Tropical Continental Shelf Research Articles

Changes in accumulation of land-based organic matter under recent climate change and anthropogenic impact: A tropical coastal perspective

Tropical continental shelves play an important role in the global carbon cycle especially in the context of increased anthropogenic interference and climate change. However, the long-term fate and response of sedimentary organic carbon (OC) in these regions remain poorly understood. In this study, we analyzed bulk OC and molecular biomarkers in a sediment core collected from the lower Gulf of Thailand, and compiled with several published records of sedimentary organic matter (OM) from other tropical Asian margins. Our results reveal a dramatic ∼ 40 % increase in terrestrial OC inputs since the 1980s, likely driven by the effects of accelerated coastal erosion from rising sea levels and the degradation of mangrove ecosystems. In addition, shifts in regional human activities, including changes in energy consumption patterns, have altered the sources of pyrogenic OM, contributing to the observed spatial and temporal variability of anthropogenic OM across tropical coastal margins. Molecular fingerprints highlight recent changes in the accumulation of land-based OM, showing an increased presence of erosion-derived and degraded OC, along with pyrogenic OM. This shift is linked to the combined effects of coastal retreat and basin-wide emissions, influenced by both natural climate forces and anthropogenic activities.

DNA barcoding of marine fish species in the waters surrounding Hainan Island, northern South China Sea

IntroductionHainan Island is encompassed within the shallow waters of the tropical continental shelf in China, which is recognized as a significant hotspot for fish biodiversity. Despite extensive research conducted on marine fish taxonomy surrounding Hainan Island, there remains a substantial gap between our current understanding and the actual fish diversity within this oceanic area.MethodsIn this study, we employed DNA barcoding and molecular identification approaches to explore the species diversity and distribution pattern of marine fish in both the northern and southern sea areas of Hainan Island in the northern South China Sea.ResultsA total of 186 sequences were obtained from the collected marine fish samples in the two sea areas surrounding Hainan Island. Through DNA identification, it was confirmed that all 186 sequences corresponded to typical fish species found in the northern South China Sea, all sequences represented a total of 56 species, 47 genera, 34 families, and 17 orders. The average Kimura 2-parameter (K2P) distances within species, genus, family, order and class were 0.15%, 6.53%, 13.17%, 16.95% and 24.81%, respectively.DiscussionOur investigation in the northern sea areas of Hainan Islands identified a total of 33 distinct species, while the southern sea areas exhibited 29 distinct species, with only 5 species found to be shared between both regions. These findings clearly indicate a significant disparity in the species composition of fish communities between the northern and southern sea regions.

Predicting global seasonal distributions and population exchange routes of a Critically Endangered shark

Oceanic sharks are of great importance to marine ecosystems but among the most endangered taxa in the world. Their conservation requires spatially and temporally explicit knowledge about distributions and migrations across oceanic basins, which are often challenging to monitor and thus little is known at the global scale. The present study addressed this challenge for the scalloped hammerhead (Sphyrna lewini) which is Critically Endangered. I gathered geo-referenced occurrences of this species from online biogeographic databases, catch data, and peer-reviewed literature. I built seasonal species distribution models (SSDMs) based on these occurrences and 11 environmental variables to predict their suitable habitats. I then predicted their seasonal migration routes for population exchanges with two connectivity analysis techniques (Circuitscape, least-cost path analysis). All SSDMs performed well and showed that there were prominent divergences in seasonal distributions driven by the species' ecological niches. Generally, sea surface temperature was the most important predictor, followed by depth, salinity, primary productivity, distance to macrohabitats, and distance to shoreline. Seasonal suitable habitats were mainly found in tropical continental shelves and deep seas. Possible seasonal migration corridors for population exchanges across ocean basins were identified and some were commonly used by oceanic sharks. The present study highlights (i) the utility of integrating SDMs and connectivity-analysis techniques to predict migration routes for oceanic sharks, and (ii) seasonal and regional changes in their habitat uses, suggesting dynamic, ocean-based conservation measures are needed to conserve these marine apex predators.

