Vertical Nutrient Fluxes Research Articles

An assessment of the vertical diffusive flux of iron and other nutrients to the surface waters of the subpolar North Atlantic Ocean

Abstract. In this study we report diapycnal diffusive fluxes of dissolved iron (dFe), dissolved aluminium (dAl) and the major macronutrients to the surface waters of the North Atlantic subpolar gyre. Turbulent diffusivities at the base of the summer mixed layer ranged from 0.01 to 0.5 (median 0.07) cm2 s−1 and daily macronutrient fluxes into the surface mixed layer typically represented < 0.5% of integrated mixed layer inventories, although fluxes were highly variable. Elevated nutrient fluxes of up to 4% of mixed layer inventories were identified on the Greenland Shelf, where integrated nutrient pools were lowest due to localised shoaling of the mixed layer. Diffusive fluxes of dFe and dAl were typically <0.1% of mixed layer inventories but were also highly variable between stations. Approximations of daily phytoplankton nutrient and Fe uptake indicate that the diffusive flux may at best represent <10% of phytoplankton macronutrient uptake, and only 1% of daily phytoplankton Fe uptake. The daily turbulent diffusive flux of dFe was comparable in magnitude to coincident estimates of aeolian Fe supply but despite shallower than normal convective mixing in winter 2010 the diffusive supply was 22 and 59 times smaller than the annual convective supply of Fe to the Irminger and Iceland basins respectively. The general picture obtained from this study is one of small magnitude diffusive nutrient and Fe fluxes to the subpolar North Atlantic during the period of annual nutrient minima and indicates that the diffusive supply mechanism is unlikely to alleviate the recently identified presence of seasonal iron limitation within the North Atlantic subpolar gyre; a condition exacerbated by low dFe:NO3− ratios in subsurface source waters.

Lagrangian evolution of DMS during the Southern Ocean gas exchange experiment: The effects of vertical mixing and biological community shift

Concentrations of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are highly variable in time and space. What is driving the variability in DMS(P), and can those variability be explained by physical processes and changes in the biological community? During the Southern Ocean Gas Exchange Experiment (SO GasEx) in the austral fall of 2008, two 3He/SF6 labeled patches were created in the surface water. SF6 and DMS were surveyed continuously in a Lagrangian framework, while direct measurements of air‐sea exchange further constrained the gas budgets. Turbulent diffusivity at the base of the mixed layer was estimated from SF6 profiles and used to calculate the vertical fluxes of DMS and nutrients. Increasing mixed layer nutrient concentrations due to mixing were associated with a shift in the phytoplankton community structure, which in turned likely affected the sulfur dynamics on timescales of days. DMS concentration as well as air‐sea DMS flux appeared to be decoupled from the DMSP concentration, possibly due to grazing and bacterial DMS production. Contrary to expectations, in an environment with high winds and modest productivity, physical processes (air‐sea exchange, photochemistry, vertical mixing) only accounted for a small fraction of DMS loss from the surface water. Among the DMS sinks, inferred biological consumption most likely dominated during SO GasEx.

Inter-annual variability of upwelling off the South-Vietnamese coast and its relation to nutrient dynamics

It is the general consensus that the South China Sea (SCS) is influenced by the El Niño southern Oscillation (ENSO). Off southern Vietnam, coastal upwelling is one of the major features of the SCS, which is consequently influenced by the ENSO. However, the processes behind this influence are under discussion. It is the general thesis that the inter-annual changes of the monsoon wind cause the changes of the strength of the upwelling. This thesis has been tested and it is discarded. Instead it can be shown that the wind stress is not the main cause for the impact of the ENSO. Therefore, also nutrient dynamics must be completely re-evaluated.This study investigates in the circulation and the hydrographical situation during the summer 2003 and 2004, an El Niño and a normal year, respectively. The influence of the circulation on the new biological production based on vertical nutrient fluxes is estimated. Numerical modelling is used to study the circulation inside the SCS. The results of the simulation discover two different states of the horizontal circulation: During the first state an anticyclonic circulation dominates the region. During the second state the cyclonic circulation in the north and the anticyclonic circulation in the south build the dipole. Two cruises during the summer 2003 and summer 2004 verify the two states of the simulated circulations. The ENSO influence on the summer circulation is by tending towards the first state in the summer after an El Niño, while during normal summers the second state is dominant.Against earlier findings, this study features one new major key of the ENSO influence on the temperature distribution and primary production in the western SCS: This influence is not via the vertical velocities, but due to the different horizontal circulations. In the dipole circulation pattern, the decreased horizontal velocities cause longer residence times which come along with the increase in the efficiency of the upwelling. In this study, for the first time, the advection of the pronounced different water masses of the northern and southern SCS is considered. The different water masses support an important preconditioning of the nutrient concentrations in the water column. An experiment, which compares primary production with and without this preconditioning proves that the originating water masses moving into the upwelling region control the primary production.Horizontal advection explains the substantial contributions of the strong ENSO signal in the region. Not the regional Ekman drift currents, but the basin wide horizontal circulation explains the inter-annual changes of the vertical nutrient uplift during the summer. It can be shown that the dipole circulation is an almost perfect nutrient pump for new production.

