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Stukel, MR, Landry MR, Selph KE.  2011.  Nanoplankton mixotrophy in the eastern equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:378-386.   10.1016/j.dsr2.2010.08.016   AbstractWebsite

Heterotrophic bacteria, cyanobacteria, and picoeukaryotic algae dominate the plankton community of high nutrient-low chlorophyll (HNLC) areas of the eastern equatorial Pacific (EEP). While grazing on these picoplankton is often attributed to aplastidic zooflagellates, mixotrophic nanoflagellates (phagotrophic phototrophs) may also exert a large grazing pressure. We assessed the relative contributions of mixotrophic nanoplankton and obligate heterotrophs to picoplankton phagotrophy in mixed-layer water of the EEP using 0.8-mu m Fluorescently-Labeled Bacteria (FLB). Obligate heterotrophs and phototrophs were distinguished from their ratios of microscopically measured red (chlorophyll a) to green (proflavin-stained protein) fluorescence. Sampling sites were located along a nutrient gradient formed by a tropical instability wave at 0.5 degrees N between 123.5 degrees W and 128 degrees W and at 1.75 degrees N, 125 degrees W. The majority of ingested particles were found within 3-5 mu m flagellates, with 54% of the demonstrated phagotrophs belonging to the high-pigment putatively phototrophic population and obligate heterotrophs responsible for 51% of the demonstrated phagotrophy due to their greater propensity to ingest multiple prey. The importance of mixotrophy as a means of alleviating nutrient stress is indicated by a strong inverse relationship between the proportion of community FLB uptake by mixotrophs and ambient nutrient concentration. Low ambient Fe concentration and a demonstrated community response to Fe-addition in shipboard grow-out experiments suggest that mixotrophs were primarily engaging in phagotrophy to offset Fe-deficiencies. (C) 2010 Elsevier Ltd. All rights reserved.

Krause, JW, Stukel MR, Taylor AG, Taniguchi DAA, de Verneil A, Landry MR.  2016.  Net biogenic silica production and the contribution of diatoms to new production and organic matter export in the Costa Rica Dome ecosystem. Journal of Plankton Research. 38:216-229.   10.1093/plankt/fbv077   AbstractWebsite

We determined the net rate of biogenic silica (bSiO(2)) production and estimated the diatom contribution to new production and organic matter export in the Costa Rica Dome during summer 2010. The shallow thermocline significantly reduces bSiO(2) dissolution rates below the mixed layer, leading to significant enhancement of bSiO(2) relative to organic matter (silicate-pump condition). This may explain why deep export of bSiO(2) in this region is elevated by an order of magnitude relative to comparable systems. Diatom carbon, relative to autotrophic carbon, was low (<3%); however, the contribution of diatoms to new production averaged 3 and 13% using independent approaches. The 4-old discrepancy between methods may be explained by a low average C:Si ratio (similar to 1.4) for the net produced diatom C relative to the net produced bSiO(2). We speculate that this low production ratio is not the result of reduced C, but may arise from a significant contribution of non-diatom silicifying organisms to bSiO(2) production. The contribution of diatoms to organic matter export was minor (5.7%). These results, and those of the broader project, suggest substantial food-web transformation of diatom organic matter in the euphotic zone, which creates enriched bSiO(2) relative to organic matter within the exported material.

Stukel, MR, Decima M, Landry MR, Selph KE.  2018.  Nitrogen and Isotope Flows Through the Costa Rica Dome Upwelling Ecosystem: The Crucial Mesozooplankton Role in Export Flux. Global Biogeochemical Cycles. 32:1815-1832.   10.1029/2018gb005968   AbstractWebsite

The Costa Rica Dome (CRD) is an open-ocean upwelling ecosystem, with high biomasses of picophytoplankton (especially Synechococcus), mesozooplankton, and higher trophic levels. To elucidate the food web pathways supporting the trophic structure and carbon export in this unique ecosystem, we used Markov Chain Monte Carlo techniques to assimilate data from four independent realizations of N-15 and planktonic rate measurements from the CRD into steady state, multicompartment ecosystem box models (linear inverse models). Model results present well-constrained snapshots of ecosystem nitrogen and stable isotope fluxes. New production is supported by upwelled nitrate, not nitrogen fixation. Protistivory (rather than herbivory) was the most important feeding mode for mesozooplankton, which rely heavily on microzooplankton prey. Mesozooplankton play a central role in vertical nitrogen export, primarily through active transport of nitrogen consumed in the surface layer and excreted at depth, which comprised an average 36-46% of total export. Detritus or aggregate feeding is also an important mode of resource acquisition by mesozooplankton and regeneration of nutrients within the euphotic zone. As a consequence, the ratio of passively sinking particle export to phytoplankton production is very low in the CRD. Comparisons to similar models constrained with data from the nearby equatorial Pacific demonstrate that the dominant role of vertical migrators to the biological pump is a unique feature of the CRD. However, both regions show efficient nitrogen transfer from mesozooplankton to higher trophic levels (as expected for regions with large fish, cetacean, and seabird populations) despite the dominance of protists as major grazers of phytoplankton. Plain Language Summary Most of the world's oceanic regions can be divided into (1) low-nutrient areas where small algae dominate and crustaceans, fish, and whales are scarce or (2) productive areas where large algae dominate, crustaceans and higher trophic levels are abundant, and substantial carbon is transported to depth as part of the biological pump. The Costa Rica Dome (CRD) is a unique natural laboratory for investigating the relationships between algae, zooplankton, and marine biogeochemistry because it is a productive region dominated by cyanobacteria (small algae) that nevertheless sustains large populations of crustaceans, fish, and whales. We used a novel data assimilation tool to constrain a food web model using at-sea rate measurements of plankton activity and nitrogen stable isotopes. We found that protists are an important intermediate trophic level linking cyanobacteria and mesozooplankton. Efficient recycling by the zooplankton community facilitates nitrogen transfer to fish, whales, and seabirds. In the CRD, vertically migrating zooplankton (which feed in the surface during the night but descend to depth during the day to escape predators) play a particularly important role in transporting nitrogen (and carbon dioxide) from the surface to the deep ocean, where it can be removed from the atmosphere.

Ota, AY, Landry MR.  1984.  Nucleic acids as growth rate indicators for early developmental stages of Calanus pacificus Brodsky. Journal of Experimental Marine Biology and Ecology. 80:147-160.   10.1016/0022-0981(84)90009-1   AbstractWebsite

Relationships among RNA and DNA contents, dry weight, and growth rate were investigated for the copepod Calanus pacificus Brodsky cultured under different temperature and food conditions. For a given culture condition, RNA/mg dry wt, RNA/DNA, and RNA/ animal tended to be positively, although weakly, correlated with growth rate. Copepods cultured at low temperature (8 °C), however, had higher RNA contents than faster growing animals from higher temperature (15 °C). Moreover, animals from low-temperature, high-food and high-temperature, low-food cultures had similar growth rates but markedly different levels of RNA. We conclude that the underlying relationships among nucleic acid contents and ratios, food, temperature, organism size, and developmental stage are too complex and contradictory for nucleic acids to be useful indicators or predictors of Zooplankton growth rates under field conditions.