Alan R. Longhurst

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.

Seasonal cycles of pelagic production and consumption

Abstract Comprehensive seasonal cycles of production and consumption in the pelagial require the ocean to be partitioned. This can be done rationally at two levels: into four primary ecological domains (three oceanic and one coastal), or about fifty biogeochemical provinces. The domains differ in their characteristic seasonal cycles of stability, nutrient supply and illumination, while provinces are defined by ocean currents, fronts, topography and recurrent features in the sea surface chlorophyll field. For each of these compartments, seasonal cycles of photic depth, primary production and accumulation (or loss) of algal biomass were obtained from the climatological CZCS chlorophyll field and other data and these, together with mixed layer depths, rendered characteristic seasonal cycles of production and consumption, which can be grouped into eight models: i — polar irradiance-mediated production peak; ii — nutrient-limited spring production peak; iii — winter-spring production with nutrient limitation; iv — small amplitude response to trade wind seasonality; v — large amplitude response to monsoon reversal; vi — canonical spring-fall blooms of mid-latitude continental shelves; vii — topography-forced summer production; viii — intermittent production at coastal divergences. For higher latitudes, these models suggest that the observed late-summer ‘blooms’ result not from a renewal of primary production rate, but from a relaxation of grazing pressure; in mid-latitudes, the observed ‘winter’ bloom represents chlorophyll accumulation at a season when loss terms are apparently smaller than during the period of peak primary production rate which occurs later, in spring. Where an episodic seasonal increase in rate of primary production occurs, as in the Arabian Sea, algal biomass accumulation may brief, lasting only until consumption is fully re-established. Only in the low latitude oligotrophic ocean are production and consumption perennially and closely coupled.

Regionally and seasonally differentiated primary production in the North Atlantic

A bio-geochemical classification of the N. Atlantic Basin is presented according to which the basin is first divided into four primary algal domains: Polar, West-Wind, Trades and Coastal. These are in turn sub-divided into smaller provinces. The classification is based on differences in the physical environment which are likely to influence regional algal dynamics. The seasonally-differentiated parameters of the photosynthesis-light curve (P-I curve) and parameters that define the vertical structure in chlorophyll profile are then established for each province, based on an analysis of an archive of over 6000 chlorophyll profiles, and over 1800 P-I curves. These are then combined with satellite-derived chlorophyll data for the N. Atlantic, and information on cloud cover, to compute primary production at the annual scale. using a model that computes spectral transmission of light underwater, and spectral, photosynthetic response of phytoplankton to available light. The results are compared with earlier, satellite-derived, estimates of basin-scale primary production.

Vertical flux of respiratory carbon by oceanic diel migrant biota

Interzonal diel migrant plankton and nekton obtain organic carbon by feeding at night above the main pycnoline of subtropical and tropical oceans, and respire part of it by day in the interior of the ocean below the pycnocline. Using data from seven oceanic stations, and conservative models to compute respiration at depth, we show that this flux of respiratory carbon ranged from 20 to 430 mg C m−2 d−1 or 13–58% of computed particulate sinking flux across the pycnocline. If this flux occurs consistently between 50°N and 50°S, it will add about 5–20% (depending on method of calculation) to current estimates of global sinking flux of organic carbon across the pycnocline.

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.

The structure and evolution of plankton communities

Abstract New understanding of the circulation of ancient oceans is not yet matched by progress in our understanding of their pelagic ecology, though it was the planktonic ecosystems that generated our offshore oil and gas reserves. Can we assume that present-day models of ecosystem function are also valid for ancient seas? This question is addressed by a study of over 4000 plankton samples to derive a comprehensive, global description of zooplankton community structure in modern oceans: this shows that copepods form only 50% of the biomass of all plankton, ranging from 70% in polar to 35% in tropical seas. Comparable figures are derived from 14 other taxonomic categories of zooplankton. For trophic groupings, the data indicate globally: geletinous predators — 14%; gelatinous herbivores — 4%; raptorial predators — 33%; macrofiltering herbivores — 20%; macrofiltering omnivores — 25%; and detritivores — 3%. A simple, idealized model for the modern pelagic ecosystem is derived from these percentages which indicates that metazooplankton are not the most important consumers of pico- and nano-plankton production which itself probably constitutes 90% of primary production in warm oceans. This model is then compared with candidate life-forms available in Palaeozoic and Mesozoic oceans to determine to what extent it is also valid for ancient ecosystems: it is concluded that it is probably unnecessary to postulate models fundamentally differing from it in order to accommodate the life-forms, both protozoic and metazoic, known to have populated ancient seas. Remarkably few life-forms have existed which cannot be paralleled in the modern ocean, which contains remarkably few life-forms which cannot be paralleled in the Palaeozoic ocean. As a first assumption, then, it is reasonable to assume that energy pathways were similar in ancient oceans to those we study today.

