Population Ecology Studies on Infaunal Polychaetes of Pauatahanui Inlet

<p>Macrofaunal polychaete densities across a stable, fine-sand, intertidal flat in Pauatahanui Inlet were examined from a set of seasonal samples, 500 micron sieve processed, from a stratified transect pair. Density Patterns had shore-normal trends, despite apparently weak tidal-cycle environmental gradients. Zones of high abundance of common species persisted unchanged, but seasonal increase and decline occurred within them, with also some population redistribution attributable to differential mortality/recruitment, or possibly to migration. Correlation analysis did not detect interspecies relationships linked to the abundance and zonation of the common polychaetes, all deposit-feeders. The population dynamics of six species was investigated from the transect-pair seasonal samples, supplemented by subsequent more finely-sieved samples during dense recruitment periods. The maldanid Axiothella serrata had three identifiable age groups O, I, and II+, with I plus II+ density to about 550 m-2. The new O group began to appear in October-November, as aggregates below 3 cm sediment depth, believed to represent lecithotropic, direct-developing, siblings from egg masses of the II+ group. Juvenile setiger-total frequencies indicated synchronous adult spawning occurred at varying intervals during an approximate six month period. Peak density of dispersed, near-surface recruits reached 29 thousand m-2. Setiger-total was the most sensitive indicator of size and age in juveniles of up to about 15 setigers. Zonation patterns were age specific. The predominantly lower-shore capitellid, Heteromastus filiformis had a short summer spawning period with settlement ending before May, when population density was up to 10 thousand m-2. Merger of O group into the adult size range occurred in about one year, and probably first spawning was at the end of the second year, with life span of three years or more. The nereidid Nicon aestuariensis had I+ and older age groups at barely detectable densities. Spawning was probably in late summer although an O group, at about 500 m-2, was not detected until May. The spionid Scolecolepides benhami had apparently unchanging size structure and density (about 400 m-2 transect-wide); new settlement was not detected. High density occurred only in a narrow near-shore strip. Nicon aestuariensis also declined down-shore, but more gradually. The spionid Microspio sp. and capitellid Capitella sp. were short-lived, near-surface species, with apparently continuous recruitment from planktotrophic and direct-development respectively. A spring recruitment increase created at first a separate modal group of juveniles, and raised Microspio sp. density to 45 thousand m-2 and Capitella sp. to 7 thousand m-2. Capitella sp. declined in density downshore, but Microspio sp. was only weakly zoned. The common polychaetes had largely concordant density cycles with settlement (or peak settlement) spring-summer orientated, although 500 micron mesh processing detected the peak of surviving adults in autumn-winter. Pauatahanui polychaetes as an assemblage, life history traits, links between population structure and zonation, and problems in polychaete population studies are discussed.</p>

Population Ecology Studies on Infaunal Polychaetes of Pauatahanui Inlet

<p>Macrofaunal polychaete densities across a stable, fine-sand, intertidal flat in Pauatahanui Inlet were examined from a set of seasonal samples, 500 micron sieve processed, from a stratified transect pair. Density Patterns had shore-normal trends, despite apparently weak tidal-cycle environmental gradients. Zones of high abundance of common species persisted unchanged, but seasonal increase and decline occurred within them, with also some population redistribution attributable to differential mortality/recruitment, or possibly to migration. Correlation analysis did not detect interspecies relationships linked to the abundance and zonation of the common polychaetes, all deposit-feeders. The population dynamics of six species was investigated from the transect-pair seasonal samples, supplemented by subsequent more finely-sieved samples during dense recruitment periods. The maldanid Axiothella serrata had three identifiable age groups O, I, and II+, with I plus II+ density to about 550 m-2. The new O group began to appear in October-November, as aggregates below 3 cm sediment depth, believed to represent lecithotropic, direct-developing, siblings from egg masses of the II+ group. Juvenile setiger-total frequencies indicated synchronous adult spawning occurred at varying intervals during an approximate six month period. Peak density of dispersed, near-surface recruits reached 29 thousand m-2. Setiger-total was the most sensitive indicator of size and age in juveniles of up to about 15 setigers. Zonation patterns were age specific. The predominantly lower-shore capitellid, Heteromastus filiformis had a short summer spawning period with settlement ending before May, when population density was up to 10 thousand m-2. Merger of O group into the adult size range occurred in about one year, and probably first spawning was at the end of the second year, with life span of three years or more. The nereidid Nicon aestuariensis had I+ and older age groups at barely detectable densities. Spawning was probably in late summer although an O group, at about 500 m-2, was not detected until May. The spionid Scolecolepides benhami had apparently unchanging size structure and density (about 400 m-2 transect-wide); new settlement was not detected. High density occurred only in a narrow near-shore strip. Nicon aestuariensis also declined down-shore, but more gradually. The spionid Microspio sp. and capitellid Capitella sp. were short-lived, near-surface species, with apparently continuous recruitment from planktotrophic and direct-development respectively. A spring recruitment increase created at first a separate modal group of juveniles, and raised Microspio sp. density to 45 thousand m-2 and Capitella sp. to 7 thousand m-2. Capitella sp. declined in density downshore, but Microspio sp. was only weakly zoned. The common polychaetes had largely concordant density cycles with settlement (or peak settlement) spring-summer orientated, although 500 micron mesh processing detected the peak of surviving adults in autumn-winter. Pauatahanui polychaetes as an assemblage, life history traits, links between population structure and zonation, and problems in polychaete population studies are discussed.</p>

