The intertidal distribution of the grey mangrove (Avicennia marina) in southeastern Australia: The effects of physical conditions,interspecific competition,and predation on propagule establishment and survival

Abstract The upper and lower limits of the distribution of mature Avicennia marina lie between mean high water and mean sea level in open estuaries in southeastern Australia. Newly established seedlings are highly variable in abundance, but are rarely found in the saltmarsh or on mudflats. Their distribution is unlikely to be limited by dispersal because propagules disperse into the saltmarsh and to intertidal mudflats, but their establishment may be limited by physicochemical conditions, interspecific competition and predation. The model that physicochemical conditions control the intertidal limits of establishment of seedlings was accepted for propagules stranding in the saltmarsh but rejected for those stranding on mudflats. No seedlings established on saltmarsh sediments but similar numbers of seedlings established within light gaps in adult mangrove stands and on intertidal mudflats. The model that interspecific interaction with freeliving macroalgae (Hormosira banksii) reduces the establishment of seedlings on mudflats covered with macroalgae or in stands with a ground cover of macroalgae was accepted. Under controlled conditions five times as many propagules established on cleared ground compared with ground covered with macroalgae. Predators also reduce seedling establishment, but the model that they preferentially act on propagules stranding on the mudflat was rejected. The low number of seedlings found on mudflats without macroalgae appears to relate to wave and current effects on establishment and the effects of waterlogging or fouling on survival.     

Suspended sediments and nutrients in water ebbing from seagrass-covered and denuded tidal mudflats in a southern Australian embayment

The concentrations of suspended solids, nitrogen (NH₄⁺, NO₃⁻, NO₂⁻), phosphorus (total phosphorus, PO₄⁻³) and silicate were measured in water ebbing from seagrass-covered tidal mudflats and from adjacent ungrassed mudflats in Western Port, Victoria, Australia. Ebb water was collected on six occasions during January and February (summer) and once during July (winter). From both types of mudflats the concentrations of suspended solids, phosphorus and silicate increased during the last hour of ebb tide with maxima in concentrations reached 0.5 to 1 h before slack water ebb. During the last 1.5–2.5 h of ebb tide the suspended solids, phosphorus and silicate concentrations were higher in water ebbing from denuded mudflats than from seagrass-covered mudflats. The concentration of nitrogen remained low in water ebbing from both types of mudflats during the day, but increased significantly during a night-time ebb; light-dependent biological uptake of nitrogen is implicated. It is suggested that denudation of seagrass-covered tidal mudflats would lead to increased efflux of suspended solids, phosphorus and silicate from sediments to overlying water.     

Foraging of a Small Planktivore (<Emphasis Type="Italic">Pseudorasbora parva</Emphasis>: Cyprinidae) and its Behavioral Flexibility in an Artificial Stream

The loss of feeding areas may pose a threat to many wintering waders because increased competition arising from reduced foraging space may force birds either to emigrate or to die. This has been demonstrated to occur in northwest European estuaries, but virtually no studies have been performed in the estuaries of southern Europe, where the loss of supratidal habitats (salines and saltmarshes), rather than intertidal habitats, are currently the main threat to waders’ habitats. If these habitats are lost, waders may be forced to move to the intertidal mudflats, perhaps increasing competition between individuals and ultimately leading to starvation or emigration. We tested this hypothesis in the Mondego estuary, a small estuary on Portugal’s west coast, which is presently under heavy human pressure. We used indirect methods to test for the occurrence of both components of intra-specific competition: interference and prey depletion. We found no evidence that either interference or depletion competition was occurring at present, either on the mudflats or in the salines. Overall, the results suggest that the intertidal mudflats may still be able to accommodate birds displaced from the destroyed supratidal salines, but modelling is required to predict the effect that the combined loss of feeding area and foraging time that this would entail would have on their fitness, and thus numbers.     