Phytoplankton patchiness during spring intermonsoon in western coast of South China Sea

Jet-like phytoplankton blooms usually occur off the southwestern coast of the South China Sea (SCS) caused by strong winds during summer monsoons. However a jet-like phytoplankton patch was observed in the western SCS in the spring intermonsoon of 2010 in both field and remote sensing data. The present study investigated the biological processes associated with this spring phytoplankton patchiness. The data showed that chlorophyll a concentrations increased in the surface water, extending out to the SCS, and the depth of the subsurface chlorophyll maximum uplifted from 75m to 50m depth; low dissolved oxygen, low pH and nutrient enrichment (nitrate+nitrite and soluble reactive phosphate) were observed in the subsurface water (50 to ~200m depth). Data analysis showed that variations in chlorophyll a, nutrients and temperature in the water column were related to wind-stress curl: the spatial distribution pattern and vertical structure of the phytoplankton patchiness were controlled by vertical flux of nutrients caused by curl-driven upwelling through Ekman pumping. There was a high correlation between chlorophyll a concentration and wind-stress curl where the influence of nutrient influx from the coast was limited. This study shows the importance of wind-stress curl in providing nutrients to support phytoplankton growth during the spring intermonsoon along the western coast of SCS. It may help to better understand the role of wind in marine biological processes.

Internal solitary waves in Upper Lake Constance

The conceptual hypothesis that internal solitary waves (ISWs) in lakes are generated at fronts of basin-scale internal waves, propagate until they break at the terminal boundary and extend over the lake width in cross-propagation direction is tested using temperature and current profiles measured in Upper Lake Constance. Trains of ISWs of depression propagate in close association with the front of a Kelvin wave at almost constant phase speed from the main basin of Upper Lake Constance to the western shore of a sub-basin, Lake Uberlingen. The trough of the ISWs extends over the entire basin width in narrow Lake Uberlingen but not in the wider main basin. A substantial part of the ISWs propagating towards the western terminal boundary did not return, suggesting wave-energy loss due to wave breaking. Close to the shorelines parallel to their propagation direction the amplitudes of the ISWs are diminished. Temperature inversions suggest that wave breaking is occurring at these boundaries, but also in the open water where breaking events extend along the width of the ISW front. The heterogeneity of ISW breaking suggests that the associated mixing and vertical fluxes of plankton and nutrients across the thermocline are highly variable in space and time.

California Coastal Upwelling Onset Variability: Cross-Shore and Bottom-Up Propagation in the Planktonic Ecosystem

The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/predation involving both large and small plankton species.

Vertical Distribution and Flux of Nutrients in the Sediments of the Mangrove Reclamation Region of Muara Angke Kapuk, Jakarta

The reclaimed mangrove estuary in Muara Angke Kapuk is a reclaimed area that has not evaded the impacted of pollution and waste in the areas surrounding Cengkareng, Jakarta. This is apparent from the fact that almost all sediments under the mangrove trees are buried under heaps of plastic trash. However, the reclaimed region still has variety of organism, which indicating that the region still has an internal carrying capacity, especially nutrients from sediment. The purpose of this research was to examine the condition of sediment nutrients in this mangrove reclamation region. The research was conducted by taking water samples using a modification of the stratified cup at a sediment depth of 0-15 cm with depth intervals of 2.5 cm, and taking sediment samples using the sediment ring. Pore water samples were measured for dissolved oxygen (DO) and concentrations of ammonia, nitrite, nitrate, and phosphate. Sediment samples were used to obtain porosity values. The data obtained is used to make vertical concentration profiles and analysis of vertical nutrient flux. Vertical nutrient flux analysis was performed with the aid of QUAL2K software version 2.11. The results showed different vertical distributions and flux of nutrients, where influx for ammonia and phosphate and an increase in line with increasing sediment depth, while nitrate efflux and a decreased concentration. The flux calculation of nitrite as transitory nutrient was not done, but the concentration decreased after a depth of 2.5 cm. This indicates that the high contamination on the surface does not prevent the natural chemical processes so the reclaimed region can still provide nutritional support for its organism.

