Intertidal Sandflat Research Articles

Porewater nutrient enrichment alters benthic-pelagic coupling on intertidal sandflats

With the intensification of urban development, agriculture, and forestry, many coastal waters are becoming enriched with nitrogen (N). Long term, excess terrestrial N accumulates in the sediment porewaters impacting benthic fauna and microphytobenthic productivity, altering the complex bio-geophysical interactions that occur on the seafloor. These interactions may influence sediment erodibility and resuspension rates on the seafloor, which have wider ecosystem consequences through changes in water column turbidity, the fluxes of organic matter and sediment porewater nutrients into the water column, and the redistribution of deposited terrestrial materials (i.e. pollutants and sediment). We conducted a field experiment to explore how bio-geophysical interactions change in the context of porewater N enrichment. The experiment enriched plots (4 m2) on an intertidal sand flat in Tauranga Harbour, New Zealand, with three concentrations of N fertiliser (0, 150 and 600 g N m−2; n = 6). After 10 weeks, we collected intact sediments from the plots, and in the laboratory, subjected these sediments to sequential increases in water flow velocity (between 0.05 and 0.4 m s−1) using annular flumes (bed area = 0.17 m−2). We calculated sediment erosion rates and thresholds, measured fluxes of NH4+ and Chlorophyll a across the sediment-water interface and characterised the sediment properties and macrofaunal communities. We anticipated complex feedbacks to occur where porewater N enrichment would increase microphytobenthic biofilm growth, stabilising the sediments (and reducing benthic-pelagic coupling), but at the same time, N enrichment might also alter the macrofaunal community, which graze the microphytobenthos and physically mix the sediment, with a potential destabilising effect (and increase in benthic-pelagic coupling). Although our enriched treatments doubled the biomass of microphytobenthos, altered the macrofauna communities and enhanced the fluxes of Chlorophyll a and NH4+ across the sediment-water interface, sediment erosion thresholds and rates were unaffected. Our results highlight a potential mismatch between the micro- and macro-biotic processes and the larger scale physical forces on dynamic intertidal sandflats, and this mismatch has implications for our ability to scale up small scale processes to entire ecosystems.

Semilunar postlarval settlement periodicity ensuing from semilunar larval release cycle in the Nihonotrypaea harmandi (Crustacea, Decapoda, Axiidea, Callianassidae) population on an intertidal sandflat in an open marine coastal embayment

AbstractMany decapod crustaceans in marine intertidal habitats release larvae toward coastal oceans, from which postlarvae (decapodids: settling‐stage larvae) return home. Decapodid settlement processes are poorly understood. Previous studies showed that in Kyushu, Japan, the callianassid shrimp population on an intertidal sandflat of an open bay joining the coastal ocean near a large estuary released eight batches of larvae basically in a semilunar cycle from June through October and that decapodids performed diel vertical migration, occurring in the water column nocturnally. We conducted (a) frequent sampling for population density and size‐composition on the sandflat through one reproductive season, (b) planktonic and benthic sampling for decapodids around the bay mouth, and (c) current meter deployment at three points across the bay mouth for tidal harmonic analysis. On the sandflat, six batches of newly‐settled decapodids (settlers) occurred in a semilunar periodicity until October, with peaks occurring 0–3 days before syzygy dates except for the first one. For larval Batches 1–4, buoyancy‐driven shoreward subsurface currents during July to mid‐October would transport some pre‐decapodid‐stage larvae (zoeae) toward the bay. The absence of expected settler Batches 7–8 would be due to the converse subsurface currents caused by water‐column mixing and seasonal winds after mid‐October, carrying zoeae offshore. Once in the bay, phasing of night and nighttime‐averaged shoreward tidal current explained the settlement pattern for Batches 1–4. For Batches 5–6 occurring in mid‐September to mid‐October, water currents generated by seasonal wind and tidal forcings may have caused peak settlement after the time expected from tidally‐driven decapodid transport.