Stable Isotopic Variability in Individual Benthic Foraminifera from the Continental Shelf of Tropical Brazil

ABSTRACT We analyzed nine benthic foraminiferal species from the Açu Reef belonging to the orders Rotaliida and Miliolida, symbiont-bearing (Rotaliida: Amphistegina gibbosa, Heterostegina depressa; Miliolida: Peneroplis carinatus, Archaias angulatus, Amphisorus hemprichii, and Archaias compressus) and symbiont-barren (Rotaliida: Buccella peruviana, Pseudononion atlanticum; Miliolida: Quinqueloculina lamarckiana). Stable isotopes measured on individual tests display a large intraspecific variability (δ18O: ∼1–1.5‰; δ13C: ∼2–3.5‰) that is not associated with site location, sampling season, or water depth. Positive correlation between δ13C and δ18O values is significant in three of the species (A. gibbosa, B. peruviana, and Q. lamarckiana), with a regression slope similar to other marine calcifiers (∼2‰ δ13C/‰ δ18O), perhaps related to the carbonate chemistry of the calcifying fluid. With the exception of A. gibbosa, offsets from δ18O equilibrium, when distinct, tend to be positive. Offsets from δ13C of ΣCO2 of sea water are negative for two of the rotaliids (A. gibbosa and H. depressa) and positive for three of the miliolids (P. carinatus, A. angulatus, A. hemprichii), with non-symbiont-bearing species of both orders being close to the sea water values (∼ 1.3–2‰). These δ13C differences are consistent with calcification pathways of rotaliids versus miliolids, with the former drawing carbon from an internal pool and the latter from ambient seawater. Our study contributes a large data set that illustrates the importance of vital and abiotic effects on the stable isotopic composition of large benthic foraminifera in a tropical continental shelf, limiting their applicability as paleoenvironmental tracers.

Assessment of the Brazilian Coast Oil Spill Impact in the fish eggs and larvae development from the Tropical Continental Shelf

The response of ichthyoplankton assemblage to the oil spill in the coastal region of Brazil Northeaster on August 2019 was investigated using data of before and after spill from plankton survey off the coast of Salvador. Differences in fish egg and larvae density was evident in all samples collected in the continental shelf of Salvador, after oil spill (Oct/2020 and Feb/2021). In general larvae larger than 5 mm tended to occur predominantly before oil spill, while smaller larvae (1–3 mm) occurred predominantly after oil spill, mainly in October/2020. Abnormalities in embryonic development, deformity in the spine and yolk sac edema were found in fish eggs and larvae sampled in the Salvador continental shelf. Rate of abnormalities in embryonic development were significantly increased after oil spill (Oct/2020, Feb/2021) compared to before oil spill period (Oct/2013, Feb/2014) and high incidence of larvae with spinal deformities was observed after oil spill, about 65% of the larvae sampled in the BA 2 station on October 2020 presented deformity in the spine. The fluid accumulation in the yolk sac (edema) were observed only after oil spill on October 2020. The results of this study indicate negative impact of oil spill in the coastal region of Brazil Northeaster on structure of ichthyoplankton assemblages in the continental shelf of Salvador.

Enhancements of Himalayan and Tibetan Erosion and the Produced Organic Carbon Burial in Distal Tropical Marginal Seas During the Quaternary Glacial Periods: An Integration of Sedimentary Records

AbstractThe Himalayan and Tibetan highlands (mountains), with high rates of physical erosion, are extreme settings for earth surface processes, generating one of the largest recent terrigenous detritus and organic carbon discharges to the ocean. However, their significance with respect to the global carbon and climate cycles during the Quaternary is still unclear, especially in quantitative terms. Here, we present comprehensive records of continental erosion and weathering, terrestrial supply, marine productivity, and organic carbon burial in the distal Arabian Sea, Bay of Bengal, and southern South China Sea since ∼700 ka over orbital timescales. These records exhibit periodicities corresponding to sea level and Indian summer monsoon intensity changes. During glacial periods, the enhanced highland surface erosion and activation of deep‐sea channels significantly increased inputs of terrigenous detritus, nutrients, and organic carbon into the Arabian Sea and Bay of Bengal, whereas strengthened continental shelf surface weathering and organic matter preservation occurred in the South China Sea. Conclusively, our integrative proxies in the study area demonstrate, for the first time, pronounced glacial burial pulses of organic carbon (∼1.12 × 1012 mol/yr), dominantly originating from the highland surface erosion and marine productivity. Together with the increased silicate weathering on the exposed tropical continental shelves and in the tropical volcanic arcs, the enhanced burial flux of organic carbon in the tropical marginal seas, therefore, highlights the large contributions that tropical regions can make within the glacial‐interglacial carbon inventory of the ocean and atmosphere and thus cause significant negative feedback on the global climate.