Restoration of the Sieperda Tidal Marsh in the Scheldt Estuary, The Netherlands

Because of land reclamation, reinforcement of dikes, and the deepening of shipping channels, large areas of tidal marshes have been removed or eroded from the Scheldt estuary during the last two centuries. Tidal wetland restoration contributes toward compensating this loss of habitat. Not all restoration projects are meticulously planned, however; some are forced by nature. During a severe storm in 1990, a dike was breached in the brackish part of the Scheldt estuary and returned tidal influence to the Sieperda polder. In the 10 years since the dike breach, the former polder has changed into a brackish tidal marsh. Here we report on the geomorphologic and ecological developments that have taken place in the marsh. Tidal intrusion into the former polder turned crop fields into mudflats and changed pastures into salty marsh vegetation. The digging of a new creek improved marsh hydrology and enhanced tidal intrusion further into the marsh. Macrofauna typical of estuarine mudflats established rapidly in the developing marsh. Vegetation succession took place rapidly. Within 5 years, large areas of mudflats became covered with marsh vegetation. Birds characteristic of salt marshes were observed breeding or seen foraging in the marsh. The number of wading birds declined as areas of mudflat became overgrown. It is demonstrated that tidal flow is the engine to tidal marsh restoration. Tidal influence caused geomorphologic changes, which directed ecological developments in the former polder.     

The role of saltwort (Batis maritima L.) in regeneration of degraded mangrove forests

While saltwort (Batis maritima L.) is common in the fringe mangrove forests of southwest Florida, its role in regeneration of degraded mangrove communities is not known. Given the potential encroachment and subsequent degradation of mangrove communities by sea-level rise, it is important to quantify the effect of early-colonizing vegetation to early mangrove seedling survival. A greater number of mangrove seedlings were observed in existing B. maritima patches compared to surrounding mudflats. A planting experiment was designed to determine whether B. maritima was responsible for the observed pattern. Black mangrove (Avicennia germinans L.) seedlings, raised in a nursery, were planted in previously established B. maritima patches and on mudflats with and without nursery-raised B. maritima. There was significantly lower mortality of A. germinans seedlings when planted in existing B. maritima patches (69%), compared to seedlings planted on the mudflats (93%), demonstrating that existing B. maritima improved A. germinans seedling survival. Nursery-raised B. maritima had lower mortality on open mudflats (28%), suggesting that it can tolerate conditions, which make it an early colonizer of newly available habitats. The primary mechanism proposed for improving seedling success is a slight increase in elevation provided by the dense root network of established B. maritima. These findings have implications for scientists and managers anticipating the response of mangroves to sea-level rise.     

Intense episodic predation by shorebirds may influence life history strategy of an intertidal amphipod

Life history theory suggests that selective predation on older or larger individuals in prey populations or a disturbance such as intense and episodic predation should lead to smaller size at maturity, and a tendency toward semelparity. Many populations of the intertidal amphipod, Corophium volutator , in the western Atlantic are subjected to an intense period of size-selective predation for about one month in summer, during the southward migration of shorebirds. We compared size at maturity and fecundity of populations of C. volutator from mudflats that are intensively used by shorebirds with populations that are visited by very few birds. We found that mature females were of similar size in May, but those from bird mudflats produced more offspring during the first reproductive episode. In July, females of the summer generation began to reproduce at a smaller size on bird mudflats, and as a consequence, produced fewer offspring that grew more slowly. The results of this correlative study suggest that shorebird predation has shaped C. volutator life history in two ways. First, females on bird mudflats concentrate their reproductive effort into a larger early brood, probably because later broods would come to maturity during the period of intense predation. Second, in summer, amphipods begin to reproduce at a smaller size so as to produce a brood before the arrival of the birds in mid-July. Predators are not present at the time that these responses occur, and thus they are probably not a plastic response to perceived predation risk. Thus the patterns we observed agree with the predictions of recent theories: individuals faced with a predictable disturbance alter their life cycles so as to minimize the effect of that disturbance.     

