Capacity of herbivorous protists to control initiation and development of mass phytoplankton blooms

Abstract

Microzooplankton, heterotrophic organisms 20 to 200 µm in size and mainly phagotrophic protists, are recognized as the dominant consumers of phytoplankton in all regions of the sea. During his long career, Fereidoun Rassoulzadegan has been a major contributor to the understanding of the importance of protists in marine pelagic food webs. In the spirit of his research, we consider here one aspect of microzooplankton ecology that, in our opinion, has been misunderstood: the capacity of protist herbivores to control initiation and development of mass phytoplankton blooms, with a focus on diatom blooms. We argue that microzooplankton grazers probably cannot prevent bloom initiation and out-growth during mesotrophic to eutrophic conditions. We base our contention on the propositions that (1) mass diatom blooms do, in fact, routinely occur in the ocean at all latitudes; (2) since prey abundance is a main factor controlling protist growth, the grazing impact of herbivorous protists will be minor in the early stages of a bloom when prey abundances and, therefore, protist growth rates, are low; (3) due to food limitation during pre-bloom conditions, protist biomass will usually be low at the onset of a bloom, which serves to limit initial grazing impact; and (4) biomass of herbivorous protists is ultimately top-down controlled, thus curtailing the potential for microzooplankton to grow to sufficient abundance to graze down a developed bloom. We conclude that herbivorous protists are not likely to be able to prevent the initiation and development of mass blooms when conditions are favorable for rapid phytoplankton growth.

The role of ciliates, heterotrophic dinoflagellates and copepods in structuring spring plankton communities at Helgoland Roads, North Sea

“…While phytoplankton competitors remain controlled by zooplankton grazing, the advance in net growth rate enables a particular phytoplankton species to form a bloom. Although microzooplankton is usually not able to prevent phytoplankton blooms under natural conditions (Sherr and Sherr 2009), its grazing can have at least a structuring influence and this circumstance has also been observed in previous studies Fonda Umani et al 2005). Irigoien et al (2005) pointed out that among other factors, defence mechanisms (e.g.…”

“…Top-down control (e.g. copepod grazing) on microzooplankton can be severe (Sherr and Sherr 2009), especially in a restricted mesocosm environment, and can cause strong trophic cascade effects (Sommer et al 2003;Sommer and Sommer 2006;Zöllner et al 2009), thus hindering exclusive investigations on the direct effects of microzooplankton grazing on phytoplankton. We therefore excluded mesozooplankton grazers from the incubations.…”

“…As food selectivity is a constant process a pre-selection of phytoplankton species will have been already taken place in the field prior to the filling of our mesocosms. In early spring, total grazing rates of microzooplankton at the start of our experiment were not sufficiently high enough to suppress a spring bloom formation (see also Sherr and Sherr 2009). However, one can assume some directive influence of microzooplankton.…”

“…The improved growth conditions at the onset of a bloom allow phytoplankton species to escape predation pressure by microzooplankton. At this time, stocks and total grazing rates of herbivorous protists are not sufficiently high enough to suppress bloom formation (Sherr and Sherr 2009). In fact, total exclusion from grazing is not required for a particular phytoplankton species for the formation of a bloom; only a positive difference between growth rate and grazing rate resulting in positive net growth, while phytoplankton competitors may remain controlled by zooplankton grazing.…”

Seasonal and Interannual Changes in Ciliate and Dinoflagellate Species Assemblages in the Arctic Ocean (Amundsen Gulf, Beaufort Sea, Canada)

“…In the Canada Basin and elsewhere, changes in the timing and character of phytoplankton production have already been reported (Li et al, 2009;Ardyna et al, 2014). Less attention has been given to the microzooplankton (ciliates and dinoflagellates) that consume from 22 to 75% of the daily phytoplankton production in the Arctic and other cold water oceans (Paranjape, 1990;Landry and Calbet, 2004;Sherr and Sherr, 2009;Sherr et al, 2013). Ciliates and dinoflagellates are also influenced by top-down processes, where changes in macrozooplankton species are linked to the successional progression of the microzooplankton communities (Riisgaard et al, 2014).…”