Land-ocean Interface Research Articles

MANGROVES – THE COASTAL GREEN BUFFER ZONE

Coastal zones are the most biologically complex and productive ecosystems. Mangroves are also termed as ‘Coastal Woodlands’ or ‘Tidal Forests’ and occur along the tropical and sub-tropical seas in bays, lagoons and estuaries region. Mangroves form a transitional complex ecosystem between the sea and land comprising of aquatic and terrestrial flora and fauna. Mangrove forests are an integral part of the coastal environment extending through out the tropics and sub-tropics of the world. Mangroves forests are of vital importance with a great socio-economic potentiality and ecological significance. Ecologically, mangroves stabilize the coastline shoreline acting as a buffer to the land mass from tidal surges, cyclonic storms and high tides. Coastal disasters are one manifestation of environmental problems and as such have ecological, social and economic dimensions. The tsunami 0f 26 th December 2004, which was triggered by a magnitude submarine earthquake in the Sumatra region that struck the shores of south and and south-east Asia resulted in the death or displacement of several lakhs of people. Keeping in view the unpredictable occurrence of such disasters the significant role of the natural mangrove vegetation along the coast in controlling these disasters is highlighted. The complexity and ecological importance of mangroves as a land-ocean interface ecosystem has been confirmed. Mangrove degradation is caused mainly due to commercialisation, economic and social pressures and failure to recognize the natural value of mangroves. Human anthropogenic activities and environmental stress significantly reduced the areal extent of mangrove vegetation. In the present study the depletion of mangroves along coastline of Guntur district has been estimated and evaluated. The devastating damage of life, property and natural biota due to tsunami disaster could have been minimal or prevented altogether had the mangroves along the coast been conserved and managed sustainably. Conservation of mangroves is essential for the benefit of socio-economic configurations and for the balance of complex eco-climates of the coastal ecosystem.

Muskrat Ecology in the Mackenzie Delta: Insights from Local Knowledge and Ecological Field Surveys

A rctic deltas have been called keystone ecosystems because of the numerous processes they influence at the land-ocean interface, such as nutrient cycling and sea ice formation (Lesack et al., 2014). Climate change is proceeding more rapidly in the Arctic than in any other region (ACIA, 2004), and low-lying Arctic deltas are expected to be significantly affected by the combined effects of changes in both terrestrial and marine environments (Walker, 1998). Terrestrial changes that are particularly concerning include increases in summer precipitation and the altered timing of spring breakup (Goulding et al., 2009; Lesack et al., 2014). Marine processes, including sea level rise, increased storm surge activity, and reduced sea ice cover will also affect Arctic deltas (ACIA, 2004; Lantz et al., 2015). The Mackenzie Delta in Canada’s western Arctic, the second largest Arctic delta in the world, is a low-lying alluvial plain that contains thousands of small lakes (Emmerton et al., 2007) (Fig. 1). The numerous water bodies in the delta offer abundant habitat for fish, birds, and mammals, including muskrats (Ondatra zibethicus). Muskrats are semiaquatic furbearers whose populations respond to water levels and may serve as an indicator species for changes in wetland ecosystems (Fuller, 1951; Weller, 1988). They are prey species for marten, mink, red fox, black bear, otter, wolves, and grizzly bears (Fuller, 1951; Jelinski, 1989) and can have significant effects on the vegetation in their habitat (Errington, 1963; Nyman et al., 1993; Smirnov and Tretyakov, 1998). Muskrats also supported a prolific fur and subsistence economy in the North throughout the 19th and 20th centuries (Stevens, 1953; Krech, 1976; Alunik et al., 2003). While participation in the fur trade has declined since the 1950s as a result of low fur prices and the high cost of trapping, muskrats remain important culturally and economically and are still used for subsistence and income (Gwich’in Elders, 1997; A. Gordon, pers. comm. 2013). Like other northern mammals, muskrats exhibit periodic cycles in population size (Errington, 1963; Krebs, 1996). It has been suggested that these cycles are caused by disease outbreaks, predator population cycles, and intrinsic and external factors (Erb et al., 2000; Krebs, 2013). Research on fur returns from the Mackenzie Delta in the late 1800s to early 1900s indicates that population cycles in the delta average around 10 years between highs (Clarke, 1944). In recent years, many residents of the Mackenzie Delta region have reported declines in muskrat abundance that are outside the normal range of variation (ABEKS, 2003, 2008). This decline is likely to have significant impacts on local communities that rely on muskrats for subsistence, trapping income, and overall wellbeing (Parlee et al., 2005; Gill et al., 2014) and may be related to changes in the climate and hydrological processes in the Mackenzie Delta. The primary objective of my research is to investigate reported declines in muskrat populations and examine linkages between habitat use and biophysical conditions in the Mackenzie Delta. The traditional knowledge held by land users, informed by the experiences of past and present generations, is a valuable resource for understanding why, when, and where changes in muskrat ecology and harvesting have taken place. In the first part of my research, I explore the insights that local knowledge holders provide regarding changes in muskrat ecology and harvesting practices in the Mackenzie Delta over the last 100 years. In the second part of my research, I use field surveys to identify and characterize biophysical conditions linked to muskrat habitat use.

