Temperate Coastal Wetlands Research Articles

Long-term trends of salinity in coastal wetlands: Effects of climate, extreme weather events, and sea water level

Coastal freshwater ecosystems play major roles as reservoirs of biodiversity and provide many ecosystem services and protection from extreme weather events. While they are of particular importance worldwide, they are affected by a large variety of anthropogenic threats, among which salinization has been less studied, particularly regarding large temporal and spatial data sets based on real case scenarios, while salinity can impact biodiversity and ecosystem functioning. In this study, we investigated the variations of salinity across long-term (1996–2020) and seasonal (monthly records) temporal scales and spatial (varying distance to the coastline) scales in water bodies of two typical temperate coastal wetlands situated on the Atlantic coast of France. We complemented our analyses with models of sea water levels computed at both sites across 2000–2020. Our detailed data set allowed for highlighting that salinity in ponds varied seasonally (higher during summer, due to decreased precipitation and higher temperature), but also spatially (higher closer to the seashore, which pattern increased through time). Over the long term, decreased precipitation but not increased temperature induced increasing salinity. We also highlighted contrasted long-term patterns of salinity changes on these two coastal wetlands, with one site were salinity decreased over time linked to the responses to marine flood, allowing to document the temporal dynamics of salinity following a massive intrusion of sea water. Complementarily, at both sites, water levels at high tides increased through time, a pattern which can induce additional salinization. To our knowledge, our study is the first to investigate long-term changes in salinity in coastal wetlands through natural processes (e.g. seaspray, seasonal variations) and ongoing climate perturbations (e.g. marine surges linked to extreme weather events, increased temperature and decreased precipitations).

An dataset of carbon and water fluxes over the reed wetlands in Panjin City (2018–2020)

With significant carbon sink capacity, active water vapor exchange and heat regulation capacity, wetlands are key ecosystems for stabilizing greenhouse gas emissions and mitigating climate change. Despite Liaohe Delta wetland being the largest warm temperate coastal wetland in Asia, its long-term carbon and water fluxes of the wetland are still not thoroughly examined, which limits the accuracy of simulating CO2 fluxes and regional carbon sinks. As a result, it is urgent to carry out long-term data monitoring and sorting in this area. This dataset is the flux data of the reed wetland ecosystem in Liaohe Delta from 2018 to 2020, collected from the Panjin Reed Wetland Research Station of the Northeast Ecological and Agrometeorological Field Experimental Base of China Meteorological Administration. Based on the data processing system of China Flux Observation and Research Network (ChinaFLUX), we established the standardized dataset of ecosystem carbon and key meteorological elements, including data files at half-hourly, daily, monthly and yearly scales. This dataset is of great significance in accurately evaluating the status and roles of carbon and water fluxes in the reed wetland ecosystem of Liaohe Delta in the regional and global carbon and water cycles.

Quantifying blue carbon and nitrogen stocks in surface soils of temperate coastal wetlands

Coastal wetlands are carbon and nutrient sinks that capture large amounts of atmospheric CO2 and runoff of nutrients. ‘Blue carbon’ refers to carbon stored within resident vegetation (e.g. mangroves, tidal marshes and seagrasses) and soil of coastal wetlands. This study aimed to quantify the impact of vegetation type on soil carbon stocks (organic and inorganic) and nitrogen in the surface soils (0–10 cm) of mangroves and tidal marsh habitats within nine temperate coastal blue carbon wetlands in South Australia. Results showed differences in surface soil organic carbon stocks (18.4 Mg OC ha–1 for mangroves; 17.6 Mg OC ha–1 for tidal marshes), inorganic carbon (31.9 Mg IC ha–1 for mangroves; 35.1 Mg IC ha–1 for tidal marshes), and total nitrogen (1.8 Mg TN ha–1 for both) were not consistently driven by vegetation type. However, mangrove soils at two sites (Clinton and Port Augusta) and tidal marsh soils at one site (Torrens Island) had larger soil organic carbon (SOC) stocks. These results highlighted site-specific differences in blue carbon stocks between the vegetation types and spatial variability within sites. Further, differences in spatial distribution of SOC within sites corresponded with variations in soil bulk density (BD). Results highlighted a link between SOC and BD in blue carbon soils. Understanding the drivers of carbon and nitrogen storage across different blue carbon environments and capturing its spatial variability will help improve predictions of the contribution these ecosystems to climate change mitigation.