Comparison of Methods for Determining Erosion Threshold of Cohesive Sediments Using a Microcosm System

Erosion of cohesive sediments is a ubiquitous phenomenon in estuarine and intertidal environments. Several methods have been proposed to determine the surface erosion threshold (τc0), which are still debatable because of the numerous and uncertain definitions. Based on erosion microcosm experiments, we have compared three different methods using (1) eroded mass (EM), (2) erosion rate (ER), and (3) suspended sediment concentration (SSC), and suggested a suitable method for revealing the variation of erodibility in intertidal sediments. Erosion experiments using a microcosm system were carried out in the Muuido tidal flat, west coast of South Korea. The mean values of τc0 for three methods were: 0.20 ± 0.08 Pa (EM); 0.18 ± 0.07 Pa (ER); and (3) 0.17 ± 0.09 Pa (SSC). The SSC method yielded the lowest τc0, due to the outflow of suspended sediment from the erosion chamber of the microcosm. This was because SSC gradually decreased with time after depleting the erodible sediment at a given bed shear stress (τb). Therefore, the regression between SSC and applied τb might skew an x-intercept, resulting in the underestimation (or “not-determined”) of τc0. The EM method yielded robust and accurate (within the range of τb step at which erosion begins) results. The EM method represents how the erodible depth thickens as τb increases and therefore seems better suited than the SSC and ER methods for representing depth-limited erosion of cohesive sediments. Furthermore, this study identified the spatiotemporal variations of τc0 by EM method in an intertidal flat. The τc0 in mud flat was about two times higher than that in mixed flat. Compared to the end of tidal emersion, the sediment was 10–40% more erodible at the beginning stage.

Geomorphological sedimentary characteristics in the coastal area of Ma river delta, Thanh Hoa province

The coastal area of the Ma river delta is formed by the interaction of continental and marine processes, between neo-tectonic activities and exogenous processes, between natural factors and human activities during the Late Holocene. Using remote sensing and geoscience research methods (granulometry, paleontology, geochemistry, clay mineralogy) and geomorphological studies (geneses, morphology, dynamics) combined with field survey, this coastal area, except the denuded mountainous remnants, could be distinguished into 12 morpho-sedimentary units formed and developed by the dynamic interactions of the river, waves and tides. The units formed by fluvial dynamics include: 1) Point bar is composed of clayey silt and sandy silt, 2) Channel bar composed of silty sand, 3) Levee with the composition of silty sand and 4) The flood plain of silty clay. The Late Holocene evolution of the Ma river delta was dominated by wave dynamics, reflected by a wave-formed association of dunes, interdune swamps and current sand beaches. The wave-formed units include 5) Dune’s sand and silty sand, 6) Back-dune depressions composed of sand silt clay, 7) Beach composed of sand, 8) Lagoon plain of silty clay and 9) Strand plain composed of silty sand. The tide-influenced units include 10) Supratidal flat with the composition of silty clay, 11) Intertidal flat characterized by clay or silty clay interbedded with thin fine sand or silty sand layers, 12) Subtidal flat of sand and silty sand.