The Role of Heterotrophic Microbial Communities in Estuarine C Budgets and the Biogeochemical C Cycle with Implications for Global Warming: Research Opportunities and Challenges.

Estuaries are among the most productive and economically important marine ecosystems at the land-ocean interface and contribute significantly to exchange of CO2 with the atmosphere. Estuarine microbial communities are major links in the biogeochemical C cycle and flow of C in food webs from primary producers to higher consumers. Considerable attention has been given to bacteria and autotrophic eukaryotes in estuarine ecosystems, but less research has been devoted to the role of heterotrophic eukaryotic microbes. Current research is reviewed here on the role of heterotrophic eukaryotic microbes in C biogeochemistry and ecology of estuaries, with particular attention to C budgets, trophodynamics, and the metabolic fate of C in microbial communities. Some attention is given to the importance of these processes in climate change and global warming, especially in relation to sources and sinks of atmospheric CO2 , while also documenting the current paucity of research on the role of eukaryotic microbes that contribute to this larger question of C biogeochemistry and the environment. Some recommendations are made for future directions of research and opportunities of applying newer technologies and analytical approaches to a more refined analysis of the role of C in estuarine microbial community processes and the biogeochemical C cycle.

Seasonal variation in sources and processing of particulate organic carbon in the Pearl River estuary, South China

Particulate organic carbon (POC) in the Pearl River estuary (PRE), South China, along a salinity gradient from freshwater to seawater in four months was studied in order to determine its temporal and spatial changes in source and processing. Analytical parameters included chlorophyll-a (Chl-a), POC, and carbon isotopic composition of POC and the dissolved inorganic carbon (DIC) (δ13CPOC, δ13CDIC). POC varied greatly from freshwater to seawater, exhibiting a significant power law distribution with a rapid decrease (from >2.5 mg l−1 to <0.9 mg l−1) in a narrow salinity range of 0–5 and then a slow decline to ∼0.4 mg l−1 along the large salinity gradient in the estuary. POC was sourced predominantly from in situ phytoplankton, and hence largely reflective of primary production, in February, August, and November as indicated by mostly lower POC/Chl-a values (<200), and significant correlation between POC and Chl-a, as well as between δ13CPOC and δ13CDIC. But in May, soil-derived OC was dominant in freshwater and low salinity estuarine water, as suggested by low POC% in total suspended substance, low Chl-a values and high POC/Chl-a ratios, and higher δ13CPOC values that was not in parallel with δ13CDIC excursion. The offset between δ13CPOC and phytoplankton δ13C (inferred from δ13CDIC) was trivial or positive in salinity <12, but then became negative downstream, which was likely suggestive of biogeochemical change from net respiration in the upper estuary to net production in the lower and outer estuary. Our results demonstrated that in situ phytoplankton was the dominant source to the estuarine POC pool during most seasons of a year, except in May in the first phase of wet season when rainfall and river flux increased abruptly causing intensive flushing effect. We further suggested that POC may be undergone intensive processing within the PRE, which is important for understanding organic carbon delivery in this vigorous land–ocean interface.