Variable behavior in pycnocline mixing over shelf seas

AbstractVertical mixing, driven by turbulence in the ocean, underpins many of the critical interactions that allow life on earth to flourish since vertical buoyancy flux maintains global overturning circulation and vertical nutrient fluxes are critical to primary production. Prediction of the ocean system is therefore dependent on accurate simulation of turbulent processes that, by their very nature, are chaotic. A growing evidence base exists that provides insight into these complex processes and permits investigation of turbulence relative to better determined, and therefore predictable, parameters. Here we examine three time series of the dissipation rate of turbulent kinetic energy (ε) in “stability space”. We reveal an ordered structure within the mean distribution of ε that compares well to a variety of proposed models of oceanic turbulence. The requirement for differing site‐specific tuning and only partial success however raises questions over “missing physics” within such models and the validity of measurement techniques.

Research Commentary: Association of Zoonotic Pathogens with Fresh, Estuarine, and Marine Macroaggregates

Aquatic macroaggregates (flocs ≥ 0.5 mm) provide an important mechanism for vertical flux of nutrients and organic matter in aquatic ecosystems, yet their role in the transport and fate of zoonotic pathogens is largely unknown. Terrestrial pathogens that enter coastal waters through contaminated freshwater runoff may be especially prone to flocculation due to fluid dynamics and electrochemical changes that occur where fresh and marine waters mix. In this study, laboratory experiments were conducted to evaluate whether zoonotic pathogens (Cryptosporidium, Giardia, Salmonella) and a virus surrogate (PP7) are associated with aquatic macroaggregates and whether pathogen aggregation is enhanced in saline waters. Targeted microorganisms showed increased association with macroaggregates in estuarine and marine waters, as compared with an ultrapure water control and natural freshwater. Enrichment factor estimations demonstrated that pathogens are 2-4 orders of magnitude more concentrated in aggregates than in the estuarine and marine water surrounding the aggregates. Pathogen incorporation into aquatic macroaggregates may influence their transmission to susceptible hosts through settling and subsequent accumulation in zones where aggregation is greatest, as well as via enhanced uptake by invertebrates that serve as prey for marine animals or as seafood for humans.

Episodic vertical nutrient fluxes and nearshore phytoplankton blooms in Southern California

Three distinct phytoplankton blooms lasting 4–9 d were observed in approximately 15‐m water depth near Huntington Beach, California, between June and October of 2006. Each bloom was preceded by a vertical NO3 flux event 6–10 d earlier. NO3 concentrations were estimated using a temperature proxy that was verified by comparison with the limited NO3 observations. The lower–water‐column vertical NO3 flux from vertical advection was inferred from observed vertical isotherm displacement. Turbulent vertical eddy diffusivity was parameterized based on the observed background (< 0.3 cycles h−1) stratification and vertical shear in the horizontal currents. The first vertical nitrate flux event in June contained both advective and turbulent fluxes, whereas the later two events were primarily turbulent, driven by shear in the lower part of the water column. The correlation between the NO3 flux and the observed chlorophyll a (Chl a) was maximum (r2 = 0.40) with an 8‐d lag. A simple nitrate–phytoplankton model using a linear uptake function and driven with the NO3 flux captured the timing, magnitude, and duration of the three Chl a blooms (skill = 0.61) using optimal net growth rate parameters that were within the expected range. Vertical and horizontal advection of Chl a past the measurement site were too small to explain the observed Chl a increases during the blooms. The vertical NO3 flux was a primary control on the growth events, and estimation of both the advective (upwelled) and turbulent fluxes is necessary to best predict these episodic blooms.

