Bot River Estuary Research Articles

The hydrodynamics of the Bot River Estuary revisited

For the past 20 years management of the Bot/Kleinmond estuarine system in the south-western Cape has been based on the premise that, barring intervention, the estuary was naturally evolving into a freshwater coastal lake. This paper presents evidence, based on a 20-year series of water-level data, updated runoff estimates from the catchment and dimensional data, that, in the absence of anthropogenic influences, the system is not progressing naturally, but artificially, towards becoming a freshwater system. It is concluded that the increasingly closed state of the Bot Estuary in recent years is most likely due to reduction in runoff from its tributaries and premature artificial breaching of the Kleinmond arm of the system. These findings, coupled with the high conservation importance of the Bot River Estuary, suggest that the current management plan needs urgent revaluation and that the two estuaries cannot be managed separately.

Conservation Problems and Management Options in Estuaries: The Bot River Estuary, South Africa, as a Case-history for Management of Closed Estuaries

Closed (or ‘blind’) estuaries are common features, practically throughout the world, of areas experiencing markedly seasonal rainfall. The Bot River vlei is a closed estuary which faces considerable problems of management and conservation. For the past 100 years or more, the estuary has been opened to the sea every 2 to 5 years by artificial means. This has resulted in a very great variability of physical conditions and a concomitantly low diversity of organisms that are able to occupy the estuary permanently. Nevertheless, those species that do exist in the Bot River vlei are mostly estuarine species.

BREACHING THE MOUTH OF THE BOT RIVER ESTUARY, SOUTH AFRICA: IMPACT ON ITS BENTHIC MACROFAUNAL COMMUNITIES

SUMMARY The changes occurring in the benthic macrofaunal communities of the Bot River estuary, after an artificial opening of its mouth on 29 June 1983, are presented and the possible short- and long-term effects of this disturbance on their ecology are described. Surveys of the benthos conducted in December 1982 and May 1983, before the mouth-breaching, are compared with similar surveys afterwards: in October 1983, a week prior to reclosure of the estuary, and three months later, in January 1984. The total number of species decreased from 23 in May to 15 in October; however, only those species which contributed less than 5 % to the total biomass in May were lost. The largest reductions in species abundance and biomass were related to the collapse of the macrophyte, Ruppia maritima, which is intolerant of high salinities. The species which were reduced most were Arcuatula capensis, Melita zeylanica and Cyathura estuaria. These species were adversely affected by the loss of their attachment substrate (Arcu...

THE EFFECT OF SALINITY AND TEMPERATURE VARIATIONS ON THE BACTERIAL POPULATION IN THE BOT RIVER ESTUARY

SUMMARY The effect of temperature and salinity variations on the suspended bacterial population of the Bot River estuary, a closed estuary, was examined. The population was found to be salt tolerant, and saline water was required for maximal growth rates. Temperatures below 15°C inhibited growth. Bacterial isolates fell into two groups as regards temperature optima, the one having an optimum of 23°C and the other 30°C. The bacterial population in the Bot River estuary is clearly adapted to estuarine conditions.

ZOOPLANKTON AND SOME ENVIRONMENTAL CONDITIONS IN THE BOT RIVER ESTUARY

SUMMARY A zooplankton study was commenced during July 1980 while the Bot River estuary was closed off from the sea, and had a mean surface salinity of 13,5%. The latter declined to 2,3‰ in September 1981 before the estuary was opened to the sea at the Sonesta Holiday Resort, after which it increased again to means of, respectively, 28,9 and 30,8‰ during November and December 1981, the last two months of the survey. Zooplankton numbers and biomass fluctuated seasonally with maximum mean values occurring during the spring of 1980. It would appear that the estuarine zooplankton population in the Bot River estuary was adversely affected in terms of both numbers and biomass by the decrease in salinity prior to the opening of the estuary at Sonesta. Zooplankton composition, however, changed relatively little. The nauplii, copepodites and adults of the calanoid copepod Pseudodiaptomus hessei dominated numerically, together forming an average of approximately 90% of the zooplankton during the study. Only two marine zooplankton organisms, Oithona similis and Paracalanus crassirostris, were collected in the estuary, appearing after it was opened at Sonesta. The prevailing salinities in the estuary prior to its opening at Sonesta appear to have inhibited the breeding of an important estuarine bait organism, the sandprawn Callianassa kraussi.

