Low Spring Tide Research Articles

Synoptic hydrography of a highly stratified estuary

This study investigates the hydrodynamic characteristics of the lower, middle, and upper sectors of a highly stratified estuary, the Itajai-Acu river estuary (south of Brazil ∼27° S/48.5° W). The study is based on a 25-h field campaign with three sampling stations positioned at 2, 17, and 38 km inward from the river mouth, during low river discharge condition and spring tide. The experimental data gathered was reduced and analyzed in terms of distribution of variables in time and space tide average vertical profiles and decomposition of the advective transport of salt and suspended particulate matter (SPM). Tidal range was nearly constant along the estuary, presenting time lag of about 2 h between lower and upper estuary. The ebb discharge peaks were about twice the discharge flood peaks and occurred simultaneously. The tide was the main determining agent in the lower estuary, where currents, salt stratification, and SPM distributions presented a repetitive behavior. In the middle estuary, the tide effects were also observed, but the presence of saline waters decreased along the time due to increasing river discharge during the campaign. The distribution of SPM in the mid- and upper estuary presented patched pattern not associated with tides and may be attributed to short-term flood contributions of tributaries. Currents presented ebb dominance in all three sectors; in the middle and upper estuary, they presented also a time asymmetry, with ebb currents longer than flood. The advective transport of salt in the lower estuary was upstream, with dominance of gravitational circulation term. In the mid-estuary, there was practically no transport, with balance between fluvial discharge (downstream) and tidal correlation (upstream). The advective transport of SPM was upstream in the lower estuary and downstream in the mid- and upper estuary, being dominated by gravitational circulation in the former and fluvial discharge in the others.

The Nutrient Status of Mgazana, a Warm Temperate Mangrove Estuary in the Transkei, Eastern Cape, South Africa

Mgazana, a rural southern African mangrove system, was visited monthly from August, 1995 to February, 1997 to collect water samples for nutrient analysis. Surface and bottom samples were taken during spring low tide at seven stations along the estuary and the following physico-chemical parameters measured: river flow, temperature, salinity, oxygen, transparency, ammonia, nitrite, nitrate, phosphate, inorganic carbon (IC), organic carbon (OC), total carbon (TC), soluble nitrogen (SN), particulate nitrogen (PN) and total nitrogen (TN). Using correlation matrix analysis and ANOVA, river flow was found to affect estuarine salinity, transparency and stratification, which influenced nutrient dynamics. Significant seasonal (winter and summer) differences were found for temperature, river flow, nitrate, SN, TN, IC and OC. Most nutrients were significantly correlated with river flow showing gradients down the estuary, indicating allochthonous input from the catchment. OC levels within the estuary were high, probably due to autochthonous mangrove leaf-fall processing by the various in-fauna, but high levels measured at the head of the estuary during high river flow suggested additional allochthonous input from coastal forest litter. Conversely, IC was negatively correlated with river flow suggesting that autochthonous faunal and microbial mineralisation of organic matter occurs within creeks, which is then diluted by increased stream-flow. An N:P ratio of 2.7:1 was obtained for this rural mangrove system, which was low compared with Spartina-based East Cape estuaries subject to urban, industrial and agricultural pollution.

Day–night variation of cohesive sediment stability

Surface sediment properties related to cohesive sediment stability were measured over 8 consecutive day- and night-time emersion periods at three upper intertidal sites on a mudflat in August 2003, during the transition from spring to neap tides. Significant differences between day- and night-time critical erosion shear stress ( τ c) and chlorophyll a were found. A high degree of temporal and spatial variability existed between the sediment properties. During the first half of the study period, a rhythmic day–night variation occurred between τ c, chl a, colloidal-S- and EDTA-extracted carbohydrate. During the second part of the study, the magnitude of variation of these parameters diminished. Results showed that sediments were more stable during the day than at night. Differences between day- and night-time sediment stability were related not only to diatom migration, but also to wave energy during preceding immersion periods. No significant relationships existed between τ c and either chl a, or colloidal-S- or EDTA-extracted carbohydrate sediment content. It is suggested that tidal phasing, in terms of both the time during the day at which low water spring and neap tides occur, as well as the duration of the emersion period, control the biomass dynamics. The tidal phasing effect is expected to be more pronounced on a cohesive intertidal flat where low water spring tides occur at noon and midnight. The results of this study will be of use in time-dependent estuarine models.