Dietary composition of myctophid larvae off Western Australia

Mesopelagic fishes of the southeastern Indian Ocean are represented by a high number of lanternfish species (Myctophidae). The horizontal distribution patterns of their larvae have been previously studied; however, research on larval feeding habits is non-existent. The goal of this study was to describe the diet and trophic-based strategies of the most abundant myctophid larvae off Western Australia through stomach content analysis. Samples were collected in autumn 2007 off southwestern Australia when this oligotrophic area was under the influence of the seasonal Leeuwin Current, and in autumn 2010 on the tropical continental shelf of northwestern Australia. The species investigated during these periods were Benthosema fibulatum, B. pterotum, B. suborbitale, Dasyscopelus asper, Diaphus “deep” and “slender” morphotypes, Hygophum hygomii, Lampadena luminosa, Lampanyctus alatus, Lampanyctus spp. and Myctophum sp. Generally, myctophid larvae showed a diurnal feeding pattern. Only postflexion stages in L. alatus, L. luminosa, B. pterotum, Diaphus “slender”, and Myctophum sp. preyed at dawn or during the night. The highest feeding incidence was found in L. luminosa and the Diaphus “deep” morphotype, while the lowest, in H. hygomii. Generally, larvae were zooplankton feeders with nauplii and copepodites being the most abundant dietary groups, except for B. suborbitale and Myctophum sp. where the small non-calanoid genus Oncaea was dominant. Soft organisms such as appendicularians were frequent items in most fish diets throughout their entire larval phase, and highly abundant in Diaphus “deep” morphotype. The wide prey-size spectrum indicates that the majority of the fish species were not size-specific feeders for most of their larval life. Nevertheless, B. suborbitale, B. fibulatum, and L. alatus preferred larger prey, ignoring the smaller items as they grew. Larval feeding patterns of similar genera from other ecosystems are discussed and compared with those in Western Australia.

Size Spectra Modeling of Mesozooplankton over a Tropical Continental Shelf

de Souza, C.S.; da Conceicao, L.R.; Freitas, T.S.S.; Aboim, I.L.; Schwamborn, R.; Neumann-Leitao, S., and Mafalda, P.d.O., Jr., 2020. Size spectra modeling of mesozooplankton over a tropical continental shelf. Journal of Coastal Research, 36(4), 795–804. Coconut Creek (Florida), ISSN 0749-0208.Size spectra analysis is a useful way of studying biomass and energy flow in ecosystems across trophic levels. The size spectrum reflects a balance of energy and matter between gains through growth and recruitment and losses though mortality in a size-structured community. Therefore, changes in biomass and normalized biomass size spectrum (NBSS) slope and intercept between time or environment can be used to assess the energy transfer efficiency within ecosystems. Linear models were developed to analyze zooplankton biomass and community size structure for the narrow continental shelf and shelf break off Salvador, Brazil, using samples that were collected during 10 oceanographic cruises, from April 2013 to October 2014, and analyzed with a ZooScan. Three descriptors of zooplankton size spectra were examined: size diversity, NBSS slope, and intercept. Copepods were always dominant in abundance. Size spectra varied considerably along a short coast-to-offshore transect of 17 km. The pattern of spatial change presented a predominance of larger size fractions nearshore (flat NBSS slope, –0.56) and the predominance of smaller sized organisms in oceanic waters (steep NBSS slope, –0.88). This gradient was mostly due to the higher abundance of large (>5 mm equivalent spherical diameter) gelatinous organisms (mainly hydromedusae) nearshore. The results suggest that the tropical zooplankton community at the continental shelf off Salvador was characterized by low productivity and low trophic efficiency, especially offshore. Although copepods were dominant in numbers, large-sized gelatinous predators were the key structuring zooplankton taxon in this area.

