Atlantic Shelf Research Articles

ANNUAL SHELL GROWTH INCREMENT FORMATION IN THE DEEP WATER PATAGONIAN SCALLOP ZAGOCHLAMYS PATAGONICA

The Patagonian scallop Zygochlamys patagonica occurs on the SW Atlantic shelf of South America between 36S to 55S in beds at depths around 100 m. Stable oxygen and carbon isotope ratio analysis in scallops from four large beds (Uruguay, 3617#S, Reclutas, 3920#S, Tango B, 4230#S, and Beagle, 5510#S) in combination with condition indexes and oceanography date strongly suggest that shell growth increments in this species are formed annually. Most translucent growth bands coincide with low values of both d 13 C and d 18 O (i.e., they are formed at times of high remineralization activity and of higher water temperature). This pattern is consistent throughout the distributional range. Given the specific Argentinean shelf ocean- ography, higher temperatures at this depth occur during austral autumn—beginning winter in the region of Reclutas and Tango B but during summer-autumn at the northern and southern limit of the distributional range. Year-round investigations at Reclutas beds indicate that translucent shell growth bands coincide with a pause in somatic growth during austral winter, when energy investment is shifted from somatic growth to gonad development.

Insights from study of the last intact neritic marine ecosystem

I am pleased to see the recent article ‘The ups and downs of trophic control in continental shelf ecosystems’ by Frank et al. [ 1 Frank K.T. et al. The ups and downs of trophic control in continental shelf ecosystems. Trends Ecol. Evol. 2007; 22: 236-242 Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar ] because it summarizes a growing literature indicating that, after all, open-water marine ecosystems can be forced by top-down factors. I would like to offer perspective garnered from working in the last yet-to-be-altered neritic marine ecosystem on Earth, the Ross Sea, which is also the largest continental shelf ecosystem of the Antarctic (the size of southern Europe). Frank et al. [ 1 Frank K.T. et al. The ups and downs of trophic control in continental shelf ecosystems. Trends Ecol. Evol. 2007; 22: 236-242 Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar ] described for the fish fauna of exploited North Atlantic shelf ecosystems an interesting pattern, whereby, depending on ocean temperature and biodiversity, food webs could be forced by either top-down (cold, low diversity) or bottom-up (warm, high diversity) factors. According to these ‘rules’, the Ross Sea neritic ecosystem – or at least the fish fauna – should be structured by predation. One portion of the Ross Sea seems to follow these rules but another does not, seemingly owing to the irregular distribution of still-existing upper-most trophic levels, that is, the cetaceans, flightless seabirds, seals and large predatory fish long since extirpated from the North Atlantic.

The influence of wind forcing on across-shelf transport in the Florida Keys

Acoustic Doppler current profiles and current meter data are combined with wind observations to describe the transport of water leaving Florida Bay and moving onto the inner shelf on the Atlantic side of the Florida Keys. A 275-day study in the Long Key Channel reveals strong tidal exchanges, but the average ebb tide volume leaving Florida Bay is 19% greater than the average flood tide volume entering the bay. The long-term net outflow averages 472 m 3 s −1. Two studies in shelf waters describe the response to wind forcing during spring and summer months in 2004 and during fall and winter months in 2004–2005. During the spring–summer study, southeasterly winds have a distinct shoreward component, and a two-layer pattern appears. Surface layers move shoreward while near-bottom layers move seaward. During the winter study, the resultant wind direction is parallel to the Keys and to the local isobaths. The entire water column moves in a nearly downwind direction, and across-shelf transport is relatively small. During the summer wet season, Florida Bay water should be warmer, fresher, and thus less dense than Atlantic shelf waters. Ebbing bay water should move onto the shelf as a buoyant plume and be held close to the Keys by southeasterly winds. During the winter dry season, colder and saltier Florida Bay water should leave the tidal channels with relatively high density and be concentrated in the near-bottom layers. But little across-shelf flow occurs with northeasterly winds. The study suggests that seasonally changing wind forcing and hydrographic conditions serve to insulate the reef tract from the impact of low-quality bay water.

