Alistar Robertson

Mangroves as nursery sites: comparisons of the abundance and species composition of fish and crustaceans in mangroves and other nearshore habitats in tropical Australia

Daytime sampling of mangrove and seagrass (Halophila/Halodule community) habitats every 7 wk at Alligator Creek, Queensland, Australia, over a period of 13 mo (February 1985–February 1986) using two types of seine net, revealed distinct mangrove and seagrass fish and crustacean faunas. Total abundance of fish and relative abundance of small and large fish also varied between habitats and seasonally. Post-larval, juvenile and small adult fish captured with a small seine-net (3 mm mesh) were significantly more abundant (4 to 10 times) in the mangrove habitat throughout the 13 mo of sampling. Mangrove fish abundance showed significant seasonality, greatest catches being recorded in the warm, wet-season months of the year. Relative abundances of larger fish (captured in a seine net with 18 mm mesh) in the two habitats varied throughout the year, but did not show a seasonal pattern. At the same site, small crustaceans were significantly more abundant in the mangroves in all but one dryseason sample. Similar comparisons for three riverine sites, sampled less frequently, in the dry and wet seasons of 1985 and 1986, respectively, showed that in general mangrove habitats had significantly more fish per sample, although the relative abundance of fish in mangroves and other habitats changed with season. Crustacean catches showed a similar pattern, except that densities among sites changed with season. Fish and crustacean abundance in mangroves varied among sites, indicating that estuaries differ in their nursery-ground value. The juveniles of two commercially important penaeid prawn species (Penaeus merguiensis and Metapenaeus ensis) were amongst the top three species of crustaceans captured in the study, and both were significantly more abundant in the mangrove habitat. By contrast, mangroves could not be considered an important nursery for juveniles of commercially important fish species in northern Australia. However, based on comparisons of fish catches in other regions, the results of the present study indicate the importance of mangroves as nursery sites for commercially exploited fish stocks elsewhere in South-East Asia.

Leaf-burying crabs: their influence on energy flow and export from mixed mangrove forests (Rhizophora spp.) in northeastern Australia

Abstract Field experiments which compared the decomposition rate of leaves of the mangrove Rhizophora stylosa Griff, in the presence and absence of the leaf-consuming crab Sesarma messa (Campbell) showed that, by burying and consuming leaves, crabs had a highly significant effect on weight loss from leaves. Seasonal measurements of the rates of leaf litter fall and leaf removal (= burial plus consumption) by crabs during day- and night-time periods of low tide in low to mid-intertidal mixed Rhizophora forest showed that crabs could remove at least 154g·m −2 ·yr −1 or 28% of the annual leaf fall of 556g·m −2 ·yr −1 . Crabs consumed >78% of the leaf material within 6 h of burial. These results show that an earlier attempt to measure detrital export from mangrove forests in northeastern Australia over-estimated export by at least 22% by ignoring leaf removal by crabs. In addition, the results indicate that current models of energy flow in mangrove forests, based mainly on work in Florida, must be altered to include the leaf litter → crab trophic link in order to describe accurately the food chains of mangrove forests in the Indo-West Pacific region.

Decomposition of mangrove leaf litter in tropical Australia

Abstract Litter bag experiments were used to follow changes in mass and chemical constituents during decomposition of leaves from three mangrove species, Rhizophora stylosa Griff., Avicennia marina (Forsk.) Vierh. and Ceriops tagal var. australis C.T. White. Losses of AFDW from leaves maintained in the subtidal were best described by double exponential models, which showed that the percentage of the initial AFDW which was relatively labile differed among species; Avicennia (61.5), Ceriops (45.4), Rhizophora (40.3). The times required for loss ofhalfthe initial AFDW (t0.5) of submerged leaves were Avicennia 11 days, Ceriops 27 days and Rhizophora 39 days. The rates of loss of carbon were slightly slower, and were more readily fitted by single exponential models. Linear models were fitted to AFDW and carbon data for leaves left to decompose in intertidal forests. Leaves in the forest decomposed much more slowly than submerged leaves; t0.5 values for AFDW were Avicennia 90 days. Rhizophora 226 days, and Ceriops 228 days. For all species in both habitats, nitrogen was lost from leaves more slowly than carbon and AFDW, and at the end of the experiment, the percentage of the original mass of nitrogen in leaves was higher than the percentage of the original AFDW or carbon. In the subtidal habitat the concentration of nitrogen in leaves of all species increased during the first 70 days of the experiment, and levelled off or dropped thereafter. In the forest nitrogen concentrations in leaves showed a steady increase to Day 348 of the experiment. Bacterial densities initially increased rapidly in both habitats as tannins were lost from leaves. After Day 40 bacterial densities in the subtidal habitat fluctuated widely (Rhizophora, Ceriops) or declined (Avicennia), while in the forest, densities increased slowly (Ceriops, Rhizophora) or declined (Avicennia). Bacterial nitrogen usually contributed less than one percent of the total leaf nitrogen concentration.