A strong CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)

Abstract. In contrast to its small surface area, the coastal zone plays a disproportionate role in the global carbon cycle. Carbon production, transformation, emission and burial rates at the land–ocean interface are significant at the global scale but still poorly known, especially in tropical regions. Surface water pCO2 and ancillary parameters were monitored during nine field campaigns between April 2013 and April 2014 in Guanabara Bay, a tropical eutrophic to hypertrophic semi-enclosed estuarine embayment surrounded by the city of Rio de Janeiro, southeast Brazil. Water pCO2 varied between 22 and 3715 ppmv in the bay, showing spatial, diurnal and seasonal trends that mirrored those of dissolved oxygen (DO) and chlorophyll a (Chl a). Marked pCO2 undersaturation was prevalent in the shallow, confined and thermally stratified waters of the upper bay, whereas pCO2 oversaturation was restricted to sites close to the small river mouths and small sewage channels, which covered only 10 % of the bay's area. Substantial daily variations in pCO2 (up to 395 ppmv between dawn and dusk) were also registered and could be integrated temporally and spatially for the establishment of net diurnal, seasonal and annual CO2 fluxes. In contrast to other estuaries worldwide, Guanabara Bay behaved as a net sink of atmospheric CO2, a property enhanced by the concomitant effects of strong radiation intensity, thermal stratification, and high availability of nutrients, which promotes phytoplankton development and net autotrophy. The calculated CO2 fluxes for Guanabara Bay ranged between −9.6 and −18.3 mol C m−2 yr−1, of the same order of magnitude as the organic carbon burial and organic carbon inputs from the watershed. The positive and high net community production (52.1 mol C m−2 yr−1) confirms the high carbon production in the bay. This autotrophic metabolism is apparently enhanced by eutrophication. Our results show that global CO2 budgetary assertions still lack information on tropical, marine-dominated estuarine systems, which are affected by thermal stratification and eutrophication and behave specifically with respect to atmospheric CO2.

The Influence of the Slope Angle of the Ocean–Land Mixed Propagation Path on the Lightning Electromagnetic Fields

In this paper, lightning electromagnetic fields in the presence of an ocean–land mixed propagation path having different configurations are evaluated using a finite-element-based full-wave approach. The simulations are conducted considering lightning strikes to ground and to the ocean. The lightning electromagnetic fields are obtained for observation points inside the ground and on the ground surface. The land–ocean interface is represented by a linearly increasing ocean depth characterized by the slope angle. Different sets of simulation results show that the electric field components (vertical and horizontal) in the immediate vicinity of the interface can be affected by the interface slope angle. The obtained results also show that, for observation points located beyond 50 m or so from the ocean, the effect of the slope angle of the ocean–land interface on the lightning electromagnetic fields can be disregarded.

English

Space-based observations offer unique capabilities for studying spatial and temporal dynamics of the upper ocean inorganic carbon cycle and, in turn, supporting research tied to ocean acidification (OA). Satellite sensors measuring sea surface temperature, color, salinity, wind, waves, currents, and sea level enable a fuller understanding of a range of physical, chemical, and biological phenomena that drive regional OA dynamics as well as the potentially varied impacts of carbon cycle change on a broad range of ecosystems. Here, we update and expand on previous work that addresses the benefits of space-based assets for OA and carbonate system studies. Carbonate chemistry and the key processes controlling surface ocean OA variability are reviewed. Synthesis of present satellite data streams and their utility in this arena are discussed, as are opportunities on the horizon for using new satellite sensors with increased spectral, temporal, and/or spatial resolution. We outline applications that include the ability to track the biochemically dynamic nature of water masses, to map coral reefs at higher resolution, to discern functional phytoplankton groups and their relationships to acid perturbations, and to track processes that contribute to acid variation near the land-ocean interface.

