Michael R. Landry

Estimating the grazing impact of marine micro-zooplankton

This paper describes a dilution technique for estimating the micro-zooplankton grazing impact on natural communities of marine phytoplankton. Experiments performed in coastal waters off Washington, USA (October, 1980), yield estimates of micro-zooplankton impact equivalent to 6 to 24% of phytoplankton standing biomass and 17 to 52% of production per day. Indirect evidence suggests that most of this impact is due to the feeding of copepod nauplii and tintinnids; in contrast, non-loricate ciliates, comprising 80 to 90% of numerical abundance, appeared to contribute little to phytoplankton mortality.

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Experimental determination of the organic carbon flux from open-ocean surface waters

The flux of biologically produced organic carbon from the euphotic zone of the ocean to the deep waters below—the ‘biological organic carbon pump’—is one of the main controls on the carbon dioxide partial pressure in the atmosphere. Accurate determination of this flux is therefore critically important for understanding the global carbon cycle and its response to climate change. Our goal is to assess how accurately the biological organic carbon pump can be determined at a single location and to constrain estimates of the global value. As there are no standards against which such environmental fluxes can be measured, we assess accuracy by comparing results from three independent experimental approaches for measuring the net annual export of organic carbon from the euphotic zone in the subtropical North Pacific Ocean near Hawaii. Mass balances of dissolved oxygen, inorganic carbon and organic carbon yield estimates of the organic carbon export flux of 2.7 ± 1.7, 1.6 ± 0.9 and 2.0 ± 0.9 mol C m−2 yr−1, respectively. These three estimates are not significantly different, and establish the present analytically attainable accuracy at this location to be about ±50%. If 2.0 mol C m−2 yr−1 is typical of the organic carbon export flux in the subtropical ocean, then this vast region, often considered to be a biological desert, may be responsible for up to half of the global-ocean biological organic carbon pump.

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

Microzooplankton grazing in the central equatorial Pacific during February and August, 1992

Abstract Dilution studies were conducted on EqPac cruises in the central equatorial Pacific (2°N to 2°S, 140°W) during February–March and August–September 1992 to determine phytoplankton growth rates and mortality rates attributable to microzooplankton grazing. Instantaneous growth rates (μ) based on bulk chlorophyll measurements were highly variable from day-to-day, but averaged 0.83 day−1 for the upper (10–20 m), 0.34 day−1 for the mid (40–50 m) and 0.22 day−1 (70–80 m) for the lower euphotic zone on the first cruise. Corresponding rate estimates for microzooplankton grazing (m) were 0.72, 0.22 and 0.21 day−1, respectively. During the second cruise, growth estimates strongly exceeded grazing estimates for the two upper (μ = 0.98 and 1.00 day−1; m = 0.57 and 0.42 day−1), but not the lower depth strata (μ = 0.32 day−1; m = 0.27 day−1). Grazing losses accounted for about 83% of depth-integrated phytoplankton growth in February–March experiments and only about 55% in August–September experiments. In addition, growth rates in the presence of added nutrients (including iron) showed evidence of limitation in February–March, which coincided with a major El Nino event, but not in August–September, which was more representative of climatological mean conditions. Differences in growth rates, implied nutrient limitation, and the balance of phytoplankton growth and microzooplankton grazing were consistent with a greater abundance of large diatoms in August–September. Despite the disparity between chlorophyll-based estimates of growth and grazing rates for this cruise, flow-cytometric analyses of specific populations (Prochlorococcus, Synechococcus, and autotrophic nanoeukaryotes) in a subset of experiments conducted in August demonstrated that microzooplankton grazing was still sufficient to balance growth rates of the smaller components of the phytoplankton assemblage.

Switching between herbivory and carnivory by the planktonic marine copepod Calanus pacificus

Adult females of the omnivorous copepod Calanus pacificus, collected from the plankton off La Jolla, California, USA (June, 1978), fed disproportionately on the prey in greatest relative abundance when given mixtures of diatoms (Thalassiosira fluviatilis) and copepod (C. pacificus) nauplii as food. This switch from herbivorous to carnivorous behavior may be significant in nature during the decline of phytoplankton blooms. More generally, the widespread omnivorous habit among pelagic animals suggests a responsive and flexible trophic organization which contributes to the resiliency of planktonic communities in a dynamic physical environment.

