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

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.

本研究测定了不同大潮差海岸生境下红树林（mangrove forests）的物理化学与生物地球化学特征。所涉及的红树林样地包括：斯特里克兰湾（Strickland Bay）溺湾湾道内的密集桐花树（Aegiceras corniculatum）林（SB1）；卡姆古尔河（Kammergoorh River）溺湾三角洲内的成熟红海榄（Rhizophora stylosa）林（SB2）；菲茨罗伊河河口外玛丽北岛陡峭岸坡上的幼龄白骨壤（Avicennia marina）群落（KS3）；以及以碳酸盐沉积物为主的罗巴克湾（Roebuck Bay）内河口处的成熟白骨壤林（BR4）。
每个红树林样地内均布设矩形样方，样方面积以100株树木占据的区域为准：SB1为48.6 m²，SB2为324.0 m²，KS3为600.0 m²，BR4为399.6 m²。
地上生物量通过已发表的红海榄与白骨壤异速生长关系（allometric relationships）估算。由于尚未明确桐花树的异速生长方程，本研究采用适配白骨壤的异速方程对其进行估算。净初级生产力（Net Primary Production）以mmol C/m²/天为单位，通过光截获法估算。
地下细根生物量通过每个样地内的3个重复柱样（长1 m、内径6 cm）估算。柱样按2 cm间隔分层至40 cm深度，之后按5 cm间隔分层至1 m深度。将根系冲洗后冷冻保存，后续采用胶体硅法分离活根与死根后进行分析。
沉积物样品采集自每个样地内的林地表层。采用截去针头的50 cc注射器，每10 cm分层采集平行双份样品，直至50 cm深度，用于分析沉积物粒度与含水率。温度、氧化还原电位与pH值通过长1 m、内径6 cm的平行柱样按2 cm间隔测定。
间隙水样品采用相同的柱状采泥器采集：在氮气（N2）氛围下切割柱样，使用特氟龙（Teflon）间隙水萃取装置挤压按2 cm间隔切割的沉积物块，以获取孔隙水。间隙水样品的分析指标包括SO4²⁻、Cl⁻、Fe、Mn、NH4⁺、NO2⁻+NO3⁻、PO4³⁻、溶解性有机碳（Dissolved Organic Carbon, DOC）与总二氧化碳（Total CO2）；另取单独样品分析甲烷（CH4）含量。
将挤压后的沉积物块与分离得到的活、死根烘干、研磨后，测定总有机碳（Total Organic Carbon, TOC）、总碳（Total Carbon, TC）与总氮（Total Nitrogen, TN）。其中TOC采用贝克曼（Beckman）总有机碳分析仪测定，TC与TN采用珀金埃尔默（Perkin Elmer）2400 CHNS/O Series II分析仪测定。
铁与锰的还原速率通过柱样培养法估算：采集2组平行柱样（长20 cm、内径7 cm），在氮气饱和箱内将其划分为4 cm长的分段。硫酸盐还原速率采用柱样注入法，通过每个样地内采集的3个重复柱样（直径2.7 cm）测定。
气体（O2、CO2、CH4）与溶质（总二氧化碳、DOC、Mn、Fe、HS⁻、Ca、SiOH、PO4³⁻、溶解性有机磷（Dissolved Organic Phosphorus, DOP）、溶解性有机氮（Dissolved Organic Nitrogen, DON）、NH4⁺、NO2⁻+NO3⁻）通量通过放置于每个样地采集的重复箱式柱样（0.027 m²）内的玻璃箱测定。样品采集后立即在船上置于遮光水浴中培养，水浴温度维持为原位海水温度。所有实验用水均取自各样地附近的红树林水道。
通过透明与不透明箱测定气-沉积物界面的气体交换速率，以估算暴露于空气中时段（每日12小时）的底栖呼吸与总初级生产力。淹没沉积物的底栖呼吸通过总二氧化碳通量估算。综合考虑沉积物每日约半日暴露于空气、半日浸没于水中的影响，通过分别取暴露条件下的CO2通量与浸没条件下的总二氧化碳通量的平均值，得到各站位的每日底栖呼吸（总碳氧化，Total Carbon Oxidation, TCOX）估算值。
本野外研究旨在明确不同海岸生境（溺湾、河流三角洲与碳酸盐主导海湾）下不同年龄与类型的红树林对沉积物生物地球化学特征的影响，研究区域属于高能环境。