The RV Investigator is currently undertaking oceanographic research along the 110°E meridian off Western Australia as part of the second International Indian Ocean Expedition. The voyage is led by Professor Lynnath Beckley of Murdoch University and the research is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility.

Date: May 29, 2019	Time: 1200 AWST
Latitude: 21.5°S	Longitude: 110°E
Wind direction: SE	Wind speed:10 knots
Swell direction: S 1 m	Depth: 5070 m
Air temperature: 25°C	Sea temperature: 25°C
Notes: Beautiful sunny weather, warm seas and lots of tropical flying fish.

Phytoplankton can go through one or two generations (cell divisions) per day and are often eaten almost as fast as they divide, by protozoan grazers. Because the size ranges of phytoplankton and microzooplankton broadly overlap, they cannot be mechanically separated from one another to determine these two different rates. We do, however, have a technique for this, which involves dilution of the grazing impact. This entails changing the encounter frequency of predators and prey using filtered water with the same chemistry, from the same depth. We create two conditions-one with natural concentrations of predators and prey and the other in which the grazing has been slowed by about 2/3rds. From the differences in the measured rates of increase of phytoplankton of the two treatments, we can solve the equations for the two unknown rates.

We run our experiments in a seawater-cooled, shipboard incubator system that has 6 light levels simulating the conditions of underwater light at the six depths where we collected our samples from the CTD. The change in phytoplankton concentration in each bottle is measured by chlorophyll and by flow cytometry analyses of dominant populations. After a lot of filtering, analyses and calculations, we will have a pretty good idea of how much of phytoplankton productivity is consumed by microzooplankton each day, how that varies with depth in the euphotic zone, and if there are significant differences among the stations or regions that we are sampling along the 110°East line.

Be sure to follow the daily posts of our One Ten East Logs from the IN2019_V03 aboard RV Investigator at https://iioe-2.incois.gov.in and https://wamsi.org.au