Routine determination of algal dry cell weight (DCW) was obtained by measuring the optical density of chlorophyll at 680 nm (OD680) using a spectrophotometer (Shimadzu UV-265). The conversion of OD680 to DCW was accomplished by generating a dilution series for each species, recording the OD680, and then collecting samples onto pre-weighed cel­lulose acetate membranes (Pall Co., Port Washington, NY, USA), which were then dried in a vacuum oven (15 in. Hg., 60 oC) for 12 h before ob­taining the final weights. To avoid optical filtering effects, samples were diluted, if need be, such that the OD680 was always less than 1.8.