Closed-mouth Conditions Research Articles

Ecological networks reveal important taxa for ecosystem function in two subtropical South African estuaries

Biodiversity is essential for estuarine ecosystems' functioning, resilience, and persistence. While biodiversity assessments provide some insight into the ecosystem state, they cannot quantify every aspect of ecosystem-level function. We assessed aspects of estuarine ecosystem function and pinpointed taxa of interest that fulfil specific roles in estuarine ecosystems, using a series of novel food web model ensembles and Ecological Network Analysis. We investigated emergent network properties of two subtropical South African estuaries, the anthropogenically impacted large temporarily closed uMdloti Estuary and the ‘near natural’ predominantly open uMlalazi Estuary, during a drought period induced by the El Niño Southern Oscillation. In addition, important network components were identified to elucidate the importance of estuarine/marine taxa in maintaining ecosystem function. The key findings included (1) estuaries exhibit system-specific functional responses under drought conditions, and (2) estuarine/marine taxa are critical for overall ecosystem function. Both estuaries showed seasonal differences in ecosystem activity and reliance on trophic level I (microalgae and detritus) to fuel the food web, and persistently low cycling (<9.3 %). However, compared to uMdloti Estuary, uMlalazi Estuary showed less variation in mean seasonal activity (1597–2095 mgC m−2 d−1), and higher reliance on detritus to fuel the food web (D:H > 2.02), indicating greater ecosystem functional resilience to external perturbations. uMlalazi Estuary ecosystem function could be attributed to predominantly open mouth conditions maintaining a salinity gradient, and subsequent large contributions to overall ecosystem function from typically estuarine/marine invertebrate taxa. In contrast, the uMdloti Estuary, dominated by freshwater/brackish taxa under prolonged closed mouth conditions, showed greater seasonal variations in mean system activity (1084–2289 mgC m−2 d−1) and generally high reliance on microalgae (D:H < 1.26), indicative of decreased resilience to external perturbations. During this study, a single mouth breach acted as a ’biological reset’ of uMdloti Estuary, enabling recruitment of estuarine/marine taxa, and a temporary recovery in ecosystem function reflected in increased system cycling (FCI >14.05 %) and decreased reliance on microalgae (D:H > 1. 41). These findings suggest that future predicted drought impacts on estuarine ecosystem function may be partially system-specific due to differing estuarine types, anthropogenic impact, and invertebrate community composition. This study provides a scientific link between ecosystem-level indicators and the role of individual taxa within the system, which is a valuable complement to biodiversity assessments and ecosystem state evaluation.

Ineffective artificial mouth-breaching practices and altered hydrology confound eutrophic symptoms in a temporarily closed estuary

Context Artificial breaching of intermittently closed estuaries has become more frequent in the face of global-change pressures. Aims This study aimed to determine whether the ecological health of the Great Brak Estuary has been affected by the prolonged loss of marine connectivity arising from below-average inflow and failed breaching attempts. Methods We characterised primary eutrophic symptoms (inorganic nutrients, dissolved oxygen, microalgae) typical of the various mouth states, i.e. open, closed and semi-closed. Key results Initially, low inflow and closed mouth conditions facilitated the widespread occurrence of macroalgal blooms (Cladophora glomerata). Phytoplankton bloom conditions (&gt;20 μg Chl-a L−1) ensued only in response to favourable hydrodynamic conditions (e.g. increased water residency, halocline formation) and increased nutrient availability from fluvial sources and macroalgal dieback. These blooms occurred in brackish conditions and comprised numerous taxa, including Cyclotella atomus var. marina, Cryptomonas sp. and Prorocentrum cordatum. Widespread hypoxia (&lt;2 mg L−1) occurred during the semi-closed mouth phase because of the reduced flushing potential associated with the preceding high flow conditions. Conclusions Global-change pressures and ineffective breaching practices will promote eutrophic conditions in intermittently closed estuaries in the future. Implications Allocating sufficient environmental flows is key to preventing ecosystem degradation.