Distributional patterns in an insect community inhabiting a sandy beach of Uruguay

Most studies of sandy beach macrofauna have been restricted to semiterrestrial species and do not include insects when providing species richness and abundance estimates. Particularly, spatio-temporal patterns of community structure of the entomofauna inhabiting these ecosystems have been scarcely documented. This study assessed spatio-temporal distributional patterns of the night active entomofauna on a beach-dune system of Uruguay, including variations in species richness, abundance and diversity, and their relationship with environmental factors. A deconstructive taxonomic analysis was also performed, considering richness and abundance patterns separately for the most abundant insect Orders (Hymenoptera and Coleoptera) to better understand the factors which drive their patterns. We found clear temporal and across-shore patterns in the insect community inhabiting a land–ocean interface, which matched spatiotemporal variations in the environment. Abundance and species richness were highest in spring and summer, concurrently with high temperatures and low values of sediment moisture and compaction. Multivariate ordinations showed two well-defined species groups, which separated summer, autumn and spring samples from winter ones. Generalized Linear Models allowed us to describe a clear segregation in space of the most important orders of the insect community, with specific preferences for the terrestrial (Hymenoptera) and beach (Coleoptera) fringes. Hymenoptera preferred the dune zone, characterized by high elevation and low sand moisture and compaction levels, whereas Coleoptera preferred gentle slopes and fine and humid sands of the beach. Our results suggest that beach and dune ecosystems operate as two separate components in regard to their physical and biological features. The high values of species richness and abundance of insects reveal that this group has a more significant ecological role than that originally considered so far in sandy beach ecology.

Metagenomic insights into particles and their associated microbiota in a coastal margin ecosystem.

Our previously published research was one of the pioneering studies on the use of metagenomics to directly compare taxonomic and metabolic properties of aquatic microorganisms from different filter size-fractions. We compared size-fractionated water samples representing free-living and particle-attached communities from four diverse habitats in the Columbia River coastal margin, analyzing 12 metagenomes consisting of >5 million sequence reads (>1.6 Gbp). With predicted peptide and rRNA data we evaluated eukaryotic, bacterial and archaeal populations across size fractions and related their properties to attached and free-living lifestyles, and their potential roles in carbon and nutrient cycling. In this focused review, we expand our discussion on the use of high-throughput sequence data to relate microbial community structure and function to the origin, fate and transport of particulate organic matter (POM) in coastal margins. We additionally discuss the potential impact of the priming effect on organic matter cycling at the land-ocean interface, and build a case for the importance, in particle-rich estuaries and coastal margin waters, of microbial activities in low-oxygen microzones within particle interiors.

Understanding Late Quaternary change at the land–ocean interface: a synthesis of the evolution of the Wilderness coastline, South Africa