Stable nitrogen isotope studies of the pelagic food web on the Atlantic shelf of the Iberian Peninsula

The structure and variability of pelagic food webs along the north and northwestern shelf of the Iberian Peninsula were analysed using natural abundance of nitrogen stable isotopes of plankton and pelagic consumers. Plankton composition was mainly studied in size-fractionated samples, but also the isotopic signatures of three copepod species, as representative of primary consumers, were considered. Several fish species were included as planktivorous consumers, with special attention to sardine ( Sardina pilchardus). Finally, top pelagic consumers were represented by the common dolphin ( Delphinus delphis). The relationship between trophic position and body size implies large variability in the ratio of predator to prey sizes, likely because widespread omnivory and plankton consumption by relatively large predators. Planktivorous species share a common trophic position, suggesting potential competition for food, and low nitrogen isotope enrichment between prey and consumers suggest nutrient limitation and recycling at the base of the food web. Both experimental and field evidences indicate that the muscle of sardine integrates fish diet over seasonal periods and reflects the composition of plankton from large shelf areas. The low mobility of sardines during periods of low population size is consistent with differential isotopic signatures found in shelf zones characterised by upwelling nutrient inputs.

The biology and ecology of Centropages typicus: An introduction

We briefly introduce the different chapters of this special issue which review the current knowledge and understanding of the biology and ecology of Centropages typicus obtained over the last 30 years. The papers are grouped into two major theme areas: the first five papers review behaviour, feeding, metabolism, growth and development, and reproduction, and the four following papers have a regional focus dealing with temporal and spatial distribution in the Mediterranean Sea and in the North Atlantic shelf areas. The findings reviewed in the two groups of papers converge to explain why C. typicus is such a very successful copepod species in these coastal areas. In the different chapters, similarities and differences with congeneric species ( C. velificatus, C. hamatus, C. brachiatus, C. chierchiae, C. furcatus, and C. abdominalis) have been explored where these are known.

Age and growth in Odontocymbiola magellanica (Gastropoda: Volutidae) from Golfo Nuevo, Patagonia, Argentina

Growth, age and somatic production of the benthic predator Odontocymbiola magellanica were studied in Golfo Nuevo (42� S6 5� W), on the South American Atlantic shelf. Stable oxygen isotope ratios confirmed semiannual formation of internal and external shell growth marks. Mean shell length (SL) of females was 115 and 112 mm for males, while popu- lation modal shell-free wet mass (SFWM) was 62.8 g. A Gompertz growth function (SL¥ = 200 mm, K = 0.197, t0 = 5.486) fitted 113 pairs of size-at-age data (12 shells) best. O. magellanica is a long-lived species, reaching up to 20 years of age. The maximum individual somatic production of 29.3 g SFWM per year is attained at 145 mm SL, which corresponds to about 12 years of age. The life span of this volutid seems to be twice compared with other large gastropods. O. magellanica is a valuable and exploitable resource regarding its large size and somatic production, but on the other hand, its slow growth, late maturity and direct devel- opment makes it extremely vulnerable to overexploi- tation.

Manmade and natural radionuclides in north east Atlantic shelf and slope sediments: Implications for rates of sedimentary processes and for contaminant dispersion

Results are presented for a study of manmade and natural radionuclides in north east Atlantic continental shelf and slope sediments to the west of Scotland. The data are interpreted in the context of sediment mixing and accumulation processes and are used to establish the westward extent of contamination of the sediment system.Offshore shelf and slope sediments were found to have post-glacial sedimentation rates of the order of 1 cm ky−1 but nearshore sediments had much higher accumulation rates of the order of 0.1 cm y−1. Surface mixed layer depths of up to 6 cm were observed and non-local mixing affected most of the slope sediments, resulting in advective transport of surface sediment to depths of up to 10 cm. Biodiffusion coefficients for offshore shelf and slope sediments were dominantly in the range 10−8 to 10−9 cm2 s−1.The study confirmed that seawater contaminated with Sellafield waste radionuclides is dominantly entrained to the east of 7° W and, consistent with this, higher levels of Sellafield derived radionuclides were confined to nearshore sediments, with lower levels to the west of 7° W. 238Pu/239,240Pu data indicated that Sellafield contributed 75–91% of the total plutonium in coastal sediment but only about 4–8% of the total in slope sediments. By analogy, it can be concluded that a similar situation will apply to other contaminants in seawater entering the north east Atlantic via the North Channel.

