Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the... more Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the zooplankton prey and feeding selectivity of early larval stages of Atlantic bluefin tuna (ABT, Thunnus thynnus) in larval rearing habitat of the Gulf of Mexico. Larvae and zooplankton were collected during two multi-day Lagrangian experiments during peak spawning in May 2017 and 2018. Larvae were categorized by flexion stage and standard length. We identified, enumerated and sized zooplankton from larval gut contents and in the ambient community. Ciliates were quantitatively important (up to 9%) in carbon-based diets of early larvae. As larvae grew, diet composition and prey selection shifted from small copepod nauplii and calanoid copepodites to larger podonid cladocerans, which accounted for up to 70% of ingested carbon. Even when cladoceran abundances were <0.2 m−3, they comprised 23% of postflexion stage diet. Feeding behaviors of larvae at different development stages were more ...
The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and mari... more The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradie...
We investigated the accuracy and precision of flow cytometric (FCM) estimates of bacterial abunda... more We investigated the accuracy and precision of flow cytometric (FCM) estimates of bacterial abundances using 4′, 6-diamidino-2-phenylindole (DAPI) and Hoechst 33342 (HO342, a bisbenzamide derivative) on paraformaldehyde-fixed seawater samples collected from two stations near Oahu, Hawaii. The accuracy of FCM estimates was assessed against direct counts by using epifluorescence microscopy. DAPI and HO342 differ in two aspects of their chemistry that make HO342 better suited for staining marine heterotrophic bacteria for FCM analysis. These differences are most important in studies of open-ocean ecosystems that require dual-beam FCM analysis to clearly separate heterotrophic bacterial populations from populations of photosynthetic Prochlorococcus spp. Bacterial populations were easier to distinguish from background fluorescence when stained with HO342 than when stained with DAPI, because HO342 has a higher relative fluorescence quantum yield. A substantially higher coefficient of varia...
During the past decade compound specific nitrogen (N) isotopic analysis of amino acids (CSIA−AA) ... more During the past decade compound specific nitrogen (N) isotopic analysis of amino acids (CSIA−AA) has become an increasingly used method for tracking the origin and fate of N in ecological and biogeochemical studies. CSIA−AA has the potential for resolving finer scale trophic dynamics than previously possible with bulk stable isotope analysis (SIA) and for reconstructing past food webs using historical archives of organismal samples. However, there is little information on the effects of chemical preservation used in historical archives on δ15NAA values, and conventional CSIA conducted on derivatized AAs using gas chromatography − combustion − isotope ratio mass spectrometry (GC−C−IRMS) has analytical errors in the range of what may be expected from chemical preservation. Here we present analytical errors across 11 underivatized AA standards analyzed by high pressure liquid chromatography followed by offline elemental analysis − IRMS (HPLC/EA−IRMS) an approach originally developed by...
We investigated the processes driving variability in primary productivity in the California Curre... more We investigated the processes driving variability in primary productivity in the California Current Ecosystem (CCE) in order to develop an algorithm for predicting primary productivity from in situ irradiance, nutrient, and chlorophyll (chl) measurements. Primary productivity data from seven process cruises of the CCE Long-Term Ecological Research (CCE LTER) program were used to parameterize the algorithm. An initial algorithm was developed using only irradiance to predict chl-specific productivity was found to have model-data misfit that was correlated with NH4+ concentrations. We thus found that the best estimates of primary productivity were obtained using an equation including NH4+ and irradiance: PP/Chl = V0m×(1-exp(−α×PAR/V0m)×NH4/(NH4+KS), where PP/Chl is chlorophyll-specific primary production in units of mg C d−1 / mg Chl, PAR is photosynthetically active radiation (units of μEi m−2 s−1), NH4+ is in units of μmol L−1, V0m = 66.5 mg C d−1 / mg Chl, α = 1.5, and KS = 0.025 μm...