Coastal barrier systems have been widely used to understand the responses of coastal margins to fluctuating Pleistocene sea levels. What has become apparent, particularly with the development of robust chronological frameworks, is that gaps in terrestrial barrier sedimentary records are not uncommon and that they most likely reflect phases of barrier construction on the now submerged continental shelf. Thus, understanding the land–ocean interface through time is critical to fully appreciate the Quaternary archives contained within the barriers and their associated back-barrier deposits. This study uses offshore and lakefloor (back-barrier) seismic profiling from the South African south coast at Wilderness to link the sub-aerially exposed barrier stratigraphy to the currently submerged geological and sedimentological record. A total of eight separate submerged aeolian units are identified at water depths of up to 130 m below mean sea level. Their approximate ages are constrained with reference to the eustatic sea-level record and the deepest units are consistent with the estimated magnitude of sea-level lowering during the Last Glacial Maximum (LGM) on the South African coastline. As previously assumed, aeolian sedimentation tracked the shoreline onto the continental shelf during the Late Pleistocene. During sea-level regressions, both the incision of fluvial channels and the deposition of back-barrier systems occurred across the continental shelf. During late low stand/early transgression periods, landward shoreface migration occurred, pre-existing channel incisions were infilled and pre-existing barriers were truncated. Rapid transgression, however, allowed the preservation of some back-barrier deposits, possibly aided by protection from antecedent topography. As sea level neared the present-day elevation, erosion of the mid-shelf sediments resulted in the development of a Holocene sediment wedge, which was augmented by Holocene fluvial sediment supply. The Holocene sand wedge is preserved in the back-barrier lakes and was deposited during the Holocene highstand inundation. Overlying middle to late Holocene terrestrial muds reflect the deposition of river-borne mud onto the shelf. These results clearly demonstrate that within transgressive–regressive sea-level cycles, accommodation space for barriers is controlled by antecedent drainage systems and gradients on the adjacent inner continental shelf.

Inorganic Carbon Fluxes in the Inner Elbe Estuary, Germany

Estuaries are strong sources for CO2 to the atmosphere. At the same time, they are intense reactors for dissolved matter at the land–ocean interface. The temporal and spatial variations of inorganic carbon fluxes in the inner Elbe estuary were studied based on 18 transects and a budget was constructed. The freshwater part of the Elbe estuary, a potential hotspot for carbon transformation, was evaluated using GIS-derived surface areas, isotopic DIC data and modelled pCO2. Calculation of pCO2 with commonly used constants yield 20 % higher values if compared to thermodynamically modelled results, integrating the actual ionic composition of the sample. With these improved calculations, it could be confirmed that this part is important for carbon turnover. Highest pCO2 values and strongest CO2 efflux per square metre compared to the rest of the inner Elbe estuary were observed. It contributes 10 % of the CO2 fluxes to the atmosphere, and 82 % of seaward transport of dissolved CO2. Overall, the estuary is net heterotrophic and releases 13.2 × 109 mol CO2 year−1 to the atmosphere. About 20 % of the DIC exports from the estuary are attributed to atmospheric CO2 exchange, while 80 % are transported seawards in dissolved form. Additionally, the Elbe estuary is a source for DIC besides CO2. The flux to the coastal zone is increased by about 18 %. Main sources are marshes and sediments. These findings should be considered in North Sea budgets, which to date rely on simple two source mixing of freshwater and seawater to derive riverine DIC inputs. Findings from this study corroborate the anthropogenic impact on carbon fluxes and clearly confirm the necessity to examine the entire estuary including the freshwater part.

Human-mediated drivers of change — impacts on coastal ecosystems and marine biota of South Africa

Coastal ecosystems are highly vulnerable to human-mediated drivers of global change because they are located at the land–ocean interface and often host centres of urbanisation and development. The South African coastline comprises several distinct coastal ecoregions that support a wide range of coastal (inshore) ecosystems, including rocky, sandy and mixed shores, kelp beds, estuaries and seagrass communities. A growing body of evidence indicates that local air and sea temperatures, wind patterns, ocean current speed and upwelling regimes are all being affected by human-mediated climate change. In addition, anthropogenic activities, such as shipping (introducing coastal bioinvasives), exploitation of coastal marine resources, industry (releasing pollutants) and urban development, act synergistically with climate change to place pressure on coastal ecosystems and their biota. The aim of this review was primarily to synthesise and update research into causes of direct and indirect human-mediated global change and their effects on South African coastal systems. It incorporates both historic and the latest regional research on climate change and anthropogenic threats across the ecosystems listed above, much of which was supported by the South African Network for Coastal and Oceanic Research (SANCOR), specifically the SEAChange programme in recent years. It is evident that all these ecosystems are vulnerable to all the drivers considered, albeit to differing degrees, depending on their location on the coast. Whereas some bioinvasives have had a dramatic impact on rocky shore systems on the West Coast, their impact has been moderate on the South Coast and minimal on the East Coast; exploitation shows the reverse pattern. Furthermore, the impacts of human-mediated drivers on coastal ecosystems are synergistic. Of major interest is the fact that the West Coast and parts of the South Coast are exhibiting cooling trends in offshore sea surface temperatures, rather than warming. Correspondingly, a geographical spread of organisms associated with West and South-West Coast rocky shores and kelp beds has tended to be eastwards around Cape Point, rather than northwards along the West Coast as would have been expected with warming sea temperatures. Overall, significant progress has been made toward a better understanding of the combined pressures on each ecosystem and knowledge gaps have been identified, thus helping to direct future research themes.