Mangrove fish-communities in tropical Queensland, Australia: spatial and temporal patterns in densities, biomass and community structure *

Regular daylight sampling over 13 mo (February 1985–February 1986) in and adjacent to intertidal forested areas, in small creeks and over accreting mudbanks in the mainstream of a small mangrove-lined estuary in tropical northeastern Queensland, Australia, yielded 112 481 fish from 128 species and 43 families. Species of the families Engraulidae, Ambassidae, Leiognathidae, Clupeidae and Atherinidae were numerically dominant in the community. The same species, with the addition ofLates calcarifer (Latidae).Acanthopagrus berda (Sparidae) andLutjanus agentimaculatus (Lutjanidae) dominated total community biomass. During high-tide periods, intertidal forested areas were important habitats for juvenile and adult fish, with grand mean (±1 SE) density and biomass of 3.5±2.4 fish m−3 and 10.9±4.5 g m−3, respectively. There was evidence of lower densities and less fish species using intertidal forests in the dry season (August, October), but high variances in catches masked any significant seasonality in mean fish biomass in this habitat. On ebb tides, most fish species (major families; Ambassidae, Leiognathidae, Atherinidae, Melanotaeniidae) moved to small shallow creeks, where mean (±1 SE) low-tide density and biomass were 31.3±12.4 fish m−2 and 29.0±12.1 g m−2, respectively. Large variances in catch data masked any seasonality in densities and biomasses, but the mean number of species captured per netting in small creeks was lowest in the dry season (July, August). Species of Engraulidae and Clupeidae, which dominated high-tide catches in the forested areas during the wet season, appeared to move into the mainstream of the estuary on ebbing tides and were captured over accreting banks at low tide. Accreting banks supported a mean (±1 SE) density and biomass of 0.4±0.1 fish m−2 and 1.7±0.3 g m−2, respectively, at low tide. There were marked seasonal shifts in fish community composition in the estuary, and catches in succeeding wet seasons were highly dissimilar. Comparison of fish species composition in this and three other mangrove estuaries in the region revealed significant geographic and temporal (seasonal) variation in fish-community structure. Modifications and removal of wetlands proposed for north Queensland may have a devastating effect on the valuable inshore fisheries of this region, because mangrove forests and creeks support high densities of fish, many of which are linked directly, or indirectly (via food chains) to existing commercial fisheries.

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

SummaryMeasurements 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.

The influence of seagrass structure on the distribution and abundance of mobile epifauna : pattern and process in a Western Australian Amphibolis bed

Patches within a mixed bed of the seagrasses Amphibolis antarctica and Amphibolis griffithii were manipulated in three ways: by the removal of epiphytes, by the removal of leaves, and by altering seagrass density. 1 month after the manipulation, large differences remained between the faunas inhabiting the variously treated patches. Leaf removal and epiphyte removal decreased total abundances by 64 and 31%, respectively, with the majority of the common species being negatively affected by one or the other of these treatments but not both. Three species significantly increased in abundance in the leaf removal treatment and were not affected by the removal of epiphytes. The fauna inhabiting Amphibolis beds therefore can be subdivided into relatively independent leaf-associated and epiphyte-associated assemblages, with a small additional assemblage of animals, possibly associated with plant stems, which was negatively affected by the presence of leaves. Four species of mollusc required both epiphyte and leaf habitats, thus providing some overlap between the two major assemblages. n nWhen seagrass plant density was reduced by half, major changes in the three faunal assemblages occurred which were almost identical to those caused by the leaf removal manipulation. The faunal densities (g−1 seagrass) of all species negatively affected by leaf removal decreased in reduced density reduction treatment while the densities of species in the epiphyte-associated assemblage showed no significant differences when compared with control patches. Species which increased in abundance in the leaf removal treatment were more abundant in the reduced density seagrass patches than in the control patches. n nField experiments carried out subsequent to the seagrass manipulation experiment showed that a change in the leaf-associated faunal assemblage similar to that identified in the seagrass density reduction treatment occurred in <4 d when seagrass plants were placed in the open, and that the settlement rates of animals onto isolated seagrass plants were generally similar to the settlement rates amongst dense Amphibolis. Caging experiments further showed that a reduction in faunal densities occurred on seagrass plants in the open even when plants were enclosed in 10-mm mesh cages, hence predation by fish or decapod predators was unlikely to be the proximate cause for the faunal decline on open seagrasses. Epifauna associated with seagrass leaves at our shallow Seven Mile Beach site appear to actively select dense seagrass habitats, possibly because of evolutionary selection to minimize predation or to avoid high levels of solar radiation.