Sedimentary Organic Matter Characterization on a Tropical Continental Shelf in Northeastern Brazil

In continental shelf areas works where the focus is Geochemistry are widely relevant, due to the vast complexity and uses of these relief features (social, environmental and economic). On the Brazilian Northeast tropical shelves, with emphasis on the Pernambuco Continental Shelf (PCS), the published studies are limited and have aimed at portions closer to the coastline. The objective of the present work is the description of the characteristics and origin of sedimentary organic matter (SOM) in the inner and middle Pernambuco shelf, defining and classifying the local geochemical sedimentary facies. The sampling stations (136) were collected in the study area, and the grain-size, contents of total organic matter (TOM), calcium carbonate (CaCO3) were analyzed. The samples were processed in an elemental analyzer coupled with a mass spectrometer after the elimination of calcium carbonate. The obtained data were the grain-size, TOM, CaCO3, elemental (C, N) and the stable isotopic ratios of δ13C e δ15N of SOM. Based on the measured values of carbon and nitrogen, the C/N ratio, associated to δ13C e δ15N, is observed that in the PCS predominates an organic matter of marine origin along its entire length: C δ13C > -21 PDB e δ15N > 8Ar. The PCS presents patches of continental origin SOM, associated to the coastal zones adjacent to estuarine systems mouths and covering the paleochannels, which may indicate that this material was carried from the coastal rivers to the offshore areas, possibly by the paleo-valleys that also act as traps of fine sediments and SOM. The statistical analysis indicates the existence of 6 different sedimentary facies and a prevalence of one of them, characterized by poorly-sorted bioclastic sandy-gravel, with low to medium organic contents of marine origin. This indicates that the coastal sedimentary material contributions are low and the local cover is autochthonous of biogenic origin, and deposited according to the surficial topography, morphosedimentary processes and meteoceanographycal conditionings of the study area, typical of moderate hydrodynamic energy environments.

Geodiversity as an indicator to benthic habitat distribution: an integrative approach in a tropical continental shelf

Marine habitat mapping provides essential information for environmental management and design of marine reserves. In tropical regions, particularly along the Brazilian coast, the spatial variability of marine habitats is poorly known. The aim of this study is to evaluate the geodiversity of the region as an indicator to benthic habitat distribution, applying an integrative approach utilizing existing broad-scale bathymetric and seafloor geological data sets. A digital bathymetric model (DBM) of the Pernambuco Continental shelf (PCS) was generated at 80 m resolution from available bathymetric data. Through the benthic terrain model (BTM), DBM derivatives and the benthic structures were generated. These structures were combined with the textural seabed classification using tools in ArcGIS™ 10.5 Spatial Analyst to identify 22 seabed geomorphic features, including a submarine canyon confirmed around 8°20′S. These geomorphological features describe the surficial characteristics of the seafloor, providing the baseline for subsequent habitat-mapping studies, and can therefore be considered a potential habitat map. Three profiles taken to describe the cross-shelf seafloor environment (North, Center, and South) extracted from interpolated seabed geomorphological map revealed that sediment grain size becomes coarser from inshore to offshore, with predominantly sand and gravel sediment grain sizes. Overall, these results indicated a great potential for PCS due to the geodiversity present in the area, which leads to a higher biodiversity as well. Based on that, the central portion of PCS is suggested as a priority area for conservation because it was the most geomorphologically diverse. These terrain features influence the environmental conditions, such as currents, waves, nutrients, and other oceanographic parameters, which results in high diversity of benthic habitats in the region. This study presents a first step in characterizing the geomorphology of PCS by using reliable, standardized data.

Limitation of fixed nitrogen and deepening of the carbonate-compensation depth through the Hirnantian at Dob's Linn, Scotland

The late Ordovician is characterized by dramatic changes in global climate concurrent with a major mass extinction and possible changes in ocean redox. To further refine our understanding of these events, we present nitrogen and carbon isotope and abundance data from the Ordovician-Silurian (O-S) Global Boundary Stratotype Section and Point at Dob's Linn, Scotland. We show that this section experienced post-depositional ammonium migration from the organic-rich to the organic-poor horizons. However, our data suggest that isotopic fractionations from ammonium substitution into illitic clay minerals are small and can be corrected. Reconstructed primary nitrogen isotope ratios indicate that unlike in tropical continental shelf sections that were transiently enriched in nitrate during the Hirnantian glaciation, the sub-tropical continental slope setting at Dob's Linn experienced persistent limitation of fixed nitrogen across the O-S boundary. Shallow subpolar settings appear to be the only environment that shows persistent nitrate availability at that time. This pattern suggests that spatial trends in marine nitrate concentrations – which are observed in the modern ocean as a result of latitudinal temperature gradients – were already established during the Paleozoic. While the average marine O2 chemocline depth may have deepened during the Hirnantian glaciation, it probably did not lead to global ventilation of the deep ocean, which may have been delayed until the Carboniferous. Furthermore, carbonate data from this and other sections suggest a deepening of the carbonate compensation depth (CCD) during the Hirnantian. This observation indicates that Pacific-style responses of the CCD to glacial/interglacial periods were operational across the O-S boundary, and that the expansion of abiotic carbonate deposition and preservation beyond the shelf break could have in-part mediated changes to surface CO2 during these extreme changes in climate.