Effect of stingray (Hemitrygon akajei) foraging on a ghost shrimp population (Nihonotrypaea harmandi) on an intertidal sandflat, western Kyushu, Japan

Callianassid shrimp residing in deep burrows have large bioturbating effects on marine soft-bottom communities. A few predators that excavate deep pits could have substantial effects on shrimp populations, as well as knock-on effects. Processes and consequences of such effects on shrimp populations are poorly understood. On a 300-m-wide intertidal sandflat area between tide marks in western Kyushu between 1989 and 1994, shrimp population densities were stable, reaching >1300individualsm–2. Dasyatid stingray feeding pits reaching depths up to 20cm occurred abruptly in large numbers in 1994, after which shrimp densities decreased yearly to hundreds of individuals per square metre in 2001. The densities of ray feeding pits formed per day were monitored every or every other spring tide between 2000 and 2001. Schools of rays were enclosed during submerged times and their body sizes recorded alive to determine size-frequency distribution. The body-size frequency distributions of shrimp were compared among the gut contents of several rays, ray feeding pits and intact sandflat. Reductions in the shrimp density per ray feeding bout compared with the density on the intact sandflat were recorded. A model of daily predation at different seasonal rates was used to simulate the yearly change in shrimp density. The result was consistent with the actual change.

Crab burrow aspect ratio influences particle capture rates on intertidal sandflats

Crabs are ecosystem engineers that dig burrows, which can trap sediment. We used a field experiment to explore how burrow aspect ratio (depth/diameter) affects the trapping efficiency of sediments and organic material. Arrays of burrows mimics were constructed using tubes of similar depths but two different opening diameters: fat tubes (aspect ratio of 3.8) and thin tubes (7.1). Different arrays were tested to examine whether the combination of burrows with two different aspect ratios affects the material capture, while retaining the same total area of openings per array. The results showed that, in general, the fat tubes trapped more organic material, especially large pieces, and more sediment than thin tubes. Furthermore, the silt-clay content of the sediment trapped in the tubes was up to 50% greater than the surrounding surficial sediment. Hence, we conducted 2D numerical simulations of flow around, and into, a single burrow to elucidate the mechanisms behind particle capture. Results showed that the flushing rate and the turbulent kinetic energy were greater in the fat tubes. However, turbulence persisted for a longer distance downstream of the thin tubes than the fat tubes. The enhanced turbulence may increase the flux of sediment into the tubes where particles can settle and, consequently, promote the sediment capture and sedimentation rate. Our results demonstrate that the aspect ratio of burrows significantly affects capture processes of organic material and sediment particles. Moreover, this capture mechanism depends on the flow regime, as faster flows are associated with larger amounts of material in the water column.

Linking changes in species-trait relationships and ecosystem function using a network analysis of traits.

A major challenge in ecology and environmental management is linking changes in community composition to ecosystem functions. We developed the network analysis of traits (NAT) to show changes in community network structure based on the changes in the composition and connectivity between clusters of species that share traits that imply shifts in functional diversity. We tested the application of NAT on a 113 species found on an intertidal sandflat that was subject to experimental nitrogen addition (control [0g N/m2 ], medium [150g N/m2 ], and high [600g N/m2 ]). This allowed us to directly link mechanistic changes in community composition and function with the trait-space network patterns revealed by NAT. We demonstrate that under medium (150g N/m2 ) N treatment, functional diversity remained consistent, whereas increasing disturbance to high (600g N/m2 ) N treatment affected the species-trait network structure and caused merging of functional clusters implying a loss of functional trait diversity.

Decomposing macroalgae (Ulva spp.) impacts benthic macrofauna and surface sediment erosion

