The phytoplankton of a eutrophic lake : community dynamics and ultrastructural studies

Sawley Dene, a small secluded lake in North Yorkshire,
supports large populations of diatoms in spring and
blue-green algae in late summer. Characteristics of
the drainage area give this shallow lake calcareous water
and a long retention time; although persistent summer
stratification does not occur, the summer algal populations
resemble those in the epilimnion of a stratified eutrophic
lake. 32 principal phytoplankton species from seven
algal classes are illustrated by light micrographs and
their periodicity recorded over two annual cycles;
a statistical method is employed to produce graphical
summaries of the patterns of species replacement. Absolute levels of algal abundance are correlated with
concentrations of chlorophyll a and with varying Secchi
disc visibility . Scale-bearing planktonic organisms
from Sawley Dene were studied as whole mounts in the
electron microscope and 30 taxa are distinguished,
including 17 species of flagellate Chrysophyceae (Mallomonas and related genera). Comparative studies on samples
from nearby lakes and pools show that a number of rare
species occur in the Sawley area but are absent from
Sawley Dene; it is suggested that Sawley Dene is too
large and too calcareous to support a very diverse
chrysophycean microflora. A systematic account of all
the scale-bearing species found in the study area is
presented and five new taxa of Chrysophyceae are distinguished, including two from Sawley Dene; these are given provisional names but will be formally described elsewhere. Mixed
phytoplankton samples were embedded for electron microscopy
and noteworthy features of four species were studied in
detail : these include scale formation and new cytological
detail in Paraphysomonas vestita , colony structure and
scale formation in Synura petersenii, serial sectioning
and 3-dimensional representation of the transverse flagellum
of Peridinium cinctum and a study of the surface morphology
of the scale-bearing Heliozoan Raphidocystis tubifera.