1,018 research outputs found
Development Of A Preliminary Lifing Analysis Tool For The F135-PW-100 Engine
No full textIn the near future the Royal Netherlands Air Force will replace their fleet of F-16’s with the F-35. In the past the NLR has aided the Air Force with life cycle and deterioration analysis work on the F100-PW-220 engine, which powers the F-16. Understanding the physical system of the engine allows for on-condition maintenance. The same is preferred for the F135-PW-100 engine powering the F-35. Therefore, a preliminary lifing analysis tool has been developed for the F135-PW-100 engine rotor blades, based on open source literature. Aerospace Engineerin
Stable isotope (δ13C) composition of the food web of the temperate Kariega estuary (Eastern Cape)
No full textStable isotope (δ13C) composition of the food web in the temperate Kariega estuary in summer and winter was analysed to determine the trophic relationships between 19 taxa. There was no seasonality in the stable isotope ratios of the dominant producers and consumers in the estuary. Two distinct zooplankton assemblages were identified. The first assemblage, comprising the different developmental stages of the copepods Acartia longipatella, A.natalensis and Pseudodiaptomus hessei and the mysid Mesopodopsis wooldridgei, had intermediate (δ13C)values ranging between -16 and -18. These values indicate a carbon source relatively depleted in carbon, most likely particulate organic matter derived mainly from the salt marsh and riparian vegetation. The second zooplankton assemblage, consisting of the caridean shrimp Palaemon peringueyi, the cumacean Iphinoe truncata and the amphipod Grandidierella lignorum, had (δ13C)values (range -9 to -18) enriched in carbon. The most likely sources of carbon for these zooplankters were the eelgrass Zostera capensis, its associated epiphytes and/or Spartina maritima. The isotope compositions of the seven most abundant fish species were also divided into two groups. The first group, comprising Atherina breviceps and Gilchristella aestuaria, had isotope values ranging between -14 and -1518, suggesting that they consume mainly zooplankton. The (δ13C)values of the second group, comprising Rhabdosargus holubi, Liza richardsonii, L. dumerilii, Caffrogobius natalensis and C. gilchristi ranged between -11 and -1218. These values suggest that these species assimilate a carbon source enriched in carbon derived either from eelgrass, Zostera capensis, its associated epiphytes or S. maritima. Results of the study suggest that there are two main carbon pathways for plankton and nekton in the Kariega estuary, carbon derived from the eelgrass and its associated epiphytes and carbon which has its origins in the salt marsh riparian vegetation and zooplankton.
Keywords: stable isotope analysis; temperate estuary; plankton; South Africa
(Afr J Aqua Sci: 2001 26(1): 49-56
Population demographics of<i>Palaemon peringueyi</i>(Macpherson 1990) (Decapoda: Palaemonidae) in a small intermittently open Eastern Cape estuary
No full textFood web structure in three contrasting estuaries determined using stable isotope (δ13C) analysis
No full textFood web structure in three contrasting estuaries, the freshwater-deprived Kariega, the freshwater-dominated Great Fish River and the temporarily open/closed Kasouga estuary, along the south-east coast of southern Africa was investigated in summer (14 November) 2000 by employing stable carbon (δ13C) isotope analysis. Results of the analysis indicated that there were distinct differences in the primary sources of carbon utilised by zooplankton in the three estuaries. In the Kariega estuary two distinct zooplankton feeding guilds were identified, the first having moderate (δ13Cvalues ranging between -16 and -18, the second having more enriched (δ13C values varying between -10 and -12. Zooplankters of the first guild utilised carbon derived mainly from the microbenthic algae, whereas those of the second guild appeared to obtain theirs from eelgrass, Zostera capensis, and its associated epiphytes. In the temporarily closed/open Kasouga estuary, zooplankton had isotopic values ranging between -17.8 and -18.1. The most likely source of carbon for the zooplankton in this estuary is the extensive microphytobenthic alga stocks usually recorded in the system. In contrast to the freshwater-deprived estuaries, the depleted isotopic values of the zooplankton samples (-28 to -26) from the Great Fish River estuary suggest that the primary source of carbon consumed there was derived from phytoplankton. Data obtained in this study are in agreement with those of previous studies conducted in the region and highlight the importance of freshwater input in determining the trophodynamics in estuarine systems in southern Africa.
