West Coast Shelf Research Articles

Enhanced Biogeochemical Cycling Along the U.S. West Coast Shelf

AbstractContinental margins play an essential role in global ocean biogeochemistry and the carbon cycle; however, global assessments of this role remain highly uncertain. This uncertainty arises from the large variability over a broad range of temporal and spatial scales of the processes that characterize these environments. High‐resolution simulations with ocean biogeochemical models have emerged as essential tools to advance biogeochemical assessments at regional scales. Here, we examine the processes and balances for carbon, oxygen, and nitrogen cycles along the U.S. West Coast in an 11‐year hindcast simulation with a submesoscale‐permitting oceanic circulation coupled to a biogeochemical model. We describe and quantify the biogeochemical cycles on the continental shelf, and their connection to the broader regional context encompassing the California Current System. On the shelf, coastal and wind stress curl upwelling drive a vigorous overturning circulation that supports biogeochemical rates and fluxes that are approximately twice as large as offshore. Exchanges with the proximate sediments, submesoscale shelf currents, bottom boundary layer transport, and intensified cross‐shelf export of shelf‐produced materials further impact coastal and open‐ocean balances. While regional variability prevents extrapolation of our results to global margins, our approach provides a powerful tool to identify the dominant dynamics in different shelf setting and quantify their large‐scale consequences.

Cetacean species richness in relation to anthropogenic impacts and areas of protection in South Africa's mainland Exclusive Economic Zone

The world's oceans are subject to the multiple impacts of human activity and to the consequent threats to the health of many and varied ocean ecosystems. Oceans around South Africa are no exception and, with the need for economic growth in the country, anthropogenic stressors on ocean resources are rapidly increasing. In this study, we investigated 14 different anthropogenic stressors that impacted ocean health between 2003 and 2013, and their cumulative anthropogenic effects on cetaceans in the South African Exclusive Economic Zone (EEZ) and South African Marine Protected Areas (MPA), Important Marine Mammal Areas (IMMA), Ecologically or Biologically Significant Areas (EBSA) and ecoregions. We determined cetacean species richness in these five area categories using ensemble models, and identified anthropogenic impacts from relevant literature. We calculated and compared the average species richness, the average trend for each anthropogenic stressor and average cumulative impact between 2003 and 2013 in the five areas. Results highlight that climate related stressors (such as sea surface temperature and ocean acidification), together with shipping stressors, are increasing more rapidly than other stressors across the EEZ. Cetacean species richness was highest along the west coast shelf and shelf edge where sea level rise, ocean acidification, shipping, and commercial pelagic fishing with low by-catch were most pronounced. The results of this study will inform marine spatial planners and policy makers in determining priority areas for cetacean conservation and in identifying anthropogenic stressors that need to be addressed to mitigate cumulative anthropogenic impacts on cetaceans.

3D seismic analysis of the AK Fault, Orange Basin, South Africa: Implications for hydrocarbon leakage and offshore neotectonics

A high-resolution 3D reflection seismic dataset was used to achieve a detailed description of the half-graben bounded AK Fault in the Ibhubesi gas field of the offshore Orange Basin. Seismic analysis of the fault involved the combination of both the conventional seismic interpretation and seismic attributes. The main objective of the study was to provide insights into hydrocarbon leakage and neotectonic activity along the west coast shelf of South Africa. The AK Fault is among the several half-graben bounded faults that formed in the Late Jurassic to Early Cretaceous during the separation of the South American and the African continents. The AK Fault consists of five seismically detectable segments, whose evolution reflects characteristics of both the “coherent” and the “isolated” end-members model of segmented fault origin. Diapiric structures that are direct indicators of vertical fluid movement were also observed along the AK Fault system, providing evidence of significant roles that the fault plays in promoting the vertical migration of hydrocarbon in the Orange Basin. Recent reactivation of the fault has also occurred along an oblique extensional regime that is dominated by normal dip-slip faulting, with minor shear components along the tips.

Differentiation of three common deep-water hermit crabs (Crustacea, Decapoda, Anomura, Parapaguridae) from the South African demersal abundance surveys, including the description of a new species of Paragiopagurus Lemaitre, 1996.

