The seasonal dyanamics of the phytoplankton in lakes

•Temperature adaptations of different algal groups

•Thermal stratification, sinking rates and nutrient dynamics

•Food-web interaction—effects of grazing zooplankton

mid-summer low biomasscommunity shifts to inedible forms

Early spring—diatoms dominate--under cold temperatures and low light conditionsplenty of nutrients in the well mixed water column

Summer—lake warms up, thermocline forms diatoms fall out of the mixing layer—low viscosity and low mixing depthAsterionella the only diatom that can still hang in.

Mid-summer—nutrients lost from mixed layer (sedimentation of algae), warm temperatures favour green algae, and zooplankton herbivory is high favouring fast growing small species eg Chlorella

Late summer—herbivores eliminate edible species, large colonial cyanobacteria dominate eg. Microcystis

Fall—water cooling, thermocline breaks up, mixing depth increases, nutrients increase, diatoms dominateWinter—low light and cold temp low biomass

Cyclotella:centric diatom (around 20 microns) and Stephanodiscus (around 50 microns)Usually abundant in spring plankton

colonial diatom—elongate cells joined at the base to form stellate colonies

Fairly large for planktonic diatoms, each cell 50-70 microns

Commonly found in dense blooms during May, prior to the onset of thermal stratification

Asterionella

Chlorella: a unicellular non-flagellated green algae

Copepods

CopepodLarvae--Nauplius

Cladocera Rotifers

Lake Zooplankton

Anabaena: cells in large colonies (filaments) with no gelatinous matrix, coiled or straight, heterocysts and akinetes usually present, cells 3-5 microns

Colony of MicrocystisA common bloom forming cynabacterium that sometimes can be highly toxic

Colonial growth pattern—cells embedded in a gelatinous matrix

Microcystis: cells in large colonies irregularly arranged within a gelatinous matrix

(3b)