Population Interactions Competition for Resources: –Exploitative competition: Both organisms competing for the same resource(s). –Interference competition (amensalism): Organism exert direct, negative effects on another (allelochemical and allelopathy) –Competitive interactions can get interesting when two species compete for more than one resource with differing capabilities. Predation (mortality): –Prey population declines when growth rates slows below predation rate (and other mortality terms) –Predator Avoidance: Mechanical defenses: spines, filaments, gelatinous aggregates. Chemical defenses: allelochemical and allelopathy (taste nasty) Life history defenses: growth rate / reproduction tradeoff Behavioral defenses: diel vertical migrations (e.g. zooplankton) –Predator-Prey (Functional Response) Models.

Two species competing for Si and P resources. Curves represent growth rate under given nutrient concentrations. Note that the two species differ in their abilities to compete for different resources. Species 1 needs higher [Si] to survive competition. Species 2 needs higher [P] to survive competition.

Predator-Prey Models Type I: e.g. Lotka-Volterra. For a given predator density, prey consumption increases linearly with prey density. Type II: e.g. Holling disc equation. Includes search and handling time of prey, following structure of Michaelis-Menton equation. (e.g. microbes, zooplankton) Type III: Introduces concept of “learning” and increase in predator efficiency with increase in prey density. (e.g. fish)

Trophic Cascades Interactions at higher levels of the food chain have a cascading influence down through lower levels. –Bottom-up control: Primary production is controlled by limitations of abiotic factors (light, nutrients, etc.) –Top-down control: Primary production is controlled by predation on herbivores. Trophic cascades in aquatic systems; e.g. piscivores and phytoplankton biomass. –With piscivore, larger population of zooplankton crustaceans, graze down phytoplankton. –Removal shifts dominance to planktivorous fish and loss of large zooplankton and switch to rotifers; phytoplankton bloom that are resistant to rotifer grazing.

3. Spring Circulation Low but increasing temperature Mixing water column with low stability Low (but variable and increasing) light availability high nutrient availability (why?) Rapid growth and increases in phytoplankton biomass, particularly diatoms. Often represents period of highest annual biomass. Increasing light is dominant contributing factor; zooplankton grazing remains low for now.