Top-down control by micro- and mesozooplankton on winter dinoflagellate blooms of Heterocapsa rotundata

Abstract

Winter dinoflagellate blooms in Chesapeake Bay tributaries can account for over 50% of a system's annual primary production, potentially more than the spring diatom bloom. Research on winter blooms has focused on environmental conditions that result in blooms, but little focus has been given to the potential importance of zooplankton grazers. We investigated the impact of microzooplankton and mesozooplankton (copepods) grazing on the population of winter phytoflagellates in the Choptank River, MD, in 2012 to 2013 and 2013 to 2014. We estimated community microzooplankton and copepod grazing rates on the dominant phytoflagellate species, and measured daily gross primary production (GPP) rates. The chlorophyll a concentration and the abundance of the dinoflagellate Heterocapsa rotundata were significantly higher in 2013 to 2014 compared with 2012 to 2013, but average daily GPP was similar between the 2 yr. However, the average percentage of daily GPP removed by grazers in 2013 to 2014 was lower than in 2012 to 2013, despite average environmental conditions and nutrient concentrations not differing between years. We hypothesize that the observed release from grazing pressure is one of the main factors controlling winter dinoflagellate bloom formation in these and other coastal temperate systems.

Nutrient- and Climate-Induced Shifts in the Phenology of Linked Biogeochemical Cycles in a Temperate Estuary

“…Recent analysis of long-term changes in phytoplankton community composition over this same timeframe have suggested that dinoflagellates have become an increasing portion of phytoplankton biomass, whereas previously the composition was dominated by diatoms (Harding et al, 2015b). Winter blooms of dinoflagellates have been documented in the estuarine turbidity maximum region of the main stem Chesapeake Bay, in the vicinity of the CB3.1 (Lee et al, 2012), as well as in multiple tributaries of Chesapeake Bay (Sellner et al, 1991;Millette et al, 2015). Many of these species tend to be mixotrophic, using phagotrophy as an alternative energy acquisition strategy during winter-spring in turbid waters, where low light availability limits photoautotrophy (Millette et al, 2017).…”

Mixotrophy in Heterocapsa rotundata: A mechanism for dominating the winter phytoplankton

“…). At this location H. rotundata is known to dominate the winter phytoplankton community and form large winter blooms (Millette et al ). On each sampling date we collected 10 L of surface water with a bucket, and immediately filtered it through 20 μm mesh to remove larger plankton.…”

“…Some studies have focused on the formation and decline of H. rotundata winter blooms in Chesapeake Bay tributaries (Cohen ; Sellner et al ; Millette et al ). These researchers found abundant H. rotundata blooms in wet, cold winters when salinity is low (Cohen ) and when there is a release in grazing pressure from microzooplankton and copepods (Millette et al ). Although wet, cold winters and a release in grazing pressure are factors that impact every phytoplankton species, it is unknown how H. rotundata can take advantage of these conditions over other species to bloom.…”

“…One particular species, Heterocapsa rotundata (Lohmann) Loeblich (Hansen 1995), is ubiquitous and occasionally forms large blooms. H. rotundata has been reported in a range of environments all over the world including Chesapeake Bay, U.S.A. (Millette et al 2015), Keum Estuary, South Korea (Seong et al 2006), Manori Creek and Manim Bay, India (Shahi et al 2015), Baltic Sea, Germany (Jaschinski et al 2015), and Kangaroo Island, Australia (Balzano et al 2015). H. rotundata tends to either dominate or be a prominent part of the phytoplankton community for at least part of the year in some of these areas (Seong et al 2006;Balzano et al 2015;Millette et al 2015).…”

Contrasting effects of high‐intensity photosynthetically active radiation on two bloom‐forming dinoflagellates

“…The chronic photoinhibition and swelling of Heterocapsa rotundata even in moderate irradiances, coupled with its high photosynthetic efficiency at low irradiances, suggests a bloom niche characterized by very low light. In Chesapeake Bay, Heterocapsa rotundata often blooms in winter when light limits the growth of other phytoplankton and predators are scarce as a result (Cohen 1985, Sellner et al 1991, Millette et al 2015. By up-regulating feeding in response to low light (Millette et al 2017) and, as shown here, by optimizing photosynthetic performance under these conditions, H. rotundata appears to gain an advantage over other phytoplankton.…”