Patches of decomposing macroalgae can be beneficial to intertidal regions, providing habitat and food, or can create anoxic conditions inhospitable to benthic organisms. These opposing outcomes warrant further investigations into biotic-abiotic processes associated with macroalgae. Here, differences in surface sediment erosion measures (erosion threshold (Ʈc; N m−2), erosion rate (ER; g m−2 s−1), and sub-surface erosion rates (me; g N−1 s−1)) were examined at sites dominated by the suspension-feeding clam or deposit-feeding bivalve (Austrovenus stutchburyi and Macomona liliana, respectively) after 30 days of exposure to decomposing macroalgae (Ulva spp.). The Austrovenus site was chosen to represent a species-rich, functionally diverse macrofaunal community. The nearby Macomona site had similar sediment characteristics, yet had a less abundant and diverse faunal community. Despite the equal amounts of Ulva recovered from both sites (< 3% of the initial 3 kg wet weight m−2 added), differences in surface erosion were measured. One day after Ulva removal, an initial increase in surface erosion (−Ʈc and +ER) was measured at the Austrovenus site, but after 14 days there was no difference compared with control plots. At the Macomona site, Ulva addition stabilized sediments (+Ʈc and -ER), an effect that persisted for 14 days. These differences were reflected in the macrofaunal community. At the Austrovenus site, the macrofaunal community recovered within 14 days whereas differences between Ulva and controls persisted at the Macomona site. This work emphasizes how subtle differences in benthic community composition and recovery can result in spatial and temporal variations in sediment erosion potential on intertidal sandflats.

Predictors of foraging plasticity by greater flamingo (Phoenicopterus roseus) in intertidal soft sediments

Numerous studies have emphasised the importance of apex predators in determining community dynamics and broader functioning in marine ecosystems. However, less understood is the ecology of plastic foraging behaviours employed by predators, with drivers of foraging plasticity being a particular knowledge gap in marine sediments. In June/July 2015, we assessed the role of traits and abundance of prey assemblages in influencing decisions made by greater flamingos Phoenicopterus roseus to employ different foraging behaviours in intertidal sandflat ecosystems in Langebaan Lagoon, on the west coast of South Africa (33°11′27″S, 18°07′37″E and 33°03′54″S, 17°58′07″E). Greater flamingos feed by either (1) creating pits, which involves flamingos stirring up deep sediments with their feet or (2) creating channels, in which their inverted bills are swept from side to side on the sediment surface. RandomForest modelling techniques indicated that both pit- and channel-foraging strategies were linked to low macrofaunal biomass in sediment patches, indicating that smaller prey items may be preferred targets for consumption. Channel foraging was linked mainly to the abundance of surface-associated fauna, suggesting that this feeding technique was predominantly a surficial foraging strategy. The probability of pit foraging increased with increasing concentrations of benthic microalgae, suggesting that high microalgal biomass may be linked to pit foraging. Overall, this study adds to knowledge on the role of flamingos as predators in marine sediments, by highlighting biotic predictors of foraging plasticity. The study demonstrates the relevance of understanding potential drivers of foraging plasticity in developing a predictive understanding of predator impacts in heterogeneous ecosystems.

Bacterial chemoautotrophic reoxidation in sub-Arctic sediments: a seasonal study in Kobbefjord, Greenland

Anoxic mineralization of organic matter releases dissolved inorganic carbon and produces reduced mineralization products. The reoxidation of these reduced compounds is essential for biogeochemical cycling in sediments and is mainly performed by chemoautotrophic microbes, which synthesize new organic carbon by dark CO2 fixation. At present however, the biogeochemical importance of chemoautotrophy in high-latitude sediments is largely unknown. Here, we determine the seasonal variation in sedimentary chemoautotrophic production in Kobbefjord (SW Greenland). Intact sediment cores from the fjord were incubated, and dark CO2 fixation was quantified by combining bacterial phospholipid-derived fatty acid analysis with 13C stable isotope probing (PLFA-SIP). Our results reveal a distinct seasonal cycle in chemoautotrophic activity, which increases after the spring bloom and shows lowest activity in the late winter when the fjord is covered by sea ice. The depth distribution of chemoautotrophic activity also varied seasonally, likely due to seasonal variation in the bioturbation activity of sediment infauna. Although chemoautotrophy rates (0.4 ± 0.2 mmol C m-2d-1) were in the low range for coastal sediments, they are comparable to those from intertidal sandflats and brackish tropical lagoons, and scale with the sulfide production through sulfate reduction in the fjord. Chemoautotrophic production in these fjord sediments thus appears to be mainly driven by sulfide oxidation and can re-fix 4% of the CO2 produced by mineralization

Normal Delayed Establishment of a Semilunar Brooding and Larval Release Cycle in the Course of the Reproductive Season of the Ghost Shrimp Population on a Warm Temperate Intertidal Sandflat

