Biological/Ecological Limnology of Lakes
 

Phytoplankton
~1 - 50 µm


 

Some major groups of algae that occur as freshwater phytoplankton:


 
If all phytoplankton species require essentially the same resources, how can so many species co-exist within the same lake?  ("paradox of the plankton")



What keeps phytoplankton from settling out?
 

Why are phytoplankton common in lentic systems but not lotic systems?


What determines the amount of biomass phytoplankton in a given lake?



Other primary producers in freshwater


 
 
 

Primary Productivity  - the amount of organic carbon fixed per unit time
 

photosynthesis:


Measuring Production and Productivity
Why is this an important measurement?

Estimates of standing crop (biomass):

 

Does high standing crop indicate high productivity?

Can you tell from these snapshots, which roads is transporting more vehicles per minute?

What conditions might result in lower than expected phytoplankton biomass despite high rates of primary productivity?

 

Estimates of productivity (rate of production):
 
 


  What are potential problems with these methodologies?
Zooplankton

TAXONOMY OF FRESHWATER ZOOPLANKTON
(extensive photo gallery from Southwest Missouri State University)
 

~1500 species, size between 0.04 and 2.5 mm
feeding: wheel of cilia (only size selective) filter feeding on bacteria, small algae, but some predators
 

Ecologic interactions may be subtle, for example release of molecules (allelopathic substances) may affect morphology.



~450 species, 0.2 - 6 mm (Leptodora kindtii 18 mm)
 
reproduction: parthogenetic so populations mostly female, eggs being incubated under carapace.
 

feeding: filter feeding on larger algae, but a few predators


What are the function of antennae and of legs?

 
 
 

  • Copepods
    • Similar size range as cladocerans

    generation time:     usually 1-3 generation per year depending on species, usually life span one year.
     

    reproduction: sexual with male tracking female. Molting with 6 nauplier stages, and 5 copepodids.  Often with diapause stage.


    feeding: filter feeders (most calanoids) on larger algae, often predators.   Typically strong swimmers.
     

    Three main groups of copepoeds:

    Species diversity in freshwater plankton communities is high for copepods, as is also the case for cladocerans and rotifers.
  • Insects - Chaoborus -non-visual predators, meroplanktonic
    •     

    In smaller ponds and in marine systems, other taxonomic groups important components of the plankton
     
     
     
     
     
     

    ECOLOGY OF ZOOPLANKTON
     


    http://www.cladocera.de/
     
     
     
     

    Vertebrate Nekton
    mostly fish, but may also include amphibians
     

    Various consumer trophic levels and feeding strategies depending on species.   Includes:

    What type of zooplankton might be selected by this type of predator?
    As turbidity increases, predator ability to dectect prey decreases faster than ability of prey to detect and respond to predators.
    What type of zooplankton might be selected by this type of predator?
    How might fisheries differ in natural lakes versus reservoirs?

    Intermediate between these modes of feeding would be buccal gulping of dense groups of prey (also light dependent)

     
     

    Trophic cascade (top-down management)
     


     
     
     
     
     
     
     

    But food webs in lakes, particularly in the southeast USA, are more complex:


     
     
     

    What are the effects of invertebrate predators?

    How might turbidity affect plankton community?
     
     
     
     
     

    Benthos
    At a macroscopic scale, generally insect larvae, mollusks, annelids...
    (How does diversity of freshwater benthos compare with marine benthos?)




    In lakes, what would be a primary factor in explaining the distribution of benthic animals?  Why?
    How might distribution and abundance differ in a reservoir?

                  

    Predation can effect body size of lake benthos as well.
         
    Introduction of zebra mussels has coincided with declines in native mussels, and greatly altered phytoplankton and macrophytes in lakes.

    In what kind of aquatic systems might benthos be more ecologically important?

    Quantitative sampling of benthos can be difficult.
           


    The microscopic     community of animals living between grains of sediment (referred to as psammon or meiofauna) differs taxonomically from the macrobenthos.