SYNOPSIS OF THE IMPACT OF ARTIFICIALLY OPENING THE MOUTH OF THE BOT RIVER ESTUARY: IMPLICATIONS FOR MANAGEMENT

SUMMARY This paper synthesizes previous research on the Bot River estuary, including that on its past and present geology, sedimentology, hydrology, history, plant life, invertebrates, birds and fish. The estuary is periodically breached to form a deep mouth and this restores estuarine conditions and allows recruitment of fish; but it also disrupts the system and results in massive losses of aquatic weeds and invertebrates, and a twentyfold reduction in bird-life. A key feature in the ecology is that the Bot is connected to the adjacent Kleinmond estuary by overflow channels which act as a spillway when water-levels in the Bot are high. Because of this the Bot will slowly become diluted by river-water and change into a fresh-water lake if it is not breached. Six proposed plans for the management of the Bot River estuary are discussed. Only two are viable: either the estuary should never be breached (and thus be managed as fresh-water lake) or breaching should be controlled and undertaken about every three...

THE BATHYMETRY, ENVIRONMENTAL PARAMETERS AND SEDIMENTS OF THE BOT RIVER ESTUARY, S.W. CAPE PROVINCE

SUMMARY The bathymetry of the Bot River estuary, water-level records from 1978 to 1983, wind records for a three-week period in August-September 1984, and investigations into the particle-size distribution, mineralogy and chemical composition of sediments in the estuary are reported and discussed. The Bot River estuary is generally shallow, too shallow for the development of a halocline. The water-levels fluctuate between approximately 0,2 m and 2,7 m above mean sea-level. Water-flow through an overflow channel into the Lamloch swamps, during periods of high water-level, is sufficient to lower the water-level by 0,11 m per week, assuming no inflow. This is probably the main factor preventing the estuary from opening naturally to the sea. From August to October 1984 wind speeds exceeded 25 km h−1 approximately 15–20% of the time, and blew from the north, west and east for approximately equal proportions of the time. The sediments in the estuary are mainly sands (51 %) or muds (33%). There is a marked lack of very fine sand in the estuary, and the mean particle size is medium sand. The sands consist mainly of quartz, except in the lower reaches near the sea where they contain a high proportion of shell fragments. The compositions of the muds in the central mud belt are remarkably homogeneous, and probably reflect thorough mixing of the finer particles by resuspension caused by wind-generated wave and current action on the sediments. The sediments in the estuary consist of quartz alone, or simple mixtures of quartz and carbonates (shell fragments) or quartz and clay minerals (with some organic matter). Only 4 out of 199 sediment samples show any mixing of carbonates with clay minerals.

HISTORICAL RECORDS OF THE BOT RIVER ESTUARINE SYSTEM

SUMMARY Historical evidence shows that the Bot River estuarine system has undergone substantial topographical changes during the past century, the most recent being the partial isolation of the Kleinmond from the Bot River estuary, of which it was once apparently a part. Despite these substantial changes, there is no evidence to confirm an early newspaper report that the estuary itself was used as a harbour.

THE ANNUAL CYCLE OF FREE-FLOATING BACTERIA IN THE BOT RIVER ESTUARY

SUMMARY The Bot River estuary is normally closed to the sea and its bacterial population fluctuates seasonally, peaking in late summer. Parameters such as temperature, salinity, organic nutrients and turbidity may directly or indirectly cause these fluctuations. The spatial distribution of this bacterial population differs from that of open estuaries studied elsewhere. However, artificial opening of the estuary results in a distribution pattern similar to that of open estuaries.