Many decapod crustaceans inhabiting estuarine and coastal shores with larvae that develop in the coastal ocean release them at nocturnal ebb tides in a semilunar or lunar spring tide cycle under semidiurnal tidal regimes. In common brachyurans on warm temperate shores, this release cycle becomes first established in their mid-reproductive season. How this occurs is poorly understood, under the dichotomous views that peak larval release in each spring tide during the mid- to late season is timed to the syzygy date or the date with the largest tidal range (TR) and fastest tidal currents a few days later. The callianassid shrimp, Nihonotrypaea harmandi, inhabiting an intertidal sandflat in midwestern Kyushu, Japan, under a mixed, mainly semidiurnal, tidal regime has a reproductive season from June through October at water temperatures ranging from 20°C to 29°C. Frequent population sampling was conducted in 1993 (to mid-August) and 1994 (to early November), with cold and hot summers, respectively. Observations and experiments on embryo deposition and larval release, and measurement on water level and velocity were performed in other years. Harmonic analysis of tides was made on the water level and velocity and applied to those in 1994. Female shrimp released larvae at nocturnal ebb tides. The population comprised three age cohorts (one 2-y-old and two 1-y-old), and eight distinct larval releases occurred during June to October, each forming a batch preceded by embryo deposition by 3 wk to 2 wk linked to 20°C to ≥ 25°C in water temperature, respectively. Syzygy-centered peak embryo deposition first occurred in late July (for the third batch in 1994) and early August (for the fourth batch in 1993), with each batch dominated by the youngest cohort. Afterward, peak larval release immediately followed by peak embryo re-deposition every spring tide occurred in 1994. In the earlier reproductive season, the older two cohorts were dominant in the mature individuals, with a longer longevity of the oldest and more dominance of the second oldest in 1993 than in 1994, and embryo deposition was only weakly synchronized with syzygies, followed by longer embryo developmental durations. The peak larval release dates for the first two (in 1994) or three (in 1993) batches were between neap and spring tides. Thereafter, the peak date for each batch approached the largest-TR date but never coincided precisely. Seaward larval transport efficiency evaluated by the product of mean water velocity per ebb and ebb duration suggests suboptimal values throughout the reproductive season.

Sexual dimorphism in the gammaridean amphipod, Urothoe carda Imbach, 1967, from an intertidal sandflat in southern Japan

AbstractSpecies of the gammaridean amphipod genus, Urothoe, commonly occur on intertidal and shallow subtidal sandy bottoms. Sexual dimorphism is reported for some species of Urothoe, but the descriptions are semi‐quantitative, lacking allometric growth models. A distinct sexual dimorphism was detected in Urothoe carda Imbach, 1967 collected from an intertidal sandflat in western Kyushu, Japan, in March, June, August, and October, 2015. The analysis for the total length–frequency distribution and maturity in each sex showed a female‐biased sex ratio (3.6 for mature individuals). Mature males had the larger eye and longer antenna 2 versus total length, expressed as generalized linear models (gamma distribution; logarithmic link‐function). Based on other studies on Urothoe spp. and peracarids, the above differences may suggest higher mate searching activity in male U. carda, with its larger eye functioning during the nocturnal excursion into the water column and its longer antenna 2 equipped with calceoli effective in detecting receptive females in the sediment.

Visual crypsis as a possible function of polymorphic shell coloration in the infaunal clam Ruditapes philippinarum

The infaunal clam Ruditapes philippinarum exhibits highly polymorphic shell coloration, but the function of the coloration remains uncertain. Here, a hypothesis that such shell coloration functions to enhance visual crypsis (i.e., background color matching) in juveniles (<15 mm in shell length) was tested with a combination of a field survey and laboratory experiments. Shell and background colorations were expressed as mean brightness values. Firstly, the association between shell and background brightness was investigated. For this, a field survey for two sympatric subpopulations with distinct substrates was conducted on an intertidal sandflat in western Kyushu, Japan. Secondly, the visual-crypsis hypothesis was tested experimentally using half-valve clam shells filled with paste of raw clam meat as prey and the pufferfish Takifugu niphobles as a visually hunting predator, in a tank with one to two dark-colored substrates and one light-colored substrate. The field survey showed that shell brightness significantly differed between the two sympatric subpopulations of juvenile clams and was positively associated with background brightness. The laboratory experiments indicated that prey items with comparatively light (dark) coloration on dark- (light-) colored substrate were consumed by predators more immediately and at a higher rate than in the color-matched combinations. Consequently, shell–background color matching could help juvenile clams avoid attack from visually hunting predators. The results provide a new insight into effective management planning for this clam species, creating sand habitats with a more matched coloration.

