zones of lakes are defined by water depth.
River and
stream system morphometry is driven by the underlying geology of the area as well as the general velocity of the water.
Thermal stratification Similar to light zonation, thermal
stratification or thermal zonation is a way of grouping parts of the water body within an aquatic system based on the temperature of different lake layers. The less
turbid the water, the more light is able to penetrate, and thus heat is conveyed deeper in the water. Heating declines exponentially with depth in the water column, so the water will be warmest near the surface but progressively cooler as moving downwards. There are three main sections that define thermal stratification in a lake. The
epilimnion is closest to the water surface and absorbs long- and shortwave radiation to warm the water surface. During cooler months, wind shear can contribute to the cooling of the water surface. The
thermocline is an area within the water column where water temperatures rapidly decrease. These lakes are often
dimictic, with a brief spring overturn in addition to a longer fall overturn. The
relative thermal resistance is the energy needed to mix these strata of different temperatures.
Lake heat budget An annual heat budget, also shown as θa, is the total amount of heat needed to raise the water from its minimum winter temperature to its maximum summer temperature. This can be calculated by integrating the area of the lake at each depth interval (Az) multiplied by the difference between the summer (θsz) and winter (θwz) temperatures or \displaystyle \intAz(θsz-θwz) In streams and small lakes, allochthonous sources of carbon are dominant while in large lakes and the ocean, autochthonous sources dominate.
Oxygen and carbon dioxide Dissolved oxygen and dissolved
carbon dioxide are often discussed together due their coupled role in
respiration and
photosynthesis. Dissolved oxygen concentrations can be altered by physical, chemical, and biological processes and reaction. Physical processes including wind mixing can increase dissolved oxygen concentrations, particularly in surface waters of aquatic ecosystems. Because dissolved oxygen solubility is linked to water temperatures, changes in temperature affect dissolved oxygen concentrations as warmer water has a lower capacity to "hold" oxygen as colder water. Biologically, both photosynthesis and aerobic respiration affect dissolved oxygen concentrations. Vertical changes in the concentrations of dissolved oxygen are affected by both wind mixing of surface waters and the balance between photosynthesis and respiration of
organic matter. These vertical changes, known as profiles, are based on similar principles as thermal stratification and light penetration. As light availability decreases deeper in the water column, photosynthesis rates also decrease, and less dissolved oxygen is produced. This means that dissolved oxygen concentrations generally decrease as you move deeper into the body of water because of photosynthesis is not replenishing dissolved oxygen that is being taken up through respiration. Therefore, most water quality studies tend to focus on
nitrate,
nitrite and
ammonia levels.
Lake trophic classification One way to classify lakes (or other bodies of water) is with the
trophic state index. An oligotrophic lake is characterized by relatively low levels of
primary production and low levels of
nutrients. A eutrophic lake has high levels of primary productivity due to very high nutrient levels.
Eutrophication of a lake can lead to
algal blooms.
Dystrophic lakes have high levels of
humic matter and typically have yellow-brown, tea-coloured waters. These categories do not have rigid specifications; the classification system can be seen as more of a spectrum encompassing the various levels of aquatic productivity. == Tropical limnology ==