Abundance,recruitment and residency of two sparids in an intermittently open estuary in South Africa

The recruitment of distinct year-class cohorts in two sparid species, Rhabdosargus holubi and Lithognathus lithognathus, were linked to records of daily mouth state in the intermittently open East Kleinemonde Estuary, South Africa, between 1995 and 2006. L. lithognathus only recruited into the estuary in years when the mouth opened between late August and January. This was attributed to a limited spawning season and inability to recruit during wave overwash events. In contrast, R. holubi recruitment was uninterrupted and not influenced by seasonality of estuary access opportunities (mouthopening and overwash events). Estuarine-dependent residency periods ranged from 27 months to 48 months for L. lithognathus and 12 months to 23 months for R. holubi. The interannual abundances of these estuarydependent sparids were determined by reproductive seasonality, recruitment strategy and seasonal timing of estuarine access opportunities.     

The effect of a storm surge event on the macrophytes of a temporarily open/closed estuary,South Africa

Temporarily open/closed estuaries typically open to the sea due to freshwater inflow coupled with storm surge events. In September 2008, in the absence of freshwater inflow, the mouth of the East Kleinemonde Estuary breached in response to a storm surge. The mouth of the estuary closed the following day at a high level. Marine overwash events following the breach introduced large volumes of saline water into the estuary and raised the water level by 0.07–0.33 m. Salinity was significantly higher in the 15 month closed phase after the breach (31 ± 0.9) compared to 21.9 ± 0.9 in the closed brackish phase before the breach. The historical average salinity for the estuary during a closed period is 23–25. The increase in salinity has reduced submerged macrophytes Ruppia cirrhosa and Chara vulgaris cover by 38.1%. Macroalgal cover of species such as Dictyota dichotoma, Caulacanthus ustulatus, Codium tenue and Ulva spp. have increased by 7.9%. The saline high water levels have also significantly reduced supratidal salt marsh cover by 15.2%, and reed and sedge cover by 19.7%. Loss of these habitats may result in bank destabilisation and erosion. This is the first record of an extended saline period in the 15 years the estuary has been monitored. Sea level rise in association with climate change, together with localised freshwater inflow reduction is likely to result in an increase in marine overwash events. The frequency and duration of closed saline periods are likely to increase in this type of estuary. A loss of submerged macrophytes may have significant impacts on faunal composition and abundance and on the subsequent functioning of temporarily open/closed estuaries. This has serious ecological implications since these estuaries represent 70% of the different types of estuaries found in South Africa.     

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

Research in South African temporarily open/closed estuaries that includes studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fishes and birds is used as a basis to review the ecology and management of this estuary type on the subcontinent. Particular attention is given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by riverine and marine events, as well as anthropogenic influences. In addition, the wider implications of these research findings for the management of temporarily open/closed estuaries in terms of freshwater supply are explored, together with the role of government legislation in maintaining the ecological integrity of these important wetland systems.     

A backbarrier overwash record of intense storms from Brigantine, New Jersey

Analysis of aerial photographs and historic charts indicates that the barrier beach at Brigantine, NJ has migrated landward 300 to 400 m since 1869, primarily as a result of overwash during hurricanes and winter storms. A series of vibracores from the backbarrier salt marsh reveals a millennial-scale stratigraphic record of overwash deposition. Carbon-14 (C-14) and Cesium-137 (Cs-137) radioisotopic methods were used to date overwash deposits (washovers). The ages of recent washovers are consistent with deposition during intense storms in 1938, 1944, 1950, and 1962. An additional overwash deposit recovered in five of the sediment cores was likely deposited by an intense hurricane strike in 1821 or possibly in 1788. Two prehistoric overwash fans were likely deposited by intense storms striking the New Jersey Coast in the 7th to 14th centuries and 6th to 7th centuries A.D. The landward barrier migration indicates that the older overwash sediments were likely transported a considerably greater distance than the more recent overwash fans. The greater distance of transport may indicate that the prehistoric storms that deposited overwash fans across the study site were more intense than the most intense storm to strike this coast in the historic period, the hurricane of 1821. The spatially variable occurrence of overwash deposition at this site points to a need for multisite stratigraphic surveys of extensive stretches of the coast in order to develop reliable records of past intense storm frequency from backbarrier environments.     

