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Size‐dependent changes in phytoplankton C and N uptake in the dynamic mixed layer of Lake Biwa
Author(s) -
FRENETTE JEANJACQUES,
VINCENT WARWICK,
LEGENDRE LOUIS,
NAGATA TOSHI
Publication year - 1996
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1046/j.1365-2427.1996.00083.x
Subject(s) - phytoplankton , mixed layer , ecology , environmental science , layer (electronics) , oceanography , biology , chemistry , nutrient , geology , organic chemistry
1. Short‐term (days) hydrodynamic effects of wind‐induced mixing on phytoplankton size structure, and C and N uptake characteristics, were studied in the surface mixed layer (epilimnion) of Lake Biwa (North Basin), before and during a period of high winds (typhoons). 2. The latter period was characterized by two major typhoon events associated with deepening of the seasonal thermocline, reduced water column stability, decreased underwater irradiance and increased dissolved reactive N and particulate P. 3. Nutrient concentrations, seston C/N ratios, and uptake rates indicated that phytoplankton biomass and production were limited by P and not N throughout the study. Higher C‐ and N‐based productivity during the typhoon period than before reflected the increased phytoplankton biomass and higher specific uptake rates due to increased nutrient supply. 4. Changes in the size‐structure of phytoplankton (< 2 and > 2 μm) were associated with variations in the stratification and mixing regime. When vertical stability was high (before the typhoons) concentrations of > 2 μm biomass (chlorophyll a , particulate organic C and N) were higher at the bottom of the mixed layer than at the surface whereas, when stability of the mixed layer was low (the typhoon period), the contribution of picoplankton (< 2 μm) to total Chl a increased at the surface and decreased at the bottom following the first high winds. 5. Photoadaptive adjustments of the phytoplankton provided further evidence of hydrodynamic control. The lower intracellular Chl a concentrations and C and N uptake efficiencies in the < 2 μm fraction suggest that they experienced, on average, higher irradiance than the larger cells because of their lower sinking rates. During the stability period, picoplankton exhibited higher photosynthetic efficiencies at the bottom of the mixed layer than at the surface. Such differences disappeared during the typhoon period indicating that the mixing rate was then probably higher than the photoacclimation rate in the small size fraction. 6. The present results stress the highly transient nature of biological homogeneity in the surface mixed layer of the lake.