A data fusion study on the impacts of the 2011 Japan tsunami on the marine environment of Sendai Bay

Visible and thermal data retrieved by Moderate Resolution Imaging Spectroradiometer-Aqua (MODIS) were analysed using an image and data fusion approach to understand the marine environment changes associated with the 2011 Japan tsunami, with special emphasis on elucidating changes in the bio-optical and thermal variables. Two days after the tsunami, the MODIS visible sensor observed the increase in phytoplankton biomass (chlorophyll-a concentration [Chl-a]) 10 times (3.3 mg m−3) pre-tsunami Chl-a. The large increases in the MODIS Chl-a in the coastal and offshore waters were likely to be, respectively, more associated with high suspended sediment and gelbstoff/detritus loaded from the land by backwash. An intense phytoplankton growth due to horizontal and vertical nutrient fluxes could not be the only mechanism underlying the Chl-a increase, as sea surface temperature was relatively low and stable during the period of study. Hence, using a data fusion approach, we found the two different mechanisms underlying Chl-a enhancements after the Japan tsunami. The image and data fusion techniques can provide valuable preliminary information for further studies dealing with environmental changes due to the tsunami.

Trends in the surface chlorophyll of the California Current: Merging data from multiple ocean color satellites

Standard remote sensing reflectance products from four ocean color sensors (OCTS, SeaWiFS, MODISA, MERIS) and over 10,000 in situ measurements of surface chlorophyll-a (Chl-a) concentration in the California Current were used to create empirical algorithms that are consistent with in situ data as well as between individual sensors. Using these algorithms, a merged multi-sensor time series of the surface Chl-a concentration in California Current region was created. The merged Chl-a time series (November 1996–December 2011) show a significant (P<0.01) increasing trend off central California and significant (P<0.01) decreasing trends in the central North Pacific gyre and off southern Baja California. Although this 15-year time series is too short to separate interannual and multidecadal cycles from climate trends, both of these trends are consistent with the predicted effects of global warming. The expected increase in vertical stratification of the water column and the resulting decreased vertical flux of nutrients would lead to lower Chl-a in the gyre but the increased upwelling-favorable winds leading to stronger upwelling off central California or the increased nitrate content of the upwelled water would lead to higher Chl-a in the upwelling region. The decreased Chl-a off southern Baja California resembles the effect of a decreased influence of strong El Nino events.

Large-scale impacts of submesoscale dynamics on phytoplankton: Local and remote effects

The sensitivity of an idealized bio-physical model of seasonally varying subtropical and subpolar gyres to increased horizontal resolution is presented. Switching from mesoscale-resolving (1/9°) to submesoscale-resolving (1/54°) allows the emergence of a denser and more energetic vortex population sustained by submesoscale physics. The experiments display a global decrease in phytoplankton abundance of ∼10–20% as the resolution is refined. This result contrasts with previous studies, which suggested that eddy-driven vertical fluxes of nutrients locally boost phytoplankton growth at mid-latitudes in the open ocean. The explanation is that the long-term (50years) modification of the large-scale, or basin-scale, circulation and distribution of nutrients by submesoscale processes (remote effects), not taken into account by previous studies because of their much shorter time integration, balance the local, small-scale effects. More precisely, dynamical remote effects (involving only the physics) affect the location of the boundary between the two gyres as well as the large-scale mixed-layer depth (MLD) and thermocline depth. Biological remote effects result from the dynamical remote effects that strongly modify the physical–biological interactions at all scales, including at large scales. These biological remote effects involve changes of the nutricline depth. In the mid-latitude subpolar gyre, phytoplankton decrease in abundance at higher resolution is due to the shallower MLD and nutricline, which promote zooplankton grazing; in the subtropical gyre, it is due to deeper MLD and nutricline, which diminishes regenerated production. In addition, remote effects modulate the dynamical supplies of nutrients to the euphotic layer through a combination of changes in mean advection, eddy advection and vertical mixing.

Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 427:29-49 (2011) - DOI: https://doi.org/10.3354/meps09043 Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model Song Hu1,*, Changsheng Chen1,2, Rubao Ji1,3, David W. Townsend4, Rucheng Tian2, Robert C. Beardsley5, Cabell S. Davis3 1Marine Ecosystem and Environmental Laboratory, College of Marine Sciences, Shanghai Ocean University, 999 Hucheng Huanlu, Lingang New City, Shanghai 201306, PR China 2School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Blvd., New Bedford, Massachusetts 02744, USA 3Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 4School of Marine Sciences, University of Maine, 5706 Aubert Hall, Orono, Maine 04469, USA 5Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA *Email: shu@shou.edu.cn ABSTRACT: SeaWiFS (Sea-viewing Wide Field-of-view Sensor) chlorophyll data revealed strong interannual variability in fall phytoplankton dynamics in the Gulf of Maine, with 3 general features in any one year: (1) rapid chlorophyll increases in response to storm events in fall; (2) gradual chlorophyll increases in response to seasonal wind- and cooling-induced mixing that gradually deepens the mixed layer; and (3) the absence of any observable fall bloom. We applied a mixed-layer box model and a 1-dimensional physical-biological numerical model to examine the influence of physical forcing (surface wind, heat flux, and freshening) on the mixed-layer dynamics and its impact on the entrainment of deep-water nutrients and thus on the appearance of fall bloom. The model results suggest that during early fall, the surface mixed-layer depth is controlled by both wind- and cooling-induced mixing. Strong interannual variability in mixed-layer depth has a direct impact on short- and long-term vertical nutrient fluxes and thus the fall bloom. Phytoplankton concentrations over time are sensitive to initial pre-bloom profiles of nutrients. The strength of the initial stratification can affect the modeled phytoplankton concentration, while the timing of intermittent freshening events is related to the significant interannual variability of fall blooms. KEY WORDS: Fall phytoplankton bloom · Surface forcing · Freshening · Interannual variability · Gulf of Maine · Modeling Full text in pdf format PreviousNextCite this article as: Hu S, Chen C, Ji R, Townsend DW, Tian R, Beardsley RC, Davis CS (2011) Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model. Mar Ecol Prog Ser 427:29-49. https://doi.org/10.3354/meps09043 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 427. Online publication date: April 12, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Effect of climate change on the thermal stratification of the baltic sea: a sensitivity experiment

The evolution in time of the thermal vertical stratification of the Baltic Sea in future climate is studied using a 3D ocean model. Comparing periods at the end of the twentieth and twenty first centuries we found a strong increase in stratification at the bottom of the mixed layer in the northern Baltic Sea. In order to understand the causes of this increase, a sensitivity analysis is performed. We found that the increased vertical stratification is explained by a major change in re-stratification during spring solely caused by the increase of the mean temperature. As in present climate winter temperatures in the Baltic are often below the temperature of maximum density, warming causes thermal convection. Re-stratification during the beginning of spring is then triggered by the spreading of freshwater. This process is believed to be important for the onset of the spring bloom. In future climate, temperatures are expected to be usually higher than the temperature of maximum density and thermally induced stratification will start without prior thermal convection. Thus, freshwater controlled re-stratification during spring is not an important process anymore. We employed a simple box model and used sensitivity experiments with the 3D ocean model to delineate the processes involved and to quantify the impact of changing freshwater supply on the thermal stratification in the Baltic Sea. It is suggested that these stratification changes may have an important impact on vertical nutrient fluxes and the intensity of the spring bloom in future climate of the Baltic Sea.

A Novel Integration of an Ultraviolet Nitrate Sensor On Board a Towed Vehicle for Mapping Open-Ocean Submesoscale Nitrate Variability

Abstract Initial results from a deployment of the SUV-6 ultraviolet spectrophotometer, integrated with the SeaSoar towed vehicle, are presented. The innovative, combined system measures nitrate concentration at high spatial resolution (4 m vertically, 5 km horizontally), high sensitivity (0.2 μM), and concomitantly with temperature, salinity, and dissolved oxygen. The authors demonstrate that this approach constitutes a powerful new tool for quantifying the role of mesoscale and submesoscale vertical nutrient fluxes to the euphotic zone, using measurements from a high-resolution survey of an eddy dipole in the Iceland Basin during the summer of 2007.

Physical Determinants of Phytoplankton Production, Algal Stoichiometry, and Vertical Nutrient Fluxes

Most phytoplankters face opposing vertical gradients in light versus nutrient supplies but have limited capacities for vertical habitat choice. We therefore explored a dynamical model of negatively buoyant algae inhabiting a one-dimensional water column to ask how water column depth and turbulence constrain total (areal) phytoplankton biomass. We show that the population persistence boundaries in water column depth-turbulence space are set by sinking losses and light limitation but that nutrients are most limiting to total biomass in water columns that are neither too shallow or too weakly mixed (where sinking losses prevail) nor too deep and turbulent (where light limitation prevails). In shallow waters, the most strongly limiting process is nutrient influx to the bottom of the water column (e.g., from sediments). In deep waters, the most strongly limiting process is turbulent upward transport of nutrients to the photic zone. Consequently, the highest total biomasses are attained in turbulent waters at intermediate water column depths and in deep waters at intermediate turbulences. These patterns are insensitive to the assumption of fixed versus flexible algal carbon-to-nutrient stoichiometry, and they arise irrespective of whether the water column is a surface layer above a deep water compartment or has direct contact with sediments.