Roles of environmental cues for embryonic incubation and hatching in mudskippers

Reproduction on mudflats requires that eggs are protected from different environmental challenges during development and hatch when environmental conditions are favorable for survival of juveniles. Mudskippers are air-breathing, amphibious gobies of the subfamily Oxudercinae, and one of a few vertebrates that reside on mudflats. They excavate burrows in mudflats and deposit eggs in them. However, these burrows are filled with extremely hypoxic water, in which eggs could not survive. To secure embryonic development within their burrows, the burrow-guarding parental fish (a male or mating pair) store fresh air in an egg chamber, located near the bottom or at mid-depth in a burrow, by transporting mouthfuls of air during each low tide. The Japanese mudskipper, Periophthalmus modestus, is the best-studied species regarding reproductive strategies. The air-supplying behavior appears to be predominantly governed by the oxygen levels within egg chambers, but also by some other factor that is possibly related to the tidal cycle. When embryonic development is complete, the burrow-guarding male P. modestus removes the air from the egg chamber and releases the air outside the burrow on a nocturnal rising tide. Consequently, the tide floods the egg chamber and induces hatching. Because P. modestus eggs only have a 5-6 day window for hatching competence, the male's initial selection of the position for the burrow in the intertidal zone and the timing of spawning relative to the tidal cycle are both important factors in hatching success. This is particularly crucial for those burrows in higher intertidal zones, which may be reached only by spring high tides. Not much is known for other mudskippers, but it is likely that they also employ similar reproductive strategies. The objective of this review is to summarize available information on reproductive strategies of mudskippers, and to discuss future directions to better elucidate mechanisms and adaptive significance for the reproduction of mudskippers. Further comparative studies with both mudskippers and other oxudercine gobies dwelling mudflats could shed new light on how vertebrates solved problems of reproduction when they expanded habitats to environments in an air-water interface.     

The surficial sediments of Königshafen-variations over the past 50 years

The investigation highlights the untypical nature of the intertidal sediments of the sheltered Königshafen embayment in the north of the island of Sylt. The area is characterized by medium sand, which might be explained by aeolian input from the adjacent dunes. Mudflats and sandy mudflats build up the western part of the bay. Other sandy mudflats, all with a small spatial distribution, are associated with mussel beds. A peculiarity of the sediments in the Königshafen embayment are the gravel and cobbles along the northern shore and parts of the southern shore. These can be interpreted as residual sediments from former stages of the “Ellenbogen” spit. A comparison of the present results with earlier studies by Wohlenberg (1937) and Felix (1981) shows a decline in mudflats and an increasing consolidation of the mud in the period up to 1981. Since 1981 the sediment distribution has not changed, hence this tidal embayment can today be qualified as representing a relatively stable depositional area.     

Patterns of vegetation along the Omo River in southwest Ethiopia

This study investigates the riverine vegetation along the perennial Omo River, which flows from the Ethiopian highlands to its terminus, in the southwestern lowlands, at Lake Turkana (formerly Lake Rudolf). Broadly defined to include the levee backslopes and adjacent mudflats (or ancient floodplains), the riverine zone in the lower Omo basin supports a relatively luxuriant vegetation compared with the dry grasslands in the surrounding plains environments. Habitat conditions along the lower Omo have changed significantly during the past century, primarily due to a period of reduced rainfall and river flow, a drop in lake level (L. Turkana) at the river's terminus and increased exposure of natural levees near the Omo's terminus. The floristic and physiognomic character of riverine vegetation were studied at selected sites, ranging from the relatively straight channel section in the modern delta to a strongly meandering (upstream) section of the river. A combination of open canopy woodland, shrub thicket and grassland (with scattered tree emergents) prevailed on the more recently exposed levees, in and near the modern delta; closed canopy woodland and forest predominated on fore levees in the meandering segment. Floristic diversity was relatively low at all study sites. Ficus sycomorus, Tapura fischeri, Melanodiscus oblongus, Celtis integrifolia and Trichilia roka were most significant in upstream forest sites, whereas Cordia sinensis, Acacia mellifera, Ziziphus mauritiana and Ficus sycomorus were more common in communities nearer the lake. Older forests generally contained greater diversity of more woody species, greater tree height and truck diameter than downstream levee vegetation, but they did not exhibit clear stratification. Species patchiness was broadly characteristic of the riverine zone, particularly the mudflats (ancient floodplains) extending away from the fore levees. Fires are frequently set by local inhabitants throughout the grasslands of the lower Omo Basin; within the riverine zone, they are particularly common in the mudflats but have only limited impact on woodland and forest vegetation. It is suggested that fire is a highly significant factor in the establishment of sharp boundaries between the closed canopy woodland/forest along the river and the more xerophytic vegetation of the mudflats.     