Facultative commensalism of a free-burrowing urothoid amphipod with a deep burrow-dwelling callianassid shrimp in intertidal sand

Species of the free-burrowing amphipod genus, Urothoe, are common on open sandy beaches. On intertidal sandflats, some species are associated with burrows or tubes of large infauna. How this link is formed and persisting under sheltered conditions was examined. On an intertidal sandflat in mid-western Kyushu, Japan, U. carda co-occurred with the deep burrow-dwelling callianassid shrimp, Nihonotrypaea harmandi, along a 300-m transect between tide marks. Amphipods resided in the surface 5-cm sediment outside shrimp burrows, as confirmed by sediment coring and burrow casting. In summers 1980 and 1981, the shrimp and amphipod populations were confined to the upper shore at mean densities of 182 and 701 inds m−2, respectively. In winter to spring, when the sediment surface mixing was caused by seasonal wind-induced waves, the amphipod but not the shrimp expanded down to the lowest shore. Later, by 1983, the shrimp increased mean density by 2.5 times and now also ranged to the lowest shore. In the summers of 1984, 2010, and 2015, the amphipod expanded to the lowest shore as well, with small variations in population size. Three marked changes in substrate properties were associated with shrimp inhabitation: thicker oxidized layer (proxy for oxygenated layer) in the sediment column; looser surface sediment, as evaluated with vane shear strength; and coarser and better-sorted surface sediment with less mud content. At least the former two changes were attributable to shrimp bioturbation, which could provide the amphipod with more permeable and softer substrates, leading to the formation of facultative commensalism.

Overproduction of microbial extracellular polymeric substances in subtropical intertidal sediments in response to endocrine disrupting chemicals

Microorganisms and their extracellular polymeric substances (EPS) in sediments are important in sediment stabilization and the fate of pollutants. However, how toxic organic pollutants affect bacteria and EPS in sediments, particularly in subtropical intertidal zones is poorly known. The present study aims to investigate the bacterial abundance and related EPS in simulated intertidal sandflat and mangrove sediments under the stress of endocrine disrupting chemicals (EDCs). Results showed that the temporal changes of the bacterial number in both sandflat and mangrove sediments were similar, increased from days 0 to 56 then became steady during the 84-days incubation. Bacteria exhibited an important role in the production of high molecular weight (HMW) EPS protein and the degradation of the low molecular weight (LMW) EPS protein. During incubation, the EPS polysaccharides changed from a colloidal-LMW fraction at the beginning to a more complex-HMW fraction at the end of the experiment. The increases in the concentration of HMW polysaccharides might contribute to sediment stabilization. Among different spiked EDCs, nonylphenol (NP) and 17α-ethinylestradiol (EE2) tended to accumulate in both sandflat and mangrove sediments and posed stresses to bacterial growth, especially the latter sediment. The persistent EDCs promoted a higher production of EPS polysaccharides and proteins in both sediments when compared to the respective control, but the EPS in the sandflat sediment was mainly in the colloidal fraction while the bound fraction was more abundant in the mangrove sediment. The present results enhance our understanding of the effects of EDCs on sediment biofilms in intertidal systems. This study also demonstrates the significance of EPS polysaccharides and proteins in sediment stabilization and provides a fundamental basis for future microbiology studies.