Deep-water hermit crabs of the family Parapaguridae can be abundant (up to 20 kg or 1000 hermit crab individuals per haul) in the trawl bycatch collected during South African demersal abundance research surveys. Until recently, only two parapagurid species had been recognized in the bycatch; Parapagurus bouvieri Stebbing, 1910, and Sympagurus dimorphus (Studer, 1883). Detailed examination of numerous samples of parapagurid specimens from research surveys revealed the existence of a third, undescribed species previously confounded with S. dimorphus, but in fact belonging to a different genus. This new species, Paragiopagurus atkinsonae sp. n. is the 25th in the genus Paragiopagurus Lemaitre, 1996, and has been found only in a small region on the West Coast shelf of South Africa, at depths of 199–277 m. The species is herein fully described and illustrated, including colour images, µCT scans of selected body parts, and CO1 barcode data. The new species is morphologically most similar to P. ventilatus Lemaitre, 2004, a species associated with hydrothermal vents, but differs in armature of the fourth antennal segment (armed with a spine on the dorsolateral distal angle vs. unarmed in P. ventilatus); setation of the antennal flagella (nearly naked vs. with dense setae in P. ventilatus); plumose setation on the third maxillipeds and basal segments of chelipeds (absent vs. present in P. ventilatus); number of rows of scales on the propodal rasp of pereopod 4 (two or three rows vs. one row in P. ventilatus); and degree of telson asymmetry (weakly asymmetrical vs. strongly asymmetrical in P. ventilatus). Paragiopagurus atkinsonae sp. n. is superficially similar to S. dimorphus, with males of the two species showing the same extreme degree of sexual dimorphism on the right cheliped, general light orange colouration, and frequent use of colonial zoanthid carcinoecia for pleonal protection. To aid in future identifications and to facilitate data gathering during surveys, a comparison of P. atkinsonae sp. n. with S. dimorphus is provided, along with descriptions of colouration and photographs of live specimens of all three parapagurid species. Information on taxonomy of the species is summarized, as well as knowledge of their distribution in the demersal research survey regions of South Africa.

Oxygen‐depleted bottom waters along the west coast of South Africa, 1950–2011

AbstractAs part of the southern Benguela upwelling ecosystem, South Africa's west coast represents a very productive marine habitat. Decadal‐scale environmental and ecosystem regime shifts have been documented based on wind and ocean temperature data, as well as biota. This study analyses the variability of oxygen‐depleted (<2 mL L−1) bottom waters on South Africa's west coast shelf using data from St Helena Bay, the most productive embayment in the ecosystem, from 1957, data from demersal and pelagic fisheries surveys since the 1980s, and monthly observations from a fixed transect off St Helena Bay from 2000 to 2011. Oxygen‐depleted waters are generally restricted to bottom depths <150 m, and occur regularly during austral summer and autumn, albeit with variable severity. The high spatial variability makes it impossible to define any single station as an indicator station for the extent of oxygen depletion. We provide estimates of the shelf area, 30–33°S, covered by oxygen‐depleted bottom water in austral summer and autumn, and show a parabolic relationship between oxygen depletion and upwelling strength. The results suggest that the cumulative upwelling divergence from the onset of the upwelling season (September) to the establishment of oxygen depletion (January) can be used as a proxy indicator for oxygen depletion in summer. Periods of better bottom ventilation are indicated for the second half of the 1960s to the early 1980s; more extensive oxygen depletion at the beginning of the 1950s, and the mid‐1990s to the mid‐2000s. Implications for environmental monitoring and marine living resources are discussed.

An assessment of the current status of the chokka squid fishery in South Africa and an evaluation of alternative allocation strategies

The South African squid fishery is based on a single species, Loligo reynaudi, locally referred to as chokka and commonly found around the Agulhas Bank and West Coast shelf of South Africa. The existing long term rights for participation in the fishery expire at the end of 2013 and a new set of rights will have to be issued. Progress made to date in achieving the objectives of the South African Marine Living Resources Act of 1998 (MLRA), including progress towards greater equity, will be an important consideration in allocation decisions. The draft Small-Scale Fisheries Policy, scheduled to be adopted in 2013, could also influence the criteria to be used. This study updates the information on the current structure and functioning of the squid jig sector and considers the likely performance of two alternative access allocation scenarios against the current and evolving policy goals. The study reaffirms that the squid fishery is an important contributor to human well-being in South Africa and particularly in the Eastern Cape Province but there is scope for improvement in the extent of transformation and equity. The study concludes that, as a result of a common lack of experience and capital amongst potential new entrants, the risks associated with transformation in this fishery are likely to be high, especially if the option of allocating rights to community-based legal entities was to be used. The risks could be reduced by ensuring that a number of pre-conditions were met. These include, amongst others, not allowing total effort in the fishery to increase; ensuring effective management by the responsible government authorities; adequate capacity building of new entrants, whether individuals or cooperatives; and ensuring new entrants have viable business plans.

