A review of the ecology and management of temporarily open/closed estuaries in South Africa, with particular emphasis on river flow and mouth state as primary drivers of these systems

Whitfield, Alan K.; Bate, G. C.; Adams, Janine B.; Froneman, P. W.; Gama, PT; Strydom, Nadine A.; Taljaard, Susan; Theron, A.K.; Turpie, Jane; Niekerk, Lara Van; Wooldridge, T.

doi:10.2989/1814232x.2012.675041

Cited by 61 publications

(32 citation statements)

References 91 publications

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“…Estuary entrance closure is a function of the relative balance between on and offshore sediment transport within the entrance channel (Fortunato et al ., ; Garside et al ., ; Hinwood and McLean, ) (Figure ). Closure occurs during periods of low river flow when wave‐driven onshore deposition surpasses the ability of ebb‐tidal currents to remove sediment from the channel (Gao and Collins, ; Morris and Turner, ; Whitfield et al ., ; Ranasinghe et al ., ). Longshore drift and cross‐shore wave processes have been broadly identified as the main drivers of entrance closure (Ranasinghe and Pattiaratchi, , ) (Figure ).…”

Section: Introductionmentioning

confidence: 99%

The daily‐scale entrance dynamics of intermittently open/closed estuaries

McSweeney

Kennedy

Rutherfurd

2017

Earth Surf Processes Landf

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Intermittently open/closed estuaries (IOCE) are a dynamic form of estuary characterised by periodic entrance closure to the ocean. Entrance closure is a function of the relative balance between on and offshore sediment transport with closures occurring during periods of low fluvial discharge whereby the estuary ebb‐tidal prism is reduced. Although the broad scale processes of entrance closure are becoming better understood, there remains limited knowledge on channel morphodynamics during an individual closure event. In this study, the entrance dynamics of three IOCE on the coast of Victoria, Australia, were monitored over a daily timescale following both artificial and natural openings. The influence of changing marine and fluvial conditions on the relative sedimentation rate within the entrance channel was examined. IOCE in Victoria showed two distinct modes of entrance closure: (a) lateral accretion, whereby the estuary gradually closes by longshore drift‐driven spit growth during low river flows; and (b) vertical accretion, where the channel rapidly aggrades under high (> 2 m), near‐normal waves. During storms, when fluvial discharge and wave heights simultaneously increase, large swells will not always close the mouth due to an increase in the ebb‐tidal prism. The estuary water depth and the maximum channel dimensions following opening were not proportional to the opening duration, with this being a function of the wave and fluvial conditions occurring following lagoon drainage. Based on the findings of this work, implementing a successful artificial entrance opening is dependent on reduced onshore sedimentation rates which occur when wave energy is low (< 2 m Hs) relative to river flow. Copyright © 2017 John Wiley & Sons, Ltd.

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Section: Introductionmentioning

confidence: 99%

The daily‐scale entrance dynamics of intermittently open/closed estuaries

McSweeney

Kennedy

Rutherfurd

2017

Earth Surf Processes Landf

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“…Zoobenthic community structure of temporarily open/ closed estuaries in the Eastern Cape is strongly driven by existing sediment type (Teske and Wooldridge 2003;Whitfield et al 2012). Temporal changes in the substratum of the sandprawn prawn beds were not monitored.…”

Section: Discussionmentioning

confidence: 99%

“…Therefore the fauna of estuarine sand and mud may have distinct compositions, even when the overlying physicochemical conditions are similar (Teske and Wooldridge 2004). These soft substrata are influenced by fine muddy sediments borne by river flow into the upper and middle reaches of the estuary and tidal sorting of coarser sediments in the lower reaches during the open phase (Whitfield et al 2012). The absence of strong currents and clear water conditions during the closed phase of such estuaries often favours the development of extensive aquatic macrophyte beds (Branch et al 1985;Whitfield et al 2008), and the stability of the zoobenthos community appears to be greatest during the closed phase cited in Whitfield et al 2008).…”

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Temporal changes in the macrobenthos of sandprawn (Callichirus kraussi) beds in Swartvlei Estuary, South Africa

