A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco Mangrove, Inhaca Island (Mozambique)

Visual counts of surface-active crabs both by binocular and burrow counting methods have been used in many studies to estimate population density. However, their reliability has not yet been assessed comparatively. Three methods for estimating the abundance of fiddler crabs Uca annulipes in a mangrove forest (Inhaca Island, Mozambique) were compared from three different sub-areas: two sub-areas inundated only during spring tides and one sub-area inundated in both spring and neap tides. Burrow, binocular and direct (excavation) counting methods were performed by plotting ten 0.25 m² quadrats in each sub-area over the four moon phases. Overall densities (per 0.25 m²) differed according to method, sub-area and lunar phase. Burrow count overestimated crab density by up to 20%, while binocular count underestimated density by up to 41%. Correlation coefficient estimated for both counting methods showed that burrow count gives better density estimates than binocular count (0.91 and 0.56, respectively). Sex ratios were also investigated within the three sub-areas and at the moon phases. Males are dominant throughout the studied period except during new moon and first quarter, indicating that when the number of gravid females is low, sex ratio bias for binocular count is minimal.     

Behavioural basis of intertidal zonation in Eurydice pulchra Leach

Eurydice pulchra Leach freshly collected at neap and spring tides exhibits an endogenous circatidal rhythm of swimming when kept without sand in constant conditions in laboratory aquaria. When provided with sand in the laboratory, tidally-phased swimming may occur spontaneously at some times of the lunar month, predominantly after the times of highest spring tides but not at neaps. In these cases, therefore, there appears to be clearly demonstrated for the first time in a marine animal a circasemilunar pattern of emergence from the substratum which is coupled to a circatidal rhythm of swimming; the isopods in the laboratory emerge from the sediment only when the swimming rhythm and emergence rhythm are at their maxima. In the field at neap tides some isopods emerge to swim at high tide, dependent upon their responses to water agitation, their feeding state and responses to light. At and just after spring tides in the field large numbers of the isopods swim at high tide, triggered by the endogenous circasemilunar rhythm of emergence which in the field is probably cued by the effects of increased water agitation. The main function of the endogenous circatidal rhythm of swimming appears to be to permit the isopods to return to their preferred zone in the sand before the fall of the tide. The coupled circasemilunar rhythm of emergence which induces greater swimming at spring tides reduces the risk of animals being stranded high on the shore during neap tides.     

Transport of microbial biomass through the North Inlet ecosystem

Tidal fluctuations and transports of total microbial biomass (measured as adenosine triphosphate [ATP]) were investigated at three marsh creeks comprising the major transfer points between the North Inlet marsh and the adjoining aquatic ecosystems. Two creeks, Town Creek and North Jones Creek, form the inlet mouth and are the only marsh-ocean exchange points. The third creek, South Jones Creek, connects to a brackish water embayment. The creeks were simultaneously sampled every 1.5 hours for 50 consecutive hours during neap tides (four tidal cycles) and 50 consecutive hours during spring tides of each season. At the inlet, ATP concentration fluctuated in phase with the tide during winter and fall and out of phase with the tide during the spring. Fluctuation patterns at South Jones Creek were irregular. The highest ATP concentrations were during the spring (mean=2.17 mg of ATP per m³) and the lowest concentrations were during the winter (mean=0.65 mg of ATP per m³). Net transports of ATP varied from tidal cycle to tidal cycle with regard to direction of transport (import or export) and magnitude. Net transports were small compared to large instantaneous transports and only 4 of 22 determinations of net transport were different from zero transport.     

Activity patterns, feeding and burrowing behaviour of the crab Ucides cordatus (Ucididae) in a high intertidal mangrove forest in North Brazil

