Enhancement of marine phytoplankton blooms by appendicularian grazers

Short-lived blooms of large phytoplankton are the main vector in the atmosphere–ocean–sediment carbon flux and are associated with high fish production. A common assumption is that grazing zooplankton can only have a negative effect on the growth of these blooms. We have found that appendicularians, the most abundant microphagous metazoans, strongly stimulate the magnitude of experimentally induced blooms and that this effect can be scaled to any appendicularian density. Moreover, during a postspring bloom cruise in May 2000 off the central Cantabrian coast, we found a high positive correlation between the biomass of large phytoplankton and the abundance of appendicularians. These results suggest that these marine microphagous zooplankton actively catalyze the biological CO2 pump by shifting the size structure of phytoplankton blooms toward large, rapidly sinking particles. Bursts of large phytoplankton chains and colonies (or net phytoplankton) associated with transient physical processes represent the main contribution to atmospheric CO2 sequestration in ocean sediments and to fish production because large particles sink fast and are readily consumed by large predators (Cushing 1989; Legendre 1990; Kiorboe 1993; Falkowski et al. 1998). It is a commonly accepted notion that grazing zooplankton can only have a negative effect on the growth of phytoplankton blooms (Cushing 1989; Legendre 1990; Kiorboe 1993; Falkowski et al. 1998). However, a long line of studies of the indirect effects in freshwater ecosystems, initiated by Sterner (1986) with cladoceran grazers, has shown that herbivores might selectively favor the growth of some algal species by a combination of selective grazing and nutrient regeneration. When the herbivore preys only on phytoplankton, its net effect on phytoplankton net growth ranges from negative to absent (Sterner 1986). In 1 Corresponding author ([email protected]).