Effects of prolonged elevated water salinity on submerged macrophyte and waterbird communities in Swartvlei Lake, South Africa

Russell, Ian; Randall, R. M.

doi:10.4314/wsa.v43i4.14

Cited by 4 publications

(3 citation statements)

References 23 publications

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“…On Swartvlei Lake, the numbers of all regularly occurring and abundant piscivores, with the exception of the regionally increasing Cape Cormorant, which is typically uncommon on this waterbody, have declined over time. These changes have been described in Russell and Randall (2017) and were the consequence of atypical meteorological conditions in 2007 and 2008 that resulted in significant increases in seawater inflows along with reduced freshwater inflows, causing substantial increases in water salinity over several years and resultant near complete loss of once abundant submerged macrophytes. Numerically dominant littoral fish species such as Cape stumpnose Rhabdosargus holubi and oval moony Monodactylus falciformis feed predominantly on filamentous algae, macrophytes, and associated invertebrates (Whitfield 1984(Whitfield , 1986, and mullet species (Mugilidae) feed mainly of plant detritus and associated unicellular algae and diatoms (Whitfield 1982).…”

Section: Changes In Piscivorous Waterbird Abundancesmentioning

confidence: 96%

“…Numerically dominant littoral fish species such as Cape stumpnose Rhabdosargus holubi and oval moony Monodactylus falciformis feed predominantly on filamentous algae, macrophytes, and associated invertebrates (Whitfield 1984(Whitfield , 1986, and mullet species (Mugilidae) feed mainly of plant detritus and associated unicellular algae and diatoms (Whitfield 1982). The prolonged senescence of macrophytes in Swartvlei Lake is thus expected to have caused substantial decline in littoral fish populations, as observed in the early 1980s by Whitfield (1984), which in turn could account for observed declines in most piscivorous waterbirds on this waterbody (Russell & Randall 2017).…”

Section: Changes In Piscivorous Waterbird Abundancesmentioning

confidence: 99%

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Waterbird community changes in the Wilderness Lakes, South Africa (Part 3 of 3): Diving piscivores and scavengers

Russell

2023

KOEDOE - African Protected Area Conservation and Science

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Estuarine waterbodies typically support diverse and abundant waterbird communities. However, global environmental degradation as a result of anthropogenic activities is leading to species level changes in biodiversity, with top predators such as piscivorous waterbirds being particularly vulnerable to ecosystem changes. The study aimed to document long-term spatial and temporal patterns of abundance of piscivorous and scavenging waterbirds in the Wilderness Lakes Complex (WLC) in the Garden Route National Park, South Africa, and where possible identify potential causes for observed trends. The abundance of waterbirds on these wetlands was determined biannually from 1992 to 2019, with counts conducted from a boat following a standardised route. Historical waterbird abundance data from the 1980s were also used to describe long-term abundance changes. Eight of the species exhibited seasonal variability in abundance with most, excluding Common Tern Sterna hirundo, being more abundant in winter. Substantial changes occurred in the abundance of several species over the four-decade study period, notably increases in Cape Cormorant Phalacrocorax capensis and Great Crested Grebe Podiceps cristatus, and decreases in Common Tern and Black-necked Grebe Podiceps nigricollis. Long-term abundance trends indicate a combination of declining, increasing, and stable populations. Although some species have undergone contrasting abundance changes on different waterbodies in the WLC the dominant pattern was a similar direction of change on most or all waterbodies and in different seasons. Local reasons for changes probably include loss of sandbanks, changing prey availability, and the absence of recreational disturbance on some waterbodies.Conservation implications: Drivers of changes in the abundances of piscivores are likely to be multifaceted, functioning on multiple spatial and temporal scales, and affecting different species in different ways. Recommended local corrective actions include managing emergent macrophyte encroachment on sandbanks, reducing recreational disturbance, managing processes affecting indigenous fish stocks, and protecting nesting sites.

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Section: Changes In Piscivorous Waterbird Abundancesmentioning

confidence: 96%

Section: Changes In Piscivorous Waterbird Abundancesmentioning

confidence: 99%

Waterbird community changes in the Wilderness Lakes, South Africa (Part 3 of 3): Diving piscivores and scavengers

Russell

2023

KOEDOE - African Protected Area Conservation and Science

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“…A significant reduction in submerged aquatic plants in the Swartvlei system from 2007 onwards following elevations in salinity (Russell & Randall 2017), is likely to have contributed to the decline in the abundance of several herbivorous waterbirds species, and particularly the previously abundant, Yellow-billed Duck, Cape Shoveler and Red-knobbed Coot. Declines in plant biomass in the WLC, although significant, are possibly not the sole causal factor for the observed changes in herbivorous waterbirds, as attested by declines in particularly Yellow-billed Duck and Cape Shoveler on Knysna Estuary over the same time period (unpublished CWAC data), and where there is no evidence of changes in plant biomass.…”

