| Popis: |
This thesis presents the results of three studies aimed at improving brine-quality at the Swartkops solar salt-works (Swartkops Sea Salt [Pty] Ltd) on the outskirts of Port Elizabeth, South Africa. This is a highly eutrophic salt-works, the management of which has become increasingly difficult in recent years. The fundamental problem is how best to operate the system at maximum capacity while limiting nutrient inputs from the nutrient-rich microtidal Swartkops Estuary. In the first study, brine-quality at several sites along the axis of the Swartkops Estuary, and the extent to which it is affected by a variety of factors, is compared. Sites were sampled on micro- and macrotidal time scales, and were selected by the management of the salt-works as possible locations for a new pump-house (for extracting brine from the estuary) for their salt-work operations at Swartkops and Missionvale. The study showed that there are incremental benefits to be had from moving the site of extraction downstream from its present position to a site closer to the mouth of the estuary, where the concentration of nutrients usually is lower and where salinity usually is higher. There is little to be gained from moving the site of extraction laterally, to the mouth of the Inlet from which brine currently is extracted, so that brine is extracted directly from the estuary itself. A set of models relating the concentrations of NH+ 4, NO{u100000}3, and PO34{u100000} to salinity is proposed. These take into account the influences of site and season and may be used to estimate the concentration of these nutrients from a measurement of salinity. The model for PO34{u100000} shows that it would be more damaging to the salt-works’ operations to pump “low”-salinity brine during the early months of summer than during autumn. Evidence is also presented to show that Wylde Bridge has no influence on nutrient concentrations in the estuary, with tidal flushing generally passing beyond the Wylde-Bridge break-point. The exceptionally heavy flooding of the estuary that occurred in September 2002 may, however, have biased this conclusion, because of its scouring effect. The second study concentrated on monitoring the effect of (1) decreasing pond depth and (2) increasing pond salinity—two readily available management tools—on brine quality at the salt-works. Pond depth throughout the salt-works was decreased by 40 cm, and the salinity of Pond 5, a pond in the middle of the system, was increased to 175 S. Both measures were kept in place for the duration of the study (Nov. 2002–Aug. 2004). The pond-depth experiment did not have the expected result, there being no evidence of the increase in microalgal growth in the water column that was predicted based on previous research. There was, however, a significant increase in benthic chlorophyll-a, and there was a general improvement in the condition of the sedimentary system of the salt-works. There was also a substantial decrease in particulate organic matter in the water column, with clear evidence that the remaining fraction was closely associated with living forms of particulate matter rather than with detritus. The pond-salinity experiment proves that there is a flourishing, and resilient, population of brine shrimp (Artemia salina L.) at the salt-works. Restocking the salina, or stocking it with a different strain of brine shrimp, is therefore not necessary. The results of this study show that the brine shrimp population at the salt-works needs salinities of greater than about 65–70 S to survive. As a living force they almost certainly need a protective salinity that is greater than about 120–140 S, perhaps even as great as 160 S. Brine shrimp thrived in the high salinity milieu of the experimental pond for the duration of the study, but dwindled from three other ponds of the system once their salinities fell to below 90 S, eventually to disappear from them, apparently completely, once salinity fell to below 65 S. The third and final study concentrated on establishing whether the products released by decomposing barley straw could be used in a solar salt-works to control macroalgal blooms without detrimentally affecting the benthic-mat. Previous research has shown that these products are effective inhibitors of macroalgal growth and that they remain effective under saline conditions. The results presented here show that the same products, or products released under similar conditions of decomposition, adversely effect both the structure and the function of the mat. Consequently, their use in a solar salt-works cannot be recommended. |