Estuary-dependence of larval fishes in a non-estuary associated South African surf zone: evidence for continuity of surf assemblages

Larval fishes were collected in the Cape Padrone surf zone on the southeast coast of South Africa, using a modified small-mesh seine net. The aim of the study was to assess the composition of fish larvae, with respect to their association with estuaries, in a surf zone that was not in close proximity to an estuary (>5 km). Sampling took place bimonthly during diurnal spring low tides between March and July 2003. In total, 544 fish were caught in the surf zone, comprising 14 families represented by 19 positively identified species, as well as an additional two species that were differentiated but remain unidentified. The families Mugilidae (65%) and Sparidae (26%) dominated the larval catch. The majority of larval fishes caught were in the postflexion stage of development, although some early juveniles were also caught. Body lengths of fish larvae ranged between 2 and 28 mm, with the majority of larvae at the recruitment size for the species. A high proportion of the fish species caught were estuary-dependent. Estuary-dependent marine fish larvae (categories I, II and IV) comprised 68% of total catch by species and 98% by number of individuals. Exclusively marine species (category III) were encountered in low numbers in the surf. The present study provides evidence for continuity in temperate South African surf zones in terms of domination by estuary-dependent larvae and reasons for this pattern are discussed.

Coral reef sedimentation on Rodrigues and the Western Indian Ocean and its impact on the carbon cycle

Coral reefs in the southwest Indian Ocean cover an area of ca. 18,530 km2 compared with a global reef area of nearly 300,000 km2. These regions are important as fishing grounds, tourist attractions and as a significant component of the global carbon cycle. The mass of calcium carbonate stored within Holocene neritic sediments is a number that we are only now beginning to quantify with any confidence, in stark contrast to the mass and sedimentation rates associated with pelagic calcium carbonate, which have been relatively well defined for decades. We report new data that demonstrate that the reefs at Rodrigues, like those at Reunion and Mauritius, only reached a mature state (reached sea level) by 2-3 ka: thousands of years later than most of the reefs in the Australasian region. Yet field observations show that the large lagoon at Rodrigues is already completely full of carbonate detritus (typical lagoon depth less than 1 m at low spring tide). The presence of aeolian dunes at Rodrigues indicates periodic exposure of past lagoons throughout the Pleistocene. The absence of elevated Pleistocene reef deposits on the island indicates that the island has not been uplifted. Most Holocene reefs are between 15 and 20 m in thickness and those in the southwest Indian Ocean appear to be consistent with this observation. We support the view that the CO2 flux associated with coral-reef growth acts as a climate change amplifier during deglaciation, adding CO2 to a warming world. southwest Indian Ocean reefs could have added 7-10% to this global flux during the Holocene.

Fecundity of the pantropical fiddler crab Uca annulipes (H. Milne Edwards, 1837) (Brachyura: Ocypodidae) at Costa do Sol mangrove, Maputo bay, Southern Mozambique

Key words: mangroves, Uca annulipes, fecundity, Mozambique Uca annulipes (H. Milne Edwards, 1837) is probably the most abundant brachyuran crab inhabiting mangrove forests in East Africa. However, its fecundity is poorly understood. Crabs were randomly sampled during spring low tides from January to April 2002 at Costa do Sol mangrove, Maputo Bay, southern Mozambique. Carapace width (CW), abdomen width (AW), weight (W) and egg numbers (EN) were recorded. The average fecundity was 1599 842 eggs, ranging from 529 (CW = 5.0 mm) to 4250 (CW = 24.1 mm) eggs respectively. Egg number increased significantly with increase in crab size. This information contributes to our knowledge of the reproductive potential and the reproductive processes occurring in this species of crab. Western Indian Ocean Journal of Marine Science Vol.3(1) 2004: 87-91

伊豆半島，青野川で出現したシオマネキ類２種について

Occurrences of rare species of fiddler crab, Uca arcuata (De Haan, 1833) and U. lactea (Dc Haan, 1835), are newly recorded from a tidal flat at the mouth of the Aono River, Izu Peninsula, Shizuoka Prefecture, Japan. Numbers of crabs and their sexes were recorded by viewing from a distance, and several of the crabs were collected for measuring their size. Observations and sampling were conducted 12 times from September, 1999, to August, 2002, at low spring tide. During this period, three individuals of U. arcuata were recorded, all males. Numerous individuals of UU lactea were found during the same period, and the maximum number recorded was 36 in September, 2000. Ages of the crabs were estimated from carapace size, based on earlier published population studies. The largest male U. arcuata on the Aono River flat was estimated to be two years old, and the other two males were one year old. Collected males of U. lactea ranged from 15.8 to 18.5mm in carapace width, and females from 13.3 to 15.8mm; the ages of these crabs were estimated as two years or more.