Lifestyle of Korean mudskipper <Emphasis Type="Italic">Periophthalmus magnuspinnatus</Emphasis> with reference to a congeneric species <Emphasis Type="Italic">Periophthalmus modestus</Emphasis>

To investigate the life history and ecology of the mudskipper Periophthalmus magnuspinnatus, observations and collection were made on coastal mudflats in southern Korea. Periophthalmus magnuspinnatus was active from May to September on the mudflats, exclusively occupying rough and elevated or sloped mudflats of the seashore or the stream mouth, usually vegetated with halophilous grasses. The congeneric species, P. modestus, mainly occurred on extensive low-elevation and level mudflats with no visible vegetation. An apparent alternation of habitat use by P. modestus took place on mudflats at the stream mouth in mid-October, when P. magnuspinnatus began wintering in its burrow and P. modestus came onto the vacated mudflats to construct burrows for wintering. The active season for P. magnuspinnatus at 17°C or higher air temperature was a little shorter than that of P. modestus. Periophthalmus magnuspinnatus constructed a burrow for the entire season in the highest area of the intertidal mudflat, where they hid themselves during high tide or when frightened, whereas P. modestus were likely to use any burrow constructed by other animals or sunken places to hide. The main stomach contents of P. magnuspinnatus were crabs and gammarids. During the reproductive season from May to July, P. magnuspinnatus performed mating behaviors and constructed a spawning burrow similar to the ones known for P. modestus, except their body color turned dark and quivering body movements were observed in the mature male instead of a pink or orange body color and wiggling body movements as in P. modestus. Eggs, measuring 1.56–1.69 mm in major axis and 0.94–1.0 mm in minor axis, were laid on the ceiling and the side wall of the “J”-shaped spawning room generally known for Periophthalmus species. Young of both species started to occur on the mudflat in June.     

Characterization of Chemical Contamination in Shallow-Water Estuarine Habitats of an Industrialized River. Part 1: Organic Compounds

The lower six miles of the tidal portion of the Passaic River (Study Area) has been heavily industrialized since the mid-1800s. The objectives of this study were to: quantify the present extent and magnitude of chemical contamination in surface sediments from the Study Area; evaluate the contamination in the Study Area relative to a reference area and surrounding regional waterways; assess the potential for adverse effects to aquatic organisms; and identify spatial gradients in concentrations that may indicate potential point-sources of chemicals. Because existing ecological communities center primarily on intertidal mudflats in the Study Area, composite surface sediment was collected from 15 mudflats to generate a realistic assessment of fish and wildlife exposure to sediment contaminants. This study showed that present concentrations of organic contaminants are elevated throughout the Study Area and are generally higher than concentrations in the reference area and regional waterways. A screening-level analysis showed that there is potential for adverse effects to sediment-associated organisms from a number of chemicals. No spatial trends were apparent for most compounds evaluated, consistent with the presence of multiple sources of chemicals in the Study Area. The exception is semivolatile compounds, which appear to be localized to specific mudflats, indicating potential point-sources of contamination for these chemicals.     

