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Size structuring of planktonic communities : biological rates and ecosystem dynamics


To fully understand the processes that influence planktonic organisms and the role they play in the marine environment requires knowledge of the rates that underlie the properties often measured. To gain this greater understanding, my dissertation focuses on measuring and modeling size-specific rates of planktonic communities. To estimate size-dependent rates under natural conditions, I developed a technique, the size-dependent dilution method, to measure in situ size-specific growth and grazing rates of phytoplankton. I also devised a method to estimate the standard deviation associated with these rates. Knowledge of the cell abundance can be used to calculate the error associated with a rate measurement and subsequently guide sampling volumes and protocols. Using a synthesis of literature data, I determined the size-dependent patterns of several planktonic rates and properties. Implementing these size-dependencies in a size-structured ecosystem model reproduced several properties observed in the natural world, including the nearly immediate response of the planktonic community to additions of nutrients and an increase in biomass with increasing nutrients. This size- structured ecosystem model and two additional, increasing complex versions were used to test the accuracy of the size-dependent dilution method. The technique was able to accurately retrieve the known model rates in each of these simulated environments. The accuracy of the size-dependent dilution method was found to be sensitive to changes in the size-dependent patterns, as opposed to the magnitude, of the parameterized model rates. Lastly, I applied the size-dependent dilution method to field data collected in the eastern Pacific. There were not distinct size- dependent trends in growth and grazing rates, contrary to much prevailing theory. The average grazing rate correlated positively with temperature, and the average growth rate varied inversely with nitracline depth. The areas that were most nutrient stressed had lower growth rates for the larger organisms. The results from this dissertation research can be used to determine how fundamental biological rates vary among different size organisms in their natural environment. Such information is critical to understand what properties affect planktonic community structure and to parameterize models to accurately predict the reaction of planktonic communities to a changing climatic environment

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