Hans G. Dam

The role of surface-active carbohydrates in the flocculation of a diatom bloom in a mesocosm

A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 μ-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3–51 urn equivalent spherical diameter) were readily formed by bubbling and became concentrated in the foam. At the beginning of the bubbling (0–0.5 h), both particles and surface-active carbohydrates were extracted at high rates; however, these rates dropped off steeply after about 0.5 h of bubbling. The rate of particle formation by bubbling could be modeled fairly well by second order kinetics. The extracted, surface-active material was enriched in deoxysugars and galactose, while the residual material was enriched in glucose. Extracted surface-active carbohydrates reached a maximum of 33% of the total dissolved sugars ( 0.99) with particle stickiness (alpha). In addition, the concentration of surface-active carbohydrates was well correlated (2 = 0.91) with the concentration of transparent exopolymer particles (TEP) in the tank, and it was demonstrated that TEP could be copiously formed by bubbling of 1.0 μm-filered seawater. The finding of a highly surface-active, deoxysugar-rich polysaccharide material that can be rapidly (<0.5 h) and selectively extracted by bubble adsorption is significant, as it is apparent that this material played important roles in particle stickiness and TEP formation in the tank, and thus it may, at times, play similar roles in particle aggregation in the sea.

The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda

Abstract Seawater samples were collected from the euphotic zone of the Sargasso Sea near Bermuda in August of 1989 and March–April of 1990. Microbial population abundances, chlorophyll concentration, particulate carbon and particulate nitrogen were measured. Calculations were performed to establish the relative and absolute importance of the various microbial assemblages. The choice of conversion factors (g C and N cell −1 , or g C and N μm −3 ) for the microbial populations dramatically affected the estimation of “living” and “detrital” particulate material in the samples, and the relative importance of the various microbial groups. Averaged over all samples on either of the two cruises, microbial biomass constituted a greater proportion of the total particulate carbon and nitrogen during March–April (55% and 63%, respectively), than during August (≈24% and 30%, respectively) using “constrained” conversion factors that were derived. Accordingly, detrital material constituted the bulk of the particulate material during August, but was similar to the amount of microbial biomass during March–April. The bacterial assemblage constituted the largest single pool of microbial carbon (35%) and nitrogen (45%) in the water, and a significant fraction of the total particulate carbon (≈10–20%) and nitrogen (≈15–30%). Phototrophic nanoplankton (microalgae 2–20 μm in size) were second in overall biomass, and often dominated the microbial biomass in the deep chlorophyll maxima that were present during both cruises. The results temper recent assertions concerning the overwhelming importance of bacterial biomass in the oligotrophic Sargasso Sea but still support a major role for these microorganisms in the open ocean as repositories for carbon and nutrients.

Coagulation efficiency and aggregate formation in marine phytoplankton

Flocculation of phytoplankters into large, rapidly sinking aggregates has been implicated as a mechanism of vertical transport of phytoplankton to the sea floor which could have global significance. The formation rate of phytoplankton aggregates depends on the rate at which single cells collide, which is mainly physically controlled, and on the probability of adhesion upon collision (=coagulation efficiency, stickiness), which depends on physico-chemical and biological properties of the cells. We describe here an experimental method to quantify the stickiness of phytoplankton cells and demonstrate that three species of diatoms grown in the laboratory (Phaeodactylum tricornutum, Thalassiosira pseudonana, Skeletonema costatum) are indeed significantly sticky and form aggregates upon collision. The dependency of stickiness on nutrient limitation and growth was studied in the two latter species by investigating variation in stickiness as batch cultures aged. In nutrient repleteT. pseudonana cells stickiness is very low (< 5 × 10−3), but increases by more than two orders of magnitude as cell growth ceases and the cells become nutrient limited. Stickiness ofS. costatum cells is much less variable, and even nutrient replete cells are significantly sticky. Stickiness is highest (> 10−1) forS. costatum cells in the transition between the exponential and the stationary growth phase. The implications for phytoplankton aggregate formation and subsequent sedimentation in the sea of these two different types of stickiness patterns are discussed.