福建洛阳江口红树林湿地大型底栖动物多样性及季节变化

基于2013-2014年福建洛阳江口红树林湿地的周年调查资料, 研究了该湿地大型底栖动物的物种多样性现状、季节变化以及红树林恢复对底栖动物群落的影响。本次调查共鉴定大型底栖动物7门78种, 环节动物和节肢动物种类最为丰富, 节肢动物对总生物量贡献最大, 短拟沼螺(Assiminea brevicula)为本区第一优势种。林区底栖动物的密度和生物量明显低于光滩, 部分优势种仅在林区出现, 此外, 林区群落与光滩有显著差异。本区底栖动物的密度高值出现在冬、春季, 生物量高值出现在春、夏季, 群落结构季节变化明显。本区底栖动物种类组成特点反映出洛阳江口红树林处在年轻期, 群落的季节变化可能跟优势种的繁殖特点有关。研究结果也表明红树林植被会影响底栖动物群落结构, 林区群落已然不同于光滩, 与2009年调查结果相比, 林区底栖动物密度虽明显下降, 但褶痕拟相手蟹(Parasesarma plicata)数量增加, 是优势种之一。洛阳江口红树林和光滩支撑着不同的生物组成, 因此, 红树林恢复应当保持栖息地的多样性/异质性。     

Diversity of the dsrAB (dissimilatory sulfite reductase) gene sequences retrieved from two contrasting mudflats of the Seine estuary, France

The diversity of sulfate-reducing microorganisms was investigated in two contrasting mudflats of the Seine estuary, by PCR amplification, cloning and sequencing of the genes coding for parts of the alpha and beta subunits of dissimilatory sulfite reductase (dsrAB). One site is located in the mixing-zone and shows marine characteristics, with high salinity and sulfate concentration, whereas the other site shows freshwater characteristics, with low salinity and sulfate concentration. Diversity and abundance of dsrAB genes differed between the two sites. In the mixing-zone sediments, most of the dsrAB sequences were affiliated to those of marine Gram-negative bacteria belonging to the order of Desulfobacterales, whereas in the freshwater sediments, a majority of dsrAB sequences was related to those of the Gram-positive bacteria belonging to the genus Desulfotomaculum. It is speculated that this is related to the salinity and the sulfate concentration in the two mudflats.     

Context‐specific effects of the identity of detrital mixtures on invertebrate communities

Many aquatic ecosystems are sustained by detrital subsidies of leaf litter derived from exogenous sources. Although numerous studies have examined the effects of litter species richness and identity on decomposition processes, it remains unclear how these effects extend to associated invertebrate communities or how these effects vary spatially according to local environmental context. Using field enrichment experiments, we assessed how the species richness, assemblage composition, and supply of detrital litter resources interact to affect benthic communities of three temperate Australian estuarine mudflats. Our experiments utilized eight litter sources that are presently experiencing human‐mediated changes in their supply to estuarine mudflats. Contrary to predictions, we did not detect effects of the species richness of detrital mixtures on benthic communities. Macroinvertebrate community structure and, in particular, abundance were, instead, influenced by the assemblage composition of detrital mixtures. At two of the three sites, plots receiving the most labile detrital mix, containing the ephemeral algae Chaetomorpha and Ulva, supported the fewest macroinvertebrates of all the experimental enrichments. The large effect of detrital mix identity on macroinvertebrate communities is of concern given present trends of proliferation of macroalgae at the expense of more refractory seagrasses and marsh grasses. As such environmental degradation continues, it will be important to more fully understand under what environmental contexts such compositional changes in detrital resources will have the most detrimental effects on important prey resources for commercially important fish and wading shorebirds.     

Native and Introduced Ecosystem Engineers Produce Contrasting Effects on Estuarine Infaunal Communities

Cordgrasses in the genus Spartina are good examples of ecosystem engineers that modify habitat structure in estuaries throughout the world. In San Francisco Bay, California, USA, marshes containing native California cordgrass (Spartina foliosa) are being invaded by a hybrid (S. alterniflora × S. foliosa) formed after introduction of S. alterniflora. This study compared vegetation, sediment structure, and infaunal invertebrates in native and invaded marshes. We hypothesized that differences in the physical structure between S. foliosa and hybrid Spartina would be reflected in differences in density, biomass, diversity, and taxonomic composition of infauna. Hybrid Spartina modifies habitat structure more than S. foliosa by producing taller stems, and greater plant biomass both above- and belowground while occupying a much wider tidal range, thereby transforming open mudflats to a vegetated habitat. In general, S. foliosa areas contained significantly higher densities of benthic infauna than adjacent mudflats, while hybrid Spartina areas never contained greater infaunal densities than mudflats. This is because S. foliosa produces a moderate level of structure that can facilitate benthic invertebrates, whereas hybrid Spartina produces so much structure, particularly belowground, that it actually excludes invertebrates. Therefore, we suggest that these two closely related species both act as ecosystem engineers, but with opposing effects on invertebrate communities.     

