High Tidal Zone Research Articles

Differential recruitment of annelids onto tidal elevations in an estuarine mud flat

The recruitment of an annelid community was examined on 2 tidal elevations in 2 kinds of sediments along an inclined intertidal mud flat in an estuary in northern Taiwan. Recruitment onto defaunated and ambient intact sediments (control) in both low and high tidal zones was followed weekly from 3 April to 1 May 1996. The overall density of recruits in the defaunated sediments reached the level of the control sediments within 3 wk in the low tidal zone, whereas in the high tidal zone the density of recruits did not reach the control level during the study period. Two polychaete species and 1 oligochaete species were the major recruits, and each exhibited a different recruitment mode and rate. The spionid Prionospio japonica and the oligochaete Doliodrilus tener were among the most rapid recruits. P. japonica simultaneously exhibited 2 recruitment modes, larval settlement and postlarval transportation, and its recruit density in the defaunated sediments in the low tidal zone reached the level of the control within 1 wk. D. tener recruited predominately by means of juvenile migration. In general, recruitment by settling larvae occurred at discrete intervals, whereas that by postlarvae or juveniles appeared to be continuous. The densities of P. japonica and of D. tener exhibited a zonation pattern wherein density increased along a down-shore gradient. The sedimentary characteristics also exhibited a similar zonation pattern, wherein the sediment particles became smaller and silt/clay content increased in the low tidal zone. This coupled phenomenon suggests that tidal action has a strong influence on the recruitment of P. japonica and D. tener. These recruits are passively entrapped in the low tidal zone due to the depositional conditions created by slow flows. However, the tidal zonation of the capitellid Capitella sp. showed an opposite trend, with greater recruitment in the high tidal zone, indicating that the recruitment of Capitella sp. possessed an active component. D. tener recruited mostly into the low tidal zone, but a small, significant proportion of the recruitment occurred in the high tidal zone. Capitella sp. and D. tener are subsurface burrowers, and thus, to some extent, their recruitment is less controlled by tidal action. Overall, the recruitment of this annelid community is influenced by both physical and biological controls.

Dynamics of herbivory in Belizean mangal

ABSTRACTWe report long-term observations on leaf herbivory in the mangroves Rhizophora mangle and Avicennia germinans, on the Placencia Peninsula of Belize. We labelled and traced leaves and recorded the amount of damage appearing over a period of 310 days. Herbivores had damaged 1–4% of total mean leaf area within the first 10 days, but damage rates levelled off thereafter, to a final total of 4–10% damage. Herbivores did not concentrate on leaves of younger age classes. Rhizophora mangle growing in the high tidal zone (HHW) exhibited the highest initial and total attack rates at all but the final sampling dates. We also documented the dynamics of injury expansion and necrosis following artificial injury to seedling and tree leaves. Artificial holes in leaves were associated with higher subsequent rates of herbivory that varied among leaf age classes, and particular damage types were more prevalent in previously injured leaves. Artificial holes expanded significantly more in young leaves of both species. Holes in young leaves of both species expanded by up to 45.1% in area over 50 days, consistent with a similar study in Australia. Substantial necrotic areas developed around the injury site; a secondary effect of damage not often considered in herbivory studies. We discuss the implications of hole expansion and necrosis in estimating error in herbivory measurements. Temporally dynamic studies of herbivory are especially needed in tropical systems that show non-deciduous phenologies and long-lived leaves.

Population processes in the ribbed mussel Geukensia demissa (Dillwyn) in a North Carolina salt marsh tidal gradient: spatial pattern, predation, growth and mortality

Empirical research and field experiments during summer and winter months on ecological processes of a North Carolina population of the rib d mussel, Geukensia demissa (Dillwyn), revealed both similarities and contrasts with previous research on this and related marine bivalves. Growth and mortality patterns along the salt marsh tidal gradient were generally consistent with other studies, although summer epibenthic predation mortality was more intensive and involved animals in larger size classes. Survivorship of all size classes was significantly higher in the low tide zone than the high tide zone, but varying densities had little effect on survivorship. In the summer clump-size experiment, predation mortality in open uncaged plots was extensive. Winter survivorship of small and medium-sized mussels increased with decreasing clump size, especially in the low tide zone, and suggested additional advantages of clumping over that of a refuge from crab predation during summer. Mussel growth was greater, but survivorship lower, in the low marsh zone where predation by crabs during summer months was also greatest. Increased longevity and thus a size structure skewed to larger animals characterized the high marsh zone. During summer, growth rate in the low zone was inversely related to increasing clump size, but the effect disappeared during the winter months. In comparing sizes of mussel vulnerable to crab predation between New England and North Carolina marshes, we suggest that mussels in the low tidal zones of more southerly marshes are less likely to escape predation through the growth process. Even though general patterns of G. demissa intertidal size structures and abundance among east coast marshes are emerging, we suggest that the mechanisms accounting for differential growth and mortality, including possible genotype-dependent processes, are still poorly understood in spite of the rather extensive research on this bivalve group.

