Larry P. Atkinson

The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean

Five large rivers that discharge on the western North Atlantic continental shelf carry about 45% of the nitrogen (N) and 70% of the phosphorus (P) that others estimate to be the total flux of these elements from the entire North Atlantic watershed, including North, Central and South America, Europe, and Northwest Africa. We estimate that 61 · 109 moles y−1 of N and 20 · 109 moles y−1 of P from the large rivers are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial of river sediments that are deposited directly on the continental slope. The effective transport of active N and P from land to the shelf through the very large rivers is thus reduced to 292 · 109 moles y−1 of N and 13 · 109 moles y−1 of P.The remaining riverine fluxes from land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental shelf is inversely correlated with the log mean residence time of water in the system. This is consistent with numerous observations of nutrient retention and loss in temperate lakes. Denitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appears to be directly proportional to the log mean water residence time. In general, we estimate that estuarine processes retain and remove 30–65% of the total N and 10–55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to the shelf amounts to 172–335 · 109 moles y−1 of N and 11–19 · 109 moles y−1 of P. These values are similar to the effective contribution from the large rivers that discharge directly on the shelf.For the North Atlantic shelf as a whole, N fluxes from major rivers and estuaries exceed atmospheric deposition by a factor of 3.5–4.7, but this varies widely among regions of the shelf. For example, on the U.S. Atlantic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4–2.2. This deficit must be met by a flux of N from the deeper ocean. Burial of organic matter fixed on the shelf removes only a small fraction of the total N and P input (2–12% of N from land and atmosphere; 1–17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings is very small. The gross exchange of N and P between the shelf and the open ocean is much larger than inputs from land and, for the North Atlantic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atlantic continental shelf appears to remove some 700–950· 109 moles of N each year from the deep ocean and to transport somewhere between 18 and 30 · 109 moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are included in the total balance, the net flux of N to the shelf is reduced by 60 · 109 moles y−1 and the P flux to the ocean is increased by 20 · 109 moles y−1. These conclusions are quite tentative, however, because of large uncertainties in our estimates of some important terms in the shelf mass balance.

Observations of a Gulf Stream frontal eddy on the Georgia continental shelf, April 1977

Abstract Satellite, hydrographic, and data from moored current meters are used to show the effect of Gulf Stream frontal disturbances on low-frequency current and temperature variability, water exchange, and nutrient flux in the outer region of the Georgia shelf. Perturbations of the Gulf Stream cyclonic front are commonly observed as folded wave patterns in routine satellite-derived analyses of the western boundary of the Gulf Stream between Cape Hatteras and Miami. The disturbances consist of southward-flowing warm filaments or streamers of near-surface Gulf Stream water, 15 to 20 m deep, which can extend 35 to 40 km over the outer shelf around a cold upwelled core. Downstream dimensions of the filaments reach 100 to 200 km in the region from Jupiter, Florida, to Charleston, South Carolina, 10 to 50 km south of Jupiter, and 200 to 300 km between Charleston and Cape Hatteras. The features are defined as cyclonic, cold-core frontal eddies due to their flow and water mass properties. They appear to form from amplified waves in the Gulf Stream cyclonic front on an annual average of one every two weeks but with considerable monthly variability. They can persist up to three weeks and travel to the north with the same phase speed as the waves, approx. 40 cm s−1. The cyclonic circulation in frontal eddies provides a means for rapid shelf-Gulf Stream water exchange. The eddies appear to control the residence time of the outer shelf waters, defined as the mean separation time between eddy events, or approx. two weeks. Upwelling in the cold core extended into the euphotic zone (45 m) and shoreward (35 to 40 km) beneath the southward-flowing warm filament in a bottom intrusion layer 20 m thick. The annual nitrogen input to the shelf waters by this process is estimated as 55,000 tons each year, about twice all other estimated nitrogen sources combined; it can support an annual carbon production by phytoplankton of 32 to 64 g C m−2y−1 with no nitrogen recycling.

