Microzooplankton Grazing Impact on Phytoplankton After a Storm in Kaneo‘he Bay, O‘ahu

Abstract

A relatively new area of research has developed over the past 30 years, that of the microbial food web and the grazing dynamics therein. Microzooplankton (the 20-200 µm size category including copepod nauplii, ciliates, and flagellates) are the principal grazers of phytoplankton. Nanoplankton (2-20 µm) were also included in this study, since they are known to graze picoplankton (0.2-2 µm), the smallest phytoplankton in the microbial food web. Our research focused on the effects the presence or absence of nutrients had on the composition of phytoplankton and grazers in water samples taken in South Kaneohe Bay, near Coconut Island. We sampled storm (KBG1) and non-storm conditions (KBG2), artificially supplementing some samples with nutrients. We used Landry and Hassett’s (1982) dilution technique to achieve the desired fraction of whole seawater to filtered seawater, and hence the desired dilution of grazers. We calculated the net phytoplankton growth in storm versus non-storm, in all dilution factors, and with varying additions of nutrients. We observed net phytoplankton growth rates (which includes mortality by microzooplankton) was higher the more diluted (and fewer grazers) there were in our water samples. The doubling rates of phytoplankton growth during storm conditions (KBG1) were largely unaffected by the artificial addition of nutrients, since growth conditions were already optimal due to nutrient loading from input streams following a storm (for example, Prochlorococcus doubled abundance in 12.7 hours with added nutrients versus an 11.6-hour doubling rate without added nutrients). For storm conditions, an average of 83% of phytoplankton growth was consumed by grazers, and so net phytoplankton growth was positive. In contrast, the doubling rates were greatly affected by artificial nutrient additions during nutrient-depleted non-storm conditions (KBG2). Non-storm samples with artificially added nutrients had doubling rates of 2-4x faster (for example, Prochlorococcus doubled in abundance in 24.0 hours) than nonstorm samples without added nutrients (Prochlorococcus doubled in 44.8 hours). For non-storm conditions, grazers consumed an average of 112% of phytoplankton growth, making net phytoplankton growth negative. Our findings agreed with the prior research; that is, non-storm conditions were dominated by the smaller cells, such as Synechococcus sp. (0.2-2 µm), and storm conditions were dominated by the larger cells such as chainforming centric diatoms Chaetoceros sp.