Donor-Control of Scavenging Food Webs at the Land-Ocean Interface.

Food webs near the interface of adjacent ecosystems are potentially subsidised by the flux of organic matter across system boundaries. Such subsidies, including carrion of marine provenance, are predicted to be instrumental on open-coast sandy shores where in situ productivity is low and boundaries are long and highly permeable to imports from the sea. We tested the effect of carrion supply on the structure of consumer dynamics in a beach-dune system using broad-scale, repeated additions of carcasses at the strandline of an exposed beach in eastern Australia. Carrion inputs increased the abundance of large invertebrate scavengers (ghost crabs, Ocypode spp.), a numerical response most strongly expressed by the largest size-class in the population, and likely due to aggregative behaviour in the short term. Consumption of carrion at the beach-dune interface was rapid and efficient, driven overwhelmingly by facultative avian scavengers. This guild of vertebrate scavengers comprises several species of birds of prey (sea eagles, kites), crows and gulls, which reacted strongly to concentrations of fish carrion, creating hotspots of intense scavenging activity along the shoreline. Detection of carrion effects at several trophic levels suggests that feeding links arising from carcasses shape the architecture and dynamics of food webs at the land-ocean interface.

Sedimentation and Organic Carbon Burial in the Yangtze River and Hudson River Estuaries: Implications for the Global Carbon Budget

One of the most important challenges in global climate change research is balancing the carbon budget within the global carbon cycle. Carbon burial in sediments at the land–ocean interface has been difficult to quantify and model because it represents non-steady-state boundary conditions that are also affected by human activities. In this study, we document carbon burial rates in the Yangtze River (1.6–4.9 × 1012 gC year−1) and Hudson River (1.8–3.6 × 1010 gC year−1) estuaries and integrate our results with carbon burial rates determined by others in the world’s 25 largest river-estuarine systems (6–11 × 1013 gC year−1). Our results indicate that carbon burial in estuaries, bays, coves, lagoons, mud flats, marshes, mangroves, and other highly productive or protected low-energy areas at the land–ocean interface along the entirety of the world’s coastlines may serve as an unrecognized sink within the global carbon budget.

Decoupling between Water Column Oxygenation and Benthic Phosphate Dynamics in a Shallow Eutrophic Estuary

Estuaries are crucial biogeochemical filters at the land-ocean interface that are strongly impacted by anthropogenic nutrient inputs. Here, we investigate benthic nitrogen (N) and phosphorus (P) dynamics in relation to physicochemical surface sediment properties and bottom water mixing in the shallow, eutrophic Peel-Harvey Estuary. Our results show the strong dependence of sedimentary P release on Fe and S redox cycling. The estuary contains surface sediments that are strongly reducing and act as net P source, despite physical sediment mixing under an oxygenated water column. This decoupling between water column oxygenation and benthic P dynamics is of great importance to understand the evolution of nutrient dynamics in marine systems in response to increasing nutrient loadings. In addition, the findings show that the relationship between P burial efficiency and bottom water oxygenation depends on local conditions; sediment properties rather than oxygen availability may control benthic P recycling. Overall, our results illustrate the complex response of an estuary to environmental change because of interacting physical and biogeochemical processes.