Activity patterns, feeding and burrowing behaviour of the economically important semi-terrestrial mangrove crab Ucides cordatus (Ucididae, L. 1763) was studied in a high intertidal Rhizophora mangle forest stand in Bragança, North Brazil. Video observations in the rainy and dry season were conducted over 24 h cycles at different lunar phases to investigate the behaviour of these litter-feeding crabs outside their burrows. During the rainy season, crabs stayed inside their burrows for 79% and 92% of the time during day and night, respectively. Time spent for feeding, burrowing and other activities outside their burrows was significantly longer during the day with 9.9% (night: 1.7%) and at waning and waxing moon with 9% (full and new moon: 0.9%). At neap tides (no tidal inundation) foraging and feeding activities outside burrows were clearly light-dependent, increasing at dawn and decreasing at dusk. Highest activities during daytime relate to the visual localisation of food. During the dry season, crabs spent less time inside burrows at neap tides than during the rainy season (80% and 91%, respectively). However, time spent for feeding activities was similar during both seasons. During almost all observation periods crabs collected leaf litter, but rarely fed on it outside burrows. At neap tides nearly all available litter was collected, suggesting that the U. cordatus population is litter-limited during these times. At spring tides (regular tidal inundation) the surface activity of U. cordatus was tide-dependent. Crabs closed their burrow entrances 2-3 h before flooding and re-emerged as soon as the tide retreated. During the day, burrow maintenance was the second most frequent behaviour after feeding. Agonistic interactions were regularly observed and were mainly related to burrow defence. The mean foraging radius of the crabs was only 19 cm (max: 1 m) underneath high Rhizophora mangle trees where crab densities were high. The results point to a high competition for burrows and show that U. cordatus is territorial. It is concluded that several exogenous factors, in particular light, leaf litter availability, flooding of burrows and the presence of conspecifics are important in controlling the crabs' activity patterns.     

Tower construction by the manicure crab <Emphasis Type="Italic">Cleistostoma dilatatum</Emphasis> during dry periods on an intertidal mudflat

Animals living on upper intertidal mudflats experience habitat desiccation during neap tides when water does not flood the habitat. Individuals of the manicure crab Cleistostoma dilatatum construct cone-shaped towers at the entrance of their burrows, in which they remain during neap tides. These towers are the tallest known structures compared to body size built by crabs living on intertidal flats. The frequency of tower construction followed semilunar tidal cycles with most building done prior to neap tides when few crabs were active on the mudflat surface. Bigger crabs tended to make taller and wider towers with a wider pinhole on the top. These towers may regulate the microclimate in burrows.     

Spring-neap cycle as a major driver of temporal variations in feeding of intertidal fishes: Evidence from the sea catfish Sciades herzbergii (Ariidae) of equatorial west Atlantic mangrove creeks

On macrotidal coasts, short- to medium term variations in feeding of intertidal fishes are influenced by several interacting time scales. To identify the driver of major variations in the feeding habits of intertidal fish, we used the pemecou sea catfish Sciades herzbergii (Ariidae), an abundant intertidal benthic second order consumer, as a model species. We analyzed the influence of the spring-neap and the day-night cycle on intertidal abundance, stomach fullness, diet composition and food consumption of S. herzbergii using block nets set at slack high tides in two mangrove creeks in north Brazil. At spring tides, intertidal abundance, stomach fullness, and total daily consumption of S. herzbergii were on average 8.8, 1.9, and 3.8 times higher than at neap tides, respectively. At spring tides, Uca spp. and Grapsidae (mostly Pachygrapsus gracilis) dominated the diet, irrespective of the time of day. Other important food items were Insecta and the semiterrestrial crab Ucides cordatus. At neap tides, Capitellidae contributed to the diet of nightly inundations while no specimens were caught at daytime. Creek location had no effect on any variable. Results from our study area and evidence from other studies suggest that the spring-neap tide pulse is likely the major driver of short- to medium term variations in feeding of intertidal fishes. This has important implications for feeding-related issues on macro- and probably also on mesotidal coasts: (i) juvenile fishes may have fortnightly growth spurts, (ii) intertidal prey populations may suffer regular fluctuations in mortality, (iii) studies of the feeding ecology of intertidal fishes should cover the combined effects of the tidal, diel, lunar and seasonal cycles, and (iv) the modeling of food webs should consider the differences between highly dynamic spring tide and quieter neap tide conditions.     

The climbing behaviour of Cerithidea anticipata (Mollusca: Gastropoda): The roles of physical and biological factors

Abstract Many molluscs in tidal wetlands climb trees as the tide rises, a behaviour usually assumed to be a means of avoiding subtidal predators. Some species are more active during neap tides, when the access of subtidal predators to the forest is limited, but rest on trees during spring tides. Cerithidea anticipata, which inhabits the mangrove forests around Darwin Harbour (Northern Territory, Australia), displayed almost exactly the opposite pattern. This species climbed higher, and was less active, during neap tides that did not flood the forest than during spring tides. In experiments with tethered snails, individuals prevented from climbing died during neap tides, apparently from physiological stress. Further, individuals resting on trees around clearings, usually selected shaded sites. These results suggest that the major reason C. anticipata climbed was to avoid physiological stress during neap tides, not subtidal predators during spring tides. There was some evidence of predation under the canopy, but the rate was relatively low and the species responsible appeared to be resident in the forest.     