Turbulent supply of nutrients to phytoplankton at the New England shelf break front

We present observations from deployments of a microstructure turbulence instrument (the Towed Microstructure and Auxiliary Sensor Instrument) aboard a pumping profiling vehicle (the Lamont Pumping SeaSoar) towed behind a research vessel at the New England shelf break front in August 2002. From these we determined coincident fine‐scale vertical eddy diffusivity and gradients of nitrate, phosphate, and silicate on several transects spanning the front. We then quantified vertical turbulent nutrient fluxes through the base of the euphotic zone (defined as the 1% light level), the base of the density transition zone, maximum nutrient gradients (the nutriclines), and the depth of maximum stratification (the pycnocline). Vertical eddy diffusivity estimates spanned a wide range from near‐molecular levels at the pycnocline to values exceeding 10−3 m2 s−1 at depth and in the surface layers. Vertical nutrient fluxes were maximal at the 1% light level and decreased by 2 orders of magnitude as they moved upward through the water column to the depth of the pycnocline. Nutrient fluxes were enhanced shoreward of the front because of high mixing rates and nutrient gradients at the depth of the 1% light level. Nitrate fluxes there averaged about 6 × 10−5 mmol N m−2 s−1, sufficient to support a net community productivity of 30 mmol C m−2 d−1. Seaward of the front, these fluxes averaged about 1 × 10−5 mmol N m−2 s−1 and would support correspondingly lower productivity. A small part of the upward flux appeared to support a silicifying community of phytoplankton that consumed phosphate in proportion to nitrate at about double the canonical Redfield stoichiometry.

Quantifying cross-shelf and vertical nutrient flux in the Coastal Gulf of Alaska with a spatially nested, coupled biophysical model

The Coastal Gulf of Alaska (CGOA) is productive, with large populations of fish, seabirds, and marine mammals; yet it is subject to downwelling-favorable coastal winds. Downwelling regions in other parts of the world are typically much less productive than their upwelling counterparts. Alternate sources of nutrients to feed primary production in the topographically complex CGOA are poorly known and difficult to quantify. Here we diagnose the output from a spatially nested, coupled hydrodynamic and lower trophic level model of the CGOA, to quantify both horizontal and vertical nutrient fluxes into the euphotic zone. Our nested model includes both nitrogen and iron limitation of phytoplankton production, and is driven by a fine-scale atmospheric model that resolves the effects of local orography on the coastal winds. Results indicate significant ‘‘rivers’’ of cross-shelf nitrogen flux due to horizontal advection, as well as ‘‘fountains’’ of vertical transport over shallow banks due to tidal mixing. Using these results, we constructed a provisional budget of nutrient transport among subregions of the CGOA. Contrary to expectations, this budget reveals substantial upwelling of nutrients over major portions of the shelf, driven by local wind-stress curl. These effects are large enough to overwhelm the smaller downwelling flux at the coast throughout the growing season. Vertical mixing by winds and tides, and horizontal flux from the deep basin, are other substantial contributors to nutrients above the 15-m horizon. These findings help to explain the productivity of this coastal ecosystem.

Cross-shelf circulation and phytoplankton distribution at the summertime New England shelfbreak front

We investigate aspects of the secondary (cross-shelf) circulation at the Middle Atlantic Bight shelfbreak front using high-resolution data collected on the New England Shelf in August 2002. The alongshore shelfbreak jet coincides with the front at the seaward edge of the cold pool (remnant winter shelf water) and there is a suggestion of a cross-stream convergence centered at the jet core. Despite indications of convergence we found no evidence of a surface subduction on the seaward side of the front. At depth 70 m near the shelfbreak there was a patch of chlorophyll, located within a local temperature–salinity maximum which, though significantly below the euphotic zone, appeared to be photo-acclimated and viable. The chlorophyll feature could be the result of a local subduction by a larger scale eddy circulation seaward of the front. Dye tracer experiments directly observed the convergence at the foot of the shelfbreak front and subsequent upwelling of bottom boundary layer water along the shoreward side of the shelfbreak front. But, we found no evidence that this upwelling influenced productivity at the front. Further, since there was no cross-shelf maximum in subsurface chlorophyll at the front, we conclude that this productivity is in general, sustained by a broadly distributed local vertical nutrient flux from an underlying nutrient reservoir.