Total electron content at low latitudes

Radio beacon from ATS-6 at 140 MHz was used to measure the changes in the polarization angle (Faraday rotation) at Bombay, Rajkot, Ahmedabad, Udaipur and Patiala during October 1975 to July 1976. In this paper, results of diurnal, seasonal and latitudinal variations in total electron content (TEC) derived from these measurements are reported. The amplitude of diurnal peak is found to be higher at Rajkot, Ahmedabad and Udaipur as compared to that at Patiala or Bombay, indicating that the peak of Appleton anomaly in the latitudinal variation of TEC was close to the latitude of Ahmedabad. The diurnal maximum of TEC occurs around the same time during summer and winter months. The peak electron content shows a semiannual variation at all the stations with large values in equinoxes as compared to winter and summer. The TEC at Bombay shows a seasonal anamoly with high values in winter as compared to summer. The paper describes the development of latitudinal anomaly with the time of the day for different seasons. This anomaly is maximum during 1000 to 1800 LT and is located between 12° and 14° N (dip latitude) in summer and equinoxes and at about 10°N in winter.     

Saldanha Bay,South Africa III: new production and carrying capacity for bivalve aquaculture

Measurements of NH₄, NO₃, urea and HCO₃ uptake using 15N and 13C stable isotope tracers were undertaken in Saldanha Bay, South Africa, between January 2012 and January 2013. These studies provide the first direct measurements of N utilisation by the plankton in the bay. Primary production in the bay is driven predominantly by the advection of nutrients from the neighbouring shelf environment during upwelling events, with terrestrial and other sources providing minor inputs. New production (NO₃-based) was calculated from the f-ratio and total primary production and was used to provide estimates of potential carrying capacity for bivalve culture. Despite the apparent light limitation of NO3 uptake in the winter, the availability of NO₃ appeared to exert the major influence on new production throughout the year. In addition, new production was modulated by NH₄ availability as shown by the suppression of NO₃ uptake by concentrations higher than 1?1.5?mmol m^?3. The estimated areal new production of 0.60?g C m^?2 d^?1 yielded a bay-wide annual estimate of 9 811 t C ha^?1 y^?1, slightly higher than previous calculations based on physical models. It is estimated that the total annual production of mussels and oysters, respectively, for a 1 000-ha cultivation area is approximately 40 000–53 000 t y^?1 (mainly Mytilus galloprovincialis) and 4 600–6 000 t y^?1 (Crassotrea gigas). The combined total production figures constitute only 24–31% of the surplus new production. A combined harvestable carrying capacity of 74 000–82 000 t y^?1 can be calculated from this surplus. However, from a management and ecological perspective, bivalve culture should be limited to well below this theoretical maximum. Even with this constraint, there appears to be considerable scope for expansion of bivalve farming over the modest, present levels with little jeopardy to ecological integrity.