The distribution of Zostera and its exploitation by wildfowl in the solent,Southern England

In the Solent estuarine system on the central south coast of England, Zostera marina L. occurs both in mainly sublittoral beds on firm substrate around the low-water mark of spring tides, and in large eulittoral beds on mudflats between the low-water mark of neap tides and the uppermost level of the shore. Zostera noltii Hornem. occurs with eulittoral Z. marina on uppershore mudflats and also on exposed sandy shores near the low-water mark of spring tides. Since 1960 the eulittoral Zostera beds have spread widely on mudflats in Portsmouth, Langstone and Chichester Harbours, where they now occupy 430–450 ha. The Zostera beds at the bottom of the shore remain winter-green, but the mudflat Zostera beds shed most of their leaves between November and January. Comparisons are made with localities further east, which show earlier and greater leaf loss.The uppershore Zostera beds in the three harbours comprise an important and favoured autumn and early winter food for Dark-bellied Brent Geese, (Branta bernicla bernicla (L.)) and Wigeon (Anas penelope L.), which consume the leaves and rhizomes, and Teal (Anas crecca L.) which consume the seed. The pattern of occurrence of these three species of birds is, to some extent, related to the availability of Zostera.     

Assessing alternative anthropogenic habitats for conserving waterbirds: salinas as buffer areas against the impact of natural habitat loss for shorebirds

Because many natural waterbird habitats are threatened by human disturbance and sea level rise, it is vitally important to identify alternative wetlands that may supplement declining natural habitats. Coastal salinas are anthropogenic habitats used for obtaining salt by evaporation of sea water. These habitats support important numbers of waterbirds around the world, but their importance as feeding habitats is poorly understood. I evaluated salinas as feeding habitats relative to natural intertidal habitats by comparing time spent foraging, prey-size selection, and net energy intake rate of four overwintering small-sized shorebird species on intertidal mudflats and on adjacent salinas. In winter, Dunlin Calidris alpina, Curlew Sandpiper C. ferruginea and Sanderling C. alba predominantly used the mudflats, whereas Little Stint C. minuta fed mainly on the salina. In the pre-migration fattening period, all species preferred to feed on the salina, significantly increasing the time they spent feeding in the supratidal pans. Net energy intake rates (kJ min^–1) were significantly higher on the salina than on the intertidal mudflats in 60% of all comparisons. On average, salina contributed 25.2 ± 24.2% (range: 4–54%) of the daily consumption in winter and 78.7 ± 16.4% (range: 63–100%) of the daily consumption in the pre-migration period. I recommend that modern active salinas maintain flooding conditions in the evaporation pans throughout winter, thus increasing the available surface for foraging waterbirds. I conclude that the conservation of salinas at coastal wetlands is a viable approach for shorebird conservation.     

<Emphasis Type="Italic">Spartina maritima</Emphasis> influence on the dynamics of the phosphorus sedimentary cycle in a warm temperate estuary (Mondego estuary,Portugal)

