Neap Tide Periods Research Articles

Salt marsh hydrology: Implications for biogeochemical fluxes to the atmosphere and estuaries

Rates of gas emissions and solute fluxes from salt marsh sediments are influenced by sediment hydrology. A comprehensive water balance study in a New England salt marsh reveals that evapotranspiration and infiltration during tidal inundation and precipitation are the dominant hydrological processes in the sediment on a marsh‐wide scale. Water loss by drainage through the sediment into tidal creeks is effectively limited to within 10 m to 15 m of the creek bank; however, drainage is responsible for 40% of the water loss within 10 m of the creek during nonflooding, neap tide periods. The rate and extent of advective transport by pore water drainage is controlled by the topography of the marsh surface. Tidal fluctuations in creek level drive larger, oscillating water fluxes across the creek bank, which results in a dispersive transport of the solutes in the sediment, but these fluxes are attenuated in the first meter. Convexities in the marsh surface, for example, the crests of the creek banks, are the location of maximum water loss by drainage and probably the highest degree of desaturation and aeration, which can, in turn, increase gas emissions locally. The spring‐neap tide cycle modulates wetting and drying of the sediment and, by inference, gas emissions in the interior of the marsh. The limited extent of solute transport by drainage implies that an as yet undescribed mechanism is responsible for controlling the concentration of conservative solutes in salt marshes.

Seasonal and semi‐lunar modulation of the endogenous swimming rhythm in the estuarine amphipodcorophium volutator(pallas)

The endogenous circa‐tidal rhythm of swimming activity shown by Corophium volutator varies seasonally, and also over the spring to neap tide cycle. Semi‐lunar modulation is evident in both phase and amplitude and analysis of the free‐swimming period using a modified correlogram technique indicates that the rhythm also undergoes slight (<0.1 phase) changes in period which are correlated with similar variations in the period of the tide. Activity maxima are greatest at or just after the time of spring tides, and these peaks show a greater phase delay relative to the predicted time of high water than those recorded during the neap tide period. This semi‐lunar variation in phase is not significant during the winter when the rhythm is generally less distinct. It is suggested that the results are best interpreted in terms of two interactive oscillators, one of circadian, the other of circa‐tidal period, and the biological significance of these variations in the swimming pattern is considered.

Plant Zonation and the Effects of the Spring-Neap Tidal Cycle on Soil Aeration in a Humber Salt Marsh

(1) Relationships were sought between species distribution, soil aeration and tidal activity in a well-zoned, ungrazed salt marsh. Redox potential was used as an index of soil aeration and was measured with platinum electrodes permanently installed in the soil to various depths in major vegetation zones. (2) Tide had considerable influence and species composition was related to aeration during summer. Rainfall effects were also evident. (3) Three main soil-aeration patterns emerged. The first, with reducing conditions persisting throughout much of the profile and indifferent to periods of exposure, was characteristic of Spartina anglica sites; only near the surface (<5 cm) and at neap periods were there phases of oxidation. The second was found in Puccinellia maritima-, creek bank (Halimione portulacoides)and Elymus pycanthus-sites; redox potentials were lowered monthly by the high spring tides but at the creek-bankand Elymus-sites potentials rarely fell below 200 mV and were responsive to high or very high spring tides only. In the 'general salt marsh' zone and its sub-sites (including Festuca hummocks) only the very high spring tides influenced aeration producing a third pattern in which there was a longer period of oxidation, free from fluctuation, than at any other site. (4) The results are discussed in relation to some of the known responses of the species to soil flooding. (5) There was no evidence of electrode poisoning during these studies and the numerical values of potential recorded in the various phases of flooding were not significantly altered at the Puccinellia sites 3 years after the electrodes had been established.

Variation in the naturally occurring rhythm of the estuarine amphipod,Corophium volutator(Pallas)

The endogenous activity rhythm of the estuarine amphipodCorophium volutatorhas been studied by direct observation and with the use of time lapse photography. The rhythm persists under constant conditions having a free running period of between 12 and 13 h, and with activity maxima occurring during the early ebb. Freshly collected animals show a rhythm which is modulated on a semi-lunar basis, the activity maxima being attenuated during the neap tide periods, and the rhythm has also been found to vary in definition throughout the year. The activity pattern is most clearly denned in early summer and autumn, the population becoming arrhythmic during the winter months. The rhythm is relatively unaffected by the ambient light intensity and temperature of the recording conditions, and is evident in all post-natal stages of development. The possibility of mutual entrainment is discussed.

