Monday 11/16: River Ecosystems (Allan)
Assigned reading: http://en.wikipedia.org/wiki/Lotic_ecosystems
Lotic ecosystems
Lotic ecosystems: ecosystems of rivers, streams, and springs
Lotic refers to flowing water
Lotic ecosystems and lentic ecosystems (ecosystems of still continental waters such as lakes and ponds) form the fields of freshwater or aquatic ecology
Characteristics of the ecology of running waters:
Flow is unidirectional
Continuous physical change
A high degree of spatial and temporal heterogeneity at all scales
High variability between lotic systems
Biota is specialized to live in flow conditions
Abiotic factors
Flow
Key factor influencing ecology
Variables: strength, speed, amount of water input, etc.
Flow creates a variety of habitats (riffles, glides, pools, etc.)
Light
Provides energy to drive photosynthesis
Beer’s Law: the shallower the angle, the more light is reflected and the amount of solar radiation received declines logarithmically with depth
Other influences: seasonal/diurnal factors, width of river systems, cloud cover, altitude, geographic position
Temperature
Most lotic species are poikilotherms: species whose internal temperature varies with their environment
Influences: shading, climate, elevation, seasonal/diurnal factors
Chemistry
Determined most directly by inputs from the geology of the ecosystem’s watershed
Other influences: precipitation, pollution
Oxygen is the most important chemical constituent
Oxygen’s solubility in water decreases as water temperature increases
Substrate
Inorganic substrate of lotic systems is composed of geologic material in the watershed
Particle size decreases downstream
Substrate can be organic as well
Biota
Bacteria
Present in large numbers in lotic waters and play a large role in energy recycling
Primary producers
Algae (phytoplankton and periphyton) are the most significant producers
Phytoplankton float freely in the water column, whereas periphyton can attach themselves to objects
Some plants such as mosses and liverworts are adapted to living in fast flow conditions
Insects and other invertebrates
Insects comprise up to 90% of invertebrates in lotic systems
Examples: mollusks (snails, limpets, clams, mussels) and crustaceans (crayfish, crabs)
Fish and other vertebrates
The ability of fish to live in flowing waters depends upon swimming speed and duration
Trophic relationships
Energy inputs
Autochthonous: energy sources derived from within the lotic system (from photosynthesis, decomposition of organisms from the lotic system itself)
Allochthonous: energy sources derived from outside the lotic system (leaves, twigs, fruits, etc.)
Invertebrates
Many feeding guilds (shredders, suspension feeders, gatherer-collector, grazers)
Fishes
Feeding guilds (planktivores, herbivore-detritovores, top predators, omnivores)
Community patterns and diversity
Local species richness
Large rivers typically have more species than small streams
Resource partitioning
Three main types
Habitat segregation: most common type, often a separation of species by substrate preferences
Dietary segregation: second-most common type, often morphological specializations or behavioral differences (i.e., feeding guilds)
Temporal segregation: less common, often due to differences in life history patterns or timing of maximum growth among guildmates
Persistence and succession
Long-term, there is a tendency for species composition in pristine systems to remain in stable states
Short-term, persistence levels decline due to flow variability and precipitation patterns decreasing habitat stability
The River Continuum Concept (RCC)
RCC: an attempt to construct a single framework to describe the function of temperate lotic ecosystems from the source to the end and relate it to changes in the biotic community
Factors to consider: size and location along the gradient from a small stream eventually linked to a large river (stream order), CPOM (coarse particulate organic matter), FPOM (fine particulate organic matter), etc.
Human impacts
Pollution
Sources: agricultural fields, urban/residential areas, acid rain
Flow modification
Dams alter the flow, temperature, and sediment regime of lotic systems
Invasive species
Organisms affect natives via competition, predation, habitat alteration, etc.