Effects of prolonged mouth closure in a temporarily open/closed estuary: a summary of the responses of invertebrate communities in the uMdloti Estuary, South Africa

Prolonged mouth closure in estuaries is often associated with hypersalinity, mainly as a result of reduced freshwater inflow. However, on the KwaZulu-Natal coast, South Africa, temporarily open/closed estuaries have a tendency towards hyposalinity during closed mouth conditions, primarily because of their perched nature reducing saltwater intrusion, but also due to persistent freshwater inputs, together with seepage of saline water through the sand berm into the sea. We investigated a system, the uMdloti Estuary, experiencing extended periods of mouth closure over two years with the aim of documenting the response of invertebrates to such conditions. Specifically, estuarine invertebrate biodiversity was severely diminished, which was attributed to persistently low salinity, rather than poor water quality due to the absence of flushing. Secondly, a distinct brackish/freshwater community established itself within the mesozooplankton and benthic invertebrate assemblages, including invasive species. Marine overwash of the berm and a short mouth breaching aided the return of estuarine species for only very brief periods (weeks). We conclude that although estuarine species are quick to re-colonise the estuary after a breach, prolonged mouth closure along extended stretches of coast might severely reduce estuarine invertebrate biodiversity. Management options are preferably the provision of an adequate freshwater inflow to allow natural breaching, or else artificial breaching, to facilitate several openings per year as well as for a long enough tidal phase to permit the establishment of an estuarine salinity gradient.

Estuarine algal responses to increasing nitrate concentrations during closed mouth conditions of oligotrophic systems: a laboratory microcosm experiment

Abstract The increasing incidence of eutrophication has potentially detrimental socio-economic and ecological consequences. This study aimed to elucidate the temporal dynamics of algal communities in response to increasing initial concentrations of inorganic nitrogen (particularly nitrate) – central components of eutrophication. A contained microcosm experiment was designed to mimic the conditions of shallow oligotrophic estuaries with high water residence times. Phytoplankton, microphytobenthos and filamentous algal community dynamics were observed over a 28-day experimental period under different nitrate regimes. Key observations included (1) accelerated filamentous algal growth, (2) rapid loss of phytoplankton biomass and abundance, and (3) reduced benthic diatom species diversity and richness in the “1.0 μm Nitrate Addition” treatment. Additionally, model results highlighted the positive relationship between filamentous algal growth and increased water temperature. From a global change perspective, the decline in microalgal abundance and diversity at the onset of filamentous algal growth in warm, N-enriched environments suggests a potential uncoupling of trophic pathways. However, the “Control” and “0.5 μm Nitrate Addition” treatments were similar in their algal responses, highlighting the ability of ecosystems to absorb small disturbances. Thus, it is critical that estuarine resilience is preserved to ensure continued provision of invaluable ecosystem services.

PS-376 Non-invasive Ventilation In Neonates: Effect Of Nasal Cannula Size, Insertion Depth And Nares Diameter On Mean Airway Pressure - An In-vitro Study

<h3>Background</h3> Increased neonatal Non-Invasive Ventilation (NIV) use, such as nCPAP and high flow nasal cannula (HFNC) prompted re-evaluation of nasal interface devices – specifically short bi-nasal prongs, and their size and fit relative to the nares. <h3>Objective</h3> To quantify the effect of nasal prong size and insertion depth on generated mean airway pressure (MAP) during NIV. <h3>Design/methods</h3> A Dräger traditional nCPAP interface, three sizes of RAM cannulae (Neotech, Valencia, CA, USA) and two Fisher and Paykel (FP) (Auckland, NZ) HFNC cannulae were tested with 8 simulated nares sizes. A simulated nasal airway was connected to an active lung model set at: Vt 8–10 ml, 60 b/m, Ti 0.35–0.40 s. A Dräger Evita XL ventilator delivered 4,5,6,7 and 8 cmH₂O to the RAM and the Dräger nCPAP cannulae and a FP HFNC system delivered 1–6 L/m flows. MAP was measured for open and closed-mouth conditions. <h3>Results</h3> MAP decreased progressively as the percent nares occlusion (%Occl) decreased. At one-half insertion and closed-mouth conditions, close fitting prongs with high% Occl yielded MAP’s of one-half of set CPAP levels. Delivered MAP’s were significantly lower during open-mouth condition using either HFNC or CPAP cannulae. MAP increased with both flowrate and%Occl. A rapid rise in pressure was associated with% Occl’s &gt;74%. Complete nares occlusions generated the highest pressures. Simulated closed-mouth produced 12 ± 7 SD higher MAP’s than open-mouth over all%Occl’s at HFNC flows &gt;1 L/m. <h3>Conclusion</h3> RAM ‘CPAP’ does not deliver MAP as effectively as standard nasal prong CPAP. Optimum delivered airway pressure is dependent on appropriate %Occl, insertion depth and NIV settings.

