Macroflocs from diatoms: in situ photography of particles in Bedford Basin, Nova Scotia

Abstract

Observations of a diatom bloom by in situ photography showed that at the beginning of the bloom the diatoms occurred only as discrete cells and were limited to the surface mixed layer. Below the thermocline, suspended particulate matter concentrations were low and dominated by 200 to 350 p-~ diameter flocs composed mainly of fine-grained detritus and terrestrial sediment. Some time after the beginning of the bloom diatoms started to clump together into aggregate particles, many mm in diameter, composed of a loose network of cells. Flocculation promoted by the high particle concentrations and the development of normal marine surface stickiness caused the diatoms to settle through the thermocline. A progressive increase in the apparent robustness and density of the flocs occurred with depth. The macroflocs could only be detected using the in situ photographs. They did not survive normal Niskin bottle sampling, and Coulter Counter analysis produced particle spectra reflecting only the size of the constituent diatoms and sediment grains. Observations indicate that studies of particle size based on water samples give misleading results and the size of particulate matter in general has been considerably underestimated in past studies based on conventional sampling methods.