The dinoflagellate Noctiluca scintillans (Noctiluca) has the ability to reproduce sexually, which... more The dinoflagellate Noctiluca scintillans (Noctiluca) has the ability to reproduce sexually, which may help to increase or restore its population size during periods of blooms or environmental stress. Here, we documented for the first time a marine ciliate Strombidium sp. that feeds on Noctiluca's progametes undergoing stages 5 to 9 of nuclear division. This ciliate frequently swam on or around gametogenic and some vegetative Noctiluca cells. The ciliates associated with gametogenic cells had significantly lower swimming speed and changed direction more frequently than those associated with vegetative cells, which overall increased their time spent around the food patches (progametes). This trophic interaction constitutes an upside-down predator-prey link, in which ciliates within the typical size range of Noctiluca prey, become the predators. Based on the phylogenetic tree (maximum-likelihood), there are 14 environmental clones similar to Strombidium sp. found in other coastal w...
SummaryAlthough chemosynthetic ecosystems are known to support diverse assemblages of microorgani... more SummaryAlthough chemosynthetic ecosystems are known to support diverse assemblages of microorganisms, the ecological and environmental factors that structure microbial eukaryotes (heterotrophic protists and fungi) are poorly characterized. In this study, we examined the geographic, geochemical and ecological factors that influence microbial eukaryotic composition and distribution patterns within Hydrate Ridge, a methane seep ecosystem off the coast of Oregon using a combination of high‐throughput 18S rRNA tag sequencing, terminal restriction fragment length polymorphism fingerprinting, and cloning and sequencing of full‐length 18S rRNA genes. Microbial eukaryotic composition and diversity varied as a function of substrate (carbonate versus sediment), activity (low activity versus active seep sites), sulfide concentration, and region (North versus South Hydrate Ridge). Sulfide concentration was correlated with changes in microbial eukaryotic composition and richness. This work also r...
Calanus pacificus, normally regarded as a passive filter‐feeding copepod, displays active predato... more Calanus pacificus, normally regarded as a passive filter‐feeding copepod, displays active predatory behavior when fed with copepod nauplii. Larger nauplii are selectively preyed upon, even though they are better able to avoid capture than small nauplii. The involvement of the first antennae in the remote detection of motile prey is suggested by the experimental result that amputation of the antennae sharply reduces predatory feeding rates without affecting filter feeding.
Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the... more Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the zooplankton prey and feeding selectivity of early larval stages of Atlantic bluefin tuna (ABT, Thunnus thynnus) in larval rearing habitat of the Gulf of Mexico. Larvae and zooplankton were collected during two multi-day Lagrangian experiments during peak spawning in May 2017 and 2018. Larvae were categorized by flexion stage and standard length. We identified, enumerated and sized zooplankton from larval gut contents and in the ambient community. Ciliates were quantitatively important (up to 9%) in carbon-based diets of early larvae. As larvae grew, diet composition and prey selection shifted from small copepod nauplii and calanoid copepodites to larger podonid cladocerans, which accounted for up to 70% of ingested carbon. Even when cladoceran abundances were <0.2 m−3, they comprised 23% of postflexion stage diet. Feeding behaviors of larvae at different development stages were more ...
The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and mari... more The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradie...
We investigated the accuracy and precision of flow cytometric (FCM) estimates of bacterial abunda... more We investigated the accuracy and precision of flow cytometric (FCM) estimates of bacterial abundances using 4′, 6-diamidino-2-phenylindole (DAPI) and Hoechst 33342 (HO342, a bisbenzamide derivative) on paraformaldehyde-fixed seawater samples collected from two stations near Oahu, Hawaii. The accuracy of FCM estimates was assessed against direct counts by using epifluorescence microscopy. DAPI and HO342 differ in two aspects of their chemistry that make HO342 better suited for staining marine heterotrophic bacteria for FCM analysis. These differences are most important in studies of open-ocean ecosystems that require dual-beam FCM analysis to clearly separate heterotrophic bacterial populations from populations of photosynthetic Prochlorococcus spp. Bacterial populations were easier to distinguish from background fluorescence when stained with HO342 than when stained with DAPI, because HO342 has a higher relative fluorescence quantum yield. A substantially higher coefficient of varia...