Observations on the tidal activity rhythm of the periwinkleLittorina nigrolineata(Gray)

The spontaneous activity pattern of the periwinkle L. nigrolineata has been studied by direct observation in the field and under controlled conditions in the laboratory. In the laboratory the animals are induced to crawl by both immersion and exposure under simulated tidal conditions, and the increased activity following emersion has been confirmed by field observations. Animals kept submerged under constant conditions show an endogenous activity rhythm with a free running period of about 12 to 13 hours. The main activity periods occur about 2 hours after the onset of inundation and the rhythm rephases almost immediately to the time of immersion. There is also evidence for a further endogenous component with peaks corresponding to the time of subjective emersion. The rhythm is modulated on a circadian basis, and is less pronounced during the neap tide period, and during the winter months.

The dynamics of estuarine manganese cycling

Seasonal observations of dissolved managnese in the Tamar Estuary,south-west England have demonstrated pronounced temporal and spatial variability in the distribution of this component. Simultaneous observations of physical and physico-chemical properties of the estuarine system, together with kinetic measurements of dissolved managenese behaviour in isolated samples, have demonstrated that the observed distributions are the instantaneous product of dynamic interactions involving external sources and internal input and removal processes. Dissolved manganese is mobilized internally from reduced sedimentsduring advective resuspension of bed material so that this input is regulated by temporal and spatial variations in tidal stress. Manganese is returned to the sediments by continuous uptake on to suspended particles. The rate of this process is responsive to changes in suspended particulate load, ionic strength pH and temperature. The reaction changes from zero-order kinetics in freshwater to first-order kinetics characteristic of the saline medium, with marked deceleration in effective, removal rate, following small increases in salt concentration. Relative changes in the first-order removal rate throughout the estuary are consistent, but there are considerable temporal variations in the absolute rates. This is partially attributable to variations in the quality, as well as the quantity, of the suspended particles. These results indicate that changes in the relative effectiveness of both the input and removal mechanisms, which lead to internal cycling of manganese and to variable distributions of dissolved managanese within the estuary, are closely coupled to the internal behaviour of particles and hence to the tidally-induced oscillations in bed stress. This observation has been confirmed by sequential recordings of dissolved manganese distributions through a spring tide to neap tide period. Furthermore, climatic events producing enhanced disturbance of bed sediment have been shown to induce a similar sequence of events to that arising from tidally induced net mobilization of bed material.

A SURVEY OF MAN-MADE TIDAL SWIMMING POOLS ALONG THE SOUTH AFRICAN COAST

A large number of man-made tidal swimming pools (two examples are shown in Figure 1) exist along the South African coast. They are usually situated on rocky outcrops in the close vicinity of popular sandy bathing beaches to provide protected bathing conditions in these areas mainly for children and elderly people. Some tidal pools, especially along rocky coast, provide the only safe bathing facilities. Besides affording protection against waves and surfzone currents the pools provide protection from sharks. A large number of tidal pools were built during the early 1950s along the Natal South Coast after the occurrence of a relatively large number of shark attacks on bathers on that coast. The semi-diurnal tide with a range of about 1,5 m along the South African coast makes it possible for pools to be built such that water replenishment can occur during every high-water (approximately every 12 hours) during both neap and spring tide periods. Presently, there exists a great need for more tidal pools as part of the demand for more recreational facilities along the South African coast. However, no information on design criteria could be found in the literature.

Tidal activity rhythms in the mangrove snailCerithidea decollata(Linn.) (Gastropoda: Prosobranchia: Cerithiidae)

Activity rhythms in Cerithidea decollata which migrates between the trees and the substrate in south-east African mangrove swamps were investigated in Durban Bay, South Africa. No light/dark rhythms were found. Activity followed a 14-day tidal rhythm. Maximum activity occurred over neap-tide periods and least over spring-tide periods when snails tended to remain on the trees over both high and low tides. Snails descended to the substrate more frequently in the upper inter- tidal area where inundation was less frequent and of shorter duration. Descent to the substrate appears to be associated with feeding. Tree-climbing is suggested to be a predator- avoidance mechanism.