A field based classification scheme for gravel beaches

A tripartite classification of gravel beaches, based upon morphodynamic properties, is proposed and demonstrated for 42 New Zealand beaches. The main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types. The three types identified from the results are: (1) pure gravel beach; (2) mixed sand and gravel beach; (3) composite gravel beach. Pure gravel beaches have steep slopes (tan β=0.08–0.24) and gravels extending from the storm berm to below mean low water spring tide level. Mixed sand and gravel beaches have moderate slopes (tan β=0.04–0.13) with sand and gravel entirely mixed both cross-shore and at depth. Composite gravel beaches have a steep gravel berm fronted by a low-angle intertidal terrace, with overall beach slopes of tan β=0.05–0.14. On composite beaches there is distinct hydrodynamic cross-shore sorting of the sand and gravel component. These broad types are tested using discriminant analysis and the three classes are shown to be highly significant ( F=16.24, df=8, P<0.00005). The key discriminating variables are Iribarren number, beach width, average grain size and storm berm height.

Derivation of a Tidal Inundation Model to Support Environmental Research in Roebuck Bay (Western Australia)

A method for deriving mean inundation times for a large expanse of intertidal mudflats is presented. Image processing techniques are used to extract waterlines from two Landsat Thematic Mapper scenes at approximately high and low tides, respectively. Along with data based on standard tidal predictions, the waterlines are utilised to derive an initial Digital Elevation Model (DEM) for the tidal flat within a GIS environment. As the model fails to fully cover the mean spring tidal range, a routine is developed to linearly extrapolate this surface to the elevation of the mean spring low tide. Tidal interpolation equations are then utilised to generate an inundation time in hours for each cell on this extrapolated DEM. Output from the generalised model is a series of maps showing isolines of inundation for both the mean spring and neap tidal cycles, as well as sets of point data that may be integrated with existing biological sample data. These data will be used to support an ongoing geologic and biologic investigation of the bay and its ecology.

Dynamics of a subtidal population of the porcellanid crab Pisidia longicornis (Decapoda: Crustacea)

A subtidal population of the anomuran crab, Pisidia longicornis was sampled quantitatively throughout 1997. Divers, using SCUBA equipment, conducted suction sampling each month, with concentrated effort directed towards the settlement season. Pisidia occurred in very high densities, constituting the major component of decapod community abundance. Gravid females were detected between March and September. Newly settled megalopae were present in benthic samples from June until late September. Six distinct settlement events were detected during this period, with pulses coinciding with low spring tides. Body size at settlement decreased as temperature during larval development increased. The rapid growth of each distinct settlement group was followed over time through each subsequent crab stage up to maximum adult size. Growth was suspended between December and April. Survival within each distinct settlement group was density-independent during the first summer's growth. Mortality increased between June and July settlement events, with the rate of decline remaining a linear function.

Shell occupancy of the intertidal hermit crab Clibanarius erythropus (Decapoda, Diogenidae) on São Miguel (Azores)

The intertidal hermit crab Clibanarius erythropus was collected at three sites on Sao Miguel (Azores) during low spring tides. Shells occupied were identified and measured. Crab sizes ranged from 1.78 to 13.67 mm (cephalothoracic shield length), with an average size of 4.40 ± 1.44 mm. Of the 19 different shells utilised, the most frequent were Littorina striata (23.8%), Nassarius incrassatus (22.5%) and Mitra sp. (22.0%). At Fenais da Luz, L. striata was most frequently occupied, while at Agua de Alto it was N. incrassatus and, at Caloura, Mitra sp. shells were most frequently used. Shell selection appears to be determined by respective sizes of hermit crab and shell species. Small size-class crabs occupy more shell species than larger crabs. The smallest crab was found at Fenais da Luz occupying a small Bittium sp., whereas the largest crab was found at Caloura inhabiting Stramonita haemastoma.

Late Holocene sea level and reef-flat progradation, Phuket, South Thailand

Fringing reefs growing in the muddy waters of SE Phuket contain fossil, former reef-front, massive corals that protrude above the intertidal reef flat. The top surfaces of some of these massive fossil corals have been dated and surveyed. The results indicate that these reefs commenced growth 6000 y ago when the sea level (low spring tide) was 1 m above present and that the average rate of reef-front progradation was 1.7 cm per year.

