Frank Tirendi

The influence of forest type on microbial-nutrient relationships in tropical mangrove sediments

Microbial productivity, nutrient chemistry and rates of nutrient regeneration were examined in muds of different mangrove forests within the Fly Delta, Papua New Guinea, to assess the effect of forest type on microbial and nutrient processes, and their interactions. Three major forest types were examined: Rhizophora-Bruguiera, Nypa and Avicennia-Sonneratia forests. For most variables, variations within a forest type were as great as, or greater than, differences between forest types. A high-intertidal Nypa site was most different in edaphic characteristics compared to five low-intertidal stations (two stations in each forest type) suggesting that differences among forest types in earlier studies were mainly a function of tidal elevation rather than species-specific ability of mangroves to influence redox and nutrient status. Dissolved inorganic nutrients were dominated by high (~ 200–500 μM) concentrations of silicates, but porewater phosphate levels were usually below detection limits (<0.02μM). Measured rates of nutrient regeneration were either slow into the sediment, or undetectable, despite a high concentration gradient for some solutes such as silicate. Rates of bacterial DNA and protein synthesis, and patterns of benthic primary production, indicate uptake of nutrients at the sediment-water interface by epibenthic microalgac and sequestering of porewater solutes by very active, subsurface bacterial communities. Rapid growth of these bacteria may be partially maintained by the decomposition and release of nutrients of mangrove roots and rhizomes, as suggested by the dominance of silicate in the porewater. Correlation analysis supports the notion of nutrient (mainly P) limitation of bacteria and microalgac in mangrove muds. It appears that a close microbe-nutricnt-planl connection serves as a mechanism to conserve scarce nutrients necessary for the existence of these tropical tidal forests.

The influence of stand age on benthic decomposition and recycling of organic matter in managed mangrove forests of Malaysia

Decomposition of sediment organic matter was examined in relation to forest age in 2-, 15- and 60-year old, managed Rhizophora apiculata (Blume) stands in the Matang Mangrove Forest Reserve of peninsular Malaysia. Rates of O2 consumption (range: 11.5–21.4 mmol m−2 d−1) and CO2 production (range: 8.9–20.9 mmol m−2 d−1) were equivalent among the forests indicating that early diagenesis is not linked to stand age and age-related differences in rates of forest production. There were, however, site differences in the dominance of specific diagenetic pathways. Rates of sulfate reduction (to 40 cm depth) averaged 8.9±3.1 mmol S m−2 d−1 and 7.2±0.3 mmol S m−2 d−1 in the 15- and 60-year old forests, respectively, accounting for most (75–125%) of the total mineralization. In contrast, sulfate reduction (3.0±0.5 mmol S m−2 d−1) constituted a considerably smaller proportion (42%) of total organic matter oxidation at the 2-year old forest. Rates of solute efflux across the sediment-water interface and vertical profiles of dissolved Mn and NO2−+NO3− suggest that manganese reduction and denitrification–nitrification, coupled with aerobic respiration, account for most oxidation of organic matter at the 2-year old forest. The loss of particulate organic matter and the increased importance of aerobic and suboxic processes in the 2 year-old forest suggest some impact from disturbance of tree removal. A shift to proportionally less sulfate reduction in sediments of regenerating forests may result in greater availability of dissolved nutrients and some trace metals, and serve to reduce exposure of R. apiculata seedlings to anoxic, toxic solutes (e.g., sulfides). This diagenetic shift may facilitate rapid seedling growth and regeneration of forests.

