Ecological Role of a Cosmopolitan Testate Rhizopod, Paulinella ovalis, in the Microbial Food Web of a Eutrophic Estuary

Paulinella ovalis is a heterotrophic nanoplanktonic rhizopod abundant in estuarine and coastal waters. In the microbial food web, heterotrophic nanoplankton (HNP) consume picophytoplankton and are grazed by larger heterotrophs–recycling organic matter and transferring carbon to higher trophic levels. This generalization of HNP’s food web role is based primarily on studies of heterotrophic nanoflagellates (HNF). Little is known about the function of P. ovalis, but this study hypothesizes a similar role to HNF based on analogous size (four-microns) and trophic mode. This study assesses the hypothesis by examining the spatial and seasonal distribution of P. ovalis and its picocyanobacterial prey in the Neuse River Estuary (NRE), by measuring its growth rates, and estimating its potential grazing pressure on picocyanobacteria. P. ovalis abundance paralleled that of picocyanobacteria with higher abundance in warmer, saltier waters. P. ovalis growth rates ranged from -0.11, 0.25, and 0.43 per day–establishing a minimum growth rate of at least 0.3 d-1. Based on a 0.3 d-1 growth rate and assuming a biomass yield of 0.3 from literature values, P. ovalis has the potential to consume ~48% of the picocyanobacteria population per day. Since ~50% of primary production in the NRE is produced by picocyanobacteria, it is possible that 25% of primary production is consumed by P. ovalis. These estimates demonstrate the potential importance of P. ovalis within the microbial food web, and should motivate future studies to directly measure the grazing rates by and on P. ovalis to further assess its role in the food web.Bachelor of Scienc