Effects of marine reserves on the reproductive biology and recruitment rates of commonly and rarely exploited limpets

We tested the effects of marine reserve status on the reproductive biology and recruitment rates of two commonly exploited (Helcion concolor, Scutellastra longicosta) and two rarely exploited (Cellana capensis, Scutellastra granularis) limpet species using two reserve and two non-reserve sites on the south coast of South Africa. Because densities of commonly exploited species are lower outside reserves, we hypothesised that limpets outside reserves will: (i) mature earlier; and (ii) show higher individual reproductive output. Rarely exploited species should show no such effects. For recruitment, we hypothesised that if recruits are attracted to adults or survive better where there are more adults, then commonly exploited (but not rarely exploited) species should show higher recruitment inside reserves. Analyses of gonadosomatic indices produced significant Month × Reserve interactions for all species (ANCOVA, p < 0.05), indicating inconsistent reserve effects even for exploited species, although months with significant differences occurred more often in the commonly exploited species, giving weak support to the hypothesis regarding reproductive output. Reserve status had no significant effect on sex ratios or size at sexual maturity for either sex of any species. Accordingly, reserves had only a minor influence on reproduction in these species, which might reflect an indirect effect of poor policing. The timing of major recruitment events differed among species, with two broad patterns: recruitment was sporadic for C. capensis and S. longicosta and protracted for H. concolor and S. granularis. Recruit densities showed strong site effects. Highest recruitment occurred at the exploited site, Xhora, with no reserve effects, except in a single month for C. capensis, indicating that recruitment is independent of whether the site is protected or not. Therefore, reserves do not enhance recruitment directly, but because these populations appear to be open on scales of tens of kilometres, generally higher densities of limpets within reserves could enhance overall recruitment, because of greater gamete output by the population. Accordingly, reserves might have an indirect ‘spillover’ effect on recruitment, if not a direct one.     

Modelling storm impacts on beaches, dunes and barrier islands

A new nearshore numerical model approach to assess the natural coastal response during time-varying storm and hurricane conditions, including dune erosion, overwash and breaching, is validated with a series of analytical, laboratory and field test cases. Innovations include a non-stationary wave driver with directional spreading to account for wave-group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion. The model performs well in different situations including dune erosion, overwash and breaching with specific emphasis on swash dynamics, avalanching and 2DH effects; these situations are all modelled using a standard set of parameter settings. The results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front. The simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching.     

Spatio-temporal patterns of coral recruitment at Vamizi Island,Quirimbas Archipelago,Mozambique

Spatial and temporal patterns of recruitment of reef corals were assessed for the first time in Mozambique by deploying settlement plates at various spatio-temporal scales between October 2012 and October 2013. The abundance of juvenile corals (5?50?mm in diameter) was assessed along transects. Settlement of acroporids was highly seasonal, with 97% of spat settling between July and October 2013. Pocilloporids settled throughout the year, peaking slightly between October 2012 and January 2013. The annual mean larval settlement of up to 1 135 spat m^?2 was comparable to that on other East African reefs, but was dominated by acroporids, which constituted over 80% of all spat, whereas pocilloporids settle in higher densities in Kenya and South Africa. The peak settlement season also differed from other African locations. A greater proportion of variance in settlement rates occurred at the spatial scale of kilometres (between sites) and centimetres (between settlement plates) than at the scale of metres (between subsites), implying that most patchiness occurs at those scales. The peak in acroporid settlement coincided with the period of multispecific spawning, with settlement occurring as early as nine days after a spawning event. As no similar spawning events have been reported for other reefs in the area, our results suggest that these spawning events strongly influence overall annual settlement rates and promote high levels of self-seeding at Vamizi. There was no relationship between settlement of larvae to settlement plates and juvenile density on adjacent reefs, suggesting either variable levels of early post-settlement mortality or high interannual variability in settlement.     