Geomorphological Classification of the Benthic Structures on a Tropical Continental Shelf

Geomorphological research offers a valuable first step to understand the complexity of seafloor patterns that are likely to support particular fauna, thereby providing value for future targeted studies on benthic communities and habitats. The objective of this study is to perform a geomorphological classification for a tropical continental shelf, located in the north of the State of Pernambuco, Brazil, using Benthic Terrain Modeler (BTM), as a first step to characterize benthic habitats. The classification of the seabed is based on the bathymetry, broad-scale and fine-scale BPI (Benthic Positioning Index), seafloor slope, and a decision table containing definitions and thresholds appropriate to the data input. The relationships between morphology and the deposition of surface sediments were also investigated. Results from the BTM analysis revealed eight types of benthic structures: Flat Plains, Depressions, Gentle Slopes, Steep Slopes, Flat Ridge Tops, Rock Outcrop Highs, Local Pinnacles in Depressions and Local Pinnacles on Broad Flats. The results from this study may therefore assist in selecting priority areas to investigate, supporting to better marine spatial planning in the region, and, in the future, collaborate to determine biodiversity patterns in Brazilian waters.

Structuring of larval fish assemblages along a coastal-oceanic gradient in the macro-tidal, tropical Eastern Indian Ocean

In the eastern Indian Ocean, the Kimberley region off north-western Australia is characterised by a wide continental shelf, complex coastal topography and very large tides (>10 m). Diversity of fishes is high although little is known about the distribution of their larvae in this dynamic region. This study investigated spatial variation in ichthyoplankton assemblages along four coastal-oceanic transects and related the results to tidal cycles and other environmental variables. Larvae representing 92 neritic and 21 mesopelagic teleost families were collected; this far exceeds the diversity described for other comparable tropical continental shelf and oceanic systems. Distinct ichthyoplankton assemblages characterised inner shelf (Engraulidae and Gobiidae were dominant families), outer shelf / slope (mixed neritic and mesopelagic taxa) and oceanic waters (mesopelagic taxa, particularly Myctophidae, Gonostomatidae and Phosichthyidae), and were correlated with changes in environmental variables (water depth, seawater density, mixed layer depth and zooplankton). The spring-neap tidal cycle and subsequent tides (same stations sampled 12 h apart) had no significant influence on larval fish assemblages. Coastal waters were delineated by a density front and larval fish assemblages were significantly different from offshore assemblages. This study provides new insights into the pelagic ecosystem in the tropical eastern Indian Ocean and contributes to the understanding of the role of environmental variables and physical forcing in the structuring of larval fish assemblages.

Primary production in the tropical continental shelf seas bordering northern Australia

Pelagic primary production (14C uptake) was measured 81 times between 1990 and 2013 at sites spanning the broad, shallow Northern Australian Shelf (NAS; 120−145°E) which borders the Australian continent. The mean of all areal production measurements was 1048±109mgCm−2d−1 (mean±95% CI). Estimates of areal primary production were correlated with integral upper-euphotic zone chlorophyll stocks (above the 50% and 20% light penetration depths) accessible to ocean color remote sensing and total water column chlorophyll standing crop, but not surface (0–2m) chlorophyll concentrations. While the NAS is subject to a well characterized monsoonal climate regime (austral summer-NW monsoon -wet: austral winter- SE monsoon -dry), most seasonal differences in means of regional-scale chlorophyll standing crop (11–33mg Chlm−2 for 12 of 15 season-region combinations) and areal primary production (700–1850mgCm−day−1 for 12 of 15 season-region combinations) fell within a 3−fold range. Apart from the shallow waters of the Torres Strait and northern Great Barrier Reef, picoplankton (<2µm size fraction) dominated chlorophyll standing crop and primary production with regional means of picoplankton contributions ranging from 45 to >80%. While the range of our post-1990 areal production estimates overlaps the range of production estimates made in NAS waters during 1960–62, the mean of post-1990 estimates is over 2-fold greater. We regard the difference to be due to improvements in production measurement techniques, particularly regarding the reduction of potential metal toxicity and incubations in more realistic light regimes.