Recruitment, growth and residence time of fishes in a tropical Australian mangrove system

Twenty fish species accounted for > 96% of the catch by numbers in mangrove habitats in Alligator Creek, in tropical Queensland, Australia. The timing of recruitment, residency status, the period of residence and growth of fish during the time they spent in the mangrove habitat was assessed by examining gonad maturity and following changes in size-frequency plots for each species over 13 months. Five species were permanent residents, completing their life-cycles in mangrove swamps; eight were ‘long-term’ temporary residents, being present for not, vert, similar 1 year as juveniles before moving to other near-shore habitats; and seven were ‘short-term’ residents or sporadic users of the mangrove habitat. Amongst the latter group, four species lived in the mangrove habitat for between 1 and 4 consecutive months, while three engraulid species appeared to move rapidly, and often, between mangrove and other near-shore habitats. One of the resident species spawned and recruited throughout the year, but recruitment for most species was highly seasonal, being concentrated in the late dry season (October) to mid wet season (February) period. Temporary resident species dominated the fish community in the wet season (December–April), but resident species comprised more than 90% of total fish numbers in the mid dry season (August) after temporary residents left the mangroves in the early dry season. Several species had more than one peak of recruitment during the wet season. The cohort of 0 + agedLeiognathus equulus which recruited in December grew more rapidly and remained in the mangroves for a shorter period than the cohort which recruited later in the wet season (February). Only nine of the 20 species examined are strictly dependent on mangrove-lined estuaries, the remaining 11 are captured in significant numbers in other near-shore habitats. Only four of the 20 species are of direct commercial importance in Australia, but most are major prey for several valuable, commercial species harvested both within mangrove habitats and in adjacent shallow shelf habitats.

Seed predation by insects in tropical mangrove forests: extent and effects on seed viability and the growth of seedlings

SummaryAlthough insects are known to be important seed predators in most terrestrial forests, their role in marine tidal (mangrove) forests has not been examined. Surveys at 12 sites in tropical Australia showed that between 3.1 and 92.7 percent of the seeds or propagules of 12 mangrove tree species had been attacked by insects. Seeds/propagules of six species (Avicennia marina, Bruguiera gymnorrhiza, B. parviflora, Heritiera littoralis, Xylocarpus australasicus and X. granatum) showed consistently high (>40%) levels of insect damage. Greater than 99% of H. littoralis seeds were attacked by insect predators. The survival and subsequent growth in height and biomass of insect-damaged and non-damaged control seeds/propagules of eight mangrove species were compared in shadehouse experiments. Mangrove species fell into 4 groups with regard to the effect of insect predators on their seeds and seedlings. Xylocarpus australasicus and X. granatum had significantly decreased survival (X 48 and 70%) and growth in height (X 61 and 96%) and biomass (X 66 and 85%). Bruguiera parviflora showed decreased survival (X 59%), but there was no effect of insects on the growth of surviving propagules. In contrast, there was no effect of insect damage on the survival of seedlings of Avicennia marina and Bruguiera exaristata, but decreased growth in height (X 22 and 25%) and biomass (X 22 and 26%). Survival and growth of seedlings of Rhizophora stylosa and Bruguiera gymnorrhiza were not affected. The influence of insect seed predators on the survival and growth of seeds of mangrove species in forests will depend on the relative abundance of seed-eating crabs and intertidal position in mangrove forests.

