The regulation of phytoplankton productivity in a shallow, turbid, oligotrophic lake.
Akhurst, Edward Gordon John.
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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.