Seasonal evolution of the nutrient pattern on Biscay Bay continental shelf over the years 1999-2000

The French Atlantic shelf is subjected to strong anthropic influences (urban, industrial and agricultural discharges) of two main rivers (Loire and Gironde). The extension and consequences of these continental loadings for the nutrient and chlorophyll distribution have never been studied before on the Bay of Biscay continental shelf as a whole. We present the first synoptic view of the nutrient distribution and evolution on the French Atlantic shelf. Nutrient concentrations of the surface layer were studied during four cruises in April, June, September 1999 and March 2000. Until June, the freshwater inputs induce a nitrate gradient from river mouths to offshore waters in the vicinity of the 100 m isobath. The Redfield’s ratio study highlights the nitrate excess in river loadings. The early spring situation is characterised by high N:P ratios in front of the two estuaries and by a potential Si-limitation in the northern part. Nitrate removal continues in spite of the P-limitation and the increase in silicate concentrations during summer supposes high regeneration processes. At the end of summer, the water column is thermally stratified and the surface mixed layer is totally depleted in nitrate.

Anisotropic field of background internal waves on a sea shelf and its effect on low-frequency sound propagation

The space-time spectral characteristics of the field of background internal waves (IW) are obtained for two oceanic shelf regions (the Atlantic shelf of the United States and the Kamchatka shelf) and analyzed. Within the framework of a numerical experiment, it is shown that the observed anisotropy of the IW field may considerably affect the low-frequency sound fluctuations in the aforementioned regions and, in particular, may change the interference invariant of the sound field.

Metal distributions and their fluxes at the coastal boundary of a semi-enclosed ria

Water column samples have been collected in the outer channel of the Ferrol Ria (NW Spain) during four occasions over a tidal cycle. The objective was to study the exchange of dissolved and particulate Cd, Cu, Pb and Zn and particulate Al, Fe and Si between the ria and the adjacent coastal waters. This study provides the first extensive dataset on dissolved and particulate metal concentrations in the water column of a Galician ria. Typical concentrations of dissolved Cd (96 ± 31 pM), Cu (8 ± 4 nM), Pb (270 ± 170 pM) and Zn (21 ± 10 nM) were similar than in other European Atlantic shelf and coastal waters. The fraction of metals in the particulate phase followed the trend: Pb > Cu ∼ Zn > Cd. The outgoing water from the ria was enriched in dissolved and particulate Cu, Pb and Zn compared with incoming waters, whereas Cd concentrations were similar for both waters. The suspended particulate matter was composed of a mixture of marine and continental material. The latter end-member was found to arise from the metal-rich ria bed sediments, which is diluted by the dominant metal-poor marine end-member. The net output flux of Cu from the channel is balanced by the freshwater inputs to the ria, and the net Zn flux gave a positive output to coastal waters. For Pb, the net flux to the coastal waters is less than that input from the rivers, as a result of its particle reactivity and deposition in sediments. On the contrary, a net input flux of dissolved Cd from coastal waters was observed, highlighting the oceanic source of this metal in the Galician rias. Results from the budget calculations are in agreement with the differential geochemical behavior of these elements in coastal waters.

Individual growth and somatic production in Adelomelon brasiliana (Gastropoda; Volutidae) off Argentina

Growth, age and somatic production of the benthic predator Adelomelon brasiliana were studied at the southern limit of its distribution on the South American Atlantic shelf. Stable oxygen isotope ratios confirmed annual formation of internal shell growth marks. Modal shell length of the population was 140 mm, while modal shell-free wet mass was 255 g. A logistics growth function (SL∞=186.28 mm, K=0.185, t0=4.601) fitted 131 pairs of size-at-age data (25 shells) best. A. brasiliana is a very long-lived species, reaching up to 20 years of age. The maximum individual somatic production of 46 g shell-free wet mass year−1 is attained at 145 mm shell length, which corresponds to about 12 years of age.