Effect of inundation on greenhouse gas emissions from temperate coastal wetland soils with different vegetation types in southern Australia

Predicted sea level fluctuations and sea level rise with climate change will lead to inundation of coastal and estuarine soils. Coastal wetlands usually contain large amounts of organic matter, which can be potential sources of greenhouse gas emissions (GHGs; CO2, CH4, N2O) during decomposition, but there are limited studies on the effects of sea level variation on GHGs in coastal wetlands. We measured the effect of brackish water inundation and wetting and drying cycles on GHG emissions from coastal wetland soil cores that supported four different vegetation types: Apium gravedens (AG), Leptospermum lanigerum (LL), Phragmites australis (PA) and Paspalum distichum (PD) from the estuarine floodplain of the Aire River in south-western Victoria, Australia. Intact soil cores were incubated under either dry, flooded, or a 14 day wet-dry cycle treatments for a total of 56 days at a constant temperature of 23 °C. CO2, CH4, and N2O fluxes were investigated in closed chambers and measured with gas chromatography. In the dry treatment, a positive correlation was found between soil organic carbon (SOC) and CO2 flux, and between SOC and CH4 flux. Higher SOC is indicative of higher amounts of soil organic matter (SOM) which acts as a source of substrate for microbes to produce CO2 or CH4 emissions under aerobic or anaerobic conditions. The NO2− and NO3− concentrations were positively correlated with N2O emissions in the wet-dry cycle treatment. NO2− and NO3− provide a supply of substrate for denitrification. The flooded treatment decreased cumulative CO2 emissions by 34%, 25% and 14% at the LL, PA, PD sites, respectively, and decreased cumulative N2O emissions by 42%, 39% and 43% at the AG, LL and PA sites, compared to the dry treatment. The wet-dry cycle treatment and dry treatment decreased cumulative CH4 emissions for all vegetation types compared to the flooded treatment. The redox potential (Eh) was negatively correlated with CH4 flux and positively correlated N2O flux at all sites. This study highlights the significance of sea level fluctuations when estimating GHG flux from coastal and estuarine floodplains which are highly vulnerable to inundation, and the role of SOC and mineral N as important drivers affecting GHG flux.

Agroforestry reclamations decreased the CO2 budget of a coastal wetland in the Yangtze estuary

How does the CO2 budget in coastal wetlands respond to dynamic and composite agroforestry reclamations is unclear. For six consecutive years, CO2 flux data were measured using the eddy covariance (EC) technique to determine the CO2 budget changes and the key factors controlling the variations of net CO2 ecosystem exchange (NEE) over a reclaimed coastal wetland in the Yangtze estuary, China. After agroforestry conversions in spring 2013, the annual NEE decreased substantially from -655.7 (day 109~365 in 2011) and -558.4 (2012) to 258.1 (2013) and -198.1(2014) and persisted on decreasing to -160.8 (2015) and -141.2 (2016) g C m−2 yr−1. |ER/GPP| ratio persistently increased from 0.40 in 2011 to 0.89 in 2016, suggesting a decoupling between ER and GPP in the coastal reclaimed site. The decrease of GPP was possibly attributed more to the decreased wetland vegetation productivity. The increased ER was due to the emergence of young forest (YF) and aquaculture pond (AP) with high net CO2 emissions in the flux footprint. A homogeneity-of-slopes (HOS) analysis showed that the interannual variability (IAV) of NEE was attributed more to the functional changes (19.0 %), rather than the interannual meteorological variations (9.2 %). The functional changes were also verified by the significant decreased Amax and the slightly increased ∂, both of which might be caused by the changed dominant wetland plant species and their growth pattern. The fluctuated Q10 and R10 during the agroforestry reclamations reflected the seasonal changes in the compositions and respiration characteristics of different land-use types within the CO2 flux footprint. Findings imply that agroforestry reclamations in temperate coastal wetlands substantially decrease the CO2 fixation service by changing the ecosystem's structure and function. Our results call for more wise land-use policy and management measures in reclaimed coastal areas around the world to pursue global warming mitigation.