Semilunar courtship rhythm of the fiddler crab<Emphasis Type="Italic"> Uca lactea</Emphasis> in a habitat with great tidal variation

Semilunar courtship rhythm is a widely distributed phenomenon among fiddler crabs in the genus Uca (Decapoda, Ocypodidae). Typically, synchronous courtship has been reported to peak near spring tides. To determine whether a region of large tidal variation shifts reproductive activity, we measured the frequency of specific courtship behaviors including claw-waving and semidome building for U. lactea males on Kanghwa Island, Korea. We found that synchronized courtship for U. lactea peaked near neap tides, whereas near the spring tides, seawater flooded the habitat and males predominantly fed on the mudflat. Although active females, which hold their burrows and usually feed on the mudflat, are abundant near to spring tides, males rarely claw-waved to attract females. This pattern is atypical for the species because other populations of U. lactea on Japan and Taiwan are synchronous around spring tides. We suggest that males invest most of their time in feeding during spring tides because foraging is limited during neap tides. During neap tides, males feed infrequently and thus expend stored energy on courtship signals. We conclude that patterns of reproductive synchrony may be dependent on food availability in periodically changing environments.     

Diel and tidal changes in the distribution and feeding habits of Japanese temperate bass Lateolabrax japonicus juveniles in the surf zone of Ariake Bay

Japanese temperate bass Lateolabrax japonicus juveniles recruit to the surf zone and grow by feeding on commonly occurring coastal copepods. However, little is known about diel and tidal patterns in their migration and feeding habits. We sampled wild juveniles during the neap and spring tides, over periods of 24 h, with small seine nets in the sand flat of the eastern part of Ariake Bay, Kyushu, western Japan. In both the neap and spring tides, abundance of juveniles significantly increased during the daytime, being highest around the time of high tide. The relative gut fullness indices of juveniles drastically increased in the morning (0700–0900) and during the flood tide in the daytime, while major prey copepod (Paracalanus spp.) density in the ambient water was relatively constant. We summarized that L. japonicus juveniles would migrate to the surf zone after sunrise to feed on copepods, and then emigrate from the surf zone after sunset. The migratory behavior of L. japonicus juveniles would be influenced by light (daytime) and feeding activity influenced by both light (morning) and tidal condition (flood tide). The intertidal region of the tidal flat was recognized to be one of the important habitats for L. japonicus during their early life history.     

Impact Scenario for an Introduced Decapod on Arctic Epibenthic Communities

The intentional introduction of a species for the enhancement of stock or establishment of new fisheries, often has unforeseen effects. The red king crabs, Paralithodes camtschaticus, which was introduced into the Barents Sea by Russian scientists, has established a self-sustaining population that has expanded into Norwegian waters. As top benthic predators, the introduced red king crabs may have possible effects upon native epifaunal scallop (Chlamys islandica) communities. These benthic communities may be a source of prey species in late spring, when the red king crabs feed most intensively. Foraging rates (consumption, killing or severely damaging) of red king crab on native prey organisms were measured by factorial manipulation of crab density (0.5, 1.5 and 3 per m ²), size classes (immature, small mature, and large mature crabs), and by evaluating prey consumption after 48 h, in order to extrapolate a scenario of the likely impacts. Foraging rates of the red king crab on scallops ranged between 150 and 335 g per m² within 48 h. These rates did not change when crab density was altered, though an increased amount of crushed scallops left uneaten at the tank floor, were correlated with high density of small mature crabs. Foraging rate changed significantly with crab size. Consequently, the susceptibility of native, shallow water epibenthic communities to red king crab predation in the early life history stages, and during the post-mating/molting spring period, must be considered significant when foraging rates are contrasted with natural scallop biomass between 400 and 1200 g scallops per m².     

