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Dynamics and Succession of the Phytoplankton in a Tropical Lake: Lake Lanao, Philippines

William M. Lewis, Jr.
Journal of Ecology
Vol. 66, No. 3 (Nov., 1978), pp. 849-880
DOI: 10.2307/2259300
Stable URL: http://www.jstor.org/stable/2259300
Page Count: 32
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Dynamics and Succession of the Phytoplankton in a Tropical Lake: Lake Lanao, Philippines
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Abstract

(1) Phytoplankton dynamics in Lake Lanao, Philippines, were studied over a 16-month period on the basis of weekly abundance measurements for seventy species and a number of physical and chemical variables. (2) An analysis of growth pulses and growth correlations in the major classes of phytoplankton provides evidence that the niche space is divided temporally on the basis of nutrient and light availability. Growth maxima of diatoms and cryptomonads occur during periods of relatively low light availability and high nutrient availability; pulses of green algae, blue-green algae and finally dinoflagellates occur successively towards the high light and low nutrient end of the spectrum. (3) Variations in total autotroph biomass over the 16-month period were more closely related to factors affecting growth than to factors affecting loss of biomass. Loss control factors nevertheless influenced succession by their differential effects on individual species. (4) Major factors controlling phytoplankton growth include light availability (computed from measurements of incident light, mixing depth and transparency) and nutrient availability (computed from measurements of nutrient concentrations, turbulence and change in depth of mixing). Individual species were arranged by statistical methods according to their growth patterns along gradients of these two factors. There is an identifiable taxonomic trend in the separation similar to that observed in the class-level analysis of succession, but a number of species showed divergent behaviour from others of their taxonomic group. (5) Measurable factors governing loss of autotroph biomass include grazing rate (analysed on the basis of herbivore biomass) and sinking rate of cells (analysed on the basis of the mean Richardson Number over the euphotic zone). Individual phytoplankton species did not separate well on the basis of grazing intensity. Overall losses of autotroph biomass to grazing are quite low ($< 7%$ per day), which probably accounts for the limited influence of grazing on succession. A good separation of species, however, was obtained on the basis of their different sinking rates along a gradient of turbulence. There is a strong tendency for diatoms and cryptomonads to thrive when turbulence is maximal, for blue-green algae and dino-flagellates to thrive when turbulence is minimal, and for green algae to occupy the broad middle range of conditions. (6) Only the two species of Melosira showed any evidence of initiating growth pulses from meroplanktonic inocula; the role of the dormant phase in Melosira appears to be similar to that in temperate lakes. (7) The surface/volume ratio of biomass units plays an important role in determining the position of a species on the factor gradients. The S/V ratio of a species is significantly correlated with its position on the gradients of nutrient availability and sinking rate. Since the different phytoplankton classes have substantially different mean S/V ratios, these correlations appear to underlie the tendency of species to segregate on the factor gradients in relation to taxonomic affinities. (8) The overall pattern of succession in Lake Lanao shows some definite contrasts with that in temperate lakes. Phytoplankton succession can be regarded as a series of episodes initiated by abrupt changes in abiotic factors, and such episodes appear to be more numerous over a year in Lake Lanao than in most temperate lakes. Events within any given episode, however, are very similar in Lake Lanao and in typical temperate lakes. This hypothesis is supported by a statistical comparison of succession in Lake Lanao and Lake Erken, Sweden.

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