In-Situ CO2 Partitioning Measurements in a Phragmites australis Wetland: Understanding Carbon Loss through Ecosystem Respiration

Common reed (Phragmites australis) is dominant vegetation of temperate coastal wetlands in northeast China. To studying the link between ecosystem respiration (Reco) and its influential factors, a multi-year in-situ experiment was carried out in a newly restored wetland during the growing seasons of 2012 to 2014. Total in-situ Reco was separated into soil microbial and belowground root respiration (Rs + r) and plant respiration (Rplant). The soil microbial respiration rate (Rs) was isolated from Rs + r, making it easier to understand each component of Reco. With the wetland restoration process, the seasonal average aboveground biomass (dry mass) increased from 411.5 g m−2 to 2048.1 g m−2 and the corresponding Reco increased from 751.78 mg CO2 m −2 h−1 to 2612.41 mg CO2 m −2 h−1. Rplant contributed averagely 69% ~ 71% to Reco on the whole seasonal scale and the plant activity was strongly seasonal. With 1 g of aboveground common reed biomass (dry weight), approximately 3.6 mg CO2 would be produced per hour during the sprouting period while it could be as low as 0.3 mg CO2 during plant senescence period. Inundation regime dominated the contribution of Rs to Reco and the flooded contribution would lower the Rs contribution to as low as 11%.

Processing of dissolved organic matter from surface waters to sediment pore waters in a temperate coastal wetland

Coastal wetlands are active transitional ecotones between land and ocean, and are considered as hot spots of organic matter processing within the global carbon cycle, which dissolved organic matter (DOM) plays a critical role. In this study, combined use of ultrahigh-resolution mass spectrometry (FT-ICR MS) and complementary optical techniques was conducted to assess the detailed molecular composition of DOM in the temperate Liaohe coastal wetland (LCW), NE China in respect to the differences in DOM composition from surface water to sediment pore water. Significant positive correlations between salinity and dissolved organic carbon (DOC) concentrations were observed in both surface waters and pore waters. Pore water DOM is generally characterized by lower protein-like fluorescence and biological index, but higher humification and humic-like fluorescent components than those in surface water DOM. Corresponding to the optical properties, FT-ICR MS measurements show that pore water DOM has higher proportions of heteroatoms, aromaticity index, O/C ratios, unsaturated aliphatics, and peptides, but lower average H/C ratios compared to surface water DOM across locations with different marsh plant species (rice (Oryza sativa), reed (Phragmites australis), Seablite (Suaeda Salsa)) and salinity (0.5 to 51.5 psu). The results suggest that selective preservation for polyphenols, lignin degradation intermediates (highly unsaturated compounds), and microbial resynthesis of heteroatomic compounds are involved in the processing of DOM from surface water to pore water, leading to the formation of higher molecular weight and sulfur-containing molecules. The abundant CHOS compounds could be related to the early diagenetic sulfurization of DOM in sediments. Our unique data set should provide new clues for a comprehensive understanding of the molecular dynamics of DOM in coastal wetlands.

Temperate coastal wetland near-surface carbon storage: Spatial patterns and variability

Carbon mitigation services provided by coastal wetlands are not spatially homogeneous, nevertheless are commonly described on the basis of vegetation distribution within the intertidal zone. Distribution of mangrove and saltmarsh varies in response to frequency of tidal inundation, resulting in environmental gradients in edaphic factors that influence vegetation structure, and subsequently affect sedimentary carbon additions by vegetation and carbon losses by decomposition. Current sampling approaches and reporting do not adequately account for variability of carbon storage within a wetland, and assessments need to capture spatial variation associated with carbon storage to improve estimates of potential carbon mitigation services by natural ecosystems. This study quantifies the variation in near-surface carbon storage (i.e. upper 30 cm) across an intertidal gradient using a stratified sampling approach that recognises vegetation structure. Vegetation distribution and structure, as well as sedimentary controls on carbon content, explained variation in carbon storage. Saltmarsh near-surface carbon storage varied considerably between structural form. This was less evident for mangrove structural forms (i.e. tall, shrub, dwarf), which may be due to mangrove roots extending to depths beyond 30 cm. Sedimentary characteristics correlated with carbon content, demonstrating considerable influence on near-surface carbon storage within a wetland. The principal finding of this study was that variation within a wetland corresponds to the variation between sites. Stable carbon isotopes offer a means to identify previous vegetation contributions to sediment, associated with an earlier stage of wetland development, likely reflecting previous environmental conditions. A stratified sampling approach that recognises vegetation structure provides the capacity to account for variability of carbon within a wetland that is inadequately described by current sampling protocols.