The influence of the Plata River discharge on the western South Atlantic shelf

The influence of the Plata, the second largest river in South America, extends along a coastal strip of 1300 km. Historical hydrographic and wind data and numerical simulations are combined to determine the seasonal and interannual variability of the Plata plume and its relationship to the magnitude of the river discharge and the intensity and direction of the wind stress. Our results indicate that the seasonal variability of the river plume is controlled by the alongshore component of the wind stress. During El Niño the effects of the wind and precipitation anomalies tend to compensate each other, preventing anomalous northeastward plume extensions associated to large outflow events. Numerical experiments confirm this finding and indicate that during El Niño the discharge from the Plata River spreads offshore.

Interdecadal variability in the Gulf of Maine zooplankton community, with potential impacts on fish recruitment

Abstract We used principal component analysis (PCA) to explore interannual changes in a time-series lasting more than 40 years of zooplankton abundance from NOAA's Continuous Plankton Recorder (CPR) survey. This analysis identified a complex of taxa, including Centropages typicus, Oithona spp., Pseudocalanus spp., and Metridia lucens that followed a common pattern of interdecadal variability characterized by a dramatic increase in these taxa around 1990, followed by a rapid decline in 2002. All of these taxa showed a large proportional increase in winter abundance between the 1980s and 1990s. These changes could be driven by increased primary productivity during winter, caused by a large-scale freshening of the Northwest Atlantic Shelf. In addition to the “community shift” mode, the analysis found a strong mode of interannual variability attributed to previously described changes in the abundance of late-stage Calanus finmarchicus. To explore the impact of these modes on higher trophic levels, we correlated the zooplankton modes with recruitment time-series from 12 fish stocks from the Gulf of Maine region. Several significant correlations were found, suggesting that the changes in the zooplankton modes may reflect broad changes in the Gulf of Maine ecosystem.

A comparison of the circulation patterns over the Southwestern Atlantic Shelf driven by different wind stress climatologies

We compare the oceanic circulation patterns over the Southwestern Atlantic Shelf (SWAS) forced by nine different wind stress climatologies. The largest differences are observed in experiments forced with the Hellerman and Rosenstein [1983, hereafter HR83] and Trenberth et al. [1990, hereafter TR90] winds. HR83 shows a general northeastward flow near the shelf break. The TR90 results shows a bifurcating path south of ∼40°S and a poleward flow north of 35°S. The most robust circulation patterns are a broad northward flow and the generation of coastal re‐circulation cells in southern Patagonia and the development of a southward jet in the inner portion of the South Brazil Bight.

A numerical study of the Southwestern Atlantic Shelf circulation: Barotropic response to tidal and wind forcing

This article analyzes the barotropic circulation in the Southwestern Atlantic Shelf using a three‐dimensional numerical model forced with winds and tides. South of 40°S, the shelf circulation is dominated by the propagation of the semidiurnal tides. In this region the diurnal tides are generally weak, except at the shelf edge where they resonate with northward propagating, continental shelf waves. North of 40°S, the tidal circulation is relatively weak, and the circulation is mainly driven by the winds. The wind‐driven annual mean circulation is characterized by a broad northeastward flow south of approximately 40°S and is characterized by a southwestward flow farther north. The intense mixing associated with the Patagonian tides enhances the bottom friction that balances the energy input from the wind stress forcing. In contrast with previous results our simulation shows a detrainment of the northward volume transport with latitude due to an offshore flow along the edge of the Patagonian shelf break. The largest seasonal variations of the shelf circulation are observed in the region between 45°S and 25°S where, during the fall, there is a development of a clockwise gyre and a northeastward flow north of 40°S. The gyre weakens toward the winter, and the northeastward flow reverses directions.

