Shelf Off Research Articles

Postglacial variability in near‐bottom current speed on the continental shelf off south‐west Spitsbergen

ABSTRACTThis study documents changes in the sedimentary environment on the outer continental shelf off south‐west Spitsbergen during the last ∼14 000 cal a BP using sedimentological, mineralogical and geochemical data from a sea‐bed sediment core. We use the sortable silt fraction to infer fluctuations of near‐bottom current speed and determine sediment provenance and sediment transport paths. For most of the record length, sediments were derived from distal sources by the West Spitsbergen Current (WSC) and the East Spitsbergen Current (ESC). However, before ∼12 000 and after ∼1800 cal a BP sediments were also sourced from proximal sources in south‐west Spitsbergen. The slowest near‐bottom currents were recorded between 10 300 and 9700 cal a BP when an increase in water stratification occurred, probably due to the strengthening of the ESC and an associated reduction in the northward water and heat flux. Near‐bottom currents related to WSC activity were at their maximum between 9000 and 7500 cal a BP, i.e. during a period of elevated water temperature, probably resulting in maximum relative heat fluxes to the Arctic.

Clay mineral stratigraphy during the last 2.4Ma at IODP Exp. 323 Site U1343 in the Bering Sea

Abstract The clay mineral composition at IODP Exp. 323 Site U1343 in the Bering Sea was analyzed so as to unravel their provenance over glacial–interglacial cycles for the last 2.4 Ma. Smectite was negatively correlated with the sum of illite and chlorite; therefore, their ratio [S/(I + C)] was used as an indicator of clay mineral composition changes. In general, the S/(I + C) ratio was rather similar for glacial and interglacial periods during most of the last 2.4 Ma. In addition, these results overlap with those of surface sediments in the modern East Aleutian Basin, which suggests that smectite-rich clay particles are delivered from the Aleutians by the northward Bering Slope Current (BSC) rather than from rivers in western Alaska. However, some clay mineral compositions of the glacial periods after the Mid-Pleistocene Transition (MPT: 1.25–0.7 Ma) were characterized by low illite and relatively high smectite. During this period, extensive ice-rafting might have transported the smectite-rich clay particles to Site U1343 from the glacial shelf off Alaska and/or from glacial rivers from that area.

Distribution of planktonic cnidarian assemblages in the southern Gulf of Mexico, during autumn

BackgroundDespite their ecological, economic and medical relevance, very little information is available on the distribution of planktonic cnidarians, this being particularly true for some regions of the ocean such as the Gulf of Mexico. In fact, the effect of locally important oceanographic features such as the offshore autumnal transport on the distribution of planktonic cnidarians has long been overlooked in this region. Because of this, the present study aimed to analyse the spatial patterns of planktonic cnidarian assemblages in the southern Gulf of Mexico during the autumn of 1998, when particularly intense conditions of convergence, offshore water transportation and productivity were recorded. The assemblages were described in terms of their composition, abundance (volume), diversity, dominance and equitability. Cluster (Bray-Curtis index) and ordination (multidimensional scaling (MDS)) analysis were performed in order to link the observed distribution of species with the environmental parameters.ResultsSixty-eight taxa were recorded, with Koellikerina fasciculata and Muggiaea atlantica representing new records for the region. The holoplanktonic species Diphyes dispar, Abylopsis spp., Liriope tetraphylla, Diphyes bojani, Aglaura hemistoma, Muggiaea kochi, Chelophyes appendiculata and Eudoxoides mitra were dominant with respect to abundance (ml/1000 m3) and frequency of occurrence and defined the differences among the groups of stations. Four groups of sampling stations were identified: (A) the mouth of the Grijalva-Usumacinta system, (B) the Campeche Bank, (C) the area of offshore oil rigs and (D) the continental shelf off Tabasco and associated oceanic waters. The parameters that defined the groups of stations were zooplankton volume, sampling station depth, salinity and dissolved oxygen concentration.ConclusionsPlanktonic cnidarians are useful as indicators of water masses, since the observed zonation was congruent with the surface circulation patterns that are present during the autumn (offshore transportation) and the presence of particular local environmental conditions, including those at the mouth of the Grijalva-Usumacinta system and those associated with the area of oil rigs. We would expect that similar conditions of strong offshore transport would lead to a similar horizontal stratification in the assemblages of planktonic cnidarians in other parts of the world, driven mainly by salinity, oxygen, depth and zooplankton gradients.

