The regulation of phytoplankton productivity in a shallow, turbid, oligotrophic lake.
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Date
1988
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Abstract
Aspects of the physical and chemical environment likely to influence
phytoplankton productivity were investigated in Lake Midmar, a shallow
(mean depth 11.4m), oligotrophic impoundment, over a three year period
to evaluate the hypothesis that "the fundamental process regulating
the functioning of a shallow lake is vertical mixing."
Energy exchange at the lake surface was similar to that reported for
other lakes. The principal components of the energy balance (net
incoming radiation and latent heat loss) and heat content of the lake
varied seasonally. However, day-to-day variation in local weather,
particularly the incidence of cloud cover, masked these seasonal
trends and contributed to the unusual pattern of almost constant
sensible heat loss throughout the year. Consequently, heat storage was
highly variable, particularly in early summer (September-December).
The combination of day-to-day variation in heat storage, the pattern
of heat loss, wind stress and the synergistic interaction between
these components together contributed to the observed pattern of weak
summer stratification (absence of vertical partitioning into an epi-,
meta- and hypolimnion) and efficient heat transfer within the water
column. The occurence of low Wedderburn numbers ( W < 1) on several
occasions during summer (September - February) resulted in Midmar
being classified as a regime 2 lake (sensu Spigel and Imberger,1980)
and the thermal structure as discontinuous polymictic (sensu
Lewis,1983 a).
Under conditions of 'normal' river flow (1980-81) phosphorus
associated with river suspended solid loads, even at times of peak
loading in late summer, had little immediate influence on the total
phosphorus content of the water column or phytoplankton productivity.
Internal phosphorus loading, resulting from sediment resuspension and
transfer of phosphorus from sediment pore water to the overlying
water, was identified as the principal source of bioavailable
phosphorus. The wind-induced circulation - sediment interaction also
exerted a strong influence on the underwater light climate. Inorganic
suspended particulate material was the principal factor regulating the
attenuation of photosynthetically active radiation (PAR). Red light
being the most penetrating component of the PAR spectral range
throughout the study period . The hypothesis that 'internal",
autochthonous processes and not external, allochthonous processes were
responsible for regulating phosphorus dynamics and PAR attenuation" was confirmed during the drought (1982-83). As lake levels decreased
the increased interaction between wind- induced circulation and the
deeper sediments led to increased PAR attenuation and total phosphorus
concentrations in the water column.
The range of turbulent conditions arising from variation in the
magnitude and frequency of wind- induced circulation created a
temporally variable environment characterised by low values of the
ratio Z[eu]:Z[m] rapidly changing light levels to which algal cells would
be exposed in any day (a result of the turbid, turbulent nature of
the environment) and nutrient patchiness (resulting from variation in
internal phosphorus loading). The subsequent variation in the
physiological state of the phytoplankton at the time of sampling
resulted in estimates of ΣA, P[max] and the physiological indices, I[K],
P[e] and assimilation number, being very variable from one sampling
occasion to the next. This variation in phytoplankton productivity and
physiological state at the time of sampling was considered to be
responsible for the general absence of any correlation between ΣA, the
physiological indices and temperature as well as the observed
variation between the real values of ΣA and those predicted using
Talling's model. Although the increased wind-induced circulation -
deeper sediment interaction during the 1982-83 drought caused changes
in the underwater light climate (as increased PAR attenuation and
shallower euphotic zones) these changes had little effect on values of
I[K], and P[e]. However, from the increase in values of ΣA, phytoplankton
standing crops and assimilation numbers recorded in 1982-83 it was
concluded that phytoplankton productivity in Lake Midmar was nutrient and
not light-limited.
The role of vertical mixing as a factor influencing temporal changes
in phytoplankton species composition is examined in the general
discussion. Two properties of the mixing regime are considered to be
important, i.e. the energy associated with a particular mixing event
and the frequency of high energy mixing events. The concept of
disturbance sensu White and Pickett (1985) was found to be useful in
distinguishing 'normal' low energy advective and vertical mixing
involving only the surface waters from disturbance, high energy
vertical mixing involving most or all of the water column which is
capable of modifying existing environmental gradients. Based on the
frequency of wind-induced disturbance a continuum can be shown ranging
from deep stratified lakes, where disturbance as changes in mixing regime associated with the onset and breakdown of stratification is
infrequent, seasonal and predictable from one year to the next, to
very shallow lakes, where disturbance as extensive vertical mixing is
more frequent, aseasonal and unpredictable.
Shallow impoundments, e.g. Lake Midmar, occupy an intermediate
position between the two extremes of the continuum. The exact location
of a particular shallow lake being determined by site-specific
properties of the energy exchange-wind stress interaction and
resultant mixing regime. It is considered that the failure to
recognise the intrinsic variation between shallow impoundments and the
importance of site-specific differences in determining the importance
of wind-induced vertical mixing in the functioning of shallow
impoundments led to the earlier classification of South African
shallow impoundments as warm monomictic systems by Walmsley and Butty
(1980). Furthermore, this property of shallow lakes (i .e. the sitespecific
differences in the energy exchange-wind stress interaction)
probably accounts for the limited success with which general lake
models have been applied in shallow lakes.
Disturbance has not been recognised as a source of temporal and
spatial heterogeneity in the physical and chemical environments of
lakes because of the failure of existing models to distinguish between
disturbance and perturbation and their roles in initiating changes in
phytoplankton species composition. Based on the results of this study
a revision of Reynolds's model describing temporal changes in
phytoplankton species composition which incorporates both disturbance
and perturbation is proposed.
The results of this study therefore confirmed the original hypothesis
that "the fundamental process regulating the functioning of a shallow
lake is vertical mixing" with summer thermal structure, the
attenuation of downwelling PAR and phosphorus dynamics all being
directly influenced by wind-induced vertical mixing. The resultant
temporally variable environment also influenced the physiological
state of the phytoplankton and estimates of primary productivity, and
is considered to exert a strong indirect influence on phytoplankton
seasonal periodicity in this shallow, oligotrophic, turbulent
impoundment.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1988.
Keywords
Phytoplankton--Natal., Theses--Botany.