The influence of fish cage aquaculture on pelagic carbon flow and water chemistry in tidally dominated mangrove estuaries of peninsular Malaysia

The impact of floating net cages culturing the seabass, Lates calcarifer, on planktonic processes and water chemistry in two heavily used mangrove estuaries in Malaysia was examined. Concentrations of dissolved inorganic and particulate nutrients were usually greater in cage vs. adjacent (approximately 100 m) non-cage waters, although most variability in water-column chemistry related to water depth and tides. There were few consistent differences in plankton abundance, production or respiration between cage and non-cage sites. Rates of primary production were low compared with rates of pelagic mineralization reflecting high suspended loads coupled with large inputs of organic matter from mangrove forests, fishing villages, fish cages, pig farms and other industries within the catchment. Our preliminary sampling did not reveal any large-scale eutrophication due to the cages. A crude estimate of the contribution of fish cage inputs to the estuaries shows that fish cages contribute only approximately 2% of C but greater percentages of N (32-36%) and P (83-99%) to these waters relative to phytoplankton and mangrove inputs. Isolating and detecting impacts of cage culture in such heavily used waterways--a situation typical of most mangrove estuaries in Southeast Asia--are constrained by a background of large, highly variable fluxes of organic material derived from extensive mangrove forests and other human activities.

A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater

Abstract Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods. This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses. This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.

Nutrient partitioning and storage in arid-zone forests of the mangroves Rhizophora stylosa and Avicennia marina

Abstract. Mangrove partitioning and storage of macronutrients and trace metals were examined in different arid coastal settings of Western Australia. Total living biomass in three Rhizophorastylosa forests, which ranged from 233 to 289 t DW ha–1, was significantly greater than biomass in three Avicenniamarina forests (range: 79–155 t DW ha–1). Although prop roots and stems were the largest single tree components for R.stylosa and A.marina, respectively, most nutrients were stored in leaves and living roots of both species. However, only a small fraction of the total nutrient pool was vested in tree biomass; the vast bulk was in soils. A large below-ground pool of dead fine roots was identified at all stands, equivalent to 36–88% DW of total living tree biomass. The amount of Ca, S, Cl, Na, Si, Fe, Mn, Zn, B, Mo and Cu vested in dead roots of both species was greater than in the total living tree biomass. The proportion of Fe and S vested in live and dead roots was exceptionally large, consistent with previous evidence of metal plaques on mangrove roots. Sulphur, iron and zinc in dead roots of both species constituted the bulk of these metals. R.stylosa trees preferentially accumulated more Mg, S, Cl, Na, Si, Fe, Mn, B and Mo than A.marina trees. Proportionally greater storage of P, N, Ca, K, Cu and Zn occurred in two of the three A.marina forests. Foliar concentrations of Mg, S, Mn, B and Mo in mangrove leaves were at the high end of the range reported for other tropical trees, but other elemental concentrations were at the low or mid-range. Nitrogen limitation in these forests is implied by a positive correlation between total tree N and net canopy production and by a lower percentage of ecosystem N in tree biomass as compared with other forests. Unlike terrestrial forests where a large proportion of nutrient capital is vested in floor litter, most elements in these mangrove forests are stored in dead roots. A large reservoir of dead roots below the forest floor may serve as a conservation mechanism, particularly in such arid oligotrophic environments.

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. The 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. Standing 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. Rates 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. Epibenthos 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. The 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.

Direct determination of organic carbon in modern reef sediments and calcareous organisms after dissolution of carbonate

Abstract An improved method for the direct determination of organic carbon in calcareous marine sediments, organisms and particulate matter is described. Samples are dissolved in phosphoric acid to remove carbonates, purged with oxygen to remove CO2, and the resulting acidic solution is analysed for total (dissolved and particulate) organic carbon. The method is rapid, involves minimal sample manipulation, and is both accurate (better than ± 2%) and precise (better than ± 0.2 mg organic carbon/g sediment). The method is especially suitable for modern carbonate sediments which have low levels of predominantly acid-soluble organic carbon.