Wind forcing controls on river plume spreading on a tropical continental shelf

AbstractThe Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river‐derived nutrients and sediments. The region is characterized by weak winds, moderate tides, and almost absent Coriolis forcing. Existing knowledge about river plume behavior in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three‐dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries, and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross‐shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well‐mixed water column. The results can be used to predict the possible impact of land‐use changes in the steadily developing Berau region on coral reef health.

High-Magnesium Calcite Dissolution in Tropical Continental Shelf Sediments Controlled by Ocean Acidification

Increases in atmospheric CO2 cause the oceanic surface water to continuously acidify, which has multiple and profound impacts on coastal and continental shelf environments. Here we present the carbonate mineral composition in surface sediments from a range of continental shelf seabed environments and their current and predicted stability under ocean acidifying conditions. Samples come from the following four tropical Australian regions: (1) Capricorn Reef (southern end of the Great Barrier Reef), (2) the Great Barrier Reef Lagoon, (3) Torres Strait, and (4) the eastern Joseph Bonaparte Gulf. Beyond the near-shore zone, these regions typically have a carbonate content in surface sediments of 80 wt % or more. The abundance of high-magnesium calcites (HMC) dominates over aragonite (Arag) and low-magnesium calcite (LMC) and constitutes between 36% and 50% of all carbonate. HMC, with a magnesium content larger than 8-12 mol %, is more soluble than both Arag and LMC, and the solubility of HMC positively correlates with its magnesium concentration. From the solubility data of Plummer and Mackenzie ( Am. J. Sci. 1974 , 274 , 61 - 83 ), 95% of HMC in the four regions is presently in metastable equilibrium relative to global mean tropical sea surface water. HMC is predicted to become destabilized in the four regions between 2040 and 2080 AD, with typical HMC decline rates between 2% and 5% per year. The range of respective estimated carbonate dissolution rates is expected to exceed current continental shelf carbonate accumulation rates, leading to net dissolution of carbonate during the period of HMC decline. In a geological context, the decline in HMC in tropical continental shelf environments is a global event triggered by reaching below-equilibrium conditions. The characteristic change in carbonate mineral composition in continental shelf sediments will serve as a geological marker for the proposed Anthropocene Epoch.

Extensive genetic population structure in the Indo–West Pacific spot-tail shark, &lt;I&gt;Carcharhinus sorrah&lt;/I&gt;

Carcharhinus sorrah (Mulle and Henle, 1839) is a coastal pelagic shark of the tropical and subtropical Indo?West Pacific, reaching 1.6 m total length. The species is widely harvested in line, net, and trawl fisheries over tropical continental shelves. We investigated mtDNA genetic differentiation in C. sorrah across the majority of the species' range, and examined the importance of six major historical and contemporary biogeographic features in shaping population genetic structure in this species. The present study includes dense sampling for a shark species across the Indo-West Pacific, with 349 specimens sampled from 21 collection locations from the northwestern Indian Ocean, Southeast Asia, New Caledonia, and to southerly distribution limits in Australia. Based on 469 base pairs of the control region, we found extensive genetic population structure, with allopatric lineages confined to Australia, New Caledonia, and the remaining surveyed area. Significant genetic subdivisions were observed over stretches of deep water dividing shelf habitat, particularly the Indonesian Throughflow-Timor Passage and Coral Sea, consistent with strong shelf habitat associated dispersal. Male length at maturity was consistent with a larger size throughout Southeast Asia and the Arabian Sea than known from Australia. Carcharhinus sorrah currently is assessed range-wide on the IUCN Red List (Near Threatened) based largely on Australian demographic data, which may under-represent overharvest risk in other parts of the species' range. The present study highlights the need for independent risk assessment and management for C. sorrah in Australia, Southeast Asia and the northwestern Indian Ocean, and New Caledonia.

Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 478:57-70 (2013) - DOI: https://doi.org/10.3354/meps10040 Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon Dirk V. Erler*,1, Lindsay A. Trott2, Daniel M. Alongi2, Bradley D. Eyre1 1Centre for Coastal Biogeochemistry, School of Environmental Science and Management, Southern Cross University, Lismore NSW 2480, Australia. 2Australian Institute of Marine Science, PMB 3, Townsville MC QLD 4810, Australia *Email: dirk.erler@scu.edu.au ABSTRACT: We provide the first reported estimates of anammox activity in tropical continental shelf sediments (southern section of the Great Barrier Reef lagoon; GBRL). The measured contribution of anammox to total N2 production was up to 70% but restricted to only 1 of the 4 (2 inshore and 2 offshore) sites assayed. Sediment characteristics (contents of total organic carbon [TOC] and manganese [Mn], C:N ratio) at this site appeared to favour anammox activity and the estimated maximum rate was 4.9 µmol m-2 h-1. Anammox bacteria may be a significant contributor to N2 production along the coastal zone of the GBRL. The availability of labile (low C:N) TOC seemed to drive denitrification to completion in the offshore sediments. However, rates of NO3- reduction to NH4+ at the offshore sites were comparable to or higher than denitrification rates. It was unclear whether dissimilatory or assimilatory processes were responsible for the observed reduction of NO3- to NH4+ at the offshore sites. At the 2 inshore sites, NO3- reduction to NH4+ was a larger sink for NO3- than denitrification. Anammox does exist in the tropical continental shelf sediments of the GBRL and should be studied further to determine its role in larger scale N cycling. The roles of assimilatory and dissimilatory nitrate reduction to ammonium also need to be assessed within the GBRL. KEY WORDS: Great Barrier Reef · Anammox · Dissimilatory/assimilatory nitrate reduction to ammonia · Denitrification Full text in pdf format PreviousNextCite this article as: Erler DV, Trott LA, Alongi DM, Eyre BD (2013) Denitrification, anammox and nitrate reduction in sediments of the southern Great Barrier Reef lagoon. Mar Ecol Prog Ser 478:57-70. https://doi.org/10.3354/meps10040 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 478. Online publication date: March 25, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Habitat Specialization in Tropical Continental Shelf Demersal Fish Assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1–10 m depth), down the fore reef slope to the reef base (10–30 m depth) then across the adjacent continental shelf (30–110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.

Comparison of the relative efficiencies of stereo-BRUVs and traps for sampling tropical continental shelf demersal fishes

The sampling efficiencies of commercial standard fish traps and baited remote underwater stereo-video systems (stereo-BRUVs) were compared by examining the diversity and relative abundance of tropical demersal fish that each method sampled on the north-western shelf (40–60m) of Western Australia. Stereo-BRUVs recorded many more species (91 species from 32 families) than commercial fish traps (30 species and 15 families). Stereo-BRUVs also sampled many more individuals (mean 36.55±5.91 SE) than fish traps (mean 12.30±1.40 SE). This suggests stereo-BRUVs would be more capable of detecting changes in the relative abundance of species over time. Data from four commercially important species (Epinephelus bilobatus, Epinephelus multinotatus, Lethrinus punctulatus and Lutjanus russelli) revealed that stereo-BRUVs had much greater statistical power to detect change than an equivalent number of samples from fish traps. In contrast, fish traps had a greater statistical power to detect change for a fifth target species, Lutjanus sebae. For two commonly sampled species, Abalistes stellatus11Usage follows CAABcodes (Rees, A.J.J., Yearsley, G.K., Gowlett-Holmes, K. and Pogonoski, J. Codes for Australian Aquatic Biota (on-line version). CSIRO Marine and Atmospheric Research, World Wide Web electronic publication, 1999 onwards. Available at: http://www.cmar.csiro.au/caab/.). and Lethrinus punctulatus, stereo-BRUVs sampled a smaller mean length than fish traps while for a third species, Lutjanus sebae, stereo-BRUVs recorded a larger mean length. The length frequencies for these species were not significantly different between methods, although stereo-BRUVs sampled a much larger range of lengths than was captured in traps. This study demonstrates that stereo-BRUVs are potentially a much more powerful technique than fish traps for assessing species richness, relative abundance and size structure in multi-species fisheries in north-western Australia.