Hanekom

Russell

2015

African Zoology

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Water quality was recorded quarterly along the length of the Swartvlei estuary (2002-2014), while seagrass biomass and macrobenthic abundance were measured annually in intertidal sandprawn beds (2009-2014). Despite relatively stable, marine-dominated conditions the abundance of the sandprawn Callichirus kraussi recorded in the lower and middle reaches decreased significantly (p  0.025), accompanied by an increase in seagrass biomass and the abundance of burrowing bivalves and the surface-grazing gastropod Nassarius kraussianus. No significant changes were noted in the upper reaches, where, unlike the lower and middle reaches, the size distribution and juvenile recruitment of C. kraussi remained relatively constant. The temporal data from the lower reaches showed a high, negative correlation between C. kraussi numbers and (a) seagrass (Zostera (Zosterella) capensis) biomass (r  −0.85), and (b) burrowing bivalve numbers (r  −0.69). These relationships were apparent in comparative samples taken from prawn and seagrass beds, where C. kraussi numbers were significantly lower and burrowing bivalve numbers were higher in a seagrass area than in the adjoining high-density prawn bed. These changes accord with previously demonstrated negative correlations between vegetation of sandbanks and abundance of C. kraussi, and with the demonstrable negative effects of sandprawns on bivalves and N. kraussianus. However, our study is novel in that it indicated that changes in enviromental conditions may shift the balance among these species.

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“…Nonperched systems are tidally dominated following a breach event until mouth closure ( Figure 3). The intrusion of marine waters into the estuary depends on numerous factors, including tide range, freshwater inflow rates, and the morphology of the channel [7][8][9]. The mouth dynamics of IOCEs play a key role in the overall ecosystem functioning of these systems.…”

Section: Introductionmentioning

confidence: 99%

“…Intermittent breaching of the sand barriers of these systems leads to rapid changes in the physico-chemical environment, which in turn triggers major biological responses, including the out-and in-recruitment of estuarine and marine breeding invertebrates and vertebrates [2,3]. The breaching process can also cause significant geomorphological changes because the strong breach outflows can scour large quantities of accumulated sediments from an estuary [2,3,9].…”

Section: Introductionmentioning

confidence: 99%

The Ecology and Food Web Dynamics of South African Intermittently Open Estuaries

Froneman¹

2018

Estuary

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This chapter provides an overview of the ecology and food web dynamics of southern African intermittently open/closed estuaries (IOCEs). Intermittently open/closed estuaries experience periodic isolation from the ocean due to a sandbar at the mouth and account for some 71% of all estuaries along the southern African coastline. Field studies indicate that the ecosystem functioning of IOCEs is strongly linked mouth phase (open vs. closed) of these systems. During the closed phase, these systems are generally characterised by low biological diversity and elevated biomass of both invertebrates and vertebrates, which are thought to be sustained by elevated biomass of microphytoplankton and zooplankton within these systems. The low diversity can be related to the virtual absence of marine species within these systems due to the presence of a sandbar at the mouth which limits recruitment. The overflow of marine waters into the estuary during winter storms or spring high tides contributes to the recruit of marine breeding species into these systems. Heavy rainfall in the catchment areas of these systems culminates in the water levels of these systems rising until such time the estuary breaches. The breaching event coincides with the outflow of biologically rich estuarine waters into the marine environment and provides an opportunity for marine breeding species to recruit into these systems. Global warming is likely to contribute to changes in the hydrodynamics of these systems with a concurrent impact on the food webs of these systems.

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A review of the ecology and management of temporarily open/closed estuaries in South Africa, with particular emphasis on river flow and mouth state as primary drivers of these systems

Cited by 61 publications

References 91 publications

The daily‐scale entrance dynamics of intermittently open/closed estuaries

The daily‐scale entrance dynamics of intermittently open/closed estuaries

Temporal changes in the macrobenthos of sandprawn (Callichirus kraussi) beds in Swartvlei Estuary, South Africa

The Ecology and Food Web Dynamics of South African Intermittently Open Estuaries

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