Spatial and temporal distribution of a deposit-feeding polychaete on a heterogeneous tidal flat

The distribution of the deposit-feeding, orbiniid polychaete, Scoloplos fragilis (Verrill, 1873), was examined in relation to differences in elevation along-shore and down-shore from August 1976 to April 1977. The sand flat at Cape Henlopen, Lewes, Delaware U.S.A., is characterized by swash bars extending perpendicular to the shore, which undergo annual vertical and lateral fluctuations and have marked differences in elevations. Six stations were monitored on a single swash bar. Three sites were sampled monthly at an area on the swash bar near the beach berm (high-tide zone) and at an area near the water's edge (low-tide zone). The sites were the swash bar ridge, its depositional slope and the adjacent trough. Distribution patterns of S. fragilis were not associated with elevational gradients or a particular tidal level. Highest numbers were found on the slopes, lowest numbers on the high-tide ridge, and second lowest numbers in the low-tide trough. On the high-tide ridge, the sediments were too unstable for habitation by S. fragilis. In the low-tide trough, predation and other biological interactions apparently keep numbers of S. fragilis low. Wave and wind activity may have concentrated S. fragilis at slope stations, where sediment deposition occurs. Mean body size of S. fragilis varied spatially and temporally. Within each tidal level, mean sizes increased significantly from ridge to trough. Body sizes were significantly larger in the high-tide zone than in the low-tide zone and varied significantly with time. Reproductive maturity appears to occur after two years and at least three age-classes are present.

Morphodynamics of a macrotidal beach

An intensive field investigation of hydrodynamic processes, processes of sediment entrainment and suspension, and morphologic change was carried out on an unprotected macrotidal beach near Broome in northwestern Australia. The spring-tide range was 9.5 m; waves had heights of 0.5–1.2 m and periods of 9–13 s. The beach had an overall concave-upward profile with low-gradient and dissipative subtidal and low-tidal zones, and steeper more reflective mid-tidal and high-tidal zones. Direct measurement of energy-flux dissipation over the intertidal profile showed dissipation rates on the order of 1–2 W m −2 of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height. Time-averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low-tidal zone and over the lower part of the high-tidal zone. Most of the work over the low- and mid-tidal zones was performed by unbroken shoaling waves rather than by surf-zone processes; surf-zone processes only dominate over the high-tidal zone. The nature of the surf-zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level. The growth of standing waves and infragravity (“surf-beat”) oscillations, as identified from spectra and cross spectra of surface elevation, η, and currents, u and v, was inhibited over most of the profile. However, well-developed secondary standing wave energy, particularly at infragravity frequencies, was observed over the high-tidal zone at spring high tide and over the mid-tidal zone at neap high tide. Over the low-tidal and subtidal zones, strong shore-parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves. Over the subtidal zone asymmetrical tidal currents, skewed toward the north, attained maximum speeds of 0.5 m s −1 just after high water. Field measurements of suspended-sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave-induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves. Northerly advection of wave-suspended sediment by asymmetrical tidal currents over the subtidal and low-tidal zones accounted for a net northerly longshore transport. The greatest morphologic mobility of the intertidal profile occurred over the lower high-tidal and upper mid-tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes. Temporally, the greatest mobility of the profile as a whole was observed on the short, within-tidal-cycle time scale. Net changes over the longest time scale of a lunar half cycle were negligible.

FORMS OF EROSION AND ACCRETION ON CAPE COD BEACHES

Frequent measurements of beach profiles have been made at sixteen areas between Maine and Long Island since September 1965 by members of the Coastal Research Center of the University of Massachusetts. This research effort has resulted in the accumulation of approximately 2000 beach profiles along the New England coastline. The detailed analysis of profiles from Monomoy Island and Nauset Spit on Cape Cod has revealed the following erosion-accretion characteristics: 1. The most active areas of the beach profile in terms of sand transport are at the low-tide, neap high-tide, and spring high-tide zones. The center of the beach face is relatively inactive. 2. An exception to this behavior occurs during severe storms when large volumes of sand are removed from the entire beach face, producing a concave upward profile shape. 3. During periods of relatively low wave activity there is much interaction in terms of sand movement between these three zones, resulting in the formation of distinctive profile shapes. 4. These profile shapes tend to maintain themselves through sand movements which cause the berm to migrate back and forth along the profile. 5. This activity is often accomplished with little or no net sand erosion or accretion to the total profile. These conclusions, combined with additional analyses, indicate that the traditional measurements of total beach width and high tide beach width (i.e., to the berm) are not a reliable indication of sand volume changes on beaches.

An analysis of some factors involved in the depth of disturbance of beach sand by waves

A study of the depth of disturbance of beach sand by waves indicates that breaker height is the dominant factor in the mid- and probably low-tide zone; slope in the higher high-tide zone. Depth of disturbance figures are much higher than those reported by King (1951), in Great Britain but agree with Otvos' (1965), in the U.S.A. Similarly no difference was found between erosional depths of fine versus coarse sand again confirming Otvos' (1965) work. Newly deposited sand found on top of the coloured sand in the sedimentation stations was mainly uniform in structure, dual sedimentation units, if occurring at all, being more common at mid-tide stations and on beaches where there was a greater range of sediment. A new method was devised for detecting those areas on the foreshore where experiments were set up, based on the principle of a change in frequency noticed when an oscillator passed over a metal plate buried in the beach face. This, on an open beach face devoid of any external features, enabled the precise experimental location to be found. From statistical analysis of over 150 samples, skewness values were found in the main to be typically negative, whilst kurtosis values tended to indicate a difference in modality between the various sedimentation units obtained. Standard deviation figures showed that moderate sorting was typical for bottom units, whilst upper and uniform units seemed to be moderately well sorted. The results obtained indicated that 82% of the factors causing depth of disturbance of sand at mid-tide position could be attributed to breaker height, slope and wave period whilst at higher high-water mark only 61% of the disturbance was due to these three factors. The latter result was possibly due to the many complex swash zone processes which were not investigated during these experiments.