Swash overtopping and sediment overwash on a truncated beach

New experimental laboratory data are presented on swash overtopping and sediment overwash on a truncated beach, approximating the conditions at the crest of a beach berm or inter-tidal ridge-runnel. The experiments provide a measure of the uprush sediment transport rate in the swash zone that is unaffected by the difficulties inherent in deploying instrumentation or sediment trapping techniques at laboratory scale. Overtopping flow volumes are compared with an analytical solution for swash flows as well as a simple numerical model, both of which are restricted to individual swash events. The analytical solution underestimates the overtopping volume by an order of magnitude while the model provides good overall agreement with the data and the reason for this difference is discussed. Modelled flow velocities are input to simple sediment transport formulae appropriate to the swash zone in order to predict the overwash sediment transport rates. Calculations performed with traditional expressions for the wave friction factor tend to underestimate the measured transport. Additional sediment transport calculations using standard total load equations are used to derive an optimum constant wave friction factor of f_w = 0.024. This is in good agreement with a broad range of published field and laboratory data. However, the influence of long waves and irregular wave run-up on the overtopping and overwash remains to be assessed. The good agreement between modelled and measured sediment transport rates suggests that the model provides accurate predictions of the uprush sediment transport rates in the swash zone, which has application in predicting the growth and height of beach berms.     

The influence of environmental factors on the abundance and recruitment of the sand crab Emerita analoga (Stimpson 1857): Source–sink dynamics?

The sandcrab Emerita analoga is the dominant species inhabiting sandy beaches along the Pacific coast of the American continent. In our study, 10 sandy beaches were sampled seasonally from 2006 to 2011, including coastal planktonic sampling from 2006 to 2008. Two major population cores were detected, the first one in the northern part of the study area and the second in the area immediately to the south of the Itata River mouth. Zoeal stages were found along the entire coastal zone. Highest densities and recruitment were found during spring and summer of each year. PLS regression indicated that source–sink habitat proxies correlated positively with morphodynamic parameters; while beach slope and total organic matter were negatively correlated. These results agree with the source–sink hypothesis, finding higher densities of adults, recruits and cohort recurrence on open coast beaches with milder physical dynamics. Furthermore, a hypoxic event and a mega-earthquake/tsunami negatively affected recruitment at the inter-annual scale.     

Sedimentary records of intense storms in Holocene barrier sequences, Maine, USA

Coastal-morphological, geophysical (ground-penetrating radar [GPR]), and sedimentological data document extreme storm events along the sandy barriers of Maine's south–central (Hunnewell and Flat Point barriers) and southwestern (Saco Bay barriers) coastal compartments. The Hunnewell barrier contains four equally spaced buried storm scarps behind the exposed scarp of the Blizzard of 1978, a 100-year storm that eroded more than 100 m of shoreline, causing extensive property loss. These scarps dip 3–5° steeper than the normal beachface slope and consist of sands with more than 50% heavy minerals. The heavy minerals produce distinct subsurface reflections that facilitate the location of buried supratidal parts of storm scarps and the mapping of ancient poststorm shoreline positions. The imaged scarps likely formed within the past 1.5–2.0 ka BP. The Flat Point barrier consists of a prograded sequence overlain by a laterally extensive, seaward-thinning layer of freshwater peat and capped by aeolian sands. This stratigraphy suggests that the bog varied in size through time, contracting during overwash events and aeolian deposition and expanding across washover sheets during extended periods of barrier stability. The main overwash event accompanied by barrier planation and wetland expansion may be linked to the first historical storm in New England, the “Great Colonial Hurricane” of 1635.

Evidence of near-modern and mid-Holocene storm events along Saco Bay includes washover units and marsh ridges. Washovers interfinger with saltmarsh peat that ranges in age from 4.5 ka BP to modern. The presence of isolated sandy ridges behind existing and former tidal inlets reflects overtopping of marshes and high intertidal mudflats during major storms. Radiocarbon ages indicate that this process took place at different locations along the Saco Bay barrier complex from 3 to 1 ka BP.     

