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Pasulka, AL, Samo TJ, Landry MR.  2015.  Grazer and viral impacts on microbial growth and mortality in the southern California Current Ecosystem. Journal of Plankton Research. 37:320-336.   10.1093/plankt/fbv011   AbstractWebsite

Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially different implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations (Prochlorococcus, Synechococcus, picoeukaryotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 +/- 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodological constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells.

Scheinberg, RD, Landry MR, Calbet A.  2005.  Grazing impacts of two common appendicularians on the natural prey assemblage of a subtropical coastal ecosystem. Marine Ecology-Progress Series. 294:201-212.   10.3354/meps294201   AbstractWebsite

The clearance rates of co-occurring appendicularian species, Oikopleura longicauda and O. fusiformis, in Kaneohe Bay, Hawaii, were investigated to evaluate and compare their roles in a tropical food web. Individual appendicularians were captured in situ and allowed to feed on the natural plankton assemblage for 60 to 180 min. Feeding rate estimates were based on flow-cytometry analyses of cell-density changes for heterotrophic bacteria (Hbact), Synechococcus spp. (Syn) and < 13.0 mu m autotrophic eukaryotes (Aeuks), Despite morphological differences, O. longicauda and O. fusiformis cleared the largest prey size-fraction at statistically indistinguish able rates. For the 3 prey categories (Hbact, Syn and Aeuks), mean clearance rates (+/- 95% CI) were 12 +/- 7, 27 +/- 6 and 34 +/- 18 ml individual (ind.)(-1) h(-1) and 25 +/- 12, 26 +/- 15 and 38 +/- 20 ml ind.(-1) h(-1) for O. longicauda and O. fusiformis, respectively, The mean clearance rates of these 2 species on total sub-micron cells in Kaneohe Bay were not significantly different; however, O. fusiformis cleared Hbact at a marginally higher rate (p = 0.07). Only O. longicauda exhibited significantly different retention efficiencies as a function of prey size, clearing the smallest prey (Hbact) at approximately 36 % the rate of the largest (Aeuks) (p < 0.01). Despite reduced efficiencies on the smallest prey categories, at high abundances in Kaneohe Bay (often 2 ind. l(-1)), O. longicauda is capable of removing > 60%, of the picoplankton standing stock from the water column daily. While generally much less abundant, during occasional peaks of 1 ind. l(-1), O. fusiformis can remove an almost equivalent amount (> 50%). Nevertheless, the consistently higher abundances of O. longicauda make this species a more significant link between picoplankton production and higher-level consumers (chaetognaths and fishes) in this coastal tropical embayment.

Landry, MR.  2009.  Grazing processes and secondary production in the Arabian Sea: A simple food web synthesis with measurement constraints. Indian ocean biogeochemical processes and ecological variability. ( Wiggert JD, Hood RR, Naqvi SWA, Brink KH, Smith SL, Eds.).:133-146.: American Geophysical Union, Washington, DC (USA)   10.1029/2008GM000781   Abstract

The Joint Global Ocean Flux Study in the Arabian Sea during the mid 1990s provides a rare opportunity to elucidate carbon flows in the lower food web of an open ocean ecosystem. Analysis of that data to date has, however, produced widely divergent perspectives on major flux pathways and roles of zooplankton: from zooplankton as controlling grazers tightly coupled to microbial processes to zooplankton as casual consumers who let a large fraction of production, mostly generated by picophytoplankton, flow directly to detritus and export. Synthesis of experimental grazing rates and production inferences for mesozooplankton and microzooplankton fit well in a conceptually simple food web, constrained by measured carbon flows through phytoplankton and bacteria. Microzooplankton dominate grazing processes, consuming over 70% of particulate primary production (PP), on average, and providing steady and significant supplemental nutrition to mesozooplankton. Direct grazing estimates of mesozooplankton, on the order of 25% of PP, are sufficient to balance the remaining particulate production, with additional transfer through a one- to two-step food chain of microzooplankton accounting for a total ingestion of 6 40% of PP required for mesozooplankton secondary production. Dissolved organic carbon fluxes to bacteria are provided mostly within the constraints of gross and net primary production. Contradictory results from inverse models are likely due to an assumption that exaggerates by approximately twofold the production contribution of picophytoplankton and to the failure to use measured rates of gross primary production as a system constraint. Grazing generally balances net particulate primary production in the Arabian Sea, but true grazer control of phytoplankton dynamics remains an open issue for further study.

Landry, MR, Lehnerfournier JM.  1988.  Grazing rates and behaviors of Neocalanus plumchrus: implications for phytoplankton control in the subarctic Pacific. Hydrobiologia. 167:9-19.   10.1007/bf00026290   AbstractWebsite

Grazing rates and behaviors of the copepod Neocalanus plumchrus were investigated in shipboard experiments during the first SUPER Program cruise (May, 1984). N. plumchrus can exploit cells in the 2 to 30 μm size range with equal clearance efficiency but displays considerable flexibility in responding to changes in concentration and size composition. Its functional response helps to stabilize phytoplankton at low densities. In 60-liter microcosms, a density of one copepod liter−1 was sufficient to maintain the ambient abundance and structure of the phytoplankton community for a week. In the absence of the copepod, phytoplankton bloomed to unnaturally high levels, and the community composition was dramatically altered. Despite its grazing potential, N. plumchrus was not present in sufficient density to control phytoplankton blooms in the subarctic Pacific. However, the copepod may have an important role in regulating the abundance of smaller grazers and the size structure of the phytoplankton community.

Liu, HB, Campbell L, Landry MR.  1995.  Growth and mortality rates of Prochlorococcus and Synechococcus measured with a selective inhibitor technique. Marine Ecology-Progress Series. 116:277-287.   10.3354/meps116277   AbstractWebsite

A selective metabolic inhibitor method has been developed to estimate growth rates and mortalities due to protozoan grazing of the photoautotrophic prokaryotic picoplankton Prochlorococcus and Synechococcus. Laboratory and field experiments show that 1 mg ml(-1) (final concentration) kanamycin inhibits the growth of Prochlorococcus and Synechococcus effectively and does not significantly affect protozoan grazing. At Station ALOHA (22 degrees 45' N, 158 degrees W) 100 km north of Oahu, Hawaii, USA, growth rates of Prochlorococcus ranged from 0.4 to 0.5 d(-1) within the surface mixed layer to about 0.1 d(-1) at the bottom of the euphotic zone. Synechococcus grew faster, with a daily growth rate of up to 1.0 d(-1) in surface waters. Grazing mortalities varied for Prochlorococcus and Synechococcus from 20 to 116% and 43 to 87% of growth rates, respectively. Growth generally exceeded grazing. Because of its high abundance (up to 2 x 10(5) cells ml(-1) in the upper 100 m), Prochlorococcus contributes significantly to phytoplankton biomass and primary production in the subtropical North Pacific Ocean. At Station ALOHA in October 1993, integrated (0 to 175 m) carbon production due to Prochlorococcus was 382.2 mg C m(-2) d(-1). In contrast, Synechococcus produced only 14.6 mg C m(-2) d(-1).