Gulf Stream frontal eddy influence on productivity of the southeast U.S. continental shelf

Weekly period meanders and eddies are persistent features of Gulf Stream frontal dynamics from Miami, Florida, to Cape Hatteras, North Carolina. Satellite imagery and moored current and temperature records reveal a spatial pattern of preferred regions for growth and decay of frontal disturbances. Growth regions occur off Miami, Cape Canaveral, and Cape Fear due to baroclinic instability, and decay occurs in the confines of the Straits of Florida between Miami and Palm Beach, between 30° and 32°N where the stream approaches the topographic feature known as the Charleston bump and between 33°N and Cape Hatteras. Eddy decay regions are associated with elongation of frontal features, offshore transport of momentum and heat, and onshore transport of nutrients. Onshore transport of new nitrogen from the nutrient-bearing strata beneath the Gulf Stream indicates that frontal eddies serve as a “nutrient pump” for the shelf. New nitrogen flux to the shelf due to Gulf Stream input could support new production of 7.4×1012 g C yr−1 or about 8 million tons carbon per year if all nitrate were utilized. Calculations indicate that approximately 70% of this potential new production is realized, yielding an annual new production for the outer shelf of 4.3×1012 g C.

The Relationship of Upwelling to Mussel Production in the Rias on the Western Coast of Spain

We have calculated an upwelling index for each month over a 17-year period (1969-1985) for a point off the western coast of Spain. We interpret April through September values of the index to indicate the flux of nitrate-rich water into the Spanish Rias. The index representing the 6-month upwelling series has been correlated with an index representing the conditions of mussels grown during that season on rafts in Ria de Arosa. Two seasons represent extreme upwelling conditions over the 17-year sampling period: 1977 when the upwelling index was the highest, and 1983 when it was the lowest. A comparison of the condition of mussels during these years showed that meat content was double in 1977. We suggest, by this study, that long range forecasts of synoptic scale weather patterns could be used to predict the potential nutritional value of mussels harvested in the rias of Spain.

The intrusion of Gulf Stream water across the continental shelf due to topographically-induced upwelling

Abstract Summer bottom temperatures along the continental shelf between Cape Hatteras and Cape Canaveral are abnormally low in regions where isobaths diverge. The regions are north of capes and shoals, which force the flow of shelf water to change vorticity and induce upwelling. Gulf Stream Water intrudes across the bottom during summer to replace the upwelled water, and accounts for the colder and more stratified water over the northern Florida and the North Carolina shelves.

Stimulation and inhibition of phytoplankton growth by low molecular weight hydrocarbons

Experiments on 4 phylogenetically different phytoplankton exposed in culture to a range of concentrations of benzene, toluene and xylene showed a variety of growth responses for marine microalgae. The degree of influence of these aromatic hydrocarbons, all components of fuel oils and crude oils, varied with concentration, compound and species. Stimulation of growth in Dunaliella tertiolecta resulted from low μg/l concentrations of all three compounds, Skeletonema costatum showed no growth enhancement, while Cricosphaera carterae and Amphidinium carterae were intermediate in their reactions. Closed culture vessels were found to be necessary to retain these volatile hydrocarbons. Many of the previous laboratory studies on oil using standard methods — cotton plugs, screw caps or beakers — have overlooked the important influence of the volatile fraction. The species-specific stimulation of low concentrations was further shown in experiments with mixtures of No. 2 fuel oil. The volatile fraction was most biologically reactive, being the source of growth enhancement at low levels and a major growth inhibitor at high concentrations. Thus, a significant environmental effect of oil on marine primary production could be the growth stimulation of particular species by low molecular weight aromatic compounds resulting in an alteration of the natural phytoplankton community structure and its trophic relationships.

