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Miller, CA, Landry MR.  1984.  Ingestion-independent rates of ammonium excretion by the copepod Calanus pacificus. Marine Biology. 78:265-270.   10.1007/bf00393012   AbstractWebsite

The relationship between food ingested and NH + 4excretion rate was investigated for female Calanus pacificus collected in August, 1982, from the San Juan Archipelago, Washington State, USA. The copepods were preconditioned to 6 densities of the diatom Thalassiosira weissflogii (0 to 104 cells ml−1) for 30 h before the experiment. The experiment was conducted with nutrients added in excess to maintain equal rates of NH + 4uptake by the diatoms at all densities. Although ingestion rates of C. pacificus varied from 0 to over 20% of body N d−1 at the different food levels, excretion was a constant 6.6 nM NH + 4copepod−1 h−1 or about 10% of body N d−1. This ingestion-excretion relationship, which is consistent with previous respiration and fecundity studies, suggests that the ecological dominance of C. pacificus only under conditions of high food abundance may be due to a dramatic increase in its growth efficiency as ingestion increases above the level supporting a constant metabolic rate. The maintenance of a constant level of metabolism during relatively short periods of low food abundance may be advantageous if it allows the copepod to exploit more effectively short-term variability in its food resulting from environmental heterogeneity or vertical migration.

Landry, MR, Brown SL, Selph KE, Abbott MR, Letelier RM, Christensen S, Bidigare RR, Casciotti K.  2001.  Initiation of the spring phytoplankton increase in the Antarctic Polar Front Zone at 170 degrees W. Journal of Geophysical Research-Oceans. 106:13903-13915.   10.1029/1999jc000187   AbstractWebsite

During austral summer 1997, satellite imagery revealed enhanced chlorophyll associated with the Antarctic Polar Front at 170 degreesW. Phytoplankton growth conditions during the early stages of the spring increase were investigated on the Antarctic Environment and Southern Ocean Process Study Survey I cruise using flow cytometry (FCM) and microscopy to characterize community biomass, composition and biological stratification and dilution experiments to estimate growth and grazing rates, Physical and biological measures showed a general shoaling of mixed layer depth from similar to 200 to < 100 m from late October to early November. Plankton assemblages on the southern side of the frontal jet (similar to0 degreesC waters) differed from those on the northern side (similar to2 degreesC) in enhanced relative importance of larger (> 20 mum) cells, greater contributions of diatoms and ciliates, and a twofold higher ratio of protistan grazers to photoautotrophs. Phytoplankton community growth rates from incubations at 10 and 23% of surface incident light showed good agreement between high performance liquid chromatography estimates of chlorophyll a (Chl a) (0.20 d(-1)) and FCM cell-based (0.21 d(-1))results. Fucoxanthin-based estimates for diatoms were 0.2-1 d(-1). Mean estimates of microzooplankton grazing from the three phytoplankton measures were 0.16, 0.12, and 0.11 d(-1), respectively. Heterotrophs typically consumed 40-100% of their body carbon per day and thus presumably grew at rates similar to phytoplankton. The low net rates of Chl a increase in shipboard bottle incubations (0.04 d(-1)) were consistent with the slow downstream accumulation of phytoplankton biomass (0.03 d(-1)) as measured with instrumented Lagrangian drifters through the month of November. Both were slightly less than the net rate estimates from SeaSoar surveys (0.05 d(-1)) because of the effects of pigment photoadaptation (bleaching) during this time of increasing light level and water column stratification.

Landry, MR.  2002.  Integrating classical and microbial food web concepts: evolving views from the open-ocean tropical Pacific. Hydrobiologia. 480:29-39.   10.1023/a:1021272731737   AbstractWebsite

Over the past half-century, and particularly the last two decades, new paradigms, perspectives and technological capabilities have greatly advanced our understanding of open-ocean pelagic ecosystems. Major new insights have come from the microbial loop concept and related discoveries, the iron limitation hypothesis and ocean time series. Focusing mainly on the tropical and subtropical Pacific Ocean, I review the influences of these new perspectives on classical views of food web complexity, phytoplankton regulation and diversity, and temporal dynamics.