Temporal dynamics of nitrogen loss in the coastal upwelling ecosystem off central Chile: Evidence of autotrophic denitrification through sulfide oxidation

Coastal upwelling areas are highly productive marine systems in which the development of oxygen‐depleted conditions and the availability of diverse electron donors (e.g., organic matter, , H2S) favor the processes involved in nitrogen (N) loss. We characterize the temporal and vertical variability of anammox and denitrification over the continental shelf off central Chile (36.5°S), through 15N and 13C tracer experiments, including amendments with H2S and along with measurements of 15N2 and 15N2O production and oceanographic variables during a year (2009 to 2010). Restricted to the bottom waters, both anammox and denitrification contributed similarly (∼ 500 nmol N2 L−1 d−1) to N loss during spring, while a marked decrease in the activity of these processes occurred in summer (103 and 14 nmol N2 L−1 d−1 for anammox and denitrification, respectively). During fall, denitrification was the only contributor to the observed nitrogen deficit (894 nmol N2 L−1 d−1). Interestingly, a substantial increase in the rates of denitrification (∼ 1200 nmol N2 L−1 d−1) and dark 13C assimilation were observed after the addition of H2S, indicating an autotrophic contribution to denitrification, which could be fueled in situ by H2S emitted from sediments or produced in the water column. The observed patterns seem to be controlled (stimulated or inhibited) by the availability of oxygen, organic matter, and H2S. This study establishes the magnitude and co‐occurrence of the different processes responsible for N removal in the coastal upwelling system of central Chile. This linkage of the nitrogen, carbon, and sulfur cycles is relevant to a global climate change scenario.

Iron speciation in the eastern tropical South Pacific oxygen minimum zone off Peru

Vertical profiles of total dissolved iron (DFe), Fe(II), and hydrographic parameters were obtained along three transects across the continental shelf off Peru in October 2005. Fe(II) and DFe concentrations were determined using luminol chemiluminescence and isotope dilution inductively coupled plasma mass spectrometry (ICP‐MS), respectively. The objective was to study the relationships among redox chemistry, nitrogen‐cycle processes, bottom topography, and the distribution and chemistry of iron in the region. The results are the most thorough survey to date of Fe geochemistry in this region. Exceedingly high DFe (∼ 50–75 nmol L−1) was measured over the wide northern continental shelf, with most of the dissolved Fe present as Fe(II) below the oxycline. In southern Peru, the shelf is narrower, and dissolved Fe concentrations were 10‐fold lower. Moreover, a smaller fraction of the dissolved Fe was present as Fe(II) in the south, even below the oxycline. At the western ends of the transects, Fe(II) maxima were coincident with deep (i.e., secondary) nitrite maxima. This suggests a relationship between nitrate reduction and Fe(II) accumulation in the water column. However, over the shelf, Fe(II) was also influenced by benthic processes. Large lateral gradients in dissolved Fe across the shelf–slope break reflect Fe removal by oxidative scavenging, and it seems plausible that much of the Fe is “trapped” by redox cycling on the shelf. Nevertheless, the maintenance of dissolved Fe(II) filaments within the secondary nitrite maxima constitutes an important mechanism for Fe transport offshore.

Biogeography of continental shelf and upper slope fishes off El Salvador, Central America

The present research provides detailed information on the geographic and bathymetric distributional patterns of fishes and describes the main species assemblages of the continental shelf and upper slope off El Salvador. The sampling was based on 673 bottom-trawl tows taken during research surveys from April to November 2003. The data analysis was based on presence-absence matrixes and was conducted with PRIMER 6 software. A total of 148 fish species were recorded during the study period: the families with the highest number of species were Sciaenidae (13) and Carangidae (10), and the highest percentages of occurrence were registered for Porichthys margaritatus (40.6%), Pontinus sp. (34.8%) and Monolene dubiosa (33.1%). The cluster analysis by depth showed three faunistic associations: (1) on the inner and part of the mid continental shelf (20–60 m), (2) one bathymetric stratum on the mid continental shelf (80–100 m), and (3) on the outer continental shelf and upper slope (120–240 m). The cluster analysis by geographic zones showed separation between three zones: Western, Central and Eastern Zones off El Salvador. The Central zone is characterized by a different fish community of mixed habitat, while the main factor that determined the fish assemblages on the continental shelf and upper slope was depth.

