Abstract
For nearly a century, phytoplankton spring blooms have largely been explained in the context of abiotic factors regulating cellular division rates (e.g., mixed-layer light levels). However, the accumulation of new phytoplankton biomass represents a mismatch between phytoplankton division and mortality rates. The balance between division and loss, therefore, has important implications for marine food webs and biogeochemical cycles. A large fraction of phytoplankton mortality is due to the combination of microzooplankton grazing and viral lysis, however, broad scale simultaneous measurements of these mortality processes are scarce. We applied the modified dilution assay along a West-to-East diagonal transect in the North Atlantic during spring. Our results demonstrate positive accumulation rates with losses dominated by microzooplankton grazing. Considering the dynamic light environment phytoplankton experience in the mixed surface layer, particularly in the spring, we tested the potential for incubation light conditions to affect observed rates. Incubations acted as short-term ‘light’ perturbations experiments, in which deeply mixed communities are exposed to elevated light levels. These “light perturbations” increased phytoplankton division rates and resulted in proportional changes in phytoplankton biomass while having no significant effect on mortality rates. These results provide experimental evidence for the Disturbance-Recovery Hypothesis, supporting the tenet that biomass accumulation rates co-vary with the specific rate of change in division.
Highlights
Half of the net primary production on Earth is due to phytoplankton in the ocean (Field et al, 1998; Friend et al, 2009)
Accumulations in biomass reflect the net balance between the specific rates of phytoplankton division (μ) and loss (l) (i.e., r = μ – l), r can be independent of μ if μ and l covary (Behrenfeld and Boss, 2014, 2018)
Mixed layer depths in this study were in the range of the climatological monthly mean values and variability expected for the North Atlantic (Monterey and Levitus, 1997; de Boyer Montégut et al, 2004; Carton et al, 2008)
Summary
Half of the net primary production on Earth is due to phytoplankton in the ocean (Field et al, 1998; Friend et al, 2009). The North Atlantic Ocean is a particular “hot spot” for production, accounting for 20% of the global net ocean CO2 uptake (Deser and Blackmon, 1993). Much of this productivity occurs during the recurrent vernal (spring) phytoplankton bloom and this event has been thoroughly studied for the past century. Accumulations in biomass reflect the net balance between the specific rates of phytoplankton division (μ) and loss (l) (i.e., r = μ – l), r can be independent of μ if μ and l covary (Behrenfeld and Boss, 2014, 2018)
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