Seasonal changes in planktonic bacterivory rates under the ice‐covered coastal Arctic Ocean

Abstract

Bacterivory was determined in surface waters of Franklin Bay, western Arctic, over a seasonal ice-covered period (winter-spring, 2003-2004). The objectives were to obtain information on the functioning of the microbial food web under the ice, during winter , and to test whether bacterial losses would increase after the increase in bacterial production following the spring phytoplankton bloom. Chl a concentrations ranged from 0.04 to 0.36 mg L 21 , increasing in March and reaching a peak in April. Bacterial biomass showed no consistent trend for the whole period, and protist biomass followed a pattern similar to that of Chl a. Bacterial production increased 1 week after Chl a concentrations started to increase, while bacterivory rates increased very slightly. Average bacterivory rates in winter (0.16 6 0.07 mg C L 21 d 21 ) were not significantly different from those in spring (0.29 6 0.24 mg C L 21 d 21 ). Average bacterial production, on the other hand, was similar to bacterivory rates in winter (0.19 6 0.38 mg C L 21 d 21 ), but higher than bacterivory in spring (0.93 6 0.28 mg C L 21 d 21 ). Therefore, bacterial production was controlled by grazers during winter and by substrate concentration in spring.

Current state and trends in Canadian Arctic marine ecosystems: II. Heterotrophic food web, pelagic-benthic coupling, and biodiversity

“…Thus, this strong and persistent signal of a parasitic component within the microbial food web seems to indicate that parasitism is indeed an important process in winter. Winter bacterivory by heterotrophic dinoflagellates and ciliates was rather low but sufficient to control bacterial production (Forest et al 2011;Vaqué et al 2008). As reported in earlier studies, sequences of the small Prasinophyte (<2 μm) Micromonas were recovered both in the rRNA gene and rRNA clone libraries in the Amundsen Gulf, indicating that this key component of the microbial food web remained active throughout winter (Terrado et al 2011).…”

Experimental evaluation of the warming effect on viral, bacterial and protistan communities in two contrasting Arctic systems

“…In the Fjord microcosms, there was a gradual increase in bacterivory as the temperature increased; however, like in the Arctic microcosms, this did not correspond to an increase in the total HF at different temperatures. Although HF are considered to be the main bacterivore microorganisms (Sherr & Sherr 2002), they also ingest prey larger than bacteria to maintain their biomass and growth (Vaqué et al 2008), and thus trophic cascades could occur (Vaqué et al 2004). For instance, HF > 5 µm could feed on bacteria, on HF ≤5 µm, and on other small prey such as Micromonas sp., which were very abundant in our experiments as in natural Arctic waters (Lovejoy et al 2007).…”

Winter-spring transition in the subarctic Atlantic: microbial response to deep mixing and pre-bloom production

“…The cell numbers of HNF we encountered were in general in the lower end of those observed globally (Sanders et al 1992) but very similar to those found in Arctic marine systems during the period of winter− spring transition (Vaqué et al 2008, Iversen & Seuthe 2011. Peak abundances of 300 cells ml −1 were observed during our study period.…”

“…They are, how ever, major grazers of picophytoplankton (Christaki et al 2001, Sherr & Sherr 2002, Brę k-Laitinen & Ojala 2011, and it remains for future studies to resolve the importance of HNF grazing. We here would suggest splitting the group into large and small HNF to test whether the size groups have different prey-size preferences as speculated by Sherr & Sherr (2002) and Vaqué et al (2008). Both of these studies suggest that heterotrophic flagellates < 5 µm are the main grazers on bacteria, while flagellates > 5 µm select for picoeukaryotes.…”