Trophic structure of benthic communities in the Cabo Frio upwelling system (southeastern Brazilian shelf): a temporal study using stable isotope analysis

In Brazil, coastal upwelling is observed in 7 areas along the southeastern/southern region and is most pronounced near Cabo Frio, Rio de Janeiro. This region is exposed to moderate seasonal Ekman-driven upwelling that brings cold water with increased nutrient levels nearshore and is more frequent and intense during the austral spring and summer, primarily due to the prevalence of northeasterly winds. Our aim was to verify the influence of this upwelling on the benthic trophic structure of the inner and outer shelf off Cabo Frio through measurements of sta- ble nitrogen (δ 15 N) and carbon (δ 13 C) isotopes. We identified 1 main pathway of organic matter (OM) transfer from the base to the top of the food web, as observed from the tendencies of the δ 13 C and δ 15 N values. The isotopic signatures of benthic consumers exhibited temporal and spatial vari- ability, with no interaction between them. As the result of a time lag, only consumers (mostly deca- pod carnivores) appeared to reflect the assimilation of 15 N-depleted and 13 C-enriched OM pro- duced and deposited during strong upwelling that occurred 4 mo earlier. Therefore, the intensity and period of the upwelling phenomenon were important for detecting upwelling in benthic food webs. Lighter nitrogen and heavier carbon isotopes were found on the inner shelf. Consumers may have exploited OM of different quality on the inner and outer shelf due to differences in sed- iment, hydrodynamics, mineralisation and assimilation of 13 C-enriched microphytobenthos. Nev- ertheless, 4 trophic levels were estimated in the benthic communities of the continental shelf off the Cabo Frio upwelling system, independent of period or area.

Chapter 16 The continental shelf of eastern India

Abstract The eastern continental margin of India is aligned NE–SW due to the trend of the Eastern Ghat orographic belt in the northern part and almost north–south in the southern part. It is of passive type, the origin of which can be traced back to its separation from Antarctica around 127 Ma ago. The 2493 km-long shoreline of eastern India is fringed with a continental shelf with a variable width of 35 km off Tamil Nadu to 60 km off north Andhra Pradesh and 120 km around Digha. The shelf has a gentle slope in the northern sector and is moderately steep in the south. Net longshore sediment transport along the east coast is from south to north. The east continental shelf of India is characterized by four major deltas; the Ganges, Mahanadi, Krishna–Godavari and Cauvery. The interdeltaic shelves are generally sediment starved. The inner shelf is silty to clayey silt and sandy, whereas the outer shelf has carbonate sands with coral debris and shell fragments. The outer shelf is characterized by carbonate sands, lime mud and ooids in the north Andhra Pradesh sector off the Kakinada–Kalingapatnam sector. The outer shelf off Chennai–Mahabalipuram, Nagapattinam–Point Calimere is characterized by a thin veneer of authigenic sediment represented by verdine, glaucony and phosphatic sediment. In Tamil Nadu, the region between 10°N and 12°30′N is characterized by two megalineaments and associated tectonics, suggesting that the area is tectonically active. The 14 C dating of relict corals, from water depths of 120 m, off Karaikal indicates an age of 18 390±210 years BP, establishing the low sea-level position of the Last Glacial Maximum. The large volume of sediment input from the deltaic system of two major drainages, Krishna and Godavari, has a significant influence on the morphology and sedimentation of the continental shelf. The influence of glacio-eustasy is noticeable in outer shelf sediments along almost the entire length of the shelf.