Keywords: estuaries; food webs; isotope analysis
(Afr J Aqua Sci: 2002 27(2): 107-115
Stable isotope (∂<sup>13</sup>C) composition of the food web of the temperate Kariega estuary (Eastern Cape)
No full textSeasonal changes in selected physico-chemical and biological variables in the temporarily open/closed Kasouga estuary, Eastern Cape, South Africa
No full textSeasonal changes in selected physico-chemical (temperature, salinity and turbidity) and biological (chlorophyll-a (chl-a), microphytobenthos and microheterotrophs) variables were measured at 79 day intervals for a period of 14 months in the temporarily open/closed Kasouga estuary. In addition, zooplankton biomass was determined monthly over the same period. During the course of the study, 13 overtopping and two breaching events were documented. Freshwater inflow into the estuary resulted in a decrease in salinity coupled with increases in total suspended solids (TSS), particulate organic matter (POM) and turbidity. Chlorophyll-a ranged between 0.29 and 8.01mg chl-a m-3 and microphytobenthic biomass between 3.87 and 209.09mg chl-a m-2. Generally, the highest chl-a concentrations were recorded during the wet season (spring to early summer). In contrast the microphytobenthic biomass attained highest levels during periods of little or no freshwater influx into the estuary. The influx of freshwater into the estuary resulted in a change in the size structure of the phytoplankton community from one dominated by nano- (220m) and picophytoplankton (20m). Densities of microheterotrophs (5 000cells l-1) recorded during winter. Total zooplankton biomass ranged between 15.91mg Dwt m-3 and 106.93mg Dwt m-3 and demonstrated a similar pattern to chl-a with maximum biomass recorded during the wet season. These data indicate that riverine input into the temporarily open closed Kasouga estuary is important for estuarine structure and function.
Keywords: temporarily open/closed estuary; plankton; Kasouga; South Africa
(Afr J Aqua Sci: 2002 27(2): 117-123
The ecological role of the suspension feeding bivalve, Austrovenus stutchburyi, in estuarine ecosystems
No full textSuspension feeding bivalves are described as key species in many aquatic ecosystems, where they can influence primary productivity and nutrient dynamics, are food for higher trophic levels, and may be harvested in commercial or recreational fisheries. In many areas where bivalve populations have declined, substantial changes to ecosystem structure and function have occurred. High-density beds of the infaunal suspension feeding bivalve, Austrovenus stutchburyi, are a dominant feature on intertidal flats in many New Zealand estuaries, but populations are declining in some areas, likely due to anthropogenic stressors such as overharvesting and sedimentation. This thesis examines the influence of Austrovenus on estuarine ecosystem function using laboratory, field and modelling studies.
The effects of bed density and flow speed on boundary layer dynamics and Austrovenus clearance rates (CR) were investigated in annular flumes. Bed shear stress increased with increasing bivalve density, and under certain conditions was also increased when Austrovenus were feeding, compared to when they were not. The bed roughness and siphonal currents generated by the bivalves may therefore influence food supply to the bed. Both individual and bed CR were significantly greater at the high flow speed. Bed CR did not scale proportionally with density, in part because individual CR decreased with increasing density, but also because the proportion of bivalves actively feeding decreased at high densities. Thus, scaling up individual CR obtained from animals at low densities will significantly overestimate population filtration capacity.
The effect of Austrovenus on ecosystem function was examined at two sites, one sandy, the other composed of muddy-sand, to determine whether sedimentary environment alters this key species’ role. Gross primary production (likely corresponding to microphytobenthos production) and ammonium uptake were significantly increased, and denitrification potential was also elevated, when Austrovenus was added, relative to removed, at the sandy site. In contrast, there was no effect of Austrovenus on any of these variables at the muddy-sand site. These results reveal the importance of considering sedimentary environment when examining the effect of key species on ecosystem function, and suggest that even moderate levels of sedimentation may reduce the positive effect of Austrovenus on primary productivity.
A food-web model was used to quantify the interactions between Austrovenus and other estuarine species, and to determine the effect of reducing Austrovenus biomass on ecosystem properties. The model revealed an estuarine system dominated by benthic species, characterised as a developing, rather than a mature ecosystem. Detritus production and export were high, while transfer efficiency and internal recycling were low. Decreasing Austrovenus biomass decreased system maturity, suggesting that reductions in Austrovenus populations may decrease ecosystem stability. The model also confirmed that within these estuarine systems Austrovenus and microphytobenthos are key species which impact greatly on all other trophic levels.
This thesis demonstrates that Austrovenus populations exert considerable influence on benthic communities and processes, and play a key role in the functioning of estuarine ecosystems. Reductions in Austrovenus populations will likely negatively impact on microphytobenthos and higher trophic levels, and may also reduce ecosystem maturity and stability
Euphausiid population structure and grazing in the Antarctic Polar Frontal Zone — austral autumn 2004
No full textThe euphausiid community structure and grazing dynamics were investigated in the West Indian sector of the Polar Frontal Zone during the austral autumn 2004. Subsurface (200m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Subantarctic Front and the Antarctic Polar Front bisected the survey area into two distinct zones, the Subantarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll a (Chl a) biomass was typical for the region ( 0.05). A multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, and (2) restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. Euphausia triacantha had the highest daily ingestion rate, ranging from 1 226.1ng pigment (pigm) ind–1 day–1 to 6 029.1ng pigm ind–1 day–1, whereas the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4–943.0ng pigm ind–1 day–1). The total grazing impact of selected euphausiids ranged fro
Fish recruitment into a South African temporarily open/closed temperate estuary during three different hydrological mouth phases
No full textPresence of microplastics in the tube structure of the reef-building polychaete <i>Gunnarea gaimardi</i> (Quatrefages 1848)
No full text- …
