George A. Jackson

A model of the formation of marine algal flocs by physical coagulation processes

Abstract Aggregates of organic matter (“marine snow”) are highly visible phenomena oceanic waters which can control material fluxes to the deep sea. These aggregates take many forms and undoubtedly have many causes. One form frequently described is that composed of algae, usually diatoms, occurring after an algal bloom. A model combining kinetic coagulation theory and simple algal growth kinetics describes the dynamics of such an algal bloom. Results show this to be a two-state system in which coagulation processes are either unimportant when algal concentrations are low or dominant when they are high, with a rapid transition between the two states. Critical algal concentration for this transition is inversely related to fluid shear, algal size and stickiness, and is similar to values observed after a bloom. The role of coagulation in controlling particle dynamics for more complicated aquatic systems will depend on other biological particle production and consumption processes, such as zooplankton feeding and defecation. These results emphasize the importance of measuring algal sizes and abundances when studying floc formation.

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.

Remineralization of upper ocean particles: Implications for iron biogeochemistry

The role of heterotrophic bacteria in iron recycling, the influence of complexation on iron remineralization, and iron mobilization rates from lithogenic vs. biogenic particulate iron (PFe) were examined using field experiments and modeling simulations. During summer, we measured the mobilization rate of algal iron by heterotrophic bacteria in the mixed layer at a polar and a subpolar site south of Australia, and conducted shipboard incubations to track the release of dissolved iron (DFe) and iron-binding ligands from subsurface settling particles sampled from 120-m depth. Bacteria mobilized . 25% PFe d21 in surface waters relative to mobilization at depth (, 2% d21). Our incubations provide the first evidence of the concurrent release of weak iron-binding ligands and DFe from sinking particles. Simulated profiles of PFe remineralization, based on proxies, point to greater dissolution from biogenic PFe than from lithogenic PFe. Together our findings point to different biogeochemical functions for lithogenic vs. biogenic PFe: biogenic PFe is probably the main source of both DFe and ligands, whereas lithogenic PFe may contribute most to DFe scavenging and ballasting of biogenic PFe. The relative proportions of lithogenic vs. biogenic PFe flux vary regionally and set the contribution of scavenging and ballasting vs. dissolution and ligand release, and hence the fate of iron in the water column. Over the last two decades the role of iron supply on the ocean’s carbon cycle has received widespread attention because of its potential function in modulating the earth’s climate during the geological past (Martin 1990). Such attention has resulted in rapid advances in this field, with the development of distinct research themes including iron and algal physiology (Morel and Price 2003), iron chemistry (Rue and Bruland 1995), dissolved iron (DFe)

Effect of coagulation on a model planktonic food web

Abstract Observations have shown that aggregates (“marine snow”) are an important fraction of the organic matter vertical flux in the ocean. There has been a separation in biological models describing this flux, with coagulation models focused on phytoplankton blooms for which particle concentrations are high and grazing is low and neglectable and with plankton models focused on food web interactions neglecting coagulation dynamics. This separation has partly resulted from the difficulty in describing the interactions among the multiple particle sources using a coagulation model for a food web. New approaches for describing particle dynamics now make it possible to do so. The present study examines the effect of combining the food web model of Fasham et al. (1990. Journal of Marine Research 34, 591–639) with a coagulation dynamics model and applying the combined model to describe the annual cycle of an oligotrophic plankton system. As part of the model formulation, the coagulation kernels had to be altered to include both the case of fractal particles interacting and the case of smaller particles being faster settling. Results show that coagulation can have an important effect on particle flux even in the low particle concentration oligotrophic environment by increasing average particle settling speed and by increasing the ratio of maximum to minimum daily vertical flux over the course of a yearly cycle. As part of this, coagulation forms large, rapidly sinking particles. Grazing and the accompanying formation of fecal pellets can compete with coagulation for particles, but the fecal pellets can also participate in the formation of large aggregates. Among the variables that can influence export rates are phytoplankton size and concentration as well as depth of the surface mixed layer. The results provide evidence for the importance of coagulation processes in enhancing particle export even in central ocean regions.

Sedimentation of phytoplankton during a diatom bloom : Rates and mechanisms

ABSTRACT Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11 -day study period, averaging ca. 0.1 d-l, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were co.07 d-t; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low ‘stickiness’ of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 3&50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms 1. Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark (email: tk@dfu.min.dk) 2. Marine Biological Laboratory, Strandpromenaden 5, DK-3000 Helsinger, Denmark. 3. Marine Sciences Institute, University of California, Santa Barbara, California, 93106, U.S.A. 4. Department of Oceanography, Texas A&M University, College Station, Texas, 77843, U.S.A. 5. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340-6097, U.S.A. 6. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02143, U.S.A. 7. Present address: School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Welling- ton, New Zealand. 8. Universidad de Cadiz, Department de Biologia Animal, Vegetal y Ecologia, Aptdo 40, E-l 1510 Puerto Real (Cadiz), Spain. 1123

