Grazing Rate Estimates Research Articles

On database biases and hypothesis testing with dilution experiments: Response to comment by Latasa

Response

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

Grazing rate estimates indicate that approximately half of the bacterivory in oligotrophic oceans is due to mixotrophic flagellates (MFs). However, most estimations have considered algae as a single group. Here we aimed at opening the black-box of the phytoflagellates (PFs) <20 μm. Haptophytes, chlorophytes, cryptophytes and pigmented dinoflagellates were identified using fluorescent in situ hybridization or by standard 4',6-diamidino-2-phenylindole staining. Their fluctuations in abundance, cell size, biomass and bacterivory rates were measured through an annual cycle in an oligotrophic coastal system. On average, we were able to assign to these groups: 37% of the total pico-PFs and 65% of the nano-PFs composition. Chlorophytes were mostly picoplanktonic and they never ingested fluorescently labeled bacteria. About 50% of the PF <20 μm biomass was represented by mixotrophic algae. Pigmented dinoflagellates were the least abundant group with little impact on bacterioplankton. Cryptophytes were quantitatively important during the coldest periods and explained about 4% of total bacterivory. Haptophytes were the most important mixotrophic group: (i) they were mostly represented by cells 3-5 μm in size present year-round; (ii) cell-specific grazing rates were comparable to those of other bacterivorous non-photosynthetic organisms, regardless of the in situ nutrient availability conditions; (iii) these organisms could acquire a significant portion of their carbon by ingesting bacteria; and (iv) haptophytes explained on average 40% of the bacterivory exerted by MFs and were responsible for 9-27% of total bacterivory at this site. Our results, when considered alongside the widespread distribution of haptophytes in the ocean, indicate that they have a key role as bacterivores in marine ecosystems.

Effects of grazing and fertilization on epiphyte growth dynamics under moderately eutrophic conditions: implications for grazing rate estimates

The effects of grazing and nutrients on epiphyte biomass in seagrass beds have received much attention, yet less is known about effects on other metrics of epiphyte growth dynamics, such as epiphyte productivity and biomass turnover rates. To help address this gap, we present here a number of mesocosm experiments in which we manipulate grazer presence and nutrient concentrations under initially moderate eutrophic conditions. We also examine the potential bias contained in estimates of epiphyte grazing that disregard the effects of grazing on epiphyte productivity. The effects of grazing and nutrient enrichment on the epiphyte growth metrics examined were disparate. Namely, the effects of grazing on epiphyte biomass predominated over those of nutrient enrichment, but grazing and nutrients had similar importance as controls of epiphyte productivity and biomass turnover rates. The results illustrate that the effects of environmental and biological factors on one given metric of epiphyte growth dynamics can be quite different from those on other metrics. Thus, we suggest that the biomass-centered view mostly used to date in studies of epiphyte dynamics should perhaps shift toward a broader approach including other metrics of epiphyte growth, such as productivity and turnover rates, to better understand epiphyte dynamics in coastal systems and cascading functional consequences in the system. Another risk of biomass-centered studies lies in the calculation of grazing rates since, as also shown here, estimates of grazing rates derived as the difference in biomass accrual rates between non-grazed and grazed conditions can be highly biased.

Reality checks on microbial food web interactions in dilution experiments: responses to the comments of Dolan and McKeon

Abstract. Dolan and McKeon (2005) have recently criticized microzooplankton grazing rate estimates by the dilution approach as being systematically biased and significantly overestimated. Their argument is based on observed mortality responses of ciliated protozoa to reduced food in several coastal experiments and a global extrapolation which assumes that all grazing in all ocean systems scales to the abundance of ciliates. We suggest that these conclusions are unrealistic on several counts: they do not account for community differences between open ocean and coastal systems; they ignore direct experimental evidence supporting dilution rate estimates in the open oceans, and they discount dilution effects on mortality rate as well as growth in multi-layered, open-ocean food webs. High microzooplankton grazing rates in open-ocean systems are consistent with current views on export fluxes and trophic transfers. More importantly, significantly lower rates would fail to account for the efficient nutrient recycling requirements of these resource-limited and rapid-turnover communities.

The reliability of grazing rate estimates from dilution experiments: Have we over-estimated rates of organic carbon consumption by microzooplankton?

Abstract. According to a recent global analysis, microzooplankton grazing is surprisingly invariant, ranging only between 59 and 74% of phytoplankton primary production across systems differing in seasonality, trophic status, latitude, or salinity. Thus an important biological process in the world ocean, the daily consumption of recently fixed carbon, appears nearly constant. We believe this conclusion is an artefact because dilution experiments are 1) prone to providing over-estimates of grazing rates and 2) unlikely to furnish evidence of low grazing rates. In our view the overall average rate of microzooplankton grazing probably does not exceed 50% of primary production and may be even lower in oligotrophic systems.

Multisensor sampling of pelagic ecosystem variables in a coastal environment to estimate zooplankton grazing impact

Sampling was conducted on the west Florida continental shelf ecosystem modeling site to estimate zooplankton grazing impact on primary production. Samples were collected with the high-resolution sampler, a towed array bearing electronic and optical sensors operating in tandem with a paired net/bottle verification system. A close biological–physical coupling was observed, with three main plankton communities: 1. a high-density inshore community dominated by larvaceans coincident with a salinity gradient; 2. a low-density offshore community dominated by small calanoid copepods coincident with the warm mixed layer; and 3. a high-density offshore community dominated by small poecilostomatoid and cyclopoid copepods and ostracods coincident with cooler, sub-pycnocline oceanic water. Both high-density communities were associated with relatively turbid water. Applying available grazing rates from the literature to our abundance data, grazing pressure mirrored the above bio-physical pattern, with the offshore sub-pycnocline community contributing ∼65% of grazing pressure despite representing only 19% of the total volume of the transect. This suggests that grazing pressure is highly localized, emphasizing the importance of high-resolution sampling to better understand plankton dynamics. A comparison of our grazing rate estimates with primary production estimates suggests that mesozooplankton do not control the fate of phytoplankton over much of the area studied (<5% grazing of daily primary production), but “hot spots” (∼25–50% grazing) do occur which may have an effect on floral composition.