Glacier‐Influenced Sedimentation on High‐Latitude Continental Margins

Glacier‐Influenced Sedimentation on High‐Latitude Continental Margins includes a collection of papers that was presented at a meeting of the Geological Society of London, held in April 2001 in Bristol, in the United Kingdom. The book follows some 11 years after another special publication (No. 53), Glacimarine Environments: Processes and Sediments, which was co‐edited by J. Dowdeswell. Hence, it is relatively easy to assess the increase in the state of knowledge represented by the recent volume. Overall, the book delivers on its quality of content and on the quantity of examples from slope and Atlantic shelf settings. Yet, given the recent results from a wide range of settings in the Antarctic, the inclusion of an invited paper or two from regions outside of the Ross Sea would have added value to the book. There are no summaries of Ocean Drilling Program legs, several of which have penetrated deep sections in the north Pacific, north Atlantic, and Antarctic margins in the last 10 years. Also, unlike its preceding volume, there are no examples of ancient shelf settings, which is a pity considering the renewed interest in ancient glacimarine sequences spurred on by the “snowball Earth“ debate. Despite these omissions, the volume is exceedingly informative and achieves high marks for communicating the significance of glacially influenced sedimentation.

Iron and other transition metals in Patagonian riverborne and windborne materials: geochemical control and transport to the southern South Atlantic Ocean

The bulk of particulate transition metals transported by Patagonian rivers shows an upper crustal composition. Riverine particulate 0.5 N HCl leachable trace metal concentrations are mainly controlled by Fe-oxides. Complexation of Fe by dissolved organic carbon (DOC) appears to be an important determinant of the phases transporting trace metals in Patagonian rivers. In contrast, aeolian trace elements have a combined crustal and anthropogenic origin. Aeolian materials have Fe, Mn, and Al contents similar to that found in regional topsoils. However, seasonal concentrations of some metals (e.g., Co, Pb, Cu, and Zn) are much higher than expected from normal crustal weathering and are likely pollutant derived. We estimate that Patagonian sediments are supplied to the South Atlantic shelf in approximately equivalent amounts from the atmosphere (∼30 × 10 6 T yr −1) and coastal erosion (∼40 × 10 6 T yr −1) with much less coming from the rivers (∼2.0 × 10 6 T yr −1). Low trace metal riverine fluxes are linked to the low suspended particulate load of Patagonian rivers, inasmuch most of it is retained in pro-glacial lakes as well as in downstream reservoirs. Based on our estimation of aeolian dust fluxes at the Patagonian coastline, the high nutrient-low chlorophyll sub Antarctic South Atlantic could receive 1.0 to 4.0 mg m −2 yr −1 of leachable (0.5 N HCl) Fe. Past and present volcanic activity in the southern Andes—through the ejection of tephra—must be highlighted as another important source of Fe to the South Atlantic Ocean. Based on the 1991 Hudson volcano eruption, it appears that volcanic events can contribute several thousand-fold the mass of “leachable” Fe to the ocean as is introduced by annual Patagonian dust fallout.

Food consumption of fish in a sub-tropical SW Atlantic ecosystem off Brazil: comparison of four Sciaenid species

The first evaluations of the daily ration and population food consumption of four Sciaenid fish, under natural conditions for the Brazilian south-western Atlantic shelf (23°35′S, 45°00′W) are presented. The average daily ration ranged from 4.38 to 12.92 % body mass (90% confidence interval) for the juvenile shortfin corvina ( Isopisthus parvipinnis), from 0.41 to 5.52 % body mass (90% confidence interval) for the juvenile shorthead drum ( Larimus breviceps) and from 3.14% to 3.15% body mass for the juvenile whitemouth croacker ( Micropogonias furnieri). Juveniles of the banded croacker ( Paralonchurus brasiliensis) showed higher estimates (2.58–2.60% body mass) than those for the adults (1.2–1.84% body mass). The mean food consumption rate of the population was estimated at about 11 and 8 year –1, respectively, for the whitemouth croacker, and banded croacker, corresponding to a gross efficiency of 13 and 10%. The estimates of daily ration were related to the feeding habits as well as to size of the fish, and probably to their specific energy budget. Estimates of population food consumption are related to population structure.