Demersal fish diversity of the isolated Rockall plateau compared with the adjacent west coast shelf of Scotland

This study compares the diversity of the demersal fish assemblage of an isolated shelf sea, the Rockall plateau, with that of the nearest adjacent continental shelf sea ecosystem, the west coast of Scotland. Bottom trawl surveys were carried out between 1986 and 2008 on the Rockall plateau and the adjacent shelf sea to the west of Scotland. All demersal fish were identified and counted. Analyses of species richness and abundance were undertaken. The fish assemblage of the Rockall plateau was less diverse and the proportional representation of species was markedly different. A number of species common at Rockall were rare on the west coast shelf and, in general, there were fewer common and temporally stable species and more rare and ephemeral species at Rockall. Some species absent from Rockall have life-history stages associated with inshore habitat. The Rockall plateau fish assemblage can be described as an impoverished subset of the north-west European shelf sea fish assemblage. It is likely that there are constraints on diversity imposed by the relatively small area of the Rockall plateau and its degree of isolation by depth, distance and ocean currents. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 138–147.

A review of the biology and ecology of Calanus agulhensis off South Africa

Calanus agulhensis is the dominant large copepod on South Africa's Agulhas Bank, and is an important food item for pelagic fish. This paper reviews current knowledge and understanding of its biology and ecology and includes comparisons with other Calanus species. Its distribution is influenced by the prevailing hydrography, being advected from the eastern to the western Agulhas Bank and then north along the edge of the west coast shelf. Highest biomass of C. agulhensis is on the central Agulhas Bank in the vicinity of a cool ridge of upwelled water that is thought to enhance local retention. Daily vertical migration on the Agulhas Bank is linked to food concentration, with a strong correlation between the extent of migration of large stages (C4-female) and food abundance in the chlorophyll-rich layer. A diel feeding rhythm has been observed, independent of whether or not animals are migrating vertically. Females offered natural assemblages show a preference for the larger particles of the dominant size classes. Development time from egg to adult (20.3 d at 15.5°C) is fast compared with other Calanus species. The long N3 stage duration suggests that it is the first feeding stage. Isochronal growth is approximated, but the equiproportional rule is not adhered to. In the field, growth rate is influenced more by food than by temperature, particularly larger stages that are more frequently food-limited. This is probably because the small phytoplankton cells that dominate at warm temperatures are generally at low concentrations (<2 mg chlorophyll a m−3), insufficient for fast growth of large stages. Egg production following short periods (1–3 d) of starvation returns quickly to normal on the reintroduction of food, an adaptation that may be beneficial in the relatively stable food environment of the Agulhas Bank. Following long periods (9 d) without food, however, many females are unable to regain normal levels of egg production. At present, there is no information on the extent of omnivory in C. agulhensis, and little on the ecology of the naupliar and young copepodite stages. These may be fruitful areas for future research.

Hydrographic observations during the Tasman boundary experiment off the west coast of South Island, New Zealand

Abstract Hydrographic observations over the west coast shelf during the Tasman Boundary Experiment (TASBEX) in 1986/87 are presented. Temperature dianges over the shelf were related to mixed‐layer development, shelf waves, and seasonal changes, with wind‐driven upwelling being important close to the coast. During upwelling, a surface coastal current was observed and at times a weak undercurrent or countercurrent developed. Changes in freshwater inflow resulted in changes in the depth of the low‐salinity surface layer. In the north, flows were generally weaker than in the south, where the shelf is narrow and the Southland Current dominates. Changes in the position and strength of the Southland Current were observed and on one occasion the response to a wind change event was a relaxation of the flow to a coastal Southland Current regime.

Hydrographic parameters as indicators of the suitability of phytoplankton populations as food for herbivorous copepods

Data collected off the south-west coast of South Africa suggest that easily measured hydrographic or biological parameters may be used as indicators of the suitability of the food environment for those mesozooplankton which preferentially utilize phytoplankton >10 μm in size. It is proposed that, in the southern Benguela during the upwelling season, there is a temperature window from 12 to 15°C when conditions are generally good for diatom population development and chlorophyll concentrations exceed 3 mg·m−3. At lower (<12°C) or higher (>15°C) temperatures, chlorophyll concentrations are <mg·m−3. There is generally a shift in size structure of the phytoplankton population at about 15°C, high biomass, diatom-dominated populations being replaced by low biomass, flagellate-dominated populations. At temperatures between 12 and 15°C when chlorophyll concentrations exceed 3 mg·m−3, the nutritional requirements for egg production by a common herbivorous mesozooplankter will be satisfied. Conversely, at lower (<12°C) or higher (>15°C) temperatures and lower concentrations of chlorophyll, the fecundity of this same mesozooplankter will be reduced severely. These parameters, in conjunction with satellite imagery, may be used to assess minimum potential secondary production of mesozooplankton in South African west coast shelf waters.