Recent biogenic traces from the coastal environments of the southern Red Sea coast of Saudi Arabia

A neoichnological study was done at a site west of Abha on the southern Red Sea coast of Saudi Arabia. This area covers Wadi Hali and its surroundings (180 49′35.27″N: 410 22′44.23″E). The objective of the study is to document biogenic traces that occur in the upper intertidal ripple-marked sand flats and supratidal flats that include microbial mats, mud flats, and scattered patches of mangrove stands and their muddy environments. Based on morphological descriptions, four different kinds of burrows are identified and their fossil equivalents are Psilonichnus upsilon (burrow type 1), Cylindricum (burrow types 2 and 3), and Arenicolites (burrow type 4). Tracks of land hermit crabs Coenobita clypeatus are described as ichnospecies Coenobichnus currani. Gastropod grazing trails and their fossil equivalent Planolites is identified as well. Supratidal algal mats and their microscopic contents are presented. Fossil plant roots (rhizomes) are reported from a localized fluvial deposit from a dry channel (wadi). The morphology of the traces, trace maker, and ethology and the environmental setting indicate that the ichnofauna and botanical traces belong to the Psilonichnus ichnofacies.

Light or tide? Effects on the emergence and recolonization of harpacticoid copepods from sand flats of the Wadden Sea (southern North Sea)

The Wadden Sea is a highly productive coastal ecosystem and an important nursery ground for various demersal fishes of the southern North Sea. Copepods are one of the major food sources of the ecosystem, yet little is known about the movement patterns of this benthic meiofaunal group within both sediment and water column in the intertidal environment, where they would be more vulnerable for predation. The diversity and abundance of harpacticoids emerging and recolonizing intertidal sand flats were investigated during two diurnal and two nocturnal high tides, using specifically designed traps. Our study revealed no significant difference in the amount of copepods emerging from the sediment into the water column during the day and the night. Furthermore, the proportion of emerging adults and copepodids did not differ from each other and most individuals persisted in the sediment during high tide. Interestingly, the most abundant species Arenosetella tenuissima and Tachidius discipes showed an opposite emergence behavior, with the former migrating in higher numbers during the night and the latter during the day.Although tidal inundation was the proximate factor initiating emergence, we suggest that sediment reworking by tidal currents, waves and bioturbation as well as water and food availability above and within the sediment may be additional factors controlling emergence of harpacticoids in the Wadden Sea, with light intensity playing a minor role.Therefore, emergence and recolonization patterns of harpacticoids from subtidal ecosystems cannot directly be transferred to intertidal areas as the time for benthic pelagic processes is limited to high tides.

Oyster reefs as carbon sources and sinks.

Carbon burial is increasingly valued as a service provided by threatened vegetated coastal habitats. Similarly, shellfish reefs contain significant pools of carbon and are globally endangered, yet considerable uncertainty remains regarding shellfish reefs' role as sources (+) or sinks (-) of atmospheric CO2 While CO2 release is a by-product of carbonate shell production (then burial), shellfish also facilitate atmospheric-CO2 drawdown via filtration and rapid biodeposition of carbon-fixing primary producers. We provide a framework to account for the dual burial of inorganic and organic carbon, and demonstrate that decade-old experimental reefs on intertidal sandflats were net sources of CO2 (7.1 ± 1.2 MgC ha-1 yr-1 (µ ± s.e.)) resulting from predominantly carbonate deposition, whereas shallow subtidal reefs (-1.0 ± 0.4 MgC ha-1 yr-1) and saltmarsh-fringing reefs (-1.3 ± 0.4 MgC ha-1 yr-1) were dominated by organic-carbon-rich sediments and functioned as net carbon sinks (on par with vegetated coastal habitats). These landscape-level differences reflect gradients in shellfish growth, survivorship and shell bioerosion. Notably, down-core carbon concentrations in 100- to 4000-year-old reefs mirrored experimental-reef data, suggesting our results are relevant over centennial to millennial scales, although we note that these natural reefs appeared to function as slight carbon sources (0.5 ± 0.3 MgC ha-1 yr-1). Globally, the historical mining of the top metre of shellfish reefs may have reintroduced more than 400 000 000 Mg of organic carbon into estuaries. Importantly, reef formation and destruction do not have reciprocal, counterbalancing impacts on atmospheric CO2 since excavated organic material may be remineralized while shell may experience continued preservation through reburial. Thus, protection of existing reefs could be considered as one component of climate mitigation programmes focused on the coastal zone.

