Time series of chlorophyll a fluorescence and temperature taken at a depth of 1 m were recorded at 4-min intervals for 136.5 h, at an anchor station located in the Middle Estuary of the St. Lawrence. The sampling program was carried out during the transition from neap tides to spring tides. Power spectral analysis was applied to the series and variance spectra were estimated. Coherency spectra between fluorescence and temperature were also computed. Results indicate that chlorophyll distribution was controlled by physical processes during neap tides. Conversely, lesser amounts of chlorophyll a per cell and lesser primary production rates were noted during spring tides. A flattening of the chlorophyll spectrum was observed for this part of the study. This result was interpreted in terms of the poor physiological state of the phytoplankton population that was sampled during the second part of the study, a different density structure being present due to the change in tidal energy. Key words: time series, chlorophyll, temperature, spectral analysis, St. Lawrence Estuary

Traditional methods of measuring surface net radiation involve point measurements that represent only a small area surrounding the instrumented sites. Remotely sensed spaceborne data offer the means by which to obtain estimates of the outgoing fluxes at the regional scale. The objective of this study was to estimate surface albedo, surface thermal exitance, and net radiation using Landsat Thematic Mapper (TM) data over wetland tundra at northern treeline near Churchill, Manitoba, Canada. Ground-based measurements of each component of the radiation balance were acquired at 5 locations coincident with 2 TM overpasses during summer 1991. Each location was representative of 1 of the major terrain types found in the Hudson Bay Lowlands (i.e. sedge-dominated wetland, upland lichenheath, tundra lakes and ponds, willow/birch wetland, and open spruce-tamarack forest). The mean absolute differences between remote sensing estimates and field measurements (all sites combined) are 0.01 for albedo, 25.7 W m -2 for thermal exitance, and 14.1 W m -2 for net radiation. The 2 summer 1991 TM images (June and August) were then used to examine within and between terrain type variations in surface net radiation during the growing season. TM imagery from August 1984 and August 1991 were also utilised to investigate differences in surface fluxes between a dry year (1984) and a wet year (1991). Results indicate that surface wetness and, to a lesser extent, phenology are the 2 main factors controlling the radiation balance during the summer period in this subarctic tundra-forest landscape.