Section: Possible Reasons For Changes In Herbivorous Waterbird Abunda...mentioning

confidence: 99%

Waterbird community changes in the Wilderness Lakes, South Africa (Part 1 of 3): Herbivores and omnivores

Russell

2023

KOEDOE - African Protected Area Conservation and Science

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Global and regional degradation and loss of aquatic systems can negatively affect wetland-dependent waterbirds. The Wilderness Lakes Complex in South Africa, which incorporates the Wilderness Ramsar site, supports populations of 68 waterbird species including eight which periodically occur in globally significant (1% population) numbers. The study aimed to document long-term spatial and temporal (including seasonal) patterns of abundance of herbivorous waterbirds in the Wilderness Lakes Complex, and where possible identify potential causes for observed trends. The abundance of waterbirds on these wetlands was determined biannually from 1992 to 2019, with counts conducted from a boat following a standardised route. Historical abundance data from the 1980s was also used to describe long-term abundance changes. Observed seasonality of four herbivorous waterbirds differed from published accounts. Long-term abundance trends indicated a combination of declining, increasing and stable populations which, in several instances, differed from regional (southern African) assessments. Significant long-term decreases have occurred in five herbivorous species in the lakes complex (Common Moorhen Gallinula chloropus, Red-knobbed Coot Fulica cristata, Yellow-billed Duck Anas undulata, Cape Shoveler Anas smithii, Cape Teal Anas capensis), whereas increases have occurred in two species (Spur-winged Goose Plectropterus gambensis, Egyptian Goose Alopochen aegyptiaca). Similar types of change occurred across multiple waterbodies, and in different seasons. The similarity of trends in spatially separated wetlands suggested either high interconnectivity of populations between wetland systems and/or some drivers of change of mobile species being broad scaled and regional, rather than system specific. Local reasons for changes potentially include increasing extent of cultivated fields, a proliferation of emergent macrophytes and loss of open sandbanks, changing food availability, increasing disturbance, and disease.Conservation implications: The fundamental character of the Wilderness Ramsar site has changed, with reduced abundance of several previously abundant herbivorous waterbirds. Drivers of change in species abundances are multifaceted, complex, and frequently poorly understood. Recommended local corrective actions include the management of emergent macrophytes, avian diseases, disturbance, and water quality.

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Differential sampling in the assessment of conservation and biodiversity merit: a comparison of the seagrass macrofauna in three nearby South African estuaries

Barnes,

Seath,

Arendse

2024

Biodivers Conserv

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To what extent is the relative biodiversity of some flagship conservation sites a result of differential attention? Knysna estuarine bay is the topmost ranked South African estuary for conservation importance and biodiversity. It is also one of the most intensively studied, and hence differential sampling effort could partly be responsible for its apparent relative richness. To assess the extent to which this might be true, identical sampling area, effort and methodology were employed to compare the benthic macrofauna of one specific major Knysna habitat (Zostera capensis seagrass beds) with equivalent ones in two nearby lesser-studied estuaries, the Keurbooms/Bitou and Swartvlei. Investigation showed all three localities to share a common species pool, but different elements of it dominated the shared habitat type in each. The seagrass and adjacent sandflat macrobenthos proved just as biodiverse in unprotected Keurbooms/Bitou as in the Protected Area of Knysna, but that in Swartvlei (also a Protected Area) was impoverished in comparison, presumably consequent on mouth closure and the prevailing lower salinity. Despite marked geomorphological and hydrological differences, all three estuaries share a suite of unusual faunal elements and such particularly close faunal similarity suggests the importance of historical biogeographic processes. The analysis emphasises the need for caution when assessing the relative conservation importance or other merits of different individual systems in a data-limited environment.

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Effects of prolonged elevated water salinity on submerged macrophyte and waterbird communities in Swartvlei Lake, South Africa

Cited by 4 publications

References 23 publications

Waterbird community changes in the Wilderness Lakes, South Africa (Part 3 of 3): Diving piscivores and scavengers

Waterbird community changes in the Wilderness Lakes, South Africa (Part 3 of 3): Diving piscivores and scavengers

Waterbird community changes in the Wilderness Lakes, South Africa (Part 1 of 3): Herbivores and omnivores

Differential sampling in the assessment of conservation and biodiversity merit: a comparison of the seagrass macrofauna in three nearby South African estuaries

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