Sedimentary facies and environments of the sedimentary fill of Southern Bida Basin, Nigeria

Six lithofacies were identified in the Lokoja Formation, Southern Bida Basin: fanglomerate/ conglomerate lithofacies (Gmc), fine to coarse-grained ferruginized weakly cross-bedded, pebbly sandstone lithofacies (Scx), fine to coarsegrained sandstone lithofacies (Sfc), silty claystone lithofacies (Csm), siltstone lithofacies (Slt) and lateritic ironstone lithofacies (Ilt). These were grouped into three lithofacies associations viz: alluvial fan, braided river channel, floodplain lithofacies association. Nine lithofacies were identified in the Ahoko Formation. These are: black-dark grey carbonaceous shale lithofacies (Shc), bioturbated ripple-laminated siltstone lithofacies (Sbr), poorly cross-laminated claystone lithofacies (Cxl), concretionary/nodular ironstone lithofacies (Icn), medium to coarse-grained sandstone lithofacies (Smc) fine grained, well-sorted, friable bioturbated herringbone cross-bedded sandstone lithofacies (Sxf), massive brownish claystone lithofacies (Clm), massive claystone with lateritic ironstone lithofacies (Cli) and lateritic ironstones lithofacies (Ilt). These have been grouped into three lithofacies associations viz: shallow marine lithofaciesassociation, tidal-intertidal flat lithofacies association and floodplain lithofacies association. Similarly, three lithofacies were identified in the Agbaja Formation and have been grouped into two lithofacies association. These are: fine to medium-grained sandstone ironstone interbedded lithofacies (Sti), oolitic–pisolitic ironstone lithofacies (Iop) and concretionary ironstone lithofacies (Icr). The lithofacies associations are: tidal-intertidal flat lithofacies association and shallow marine lithofacies association. Result of lithofacies analysis helped in interpreting the depositional environments. The Lokoja Formation is a product of a fluvial dominated alluvial system from debris/gravity flow in alluvial fan. This developed further into braided river channels and later meandering river during the closing stages. Sediments of the Ahoko Formation were deposited in tidal/intertidal flats and shallow marine environments while sediments of the Agbaja Formation were produced by a shallow marine system with a high tidal influence.

Mid-Late Holocene Paleoenvironmental and Sea Level Reconstruction on the Al Lith Red Sea Coast, Saudi Arabia

Late Quaternary paleoenvironments are of particular interest to understand how the Earth System’s climate will respond to the undramatic changes during this period, compared with the broader glacial-interglacial variations. In this study, a shallow sediment core (2.84 m long) retrieved from the Red Sea coastal zone in northern Ghubbat al Mahasin, south of Al-Lith, Saudi Arabia, is used to reconstruct the mid-Late Holocene paleoenvironments and sea level based on a multiproxy approach. Remote sensing data, sedimentary facies, benthic foraminiferal assemblages, δ18O and δ13C stable isotopes, elemental composition and 14C dating were utilized. The stratigraphy of the core shows three distinctive depositional units. The basal pre 6000 year BP unit consists of unfossiliferous fine to medium sand sharply overlain by black carbonaceous mud and peat, suggesting deposition in a coastal/flood plain under a warm and humid climate. The middle unit (6000-3700 year BP) records the start and end of the marine transgression in this area. It consists of gray argillaceous sand containing bivalve and gastropod shell fragments and a benthic foraminiferal assemblage attesting a lagoonal or quiet shallow marine environment. The upper unit (&lt;3700 year BP) consists of unfossiliferous yellowish-brown argillaceous fine-grained sands deposited on an intertidal flat. Both middle-and upper-units stack in a regressive shallowing upward pattern although they may be separated by a hiatus. The overall regressive facies and the stable isotopic data are consistent with a late Holocene sea-level fall and a change to a more arid climate.

Environmental Characteristics and Macrobenthic Community Structure of Yubu Island, a UNESCO-Designated Migratory Bird Sanctuary in the South Korea

This study investigated the environmental characteristics and macrobenthic community structures of an intertidal flat located on Yubu Island in the western coastal area of the Yellow Sea of South Korea. The study site has been well known for the migratory bird protection designated by UNESCO. We collected the samples of surface and depth core sediments and macrobenthos from the unused saltern and the area surrounding it in July 2017. Our results demonstrated that the means of mud, total carbon, and total nitrogen contents were 50%, 0.35% and 0.05%, respectively, and the origin of sediment composed of marine sources. The sedimentation rate ranged from 0.99 to 1.66 cm y−1, which was higher than other tidal flats in the Yellow Sea. The result of a topographic analysis indicated that the area in the unused saltern was 289.489 m2, and the length and width were 625 m and 500 m, respectively. A total of 13 species occurred with a mean density of 92 ind. m−2 and a mean biomass of 58.6 gWWt m−2. The Heteromastus filiformis (23.1%, 21 ind. m−2) was the most dominant species, followed by Perinereis linea (19.2%, 18 ind. m−2) and Tubuca arcuata (17.3%, 16 ind. m−2). Thus, this environmental information will help to establish policies to protect migratory birds in the future.