Vertical distributions of chlorophyll in deep, warm monomictic lakes |
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| Authors: | David P Hamilton Katherine R O’Brien Michele A Burford Justin D Brookes Chris G McBride |
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| Institution: | (1) Centre for Biodiversity and Ecology Research, University of Waikato, Hamilton, 3240, New Zealand;(2) Division of Environmental Engineering, University of Queensland, St Lucia, 4072, Australia;(3) Australian Rivers Institute, Griffith University, Nathan, QLD, 4111, Australia;(4) Water Research Cluster, University of Adelaide, Adelaide, SA, 5005, Australia |
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| Abstract: | The factors affecting vertical distributions of chlorophyll fluorescence were examined in four temperate, warm monomictic
lakes. Each of the lakes (maximum depth >80 m) was sampled over 2 years at intervals from monthly to seasonal. Profiles were
taken of chlorophyll fluorescence (as a proxy for algal biomass), temperature and irradiance, as well as integrated samples
from the surface mixed layer for chlorophyll a (chl a) and nutrient concentrations in each lake. Depth profiles of chlorophyll fluorescence were also made along transects of the
longest axis of each lake. Chlorophyll fluorescence maxima occurred at depths closely correlated with euphotic depth (r
2 = 0.67, P < 0.01), which varied with nutrient status of the lakes. While seasonal thermal density stratification is a prerequisite
for the existence of a deep chlorophyll maximum (DCM), our study provides evidence that the depth of light penetration largely
dictates the DCM depth during stratification. Reduction in water clarity through eutrophication can cause a shift in phytoplankton
distributions from a DCM in spring or summer to a surface chlorophyll maximum within the surface mixed layer when the depth
of the euphotic zone (z
eu) is consistently shallower than the depth of the surface mixed layer (z
SML). Trophic status has a key role in determining vertical distributions of chlorophyll in the four lakes, but does not appear
to disrupt the annual cycle of maximum chlorophyll in winter. |
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