Landry, MR, Barber RT, Bidigare RR, Chai F, Coale KH, Dam HG, Lewis MR, Lindley ST, McCarthy JJ, Roman MR, Stoecker DK, Verity PG, White JR.  1997.  Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis. Limnology and Oceanography. 42:405-418.   10.4319/lo.1997.42.3.0405   AbstractWebsite

Recent studies in the central equatorial Pacific allow a comprehensive assessment of phytoplankton regulation in a high-nutrient, low-chlorophyll (HNLC) ecosystem. Elemental iron enters the euphotic zone principally via upwelling and is present at concentrations (less than or equal to 30 pM) wen below the estimated half-saturation constant (120 pM) for the large cells that bloom with iron enrichment. In addition, the meridional trend in quantum yield of photosynthesis suggests that even the dominant small phytoplankton are held below their physiological potential by iron deficiency. Grazing by microzooplankton dominates phytoplankton losses, accounting for virtually all of the measured phytoplankton production during El Nino conditions and similar to 66% during normal upwelling conditions, with mesozooplankton grazing and lateral advection closing the balance. Nitrate uptake is strongly correlated with the pigment biomass of diatoms, which increase in relative abundance during normal upwelling conditions. Nonetheless, the f-ratio remains low (0.07-0.12) under all conditions. Iron budgets are consistent with the notions that new production is determined by the rate of new iron input to the system while total production depends on efficient iron recycling by grazers. Although the limiting substrates differ, the interactions of resource limitation and grazing in HNLC regions are conceptually similar to the generally accepted view for oligotrophic subtropical regions. In both systems, small dominant phytoplankton grow at rapid, but usually less than physiologically maximal, rates; they are cropped to low stable abundances by microzooplankton; and their sustained high rates of growth depend on the remineralized by-products of grazing.

Bidigare, RR, Hanson KL, Buesseler KO, Wakeham SG, Freeman KH, Pancost RD, Millero FJ, Steinberg P, Popp BN, Latasa M, Landry MR, Laws EA.  1999.  Iron-stimulated changes in C-13 fractionation and export by equatorial Pacific phytoplankton: Toward a paleogrowth rate proxy. Paleoceanography. 14:589-595.   10.1029/1999pa900026   AbstractWebsite

We present delta(13)C values for phytol, an algal biomarker, which document up to 7 per mil isotopic enrichment during the IronEx II iron fertilization experiment. We evaluate these data using a laboratory-derived C-13 fractionation model and show this variability is largely the result of elevated growth rates. Isotopic enrichment and stimulation of growth rate were accompanied by a sevenfold increase in the export of particulate organic carbon as estimated from Th-234 activities. This is the first direct evidence that enhanced productivity following iron enrichment can lead to both increased export of organic matter and an associated isotopic signal in an algal biomarker. On the basis of these results, we propose biomarker isotopic data be used in conjunction with paleo-CO2 records to reconstruct paleogrowth rates. This approach provides a means to test for iron-stimulated changes in algal growth in sedimentary records.

Gutierrez-Rodriguez, A, Decima M, Popp BN, Landry MR.  2014.  Isotopic invisibility of protozoan trophic steps in marine food webs. Limnology and Oceanography. 59:1590-1598.   10.4319/lo.2014.59.5.1590   AbstractWebsite

According to modern oceanographic perspectives that emphasize microbial pathways, phagotrophic protists comprise one to several levels of intermediate consumers between phytoplankton and larger metazooplankton (copepods and krill). However, recent attempts to quantify pelagic trophic structure in the open ocean using nitrogen stable isotope techniques have brought into question whether such measurements adequately account for protistan trophic steps. Here, we use a two-stage chemostat system, with Dunaliella tertiolecta and Oxyrrhis marina as a predator-prey model, to address this question experimentally. To investigate N-15 trophic discrimination under different conditions of nitrogen availability and recycling, Oxyrrhis was fed in the light and in the dark on phytoplankton provided with high and low nutrient ratios of N : P. We used both bulk and amino acids-compound specific isotopic analysis (AA CSIA) to distinguish trophic fractionation from changes in the delta N-15 values of phytoplankton (isotopic baseline). Results demonstrate that protistan consumers are not, in fact, significantly enriched in N-15 relative to their prey, a marked departure from the general findings for metazoan consumers. In addition, we show that changes in the isotopic baseline propagate rapidly through the protistan food chain, highlighting the need to account for this variability at ecologically relevant time scales. If protistan trophic steps are largely invisible or significantly underestimated using nitrogen isotope measurements, research that utilize such measurements in ecological, fisheries, and climate change studies may miss a large part of the ocean's variability in food-web structure and ecosystem function.