Distribution and Abundance of the Macrobenthic Fauna of the Kariega Estuary

S. Afr. J. Zool. 1987, 22(2) 153 Distribution and abundance of the macrobenthic fauna of the Kariega estuary A.N. Hodgson Department of Zoology and Entomology, Rhodes University, Grahamstown, 6140 Republic of South Africa Received 23 October 1986; accepted 8 December 1986 The Kariega estuary is an open estuary which is about 18 km in length. The freshwater inflow into the estuary is very sporadic and consequently the water Is often hypersaline at the head. Quantitative and qualitative sampling has been undertaken to describe the distribution and abundance of the macrobenthos along the estuary. Of the 107 species that have been recorded, 800/0 can be classified as euryhaline. One of the possible reasons for the high species diversity, when compared to other eastem Cape estuaries, is the growth of Zostera capensis along the entire length of the estuary. The major contributors to the macrobenthic biomass are the crustaceans, Cleistostoma edwardsii, C. algoense, Upogebla afrlcana, Sesarma catenata and Uca urvillei and the bivalve Solen cylindraceus. The latter is particularly abundant in the middle reaches of the estuary where it can reach denSities of 400 m-2• Its successful colonization is attributed to food availability and stable physical conditions within the estuary which in tum can be linked to the low freshwater input. Die Kariegariviermond is 'n oop riviermonding ongeveer 18 km lank. Die varswaterinstroming in die monding geskied baie sporadies en die water aan die bopunt Is gevolglik dikwels hipersoutagtig. Die verspreiding en voorkoms van die makrobentos in die monding is deur kwantitatiewe en kwalitatiewe monsterneming bepaal. Van die 107 spesies wat opgeteken is, kan 80% as eurlhallen beskryf word. Een van die moontlike redes vir die ho& spesies verskeidenheid, vergeleke met ander Oos-Kaaplandse riviermondings, is die voorkoms van Zostera capensis iangs die hele lengte van die riviermonding. Die grootse bydrae tot die makrobentos word deur die krustacea, Cleistostoma edwarrJsii, C. a/goense, Upogebia africana, Sesatma catenata en Uca urvillei en die tweekleppige Solen cyllndraceus gemaak. Laasgenoemde kom vera! in die middelste gedeelte van die monding voor, waar dit digthede van 400 m - 2 kan bereik. Die suksesvolle kolonisasie van die monding deur Solen cylindraceus kan veral toegeskryf word aan die bestendige fisiese toestande binne die monding vanw~ die lae varswaterinstroming. ) Considerable ecological research has been carried out on many Materials and Methods 0 1 South African estuaries (see Day 1981 for review). However Ten transects (fl to nO) were undertaken during December 0 2 the estuaries of the eastern Cape have to a large extent been 1983 to February 1984. Further sampling was carried out in d overlooked. In a report on the S.A.N.C.O.R. estuaries e September 1984 and 1985. The transects were spaced at t a programme (1983), only the Kronune and Swartkops were approximately I-km intervals from the mouth to the head of d ( categorized as estuaries with a good data base. Apart from the estuary (Figure 1). A preliminary survey had shown that r e the pollution survey of Gardner, Connell, Eagle, Moldan & from LWS to HWS, four distinct zones or habitats, charac h s Watling (1985), who determined that the water of the Kariega i terized by plant species, could be recognized on the intertidal l b had no metal pollution, there have been no published studies banks. Zone I, lowest down the bank, consisted of a band u on this estuary. P of Zostera capensis, Zone II of mud or sand which lacked e The Kariega estuary (33°4I'SI26°42'E) is situated 120 km macrophytes, Zone III a belt of Spartino maritima and Zone h t north-east of Port Elizabeth, its mouth opening just east of IV of Sorcocomia sp. Although the width of these zones y b Kenton-on-sea (Figure 1). The characteristic features of the varied along the length of the estuary, each zone was present d e Kariega, which ultimately affect the physical conditions and for most of the length. For each transect, two quantitative t n therefore faunal distribution within the estuary, are a mouth samples were taken within each of the four zones. Quantitative a 2 r which is always open to the sea, and a freshwater input which samples were obtained from a 0,25-m quadrat dug to a depth g e is very sporadic. Prior to October 1985, there had not been of about 60 em, and sieved through a I-nun mesh sieve. Each c n any significant freshwater inflow for four years. A number transect was continued into the subtidal area to the middle e c of factors have contributed to this insignificant freshwater i of the channel, samples being taken with a Van Veen grab l 2 r inflow; poor rainfall in the catchment, the