Benthic hydroids (Cnidaria, Hydrozoa) from the continental shelf and slope off Queen Mary Coast (East Antarctica)

Hydrozoans are one of the main, and more characteristic, zoological groups in Antarctic benthic communities, yet there are still vast Antarctic areas where the hydrozoan fauna is scarcely known, or even completely unknown, particularly in East Antarctica. The main goal of this study is to contribute to fill that gap of knowledge by studying the benthic hydroids present in the material collected at depths between 404 and 1,156 m off Queen Mary Coast (East Antarctica) during an Australian Antarctic expedition in 2009–2010. A total of 44 species, including Acryptolaria frigida sp. nov. and Halisiphonia prolifica sp. nov., has been found, belonging to the orders Anthoathecata and Leptothecata. The hydrozoan fauna of the studied area is composed of typical representatives of the Antarctic benthic hydroid fauna, with a good representation of some of the most characteristic Antarctic genera, such as Oswaldella and Staurotheca, but with scant representation of others, such as Schizotricha. The dominant species in terms of relative abundance are Symplectoscyphus glacialis with nine records (60 %), Abietinella operculata and Halecium antarcticum with seven records (47 %) and Acryptolaria frigida sp. nov. and Staurotheca nonscripta with six (40 %). A few species were found at a single station, being particularly remarkable the typical Antarctic genus Oswaldella, represented by three species, each present at a single station. Twenty-six species (65 %) are endemic to the Antarctic Region, 22 of them having a circum-Antarctic distribution and four being endemic to East Antarctica. Most of the species studied (93 %) are restricted to Antarctic or Antarctic/sub-Antarctic waters in their distribution.

Recent benthic foraminiferal assemblages and their relationship to environmental variables on the shoreface and inner shelf off Valencia (Western Mediterranean)

The environmental variables that determine the distributions of benthic foraminiferal assemblages on the shoreface and inner shelf of the north of Valencia province (Western Mediterranean) are identified. The possible influence of variables such as water depth, distance from shore, hydrodynamics, substrate type, carbonate content, organic matter content and human activity is evaluated. Multivariate cluster-Q-type analysis and redundancy analysis (RDA) are used to identify the environmental variables that have the greatest influence on the assemblage distribution.The spatial distribution of the assemblages is closely associated with water depth and substrate. The diversity and abundance of foraminifera shells increase with depth and their conservation improves. The most common species in the study area are Ammonia beccarii, Rosalina globularis, Buccella granulata, Planorbulina mediterranensis, and Lobatula lobatula. The presence of wastewater in the study area has not polluted the foraminiferal assemblages (absence of anomalous shells). The direction of the discharge plume is a potential source of nutrients for deep water.

Modification of the Quaternary stratigraphic framework of the inner-continental shelf by Holocene marine transgression: An example offshore of Fire Island, New York

Abstract The inner-continental shelf off Fire Island, New York was mapped in 2011 using interferometric sonar and high-resolution chirp seismic-reflection systems. The area mapped is approximately 50 km long by 8 km wide, extending from Moriches Inlet to Fire Island Inlet in water depths ranging from 8 to 32 m. The morphology of this inner-continental shelf region and modern sediment distribution patterns are determined by erosion of Pleistocene glaciofluvial sediments during the ongoing Holocene marine transgression; much of the shelf is thus an actively forming ravinement surface. Remnants of a Pleistocene outwash lobe define a submerged headland offshore of central Fire Island. East of the submerged headland, relatively older Pleistocene outwash is exposed over much of the inner-continental shelf and covered by asymmetric, sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment. Erosion of the eastern flank of the submerged Pleistocene headland over the last ~ 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface-attached ridges offshore of western Fire Island. West of the submerged headland, erosion of Pleistocene outwash continues in troughs between the sand ridges, resulting in modification of the lower shoreface. Comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of ~ 1000 m westward since formation. Comparison of modern sediment thickness mapped in 1996–1997 and 2011 allows speculation that the nearshore/shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges.

