Using a protocol of numerical experiments where horizontal resolution is progressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one e fth of the internal Rossby radius of deformation, reinforce the mesoscale eddy e eld and contribute to double the primary production and phytoplankton subduction budgets. This enhancement is due to the reinforced mesoscale physics and is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and between the eddies. It triggers a signie cant small-scale nutrient injection in the surface layers, leading to a phytoplankton e eld mostly dominated by e ne spatial structures. It is believed that, depending on wind forcings, this scenario should work alternately with that of Abraham (1998) which invokes horizontal stirring of nutrient injected at large scales. Results also reveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the eddies.

/pdf/impact-of-sub-mesoscale-physics-on-production-and-subduction-5b5njwptp8.pdf

Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime

The silicon cycle within Gulf Stream warm-core ring 82-B was examined before and after spring stratification using silicon isotope tracers In April, prior to stratification, the ring was isothermal and isohaline to 400 m Ambient silicic acid concentrations were ca 3 μmol l−1 and biogenic silica concentrations ranged from 80 to 100 nmol l−1 throughout this depth range Specific production rates tended to be greatest at the surface and were vertically uniform below 20 m Net silica production occurred from the surface to 300 m on 25 April, about three times deeper than has been observed before in the sea The zone of net production shoaled to 4 times the 1% light depth, indicating that the diatom assemblage was producing substantial amounts of silica at depths where little or no photosynthesis was taking place

By June, a seasonal pycnocline had developed and silicic acid concentrations were depleted to ⩽02 μmol l−1 in the upper 20–30 m Vertically integrated particulate silica concentrations in the upper 80 m had increased four-fold compared to those for the upper 110 m in April/May Siliceous biomass was concentrated in the upper 40 m, with a subsurface maximum of ca 1000 nmol Si l−1 within the seasonal pycnocline The upper 80 m remained a zone of net silica production, but net dissolution was occurring at all depths >100 m The average specific production rate of biogenic silica (00098 h−1) was five times less than observed during April/May (0041 h−1), suggesting slower diatom division rates in the stratified water column The decrease in specific production rates following stratification was nearly offset by the increased particulate silica levels such that the vertically integrated production rate of biogenic silica for the upper 80 m was essentially unchanged from that observed for the upper 110 m during April/May

The percentage of silica production that could be supported by in situ dissolution increased from ⩽30% for the upper 110 m during April/May to 68% for the upper 80 m in June This transition from a “new production” cycle to a “regenerated production” cycle is the first observation of a seasonal trend in the silicon cycle in the surface ocean Furthermore, while net silica production occurred throughout the upper 110–300 m in April, net silica production in June was confined to the relatively nutrient-rich waters below seasonal pycnocline, wih a near balance between silica production and dissolution in the oligotrophic watersabove (average production:dissolution = 103) These changes in the production:dissolution ratio and the increased reliance on new nutrients with depth imply that the cycling of particulate silica in June fit a two-layer model of stratified oceanic systems where nearly complete and rapid recycling of materials occurs within a nutrient-poor surface layer with a zone of net production below

Seasonal changes in the silicon cycle within a Gulf Stream warm-core ring

A detailed description of the physical, chemical and biological structure of the Cyprus eddy is given from cruises carried out in February, May, September and November 1989. The chemical data support the suggestion ( Brenner et al., Dynamics of Atmospheres and Oceans , 15 , 457–477, 1991) that this eddy forms essentially in situ by the injection of 250–300 m of Levantine Intermediate Water into the upper layers of the water column. The nitrate vs temperature profile is characteristic of this longitude and not of one transported rapidly from farther west. The silicate vs depth profile at the boundary can be superimposed on that from the core if its is displaced downward by 250 m. The depth of the nitrate maximum in the core also is displaced downward by 250–300 m relative to the boundary. There was no evidence of a different origin of the core water based on nutrient profiles on Sigma-T surfaces. These data suggest that the Cyprus eddy is similar in many respects to a mid-ocean eddy, not a ring. There was enhanced productivity in the core of the eddy in winter. This bloom occurred simultaneously with deep (> 350 m) mixing of the water column, and possibly in part as the result of algal growth at unusually low light levels. There was no evidence of enhanced productivity at the boundary of the eddy, suggesting that the upwelling of nutrients was weak and that the enhanced productivity observed in the boundary of rings is more related to the strong temperature front or entrainment of shelf waters. It is cautioned that several important physical and biological features of this eddy would not have been seen using satellite imagery.

