The influence of mangrove biomass and production on biogeochemical processes in the Kimberley Region, Western Australia

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The physiochemical and biogeochemical properties of mangrove forests in different macrotidal coastal settings were measured. The forests were: a dense Aegiceras corniculatum forest in a ria embayment of Strickland Bay (SB1); a mature Rhizophora stylosa forest in a ria delta of the Kammergoorh River (SB2); an immature Avicennia marina stand on the steep banks of Mary Island North, off the mouth of the Fitzroy River (KS3); and a mature Avicennia marina forest at a creek mouth in Roebuck Bay, a bay with sediments dominated by carbonate deposits (BR4).Rectangular plots were marked out in each forest and plot size was determined by the area occupied by 100 trees: 48.6 m² at SB1, 324.0 m² at SB2, 600.0 m² at KS3, and 399.6 m² at BR4. Above ground biomass was estimated using previously established allometric relationships for Rhizophora stylosa and Avicennia marina. The allometric equations derived for Avicennia marina were also used for Aegiceras corniculatum, for which allometric relationships were not known. Net primary production (mmol C/m²/day) was estimated using the light interception method. Below ground fine root biomass was estimated from 3 replicate cores (1 m long x 6 cm internal diameter) in each plot. Cores were subdivided at 2 cm intervals to 40 cm and 5 cm intervals to 1 m. Roots were washed and frozen until analysis, when live and dead roots were separated using the colloidal silica method. Sediment samples were collected from the forest floor within each plot. Duplicate samples for grain size and water content were taken every 10 cm to a depth of 50 cm using 50 cc syringes with the needle ends cut off. Temperature, redox potential, and pH were measured at 2 cm intervals from duplicate cores (1 m long x 6 cm internal diameter). Samples for interstitial water were taken using the same corer. Porewater samples were obtained by cutting cores under a N2 atmosphere and squeezing sediment cakes (cut at 2 cm intervals) in a Teflon porewater apparatus. Samples were analyzed for SO4 Cl, Fe, Mn, NH4, NO2 + NO3, PO4, DOC, and Total CO2. Separate samples were analyzed for CH4. The same squeezed cakes and live and dead roots were dried, ground, and analyzed for total organic carbon (TOC), total carbon (TC), and total nitrogen (TN). TOC was measured on a Beckman TOC Analyzer, and TC and TN on a Perkin Elmer 2400 CHNS/O Series II Analyzer.Rates of iron and manganese reduction were estimated, using a core incubation method, from two sets of duplicate cores (20 cm long x 7 cm internal diameter), which were subdivided into 4 cm long sections in a N2 saturated box. Rates of sulfate reduction were measured on triplicate 2.7 cm diameter cores taken from each plot using the core injection technique. Gas (O2, CO2, CH4) and solute (Total CO2, DOC, Mn, Fe, HS-, Ca, SiOH, PO4, DOP, DON, NH4, NO2 + NO3) fluxes were measured via the glass chambers placed in replicate box core (0.027 m²) samples taken from each plot. The samples were immediately incubated on board ship in a shaded water bath maintained at ambient seawater temperature. Water used in all experiments was taken from the mangrove waterways closest to each site.Gas exchange across the air-sediment interface was measured in clear and opaque chambers to estimate benthic respiration and gross primary production during air-exposed periods (12 hours/day). Benthic respiration from submerged sediments was estimated from the total CO2 flux. An estimate of daily benthic respiration (total carbon oxidation, TCOX) at each station, taking into account the effect on sediment of roughly one-half day exposure to air and one-half day submergence, was derived by averaging the CO2 (exposed condition) and total CO2 (submerged condition) flux rates.
Field studies were undertaken to determine the influence on sediment biogeochemistry of various ages and types of mangroves, located in different coastal settings (ria, riverine delta and a carbonate dominated bay) in a high energy environment.

