Microbial food web connections and rates in a subtropical embayment

ABSTRACT: Kāne’ohe Bay, Hawai‘i, USA, an oligotrophic subtropical coastal embayment, is a stable ecosystem dominated by picophytoplankton through most of the year (dry state). After heavy rains, macronutrients from high stream flow result in elevated phytoplankton biomass and change the community to one dominated by diatoms (wet state). However, the effect of this perturbation on the overall productivity of the community, in particular linkages between lower trophic levels and planktonic metazoan consumers, is unknown. Given that microzooplankton are the main intermediaries between phytoplankton and higher trophic levels, this study examined phytoplankton growth and mortality rates using microscopy and flow cytometry, in conjunction with seawater dilution experiments during wet and dry system states. During low stream flow (dry state), Synechococcus dominated the phytoplankton, with growth rates of 1.1 ± 0.4 d-1, and pico- to nano-sized autotrophic eukaryotes were either growing slowly or dying. Growth increased rapidly in experimental bottles with macronutrient (ammonium and phosphorus) addition. Sixty percent of Synechococcus were consumed daily, whereas autotrophic eukaryote mortalities (0.5 d-1) greatly exceeded their growth rates. Microzooplankton were dominated by <30 μm oligotrich ciliates and gymnodinoid dinoflagellates. During the wet state, autotrophic biomass doubled, but microzooplankton biomass increased ~7-fold, including large increases in naupliar and larval stages of metazoans. During wet conditions, the path from primary producers to metazoan consumers had fewer trophic levels and episodically increased the overall productivity of the bay over the dry system state.