W. Glen Harrison

The biological pump: Profiles of plankton production and consumption in the upper ocean

Abstract The ‘biological pump’ mediates flux of carbon to the interior of the ocean by interctions between the components of the vertically-structured pelagic ecosystem of the photic zone. Chlorophyll profiles are not a simple indicator of autotrophic biomass or production, because of non-linearities in the physiology of cells and preferential vertical distribution of taxa. Profiles of numbers or biomass of heterotrophs do not correspond with profiles of consumption, because of depth-selection (taxa, seasons) for reasons unconnected with feeding. Depths of highest plant biomass, chlorophyll and growth rate coincide when these depths are shallow, but become progressively separated in profiles where they are deeper — so that highest growth rate lies progressively shallower than the chloropyll maximum. It is still uncertain how plant biomass is distributed in deep profiles. Depths of greatest heterotroph biomass (mesozooplankton) are usually close to depths of fastest plant growth rate, and thus lie shallower than the chlorophyll maximum in profiles where this itself is deep. This correlation is functional, and relates to the role of heterotrophs in excreting metabolic wastes (especially ammonia), which may fuel a significant component of integrated algal production, especially in the oligotrophic ocean. Some, but not all faecal material from mesozooplankton of the photic zone appears in vertical flux below the pycnocine, depending on the size of the source organisms, and the degree of vertical mixing above the pycnocline. Diel, but probably not seasonal, vertical migration is significant in the vertical flux of dissolved nitrogen. Regional generalisations of the vertical relations of the main components of the ‘biological pump’ now appear within reach, and an approach is suggested.

Vertical Nitrate Fluxes in the Oligotrophic Ocean

The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the worlds ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of 15N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean.

Vertical nitrogen flux from the oceanic photic zone by diel migrant zooplankton and nekton

Abstract Where the photic zone is a biological steady-state, the downward flux of organic material across the pycnocline to the interior of the ocean is thought to be balanced by upward turbulent flux of inorganic nitrogen across the nutricline. This model ignores a significant downward dissolved nitrogen flux caused by the diel vertical migration of interzonal zooplankton and nekton that feed in the photic zone at night and excrete nitrogenous compounds at depth by day. In the oligotrophic ocean this flux can be equivalent to the flux of particulate organic nitrogen from the photic zone in the form of faecal pellets and organic flocculates. Where nitrogen is the limiting plant nutrient, and the flux by diel migration of interzonal plankton is significant compared to other nitrogen exports from the photic zone, there must be an upward revision of previous estimates for the ratio of new to total primary production in the photic zone if a nutrient balance is to be maintained. This upward revision is of the order 5–100% depending on the oceanographic regime.

Primary productivity and size structure of phytoplankton biomass on a transect of the equator at 135°W in the Pacific Ocean

The distribution of total and size-fractioned phytoplankton biomass was studied in a transect across the equatorial Pacific at ca 135°W from 15°S to 15°N in April 1988. Cold, nutrientrich surface waters characterized the equatorial region, whereas north and south of this region warmer temperatures prevailed and nitrate was low or undetectable in surface waters. Highest primary productivity and chlorophyll concentration were found in the equatorial region. A subsurface chlorophyll maximum (SCM) was a consistent feature along the transect. The depth of the SCM and integrated production were not related to the depth of the nitracline as has been found in other oceanic regions. An analysis of variance of the chlorophyll concentrations along the transect showed a significant spatial heterogeneity in both the horizontal and vertical. Picoplankton (cells 10 μm) accounted on average for ca 50, 40 and 10% of the total chlorophyll, respectively. Although a slight increase in the contribution of netplankton was observed at the equator, at all the stations along the transect more than 75% of the chlorophyll was within organisms which passed through a 10 μm Nucleopore ® filter. No significant differences were observed in the relative size distribution of chlorophyll between the surface layer and the chlorophyll maximum. However, below the maximum a significant increase in the contribution of nanoplankton and a decrease in picoplankton was observed. The horizontal pattern observed in the size distribution of phytoplankton along the transect seems to be better explained by the effect of circulation rather than nitrate availability. The lack of correlation of primary productivity and chlorophyll concentration with the vertical nitrate distribution suggests that an important proportion of the nutrient supply may be due to meridional advection of rich nitrate water from the equatorial upwelling region.

Coherent assembly of phytoplankton communities in diverse temperate ocean ecosystems

The annual cycle of phytoplankton cell abundance is coherent across diverse ecosystems in the temperate North Atlantic Ocean. In Bedford Basin, on the Scotian Shelf and in the Labrador Sea, the numerical abundance of phytoplankton is low in spring and high in autumn, thus in phase with the temperature cycle. Temperature aligns abundance on a common basis, effectively adjusting apparent cell discrepancies in waters that are colder or warmer than the regional norm. As an example of holistic simplicity arising from underlying complexity, the variance in a community variable (total abundance) is explained by a single predictor (temperature) to the extent of 75% in the marginal seas. In the estuarine basin, weekly averages of phytoplankton and temperature computed from a 13 year time-series yield a predictive relationship with 91% explained variance. Temperature-directed assembly of individual phytoplankton cells to form communities is statistically robust, consistent with observed biomass changes, amenable to theoretical analysis, and a sentinel for long-term change. Since cell abundance is a community property in the same units for all marine microbes at any trophic level and at any phylogenetic position, it promises to integrate biological oceanography into general ecology and evolution.

Coherent Sign Switching in Multiyear Trends of Microbial Plankton

Since the 1990s, phytoplankton biomass on the continental shelf of Nova Scotia and in the Labrador Sea has undergone sustained changes in the spring and fall, which are accompanied by changes in bacterioplankton that are dampened in amplitude but coherent in the direction of change. A reversal of trend in biomass change, so-called sign switching, occurs both in time and in space. Thus, whenever (spring or fall) and wherever (Scotian Shelf or Labrador Sea) phytoplankton increase or decrease, so also does bacterioplankton. This tandem sign switch indicates coupling of the trophic levels at a multiyear time scale and contributes to an ecological fingerprint of systemwide forcing.

REVERSIBLE KINETIC MODEL FOR THE SHORT‐TERM REGULATION OF METHYLAMMONIUM UPTAKE IN TWO PHYTOPLANKTON SPECIES, DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) AND PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Methylammonium, an ammonium analog, was used to study the short‐term kinetics of ammonium uptake in a diatom, Phaeodactylum tricornutum Bohlin, and a green alga, Dunaliella tertiolecta Butcher. Time courses of methylammonium disappearance were measured over a wide range of initial substrate concentrations for the two species. It was shown that feedback inhibition, described mathematically by a reversible enzyme kinetic model, can be used to explain the data. For the two species, there was good agreement between the kinetic parameters obtained from the analysis of the uptake versus substrate curve and those from the fit of the reversible kinetic model to the time‐course data. All time courses of CH3NH3+ disappearance could be described by constants Vm and ks. Ammonium time‐course data show some similarities to its analog, methylammonium. Our study suggests that the apparent change in Vm and ks with time measured after the addition of saturating ammonium concentrations reflects an uncoupling between transport and assimilation of the substrate rather than a real change in the kinetic parameters of the transport mechanism.