A sea‐going continuous culture system for investigating phytoplankton community response to macro‐ and micro‐nutrient manipulations

Abstract

Continuous cultures can provide refined insights on the response of phytoplankton communities to small changes in nutrient flux, however are logistically more challenging to perform at sea than batch cultures. Here we describe the design and successful testing of a new continuous culture system for shipboard experiments using natural phytoplankton communities. Using this system, we studied the effects of nitrate amendments in coastal waters of the Pacific Northwest, and low‐level iron additions in High Nitrate, Low Chlorophyll (HNLC) waters of the subarctic Pacific. With nitrate amendments the coastal phytoplankton community showed proportional increases in chlorophyll biomass and appeared to achieve dynamic steady state as biomass, nutrient drawdown, and photo‐physiology stabilized after 4 d. In contrast, biomass did not reach steady state with iron amendment in the 10‐d HNLC experiment, however a major transition in dominant phytoplankton from small autotrophic flagellates to the toxigenic diatom Pseudo‐nitzschia was observed. This new continuous culture design demonstrated high precision in flow rates, good mixing within culture vessels, and was simple to operate at sea. This system provides an effective platform for investigating small changes in macro‐ and micro‐nutrient flux on the growth of individual phytoplankton species and, in turn, the trajectory of planktonic ecosystems.