Protozoan Grazing and Nano-Plankton Producer Growth Rates in Varying in Situ Nutrient Concentrations and Temperature in the West Equatorial Pacific

Abstract

Protozoa are a diverse group of organisms that impact trophic transfer in marine ecosystems, constituting an important link between producers and higher trophic levels. In this study, I focused on determining how protozoan grazing rates differ in nutrient-rich and poor ecosystems. I used a CTD rosette to collect six seawater samples along the equatorial transect of five degrees south to five degrees north at stations: 5°S,2°S, 1°S, 0°, 1°N, 4°N, and 5°N along 167°W in January 2024. These samples were filtered to 10 microns and divided into isolated incubation cultures with 0, 25, and 90 percent dilution of 0.2 micron filtered seawater. Change in Chlorophyll was used to calculate the phytoplankton growth rate across the dilution factors. Using a linear model of growth rate by dilution factor, a grazing rate was determined for each sample. Nutrients from the water samples were measured for nitrate, phosphate, and silicate concentrations. A series of linear regression analyses of the protozoan grazing rates by in situ nutrient concentrations revealed negative linear trends with temperature in both growth and grazing rates. The growth rates of phytoplankton ranged from -0.09 day-1 to 4.62 day-1. Ambient nitrate, silicate, and phosphate concentrations reached 2.25 μM, 2.18 μM, and 0.46 μM respectively. Surface temperatures reached 30.46 centigrade, and the grazing rate exhibited a decreasing trend with higher temperatures, eventually reaching zero at 30.3 degrees. As eutrophication events become increasingly common due to climate change and anthropogenic pollution, it is important to determine how protozoan communities respond to changes in dissolved nutrients and temperature.