Bacterial community composition during two consecutive NE Monsoon periods in the Arabian Sea studied by denaturing gradient gel electrophoresis (DGGE) of rRNA genes

Abstract Horizontal and vertical variations in bacterial community composition were examined in samples collected during two Joint Global Ocean Flux Study (JGOFS) Arabian Sea cruises in 1995. The cruises, 11 months apart, took place during two consecutive NE Monsoon periods (January and December). Bacteria were harvested by filtration from samples collected in the mixed layer, mid-water, and deep sea at stations across the study area. Total bacterial community genomic DNA was analyzed by PCR amplification of 16S rRNA gene fragments, followed by denaturing gradient gel electrophoresis (DGGE). In total, 20 DGGE bands reflecting unique or varying phylotypes were excised, cloned and sequenced. Amplicons were dominated by bacterial groups commonly found in oceanic waters (e.g., the SAR11 cluster of α -Proteobacteria and cyanobacteria), but surprisingly none of the sequenced amplicons were related to γ -Proteobacteria or to members of the Cytophaga-Flavobacter-Bacteroides phylum. Amplicons related to magnetotactic bacteria were found for the first time in pelagic oceanic waters. The DGGE banding patterns revealed a dominance of ≈15 distinguishable amplicons in all samples. In the mixed layer the bacterial community was dominated by the same ≈15 phylotypes at all stations, but unique phylotypes were found with increasing depth. Except for cyanobacteria, comparison of the bacterial community composition in surface waters from January and December 1995 showed only minor differences, despite significant differences in environmental parameters. These data suggest a horizontal homogeneity and some degree of seasonal predictability of bacterial community composition in the Arabian Sea.

Discrimination between living and heat-killed prey by a marine zooflagellate, Paraphysomonas vestita (Stokes)

Abstract A recombination-deficient strain (EM 1035) of Escherichia coli (Migula) was used to test the ability of the marine zooflagellate Paraphysomonas vestita (Stokes) to discriminate between living and heat-killed prey of similar size and morphology. Cell division of EM 1035 was prevented by relatively short exposure to UV-irradiation. Fluorescent staining with rhodamine isothiocyanate did not affect the viability of the bacterial cells or the growth and feeding rates of the zooflagellate. P. vestita fed preferentially on living cells when presented with an equal density mixture of heat-killed and UV-irradiated (nondividing) cells. The flagellate discriminated between living and dead cells with a preference ratio of about 20 suggesting that chemosensory cues may be important in the feeding selectivity of some marine protozoans.

Abundances and distributions of picoplankton populations in the central equatorial Pacific from 12°N to 12°S, 140°W

Abundances and distributions of picoplankton were studied on two cross-equatorial transect cruises (12°N, 140°W-12°S, 135°W) during February–March (TT007) and August–September 1992 (TT011). Samples were collected in the upper 200 m from early-morning and late-afternoon hydrocasts at 15 stations on each cruise (60 depth profiles, 820 samples). Populations of heterotrophic bacteria, Prochlorococcus, Synechococcus and small autotrophic eukaryotes were enumerated by dual-beam flow cytometry. At the northern end of the transect (7–12°N), abundances and vertical distributions were similar to those reported for the oligotrophic North Pacific gyre, with Prochlorococcus and heterotrophic bacteria dominating in the upper euphotic zone, and Synechococcus and eukaryotes exhibiting peaks in cell abundance at depth. All populations were abundant throughout the euphotic zone close to the equator and at the southern end of the transect. Heterotrophic bacteria and Synechococcus were generally more abundant in late-afternoon hydrocasts. The diel enhancement followed the temporal pattern in beam c and suspended particulates, and was particularly strong in the equatorial upwelling region where it averaged 13.6% of the morning population for heterotrophic bacteria and 22.3% for Synechococcus. Conservative estimates of daily growth rates from these data are 0.25 and 0.40 day−1, respectively, for the two populations. Near-surface maxima in heterotrophic bacteria were symmetrical around the equator, centered around 5°S and 5°N. Prochlorococcus was most abundant during local summer conditions at the respective ends of the transect. A minimum occurred in association with a dense aggregation of buoyant diatoms at the convergent front of a tropical instability wave (2°N, TT011). The ratio of Prochlorococcus to total bacteria was generally in the range of 0.15-0.2 for the upper water column, but varied during TT011 from > 0.3 for the most northern stations to < 0.1 at the 2°N front. At higher latitudes, Synechococcus was more numerous during El Nino conditions (TT007) on both sides of the equator and at southern stations on both cruises. Autographic eukaryotes were more abundant during local winters at the ends of the transect and during the “cold tongue” conditions (TT011) at the equator. Picoplankton account for most of the chlorophyll biomass and primary production in the central equatorial Pacific. Nonetheless, their abundances and distributions are relatively stable and conservative while other populations, such as diatoms, respond more dramatically to environmental forcing.

Iron‐stimulated changes in 13C fractionation and export by equatorial Pacific phytoplankton: Toward a paleogrowth rate proxy

We present δ13C 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 13C 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 234Th 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.