Characteristics of Vegetation Carbon, Nitrogen, and C/N Ratio in a Tamarix chinensis Coastal Wetland of China

AbstractVariations in carbon (C) and nitrogen (N) of the vegetation in a Tamarix chinensis coastal wetland located in Laizhou Bay, China, are analyzed. It is found that T. chinensis accumulates more C and N than the surrounding herbs, and it allocates more C and N in the aboveground parts but less in the roots. Branches store more C, whereas leaves and flowers accumulate more N than other tissues. The C and N contents in the aboveground parts of T. chinensis are mainly influenced by pH, electrical conductivity (EC), water content, and clay content in the top 100 cm of soil as well as the distance from the sea. For the herbs, their C contents vary little in the supratidal zone, but they are higher than those in the intertidal zone. However, N contents in the herbs are higher in the aboveground parts and varied among different communities. The contents of C and N in the herbs are affected by EC, water content as well as the contents of organic C and total nitrogen (TN) in the top 10 cm of soil. The findings confirm that increasing the biomass of T. chinensis is an effective way to increase C sequestration in temperate coastal wetland.

A 3-Year In-Situ Measurement of CO2 Efflux in Coastal Wetlands: Understanding Carbon Loss through Ecosystem Respiration and its Partitioning

Understanding the link between ecosystem respiration (Reco) and its influential factors is necessary to evaluate the sources of gaseous carbon loss in coastal wetlands. Seablite (Suaeda salsa Pall.) is the main vegetation type pioneering temperate coastal wetlands in northeast China, and is generally an understudied wetland type. To evaluate the influence of environmental factors on Reco, a multi-year in-situ experiment was carried out during the growing seasons of 2012 to 2014. Total CO2 efflux was measured and separated further into soil microbial and belowground root respiration (Rs + r) and plant respiration (Rplant). Reco displayed strong seasonal variation, with effluxes as high as 845 to 1150 mg CO2 m−2 h−1 during summer months and as low as 32 to 111 mg CO2 m−2 h−1 during spring (when new shoots are sprouting) and fall (when plants are senescing) months. Aboveground plant structures contributed on average 79% to total plant biomass, and accounted for most of the Reco measured; i.e., 62–96% was associated as Rplant. Plant activity was strongly seasonal, accordingly driving Reco, with 1 g of soil-emergent S. salsa biomass (dry weight) producing approximately 1.58 mg CO2 per hour toward Reco during mid-summer. When water level was below the soil surface, Rs + r was exponentially correlated to air temperature. Because Reco for S. salsa marsh in the Liaohe Delta is controlled by plant growth cycles, inundation regime, and air temperature, this finding may be applied for national carbon budget estimation purposes from S. salsa wetlands throughout Northeast China and potentially close a key gap in understanding the role of this large wetland area in contributing to respiratory CO2 emissions globally.

Vegetation patterns in a South American coastal wetland using high-resolution imagery

ABSTRACT The aim of this study was to identify and characterize the main plant communities in a temperate coastal wetland using high-resolution imagery. We produced a map of Samborombón Bay at 1:25,000 scale using a WorldView-2 image. An Object-based Image Analysis approach was chosen, and an unsupervised classification algorithm was applied. Overall classification accuracy was 81%, and the Kappa index was 78.1%. Six land cover types were mapped including four main natural monospecific plant communities. The lower intertidal area was dominated by mudflats without vegetation and stands of Bolboschoenus maritimus. The middle intertidal area was dominated by Sarcocornia ambigua, while, in the higher intertidal area, Sporobolus densiflorus and grasslands with Cortaderia selloana prevailed. We found four spatial patterns at a landscape scale, based on the presence and spatial distribution of the natural plant communities. This map represents a valuable tool for future studies on wetland environmental indicators.