Florida current meanders and gyre formation in the southern Straits of Florida

Moored current measurements, shipboard hydrography, and a time sequence of satellite-derived surface thermal images are used to show the formation and evolution of cold, cyclonic gyres coupled to large offshore meanders of the Florida Current in the southern Straits of Florida (SSF). Gyre formation is dependent upon the orientation of the Loop Current as it enters the SSF. With a well-developed Loop Current the flow overshoots the entry to the SSF causing the formation of a cold recirculation off the Dry Tortugas, approximately 200 km in size, that persists over timescales of about 100 days. The demise of the gyre occurs as it moves to the east at about 5 km d−1, reducing to half its original size off Big Pine and Marathon Keys and unobservable off the northern Keys. Previously observed local gyres between Key West and Islamorada, Florida (Lee et al., 1992), are identified as a latter stage in the downstream evolution of the gyres formed off the Tortugas. When the Loop Current is not developed, flow from the Yucatan Channel turns anticyclonically into the SSF, causing strong eastward flow over the slope off the Dry Tortugas and lower Florida Keys, and gyre formation does not occur.

Transport, potential vorticity, and current/temperature structure across Northwest Providence and Santaren Channels and the Florida Current off Cay Sal Bank

Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one-half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low-potential-vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.

Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf

Abstract Gulf Stream frontal disturbances cause nutrient-rich waters to frequently upwell and intrude onto the southeastern United States continental shelf between Cape Canaveral, Florida and Cape Hatteras, North Carolina. Phytoplankton response in upwelled waters was determined with three interdisciplinary studies conducted during April 1979 and 1980, and in summer 1978. The results show that when shelf waters are not stratified, upwelling causes productive phytoplankton (diatom) blooms on the outer shelf. Phytoplankton production averages about 2 g C m −2 d −1 during upwelling events, and ‘new’ production is 50% or more of the total. When shelf waters are stratified, upwelled waters penetrate well onto the shelf as a subsurface intrusion in which phytoplankton production averages about fives times higher than the nutrient-depleted overlying mixed layer. Phytoplankton within the intrusion deplete upwelled NO 3 in about 7 to 10 days, at which point no further net increase in phytoplankton biomass occurs. Current meter records show that upwelling occurs roughly 50% of the time on the outer shelf during November to April (shelf not stratified), and we estimate that seasonal primary production in upwelled waters is 175 g C m −2 6 months −1 of which at least 50% is ‘new’ production. More than 90% of outer shelf primary and ‘new’ production occurs during upwelling and thus upwelling is the dominant process affecting primary productivity of the outer shelf. Our seasonal estimates of outer shelf primary and ‘new’ production are, respectively, three and ten times higher than previous estimates that did not account for upwelling.

Hydrographic properties and circulation of the North Carolina shelf and slope waters

Abstract Processes affecting the renewal of the North Carolina Shelf Waters are discussed on the basis of temperature, salinity, dissolved oxygen, nutrients, runoff and wind data collected during 1965–1967. These consist of horizontal advection near the coast from the north, meanders of the Gulf Stream, subsurface intrusion, cascading and runoff. The wind-driven Virginia Water transport past Cape Hatteras may markedly reduce the temperature and salinity, and accelerate the freshwater exchange. The runoff from the watershed north of Cape Hatteras, when driven south by northerly winds, especially during the peak period in early spring, has a much greater influence on the circulation in Raleigh and Onslow Bays that does that from the adjacent rivers south of Cape Hatteras. Meanders in the Gulf Stream may renew the shelf waters with warm, saline water. Intrusion of subsurface Caribbean Water on to the outer part of the shelf takes place most frequently during the late summer following a period of southerly winds. It is postulated that during the cold part of the year when the surface layers are only partly stratified, such an intrusion may lead to upwelling near the shelf break. Cascading from the shelf down the slope may occur during the cold season following periods of low air temperature. Intrusion of subsurface water replenishes nutrients in the area, while cascading reduces the available supply of nutrients.