During the last decades the Mondego estuary has been under severe ecological stress mainly caused by eutrophication. In this salt march system, Spartina maritima covers about 10.5 ha of the intertidal areas. The objective of the present study was to evaluate the effect of Spartina maritima marshes on the dynamics of phosphorus (P) binding in the surface sediment. We compare phosphate and oxygen fluxes, P-adsorption capacity, phosphate concentrations and total amount, and the extractable P forms in the upper 20 cm of sediment in vegetated sediment with adjacent mudflats without vegetation. Sediment pore-water profiles followed a clear trend, with lower P concentrations in more superficial layers, and increasing with depth. The vegetated mudflats presented lower concentrations of dissolved inorganic phosphorus than adjacent bare bottom mudflats, lower phosphate total amount, as well as higher P-adsorption capacity. Results from the extraction procedure show that the superficial layers are the most important for estuarine phosphorus dynamics, since maximum concentrations of labile P pools are present here. In contrast, higher proportions of refractory P pool are found in deeper layers. Spartina marsh sediments had less total P, less iron bound P, and less exchangeable P than adjacent bare bottom mudflats. Also the pool of loosely sorbed P is lower in the Spartina marsh. Phosphate regeneration from the sediment to the overlying water was only 11.8 kg ha⁻¹ year⁻¹ in vegetated sediment while 25.8 kg ha⁻¹ year⁻¹ in the bare mud flat. Plant uptake for growth combined with an enhanced P-adsorption capacity of the sediment, may explain these differences. Therefore, Spartina marshes are very important agents in the sedimentary P cycle worldwide, and can be considered a useful management tool in estuarine ecosystem recovery efforts.     

The feeding strategy of the dunlin (Calidris alpina L.) in artificial and non-artificial habitats at Ria de Aveiro,Portugal

Usually waders (Aves, Charadrii) feed in the inter-tidal zone over the low tide period and roost over high tide. But occasionally when their energetic needs increase, due to adverse weather conditions or to a need to store fat to fuel migration, the birds also feed during high tide, sometimes in agricultural land. At the Ria de Aveiro, about 44% (corresponding to 4500 birds) of the wintering dunlin (Calidris alpina) use artificial salt pan habitats throughout the winter whilst the others utilize natural intertidal habitats, mainly mudflats. The dunlin that feed intertidally in natural areas, such as the Canal de Mira, do not feed over high tide. In contrast, the birds that use the salt pan area feed both during low tide, in abandoned salt pans whose walls are breached and thus allow access to the tide, and high tide, in active salt pans, and keep on doing so throughout the winter. The energetic needs of both groups of birds are thought to be similar. However, the benthos of the abandoned salt pans seem to be different, and poorer in energetic terms, from those of the natural mudflats. It is argued that dunlin wintering in the salt pan area must compensate for this by feeding over high tide in active salt pans. Rather than just being a means of obtaining more energy at certain times of high energy demand, it would seem that feeding in active salt pans over high tide should be considered as a normal component of the feeding strategy of these birds, a strategy that is different from those wintering in the natural mudflats. These findings highlight the importance of the salt exploitation industry to waders and also show that artificial habitats can support large numbers of wintering waders.     

Spatial analysis of stable isotope data to determine primary sources of nutrition for fish

Carbon and nitrogen stable isotopes were used to determine the ultimate autotrophic sources supporting production of three commercially important fish species over unvegetated mudflats in a subtropical estuary. Mean isotope values over the whole estuary for fish and autotroph sources were modeled to indicate feasible combinations of sources. Variability in isotope values among nine locations (separated by 3-10 km) was then used as a further test of the likelihood that sources were involved in fish nutrition. A positive spatial correlation between isotope values of a fish species and an autotroph indicates a substantial contribution from the autotroph. Spatial correlations were tested with a newly developed randomization procedure using differences between fish and autotroph values at each location, based on carbon and nitrogen isotopes combined in two-dimensional space. Both whole estuary modeling and spatial analysis showed that seagrass, epiphytic algae and particulate organic matter in the water column, including phytoplankton, are likely contributors to bream (Acanthopagrus australis) nutrition. However, spatial analysis also showed that mangroves were involved (up to 33% contribution), despite a very low contribution from whole estuary modeling. Spatial analysis on sand whiting (Sillago ciliata) demonstrated the importance of two sources, mangroves (up to 25%) and microalgae on the mudflats, considered unimportant based on whole estuary modeling. No spatial correlations were found between winter whiting (Sillago maculata) and autotrophs, either because fish moved among locations or relied on different autotrophs at different locations. Spatial correlations between consumer and source isotope values provide a useful analytical tool for identifying the role of autotrophs in foodwebs, and demonstrated here that both in situ production of microalgae and organic matter from adjacent habitats were important to fish over mudflats.