Sedimentary controls on modern sand grain coat formation

Coated sand grains can influence reservoir quality evolution during sandstone diagenesis. Porosity can be reduced and fluid flow restricted where grain coats encroach into pore space. Conversely pore-lining grain coats can restrict the growth of pore-filling quartz cement in deeply buried sandstones, and thus can result in unusually high porosity in deeply buried sandstones. Being able to predict the distribution of coated sand grains within petroleum reservoirs is thus important to help find good reservoir quality. Here we report a modern analogue study of 12 sediment cores from the Anllóns Estuary, Galicia, NW Spain, collected from a range of sub-environments, to help develop an understanding of the occurrence and distribution of coated grains. The cores were described for grain size, bioturbation and sedimentary structures, and then sub-sampled for electron and light microscopy, laser granulometry, and X-ray diffraction analysis. The Anllóns Estuary is sand-dominated with intertidal sand flats and saltmarsh environments at the margins; there is a shallowing/fining-upwards trend in the estuary-fill succession. Grain coats are present in nearly every sample analysed; they are between 1μm and 100μm thick and typically lack internal organisation. The extent of grain coat coverage can exceed 25% in some samples with coverage highest in the top 20cm of cores. Samples from muddy intertidal flat and the muddy saltmarsh environments, close to the margins of the estuary, have the highest coat coverage (mean coat coverage of 20.2% and 21.3%, respectively). The lowest mean coat coverage occurs in the sandy saltmarsh (10.4%), beyond the upper tidal limit and sandy intertidal flat environments (8.4%), close to the main estuary channel. Mean coat coverage correlates with the concentration of clay fraction. The primary controls on the distribution of fine-grained sediment, and therefore grain coat distribution, are primary sediment transport and deposition processes that concentrate the clay fraction in the sediment towards the margins of the estuary. Bioturbation and clay illuviation/mechanical infiltration are secondary processes that may redistribute fine-grained sediment and produce grain coats. Here we have shown that detrital grain coats are more likely in marginal environments of ancient estuary-fills, which are typically found in the fining-upward part of progradational successions.

Sedimentary processes and products in a mesotidal salt marsh environment: insights from Groves Creek, Georgia

Southeastern salt marshes are important repositories of sediment and carbon, and their formation is heavily dependent on deposition and accumulation of inorganic sediment. This study examined Groves Creek marsh near Savannah, GA, a typical Spartina alterniflora salt marsh of the southeastern US. Analyses were focused on the character, deposition and accumulation of material within the marsh on daily, monthly, decadal and centennial timescales, to determine the dominant factors in material supply and redistribution, and on its stratigraphy to determine the 1,000-year history of Groves Creek salt marsh development. Modern processes create gradients in grain size, which shows little variation from the tidal channel flanks up to mean sea level, and which coarsens with distance into the marsh from mean sea level to mean high water. This unexpected result suggests that, although floc transport is an important mechanism of sediment supply near the channel margins, energetic events must supply coarser materials to the marsh platform, where they are not readily removed by typical energy regimes. Daily deposition can approach ~3 g/cm2 year; however, centennial accumulation rates are orders of magnitude lower (0.11±0.05 g/cm2 year) and are similar to those present over the past 300 years (0.05–0.2 g/cm2 year), indicating that much of the daily deposition is remobilized. Stable isotopic δ13C (average –18.7‰) and δ15N (average 5.7‰) values most likely indicate a large contribution from S. alterniflora as a carbon source throughout the marsh, although heavier δ15N on the channel flanks suggest that benthic algae may be locally important. Geologic, geochemical and microfossil evidence suggests that depositional conditions in the Groves Creek marsh have changed significantly over the past ~1,000 years, creating a distinct fining-upward sequence. This sequence preserves the signature (from bottom to top) of subtidal flats grading to intertidal sandflats, an erosional lag created by a migrating tidal channel, point bar deposits, channel fill deposits, and a capping salt marsh. A significant change in environmental conditions occurred between 360–500 cal years BP, which changed the character of geological, geochemical and microfossil components in the marsh, and, because this observation is robust throughout the southeastern US, hints at a major change in estuarine and coastal systems at this time.