FORAGING ACTIVITY OF HELCION PRUINOSUS (PATELLOGASTROPODA) ON A SOUTH AFRICAN BOULDER SHORE

Foraging activity of the mid-shore limpet Helcion pruinosuswas monitored on an exposed boulder beach in the Eastern Capeof South Africa. Numbers of limpets active in 10 fixed quadratswere monitored and compared in different seasons and during springand neap tides. Foraging excursions were mainly associated with nocturnallow tides, limpets only emerging from beneath rocks when theywere completely uncovered by the receding tide, and they hadall retreated to beneath rocks before being splashed by theadvancing tide. The number of animals active was greater onfull moon spring tides. Significantly more limpets were activeon an autumn spring tide than on similar tides in other seasons. ^* To whom correspondence should be addressed. (Received 27 March 2000; accepted 5 July 2000)     

Solute dynamics in a North Brazilian mangrove: the influence of sediment permeability and freshwater input

Although water in mangrove sediments influences nutrient cycling in both, mangrove forest and estuary, little information exists on seasonal and vertical distribution of dissolved organic and inorganic compounds in the sediment column. We studied the influence of sediment texture and chemistry, permeability (K), tides, and rainfall on dissolved organic carbon (DOC) and nitrogen (DON), dissolved inorganic phosphate (DIP) and salinity in creek and sediment waters of a mangrove in Pará, Brazil. Water samples were taken from boreholes and piezometers in the mangrove forest and from an adjacent tidal creek at neap and spring tides, during the dry and rainy season. Forest sediment was analysed for carbon (C), nitrogen (N), salinity and permeability. Clay, C and N decreased with depth. Sediment permeability (K) was lowest (<0.1 m day⁻¹) in the upper, clay-rich and crab-burrow-free mud layer. In the deeper, fine sand strata, K ranged from 0.7 to 1.8 m day⁻¹. Tidal range in the creek was 3.5 and 5.5 m for neap and spring tides, respectively. Salinity, DOC, DON and DIP in creek water were inversely related to tidal height. Piezometer data revealed significant water level changes in deeper, sandy sediment layer, which followed, time-lagged, the tidal fluctuations. In contrast, tide did not affect the water level in the upper sediment due to low permeability. Compared with creek water, sediment water was enriched in DOC, DON and DIP because of organic matter input and mineralization. In deeper layers, solute concentration was most likely affected by sorption processes (DOC and DIP) and reduction reactions (DIP). During the rainy season, DOC and DON in creek and sediment water were higher than in the dry season. DIP appeared invariant to seasonal changes. In the rainy season, salt flushing from surface sediments resulted in higher salinities at intermediate sediment depths, while in the deeper layers salinity was lower due to exchange with water from the tidal creek.     

Coupling between microphytobenthic biomass and fiddler crab feeding

Among the organic matter ingested by fiddler crabs, microphytobenthos is of fundamental importance because it is their main N source. Microphytobenthos abundance generally develop semilunar changes as the dynamics of tidal exposures and day-night cycle are not held constant across days, modifying the balance between growth and mortality. In this study we explored the coupling between temporal dynamics in microphytobenthos abundance and crab feeding activity. We measured the Chlorophyll a content in the 2 mm surficial sediment surrounding the burrows and the crab feeding activity over two semilunar cycles. Chlorophyll a and crab feeding activity showed biweekly cyclic dynamics. Crabs did not concentrate feeding activity around days with maximum abundance of microhytobenthos. This phase difference between both dynamics could be the result of the crab feeding impact, but a crab experimental exclusion showed that the temporal dynamics of Chlorophyll a content stayed unchanged when feeding activity was removed. Comparisons between fed and unfed sediment suggest that the feeding efficiency changes with tidal dynamic. Crabs achieved more than 50% of Chlorophyll a extraction during days of highest feeding activity, and less than 30% during days of low feeding activity or low microhytobenthos abundance. Furthermore, comparisons of fed sediment between consecutive days indicated that Chlorophyll a was completely replenished during days with high flooding tides, but partially replenished during days near neap tides. Environmental conditions affecting feeding efficiency may select crabs to concentrate feeding activity before days with the highest microhytobenthos abundance. The low feeding impact on microphytobenthos dynamics suggests that fiddler crabs would not control microhytobenthos abundance and thus unable to absorb the increasing eutrophication of studied estuarine areas.     