During the past decade compound specific nitrogen (N) isotopic analysis of amino acids (CSIA−AA) ... more During the past decade compound specific nitrogen (N) isotopic analysis of amino acids (CSIA−AA) has become an increasingly used method for tracking the origin and fate of N in ecological and biogeochemical studies. CSIA−AA has the potential for resolving finer scale trophic dynamics than previously possible with bulk stable isotope analysis (SIA) and for reconstructing past food webs using historical archives of organismal samples. However, there is little information on the effects of chemical preservation used in historical archives on δ15NAA values, and conventional CSIA conducted on derivatized AAs using gas chromatography − combustion − isotope ratio mass spectrometry (GC−C−IRMS) has analytical errors in the range of what may be expected from chemical preservation. Here we present analytical errors across 11 underivatized AA standards analyzed by high pressure liquid chromatography followed by offline elemental analysis − IRMS (HPLC/EA−IRMS) an approach originally developed by...
We investigated the processes driving variability in primary productivity in the California Curre... more We investigated the processes driving variability in primary productivity in the California Current Ecosystem (CCE) in order to develop an algorithm for predicting primary productivity from in situ irradiance, nutrient, and chlorophyll (chl) measurements. Primary productivity data from seven process cruises of the CCE Long-Term Ecological Research (CCE LTER) program were used to parameterize the algorithm. An initial algorithm was developed using only irradiance to predict chl-specific productivity was found to have model-data misfit that was correlated with NH4+ concentrations. We thus found that the best estimates of primary productivity were obtained using an equation including NH4+ and irradiance: PP/Chl = V0m×(1-exp(−α×PAR/V0m)×NH4/(NH4+KS), where PP/Chl is chlorophyll-specific primary production in units of mg C d−1 / mg Chl, PAR is photosynthetically active radiation (units of μEi m−2 s−1), NH4+ is in units of μmol L−1, V0m = 66.5 mg C d−1 / mg Chl, α = 1.5, and KS = 0.025 μm...
The dinoflagellate Noctiluca scintillans (Noctiluca) has the ability to reproduce sexually, which... more The dinoflagellate Noctiluca scintillans (Noctiluca) has the ability to reproduce sexually, which may help to increase or restore its population size during periods of blooms or environmental stress. Here, we documented for the first time a marine ciliate Strombidium sp. that feeds on Noctiluca's progametes undergoing stages 5 to 9 of nuclear division. This ciliate frequently swam on or around gametogenic and some vegetative Noctiluca cells. The ciliates associated with gametogenic cells had significantly lower swimming speed and changed direction more frequently than those associated with vegetative cells, which overall increased their time spent around the food patches (progametes). This trophic interaction constitutes an upside-down predator-prey link, in which ciliates within the typical size range of Noctiluca prey, become the predators. Based on the phylogenetic tree (maximum-likelihood), there are 14 environmental clones similar to Strombidium sp. found in other coastal w...
SummaryAlthough chemosynthetic ecosystems are known to support diverse assemblages of microorgani... more SummaryAlthough chemosynthetic ecosystems are known to support diverse assemblages of microorganisms, the ecological and environmental factors that structure microbial eukaryotes (heterotrophic protists and fungi) are poorly characterized. In this study, we examined the geographic, geochemical and ecological factors that influence microbial eukaryotic composition and distribution patterns within Hydrate Ridge, a methane seep ecosystem off the coast of Oregon using a combination of high‐throughput 18S rRNA tag sequencing, terminal restriction fragment length polymorphism fingerprinting, and cloning and sequencing of full‐length 18S rRNA genes. Microbial eukaryotic composition and diversity varied as a function of substrate (carbonate versus sediment), activity (low activity versus active seep sites), sulfide concentration, and region (North versus South Hydrate Ridge). Sulfide concentration was correlated with changes in microbial eukaryotic composition and richness. This work also r...
Calanus pacificus, normally regarded as a passive filter‐feeding copepod, displays active predato... more Calanus pacificus, normally regarded as a passive filter‐feeding copepod, displays active predatory behavior when fed with copepod nauplii. Larger nauplii are selectively preyed upon, even though they are better able to avoid capture than small nauplii. The involvement of the first antennae in the remote detection of motile prey is suggested by the experimental result that amputation of the antennae sharply reduces predatory feeding rates without affecting filter feeding.
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Papers by Michael Landry