Input Of New Nitrogen Research Articles

The balance of supply of nutrients and demands of biological production and denitrification in a temperate latitude shelf sea — a treatment of the southern North Sea as an extended estuary

Abstract Data from the NERC North Sea Programme (1988–1989) and the OMEX project (1993–1997), and subsequent numerical modelling based on that data, enables budgets to be drawn up which allow the scales of a number of processes which are significant in shelf seas to be quantified. In winter the N:P ratio is lower in many areas of the southern North Sea than in any of the source waters. The deficit in the load of nitrate-N relative to that which would be present assuming conservative mixing of river and ocean waters is 580 ktonnes. This is probably due to denitrification. Considering the flushing rate it is equivalent to a maximum rate of loss of 0.7 mmol N m−2 day−1. A loss of this magnitude confirms the suggestion of Seitzinger and Giblin (Seitzinger, S.P., Giblin, A.E., 1996. Biogeochemistry 35, 235–260.) that the relatively high productivity in shelf seas such as the North Sea fuels significant levels of denitrification. The wide area of the shelf involved makes these areas net sinks for nitrogen in global budgets. A net flow of ocean water onto the shelf equivalent to 0.6 Sv (1 Sv=106 m3 s−1) containing nitrate at the winter concentration of 7.5 μM is required to maintain the nitrogen balance in the North Sea. An analysis of the sources of dissolved inorganic nitrogen (DIN) (and silicon) contributing to the change in dissolved nitrogen load between November 1988 and January 1989 (526 ktonnes N, 242 ktonnes Si) shows that between November and January the principal source of dissolved nitrate over the southern North Sea is probably from recycling of detrital or dissolved organic nitrogen (DON) already present in the system (383 ktonnes N) and not from direct inputs of new nitrogen. In one region, the German Bight, inputs of new nitrogen from rivers and the atmosphere approach 50% of the observed change. The ratio of annual primary production in different areas of the North Sea to the estimate of available nitrogen varies between five off the Dutch and German coasts and two off the UK coast. This indicates that the high productivity of the North Sea is maintained by both the total amount of nitrate that is supplied to the system and recycling in its shallow waters. Both the ratio and the efficiency of recycling are lower off the UK coast where light limitation of production is more likely.

Investigating the seasonal vertical structure of phytoplankton in shelf seas

A 1-dimensional (vertical) physical-biological coupled model is presented. The model is designed for investigations into the link between the vertical turbulent structure of a coastal or shelf sea water column, and the primary production. The physical model employs a turbulence closure scheme to provide the link between local vertical stability (driven by seasonal solar heating) and the vertical turbulent mixing (driven by tidal currents and surface wind stress). The biological component of the model is a simple cell quota, threshold limitation model, with either 1 or 2 taxa/species of phytoplankton growing in response to light and dissolved inorganic nitrogen. The user has control over all model physical and biological driving parameters. Graphical screen output, suitable for basic visualisation and teaching purposes, is generated as the model operates, and more detailed data are written to files for later analysis. In order to demonstrate the model's use in simple hypothesis investigation, an example of the model operation is illustrated. This focuses on the effect that the springs-neaps tidal cycle has on the production within the summer sub-surface biomass maximum, illustrating the fortnightly input of new nitrogen into the thermocline and the subsequent new production.

Unbalanced particle flux budgets in Crater Lake, Oregon: Implications for edge effects and sedimentfocusing in lakes

Flux estimates show that upward mixing of the deep‐water nitrate pool accounts for more than 85% of the total new nitrogen input to the euphotic zone of Crater Lake. Because measured primary productivity (360 mg C m−2 d−1) is 10–30 times higher than a level supported solely by the input of new nitrogen into the euphotic zone, nitrogen must be recycled in the euphotic zone many times before it is transferred by particles to the deep lake. Nitrogen recycling in the deep lake is also very efficient. Sediment trap measurements of particulate nitrogen fluxes reveal a major imbalance in our estimates of the lake’s internal nitrogen budget. We propose the imbalance reflects an “edge effect,” whereby enhanced biological production occurs near the lake margin and the shallower portions of the lake. Our measurements also reveal that Al, an element carried by refractory phases, is accumulating in basin sediments at a rate 15–30 times higher than the flux we measure with sediment traps. This difference is maintained by the near‐bottom transport of lithogenic particles from the lake margins to the deep basins of the lake. These comparisons of nutrient and refractory element fluxes reflect two important lake processes— enhanced productivity at the lake margin that may be due to greater availability of macro‐ and micronutrients at the lake edges and focusing of particulate material into the deep lake basins.