Spatial variation of above-ground carbon storage in temperate coastal wetlands

Carbon mitigation services provided by coastal wetlands are not spatially homogeneous. The scale of assessment at which above-ground biomass is measured will directly influence carbon storage estimates. Greater confidence in estimates is obtained with approaches that describe more variation. There is a need to improve accuracy while optimising assessment effort efficiency. Accurate quantification of carbon storage is dependent upon accurate assessment of biomass, carbon content and the extent of vegetation for which carbon storage is being assessed. This study demonstrates that vegetation structure influences above-ground biomass of mangrove and saltmarsh, resulting in considerable variability in biomass estimates and associated carbon storage of temperate coastal wetlands in southeast Australia. For mangrove, variability in above-ground biomass (Mg ha−1 ± SE) was best described by measuring height, stem diameter, crown area and vegetation density, whereby tall mangrove (3–8 m in height; 71.50 ± 12.53 Mg ha−1) had higher biomass than both shrub (1.3–3 m in height; 53.06 ± 6.94 Mg ha−1) and dwarf mangrove (<1.3 m in height; 10.68 ± 1.77 Mg ha−1). Saltmarsh above-ground biomass was best described by height, species and vegetation density, which demonstrated significant differences between rush saltmarsh (15.97 ± 2.35 Mg ha−1) and herbs, grasses and sedges saltmarsh (7.51 ± 0.91 Mg ha−1). The effect of this variation was compounded by carbon content (% C), which varied markedly between vegetation structural form and species (30.9–49.8% C). Maintaining accuracy when assessing carbon storage requires mapping units that correspond to the scale of biomass assessments. Results from this study suggest that recognition of variation in biomass and carbon content of mangrove and saltmarsh vegetation structure will enhance the accuracy of estimates of carbon storage, and provide the confidence necessary for carbon storage inventories.

Opening the floodgates to the recovery of nektonic assemblages in a temperate coastal wetland

Tidally active wetlands are important nurseries for fish and crustaceans; however, urban development involving structural flood mitigation has fragmented and destroyed much of this habitat, often leading to losses in fisheries productivity. The present study demonstrated that some of these impacts may be reversible if rehabilitation projects are implemented over a sufficiently long time. The incremental opening of eight floodgates at a temperate coastal wetland in Australia afforded the opportunity to monitor water quality and nekton assemblages in a tidal creek over 11 years, encompassing pre-floodgate opening, opening of one gate, three gates and finally all eight gates. Floodgate opening enhanced both water quality and nektonic assemblages relative to a control creek whose barrier remained in place. A doubling in species richness occurred and many commercially important species increased in abundance, including Metapenaeus macleayi (15 times more), Acanthopagrus australis (62 times more), Liza argentea (10 times more) and Gerres subfasciatus (19 times more). Recovery to a condition similar to unrestricted reference creeks was achieved once all eight gates were opened. The study illustrated that reinstating tidal flushing can recover nursery habitats and enhance populations of economically valuable species.

Effects of Tamarisk shrub on physicochemical properties of soil in coastal wetland of the Bohai Sea

There are many different and even controversial results concerning the effects of Tamarisk on the physicochemical properties of soil. A year-round monitoring of soil salinity, pH and moisture is conducted beneath the Tamarisk shrub in a coastal wetland in the Bohai Sea in China, to ascertain the effects of Tamarisk on the physicochemical properties of soil in coastal wetland. Compared with the control area, the soil moisture content is lower around the area of the taproot when there is less precipitation in the growing season because of water consumption by Tamarisk shrub. However, the soil moisture content is higher around the taproot when there is more precipitation in the growing season or in the non-growing period because of water conservation by the rhizosphere. The absorption of salt by the Tamarisk shrub reduces the soil salinity temporarily, but eventually salt returns to the soil by the leaching of salt on leaves by rainfall or by fallen leaves. The annual average soil moisture content beneath the Tamarisk shrub is lower than the control area by only 6.4%, indicating that the Tamarisk shrub has little effect on drought or water conservation in soils in the temperate coastal wetland with moderate annual precipitation. The annual average salinity beneath the Tamarisk shrub is 18% greater than that of the control area, indicating that Tamarisk does have an effect of rising soil salinity around Tamarisk shrubs. The soil pH value is as low as 7.3 in summer and as high as 10.2 in winter. The pH of soil near the taproot of the Tamarisk shrubs is one pH unit lower than that in the control area during the growing season. The difference in pH is less different from the control area in the non-growing season, indicating that the Tamarisk shrub does have the effect of reducing the alkalinity of soil in coastal wetland.

