BACTERIAL DYNAMICS IN RESPONSE TO PHYTOPLANKTON, HETEROTROPHIC MICROFLAGELLATES, AND AMINO ACID POOLS IN THE DELAWARE ESTUARY (TROPHODYNAMICS, GRAZING, UPTAKE, FOOD CHAIN)

This study examined the relationship between bacteria, phytoplankton and heterotrophic microflagellates spatially and seasonally in the Delaware Estuary. In the classical paradigm of marine food chains phytoplankton are grazed by zooplankton, which are subsequently grazed by larger organisms; thus energy and matter are transferred to higher trophic levels. A new hypothesis suggests that bacteria and heterotrophic microflagellates are important components of the marine food web. This hypothesis states that bacteria build biomass with organic matter lost from primary production, bacteria are subsequently grazed by microflagellates, and the microflagellates are then grazed by larger zooplankton. Thus, an energy conserving sub-loop returns lost primary production to the main food chain.;This study suggested that phytoplankton were the source of amino acids in the estuary. Amino acid pools were highest where phytoplankton production was high. Molecular weight analysis indicated that peptides were generally less than 1000 daltons. Bacterial substrate uptake indicated that dissolved combined amino acids were consumed as fast as the dissolved free amino acids. Although bacterial abundances did not vary greatly spatially, bacterial production did vary. However, bacterial biomass did not accumulate because microflagellates rapidly consumed bacteria. Enclosure experiments indicated that there was a close coupling between these trophic levels. Further analysis suggested that small amounts of lost primary prouction could be returned to the main food chain. This suggests the role of the microbial loop, instead, may be mineralization of organic matter.;The questions addressed in the research were: (1) What are the concentrations of particulate, dissolved combined, and dissolved free amino acids in the estuary and are phytoplankton a major source of amino acids? (2) What is the relative bacterial uptake rate of dissolved free and combined amino acids? (3) What are the spatial and seasonal changes of bacterial production and abundance? (4) How do microflagellates vary spatially and seasonally and how fast do they graze bacteria? (5) How closely coupled are phytoplankton, bacteria and microflagellates? (6) What is the role of these trophic levels in the marine food web?...