THE ECOLOGICAL ROLE OF THE RED-KNOBBED COOTFULICA CRISTATAGMELIN AT THE BOT RIVER ESTUARY SOUTH AFRICA: A PRELIMINARY INVESTIGATION

SUMMARY The red-knobbed coot Fulica cristata constitutes over 90% of the waterfowl numbers at the Bot River estuary, South Africa. In order to determine the impact of the birds on the submerged aquatic macrophytes at the estuary, aspects of the bioenergetics of the red-knobbed coot were studied. Captive birds of mean mass 720,4 g had a daily metabolized energy of 516 kJ per bird. The daily energy expenditure of the coots in the wild was estimated to be 691 kJ per bird. The birds assimilated 57,9% of their gross energy intake when feeding on Ruppia maritima L. and Potamogeton pectinatus L. and consumed approximately 82,4 g dry weight of macrophyte per bird per day during free existence. On the basis of their energy requirements, it is estimated that the coot population on the Bot River estuary consumed 759,5 tonnes dry weight, approximately 9,9-12,5% of the submerged macrophyte production in 1980–1981. Guano released into the water during the same period is estimated at 470,1 tonnes dry weight.

PRIMARY PRODUCTIVITY OF THE BOT RIVER ESTUARY, SOUTH AFRICA

SUMMARY The primary production of the Bot River estuary, a large closed estuary on the south-western coast of South Africa has been estimated at 11 059 t dry mass y−1. The main contributions to this value come from the submerged aquatic macrophytes (7 987 t dry mass y−1 or 72% of total calculated production), reeds (1 503 t dry mass y−1 or 14%), benthic diatoms (787 t dry mass y−1 or 7%) and phytoplankton (782 t dry mass y-1 or 7%). These four categories are believed to contribute over 90% of the primary production during periods of stability when the estuary is closed and full. Much of this production is not directly utilized by herbivores and forms the basis of a bacterial system producing very fine, organically rich material deposited on the estuary bed. Primary production is affected by mouth opening in a number of different ways including changes in volume of water, available surface area for attachment of macrophytes, and loss of macrophytes which are stranded or washed out to sea.

A COMPARISON OF THE PHYSIOLOGICAL CONDITION OF THE SOUTHERN MULLETLIZA RICHARDSONI(SMITH), IN A CLOSED ESTUARY AND THE SEA

SUMMARY The physiological condition of the mullet, Liza richardsoni (Smith), from the closed Bot River estuary is compared with that of the same species from False Bay. Levels of protein, lipid, water and ash, as well as energy content were determined in fish from the two environments. Protein (17,94% vs. 18,53 %) and ash levels (3,41 % vs. 4,81 %) were similar in both estuarine and marine fish. Lipid (9,95 % vs. 2,23%) and energy levels (18,35 kJ g-1 dry weight vs. 14,18 kJ g-1 dry weight), were significantly higher, while levels of water (67,73% vs. 74,87%) were lower in the estuarine fish. On average the estuarine L. richardsoni were in better condition than their marine conspecifics. Liza richardsoni does not spawn in estuaries and their better condition in the estuary could possibly be attributed to the retention of energy usually lost through spawning in the sea. On the other hand, estuarine subadults were already in an improved condition, which could only be due to better feeding conditions. It is therefore concluded that estuarine L. richardsoni can maintain a better condition due to both more favourable feeding conditions and the retention of energy usually lost through spawning.

THE HYDROLOGY OF THE BOT RIVER ESTUARY

SUMMARY During the flood year of 1981 and in 1983 the Bot River estuary, one of the largest lagoons in the south-western Cape was opened to the sea by three artificial breachings of the sand bar which were aimed at lowering the excessively high water levels in the lagoon and at the re-establishment of estuarine conditions. Surveys were undertaken in the lagoon and at the mouth channels in an attempt to formulate a future viable management policy, taking into account the conflicting interests of the defenders of continued artificial breachings and of those who advocate a closed estuary which, if left unbreached, will eventually change into a coastal fresh-water lake. Artificial breachings cause a rapid lowering of the water-level in the vlei associated with the sudden loss of large areas of water surface; they also lead to the effective discharge of silt-laden flood waters and an influx of clear sea-water. It is recommended that breachings only be done after the vlei has reached a water-level of more than ...

VARIATIONS IN THE USE OF THE BOT RIVER ESTUARY BY WATER-BIRDS

SUMMARY The Bot River estuary is an important wetland for various water-birds, and is especially of value as a safe refuge during the dry season. Its use by birds, however, varied widely over the study period, and was largely related to changing hydrological conditions in the estuary. Indications are that artificial breaching of the sand-bar in particular affects the bird community, and that favourable environmental conditions for most species develop only after prolonged periods of three or more years after breaching. It is recognized, however, that the establishment of periodic estuarine conditions by means of direct contact with the sea would be in the interest of the water-birds. It is therefore proposed that a management strategy of infrequent artificial breaching be pursued.

