The benthos of the Siyaya estuary : species composition, density and distribution
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Date
1996
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Abstract
This study describing the zoobenthos of the Siyaya Estuary is the result of data collection spanning three consecutive years (1992 - 1994). The investigation of the zoobenthic component of the estuary, forms part of a multidisciplinary study to monitor the effects of catchment rehabilitation. The abiotic and biotic characteristics of the estuary have been used as a tool to determine the effectiveness of improved management practices in the catchment. The response of the zoobenthos was first examined in 1983, and this showed 37 zoobenthic taxa with a strong marine/estuarine component.
From 1992 to 1994, the zoobenthos was sampled on a seasonal basis at each of five sites along the length of the estuary. A total of 88 taxa representing four phyla were collected over three years. During 1992, 50 taxa were recorded in samples and the impression was of a wide faunal assemblage, with representatives from Nematoda, Annelida, Crustacea, Insecta, and Gastropoda. Five less taxa were recorded during 1993, although the faunal assemblage was still fairly wide with the inclusion of several more insect taxa. The greatest number of taxa were recorded during 1994. Of the 59 zoobenthic taxa, 40 represented the freshwater component. Over the three year period, few decapod invertebrates were sampled, the majority were primarily post prawn larvae. Results of this study have therefore shown that the number of taxa have increased. However, the majority of new taxa added to the total species list belong to the freshwater component of the zoobenthos. This is due to prolonged closure of the estuary mouth as a result of the prevailing drought conditions.
A comparison of zoobenthic densities from the estuarine and freshwater components revealed that the estuarine component still dominated the benthos, and that this dominance decreased from 1992 to 1994. During 1992, the estuarine component constituted 97.2% of the total zoobenthos, while this decreased to 84.6% and 54.4% in 1993 and 1994, respectively. Of the overall mean density of 29 623 invertebrates m"2 calculated from the sum of five sites, on a seasonal basis over three years, 48.7% (J4 433 m"2) of the total was from 1992. Zoobenthic densities declined in 1993 and 1994, to
28.2% (8 340 m"2) and 23.1% (6 850 m"2) of the total for the three years. Zoobenthic densities were generally highest in winter, and lowest in autumn. However, a multifactor ANOVA showed that season alone was not a significant factor governing the increase or decrease of zoobenthic densities. The relict estuarine amphipods Granclidierella lignorum and Corophium triaenonyx, and the tanaid Apseudes digitalis were among the taxa dominating the benthos in all seasons.
Various parameters were used to determine the physico-chemical condition of the estuary, and several were used as an input matrix to determine their effects on the distribution and abundance of the zoobenthos. Salinity, temperature, dissolved oxygen concentration, depth and turbidity were measured each year. In 1994, a more detailed water quality and sediment analysis was performed. A gradient of turbidity, salinity and oxygen existed from the upper to lower reaches of the estuary, and the substratum constituted medium sands in the lower reaches to detrital muds with silt patches in the upper reaches. The estuary became increasingly fresh over the study period, as salinity declined in the upper reaches from 6%o in 1992 to 0%o at the end of 1994. A suite of multivariate techniques involving classification and ordination methods revealed that sediment particle size was not the most important environmental factor determining species distributions. A combination of turbidity, dissolved oxygen, pH and the percentage organic content proved to be the most important effect of the environment, accounting for most of the variability in the distribution and abundance of the zoobenthos.
On the basis of the results obtained, it is concluded that the effects of improved catchment management practices have had a positive effect on the ecology of the estuary, and particularly on the state of the zoobenthos. However, the prevailing drought conditions also had an effect on the benthos, in terms of a change in species composition from 1992 to 1994. Despite this, the estuarine taxa still dominate the estuarine benthic fauna.
Description
Submitted to the Faculty of Science in fulfilment of the requirements for the degree of
Master of Science in the Department of Zoology at the University of Zululand, 1996.
Keywords
Benthos., Estuaries -- South Africa -- KwaZulu-Natal, Zoobenthos