Phytoplankton growth, microzooplankton herbivory and community structure in the southeast Bering Sea: insight into the formation and temporal persistence of an Emiliania huxleyi bloom

Abstract

Using the seawater dilution technique, we measured phytoplankton growth and microzooplankton grazing rates within and outside of the 1999 Bering Sea coccolithophorid bloom. We found that reduced microzooplankton grazing mortality is a key component in the formation and temporal persistence of the Emiliania huxleyi bloom that continues to proliferate in the southeast Bering Sea. Total chlorophyll a (Chl a) at the study sites ranged from 0.40 to 4.45 mg C l À1 . Highest phytoplankton biomass was found within the bloom, which was a mixed assemblage of diatoms and E. huxleyi. Here, 75% of the Chl a came from cells >10 mm and was attributed primarily to the high abundance of the diatom Nitzschia spp. Nutrient-enhanced total phytoplankton growth rates averaged 0.53 d À1 across all experimental stations. Average growth rates for >10 mm and o10 mm cells were nearly equal, while microzooplankton grazing varied among stations and size fractions. Grazing on phytoplankton cells >10 mm ranged from 0.19 to 1.14 d À1 . Grazing on cells o10 mm ranged from 0.02 to 1.07 d À1 , and was significantly higher at non-bloom (avg. 0.71 d À1 ) than at bloom (avg. 0.14 d À1 ) stations. Averaged across all stations, grazing by microzooplankton accounted for 110% and 81% of phytoplankton growth for >10 and o10 mm cells, respectively. These findings contradict the paradigm that microzooplankton are constrained to diets of nanophytoplankton and strongly suggests that their grazing capability extends beyond boundaries assumed by size-based models. Dinoflagellates and oligotrich ciliates dominated the microzooplankton community. Estimates of abundance and biomass for microzooplankton >10 mm were higher than previously reported for the region, ranging from 22,000 to 227,430 cells l À1 and 18 to 164 mg C l À1 . Highest abundance and biomass occurred in the bloom and corresponded with increased abundance of the large ciliate Laboea, and the heterotrophic dinoflagellates Protoperidinium and Gyrodinium spp. Despite low grazing rates on phytoplankton o10 mm within the bloom, the abundance and biomass of small microzooplankton (o20 mm) capable of grazing E. huxleyi was relatively high at bloom stations. This body of evidence, coupled with observed high grazing rates on large phytoplankton cells, suggests the phytoplankton community composition was strongly regulated by herbivorous activity of microzooplankton. Because grazing behavior deviated from size-based model predictions and was not proportional to microzooplankton biomass, alternate mechanisms that dictate levels of grazing activity were in effect in the southeastern Bering Sea. We hypothesize that these mechanisms included morphological or chemical signaling between phytoplankton and micrograzers, which led to selective grazing pressure. r

Environmental constraints on the production and removal of the climatically active gas dimethylsulphide (DMS) and implications for ecosystem modelling

“…However, it was recently suggested that DMSP rather than acrylate is the active deterrent of microzooplankton grazing (Strom et al 2003b) and readers are directed to the paper presented by Nejstgard et al (this issue) for a discussion on this subject. A few field experiments have combined the determination of microzooplankton grazing rates (the Landry-Hassett dilution technique) with DMS(P) analyses and have shown that DMSP-containing algal species may be proportionally less grazed than total algal biomass (Archer et al 2001b;Olson and Strom 2002;Wolfe et al 2000). However, several problems are associated with this technique and we refer to Archer et al (2001b) for a discussion on the potential pitfalls.…”

Short-term changes in a microplankton community in the Chukchi Sea during autumn: consequences of a strong wind event

“…and Protoperidinium spp. are known to prey on diatoms and, thus, strongly regulate the phytoplankton community (Olson and Strom, 2002). Therefore, these increases in microzooplankton and the decrease in diatoms may be caused by microzooplankton grazing on diatoms.…”

“…In the marine ecosystem of the western Arctic Ocean, microplankton, including diatoms, dinoflagellates and ciliates, play several roles, such as primary producers, consumers and food resources for mesozooplankton Sherr, 1988, Sherr et al, 1997;Olson and Strom, 2002). The microplankton community in the western Arctic Ocean is divided into three groups -shelf, continental slope and basin (Sukhanova et al, 2009;Matsuno et al, 2014).…”

Nitrate : phosphate ratios and Emiliania huxleyi blooms

“…If micronutrients were scarce, the relatively low iron and zinc requirements of E. huxleyi (reviewed in Paasche 2002) may have been a factor in its success. It is interesting to note as well that E. huxleyi was preferentially grazed in the North Atlantic bloom , whereas data (Olson and Strom 2002) and modeling ) of the coastal Bering Sea bloom indicated that reduced grazing was important in bloom initiation and duration. It may well be that factors that govern the dynamics of oceanic blooms of E. huxleyi are different from those of coastal blooms.…”