Comparing observed changes in particle size spectra with those predicted using coagulation theory

The increasing appreciation of the importance of aggregates in transport of material through the ocean makes understanding their formation and movement crucial. Coagulation theory has the potential to explain key aspects of oceanic particle dynamics. Unfortunately, there are uncertainties about the best way to formulate coagulation theory in marine systems. A mesocosm experiment in which members of the SIGMA (Significant Interactions Generating Marine Aggregates) program grew marine phytoplankton and observed the changes in particle size spectra provides information needed to test different formulations of coagulation rates. Computer simulations incorporating coagulation theory were able to predict aspects of the particle size spectral evolution. The rectilinear formulation of coagulation kernels over-predicted coagulation rates, particularly because of high interaction rates between particles of much different sizes. The curvilinear formulation worked better, especially if used in conjunction with a disaggregation model. Disaggregation was a very important process regulating particle size spectra that was necessary to include in the simulations. The derived disaggregation rates were consistent with past observations of marine snow disaggregation. The simulations in which coagulation and disaggregation predicted the observed results best were those for which the dominant aggregation was between similar sized particles. The largest discrepancies between observed and predicted particle size spectra occurred when theory predicted that dominant interactions were between widely different particle sizes. While coagulation theory was useful in predicting particle size spectra for this experiment, there are still improvements needed in the theory.

Currents in the high drag environment of a coastal kelp stand off California

Abstract The physical environment within a California kelp bed is modified from that of the region by the large drag of the massed plants. The change is greatest for longshore motions, where velocities diminish with distance upcoast of the leading edge of the bed in a manner consistent with the damping expected for coastally-trapped waves. Cross-shore velocity fluctuations are more important for the movement of material between offshore waters and the kelp bed interior. Results suggest that penetration of water for distances of 400 m into the kelp is common. The importance of water motions for ecological processes ranging from enhancing boundary layer transport to kelp blades to moving planktonic larvae implies that size of a kelp bed and position within it are important for the kelp ecosystem.

Food web analysis of a planktonic system off Southern California

Abstract The California Basin Study Program (CaBS) has measured carbon and nitrogen flows between some of the compartments of a planktonic system of Southern California. We have used an inverse analysis approach to infer those which were not measured in order to develop a more complete description of the planktonic food web. We then analyzed these results by using network analysis and by following the movement of imaginary tracers through the food web.Results suggest that detritus was a crucial part of the food web, with a substantial fraction of algal production flowing directly to it. Detrital production of dissolved organic matter (DOM), along with leakage from grazers, were the dominant sources of bacterial food. Bacteria did not provide an important path for the movement of organic matter to the larger mesozooplankkton. Although mesozoa were not the dominant grazers of algal production, they were important for the movement euphotic zone, accounting for nearly half of the downward flux. Flows of carbon and nitrogen were quite different because of carbon respiration. Nitrogen removal from the euphotic zone was influenced by high dissolved organic nitrogen (DON) concentrations. The effects of DON and dissolved organic carbon (DOC) in euphotic zone elemental cycling imply that an element used to trace new production will have a different removal rate from the euphotic zone unless it also has a comparable dissolved organic phase. More experimental effort needs to be devoted to detrital dynamics, as well as those of sub-copepod sized grazers.

Sediment denitrifier community composition and nirS gene expression investigated with functional gene microarrays.

A functional gene microarray was used to investigate denitrifier community composition and nitrite reductase (nirS) gene expression in sediments along the estuarine gradient in Chesapeake Bay, USA. The nirS oligonucleotide probe set was designed to represent a sequence database containing 539 Chesapeake Bay clones, as well as sequences from many other environments. Greatest nirS diversity was detected at the freshwater station at the head of the bay and least diversity at the higher salinity station near the mouth of the Bay. The most common OTUs from the sequence database were detected on the array with high signal strength in most samples. One of the most abundant OTUs, CB2-S-138, was identified as dominant at the mid-bay site by both microarray and quantitative PCR assays, but it comprised a much smaller fraction of the assemblage in the north and south bay samples. cDNA (transcribed from total RNA extracts) targets were hybridized to the same array to compare the profiles of community composition at the DNA (relative abundance) and mRNA (gene expression) levels. Only the three dominant denitrifying groups (in terms of relative strength of DNA hybridization signal) were detected at the mRNA level. These results suggest that the most actively denitrifying groups are responsible for most nirS expression as well.

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

correlated with the hypoxic area at r 2 = 0.32 for the 1985–2010 period, and r 2 = 0.52 for the 1993–2010 period. Multilinear regressions using both wind duration and May-June nitrate loading improve the statistical relationships for both periods to r 2 = 0.69 and 0.74 for the long and short time periods, respectively. Mechanistically, the statistical relationships reflect the movement and changes in horizontal river plume position associated with the wind and the influence of stratification on the hypoxic area. Citation: Feng, Y., S. F. DiMarco, and G. A. Jackson (2012), Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico, Geophys. Res. Lett., 39, L09601, doi:10.1029/2012GL051192.