Benthic macrofauna of an estuarine lake during a drought: spatio-temporal drivers under different hydrological states

Mouth state is a key determinant of estuarine processes in arid and semi-arid regions. While this factor is well researched for periodically open systems, it is much less researched for estuarine lakes. Here we used a 4-year data set to determine spatio-temporal patterns in responses of benthic macrofauna to changes in mouth state in a subtropical estuarine lake system during a drought cycle, with a focus on understanding the ecological mechanisms driving community change. We also assessed the effects of changes in mean levels of environmental factors relative to their statistical variability (measured as standard deviation, SD) in driving changes in species richness and abundance of dominant macrofauna. Results showed that greatest variability in physico-chemical factors and macrofaunal assemblages occurred in the upper lake sections, especially under closed mouth conditions. At the community level, changes in salinity under different mouth states played an important structuring role, but were less important at the level of individual species, being important for only 1 of the 6 dominant species. Low oxygen levels under closed mouth conditions, particularly in the lake complex, were also an important determinant of macrofaunal community structure and species richness. A major finding was that variability (SD) in environmental factors was more important in determining species richness and abundance of dominant macrofauna than changes in mean levels. This suggests that, while dominant species in the systems can tolerate significant environmental changes, the rate and magnitude of change exert an important control over species abundance.

Influence of habitat structure and mouth dynamics on avifauna of intermittently-open estuaries: A study of four small South African estuaries

Habitat composition was a major factor in determining waterbird species composition and abundance, particularly the area of floodplain and vegetated channel in four intermittently open estuaries (IOEs) in the warm-temperate coastal biogeographical region of South Africa. Average bird densities on the four estuaries varied from 0.5 to 4.2 birds per hectare, and community composition differed significantly between estuaries. However, the considerable variation in abundance of macrophytes did not have a detectable effect on waterbirds. Under closed mouth conditions, piscivorous birds dominated the avifauna. Each estuary's avifauna responded differently in terms of changes in feeding guild composition when the mouth opened. Bird abundance changed immediately after breaching, but not consistently. Diversity was significantly higher under open-mouth conditions for three of the four estuaries, and species composition was significantly different from that under closed-mouth conditions at all four estuaries. Changes in mouth dynamics as a result of climate change, water abstraction and artificial breaching could lead to significant changes in estuarine fauna.

Analysis of the characteristics of mouthguards that affect isokinetic muscular ability and anaerobic power

PURPOSEThe purpose of this study was to estimate the effects of occlusal stability to identify action mechanisms of mouthguards, known to have a modulatory effect on limb muscle function.MATERIALS AND METHODSThis study included 20 male subjects to perform the isokinetic muscle tests and the Wingate anaerobic power test on both knee joints under five closed-mouth conditions: without or with 4 types of mouthguards with thickness of 2 mm based on premolar area: (1) full-coverage, (2) anterior partial-coverage, (3) right posterior partial-coverage, and (4) left posterior partial-coverage. The obtained results were subjected to One-way ANOVA with repeated measures, followed by post hoc test of the contrast method (α=.05).RESULTSThere was no significant difference between the closed position with and without a full-coverage mouthguard in all variables. However, significant differences were observed between with and without a partial-coverage mouthguard in muscular endurance during extension of the left knee, muscular power and endurance during flexion of the right knee. Additionally, significant differences were found between occlusal states with full- and partial-coverage mouthguards in muscular power and endurance during extension of the left knee.CONCLUSIONThese findings indicate the elevation of vertical dimension by 2 mm or the inducement of occlusal stability had little effect on isokinetic muscle strength and anaerobic performance, while uneven distribution of occlusal force might have some positive effects.

Observations on the Bloom-Forming Jellyfish Crambionella stuhlmanni (Chun, 1896) in the St Lucia Estuary, South Africa

ABSTRACT Opportunistic observations made in the St Lucia estuarine lake, mainly during the period 1991 to 2000, show that the rhizostomatid scyphozoan Crambionella stuhlmanni often forms aggregations with densities in excess of 1 ind.m-3. Its salinity tolerance varies in the range of 12–65 ‰, with the highest jellyfish concentrations often observed at hypersaline levels of 40–65 ‰. Considering that the species has persisted in the system under the present closed mouth conditions, it is suggested that it is capable of completing its life cycle within the estuarine lake. Its role in the ecosystem needs to be investigated, with emphasis on its potential ability to remove suspended sediment particles from the water-column.