Trophic Consistency of Benthic Invertebrates Among Diversified Vegetational Habitats in a Temperate Coastal Wetland of Korea as Determined by Stable Isotopes

The relative importance of different organic matter sources to macrobenthic consumers was investigated at two vegetational habitats (Phragmites australis and Suaeda japonica) and at a bare intertidal flat in Suncheon Bay in May and August 2010, using stable carbon and nitrogen isotope tracers. The δ13C of consumers was much narrower in range than that of potential food sources (microphytobenthos, marsh plants, and riverine and marine particulate organic matter), and values were similar between sites. However, cluster analysis based on the consumer’s isotope values showed that they are divided according to functional groups rather than habitat type and taxonomical group, suggesting that the exploitation of different dietary sources depends on feeding strategy. Except for two suspension feeders and nonselective deposit feeders at the Phragmites bed, most consumers had similar δ13C to those of microphytobenthos. An isotopic mixing model calculation revealed the consistent importance of microphytobenthos to the nutrition of benthic invertebrates among wetland habitats in different seasons. In contrast, very little contribution of marsh plant-derived organic matter to consumer nutrition was detected at all sites. Furthermore, our results suggest that the summer monsoon event does not lead to a seasonal shift in dietary sources of consumers in this coastal wetland system.

Succession of fish and crustacean assemblages following reinstatement of tidal flow in a temperate coastal wetland

Fish and decapod crustacean assemblages were sampled from two manipulated creeks in which tidal flow had been increased through culvert removal. Assemblages were compared to those of two control creeks where culverts remained and two reference creeks without culverts. The presence of culverts reduced the richness and abundance of estuarine–marine dwelling species. Successional changes over a 16 year period (2 years prior to culvert removal and 14 years after) showed an immediate response at one manipulated creek but many years to attain reference condition. The other manipulated creek showed a completely different trajectory: oscillation between control and reference status with no clear intermediary condition. We concur with other investigators that short-term studies may give only a partial indication of response to rehabilitation efforts in coastal wetlands and longer-term studies (5–10 years) are recommended. There is some evidence that over longer-time frames, the presence of distinct groups of fish and/or decapods may indicate stages in the maturation of a rehabilitated wetland.

Coastal wetlands of temperate eastern Australia: will Cinderella ever go to the ball?

The many sheltered embayments, riverine estuaries and drowned river valleys of temperate eastern Australia support a large area and a wide diversity of coastal wetlands. This region also supports over one-half of the country’s population and includes its major tourist and industrial centres. The story of Cinderella provides an excellent simile for the status of coastal wetlands in this region of Australia: coastal wetlands provide extremely valuable ecosystem services (indeed, of considerably greater value than those provided by an equivalent area of inland wetland), yet they are undervalued by the general community, by natural-resource managers and by funding agencies. The selective investment of ecological research (including rehabilitation studies) into inland wetlands – and in particular into floodplain wetlands of the Murray–Darling Basin – has left us inadequately equipped to understand how temperate coastal wetlands function, how they will respond to climate change, and how degraded sites can be rehabilitated. Not only is an increase in research funding required, but funding has to be coordinated and focussed. The problem, however, is not only a lack of research funding; at a more fundamental level, the widespread ignorance of the value of temperate coastal wetlands needs also to be addressed.

Energetic Bottlenecks and Other Design Constraints in Avian Annual Cycles

The flexible phenotypes of birds and mammals often appear to represent adjustments to alleviate some energetic bottleneck or another. By increasing the size of the organs involved in digestion and assimilation of nutrients (gut and liver), an individual bird can increase its ability to process nutrients, for example to quickly store fuel for onward flight. Similarly, an increase in the exercise organs (pectoral muscles and heart) enables a bird to increase its metabolic power for sustained flight or for thermoregulation. Reflecting the stationary cost of organ maintenance, changes in the size of any part of the "metabolic machinery" will be reflected in Basal Metabolic Rate (BMR) unless changes in metabolic intensity also occur. Energetic bottlenecks appear to be set by the marginal value of organ size increases relative to particular peak requirements (including safety factors). These points are elaborated using the studies on long-distance migrating shorebirds, especially red knots Calidris canutus. Red knots encounter energy expenditure levels similar to experimentally determined ceiling levels of ca. 5 times BMR in other birds and mammals, both during the breeding season on High Arctic tundra (probably mainly a function of costs of thermoregulation) and during winter in temperate coastal wetlands (a function of the high costs of processing mollusks, prey poor in nutrients but rich in shell material and salt water). During migration, red knots phenotypically alternate between a "fueling [life-cycle] stage" and a "flight stage." Fueling red knots in tropical areas may encounter heat load problems whilst still on the ground, but high flight altitudes during migratory flights seem to take care of overheating and unacceptably high rates of evaporative water loss. The allocation principles for the flexible phenotypes of red knots and other birds, the costs of their organ flexibility and the ways in which they "organize" all the fast phenotypic changes, are yet to be discovered.