small catchment which sampled an area of 0,026 m • Qua1itative sampling was e 2 d area (685 km ) and fmally the fact that there are two major undertaken from the rocky areas of the estuary (RI to R7, n u reservoirs in the catchment. Figure 1). At each of these areas random collections were y Although the catchment of the Kariega estuary has been made from HWS to LWS. a w greatly disturbed by the construction of dams, the estuary itself The collected samples were immediately frozen and stored e t is undisturbed. Recreational activities are almost restricted to a until sorting. After sorting to determine species composition, G one month of the year (mid-December to mid-January), and specimens were dried at 60°C to constant weight. All biomass t e unlike the adjacent Bushmans estuary, farming activities have figures are expressed as dry weight. - n i not encroached upon the intertidal banks. The Kariega there b a fore provided an opportunity to study an undisturbed, open S Results y estuary with a sporadic freshwater input. This paper reports b on the distribution of invertebrates within the estuary and is Summary of physical conditions d e c intended as a base-line study for future research. In addition The following account of the physical conditions within the u d the paper is one of several (to be published by other workers estuary includes personal observations and a summary of o r from Rhodes University) dealing with the ecology of the information provided by Dr G. Read and Mr D. Taylor of p e Kariega estuary. Rhodes University. I R 154 S.·Afr. Tydskr. Dierk. 1987,22(2) along the length of the estuary. Using a combination of aerial photographs and on site measurements it is estimated that there is approximately 24 ha of Zostera within the Kariega. The plant's biomass (leaves and roots) is greatest in the middle reaches of the estuary, between T5 and T8 (Figure 2), with a maximum value of 391 g m - 2. The mean biomass for the entire estuary of 190 g m - 2 is only slightly lower than that recorded by Grindley (1978) for Zostera in Knysna lagoon (206 g m - 2) but is over 100 g m - 2 lower than that recorded (Hodgson, unpublished data) in the adjacent Bushmans estuary (fable I). Just below the Zostera, and only exposed on the lowest spring tides, are small patches of Halophila ovalis (R.Br.). Phty ClSt~~ . ) 0 1 lRANSECT 0 figure 1 The Kariega estuary in the eastern Cape showing transects 2 and sampling stations. TI-TlO transects taken in sand or mud. Rl- R7 d Figure 2 Distribution and biomass of four macrophytes along the e rocky areas sampled. Heavy stipple indicates areas of rocky shore. Light t Kariega estuary. a stipple identifies larger areas of salt marsh not sampled in this study. d ( r e h s Towards the top of the intertidal zone is a belt of rice grass i l Spartina maritima (Curtis), which occupies approximately b In winter, water temperatures are stable at 15 -16°C within theu water column and along the estuary. During summer, 22 ha of mud bank. Spartina has its greatest biomass (c. P thee water temperature reaches a maximum of 23°C at the 2100 g m - 2) in the lower and middle reaches (f2 - 6) of the h t to mouth, whereas in the upper reaches, temperatures are about estuary (Figure 2). Such a standing biomass is similar that y 5°Cb higher. The salinity throughout the water column of the recorded in the adjacent Bushmans (fable I). Towards the d has ~/00 head of the estuary (above TS), Spartina is more patchy in lowere reaches remained stable at 35 - except during t its distribution and is gradually replaced by Scirpus litoralis an brief period of floods in 1981. In the upper reaches, how a ever,r the salinity has increased slowly to 40 - 4t:> /00. Oxygen Schrad. and Phragmites australis (Cav.) (Figure 2). g concentrationse within the water column range from 7 - 8 mg c 1 • 021-n e c The intertidal of the lower reaches of the Karlega 2 i banks Table 1 Mean and maximum biomasses (g m- dry l estuaryr have a shallow gradient. The sediments for the frrst weight) of Zostera capensis, Spartina maritima and e 2,5d kJ!1 are of well sorted marine sands which are clean and Sarcocomia sp. recorded in the Kariega and Bushmans n haveu a low organic content « 0,61110). The sand flats at estuaries Transecty I are about 300 m wide. Three hundred metres above a Zostera Spartina Sarcocornia thew national road bridge (fransect 2, Figure I), the sediments e aret slightly finer with a greater proportion of mud. Further Mean Max Mean Max a Mean Max G upstream the intertidal mud narrow; at Transect 3 they banks t Kariega 190 391 1186 2194 522 1469 aree 30 m wide, Transect 4, 10 m and thereafter (f5 - 10) 5 m n Bushmans 319 409 1570 2S58 400 490 wide.i In addition the banks become steeper.

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