Nitrogen transport by vertically migrating diatom mats in the North Pacific Ocean

PHYTOPLANKTON production in the surface waters of stratified oceans is fed mainly by nitrogen that has been recycled within the euphotic zone1. The nitrogen that is lost from surface waters as organic matter exported to the deep ocean must be balanced by inputs of new nitrogen to the upper ocean2,3. Sediment trap studies2 have shown that the 15N/14N ratio (δ15N) of the exported organic matter is higher than that of the suspended particulates, and suggest that the rich nitrate pool below the euphotic zone is the source of 'new' nitrogen for the upper ocean. Yet steep vertical concentration gradients suggest that diffusive upward transport of nitrate is extremely limited, raising the question of how the nitrate reaches the surface waters. Here we present evidence that abundant diatom (Rhizosolenia) mats migrate vertically between surface waters and deep nitrate pools in the central North Pacific Ocean. Rising mats contain significantly larger internal nitrate pools than sinking mats. Mat δ15N is similar to that of the sub-nitricline nitrate, and consistently heavier than that of near-surface particulate organic matter. We conclude that Rhizosolenia mats may transport the equivalent of 50% of the new nitrogen requirements into the surface waters of the North Pacific gyre.

The size-based dynamics of plankton food webs. II. Simulations of three contrasting southern Benguela food webs

Size-based models are used to simulate planktonic food webs for the euphoric zones of three contrasting southern Benguela ecosystems, coastal stratified waters of the Agulhas Bank, a coastal upwelling area, and an oligotrophic area representative of SE Atlantic oceanic waters. In the models, phytoplankton are divided into four size categories and zooplankton into five All parameters describing flows are derived using body-size relationships Nitrogen is assumed to be limiting in ail three ecosystems, and other physical factors, such as light, are kept constant for simplicity. Initial pulses of new nitrogen in the Agulhas Bank (10 mg-at N m −3 ) and coastal upwelling models (25 mg-at N m −3 ) respectively simulate mixing and upwelling. Continuous inputs of new nitrogen into the Agulhas Bank (0.6 mg-at N m −3 day −1 ) and oceanic models (0 1 mg-at N m −3 day ) simulate turbulent diffusion into the euphotic zone across the nutricline The Agulhas Bank simulations depict a fluctuating standing stock of phytoplankton, dominated by different size classes at different times Large (net-) phytoplankton are important in this model only during periods of enhanced nitrogen supply. The phytoplankton bloom in the upwelling simulation is dominated by nano- and net-phytoplankton. The oceanic simulation depicts a phytoplankton community dominated by small cells (<5 μm), the populations of which remain at a relatively constant biomass of ∼40 mg C m −3 . In all three simulations the smallest autotrophs generally are responsible for the greatest proportion of primary production. The simulated heterotroph communities consist of varying biomasses of different-sized organisms. The Agulhas Bank and coastal upwelling simulations become dominated by micro- and meso-zooplankton, whereas small beterotrophs dominate the oceanic simulation. Different size classes of heterotrophs are important at different times in nitrogen regeneration, although zooflagellates (1–5 μm) were shown to be the most important regenerators on average. In the stratified Agulhas Bank and SE Atlantic oceanic simulations, regenerated production is generally greater than new production. In the Benguela upwelling simulation, new production is replaced by regenerated production as the phytoplankton bloom matures after upwelling and new nitrogen is exhausted. Average f -ratios were greatest (∼0 75) in newly upwelled water, decreasing for coastal stratified (0.25) and oligotrophic oceanic regions (0.14). Simulated model communities compare well with field observations in terms of standing stocks and size composition, suggesting that the model results accurately reflect nature. Up to seven-step ‘food chains’ occur in all three simulated ecosystems, but in practice most of the carbon is transferred via three effective trophic categories Long transfer pathways are important for planktivorous pelagic fish production in the oceanic simulation, short pathways in the coastal upwelling simulation, and intermediate-length pathways in the simulation of stratified coastal waters. Very little (<13%) of total photosynthetically fixed carbon potentially is available to pelagic fish in the simulations, and most carbon is lost through respiration and sinking. Nevertheless, the carbon potentially available to pelagic fish in the upwelling simulation is sufficient to sustain the pelagic fish production measured in the southern Benguela region. A revised food web model of plankton communities depicts the relative importance of different carbon and nitrogen flow pathways in different marine systems.

Introduction to the short communications

The shift in phytoplankton community structure towards an increased proportional abundance of cyanobacteria∗ has wide‐ranging ecological as well as water management implications. Certain species, typically representatives of the genera Ana‐baena, Aphanizomenon, Nodularia, and Nostoc, can fix gaseous nitrogen and their rise to dominance within the community can result in a major input of new nitrogen to the ecosystem. Most bloom‐forming species consist of long trichomes or large colonies which may inhibit grazing by zooplank‐ton, and dominance by these forms may reduce the transfer of carbon and energy through to higher trophic levels. Bloom‐forming cyanobacteria contain gas vesicles within their cells which allow them to float to the top of the water column, and their sudden and striking appearance at the surface during conditions of low water turbulence often creates public concern. Cyanobacteria can additionally produce unpleasant tastes and odours in drinking water by releasing volatile organic substan...