Bio-Optical Properties of the Inner Continental Shelf off Santos Estuarine System, Southeastern Brazil, and their Implications for Ocean Color Algorithm Performance

Optical characterizations of coastal water masses are important tools for a better understanding of physical and biochemical processes and aid the optimization of ocean color algorithms. In this study we present three optical classes of water observed during October/2005 and March/2006 on the inner continental shelf adjacent to Santos Bay (Brazil), based on remote sensing reflectance. ANOVA indicated a crescent estuarine influence in classes 1 to 3. Class 3 presented the highest chlorophyll-a and nutrient concentration and highest light absorption coefficients. Colored dissolved organic matter (CDOM) dominated the light absorption in all classes and was strongly correlated to salinity in October/2005 due to the influence of the La Plata plume. The results indicated that CDOM dynamics in the Santos inner shelf are very complex. The performance of global chlorophyll algorithms was significantly smaller for October/2005 than for March/2006. As inconsistent changes in light absorption spectra by phytoplankton were detected between samplings, the results show that future bio-optical algorithms for this region must be optimized preferentially considering CDOM optical parameters.

Structuring Factors Of The Cumacean Communities Of The Continental Shelf Of Southeastern Brazil

Cumaceans are benthic crustaceans distributed worldwide, from intertidal to abyssal plains. They live in sandy and muddy bottoms and can sometimes be found in the water column. Despite their ecological importance as food items for fishes and other organisms and their usefulness as environmental indicators, very few studies have investigated their biodiversity and distribution in Brazil. The present paper reports an ecological study of the cumacean species found on the continental shelf off Santos, southeastern Brazil. It investigates the composition and spatio-temporal structure of the communities, the main environmental factors responsible for the maintenance of that structure and analyzes the effects of the seasonal oceanographic changes on the community descriptors. A grid of 21 stations located between 10 and 100 m depth was created in winter 2005 and summer 2006 and the sediment samples collected with a 0.1 m2 box corer. Twenty four species were obtained, five of them being the most numerous: Diastylis sexpectinata, D. sympterygiae, Eudorella sp., Oxyurostylis salinoi and Anchistylis notus, the latter highly dominant in summer. Diastylis sympterygiae was characteristic of the shallowest and least saline stations (< 16 m depth; S = 33.07 - 34.2) in both seasons. Of the total of 919 individuals, nearly 70% occurred in summer. The area was divided into four depth bands and no significant temporal difference was found as regards the density distribution of cumaceans in those bands as between the samples of the two campaigns. There were, however, spatial differences within bands in the summer campaign: the higher values of density, diversity and richness were close to the 50 m isobath in both campaigns, a fact that could well be attributed to the ongoing effect of the South Atlantic Central Water in this area. The results showed also that depth and the sediment particle size were the main structural factors determining the composition and distribution of the cumacean species.

Late Pleistocene–Holocene seismic stratigraphy of Nha Trang shelf, central Vietnam

The late Pleistocene–Holocene stratigraphic architecture on the steep and narrow shelf off Nha Trang, central Vietnam has been explored by high resolution seismic profiles integrated with sediment core data. Sequence stratigraphic results reveal five major seismic units and three bounding surfaces which are composed of two distinctive sequences. Those sequences are bounded by two regional unconformities (SB1, SB2) which have been formed in respond to different sea-level regimes. The revealed relict beach–ridge deposits at water depth of about ∼130 m below the present water depth indicate that the Last Glacial Lowstand (LGM) sea-level in this area was lower than in neighboring areas and it probably resulted from subsidence due to high sedimentation rate and/or neotectonic movements of the East Vietnam Fault System. The late Pleistocene high amplitude of sea-level change during a long fourth-order and superimposed by shorter fifth-order cycle is the principal factor in reorganizing the formation of the Nha Trang continental shelf sequence. Other local controlling factors as fluctuations in sediment supply, morphological variations of the LGM surface, subsidence rate and hydrodynamic conditions provided the distinctive features of the Nha Trang shelf sequence stratigraphic model in comparison with neighboring other areas.