Quantifying macrodetritus fluxes from a small temperate estuary

Empirical measurements of estuary-to-coast material fluxes usually exclude the fraction of primary production that is exported as macrodetritus (marine plant litter), potentially leaving a gap in our understanding of the role of estuaries as outwelling systems. To address this gap, we sampled water and suspended material seasonally from the mouth of Pepe Inlet, Tairua Estuary, New Zealand. From samples collected hourly over 24h, we calculated the lateral tidal fluxes (import, export, net flux) of macrodetritus, particulate and dissolved forms of nitrogen (N) and phosphorus (P). Annually, the inlet was a net exporter of N and P (5145 and 362kg respectively). However, macrodetritus accounted for &lt;13 and &lt;3% of seasonal N and P exports respectively. Macrodetritus is an obvious and visible source of estuary-to-coast subsidy, but our derived nutrient budgets suggest the dissolved and particulate forms dominate the net export of N and P (&gt;87%). Nevertheless, seasonal pulses in the source and supply of macrodetritus may have consequences for the temporal scales over which this resource subsidy affects receiving ecosystems (e.g. intertidal sandflats). These mensurative investigations are useful to inform estuarine nutrient budgets that quantify the ecosystem services provided by temperate estuaries (e.g. contribution to fisheries food webs).

Persistence, Extinction, and Recolonization of an Epibenthic Gastropod Population on an Intertidal Sandflat: 35-Y Contingent History of a Key Species of the Benthic Community in Metapopulation and Metacommunity Contexts

For marine benthos with planktonic larval stages, demographic metapopulation and empirical metacommunity studies are scarce. Asink nature was shown for a local intertidal sandflat population of the gastropod Umbonium moniliferumin an estuarine to coastal oceanic region in Amakusa, Kyushu, Japan. The gastropod has a 3-day planktonic larval duration (PLD). It inhabited 11 sandflats, which were shared by the ghost shrimp Nihonotrypaea harmandi. The shrimp is a burrow-dwelling bioturbator with a 30-day PLD. On one sandflat (hereafter U0) in Amakusa, the density and distribution of the two species was monitored from 1979 to 2014. All their main local populations were surveyed in 1998. In July 1979, the gastropod and shrimp were distributed on the lower and upper zones of U0 separately, with mean densities of 1,740 and 170 ind/m2. The gastropod population declined to extinction in 1986, associated with the increase in shrimp density to 440 ind/m2 and distribution to the lowest shore. Shrimp sediment destabilization was most likely to inhibit gastropod recruitment. The gastropod population began to recover in 1997 owing to shrimp population shrink. The regional survey results showed that (1) eight local populations (U0–U7 sandflats) of the two species along a 25-km shoreline of Amakusa, presumably, constitute each substantial metapopulation, and (2) their largest populations were at the shoreline opposite ends. These features would be generated by the interaction of speciesspecific larval dispersal traits and shoreline physiography. The threshold shrimp density of 160 ind/m2 precluding gastropod recruitment was detected from U1–U7. In 2001 and 2009, gastropod population density on U0 reached respective peaks comparable to that in 1979. This population fluctuation was due to varying recruit abundance, which corresponded well with the year-to-year change in shrimp population density across that threshold. The recolonization of the U0 gastropod population ought to be made by allochthonous larvae from the U1–U7 populations, especially U6–U7, suggesting a rescue effect in the source—sink relationship. The degree of self-persistency of the U0 gastropod population was estimated by calculating net reproductive rate (R0) based on the parameters about pre-settlement larval return-home and survival rates and post-settlement juvenile and adult survival rates. The availability of data set restricted the estimation to potential R0 for four year-segments, with each 1-y parameter set extrapolated to the gastropod 4-y life span. On U0 in 1979 to 1981, despite with minimum shrimp influences, the gastropod population could still be a demographic sink probably due to a limited larval return-home rate and due to a low post-settlement survival rate of juveniles caused by epibenthic predation or storm for their first 1 y to reach sexual maturity.