Organic matter oxidation and sediment chemistry in mixed terrigenous-carbonate sands of Ningaloo Reef, Western Australia

The oxidation of organic matter in relation to porewater and solid-phase element chemistry was examined in mixed terrigenous-carbonate sediments in sheltered and exposed lagoons of Ningaloo Reef, Western Australia. Rates of O2 consumption and CO2 release were faster in the very fine sand (41–43% CaCO3 content) of the sheltered lagoon of Mangrove Bay (means = 10.5 mmolm−2day−1 O2 and 9.4mmolm−2day−1 CO2) than in the carbonate-rich (73% CaCO3) sand of the exposed back-reef lagoon of Ningaloo Reef (means = 2.1 mmolm−2 day−1 O2 and 3.5 mmolm−2 day−1 CO2). Rates of sulfate reduction (ΣSRR) were similarly faster in the Mangrove Bay sediments (6.1–25.3 mmolm−2day−1 S), sufficient to account for all of the organic matter mineralization. In Ningaloo reef sands, ΣSRR rates (1.0 mmol m−2 day−1 S) accounted for a significant fraction (57%) of total organic carbon oxidation. In Mangrove Bay, in contrast to previous measurements of sulfate reduction in tropical sediments, most (mean = 64%) of the reduced 35S was incorporated into the acid-volatile sulflde fraction with a buildup of iron sulfides. In contrast to most carbonate-bearing sediments, the production and accumulation of Fe sulfides (most evident in Mangrove Bay) increased pH to levels promoting carbonate precipitation. Higher decomposition rates in Mangrove Bay are attributed to restricted water circulation, a richer benthic community, and geomorphology conducive to greater input and retention of mangrove- and macroalgae-derived detritus. At both sites, the lack of a clear zonational sequence of porewater solutes, discrepancies between depth profiles of solutes and solid-phase elements, and high core-to-core variation in conservative element concentrations and in rates of bacterial activity, suggest non-steady state diagenesis. Non-steady state conditions may be fostered by a combination of factors, such as physical disturbances, temporal changes in rates and quality of organic sedimentation, and tidal advection.

Rates and pathways of benthic mineralization in extensive shrimp ponds of the Mekong delta, Vietnam

Abstract Benthic mineralization rates and pathways were measured in two extensive shrimp ponds of the Mekong delta, Vietnam. Sediments of both ponds were unconsolidated, oxic to suboxic silt-clays of neutral pH (6.8–7.3). Free sulfides and methane were not detected in the porewater, which was dominated by NH 4 + (up to 400 μM); other interstitial solutes were very variable among replicate cores, ponds, and sediment depths. Particulate C and N concentrations ranged from 2–8% and 0.15–0.36% by sediment dry weight (DW) with few depth differences between ponds. Pyrite was abundant (0.3–5.6% of sediment DW) in both ponds. Total carbon oxidation rates were not significantly different between the pond located separate from mangroves (separate pond) and the pond located within a mangrove forest (mixed pond). Fluxes of O 2 and CO 2 (=total carbon oxidation, T COX ) were highly variable, with slow rates of CO 2 release (range: 7.7–30.5 mmol m −2 day −1 ) but higher rates of O 2 consumption (range: 9.8–135.9 mmol m −2 day −1 ), especially in the separate pond. A budget of the contribution of the various diagenetic pathways to total carbon oxidation indicates that aerobic respiration accounted for 41–60% of T COX , with active manganese and iron reduction in the mixed and separate ponds, respectively. No denitrification or methane flux was detected from sediments of either pond. Rates of sulfate reduction were slow (range: 0.94–2.73 mmol S m −2 day −1 ) and highly variable, accounting for 13–26% of T COX . Rates of solute flux across the sediment–water interface were dominated by DOC, NO 2 − +NO 3 − , and Mn. There was no measurable NH 4 + flux. Most light–dark bottle O 2 fluxes indicated no net benthic primary productivity. The slow rates of benthic decomposition and the dominance of oxic and suboxic pathways reflect the slow rates of organic matter input, and phytoplankton and shrimp production in these extensive ponds.

Vertical profiles of copper in sediments from the Fly delta and Gulf of Papua (Papua New Guinea)

We examined the vertical profiles of disolved and particulate copper concentrations in the sediments within this region to determine whether any metal pollution has yet occurred as a result of the Ok Tedi mining operation, and to evaluate the spatial extent of copper levels in relation to sediment organic content, grain size and nutrient chemistry