NATIONAL OCEANOGRAPHIC PROGRAM—1965 415 
Diaptomus minutus, D. sicilis, D. ashlandi, and D. oregonensis are 
especially plentiful in Lake Erie and in the upper strata of the other 
lakes. The coldwater copepods Limnocalanus macrurus and Senecella cala- 
noides are taken in the deep waters. 
Little is knows of the abundance of plankton in the lakes. Most studies 
have been made on Lake Erie. Generally speaking, peak plankton abundance 
occurs in the spring and again in the fall. The spring pulse consists al- 
most entirely of diatoms. Populations of blue-green and green algae build 
up during August and September and contribute to the fall pulse, although 
diatoms are still dominant. 
Large zooplankters of Lake Erie are abundant in summer and the smaller 
ones in spring and fall. Most of the Crustacea in Lake Michiyyan reach only 
one population peak a year. Cladoccrans and the copepods Mesocyclops cdax 
and Epischura lacustris probably overwinter as eggs. Cyclops bicuspidatus, 
which is a winter and early spring form in Lake Erie, seems to be plentiful 
throughout the entire year in the colder environment of Lake Michigan. 
The vertical distribution of eight species of cladocerans, nine 
species of copepods, and the two benthic malacostracans, Pontoporeia affinis 
and Mysis relicta was studied by the Bureau in Lake Michigan. All species 
migrated toward and attained their greatest numbers at the surface late in 
the day; apparently in response to diurnal changes in light intensity. The 
number of individuals at the surface decreased toward midnight and limited 
evidence indicated an increase for some species at the surface again slight- 
ly before dawn. A more detailed study of the mipratory behavior of M. 
relicta indicated that light "trireers" and controls their migrations, while 
thermal conditions interact with und modify the influence of lijht. Fol- 
lowing the summer solstice the mysids ascended progressively earlier each 
evening and descended later each morning. Moonlight and fog influenced the 
time as well as the amplitude of the vertical migration. The mysids fre- 
quently migrated through the thermocline when first ascending, but later 
in the night the majority occurred in or immediately below this layer. 
Attempts to obtain estimates of the productivity of Great Lakes 
phytoplankton populations have met with certain problems. The low plankton 
populations in Lake Superior make it necessary to concentrate samples for 
productivity measurements. Estimates of productivity made at a few stations 
are obviously not representative of conditions in an entire lake or even 
in the larger bays. Consequently, methods are being developed that will apply 
to large lakes. 
The deep-water bottom fauna of the Great Lakes is dominated by the 
amphipod, P. affinis, and the opossum shrimp, M. relicta. Olifochaetes, 
especially Limnodrilus, and the Sphocridiidac, Pisidium ond Sphnerium, are 
also numerous. Midge larvae of the menus Spaniotoma, are found at depths 
greater than 30 fathoms. The shallow-water fauna has many of the species 
common in the.smaller inland lakes. Hirudin-a and various Tendipedids are 
