Lotic / lentic habitats
- Lotic: flowing waters (steams, rivers)
- Lentic: standing water (ponds, lakes)
Discharge of a river
Water flow expressed as volume per unit time (e.g. m3/s)
What are the main characteristics of lotic habitats (main differences with lentic habitats)
Lakes are island-like habitats whereas rivers form a continuum of habitats that are in open connection with each other. Water motion is a dominant environmental feature since it controls physical structure of the streambed (e.g. substrate size). Streams/rivers are closely linked to their catchments - they have a larger input of organic matter from the drainage basin (while lakes are more isolated from their environments). Year to year variations in nutrients are more pronounced than in lakes since in lakes the large volume dilutes the effects of inflows.
Black / white rivers. Why are black rivers not productive? And white rivers?
- Black water rivers are acidic + contain high amounts of humic substances (decomposed plant and animal matter). Low pH and low nutrient concs reduces primary productivity of phytoplankton and macrophytes
- White rivers: Higher nutrient conc due to suspended sediment load but primary productivity is reduced by high turbidity
- Junction of black and white rivers may create an area of high productivity (white = high nutrient and black = less light limited) e.g. main channel of the Amazon
River order
Smallest trickles are first order streams. When two first order streams join they become a second order stream. When 2 second-order streams join they make a 3rd order stream e.g. Mississippi river reaches 12th order before it reaches the ocean
River Continuum Concept: explain
- The structure and function of the benthic invertebrate gradually changes downstream with the food supply. Shredders common in lower order streams and grazers more abundant in middle order waters.
> For small streams (orders 1 to 3), high inputs of coarse particulate organic matter (CPOM) constitute the food supply for large shredders (e.g. crayfish, stoneflies). Fine particulate organic matter (FPOM) produced by CPOM users dominates central-lower rivers (orders 4-7) so sediment collectors or open water filtering species dominate (e.g. larvae). In large rivers (orders 8+), FPOM and grazers dominate. Primary production is very low because of low light levels associated with high turbidity. In large rivers, phytoplankton and zooplankton become more important because of low water velocity
P/R ratio’s in rivers
P/R indicates the ratio between primary production and respiration. In headwaters and large rivers, the system is primarily heterotrophic because of low production of organic material (P/R less than 1). In very small rivers the high input of organic material + speed of water movements impede the growth of phytoplankton. In very large rivers turbidity severely limits primary production. Only in mid sized rivers (order 4-6) is there substantial primary production (P/R > 1) by attached macrophytes and periphyton
What is the main difference between the food web of a lotic and lentic habitat?
- Organic matter is found as CPOM, FPOM and DOM leaches from soils or particulates and is modified by bacteria and fungi and then utilized by invertebrates
- Invertebrates dominate the benthic community whereas fish dominate the open water. Benthic invertebrate community is very diverse and consists of grazers (scraping off microbes and algae from solid surfaces), shredders (of CPOM), filter feeders (of FPOM) and predators (e.g. insects, crustaceans, molluscs)
- Crucial role of detritus (dissolved and particulate organic matter) - only fish and grazers are animals not directly involved in decomposition of detritus
- Phyto and zooplankton are only minor components of undisturbed lotic systems but can become important when current is reduced by dams
Benthic macro-invertebrates are often used to determine water quality in rivers. Explain why.
Abundant and found in water bodies throughout the world. They are not extremely mobile and can be easily caught. They carry out part or all of their life cycle within the river
Explain: drift (in rivers)
Drift contains many small living organisms + pieces of detritus = it is the major food of many stream fish. Since it is main food source for fish, it is important for invertebrates to keep attached to their substrates - adapted to have flat bodies, have hooks at the end of their legs
Why does one find most benthic macro-invertebrates in rivers at the underside of stones?
Most predations occur during the day so herbivores hide under the stones during the day and then feed at night
Hyporheic zone. Interstitial fauna
- Interstitial fauna: fauna of organisms living in gravel and sand
- Hyporheic zone: if gravel is coarse enough, zone in which water is still flowing slowly extends through the sediment well away from the main river bed. Water in this region is similar in chemical composition to that of the river
Flood pulse concept.
River continuum applies best to small streams and rivers. Big rivers are in intrinsic contact with surrounding land and often flooding a large area = led to the development of the flood pulse concept which stresses the importance of regular flooding in structuring the ecosystems of big rivers and their floodplains. Flood pulse concept states that floods are the principal driving force in the existence, productivity and interactions of the major biota in river floodplain systems
Varzea lakes: their importance and characteristics.
- Floodplain of the amazon is called the varzea and numerous varzea lakes mark the landscape. The white water rivers and black water tributaries are unproductive whereas the flooded varzea is extremely productive. Although the water is unsuitable for macrophytes (due to large water level fluctuations and absence of good substratum) or algae, there are lots of aquatic plants
- Flooding increases the nutrient content of rivers (mass input of organic matter) and the productivity of the floodplain (by deposition of suspended sediments from the river). Nutrients provided by the river leads to abundant vegetation in the floodplain, floodplain becomes a habit rich in food for fish and provides fish for a protected place to feed and spawn
- Very productive, rich in vegetation and extremely diverse
What is the importance of floating meadows in tropical lowlands
In early flood stages the overflowing waters become enriched with N and P leaching from the forest floor. The varzea lakes, which become depleted in the dry season, become more fertile as they receive nutrient-rich flood waters. Most organic production in varzea is recycled locally and through the terrestrial food chain. Using these nutrients, large free floating plants form a community unique to the tropics = the floating meadow - composed of water cabbage, water fern, duckweed.
Definition: estuary
Partially enclosed body of water with a freshwater inflow at one end and seawater introduced by tidal action at the other end. Features: variabile salinity; tidal action; interface between salt and freshwater; large areas of shallow, turbid water overlying mud flats and salt marshes
What is the typical vegetation in a temperate / tropical estuary?
Temperate: salt marshes
Tropical: mangroves
Why are estuaries productive?
- Tidal action brings nutrients and food to the organisms - particles get recycled due to tidal action rather than being lost to sediments like in stratified lakes
- Mixing of seawater and freshwater causes flocculation of particles into larger particles that are trapped at the salt and freshwater interface - that together with the bacteria growing on them make them very good food for zooplankton and filter-feeding benthos
- The shallow water and dark mud exposed at low tide is heated by the sun, increasing nutrient recycling through bacterial decay and accelerating the growth of benthic animals
- Presence of adjacent sea tends to reduce extremes in temperature
What is the ecological / economic importance of estuaries?
- Estuaries are important for the conservation of biodiversity - some species are unique to estuaries. Estuaries are important nursery areas - shallow, well mixed waters are ideal for high rates of photosynthesis and secondary production. Many fish and crabs spawn at sea and the eggs and juveniles are carried with the tides into the estuary
- Many estuaries are productive = fisheries and economically attractive for tourism and access to transport over sea
What is the ecological / economic importance of mangroves?
- Extremely productive systems
- Cut down for wood or drained and filled to provide industrial or residential sites
Why do mangroves have such extensive root systems?
- Mangroves support large growths of attached animals and plants on their root systems - roots are exposed at each tide. Roots form massive branches that grow away from the trunk of the tree and penetrate anoxic muds to obtain water and nutrients - the upper part of the roots supplies oxygen for the respiring root hairs below.
- The extensive root systems are essential to success of mangroves: provide oxygen, nutrients, water and protect the trees from being dragged away with tides or currents