Feeding of hake and a first attempt at determining their trophic role in the South African west coast marine environment

During intensive biannual demersal fish-sampling cruises since January 1983 over the South African west coast shelf region (Orange River to Cape Agulhas, coast to 500-m isobath), feeding studies on both species of Cape hake (Merluccius capensis and M. paradoxus) were undertaken. These studies were of two forms, random sampling of the dietary preference and constituents of the hake caught during daylight only and intensive sampling of stomachs with food throughout a 28-hour cycle. From the index of relative importance, crustaceans were shown to be most important in the majority of length classes of hake studied, with mesopelagics (fish and cephalopods) and other hake also important. Differences by hake species, by predator size, by geographical location and by season were evident. The Cape hakes, which constitute some three-quarters of the demersal fish biomass in the area, are deduced to play a major role in the marine faunal environment, both as predators and prey, and some base-line figures of the quantities of different food items consumed by the resource are presented. Hake are largely opportunistic feeders and it is concluded that they can adapt to perturbations in the availability of prey, whether they be triggered by environmental cause or by, for example, purse-seining of particularly mesopelagic fish.

The Coastal Ocean Dynamics Experiment collection: An introduction

The Coastal Ocean Dynamics Experiment (CODE) was designed to identify and study those dynamical processes which govern the wind‐driven motion of water over the continental shelf. The initial effort in this multiyear, multi‐institutional research program was to obtain high‐quality data sets of all the relevant physical variables needed to construct accurate kinematic and dynamic descriptions of the response of continental shelf waters to strong wind forcing in the synoptic band covering 2‐ to 20‐day variability. Two small‐scale, densely instrumented field experiments, each approximately 4 months long, were conducted in spring and summer 1981 (CODE 1) and 1982 (CODE 2). A more sparsely instrumented, long‐term, large‐scale component was also conducted in conjunction with a separate but related Large‐Scale West Coast Shelf Experiment (informally called “SuperCODE”) to help separate the local wind‐driven response in the region of the small‐scale experiments from motions generated in some distant region along the coast and to investigate the seasonal cycles of atmospheric forcing, water structure, and coastal currents between 35° and 48°N.

Sediment texture of the continental shelf and upper slope off the west coast of South Island, New Zealand

Abstract Surface sediments on the continental shelf and upper slope off the west coast of the South Island, New Zealand, from north of Karamea Bight (40°45'S) to the Whataroa River mouth (43°07'S) consist almost entirely of sands and muds. Inshore (< 50 m depth) sediment is predominantly fine sand to coarse silt. Silt‐clay content increases towards mid‐shelf depths, and areas of mud occur at depths of about 90–140 m. Sediments tend to become coarser again on the outer shelf, and sandy mud or muddy sand covers the outer shelf and upper slope. The west coast shelf experiences a high input of fluvial sediment, probably c. 12–26 × 106 tonnes y‐1 for the study area. Sedimentation rate on the shelf is estimated to average at least 1–2 mm y‐1. Sedimentation is probably appreciably lower than this in the north of the area and higher in the south, and this is reflected in shelf grain‐size parameters and sediment carbonate content. There are north‐to‐south decreases in mean grain size and in the ranges of sorting a...

The signature of El Niño off Oregon, 1982–1983

Current and conductivity, temperature, and depth (CTD) measurements were made over the Oregon shelf near 43°N between February 1981 and April 1984 as part of a large‐scale west coast shelf experiment (SuperCODE). The data set includes a nearly continuous record of current velocity and temperature over the continental shelf off Coos Bay from May 1981 through January 1984, CTD sections off Coos Bay in January or February of each year from 1981 to 1984, and CTD sections off Newport (44.6°N) in April 1983, July 1983, and April 1984. The latter are compared with sections off Newport made during the previous two decades. Sea level from the Newport tide gage, daily sea surface temperature at Charleston (43.3°N), the alongshore component of the wind stress at 45°N and the large‐scale North Pacific atmospheric pressure pattern provide a climatological perspective. The initial manifestation of El Niño off Oregon was in October 1982: anomalously high sea level, high coastal sea surface temperature, and increased poleward flow. These effects occurred within 1 month of the onset of El Niño off Peru and preceded any local (North Pacific) atmospheric effect by 2–3 months. The anomalous local meteorological conditions, which became manifest in December and January, greatly enhanced the initial effects and inserted their own signal. The first signals of El Niño probably arrived by an oceanic path, but there is no doubt they were subsequently reinforced by anomalous atmospheric conditions.