Daytime surface swarming by Meganyctiphanes norvegica (M. Sars) (Crustacea, Euphausiacea) off Brier Island, Bay of Fundy

In July–September the euphausiid Meganyctiphanes norvegica (M. Sars) swarms at the surface in daytime off Brier Island, Nova Scotia (ca. 44°15′ N 66°23′ W). Systematic observations of swarming were made, mainly in late August 1974, 1975, 1976, and 1978, supplemented in 1974 and 1975 by the frequency of occurrence of the euphatisiids in the proventriculi of collected greater shearwaters, Puffinus gravis (O'Reilly). Swarms, consisting mostly of immature animals, occurred near an area of turbulence where strong tidal streams ran up against steep underwater ledges, bringing cool subsurface water and also copepods to the surface. Swarming was apparently not an attempt to avoid underwater predators or unfavourable oceanographic conditions, nor was it a means of advancing gonad maturation through exposure to solar irradiation. Swarming was most common in spring tide periods when tidal streams were strongest, and did not occur at all during a neap tide period in late August 1975. It is suggested that M. norvegica swarms were transported passively to the surface by strong vertical turbulence. However, the possibility that they swam actively to the surface in search of copepods and other food passively transported in that way could not be ruled out. It is suggested that the surface swarming of Euphausia pacifica Hansen, the other well-documented survey of the phenomenon, by Y. Komaki in 1967, be reexamined in the light of these conclusions. The significance of surface swarms as food sources for a variety of marine predators is discussed.

Effets des marées sur la variation circadienne de la capacité photosynthétique du phytoplancton de l'estuaire du saint-laurent

The photosynthetic capacity of phytoplankton was measured at an anchor station in the Middle Estuary of the St Lawrence, over a period of 120 h. Hourly incubations were made in constant light conditions, under three different colour filters of 3, 4 and 23 W · m −2. Differences in the oscillations of the photosynthetic capacities were observed in relation to the tidal regime. During spring tide, variations in the photosynthetic capacity are circadian, whereas a tidal pattern was also observed during the neap tide period. Differences in the patterns of photosynthetic capacity were probably due to changes in the gradients of physical factors and to the physiological state of phytoplankton relative to these gradients. The circadian rhythms appeared to be endogenous since they were apparent at non-saturating light intensities and since the photosynthetic potential ( U max ) changed during the day.

Effects of Change in Catchability on Sampling of Juvenile and Adolescent Banana Prawns, Penaeus merguiensis de Man

To provide the information necessary for optimizing sampling strategies, the effects of tidal, diurnal, lunar and seasonal cycles on the catchability of juvenile and adolescent banana prawns, P. merguiensis, were examined in the Norman River and in the inshore coastal zone of the Gulf of Carpentaria. A major part of the large variability in catches in both environments was attributable to marked tidal periodicity in catchability. In the river, large catches of juvenile P. merguiensis were taken only at or near the time of low water during both the neap and spring tide periods. Smaller prawns (&lt; 12 mm carapace length) tended to be even more restricted to the time of low water than were larger prawns. This phase shift in catchability of larger prawns was particularly evident in the offshore sampling where large catches of adolescent prawns were made only at high water. Both the number and size of prawns caught also varied considerably with water depth. The reasons for the marked differences in catchability throughout a tidal cycle are not clear, but it is obvious that sampling can be restricted in time, thus providing much more consistent data with a minimum of effort.

STUDIES ON THE SEASONAL VARIATION OF THE SUSPENDED MATTER IN THE MENAI STRAITS. I. THE INORGANIC FRACTION

The ash weight of material suspended in the waters of the Menai Straits was measured twice weekly between May 1962 and May 1963. The amount was much greater in winter than in summer and greater during spring than during neap tide periods. No differences could be detected between the ebb and flood periods within a single tidal cycle.The variations in ash weight correlated strongly with temperature but not with rainfall. There was a significant correlation between ash weight and the temperature‐related variables, σt and estimated freshwater runoff, but neither was significant after the effect of temperature was eliminated. The suspended burden was greater during periods of easterly winds, but no relationship could be established with wind velocity. The seasonal changes could not be attributed to changes in thermal instability or viscosity. The variations are thought to be caused by biological rather than by physical processes.