Tidal Time-scale Changes in the Composition of Particulate Organic Matter Within the Estuarine Turbidity Maximum Zone in the Macrotidal Seine Estuary, France: the Use of Fatty Acid and Sterol Biomarkers

During low flow and spring tide conditions of September 1995, the organic character of suspended particulate matter (SPM) was investigated in the estuarine turbidity maximum (ETM) zone located within the inner Seine River estuary, France.The appearance of the ETM was observed 2–4h before low tide, during ebb resuspension into the water column. During the flood resuspension, a second ETM was observed but only in bottom waters. This suggests separation of the surface (fresh waters) from the bottom layer (brackish/marine waters) at maximum flood velocities. Suspended particulate matter composition was relatively constant in terms of organic carbon (2%) and only a slight decrease was observed during the high SPM load.Lipids (i.e. fatty acids and sterols) were used as biomarkers to assess different sources of organic matter including terrestrial inputs, phytoplankton, bacteria and domestic sewage. Given specific assumptions, source-specific biomarker concentrations were used to trace quantitatively temporal changes in the relative and absolute importance of source-derived organic material. Terrestrial matter was a very important part of ETM organic matter [35–58% particulate organic carbon (POC) in ebb ETM at low tide; 10–20% POC at high slack tide], whereas sewage organic matter was a relatively minor (<4% of POC) component. Although, the proportion of phytoplankton matter in the high SPM load was low, phytoplankton was a predominant component of SPM during the flood slack (12–31% of POC). This prominence of SPM was associated with the oxygen saturation due to the diatom bloom in the estuary.

Note on the Trophic Relationships of the Stauromedusa Haliclystus Antarcticus from Subantarctic South Georgia

Stauromedusae are cnidarians that have attracted relatively little ecological study, especially in the southern hemisphere. They are Scyphozoa that develop directly from the scyphistoma, and each consists of a calyx and a more or less distinct aboral peduncle that attaches to the substratum by an adhesive disc. The animals are mobile on the substratum, but have no pelagic phase. The present note originates from observations made on stauromedusae living in intertidal and shallow subtidal waters at Husvik Harbour, South Georgia (54°11′S 38°40′W) in early 1994. The species concerned was identified from Kramp (1961), Carlgren (1930) and Pfeffer (1889) as Haliclystus antarcticus Pfeffer, 1889. This species has attracted little previous study. The most detailed anatomical description was given by Carlgen (1930), while distributional details are given in O'Sullivan (1982) who followed Pfeffer (1889); at present there is no evidence that the species occurs other than at South Georgia.Medusae were found on two substrata. Large animals (~30mm high, inconspicuous dark brown in colour) were found on the underside of boulders at extreme low spring tide level in a bay on the north of Husvik Harbour about 1·2 km from the whaling station and close to Brain Island. Locally they were common, often being close enough to touch one another. Smaller animals (pink-orange in colour; cryptic on the macroalga) were found attached to the brown macroalga Desmarestia menziesii J. Aghardh (Phaeophyceae) collected at low-water spring tide level from Kanin Point on the southern shore of the harbour. Medusae on boulders were briefly emersed on particularly low tides: they collapsed, hanging from the peduncle, when out of water, but did not risk desiccation as their habitat was wet and not exposed to the sun.

Estado morfodinâmico praial no instante da observação: uma alternativa de identificação

Determinations of the morphodynamic state of beach and surf zone were made on four beaches in Rio de Janeiro state, with two beaches located northward and two westward of Cabo Frio. Twenty four monthly measurements of beach and surf zone profiles associated to measurements of wave height and period, sediment settling velocity as well as swash climate were made during low spring tide. The beaches were classified by their modal morphodynamic state, a differentiation due to different wave climate and sediment characteristics. With exception for the reflective state, low correlation between the predicted and observed Morphodynamic State were obtained using Dean's parameter (Ω). This is a consequence of the low correlation between the grain size (and settling velocity) of the beach face sediments and the topographic gradient, when applied to a single beach. A new approach for the identification of the Morphodynamic State is proposed by inferring the beach and nearshore morphology through the comparison of the wave height and period in the outer breaker zone to the height and semi-period of wave in the swash zone.

