mid-Atlantic Ocean Research Articles

Pelagic community structure of the subtropical convergence region south of Africa and in the mid-Atlantic ocean

Cross-frontal changes in the microphytoplankton, zooplankton and micronekton species composition and biomass were investigated in two sectors of the Subtropical Convergence region (STC) to evaluate patterns in the pelagic community in areas of contrasting hydro- dynamic structure. The first sector was south of Africa (±20°E, winter 1993) where the frontal zone is relatively permanent and intense. The other sector was in the mid-Atlantic ocean (±2°E, summer 1994) where the STC is ephemeral and weak. Higher biological diversity and weaker zonation patterns were observed in the mid-Atlantic sector, relative to the sector south of Africa. This indicates that the boundaries of the STC were more relaxed in the former region, suggesting that the structure in the mid-Atlantic community is less controlled by hydrodynamic forcing. In both sectors, species of Antarctic and subtropical origin were present on both sides of the convergence, suggesting that cross-frontal mixing was prevalent. Changes in the relative proportion of microphytoplankton, micro- and mesozooplankton in both regions appear to reflect the seasonality of sampling, rather than regional differences in the pelagic food web structure. Despite the marked contrast in the intensity of the hydrographic front between the two sectors, higher phytoplankton, zooplankton and mesopelagic fish abundances were consistently associated with the Subtropical Convergence, reflecting the importance of this region in the pelagic production of the south Atlantic Ocean.

Market Share, Capacity Utilization, Resource Conservation, and Tradable Quotas

This paper examines the impact of the introduction of Individual Transferable Quotas (ITQs) on catch, market share, and capacity utilization of firms in the Mid Atlantic Surf Clam and Ocean Quahog (SCOQ) Fishery. Via the production function framework, catch and market share regression models are utilized in examining the effects of operator size, vessel age, and alternative product catch variables on industrial structure and how such effects changed after ITQs were introduced. Results indicate that in both fisheries, the ITQ system enhanced the value of each vessel by allowing vessel owners to apply greater effort to fewer boats, thus reducing excess capacity in the fishery. Results also indicate an overall resource conservation effect of ITQ introduction in the surf clam fishery. These results suggest that in the presence of ITQs, overall efficiency was enhanced in the SCOQ fishery.

Diverse origins of fluid in magmatic inclusions at Bingham (Utah, USA), Butte (Montana, USA), St. Austell (Cornwall, UK), and Ascension Island (mid-Atlantic, UK), indicated by laser microprobe analysis of Cl, K, Br, I, Ba + Te, U, Ar, Kr, and Xe

Saline fluid inclusions (FI) trapped at close to the temperature of final solidification of a granitic melt occur in rocks from Bingham, Utah, Ascension Island, mid-Atlantic Ocean, and St. Austell, Cornwall. Slightly lower temperature FI occur at Butte, Montana. Argon, Kr, and Xe were extracted from FI by laser microprobe decrepitation of minute portions of the samples after neutron irradiation (along with synthetic FI of known composition), and measured in a low blank, high sensitivity, pulse-counting mass spectrometer. Results enable measurement of Cl, K, Br, and I simultaneously with 36r, 40Ar, 84Kr, and 129Xe, in ∼10 −7 cc of fluid, the contents of a single spherical inclusion ∼57 μm in diameter. Average K Cl in Bingham and St. Austell FI are ∼0.25 and ∼0.15, respectively, broadly consistent with the composition of fluids equilibrated with rocks at temperatures close to the eutectic in the granite system (400–600°C and 0.5 – 2 Kb). Within-sample variations in K Cl are significant and may be a result of exsolution of fluids from magmas over a range of temperatures and/or pressures. Halogen ratios are confined to a narrow range, with I Cl and Br Cl in Bingham, Ascension, and some Butte FI ∼ 1 − 8 × 10 −5 and 1 − 3 × 10 −3, respectively, probably evidence of a common source of salinity, presumably part of the Earth's mantle. A component of salinity derived from continental crust may be indicated by higher I Cl and lower Br Cl in St. Austell Fl. Radiogenic 40Ar produced in situ from K in FI after trapping is usually insignificant. 40Ar e is defined as 40Ar in excess of the amounts attributable to atmospheric gases and that produced by decay of K in FI. Variations in 40Ar e Cl in Bingham, Ascension, and St. Austell FI are greater than can be explained by just different Cl concentrations in FI, (typically between ∼5 × 10 −7 and 8 × 10 −6 ), probably because Ar and Cl have been fractionated within these systems by outgassing of a magma in the interval between exsolution of different FI generations. Concentrations of Ar and 84Kr in most Bingham, Ascension, and St. Austell FI are ∼2 orders of magnitude greater than in most lavas and granites, with 84Kr 36Ar ranging between ∼0.015 and ∼0.08, grossly consistent with the composition of fluids exsolved from magmas. Several distinct fluid types are present at Butte: (1) rare “magmatic” FI having halogen and noble gas abundances similar to Bingham and Ascension and (2) abundant FI having 36Ar and 84Kr concentrations similar to air-saturated fresh waters, with slightly lower Br Cl than FI at Bingham and Ascension (∼5–10 × 10 −4) and relatively high 40Ar e Cl (∼1 × 10 −5 and 2.8 × 10 −5). Mixing prior to trapping between fluid derived from magmas and meteoric water that acquired 40Ar e plus some halogens by interaction with country rocks is the most likely origin of most Butte FI.

