Cultural eutrophication has become the primary water quality issue for most of the freshwater and coastal marine ecosystems in the world. However, despite extensive research during the past four to five decades, many key questions in eutrophication science remain unanswered. Much is yet to be understood concerning the interactions that can occur between nutrients and ecosystem stability: whether they are stable or not, alternate states pose important complexities for the management of aquatic resources. Evidence is also mounting rapidly that nutrients strongly influence the fate and effects of other non-nutrient contaminants, including pathogens. In addition, it will be important to resolve ongoing debates about the optimal design of nutrient loading controls as a water quality management strategy for estuarine and coastal marine ecosystems.

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Eutrophication science: where do we go from here?

An estimate of global net primary production in the ocean has been computed from the monthly mean near-surface chlorophyll fields for 1979-1986 obtained by the Nimbus 7 CZCS radiometer. Our model required information about the subsurface distribution of chlorophyll, the parameters of the photosynthesis-light relationship, the sun angle and cloudiness. The computations were partitioned among 57 biogeochemical provinces that were specified from regional oceanography and by examination of the chlorophyll fields. Making different assumptions about the overestimation of chlorophyll by the CZCS in turbid coastal areas, the global net primary production from phytoplankton is given as 45-50 Gt C year"1. This may be compared with current published estimates for land plants of 45-68 Gt C year"' and for coastal vegetation of 1.9 Gt C year"1.

An estimate of global primary production in the ocean from satellite radiometer data

Planktonic marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans. Their unusual adaptations, from the molecular to the macroscopic level, contribute to their ecological success and biogeochemical importance. Trichodesmium fixes nitrogen gas (N2) under fully aerobic conditions while photosynthetically evolving oxygen. Its temporal pattern of N2 fixation results from an endogenous daily cycle that confines N2 fixation to daylight hours. Trichodesmium colonies provide a unique pelagic habitat that supports a complex assemblage of consortial organisms. These colonies often represent a large fraction of the plant biomass in tropical, oligotrophic waters and contribute substantially to primary production. N2 fixation by Trichodesmium is likely a major input to the marine and global nitrogen cycle.

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Trichodesmium, a Globally Significant Marine Cyanobacterium

Seven years of time-series observations of biogeochemical processes in the subtropical North Pacific Ocean gyre have revealed dramatic changes in the microbial community structure and in the mechanisms of nutrient cycling in response to large-scale ocean–atmosphere interactions. Several independent lines of evidence show that the fixation of atmospheric nitrogen by cyanobacteria can fuel up to half of the new production. These and other observations demand a reassessment of present views of nutrient and carbon cycling in one of the Earth′s largest biomes.

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The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean

It is problematic that geochemical estimates of new production - that fraction of total primary production in surface waters fueled by externally supplied nutrients - in oligotrophic waters of the open ocean surpass that which can be sustained by the traditionally accepted mechanisms of nutrient supply In the case of the Sargasso Sea, for example, these mechanisms account for less than half of the annual nutrient requirement indicated by new production estimates based on three independent transient-tracer techniques Specifically, approximately one-quarter to one-third of the annual nutrient requirement can be supplied by entrainment into the mixed layer during wintertime convection, with minor contributions from mixing in the thermocline and wind-driven transport (the potentially important role of nitrogen fixation - for which estimates vary by an order of magnitude in this region - is excluded from this budget) Here we present four lines of evidence - eddy-resolving model simulations, high-resolution observations from moored instrumentation, shipboard surveys, and satellite data - which suggest that the vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget in the Sargasso Sea Additional information is contained in the original extended abstract

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Influence of mesoscale eddies on new production in the Sargasso Sea

The biological pump: profiles of plankton production and consumption in the upper 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 world's 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 (15)N-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.

https://science.sciencemag.org/content/sci/234/4778/870.full.pdf

Vertical Nitrate Fluxes in the Oligotrophic Ocean

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.

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

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.

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

The global association between heterotrophic bacteria and phytoplankton in the open ocean was examined to identify the macroecological limits of bacterial abundance. A consolidated dataset was constructed using 13,973 paired measurements of bacterial abundance and chlorophyll concentration from 15 years of observation in various biogeochemical provinces covering the Atlantic, Pacific, Arctic and Antarctic Oceans as well as the Arabian Sea. The bivariate relationship between bacteria and chlorophyll is a filled polygon whose upper boundary undergoes a change in slope from positive to negative at about 1 mgChl m −3 , suggesting a transition from bottom-up to top-down control of bacteria along a resource supply gradient. The upper limit of bacterial abundance in the ocean is everywhere set by phytoplankton, but the limit is not realized in productive waters because of mortality losses. A carrying capacity of about 7 trillion bacteria for 1 mg of chlorophyll is indicated by empirical observations, consistent with theoretical considerations and predictions from food web modelling.

W. Glen Harrison

Papers

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

Vertical Nitrate Fluxes in the Oligotrophic Ocean

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

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

Macroecological limits of heterotrophic bacterial abundance in the ocean