Seasonal cooling and blooming in tropical oceans

Abstract The relative importance of tropical pelagic algal blooms in not yet fully appreciated and the way they are induced not well understood. The tropical Atlantic supports pelagic blooms together equivalent to the North Atlantic spring bloom. These blooms are driven by thermocline tilting, curl of wind stress and eddy upwelling as the ocean responds to intensified basin-scale winds in boreal summer. The dimensions of the Pacific Ocean are such that seasonal thermocline tilting does not occur, and nutrient conditions are such that tilting might not induce bloom, in any case. Divergence at the equator is a separate process that strengthens the Atlantic bloom, is more prominent in the eastern Pacific, and in the Indian Ocean induces a bloom only in the western part of the ocean. Where western jet currents are retroflected from the coast off Somalia and Brazil, eddy upwelling induces prominent blooms. In the eastward flow of the northern equatorial countercurrents, positive wind curl stress induces Ekman pumping and the induction of algal blooms aligned with the currents. Some apparent algal bloom, such as that seen frequently in CZCS images westwards from Senegal, must be due to interference from airborne dust.

Relationship between diversity and the vertical structure of the upper ocean

Abstract The sources of diversity in the plankton ecosystem of the upper 250 m in the eastern tropical Pacific Ocean are explored in the data from LHPR plankton profiles. Though there is good evidence for resource partitioning among feeding guilds of congeners, and for specialization in predation—both known to create diversity in simple aquatic ecosystems—the existence of a stable vertical structure, including a thermocline, may be one of the more important causes of variation in regional plankton diversity in the euphotic zone.

The distribution and metabolism of urea in the eastern Canadian Arctic

Urea concentrations, uptake, and excretion were measured at various locations in northern Baffin Bay and surrounding waters during the summer of 1980. Concentrations were variable ( 2.00 mg-at. N m−3) but followed patterns of decreasing concentration with depth in the euphotic zone and with distance from land. Urea accounted for > 50% of the total dissolved nitrogen in the upper mixed layer at most stations. Urea uptake rates showed generally the same distributional patterns as did concentrations and on the average accounted for 32% of the total nitrogen (NO3− + NH4+ + urea) productivity in the eupholic zone. Ammonium, and frequently NO3−, were utilized in preference to urea. Dual isotope (14C and 15N-urea) labelling experiments suggested that most urea-C was respired as CO2 while 50 to 80% of the urea-N was incorporated by the phytoplankton. Excretion measurements suggested that the four dominant macrozooplankton species (Calanus hyperboreus, C. finmarchicus, C. glacialis, and Metridia sp.) supplied only −3% of the urea-N but –40% of the NH4+-N requirements of the primary producers.

The western North Atlantic bloom experiment

An investigation of the spring bloom was carried out in the western North Atlantic (40–50°W) as one component of the multi-nation Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE). The cruise track included an extended hydrographic section from 32 to 47°N and process studies at two week-long time-series stations at 40 and 45°N. Biological and chemical data collected along the transect indicated that the time-series stations were located in regions where the spring bloom was well developed; algal biomass was high and surface nutrient concentrations were reduced from maximum wintertime levels. Despite similarities in the vertical structure and magnitude of phytoplankton biomass and productivity, the two stations clearly differed in physical, chemical and other biological characteristics. Detailed depth profiles of the major autotrophic and heterotrophic microplankton groups (bacteria, phytoplankton, microzooplankton) revealed a strong vertical coherence in distribution at both sites, with maximum concentrations in the upper 50 m being typical of the spring bloom. Ultraplankton (< 10 μm) were an important component of the primary producers at 40°N, whereas larger netplankton (diatoms, dinoflagellates) were more important at 45°N. Silicate depletion was clearly evident in surface waters at 45°N, where diatoms were most abundant. Despite the relative importance of diatoms at 45°N, dinoflagellates dominated the biomass of the netplankton at both sites; however, much of this community may have been heterotrophic. Bacterial biomass and production were high at both stations relative to phytoplankton levels, particularly at 45°N, and may have contributed to the unexpectedly high residual ammonium concentrations observed below the chlorophyll maximum layer at both stations. Microzooplankton grazing dominated phytoplankton losses at both stations, with consumption as high as 88% of the daily primary production. Grazing losses to the mesozooplankton, on the other hand, were small (<10%), but mesozooplankton contribution to the vertical flux of organic matter (fecal pellets) was important at 45°N. F-ratios estimated by 15N tracer methods and sediment trap fluxes were similar and suggeste that ∼30% of the daily primary production was lost by direct sedimentation during the observation period. Numerous similarities in bloom characteristics were noted between the western and eastern Atlantic study sites.