Particle size spectra between 1 μm and 1 cm at Monterey bay determined using multiple instruments

Abstract Particles are responsible for the vertical transport of material in the ocean. Size is an important characteristic of a particle, determining its fall velocity, mass content, scattering crosssection, and food value, as well as other properties. The particle size spectrum describes the distribution of particles in a volume of water as a function of their sizes. We measured particle size spectra in Monterey Bay, CA, using six different instruments that examined particles ranging from approximately 1 μm to 10 mm. Before the results could be combined, they had to be adjusted for the different particle properties actually measured. Results from different optical instruments were similar, although the spectral values were sensitive to minor variations in the diameter assigned to particles. Sample volume was crucial in determining the effective upper size limit for the different techniques. We used fractal scaling to piece the results together, deriving fractal dimensions of 2.26–2.36. Diver observations of visible particles showed that they were composed mostly of aggregated diatoms. The particle size spectra n I were remarkably well fitted with a power law function n I = ad I − b I , where d I is the image diameter and b I = 2.96–3.00 . The equivalent slopes for particles measured with an aperture impedance instrument were 3.50–3.61. The particle volume distribution showed that most of the particle mass was in the 0.1–3 mm range. This volume distribution is consistent with theories that assume particle sizes are controlled by simultaneous coagulation and disaggregation.

Mesozooplankton grazing and metabolism at the equator in the central Pacific: Implications for carbon and nitrogen fluxes

Primary productivity and chlorophyll in the equatorial Pacific are lower than expected based on ambient nutrient concentrations. We tested the hypothesis that these conditions are due to a balance between phytoplankton growth and mesozooplankton grazing. Grazing rates and biomasses of three size classes of zooplankton in the size range of 200-200 μm were measured during March-April and October of 1992 at the equator (140°W). El Nino conditions prevailed in March–April, whereas in October a tropical instability wave (TIW) passed through the study area. Weight-specific pigment ingestion rates of mesozooplankton tended to be higher in March–April than in October while the opposite was the case for biomass. There were no discernible temporal trends in mesozooplankton community grazing rates in March–April, but there were changes in October associated with the passage of the TIW. Mesozooplankton grazing removed 1–9% day−1 of the total chlorophyll and 1–12% of the primary production within the euphotic zone. Therefore, the grazing hypothesis as stated above is rejected, although a simple chlorophyll budget suggests that grazing (mostly by microzooplankton) and phytoplankton growth are roughly in balance. Most of the phytoplankton was 2-μm phytoplankton, we estimate removal rates of the > 2-μm chlorophyll standing stock of up to 27% day−1. Another question of biogeochemical importance is the rate of the production of large diatoms. Mesozooplankton grazing could have balanced growth of the large (> 20-μm) diatoms in March–April, but not in October. From estimates of respiration and excretion based on water temperature and body size, we infer that: (1) > 80% of the carbon ingested by mesozooplankton is not phytoplankton; (2) mesozooplankton excretion supports < 7% of the nitrogen demands of phytoplankton; (3) the flux of carbon passing through the mesozooplankton would be equivalent to 23% of the primary production; (4) mesozooplankton fecal carbon could account for 100% of the sinking POC flux in this region. Finally, a simple food chain analysis suggests that a significant fraction of the microzooplankton production probably passes through mesozooplankton.

The trophic role of mesozooplankton at 47°N, 20°W during the North Atlantic Bloom Experiment

The biomass and grazing rates of three size classes of mesozooplankton—0.2–0.5 mm (small), 0.5–1.0 mm (medium) and 1.0–2.0 mm (large)—were quantified in the vicinity of 47°N, 20°W, from 25 April to 7 May (leg 4) and from 18 to 31 May 1989 (leg 5) as part of the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS). Biomass was inversely related to body size, with the small size fraction accounting for > 50% of the entire mesozooplankton biomass. Diel differences in biomass, however, were directly related to body size, indicating that vertical migration became more pronounced as the size of the animals increased. Total zooplankton biomass increased by almost a factor of 3 from the beginning to the end of the study. The average carbon-weight of individuals increased six-fold from leg 4 to leg 5 of the study. Carbon-specific rates of phytoplankton ingestion were (1) inversely related to body size; (2) greater at night for all size fractions; and (3) generally greater on leg 4 than on leg 5, particularly for the small size fraction. Grazing was dominated by the small size fraction (66% of the total grazing) on leg 4 and by the medium size fraction (44% of the total grazing) on leg 5. The removal of the daily primary production by mesozooplankton was not different from leg 4 to leg 5, averaging 2.7% day−1 (range 0.6–5.2% day−1). Comparisons of (1) estimated metabolic rates and (2) measured nitrogen excretion rates with daily rations of carbon and nitrogen, respectively, for zooplankton suggest that a phytoplankton diet only contributed about 50% of the daily carbon and nitrogen rations of animals. We hypothesize that mesozooplankton fecal pellets contributed < 5% of the POC flux out of the euphotic zone measured with particle traps. However, we estimate that during leg 5, the active flux of dissolved nitrogen out of the euphotic zone due to mesozooplankton diel vertical migration was 26% of the passive PON flux.