Alternative nursery habitat for estuarine associated marine fish during prolonged closure of the St Lucia estuary,South Africa

The Mfolozi estuary, located on the east coast of South Africa, was historically directly linked to the adjacent St Lucia estuarine system, the largest estuarine system in Africa and a World Heritage Site. The Mfolozi used to be the main feeder system to maintain lake levels in St Lucia, but increased siltation from sugar cane farming in the Mfolozi floodplain led to artificial separation of the two systems in 1950. Reduced freshwater inflow due to drought conditions caused the St Lucia mouth to remain closed from June 2002 to present, coinciding with low lake levels and hypersaline conditions, except for a brief period during 2007 after the St Lucia mouth breached. These conditions led to disruption of larval recruitment into the system and major changes in biotic communities. Due to the importance of the St Lucia – Mfolozi System link, a study was initiated in 2007 on the fish community of the Mfolozi system, which was sampled using seine and gill nets. The 48 species recorded were dominated by juveniles of marine spawners, particularly Leiognathus equula and Valamugil cunnesius and the estuarine spawners Ambassis dussumieri and Ambassis natalensis. Estuarine dependent marine spawning species formed 68% of both the species numbers and CPUE, an indication of the regional importance of the Mfolozi estuary as an alternate refuge for juvenile marine fish during periods when the St Lucia system remained closed. Post-larval recruits of marine spawning species were particularly abundant, but low zoobenthic densities caused a rapid decline in numbers of benthic feeders shortly after their recruitment into the system. The importance of the Mfolozi estuary in maintaining marine brood stocks of estuarine dependent marine fish is discussed with particular reference to estuarine degradation and the ecological integrity of the St Lucia system.     

Modeling of storm-induced coastal flooding for emergency management

This paper describes a model package that simulates coastal flooding resulting from storm surge and waves generated by tropical cyclones. The package consists of four component models implemented at three levels of nested geographic regions, namely, ocean, coastal, and nearshore. The operation is automated through a preprocessor that prepares the computational grids and input atmospheric conditions and manages the data transfer between components. The third generation spectral wave model WAM and a nonlinear long-wave model calculate respectively the wave conditions and storm surge over the ocean region. The simulation results define the water levels and boundary conditions for the model SWAN to transform the storm waves in coastal regions. The storm surge and local tides define the water level in each nearshore region, where a Boussinesq model uses the wave spectra output from SWAN to simulate the surf-zone processes and runup along the coastline. The package is applied to hindcast the coastal flooding caused by Hurricanes Iwa and Iniki, which hit the Hawaiian Island of Kauai in 1982 and 1992, respectively. The model results indicate good agreement with the storm-water levels and overwash debris lines recorded during and after the events, demonstrating the capability of the model package as a forecast tool for emergency management.     

Long-term stability of the fish assemblages in a warm-temperate South African estuary

Little is known about long-term changes in estuarine fish populations and related environmental variations. Fishes in the temporarily open/closed East Kleinemonde Estuary were sampled bi-annually, in summer and winter, using seine and gill nets between December 1994 and July 2005. A total of 18 families, represented by 33 species, were recorded. The 10 most abundant species caught were consistently recorded in catches each year, but CPUE of individual species varied on an annual basis and this can often be related to mouth state. Multivariate analyses of the annual marine fish community identified two distinct groups, with more species recorded during years that succeeded spring (September to November) mouth-opening events than in years following no mouth-opening events in spring. Interannual community stability (IMD) and seriation (IMS) also increased from the years following no opening events in spring to the years that succeeded spring opening events. These results highlight the importance of the timing of mouth opening to the marine fish community in a temporarily open/closed estuary. This study reinforces the importance of long-term studies to understanding community changes in estuaries caused by environmental variations over different time scales.     