Multiple scavengers respond rapidly to pulsed carrion resources at the land–ocean interface

Sandy beaches are the globe's longest interface region between the oceans and the continents, forming highly permeable boundaries across which matter flows readily. Stranded marine carrion supplies a high-quality food source to scavengers, but the role of animal carcasses is generally under-reported in sandy-beach food webs. We examined the response of scavengers to pulsed subsidies in the form of experimental additions of fish carcasses to the dune–beach interface in eastern Australia. Ghost crabs (Ocypode spp.) are the dominant invertebrate scavengers in these habitats and they responded strongly and consistently to changed resource availability: densities increased significantly within days of carrion augmentations. Carcasses added experimentally also formed local nuclei for a diversity of vertebrate scavengers that aggregated at food falls; these included large lizards, several species of birds (including raptors), and foxes. Consumption of fish carrion by the vertebrate scavengers was rapid and often complete. There is also evidence for higher-order interactions, where responses of invertebrate scavengers became depressed in plots where vertebrate scavenger activity was intense. Our findings emphasize that carrion can be a pivotal component of beach food webs.

Climate Change Impacts on the Organic Carbon Cycle at the Land-Ocean Interface

Estuaries are among the most altered and vulnerable marine ecosystems. These ecosystems will likely continue to deteriorate owing to increased population growth in coastal regions, expected temperature and precipitation changes associated with climate change, and their interaction with each other, leading to serious consequences for the ecological and societal services they provide. A key function of estuaries is the transfer, transformation, and burial of carbon and other biogenic elements exchanged between the land and ocean systems. Climate change has the potential to influence the carbon cycle through anticipated changes to organic matter production in estuaries and through the alteration of carbon transformation and export processes. This review discusses the effects of climate change on processes influencing the cycling of organic carbon in estuaries, including examples from three temperate estuaries in North America. Our goal is to evaluate the impact of climate change on the connectivity of terrestrial, estuarine, and coastal ocean carbon cycles.

Chemical Ecology of Red Mangroves, Rhizophora mangle, in the Hawaiian Islands

The coastal red mangrove, Rhizophora mangle L., was introduced to the Hawaiian Islands from Florida 100 yr ago and has spread to cover many shallow intertidal shorelines that once were unvegetated mudflats. We used a field survey approach to test whether mangroves at the land-ocean interface could indicate watershed inputs, especially whether measurements of leaf chemistry could identify coasts with high nutrient inputs and high mangrove productivities. During 2001–2002, we sampled mangroves on dry leeward coasts of southern Moloka'i and O'ahu for 14 leaf variables including stable carbon and nitrogen isotopes (δ13C, δ15N), macronutrients (C, N, P), trace elements (B, Mn, Fe, Cu, Zn), and cations (Na, Mg, K, Ca). A new modeling approach using leaf Na, N, P, and δ13C indicated two times higher productivity for mangroves in urban versus rural settings, with rural mangroves more limited by low N and P nutrients and high-nutrient urban mangroves more limited by freshwater inputs and salt stress. Leaf chemistry also helped identify other aspects of mangrove dynamics: especially leaf δ15N values helped identify groundwater N inputs, and a combination of strongly correlated variables (C, N, P, B, Cu, Mg, K, Ca) tracked the mangrove growth response to nutrient loading. Overall, the chemical marker approach is an efficient way to survey watershed forcing of mangrove forest dynamics.