Coagulation efficiency, organic-matter glues and the dynamics of particles during a phytoplankton bloom in a mesocosm study

Abstract The combination of phytoplankton aggregate formation and subsequent sedimentation has been proposed as a mechanism for termination of phytoplankton blooms. To test this hypothesis, the evolution of a phytoplankton bloom was studied in a mesocosm. During a seven-day period, the concentration and size of small particles (

Downward export of respiratory carbon and dissolved inorganic nitrogen by diel-migrant mesozooplankton at the JGOFS Bermuda time-series station

Published calculations suggest that downward fluxes of respiratory carbon (RC) and dissolved inorganic nitrogen (DIN) associated with diel-migrant zooplankton are significant forms of export production. In this study, we examined variability in downward fluxes of RC and DIN due to diel-migrant mesozooplankton (200–2000 μm) on seven occasions between 25 March and 8 April 1990 at the JGOFS (Joint Global Ocean Flux Study) station off Bermuda. Downward fluxes due to migrators crossing 150 m ranged from 6 to 41 mgC m−2 day−1 (RC) and 2 to 5 mgN m−2 day−1 (DIN). In comparison to gravitational fluxes of POC and PON measured with particle traps at 150m during the same period, mesozooplankton RC flux was 18–70% of POCflux and mesozooplankton DIN flux was 17–82% of PON flux. If the RC and DIN fluxes due to migrators are considered in calculations of export production, the percentage of the production exported below 150 m over the study period would increase by an average of 25% for carbon and 21% for nitrogen. These results support the contention that metabolic activities of diel-migrant zooplankton should be included in calculations of export of carbon and nitrogen out of the mixed layer.

Evolutionary Adaptation of Marine Zooplankton to Global Change

Predicting the response of the biota to global change remains a formidable endeavor. Zooplankton face challenges related to global warming, ocean acidification, the proliferation of toxic algal blooms, and increasing pollution, eutrophication, and hypoxia. They can respond to these changes by phenotypic plasticity or genetic adaptation. Using the concept of the evolution of reaction norms, I address how adaptive responses can be unequivocally discerned from phenotypic plasticity. To date, relatively few zooplankton studies have been designed for such a purpose. As case studies, I review the evidence for zooplankton adaptation to toxic algal blooms, hypoxia, and climate change. Predicting the response of zooplankton to global change requires new information to determine (a) the trade-offs and costs of adaptation, (b) the rates of evolution versus environmental change, (c) the consequences of adaptation to stochastic or cyclic (toxic algal blooms, coastal hypoxia) versus directional (temperature, acidification, open ocean hypoxia) environmental change, and (d) the interaction of selective pressures, and evolutionary and ecological processes, in promoting or hindering adaptation.

Downward export of carbon by diel migrant mesozooplankton in the central equatorial Pacific

Abstract Using data collected during cruises of the JGOFS equatorial Pacific Study in March/April and October of 1992 at the equator (140°W), we examine the downward transport of carbon by three size classes of die] migrant mesozooplankton (200–500 gm, 500–1000 μm and 1000–2000 gm). In addition to respiratory carbon flux, we consider the flux due to mortality of migrators below the euphotic zone. Diel migrant mesozooplankton biomass was estimated from the difference between nighttime and daytime biomass within the euphotic zone. Except for a four-day period early in the March/April cruise, mesozooplankton nighttime biomass was significantly larger than daytime biomass within the euphotic zone during both cruises. We estimate that the downward flux of carbon from the euphotic zone due to mesozooplankton die] vertical migrators was an average of 0.6 mmol Cm −2 d −1 and 1.1 mmol C m −2 d −1 during the March/April and October cruises, respectively. Addition of this flux to the gravitational particle sinking flux estimated from 234 Th measurements during the same period results in a 31 % increase in the carbon export flux from the euphotic zone in the equatorial Pacific during the March/April cruise and a 44% increase in the October cruise. The migratory flux is strongly dependent on whether feeding takes place below the euphoric zone, the length of time migrators spend in the deep waters, and the mortality rate of migrators.