疏浚物海洋倾倒区封闭条件探讨

海洋倾倒区封闭是海洋倾废管理的主要内容之一。目前疏浚物倾倒活动是一项比较频繁的海上倾倒活动。疏浚物海洋倾倒区的封闭,需要有一个相对统一的标准和判断方法,本文提出用外溢作为一种判断方法,且将倾倒 溢发为无外溢、少量外溢、显著外溢、严重外溢和最严重外溢五类,认为当倾倒区出现显著外溢时,应考虑限制使用;当倾倒区出现严重外溢时,考虑封闭;当出现危害性外溢时,就立即封闭。     

Response of the mesozooplankton community of the St Lucia estuary,South Africa,to a mouth-opening event during an extended drought

Mesozooplankton samples were collected between March 2005 and November 2008 in St Lucia, the largest estuarine lake system in South Africa. St Lucia experienced an extended period of drought before and during the present study. This drought led to natural closing of the estuary mouth as a result of flood-tide marine sediment deposition in 2002. In March 2007 the mouth was washed open by exceptionally high tidal and wave conditions. This resulted in an influx of a large volume of seawater. The mouth closed again in August 2007. Before opening of the mouth salinities in the Estuary were below 10 and large parts of North Lake dried up while South Lake retained a relatively stable waterbody with salinities between 10 and 30. When the mouth opened seawater flooded the system and salinities changed to about 35. After the mouth had closed again in August 2007 salinities increased in the lakes and decreased in the Estuary. The mesozooplankton community was dominated by copepods during all sampling sessions, especially by the estuarine calanoids Pseudodiaptomus stuhlmanni and Acartia natalensis. Mean mesozooplankton densities were significantly higher in South Lake before the mouth opened in March 2007. While zooplankton density decreased when the mouth opened species richness increased with the influx of coastal marine species, especially in the Estuary. Overall zooplankton densities declined progressively as salinity increased to hypersaline levels after mouth closure. Multivariate analyses supported significant differences between the lakes and the Estuary in terms of mesozooplankton community composition. Taxa mostly responsible for the similarities within and dissimilarity between sections of the system were the copepods P. stuhlmanni and A. natalensis with the meroplankton, crab zoeae and mollusc larvae, also contributing significantly after the mouth-opening event.     

Pacific Basin climate variability and patterns of Northeast Pacific marine fish production

A review of oceanographic and climate data from the North Pacific and Bering Sea has revealed climate events that occur on two principal time scales: a) 2–7 years (i.e. El Niño Southern Oscillation, ENSO), and b) inter-decadal (i.e. Pacific Decadal Oscillation, PDO). The timing of ENSO events and of related oceanic changes at higher latitudes were examined. The frequency of ENSO was high in the 1980s. Evidence of ENSO forcing on ocean conditions in the North Pacific (Niño North conditions) showed ENSO events were more frequently observed along the West Coast than in the western Gulf of Alaska (GOA) and Eastern Bering Sea (EBS). Time series of catches for 30 region/species groups of salmon, and recruitment data for 29 groundfish and 5 non-salmonid pelagic species, were examined for evidence of a statistical relationship with any of the time scales associated with Niño North conditions or the PDO. Some flatfish stocks exhibited high autocorrelation in recruitment coupled with a significant step in recruitment in 1977 suggesting a relationship between PDO forcing and recruitment success. Five of the dominant gadid stocks (EBS and GOA Pacific cod, Pacific hake and EBS and GOA walleye pollock) exhibited low autocorrelation in recruitment. Of these, Pacific hake, GOA walleye pollock and GOA Pacific cod exhibited significantly higher incidence of strong year classes in years associated with Niño North conditions. These findings suggest that the PDO and ENSO may play an important role in governing year-class strength of several Northeast Pacific marine fish stocks.     