A MASS MORTALITY OF FISH ASSOCIATED WITH LOW SALINITY CONDITIONS IN THE BOT RIVER ESTUARY

SUMMARY The Bot River estuary remains closed for prolonged periods, during which the salinities decline progessively. In October 1981, after four years of isolation from the sea, the maximum salinity in the estuary fell to 3% resulting in a mass fish mortality. The extent of this is compared with the species composition and relative abundance of fish prior to the mortality and with published records of the minimum salinities these species were previously known to tolerate.

BARRIER/ESTUARINE PROCESSES; BOT RIVER ESTUARY—AN INTERPRETATION OF AERIAL PHOTOGRAPHS

SUMMARY Bot River estuary can be classified as an end-member stage of a bar-built estuary, occupying a drowned river valley. The modern barrier consists of fairly large back-barrier dunes, hummocky foredunes and a dissipative beach. Aerial photographs were interpreted to assess whether the estuary has during modern times always had its present configuration, and to interpret the present physical processes based upon changing morphological features. Lowering of sea-level following the Late Holocene high-water stand exposed a sandy strand plain between Kleinmond and Hawston. Wind action forced an eastward transport of these sands creating a large dune field between Sonesta and Hawston, and a deflation surface at Rooisand. In 1938 the Lamloch swamps were well vegetated but by 1981 large formerly vegetated areas had been converted to open water as a result of subsidence. Long periods of mouth closure forced high water-levels in the estuary which flooded the deflated Rooisand area. Eventually estuarine waters ...

THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE WATERS OF THE BOT RIVER ESTUARY, SOUTH AFRICA

SUMMARY The physical and chemical properties of the waters of the Bot River estuary have been examined over a period of five years. The parameters measured were: electrical conductivity, pH, alkalinity, temperature, dissolved oxygen, total dissolved salts, sodium, magnesium, calcium, potassium, fluoride, chloride, nitrate, ammonia, sulphate, phosphate and silicate. The physical and chemical properties are mostly affected by artificial openings of and water-levels in the estuary.

CHANGES IN THE FISH FAUNA OF THE BOT RIVER ESTUARY IN RELATION TO OPENING AND CLOSURE OF THE ESTUARY MOUTH

SUMMARY The fish community of the Bot River estuary was sampled with seine and gill nets between April 1980 and July 1983. During part of this period the estuary, which is usually closed, was in contact with the sea via an artificially opened deep-water mouth as well as the natural spillway into the adjacent Kleinmond estuary. This paper describes changes in the species composition, abundance and size-structure of the fish community that occurred as a result of these connections with the sea, as well as seasonal and longer-term changes that occurred during the closed phase. The implications of opening the mouth, for the fish and the fishermen that utilize the lagoon, are outlined and management options discussed.

A NUMERICAL MODEL FOR THE WIND-DRIVEN CIRCULATION IN THE BOT RIVER ESTUARY

SUMMARY A numerical model for the wind-driven circulation for the Bot River estuary was established. The model was run for various wind-directions and the resulting current patterns are briefly discussed.

THE BENTHIC MACROFAUNA OF THE BOT RIVER ESTUARY, SOUTH AFRICA, WITH A NOTE ON ITS MEIOFAUNA

SUMMARY The species composition, abundance, biomass and distribution of the benthic macrofauna of the Bot River estuary are discussed. In general, the fauna is impoverished, comprising mainly amphipods, isopods, polychaetes and molluscs. Average total biomass figures are 8,29 g dry mass m-2 for December 1982 and 4,32 g dry mass m-2 for May 1983. The tanaid, Apseudes digitalis, was the most abundant species, while the bivalve, Arcuatula capensis, had the highest biomass. Low diversities and abundances are attributed to the lack of contact with the sea. The estuary can be divided roughly into four major zones, based on the distribution and abundance of the benthic macrofauna. Faunistically, the zones differ with respect to the characteristic and numerically dominant species. Highest total biomass figures (91,63 g dry mass m-2) are associated with the Ruppia beds, whereas the lowest figures were usually found in the deeper areas. The meiofauna of the system is noticeably impoverished, in terms of both number...