Estado morfodinâmico praial no instante da observação: uma alternativa de identificação

Para a determinação do estado morfodinmico modal de algumas praias do estado do Rio de Janeiro, foram realizados perfis topográficos transversais acompanhado de medições oceanográficas e sedimentológicas durante vinte e quatro campanhas mensais em condições de baixa-mar de sizígia em quatro praias. estando duas localizadas a norte e duas a oeste do Cabo Frio. As mesmas se diferenciaram pelo clima de ondas, pelas suas características sedimentológicas expressas em termos de velocidade de decantação e conseqüentemente pelo estado modal. A utilização do parmetro Ω de Dean para a identificação do estado morfodinmico no momento da observação apresentou resultados geralmente insatisfatórios, com exceção do estado refletivo, devido à baixa correlação entre a velocidade de decantação dos sedimentos da face da praia e o gradiente topográfico. Uma nova abordagem de determinação do estado morfodinmico, no momento da observação, é proposta através da inferência da morfologia da praia e zona de surfe e arrebentação por meio da comparação da altura e período da onda significativa na zona de arrebentação externa e a altura e duração do espraiamento da onda na face da praia.

Morphodynamcis of beaches between Saquarema and Macaé, RJ

The morphodynamic response to oceanographic variables of beaches located between Saquarema and Macae, Rio de Janeiro State, were monitored trough monthly topographic profiles during low water spring tide. The results showed a high topographic stability ofthe beaches located north of cabo Buzios (Macae and Barra de Sao Joao) and high topographic variability ofthe profiles at Massambaba beach, located westward of cabo Frio.

Temporal and spatial variations in the pattern of shoreline utilization in a region subject to subsistence exploitation

Differences in the pattern and intensity of utilization of rocky intertidal reefs along a rural South African coastline were investigated. Regular surveys at one particular locality, Nqabara, indicated that tidal phase had a major influence on activities; during spring low tides approximately 56% of the visitors were engaged in shellfish‐gathering, whereas at neap lows the most popular activity was swimming. Although factors such as year, season and month of observation had little influence on the deasitites of the major user‐groups, significant differences in the numbers of shellfish‐gatherers and walkers were observed during holidays and term‐time. Much of the variation in the pattern of shoreline utilization at this site was attributed to unpredictable societal influences. Additional surveys, conducted at nine study sites, revealed marked regional differences in shoreline utilization; the central region supported the highest numbers of anglers, seaweed‐harvesters and shellfish‐gatherers. Although the intensity of exploitation observed in this region appears to be comparable with that reported previously in popular metropolitan areas it is likely to have a much greater impact on intertidal biota simply because the activities of rural shore‐goers are less constrained than those of the urban shore‐goers.

Mortality of shallow reef corals in the western Arabian Gulf following aerial exposure in winter

Aerial exposure of patch reef corals occurred in Tarut Bay, western Arabian Gulf, (Saudi Arabia) between December 1991 and May 1992, and coincided with extreme low spring tides (below the predicted lowest astronomical tide-LAT). Colonies of Acropora and Stylophora occurring at the highest levels on the tops of patch reef platforms were most affected by the low tides. Corals fully exposed to air suffered total mortality, whereas those not fully exposed suffered tissue damage to their upper parts. Exposure occurred during winter months when air and water temperatures are at their lowest in the gulf. Coupling of extremely low spring tides with wind-induced negative surges (below LAT) are not regular events but are not infrequent. Cold temperatures and exposure may act in concert to produce disproportionate mortalities of reef flat corals in the shallow coastal areas of eastern Saudi Arabia. It is highly unlikely that the Gulf War oil spill played any role in the observed damage to reef corals in the Gulf in 1992.

Modelling transport processes in the Ribble estuary

Field measurements of velocity and salinity at three stations along a reach of the Ribble estuary were used to evaluate the temporal variations of the vertical shear induced component of the longitudinal dispersion coefficient. The results yield clear temporal trends which lead to the conclusion that vertical shear-induced dispersion, resulting from differential advection and density gradients, is the most important component of the longitudinal mixing on the ebb tide. Turbulent diffusion, due to the high velocities, dominates the mixing mechanisms on the flood tide. The sensitivity to the dispersion coefficient in a one-dimensional model based on a schematised form of the channel geometry of the Ribble estuary revealed the need for a time dependent dispersion coefficient. Measurements of suspended solids revealed low concentrations compared to previous records. It is proposed that this is due to a pumping effect in the estuary, induced by a combination of the tidal asymmetry and the ebb tide vertical density gradients leading to upstream storage during low river flow spring tide periods. Modelling of the turbidity maximum is shown to be possible by adjustment of critical erosion and deposition shear stress terms.