Spatial and temporal distribution of floating kelp in the channels and fjords of southern Chile

Numerous studies suggest that floating macroalgae contribute to population connectivity of the algae themselves as well as of associated organisms. In order to evaluate the importance of floating macroalgae for population connectivity in southern Chile we analysed their spatial and temporal distributions. We estimated the abundance of floating kelp in the Interior Sea of Chiloé and the Los Chonos Archipelago via ship surveys conducted during austral spring (November) and winter (July) of the years 2002–2005. Highest densities of floating kelp (Macrocystis pyrifera and Durvillaea antarctica) were found during spring. Generally, the density of floating kelp was relatively low in areas with high supply of freshwater, and highest abundances were found in large channels opening to the open ocean. We suggest that this pattern is caused by the scarcity of natural kelp beds combined with rapid degradation of floating kelp in the interior fjords and by seaward surface outflow. The maximum densities of floating kelps were found in an area known to be an extensive convergence zone (i.e. estuarine front), which appears to act as a retention zone. In accordance with this distribution pattern, we propose that connectivity between local populations via floating kelp is higher in the outer channels with oceanic influence than in the inner fjords with high freshwater influx.     

The effects of wastewater discharges on the functioning of a small temporarily open/closed estuary

Wastewater discharges affect the functioning of small temporarily open/closed estuaries (TOCEs) through two main mechanisms: (1) they can significantly change the water balance by altering the quantity of water inflows, and (2) they can significantly change the nutrient balance and hence the water quality. This study investigated the bio-physical responses of a typical, small TOCE on the east coast of South Africa, the Mhlanga Estuary. This estuary receives significant inflows of treated effluent from upstream wastewater treatment works. Water and nutrient budgets were used together with biological sampling to investigate changes in the functioning of the system. The increase in inflows due to the effluent discharges has significantly increased the mouth breaching frequency. Furthermore, when the mouth closes, the accumulation of nutrients leads to eutrophication and algal blooms. A grey water index, namely the proportion of effluent in the estuary and an indicator of the additional nutrient inputs into the estuary, reached high values (?50%) during low flow regimes and when the mouth was closed. In these hyper-eutrophic conditions (DIN and DIP concentrations up to 457 μM and 100 μM respectively), field measurements showed that algal blooms occurred within about 14 days following closure of the mouth (chlorophyll-a concentrations up to 375 mg chl-a m⁻³). Water and nutrient balance simulations for alternative scenarios suggest that further increases in wastewater discharges would result in more frequent breaching events and longer open mouth conditions, but the occurrence of hyper-eutrophic conditions would initially intensify despite more frequent openings. The study indicates how water and nutrient balance simulations can be used in the planning and impact assessment of wastewater treatment facilities.     

Hurricanes and coral bleaching linked to changes in coral recruitment in Tobago

Knowledge of coral recruitment patterns helps us understand how reefs react following major disturbances and provides us with an early warning system for predicting future reef health problems. We have reconstructed and interpreted historical and modern-day recruitment patterns, using a combination of growth modelling and in situ recruitment experiments, in order to understand how hurricanes, storms and bleaching events have influenced coral recruitment on the Caribbean coastline of Tobago. Whilst Tobago does not lie within the main hurricane belt results indicate that regional hurricane events negatively impact coral recruitment patterns in the Southern Caribbean. In years following hurricanes, tropical storms and bleaching events, coral recruitment was reduced when compared to normal years (p = 0.016). Following Hurricane Ivan in 2004 and the 2005–2006 bleaching event, coral recruitment was markedly limited with only 2% (n = 6) of colonies estimated to have recruited during 2006 and 2007. Our experimental results indicate that despite multiple large-scale disturbances corals are still recruiting on Tobago’s marginal reef systems, albeit in low numbers.     

ENSO事件中热带太平洋上层海洋热含量变化分析

利用1955-2001年海洋上400m层的热含量资料，分析了20世纪60年代以来全部El Nino／La Nina事件期间上层海洋热含量异常的空间分布规律和传播特性，发现如下3个重要观测事实：(1)在ENSO循环期间存在着关于赤道对称的南北2个热含量异常信号的闭合传播路径；(2)北半球的信号闭合回路非常清晰，而南半球回路较为混乱；(3)这种闭合回路在El Nino／La Nina年份表现得非常明显，但在其它年份可能由于海洋或大气的条件不匹配而中途夭折。上述观测事实对于ENSO研究具有重要价值，其物理机制值得进一步探讨。