Oceanographic conditions over the continental shelf off Magdalena Bay (Mexico) in 2011–2012

The hydrophysical results obtained during six oceanographic cruises in 2011 (February, April, July, and October) and 2012 (February and April) over a 50-station sampling grid in front of the Magdalena-Almejas lagoon system (Mexico) are presented. MODIS-Aqua sensor data (1.1 km) of sea surface temperature (SST) and chlorophyll a, and geostrophic current fields were analyzed during the sampling periods. In the northern part of the study area the pattern of the thermohaline structure shows subsurface intrusion of modified subarctic water, especially during February and April of both years. To the south, in July and October 2011, evident indications of subtropical subsurface water were found. The geostrophic current fields in February and April show a surface southward flow along the coast, which changes direction towards the North Pole in July and October. In spring, the satellite-derived SST showed negative anomalies of up to 4.5 ºC in the coastal zone, corresponding to cold upwelled water rising to the surface from a depth of 60–70 m. Based on the results, the study area can be considered the southern boundary of the transition zone between the subarctic and subtropical waters of the northeastern Pacific Ocean, with marked seasonality.

Use of ship traffic density from Automatic Identification System (AIS) and Vessel Monitoring Systems (VMS) in marine spatial planning: a case study in New England coast, USA

Analyzing current and anticipated uses of ocean and coastal areas based on scientific understanding and information is quite critical to achieve a successful Marine Spatial Planning (MSP). One of key elements in ocean and coastal use is navigation and thus vessel traffic density should be included as part of MSP. One of emerging techniques to quantify ship traffic density is to analyze Automatic Identification System (AIS) and Vessel Monitoring System (VMS) data for a designated area. This study presents a case study conducted in New England area, USA, where the ship traffic density was estimated by analyzing AIS/VMS data. The results of ship traffic density were used in MSP (e.g., Northeast Ocean Data Portal), which were later used by one of stake holders, U.S. Department of Interior, Bureau of Ocean Energy Management when they issued wind lease blocks in outer continental shelf off Massachusetts.

Semi-annual cycle of sea-surface temperature in the East/Japan Sea and cooling process

Semi-annual cycles of the sea-surface temperature (SST) in the East/Japan Sea (EJS) and their forcings were examined by analysing National Oceanic and Atmospheric Administration/Advanced Very-High-Resolution Radiometer data, scatterometer wind vectors, and heat flux data. The semi-annual cycle contributed to the total variance of the SST by 8% and amounted to 25% of the amplitudes of the annual SST cycle, particularly in the Tatarskiy Strait and along the continental shelf off Russia. The lowest phase, corresponding to the minimum SST, occurred during early November and 6 months earlier in May or June depending on the position. The forcings of the semi-annual cycle were not semi-annual but substantially annual with a lag of 6 months, which gave rise to SST cooling in spring and autumn. Our analyses illustrated that SST cooling in autumn was caused by direct, local atmospheric wind forcings, whereas the cooling with large amplitudes of the semi-annual cycle in spring was caused by the non-local, remotely forced cold water advection of the Liman Current associated with sea-ice melting in the Tatarskiy Strait. The phase lag of 1–2 months between the complete melting of the sea ice in the Tatarskiy Strait and the surface cooling along the Russian continental shelf was related to the advection of cold water from sea ice in the form of the Liman Cold Current. The semi-annual cycle also resulted from asymmetry of the time series of the SST due to a long cold winter and a short warm summer. To understand how SST curves are distorted and asymmetric, we suggested two mathematical parameters of kurtosis and skewness. In addition, we suggest that the ratio of the semi-annual amplitude to the annual amplitude of SST harmonics can be used as a typical indicator of the asymmetry in year-to-year SST variations.

Benthic mineralization and nutrient exchange over the inner continental shelf of western India

