Phytoplankton and bacterial dynamics on the Chukchi Sea Shelf during the spring-summer transition

Climate warming is exerting significant change on the physical properties of the Arctic Ocean, which in turn has marked consequences for the biology of the region. The Chukchi Sea is notable for its species richness as a consequence of a nutrient-rich shelf region that supports substantial primary production. However, little is known about the carbon transformations at the base of the food web in the Chukchi Sea, and in particular the relative amounts of primary production that are transferred directly to higher trophic levels or remineralized within the microbial loop. We measured microbial standing stocks (bacteria to microplankton), phytoplankton growth and mortality rates, and bacterial production and mortality rates at 10 stations in the Chukchi Sea and Bering Strait during the spring-summer transition. Our study revealed that protistan grazers consumed substantially more phytoplankton carbon than bacterial carbon. Phytoplankton growth rates were variable, but at times considerable (range: -0.06 to 0.71 d(-1)), with protistan grazers consuming an average of 46% of the daily primary production. Heterotrophic protists exerted significant grazing pressure on phytoplankton despite low environmental temperatures. Bacterial production and mortality rates were low (generally <1 mu g C 1(-1) d(-1)) and at times in balance, but overall bacterial production exceeded mortality. This study improves our understanding of carbon cycling in the Chukchi Sea during the spring-summer transition, demonstrating a significant transfer of primary production to heterotrophic protists at that time of year.