Tidal amplitude and fish abundance in the mouth region of a small estuary

Using an acoustic underwater camera (Dual Frequency IDentification SONar, DIDSON), the abundance and direction of movement of fishes > 80 mm total length (L_T) in the mouth of a small South African estuary during spring and neap tidal cycles were observed. While the sizes of fishes recorded were consistent across both tide cycles, the number of fishes passing the camera was significantly greater during the smaller neap tides. Schooling behaviour was more pronounced for fishes that were travelling into the estuary compared to fishes swimming towards the ocean.     

Feeding rhythms of the green-lipped mussel, Perna viridis (Linnaeus, 1758) (Bivalvia: Mytilidae) during spring and neap tidal cycles

Food availability and feeding responses of the green mussel Perna viridis were investigated for two complete tidal cycles during both spring and neap tides. Temporal changes in total particulate matter (TPM), particulate inorganic matter (PIM) and particulate organic matter (POM), were smaller during neap than spring tides. Seston characteristics at different times of a tidal cycle were compared for both spring and neap tides. Only during spring tides were TPM and PIM significantly higher at high tides while POM remained relatively constant (P>0.05). The clearance rate of the mussels underwent temporal variations with tides, and was a negative power function of TPM and a positive linear function of f (organic content), during both spring and neap tides. f was the key factor influencing filtration rate, organic ingestion rate, absorption rate and absorption efficiency. All feeding rates increased linearly with increases in organic content. Pseudofaeces were produced only during spring but not neap tides. Feeding rates and absorption efficiency were highest at low and lowest at high tides (P<0.01). There was no significant temporal change in the wet weight and protein content of the crystalline style with the tidal regime. For the digestive gland, alpha-amylase activity was higher at spring than at neap tides, and higher during high tides in a tidal cycle. The digestive gland cellulase activity did not change significantly with the tides. For the crystalline style, both the activity of cellulase and alpha-amylase were not significantly different (P>0.05) between spring and neap tides. Tidal rhythms in feeding and digestion in this species were likely controlled by temporal variations in food availability in the seawater. By adjusting feeding rates and enzymatic activities, absorption in Perna viridis remained constant, irrespective of the changes in food availability.     

The influence of crabs on litter processing in high intertidal mangrove forests in tropical Australia

Summary Measurements of litter fall and litter removal by crabs, in conjunction with estimates of litter decay by microbes and tidal export of litter from three high-intertidal mangrove forests were made during a year-long study in tropical northeastern Australia. In forests dominated by Ceriops tagal and Bruguiera exaristata, litter standing stocks remained low on the forest floor (mean 6 g·m^-2), although litter fall was high; 822 and 1022 g·m^-2·y^-1, respectively. Sesarmid crabs removed 580 (Ceriops) and 803 (Bruguiera) g·m^-2·y^-1, or 71 and 79%, of the total annual litter fall from the forest floor. Relative to the rate of litter removal by crabs, microbial turnover of whole, unshredded litter was insignificant, accounting for <1% of annual litter fall. Export of litter by tides was estimated to remove 194 (Ceriops) and 252 (Bruguiera) g·m^-2·y^-1 or 24 and 25% of annual litter fall. In a forest dominated by Avicenniamarina, in which an ocypodid crab was more abundant than sesarmids, litter standing stocks were higher (mean 84 g·m^-2) and crabs removed less litter; 173 g·m^-2·y^-1 or 33% of the annual litter fall of 519 g·m^-2·y^-1. Microbial turnover of intact litter was more important in the Avicennia forest (168 g·m^-2·y^-1 or 32% of annual litter fall), and tides exported 107 g·m^-2·y^-1 or 21% of litter production. In areas where sesarmid crabs were absent or rare in Ceriops forests, there were significantly higher standing stocks of litter and slower rates of leaf removal. Taking into account the probable assimilation efficiencies of sesarmid crabs feeding on mangrove leaves, we estimate that in Ceriops and Bruguiera forests leaf processing by crabs turns litter over at >75 times the rate of microbial decay alone, thus facilitating the high sediment bacterial productivity in these forests. The importance of litter processing by crabs increases with height in the intertidal in tropical Australia, in contrast to New World mangrove forests, where the reverse is true.Contribution No. 445 from the Australian Institute of Marine Science     