Diversity and community structure of epibenthic invertebrates and fish in the North Sea

The structure of North Sea benthic invertebrate and fish communities is an important indicator of anthropogenic and environmental impacts. Although North Sea fish stocks are monitored regularly, benthic fauna are not. Here, we report the results of a survey carried out in 2000, in which five nations sampled the epibenthic and fish fauna at 270 stations throughout the North Sea. The aim of the survey was to investigate the diversity and community structure of epibenthic and fish communities and to identify relationships with environmental factors, including the frequency of commercial otter and beam trawling disturbance. Epibenthic species diversity was lower in the southern North Sea than in central and northern areas. Fish, conversely, were more diverse in the south. The 50 m, 100 m and 200 m depth contours broadly defined the boundaries of benthic and fish communities. The abundance of epibenthos of the southern North Sea was dominated by free-living species, whilst north of the 50 m contour sessile species prevailed. A hybrid area, with sessile species typical of the north and free-living species characteristic of the south, was found off the Norfolk and Flamborough coast stretching towards the Dogger Bank. Large-scale hydrodynamic phenomena were most likely to be responsible for the main divisions between communities, especially the boundary between mixed and stratified water masses. However, bottom temperature, sediment parameters and beam trawling were closely correlated with species richness and diversity, as well as community patterns, and may modify regional species composition. Our study shows that effective large-scale sampling of benthic communities can be conducted during existing fisheries surveys. Since annual fisheries surveys are conducted throughout the northeast Atlantic shelf seas, concurrent benthic surveys would allow benthic sampling on unprecedented spatial and temporal scales. The samples would help to monitor the environmental impacts of trawling disturbance, climate change, pollution and other natural and anthropogenic factors.

NEW CRABS FROM THE EOCENE AND OLIGOCENE OF BAJA CALIFORNIA SUR, MEXICO AND AN ASSESSMENT OF THE EVOLUTIONARY AND PALEOBIOGEOGRAPHIC IMPLICATIONS OF MEXICAN FOSSIL DECAPODS

A new collection of fossil decapod crustaceans from the Cretaceous Rosario Formation, the Eocene Tepetate Formation and the Oligocene El Cien Formation, Baja California Sur, Mexico, has yielded two new genera and several new species, Amydrocarcinus dantei n. gen. and sp., Levicyclus tepetate n. gen. and sp., Eriosachila bajaensis n. sp., Oregonia spinifera n. sp., Archaeopus mexicanus n. sp., and Necronectes nodosa n. sp. Additionally, new occurrences of the previously described Lophoranina bishopi, Xandaros sternbergi, Icriocarcinus xestos, and Lobonotus mexicanus as well as Dardanus cf. D. mexicanus are reported. As part of ongoing work on global evolutionary and paleobiogeographic patterns within the Decapoda, the work has prompted a review and synthesis of decapod occurrences in the tropical and subtropical Americas including the southern United States, the Caribbean, Mexico, Central America, and northern South America. As a result of the systematic review, several new combinations are reported herein which include Eriosachila bartholomaeensis (Rathbun, 1919), Lobonotus sandersi (Blow and Manning, 1996; 1998), and Matutites americanus (Rathbun, 1935). Icriocarcinus is transferred to the Goneplacidae, extending the range of that family into the Cretaceous. Most Cretaceous through Miocene tropical and subtropical American taxa appear to have originated within the area and a large number were endemic. Most of the immigrants to the central Americas appear to have evolved along North Atlantic shelves and subsequently dispersed to the Americas, probably via continental shelf routes. In addition, as demonstrated by several previous studies, decapod crustaceans appear to have evolved in numerous middle- and high- latitude areas with subsequent dispersal to lower latitudes, contrary to the long held notion that the tropics are areas of origin with subsequent dispersal to other regions. Low-latitude decapod taxa tend to remain in low-latitude areas. The Maastrichtian and the Eocene appear to have been times of elevated extinctions within the Decapoda; however, the extinction patterns for those two time intervals are very complex.