A new species of Nitocra Boeck 1865 (Harpacticoida, Ameiridae, Ameirinae) from South Africa, with notes on its ecology and remarks on the status of Nitocra sewelli husmanni Kunz 1976

A new species of the genus Nitocra Boeck, 1865, Nitocra taylori sp. n. is described from the St Lucia Estuary, Africa’s largest estuarine lake. It is also suggested that Nitocra sewelli husmanni Kunz, 1976 and Nitocra reducta fluviatilisGalhano, 1968 are granted full species rank as Nitocra husmanni stat. n. Kunz, 1976 and Nitocra fluviatilis stat. n. Galhano, 1968. Nitocra taylori sp. n. appears to be closely related to Nitocra husmanni. Unfortunately, the original description of the micro-characters of the species lacks the detail needed to make reliable comparisons between species of the genus Nitocra. The main differences observed are the number of spinules along the posterior margin of the anal operculum, length ratio of the exopod and endopod of the first swimming leg, shape of the outer spine on the male third endopodal segment of the third swimming leg, number of segments of the male antennule, relative length of the setae on the male baseoendopod of the fifth leg, shape of the male exopod of the fifth leg, relative length of the two setae of the male sixth leg, and shape of the female baseoendopod of the fifth leg. The current distribution of Nitocra taylori sp. n. is limited to the lake part of the estuary, an area which is most severely affected by the current freshwater deprivation crisis. During closed mouth conditions, these regions (South/North Lake and False Bay) are characterized by low water levels, high salinities and high turbidity levels. This suggests that Nitocra taylori sp. n. may favor these environmental conditions and the significant correlations found between the abundance of Nitocra taylori sp. n. and salinity and turbidity confirm this to a degree. Nitocra taylori sp. n. individuals are also able to withstand a wide range of fluctuations. They were recorded at turbidities ranging from 2 to 102 NTU, temperatures from 20.9 to 34.8 ºC and salinity levels ranging from 9.81 to 53.7 psu. However, in the current state of the system, salinity and temperature levels in the northern regions frequently exceed this value. Continued freshwater deprivation may, therefore, further limit the distribution range of this species.

Lake St Lucia, Africa’s largest estuarine lake in crisis: Combined effects of mouth closure, low levels and hypersalinity

The Lake St Lucia estuarine system is the most important nursery ground for juvenile marine fish and prawns along the KwaZulu-Natal coast. The estuary mouth closed in June 2002 because of drought and remained so for 4 years and 9 months. A study to determine the impacts of extended mouth closure, hypersalinity and low lake levels on the mesozooplankton, macrobenthic invertebrates and fish fauna was initiated in 2004. Zooplankton and benthic invertebrate diversity declined, benthic invertebrate community composition changed and the diversity and abundance of fish decreased between 2004 and 2007. In the case of fish, the declines were related to die-offs in the lake and the failed recruitment of post-larvae and juveniles from the marine environment as a result of the mouth having been closed. Options for management intervention under closed-mouth conditions are limited at this time, particularly in the short term, to breaching the mouth and facilitating the inflow of sea water. In the medium term, as was the historical situation, the reconnection of the Mfolozi system to St Lucia should be viewed as a major priority.

