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Spectra1 absorption of phytoplankton from cultures and natural samples was measured on filters with various optical setups including collimated and diffuse irradiation and measurements ofwetted filters within an integrating sphere. In suspensions within an integrating sphere, specific absorption coefficients for laboratory cultures varied by a factor of only two.Measurements on filters yielded values dependent on filter load. Specific absorption coefficients derived from measurements of sample filters were considerably higher than values obtained from suspensions in an integrating sphere due to increased diffuseness of irradiance and to pathlength amplification by filter-particle and particle-particle interactions. Measured absorption of phytoplankton in the blue can be increased greatly by absorption of detritus, evident from absorption spectra of depigmented samples on filter. After subtracting detrital absorption, absorption spectra of phytoplankton are qualitatively similar to the corresponding quantum-corrected fluorescence excitation spectra. The detritus-corrected ratio of absorption at 440 vs. 675 nm shows average values between 1 and 1.5.Comparison with published values shows that specific in vivo absorption coefficients of phytoplankton arc mostly overestimated as a result of the methodology applied. In the blue region of the spectrum, overestimation of phytoplankton absorption in field samples is possible if detrital absorption is neglected.

Spectral absorption of phytoplankton from cultures and natural samples was measured by means of an integrating-sphere photometer. Measurements on suspension yielded specific absorption coefficients for healthy phytoplankton cultures in collimated light at 675 nm between 0.007 and 0.0 13 m2 (mg Chl a) I. These values were independent of cell density and cell wall material.Natural samples of Kiel harbor water yielded specific absorption coefficients at 675 nm of 0.009 and 0.017 m2 (mg Chl a)-I. The specific absorption coefficients in suspension were dependent on the optical properties of the individual cells (packaging effect), decreasing (ca. l/r) with increasing cell size (r radius) and increasing chlorophyll a content per cell as predicted by theory, but are significantly lower than theoretically expected. Comparison with published values shows that specific absorption coefficients of cell suspensions are often overestimated, presumably because of methodological problems.

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'Burst' series consisting of 5 chlorophyll-fluorescence profiles at 3 rnin intervals were taken every hour at 3 drift stations over periods of about 15 h each in the transition zone between North and Baltic Seas using a fluorescence sonde, pre-calibrated into i n vivo Dunaliella chlorophyll units. Differences in the field between fluorometric readings and water samples analysed photometrically were not significant. Chlorophyll layers in the Kattegat were about l m thick, floating on isopycnal surfaces. Microscale horizontal variability (= standard deviation of 1 burst of profiles) was generally high, reaching a maximum in the pycnocline at high chlorophyll concentrations. Mesoscale variability ( = standard deviation of all profiles) observed in chlorophyll, temperature and salinity, may be due to horizontal displacement, advection, and internal waves. It appears lrnpossible for the data presented here to separate the spatial and temporal components of the chlorophyll distribution pattern.