Nutrient distributions during an annual cycle across a warmcore eddy from the E. Mediterranean Sea

Particulate matter variability is described in the contect of biological and physical processes in the core waters of WCR 82B between February and late June 1982. WCR 82B formed in mid-February with a probable mixed layer depth of 50 m. A series of heat loss and convective mixing events deepened the mixed layer to >300 m by March and to 400 m by early April 1982. Periods of convective mixing and transient stratification in the deep mixed layer were indicated by chlorophyll, nutrient, and temperature data collected during the last 10 days of April. Seasonal stratification was established by early May, and the core waters of WCR 82B became strongly stratified shallower than 40 m by June. Particulate matter samples were collected from the upper 1000 m by large volume in situ filtration in April and June 1982. Vertical distributions were obtained for particulate dry weight, organic carbon, calcium, biogenic silicon, and phosphorus as well as abundances of >1 mm size fecal matter and fecal pellets. The data show that particle production exceeded particle consumption by zooplankton in the euphotic zone from February to April. However, the particle concentration remained nearly constant in the euphotic zone, but increased between 50 and 400 m during this period. These observations suggest that mixed layer convection in March and April removed a significant fraction of particles from the euphotic zone into the deep thermostad. Analysis of the data shows that 67% of primary produced carbon was mixed into the thermostad during this time. Such conditions were favorable for the growth of herbivorous zooplankton which were dispersed several hundred meters below the surface. Comparisons of the standing stock of particles present in the upper 400 m in late April with supply rates of material due to down mixing suggest that the particle populations in this zone turn over on time scales of 10 days. After onset of seasonal stratification in early May, the down-mixed supply of newly produced carbon to deep thermostad waters was eliminated. Because the rates of euphotic zone particle production and loss were no longer balanced, a bloom of phytoplankton peaking in mid-May may have occurred. Observations showed that particles were removed from the thermostad between April and June, and calculations suggest that this loss (by zooplankton consumption) may have occurred in as short a time as one week following seasonal stratification. Following the removal of utilizable particulate matter, zooplankton shoaled to become concentrated in the upper 50 m by June. The continued high rate of primary production and the strongly increased zooplankton biomass in the upper 50 m in June resulted in enhanced production of fecal material in the upper 40 m. The high rates of zooplankton grazing in June resulted in the removal of most aggregate material by 110 m. These observations demonstrate that the coupling of physical, biological and chemical processes in the upper ocean occurs on time scales as short as 10 days. They also show that particulate matter is a sensitive indicator of the balance between production and removal processes in the upper 1000 m. Our data suggest that (1) zooplankton consumption as opposed to dark respiration is the dominant loss mechanism for phytoplankton carbon mixed below the euphotic zone into deep mixed layers, and (2) the imbalance between production and removal processes in the euphotic zone at the time of stratification caused by the cessation of mixing to depths below the euphotic zone leads to the development of the spring phytoplankton bloom.

Particulate matter production and consumption in deep mixed layers: observations in a warm-core ring

The impact of mesoscale activity on phytoplankton and nutrient distribution in the Mozambique Channel was simulated by coupling a biogeochemical model (PISCES) with a regional oceanic model (ROMS). Examples of the effects of eddies on the biogeochemistry of the Mozambique Channel are presented to illustrate the complexity of the system. In the model, several cyclonic eddies were found with low concentrations of chlorophyll at their cores, which contrasts with previous studies in the open ocean. In addition, several anticyclonic eddies were simulated with high concentrations of chlorophyll at their cores. Phytoplankton growth within these mesoscale features (both cyclonic and anticyclonic eddies) occurred in response to nutrient injection into the euphotic zone by advection, and subsequent retention of surrounding nutrient-rich waters within eddies. Offshore nutrient distributions depended strongly on lateral advection of nutrient-rich water from the coastal regions, induced by eddy interaction with the shelf. The environmental conditions at the locations where eddies were generated had an important effect on nutrient concentrations within these structures.

/pdf/influence-of-mesoscale-eddies-on-biological-production-in-mb2c8whd7d.pdf

Influence of mesoscale eddies on biological production in the Mozambique Channel: Several contrasted examples from a coupled ocean-biogeochemistry model

We believe that two mechanisms are important in regulating primary production and hence the abundance of phytoplankton in warm core rings. The first mechanism, anticyclogenesis, is associated with the rotary motion of rings since the resultant geostrophic forces are believed to be the basis for nutrient enrichment in the high velocity region. Phytoplankton populations in this peripheral region experience a near steady state growth closely coupled with the rotational velocity of the ring. The second mechanism is that phytoplankton populations in the ring center rely on seasonal changes in the depth of the mixed layer due to convection and stabilization. This process regulates both the mean light energy reaching phytoplankton and equalizes the nutrient distribution over the water column. Growth in populations at ring center occurs as pulses, responding to changes in the depth of the mixed layer.

Rotary motions and convection as a means of regulating primary production in warm core rings. [of ocean currents]

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Rotary motions and convection as a means of regulating primary production in warm core rings. [of ocean currents]