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Statement: Statement: Above ground biomass were estimated using the procedures and allometric equations in:Clough BF, Dixon P and Dalhaus O (1997) Allometric relationships for estimating biomass in multistemmed mangrove trees. Australian Journal of Botany 45: 1023-1031.Clough BF and Scott K (1989) Allometric relationships for estimating above-ground biomass in six mangrove species. Forest Ecology and Management 21:111-121.Net primary production estimates were made using the light interception method described in:Clough BF (1991) Mangrove ecosystems, p. 119-196. In: English S, Wilkinson C and Baker V (eds.) Survey Manual for Tropical Marine Resources, 2nd ed. Australian Institute of Marine Science, Townsville, Australia.The colloidal silica method of separating live and dead roots is described in:Robertson AI and Dixon P (1993) Separating live and dead fine roots using colloidal silica: An example from mangrove forests. Plant and Soil 157: 151-154.Field and laboratory method used for the collection and analyses of sediment samples are detailed in:Alongi DM (1996) The dynamics of benthic nutrient pools and fluxes in tropical mangrove forests. Journal of Marine Research 54:123-148.Alongi DM, Tirendi F and Goldrick A (1996) Organic matter oxidation and sediment chemistry in mixed terrigenous-carbonate sands of Ningaloo Reef, Western Australia. Marine Chemistry 54:203-219.Alongi DM, Sasekumar A, Tirendi F and Dixon P (1998) The influence of stand age on benthic decomposition and recycling of organic matter in managed mangrove forests of Malaysia. Journal of Experimental Marine Biology and Ecology 225:197-218.Alongi, DM, Tirendi F, Dixon P, Trott LA and Brunskill GJ (1999) Mineralization of organic matter in intertidal sediments of a tropical semi-enclosed delta. Estuarine, Coastal and Shelf Science 48:451-461.Methods for methane analysis are described in:Aller RC, Blair NE, Xia Q and Rude PD (1996) Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments. Continental Shelf Research 16:753-186.Ferdelman TG, Lee C, Pantoia S, Harder J, Bebout BM and Fossing H (1997) Sulfate reduction and methanogenesis in a Thioploca dominated sediment off the coast of Chile. Geochimica et Cosmochimica Acta 61:3065-3079.The core incubation method used to estimate rates of Fe and Mn reduction is described in Aller et al (1996) above. Each subsection from each set of cores was placed into a sterile plastic box containing 20 ml of a deoxygenated Na2MoO4 (20 mM) solution to inhibit sulfate reduction. Sediments in each box (two boxes per 4 cm interval) were mixed and subsamples were taken for determination of porewater Fe and Mn on days 0, 1, and 3. Total dissolved Fe and Mn were determined on a Varian Liberty 220 ICP-AES.The core injection technique used to measure rates of sulfate reduction is described in:Fossing H and Jorgensen BB (1989) Measurement of bacterial sulfate reduction in sediments: Evaluation of a single-step chromium reduction method. Biogeochemistry 8:205-222.A two step distillation procedure was used to determine the fraction of reduced radiolabel in the acid volatile sulfide (AVS = free sulfide, FeS) and chromium reducible sulfur (CRS = S°, FeS2) pools. Due to compaction, core depths varied among sites as follows: SB1 = 32 cm, SB2 = 50cm, KS3 = 100 cm and BR4 = 38 cm.The methods used to measure gas and solute fluxes are described in Alongi et al (1996), Alongi et al (1998) and Alongi et al (1999) above.Gas exchange across the air-sediment interface: Gas measurements were made using a MTI Analytical Instruments P200 gas chromatograph and checked using certified standards. Solutes were measured across the sediment-water interface in separate sets of opaque chambers. Total CO2 was determined by the difference in dissolved carbon values pre and post acid digestion (Alongi et al. 1998). All flux measurements were made at 30 minute intervals for 3 hours.

Notes

Credit
Alongi, Daniel M, Dr (Principal Investigator)