The keystone role of leaf-removing crabs in mangrove forests of North Brazil

Principle factors which influence mangroveleaf litter turnover, in particular therole of leaf-removing crabs, were evaluatedin a riverine mangrove site nearBragança (Pará, North Brazil). Ourspecial interest was focussed on the roleof the leaf-removing crab Ucidescordatus. Leaf litter fluxes between themangrove forest and the adjacent estuarywere investigated by estimating the biomassand fate of leaf litter material and propagules. Vegetation is dominated by Rhizophora mangle, with Avicenniagerminans trees, both up to 25 m high,found intermittently. During 1997, Rhizophora trees produced around 1.40 gDW m^-2 d^-1 of leave fall and0.75 g DW m^-2 d^-1 of propagules.Leaf decomposition rates on the ground wereabout 0.06 g DW m^-2 d^-1,irrespective of species, habitat or siteexposure. This amount accounts for <3%of total leaf fall. Average leaf litterbiomass present on the ground was 0.01 gDW m^-2 d^-1. When the mangroveforest was flooded (on average 10 days permonth) the quantity of leaf litterand propagules washed out with the springtide was 10 and 17 times greater thanduring neap tide. Nevertheless, tidalexport and decomposition together made upless than 39 percent of annual leaf litterfall. The bulk of the remaining amount isapparently removed by Ucides. Eachcrab consumed about 1.30 g DW of leaflitter material and propagules per day.Since the average density of these crabswas 1.38 crabs m^-2, it is proposedthat Ucides is a keystone species inBragantinian mangroves.     

辽河口滨海湿地CH4排放特征及其影响因素

采用静态箱-气相色谱法,于2016年6—11月连续观测辽河口芦苇湿地、翅碱蓬湿地和裸滩湿地的CH_4排放速率,同时测定温度、氧化还原电位(Eh)、pH值和电导率(EC)等相关环境因子的动态变化。结果表明,3种类型湿地的CH_4排放具有明显的季节变化特征,均呈先上升后下降趋势。芦苇湿地、翅碱蓬湿地(涨潮前)和裸滩湿地(涨潮前)CH_4排放通量变化范围分别为0.447—10.40、0.045—0.509 mg m~(-2) h~(-1)和0.016—0.593 mg m~(-2) h~(-1),观测期内排放通量均值相应为(3.699±3.679)、(0.165±0.156) mg m~(-2) h~(-1)和(0.198±0.191) mg m~(-2) h~(-1),不同类型湿地之间差异显著(P0.01),芦苇湿地裸滩湿地(涨潮前)翅碱蓬湿地(涨潮前)。涨潮过程中,翅碱蓬湿地和裸滩湿地的排放速率分别变化在0.009—0.353 mg m~(-2) h~(-1)和0.018—0.335 mg m~(-2) h~(-1),观测期间其排放速率均值分别为(0.119±0.132) mg m~(-2) h~(-1)和(0.131±0.103) mg m~(-2) h~(-1),明显低于涨潮前(P0.01)。不同湿地类型间CH_4排放通量与电导率(EC)呈显著负相关(P0.01)。研究结果表明,潮汐和电导率均为影响辽河口不同类型湿地中CH_4排放的关键因子。     

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m⁻²) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m⁻² d⁻¹ and 4.80 mg m⁻² d⁻¹ of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.     

Linkages between seagrass,mangrove and saltmarsh as fish habitat in the Botany Bay estuary,New South Wales

Shallow-water vegetated estuarine habitats, notably seagrass, mangrove and saltmarsh, are known to be important habitats for many species of small or juvenile fish in temperate Australia. However, the movement of fish between these habitats is poorly understood, and yet critical to the management of the estuarine fisheries resource. We installed a series of buoyant pop nets in adjacent stands of seagrass, mangrove and saltmarsh in order to determine how relative abundance of fishes varied through lunar cycles. Nets were released in all habitats at the peak of the monthly spring tide for 12 months, and in the seagrass habitat at the peak of the neap tide also. The assemblage of fish in each habitat differed during the spring tides. The seagrass assemblage differed between spring and neap tide, with the neap tide assemblage showing greater abundances of fish, particularly those species which visited the adjacent habitats when inundated during spring tides. The result supports the hypothesis that fish move from the seagrass to the adjacent mangrove and saltmarsh during spring tides, taking advantage of high abundances of zooplankton, and use seagrass as a refuge during lower tides. The restoration and preservation of mangrove and saltmarsh utility as fish habitat may in some situations be linked to the proximity of available seagrass.