The Relationship Between Nitrogen Fixation and Tidal Exports of Nitrogen in a Tropical Mangrove System

Various components (sediments, algal mats, decomposing logs and algal-covered prop roots) of a tropical mangrove forest showed low to moderate nitrogen fixation (acetylene reduction) rates. Measurements carried out in March, May and October 1989 revealed negligible or inconsistent seasonal variation in activity for all components. Prop roots with their associated algae showed much greater activity during night-time, typical of many substrates with associated cyanobacteria, whereas all other components had constant activity over 24 h periods. Extrapolation of the nitrogen fixation rates for each component to a ‘whole-forest’ basis, using field estimates of the densities of each component, indicated that sediments, prop roots and decomposing logs contributed approximately 3·5, 1·6 and 1·0%, respectively, of the nitrogen requirements for forest net primary production (FPPN). Blue-green algal mats on bare saltpan areas showed no significant nitrogen-fixing activity (in excess of that for the bare sediments in the same area). The total nitrogen contribution of the various components (6% of FPPN) closely matches previous esimates of the net nitrogen loss from the system through tidal fluxes of particulate and dissolved materials. The possible relative importance of other nitrogen input and loss mechanisms for this mangrove system is also discussed.

The influence of freshwater and material export on sedimentary facies and benthic processes within the Fly Delta and adjacent Gulf of Papua (Papua New Guinea)

Large volumes of freshwater and suspended material debouch from the Fly River in southwestern Papua New Guinea into the Gulf of Papua, greatly influencing the hydrography and sedimentary processes within the river delta and adjacent shelf region. Sedimentary facies within the subtidal regions of the Fly Delta are composed mainly of compacted and eroded very fine black sand, and highly laminated, muddy sand and sandy mud, progressing to prodelta mud with intermixed primary and biogenic structures in the inner Gulf of Papua. These prodelta muds grade further to mixed terrigenous-carbonate deposits southwards into the northern Great Barrier Reef and Torres Strait, and to well-bioturbated, fluid mud northwards into the Gulf of Papua. n nThe transition from physically-dominated, estuarine conditions within the delta to more quiescent, marine conditions on the shelf leads to concomitant changes in sediment chemistry, microbial activity and infaunal and epifaunal communities. Particulate (C, N, P) and dissolved inorganic and organic nutrient concentrations were a function of sediment type (higher in finer deposits) rather than location (delta vs gulf). C: N: P ratios of solid-phase nutrients varied greatly, but were usually less than those predicted by the Redfield ratio. Mean interstitial concentrations of dissolved inorganic nutrients were low (μM range), but dissolved organic carbon, nitrogen and phosphorus levels were equivalent to those found in higher latitude systems. Fluxes of dissolved inorganic nutrients were generally low (μmol m−2 day−1). Flux rates were mostly negative (into the sediment) in the delta suggesting that these deposits are a sink for nutrients. In the offshore deposits, dissolved inorganic fluxes were higher and mostly positive indicating that they are a source for dissolved nutrients. n nStanding crops of bacteria (range: below detection limits— 2.5 × 1010 cells g−1 dry wt), meiofauna (range: 5–750 individuals 10 cm−2; 9–1006 μg dry wt 10 cm−2) and infauna (range: 86–5555 individuals m−2; 0.10-5.85 g AFDW m−2) were generally lower in the delta than in the gulf. The infauna was dominated by nematodes, copepods, foraminifera and small, tube-building, deposit- and suspension-feeding polychaetes and amphipods. n nRates of bacterial productivity were very erratic with sediment depth across stations, ranging from 0–2108 mg C m−2 day−1 (DNA synthesis) and from 0–228 mg C m−2 day−1 (protein synthesis), respectively. Rates of benthic respiration and DOC flux across the sediment-water interface were generally high, ranging from 63–780 mg C m−2 day−1 and from −797 to 514 mg C m−2 day−1, respectively. n nEpibenthos were more diverse (at the phyletic level) at the mid-shelf than inshore, composed mainly of sponges, crabs, crinoids, echinoids, bivalves, hydroids and asteroids. Demersal nekton abundance was low, dominated by the leatherjacket, Paramonacanthus filicauda, the pony fish, Leiognathus splendens and the grunter, Pomadasys argyreus, suggesting limited transfer of infaunal biomass to higher trophic levels. n nThe response of the benthic regime to the export of freshwater and material from the Fly River generally conforms to the Rhoads et al. [(1985) Continental Shelf Research, 4, 189–213] model of benthic response to effluent derived from the Changjiang River in the East China Sea and is similar to infaunal and sedimentary patterns off the Amazon. Nutrient release from the delta sediments contributes little to water-column production, but in the gulf, nutrient efflux from the benthos contributes, on average, 38 and 61% of the annual N and P requirements of phytoplankton production, reflecting closer benthic-pelagic coupling and enrichment of biological productivity in the Gulf of Papua due to nutrient export from the Fly River.