Dilution effects on microzooplankton in dilution grazing experiments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 200:127-139 (2000) - doi:10.3354/meps200127 Dilution effects on microzooplankton in dilution grazing experiments J. R. Dolan1,2,*, C. L. Gallegos1, A. Moigis1,** 1Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA 2Marine Microbial Ecology Group, CNRS ESA 7076, Université Paris VI, Station Zoologique, BP 28, 06230 Villefranche-sur-Mer, France *E-mail: dolan@obs-vlfr.fr **Present address: Ulmenstrasse 73, 24306 Ploen, Germany ABSTRACT: In dilution experiments, filtered seawater is used to create a gradient of grazing pressure on phytoplankton. Microzooplankton grazing is estimated by examining phytoplankton growth within the gradient. However, the dilution series also represents a resource gradient for microzooplankton. Here we report the effects of dilution on grazers. In 2 standard dilution experiments, using communities from the eutrophic (chlorophyll a = 12 to 15 µg l-1) Rhode River Estuary, we examined the effects of dilution on different groups of microzooplankters: rotifers, tintinnid ciliates, oligotrich ciliates, predacious ciliates, and Mesodinium rubrum. Apparent growth rates of tintinnids and oligotrichs varied with prey concentration, decreasing with the dilution factor from about +0.5 d-1 in undiluted whole water to about -1 d-1 in the 5% whole water, closely resembling numerical response curves. Among tintinnids, there was an increase in the relative abundance of larger tintinnids in the time 24 h samples of dilute treatments compared to the less dilute treatments. No consistent dilution effect was shown by rotifers or predacious ciliates. The growth rates of the photosynthetic ciliate M. rubrum increased with dilution, resembling the typical pattern of chlorophyll a and autotrophic nanoplankton. Grazer growth in undiluted waters and grazer mortality in dilute water may be common and result in uncertainty in measured grazing rates. We urge that grazers be examined in grazing experiments not only to assess possible artifacts in grazing rate estimates, but also to provide information, beyond a simple grazing rate, on the grazer populations. KEY WORDS: Ciliate · Oligotrich · Tintinnid · Phytoplankton · Nanoplankton · Rotifer · Flagellate Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 200. Publication date: July 14, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Temporal and spatial dynamics of the pelagic microbial food web in an atoll lagoon

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 16:53-64 (1998) - doi:10.3354/ame016053 Temporal and spatial dynamics of the pelagic microbial food web in an atoll lagoon J. M. González1, J.-P. Torréton2,*, P. Dufour3, L. Charpy3 1Institute of Microbial Studies, La Roza 56, Quintanar de la Sierra, E-09670 Burgos, Spain 2Centre ORSTOM de Tahiti, BP 529 Papeete, French Polynesia 3ORSTOM, Centre d'Océanologie de Marseille, Traverse de la Batterie des Lions, F-13007 Marseille, France *Addressee for correspondence. E-mail: torreton@orstom.orstom.fr ABSTRACT: We examined the influence of diurnal cycle, depth, exchange and proximity to land on the dynamics of microbial populations (bacteria, cyanobacteria, nanoplankton, phagotrophic nanoflagellates, ciliates and dinoflagellates) in Tikehau atoll lagoon (French Polynesia). Microbial populations increased over this 15 d study. Their abundances in the lagoon ranged from 1.2 to 2.6 x 106 bacteria ml-1, 1.1 to 4.0 x 105 cyanobacteria ml-1, 200 to 1090 eukaryotic phytoplankton ml-1, 380 to 1500 phagotrophic nanoflagellates ml-1, 0.3 to 4.0 ciliates ml-1, and 11 to 195 nanodinoflagellates ml-1. Microbial abundances revealed that the intrusion of oceanic water into the lagoon only affected bacterial and cyanobacterial distributions in a narrow band around the lagoon. Grazing rate estimates and population dynamics showed that phagotrophic nanoflagellates were the major grazers of picoplankton (mainly bacteria and cyanobacteria). Cyanobacteria contributed about twice as much as bacteria to this grazed biomass and are therefore supposed to have a higher contribution to the transfer of organic carbon to the upper trophic levels. Hetero- and autotrophic nanoflagellate abundance appeared to be regulated primarily by predators. Ciliates, and perhaps heterotrophic dinoflagellates, appeared to be grazing mostly on nanoplankton, both autotrophic and heterotrophic cells. Autotrophic and heterotrophic dinoflagellate populations increased at net rates comparable to, or even more rapidly than, other microbial communities; this suggests that dinoflagellates also play a significant role in the lagoon microbial foodweb. Additional aspects of the spatial and temporal variability of the microbial foodweb in Tikehau lagoon, as well as the role of grazers and dinoflagellates, are analyzed. KEY WORDS: Atoll lagoon · Microbial food web · Grazing · Bacterioplankton · Cyanobacteria · Protists · Dinoflagellates · Ciliates · Nanoflagellates Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 16, No. 1. Publication date: October 15, 1998 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1998 Inter-Research.