Abstract. The western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyclical monsoonal influence. In order to understand the impact of the seasonally varying oxygen regime on benthic mineralization, nutrient exchange and, in turn, on the shelf ecosystem, we carried out the first ever intact-core incubations during two contrasting seasons – spring intermonsoon and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on the inner shelf off Goa, dominated by fine-grained cohesive sediments. The results showed that incomplete sediment oxygen consumption (SOC) occurred during April as opposed to the complete SOC and subsequent sulfide flux observed in the fall intermonsoon incubations. The sediments acted as a perennial net source of DIN (dissolved inorganic nitrogen i.e. NO3− + NO2− + NH4+), PO43− and SiO44− to the overlying water column. The efflux of DIN increased from 1.4 to 3.74 mmol m−2 d−1 from April to October, of which NH4+ flux comprised 59–100%. During the oxic regime, ∼75% of diffusing NH4+ appeared to be nitrified (2.55 mmol m−2 d−1), of which ∼77% remained coupled to benthic denitrification. Consequently, 58% of NH4+ flux was lost in active coupled nitrification–denitrification, resulting in substantial N loss (1.98 mmol m−2 d−1) in the sediments. The continental shelf sediments switched over from being a NO3− source during the oxic regime to a NO3− sink during the anoxic regime. During suboxia, benthic denitrification that is fed by NO3- from the overlying water caused N loss at the rate of 1.04 mmol m−2 d−1. Nitrogen loss continued even under sulfidic conditions during October, possibly through the chemolithoautotrophic denitrification, at a potential rate of 3.21 mmol m−2 d−1. Phosphate flux increased more than 4-fold during October as compared to April, due to reductive dissolution of Fe- and Mn oxides. The SiO44− flux increased during October apparently due to the higher availability of siliceous ooze from diatom blooms commonly occurring in the monsoon season. Slow oxidation of organic carbon (Corg) under anoxia, lower temperature and reduced benthic faunal activity appeared to decrease benthic mineralization by 25% as suggested by the drop in the Corg oxidation rate from 63.8 mmol C m−2 d−1 in April to 47.8 mmol C m−2 d−1 in October. This indicated a higher preservation of Corg during the late southwest monsoon. Sediment porosity, Corg content and nutrients did not show significant variations from April to October. Porewaters were found to be enriched with NH4+, PO43− and SiO44− but depleted in NO3− and NO2− in these organic-rich sediments. Significant DIN, PO43− and SiO44− effluxes indicate the potential of benthic input in meeting nutrient demand of the phytoplankton community in this seasonally N-limited shelf system.

Morphology and morphometry of Doryteuthis plei (Cephalopoda: Loliginidae) statoliths from the northern shelf off São Paulo, southeastern Brazil

This study aims to describe and evaluate statoliths of Doryteuthis plei from the northern shelf off São Paulo (23° S), to provide details useful for taxonomic identification, sexual differentiation and estimation of individual size from statolith dimensions. A total of 390 statolith pairs were analysed and their morphology and morphometry were described according to growth and maturity stage. There was a tendency for the extremity of the rostrum and the domes to narrow with growth. A morphological comparison with the statoliths of the congeneric Doryteuthis sanpaulensis was also undertaken. No obvious gross morphological difference among statoliths between sexes was found. A logarithmic model provided the best fit of the relationship between statolith length and mantle length, which can be useful to estimate the squid’s length. Power function equations indicating the allometric relationships are also shown, allowing comparison with other squid populations elsewhere.

Reproductive potential and stock recruitment of the Caribbean and Brazilian metapopulations of the spiny lobster, Panulirus argus (Latreille, 1804)

The spiny lobsterPanulirus argus(Latreille, 1804) is the most important commercial fishing resource in the western central Atlantic and Brazil. Field studies covering the waters off southwestern Cuba and northern Brazil have improved our understanding of the variations in the reproductive potential (number of eggs), stock recruitment and reproductive efficiency of spiny lobsters according to location, depth and size class. Using the spawner-recruitment model, the reproductive potential index was correlated with the index of subsequent recruitment based on field sampling. Spiny lobster habitats in deeper waters need special attention in order to protect the species from overfishing of the recruitment. Considering the longevity and absence of reproductive senility in spiny lobsters, management strategies should ideally include the creation of spawner sanctuaries (marine protected areas) capable of restoring and maintaining the biomass of the spawning stock and the establishment of a maximum catch size of 135 mm (CL) for both sexes along the entire Brazilian coast. Based on our findings, we propose to establish spiny lobster sanctuaries (50-100 m) on the continental shelf off northern Brazil, from Amapá (5°25′N 51°0′W) to the western reaches of the coast of Pará (1°11′N 46°27′W, 0°42′N 46°45′W), covering a total surface area of 64 230 km2.