Zooplankton of the St. Lucia Estuary during the current drought cycle: a comparison between open- and closed-mouth conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 399:157-171 (2010) - DOI: https://doi.org/10.3354/meps08355 Zooplankton of the St. Lucia Estuary during the current drought cycle: a comparison between open- and closed-mouth conditions Nicola K. Carrasco1,*, Renzo Perissinotto1, Deena Pillay2 1School of Biological and Conservation Sciences, University of KwaZulu-Natal, Westville Campus, Durban 4000, Private Bag X5 4001, South Africa 2Marine Research Institute, Zoology Department, University of Cape Town, Rondebosch 7701, Private Bag X3, South Africa *Email: nicola.carrasco@gmail.com ABSTRACT: The St. Lucia Estuary is currently experiencing a drought-induced crisis, resulting in the system having been closed off from the sea for approximately 8 yr. This closure was interrupted by a brief open-mouth phase, induced by a unique combination of extreme climatic events. The primary aim of the present study was to compare zooplankton dynamics during open- and closed-mouth conditions. Sampling was undertaken during quarterly surveys from February 2006 to November 2008. During the closed-mouth phase, up to 70% of the lake bed was dry and salinities in the northern lakes often exceeded 90, making these areas largely uninhabitable for zooplankton. However, in the lower regions where drought effects were less harsh, zooplankton were characterized by high densities and biomass of typical estuarine taxa such as the copepods Pseudodiaptomus stuhlmanni, Acartia natalensis and the mysid Mesopodopsis africana. Of the 69 taxa recorded during the study period, only 27 were present during the closed-mouth phase. Under open-mouth conditions, previously excluded marine taxa (e.g. the prawn Penaeus indicus and fish larvae), once again re-entered the system, increasing its diversity significantly. A unique occurrence after mouth re-closure was the colonisation of the mouth area by swarms of the tunicate Oikopleura dioica (>103 ind. m–3), while previously dominant zooplankton grazers virtually disappeared. These findings emphasize the complexity of the system and stress the need for further research into the potential impacts of environmental and climate changes on this key African estuarine lake. KEY WORDS: Zooplankton · Drought · St. Lucia Estuary · Extreme events · Oikopleura dioica Full text in pdf format PreviousNextCite this article as: Carrasco NK, Perissinotto R, Pillay D (2010) Zooplankton of the St. Lucia Estuary during the current drought cycle: a comparison between open- and closed-mouth conditions. Mar Ecol Prog Ser 399:157-171. https://doi.org/10.3354/meps08355 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 399. Online publication date: January 28, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Historic and recent (2006) state of mangroves in small estuaries from Mlalazi to Mtamvuna in KwaZulu-Natal, South Africa

The new forest type classification considers mangroves to be the rarest and most threatened of forest types in South Africa. The aim of this study was to determine the change in distribution and the current population and community structure of mangrove forests along the KwaZulu-Natal coastline. Potential threats to mangroves in South Africa include wood harvesting, altered water-flow patterns coupled with salinity changes, and the potential for permanently open-mouth conditions to become prolonged closed-mouth conditions. A literature review and field sampling were undertaken to gather information on historic and current mangrove forest status. Population structure was assessed using height and diameter at breast height in three 25 m2 quadrats per site in four estuaries with mangroves. Mangroves were completely lost from 11 estuaries between 1982 and 1999. These losses can be related to changes in open-mouth conditions and intertidal habitat, which are important for mangrove survival. The mouth condition of these estuaries was observed over a nine-year period from 1996 to 2005. Mangroves only occurred in those estuaries where the mouth was open for more than 56% of the time. Rhizophora mucronata has been lost from the Mkomazi Estuary and almost completely from the Mlalazi Estuary; the distribution of this mangrove species is very limited and density is low at 0.5 ± 0.4 m-2. Most mangrove forests were regenerating as they had inverse J-shaped curves as well as high adult:seedling ratios (1:2 for Bruguiera gymnorrhiza, 1:19 to 1:7 for Avicennia marina, and 1.8 to 3:1 for R. mucronata). None of the forests showed signs of harvesting for poles, and the greatest threat seems to be altered water-flow patterns due to freshwater use in the catchments and the change of land use from wetland to sugar-cane plantations. Effective management of these mangroves begins with determining the freshwater requirements of the estuaries to maintain the mouth dynamics and biotic communities. Further management is needed to ensure that mangroves are cleared of pollutants (plastic and industrial), and any further developments near them should be minimised. Management plans for each mangrove forest should be drafted to ensure long-term conservation of the mangroves in KwaZulu-Natal.

Status of the estuarine fish fauna in the St Lucia Estuarine System, South Africa, after 30 months of mouth closure

The Lake St Lucia Estuarine System, South Africa, has been under pressure due to recent drought conditions, which led to extreme low lake levels, hypersaline conditions and closure of the mouth. The estuary mouth closed in June 2002 and has remained so for more than two-and-a-half years. A lack of information on biotic responses during extended closed-mouth conditions prompted a study to ascertain the current status of the fish fauna in the system. Fish were sampled with seine and gill-nets at six sites spread throughout the system. At the time of sampling in December 2004, the system had become partitioned into four isolated compartments. A total of 4 475 fish comprising 30 species were recorded during the survey. The CPUEs were generally low, except for those of Oreochromis mossambicus, Liza dumerili and Ambassis ambassis. The freshwater cichlid O. mossambicus made up 45% of the total catch. The species composition in each of the four compartments of the system were very different. Historical data indicated a loss of about 40% of the commonly-occurring species from the lake since mouth closure, reflecting a substantial change in species composition from that which occurs under 'normal' conditions. It is expected that the decline in the number of species and the abundance of remaining taxa will continue if drought conditions persist and the mouth remains closed.