Understanding grazing experiments: length and nutrients addition effects

Abstract

The dilution experiment is the most used method for measuring microzooplankton grazzing rates in the ocean. The 2-point method, a simplification of the dilution method, has achieved less tedious and more workable procedures during oceanographic cruises. Standard experiments consist in incubating whole seawater (WSW) and a dilution of 5% WSW for 24 hours. However, there is scarce information about how the length of experiments and nutrient additions affects grazing rates. Here we show that mortality raate varied depending on the experiment length and nutrient addition. During the first 12 h, mortality rates were variable, especially for Synechoccocus. However, higher grazing rates were achieved during the last 122 h of incubation for picoeukaryotes, Synechoccocus, Prochloroccocus and total phytoplankton. Moreover, the longer incubation, the lower mortality rates. Grazing rates over 24 h were not significantly different in nutrient amended or nonamended experiments. However, enriched nutrient experiments showed higher and more variable grazing rates during the first 12 h. These results suggest that longer experiments may underestimate mortality rates, and nutrient addition affects phytoplankton in the early hours of incubation.