Diel vertical migration and nutrient transport of the
dinoflagellate Peridinium bipes f. occultatum in a
thermally stratified reservoir
T. Inoue and Y. Iseri
ABSTRACT
Field observations were conducted with respect to the vertical migration of the dinoflagellate
Peridinium bipes and its influence on vertical nutrient transport in a stratified reservoir. P. bipes
migrated vertically with circadian rhythms; that is, P. bipes migrated downward and accumulated in
the top layer of the hypolimnion from evening to night and then migrated upward and accumulated
in the epilimnion in the early morning. In the top layer of the hypolimnion, a rapid decrease in
nutrient concentration was observed in the evening, presumably due to uptake by P. bipes. Active
migration from the epilimnion to the top of the hypolimnion was also suggested by estimation of
the descent velocity of P. bipes, which was advantageous for the uptake of nutrients. Moreover,
based on the equilibrium of forces during migration, the energy cost of vertical migration could
be inferred to be negligible. The results suggest that active vertical migration of P. bipes played an
important role in nutrient transport between the hypolimnion and the epilimnion in a stratified reservoir.
T. Inoue (corresponding author)
Coastal and Estuarine Environment Research Field,
Port and Airport Research Institute,
3-1-1 Nagase,
Yokosuka 239-0826,
Japan
E-mail: inoue-t@ipc.pari.go.jp
Y. Iseri
West Japan Engineering Consultants, Inc.,
1-1-1 Watanabe-dori,
Chuo-ku,
Fukuoka 810-0004,
Japan
Key words | dinoflagellate, nutrient transport, stratified reservoir, vertical migration
INTRODUCTION
The water blooms of specific phytoplanktons and freshwater
red tides cause serious problems in enclosed waters, and
nutrient concentration is considered as one of the most
important parameters for the problem. As phosphorus is
supplied due to the benthic release or microbial degra-
dation, phosphorus depletion will be the most important
limiting factor for primary production when the vertical den-
sity stratification is secure in a water column. Dinoflagellates
overcome such nutrient depletion by vertical migration to
access a layer in which the nutrient concentration is high
enough for primary production (Lunven et al. ; Town-
send et al. ; Ralston et al. ). Their motility allows
them to accumulate not only in a nutrient-rich layer (nutri-
cline), but also in a preferred layer where the light
conditions are optimal for reproduction ( Ji & Franks
). The vertical migration of dinoflagellates has fre-
quently been investigated, and their circadian rhythm is
considered to be related to the photoresponse, endogenous
rhythm, and internal nutritional state (Forward & Daven-
port ; Ji & Franks ; Ralston et al. ). However,
there are few studies on nutrient transport via the vertical
migration of freshwater phytoplankton (Yamada et al.
). This led us to investigate nutrient uptake and trans-
port due to the vertical migration of Peridinium bipes,
which is often dominant in oligotrophic or mesotrophic
lakes and reservoirs, based on field observations at a
small, stratified reservoir with no obvious current.
MATERIALS AND METHODS
Field observations
Field observations were conducted at Imuta Reservoir, a
man-made reservoir located in Fukuoka prefecture, Japan
(15,000 m
2
in area, 5.0 m in maximum depth, 33
W
38.9
0
N,
130
W
30.1
0
E). The reservoir, surrounded by paved roads
and a golf course, is in a predominately residential area.
Because there are no inflowing or outflowing rivers, the
water quality is mainly controlled by weather conditions,
and horizontal water-quality profiles are homogeneous. On
average, conductivity, pH, and oxidation–reduction poten-
tial during the summer season are 150 μS cm
–1
, 9.2, and
140 mV in the surface layer and 200 μS cm
À1
, 6.6, and
1212 © IWA Publishing 2012 Water Science & Technology | 66.6 | 2012
doi: 10.2166/wst.2012.302