Mitochondrial DNA differentiation in North Atlantic eels: Population genetic consequences of an unusual life history pattern

A survey of restriction site polymorphism in the mitochondrial DNA (mtDNA) of the American eel Anguilla rostrata showed no genetic divergence among samples from a 4000-km stretch of North America coastline. Lack of geographic differentiation in mtDNA over such a large area contrasts sharply with results for terrestrial and freshwater vertebrates and is most likely attributable to the extraordinary life history of these catadromous fishes, which involves perhaps a single spawning population in the western tropical mid-Atlantic Ocean and subsequent widespread dispersal of larvae by ocean currents. However, samples of the European eel (nominally Anguilla anguilla) are highly distinct from A. rostrata in mtDNA genotype (distinguishable by 11 of 14 restriction endonucleases), contradicting some previous suggestions that the two forms belong to the same panmictic population. Results of this study emphasize the importance of life history in shaping population genetic structure.

Small-scale oxygen variations in the thermocline

An in situ dissolved oxygen sensor was used in conjunction with an STD system to map the oxygen distribution in the thermocline of a 40 km region of the mid-Atlantic Ocean. Features of the oxygen distribution were resolved to better than 0·07 ml 1.−1 and spatially to better than 10 m vertically and 2 km horizontally. An oxygen minimum region was observed at 350 m depth which contained 0·4 ml 1.−1 less oxygen than its surroundings. This layer was 80 m thick, 15 km in the east-west dimension and more than 37 km in its north-south extent. One of the dynamic characteristics of this oxygen minimum over a one-day period included a westward shift of about 4 km. This study demonstrates the small-scale variability which can exist in the oceanic oxygen distribution, and it illustrates the value of continuous in situ oxygen measurements in examining advective and mixing processes.

Global variation in the atmospheric electric field

An analysis of the global nature of the atmospheric electric field is presented on the basis of comparison of measurements on the research vessel, Hakuho-Maru, on the Mid- and South-Pacific Ocean, with those at Syowa Station in Antartica and on two vessels on the Mid-Atlantic Ocean. The comparison of daily averages gave a different type of latitude dependence, which was characterized by a gradual decrease toward the antarctic region. Diurnal variations at these globally representative stations on the same day were checked with each other for the first time, and the correlation between them was found much higher than that between land stations. The regional effect, which might depend on the distance from the thunderstorm area, was not evidently detected. So the influence of the generator area was considered to propagate over the entire globe without significant attenuation.

A method of correctingP-wave magnitudes for the effect of earthquake focal mechanism

abstractThis paper presents a methodology for correcting body-wave magnitudes for the effect of focal mechanism in a routine manner. The method requires a knowledge of the prevailing or dominant mechanism for a geographic region, from which tables are constructed which enable one to make the necessary correction. Included in the paper are tables for Aleutian Island, Kamchatka and mid-Atlantic Ocean earthquakes.From a study of seven earthquakes, it is concluded that the present method gives essentially the same average magnitude with the same standard deviation as a more exact method of correcting for the focal mechanism. The latter method uses the focal-mechanism parameters of the earthquake, which must be determined independently for each earthquake.The existing distribution of seismograph stations is such that transform-fault earthquakes of the mid-Atlantic Ocean will consistently have their P-wave magnitudes underestimated by about 0.2 magnitude units, if no correction is made for the focal mechanism. On the other hand, P-wave magnitudes of earthquakes in Kamchatka and south of the axis of the Aleutian Trench will be overestimated by about 0.2 units.

Young Black Drum, Pogonias cromis, in Tidal Fresh and Brackish Waters, Especially in the Chesapeake and Delaware Bay Areas

Young black drum, ranging from 15–267 mm, were found in virtually fresh tidal waters (4 out of 86 records), but most often under brackish conditions in the Chesapeake and Delaware Bay areas. The sporadic records indicate that young are relatively uncommon. No relationship was found between size and salinity. Records suggest thatPogonias cromis spawns in offshore waters of the mid-Atlantic Ocean from March to May, and that the Chesapeake and Delaware Bay areas are at the northern limits of the effective spawning range. The larvae probably drift into the estuarine nursery areas thereafter by way of tidal currents, but these may be only a small part of a crop spawned offshore. The young apparently move offshore and perhaps southward agains during the late fall. A marked low ratio of records of the young to adults was apparent. Young may be subjected to high mortality from unknown environmental factors; changing low temperatures in spring may be an important one.