The particulate 7Be/ 210Pb xs and 234Th/ 210Pb xs activity ratios as tracers for tidal-to-seasonal particle dynamics in the Gironde estuary (France): Implications for the budget of particle-associated contaminants

The short-lived natural radionuclides 7Be ( T 1/2 = 53 days), 234Th xs ( T 1/2 = 24.1 days) and 210Pb xs ( T 1/2 = 22.3 years), i.e. 234Th and 210Pb in excesses of that supported within particles by the decay of their parent isotopes, were analysed in suspended particulate matter (SPM) to study the particle dynamics in the Gironde fluvial estuarine system (France), strongly impacted by heavy metal pollution. From surveys of this land–ocean interface in 2006 and 2007, we established a times series of these radioisotopes and of their activity ratios ( 7Be/ 210Pb xs and 234Th/ 210Pb xs ARs) in particles sampled under different hydrological conditions. The particulate 7Be/ 210Pb xs AR varies along the fluvial estuarine system mainly due to variations in 7Be activities, controlled by riverine, oceanic and atmospheric inputs and by resuspension of old 7Be-deficient sediments. These processes vary with river discharge, tidal cycle and season. Therefore, seasonal particle transport processes can be described using variations of the SPM 7Be/ 210Pb xs ARs. During high river discharge, the SPM 7Be/ 210Pb x ARs decrease from river to the ocean. The turbidity maximum zone (TMZ) is dispersed and the particles, and the associated contaminants, are rapidly transported from river to coastal waters, without significant retention within the TMZ. During low river discharge, the TMZ intrudes into the fluvial estuary, and the lowest 7Be/ 210Pb x ARs are observed there due to resuspension of 7Be-deficient sediments. Away from the TMZ, from the middle to lower estuary, SPM 7Be/ 210Pb x ARs increase, indicating that the particles have been recently tagged with 7Be. We explain this trend as being caused by marine input of dissolved radionuclides, as traced by SPM 234Th/ 210Pb xs ARs, followed by scavenging in the estuary. This result indicates that particle transport models based on 7Be and trace-metal budgets must consider oceanic dissolved inputs as an additional source of 7Be and, possibly, of contaminants to estuaries.

The Rhine delta—a record of sediment trapping over time scales from millennia to decades

PurposeAt the land–ocean interface, large river deltas are major sinks of sediments and associated matter. Over the past decennia, many studies have been conducted on the palaeogeographic development of the Rhine delta and overbank deposition on the Rhine floodplains. This paper aims to synthesise these research results with special focus on the amounts and changes of overbank fines trapped in the Rhine delta and their controls at different time scales in the past, present and future.Materials and methodsSediment trapping in the Rhine delta throughout the Holocene was quantified using a detailed database of the Holocene delta architecture. Additional historic data allowed the reconstruction of the development of the river’s floodplain during the period of direct human modification of the river. Using heavy metals as tracers, overbank deposition rates over the past century were determined. Measurements of overbank deposition and channel bed sediment transport in recent years, together with modelling studies of sediment transport and deposition have provided detailed insight in the present-day sediment deposition on the floodplains, as well as their controls.Results and discussionEstimated annual suspended sediment deposition rates were about 1.4 × 109 kg year–1 between 6,000 and 3,000 years BP and increased to about 2.1 × 109 kg year–1 between 3,000 and 1,000 years BP. After the rivers were embanked by artificial levees between 1100 and 1300 AD, the amount of sediment trapped in the floodplains reduced to about 1.16 × 109 kg year–1. However, when accounting for re-entrainment of previously deposited sediment, the actual sediment trapping of the embanked floodplains was about 1.86 × 109 kg year–1. Downstream of the lower Waal branch an inland delta developed that trapped another 0.4 × 109 kg year–1 of overbank fines. Since the width of channel was artificially reduced and the banks were fixed by a regular array of groynes around 1850, the average rates of deposition on the embanked floodplains have been 1.15 × 109 kg year–1. Scenario studies show that the future sediment trapping in the lower Rhine floodplains might double.ConclusionsThe variations in amounts of sediment trapped in the Rhine delta during the past 6,000 years are largely attributed to changes in land use in the upstream basin. At present, the sediment trapping efficiency of the floodplains is low and heavily influenced by river regulation and engineering works. Upstream changes in climate and land use, and direct measures for flood reduction in the lower floodplains, may again change the amounts of sediments trapped by the lower floodplains in the forthcoming decades.