长期施肥对水稻土土壤有机碳矿化的影响

以湖南省3个国家级稻田肥力变化长期定位监测点的土壤为材料,通过室内分析和培养试验,研究了不同施肥处理下土壤有机碳矿化特征及土壤总有机碳、微生物量碳和水溶性有机碳对土壤有机碳矿化的影响.结果表明：3个监测点各施肥处理的土壤CO₂累积排放量为448.64～1 516.77 μg·g^-1,CH₄累积排放量为15.60～33.34 μg·g^-1,在58 d的培养期内土壤有机碳矿化量占总有机碳的3.59%～5.57%;不同处理CO₂的产生速率均在前期保持较高水平,之后迅速下降,后期较慢并趋于平稳,CH₄的产生速率表现为先缓慢升高后迅速降低的变化趋势;化肥配施有机肥处理显著增加了CO₂和CH₄的累积排放量;不同施肥处理土壤有机碳矿化量与总有机碳、微生物量碳和水溶性有机碳含量之间的相关性达到了极显著水平,而与矿化量所占土壤总有机碳的比例无明显相关关系.     

刺参对浅海筏式贝类养殖系统的修复潜力

浅海筏式养殖滤食性贝类产生大量的粪便和假粪（总称生物沉积物）,对海水养殖环境产生一系列影响;而沉积食性海参能够有效清除颗粒有机物,在海水养殖系统中扮演“清道夫”的生态角色.为评估刺参在浅海筏式贝类养殖系统中的生物修复潜力,本文在不同季节现场研究了贝 参混养模式下刺参对贝类生物沉积物的摄食及生长和排泄特征.结果表明: 刺参能够在新设计的养殖设施中与滤食性贝类混养,最大生长率达0.34%·d^-1; 并可通过摄食有效清除贝类生物沉积物, 摄食率为0.1746 g·g^-1·d^-1（夏季,21.2 ℃）、0.0989 g·g^-1·d^-1（秋季,19.2 ℃）和0.0050 g·g^-1·d^-1（冬季,7.7 ℃）;刺参主要通过排泄溶解形态的NH₄⁺N和PO₄^3- -P来促进沉积物中营养盐的再生,其排泄率也呈现明显的季节变化.基于现场试验数据,估算了刺参在桑沟湾的生物修复潜力, 刺参与贝类混养可摄食4.5～159.6 kg·hm^-2·d^-1生物沉积物、排泄1 382.5～3 678.1 mmol·hm^-2·d^-1NH₄⁺ -N及74.6～335.7 mmol·hm^-2·d^-1PO₄^3--P.表明刺参对浅海筏式贝类养殖系统具有较大的生物修复潜力,贝-参混养模式不仅能够取得较大的生态效益,而且能显著增加养殖生产的经济效益.     

二氧化碳和臭氧浓度升高对春小麦生长及次生代谢的影响

通过开顶式气室(OTCs)研究了OTC对照（自然CO₂浓度约342 μmol·mol^-1,O₃浓度约30 nmol·mol^-1）、高浓度CO₂（550 μmol·mol^-1）、高浓度O₃（浓度为80 nmol·mol^-1）及其交互作用（CO₂ 550 μmol ·mol^-1,O₃ 80 nmol·mol^-1）对春小麦不同发育时期生物量、总酚量、黄酮含量及成熟期产量性状的影响.结果表明：CO₂浓度增加条件下,春小麦生物量和产量性状都显著高于OTC对照（P<0.05）;而O₃浓度升高条件下,小麦生物量降低,株高、穗长、穗粒质量及千粒重也显著低于对照;CO₂和O₃交互作用下各项指标处于二者之间.说明CO₂可以缓解O₃对小麦的负效应,而O₃对CO₂的正效应具有削弱作用,但二者的作用并非简单的叠加.CO₂、O₃浓度增加及其交互作用显著增加了春小麦叶片中的总酚含量,其中两者交互作用的效应更大,但在小麦生长后期,总酚含量增加量比对照有所降低.在小麦生长前期,各处理总黄酮含量均低于对照;而在成熟期,各处理都显著高于对照.     

FACE水稻茎蘖动态模型

借助农田开放式空气CO₂浓度增高（FACE）技术平台,以武香粳14为供试水稻品种,设置不同施N量处理,研究大气CO₂浓度为570 μmol·mol^-1（比对照高200 μmol·mol^-1）的FACE处理对水稻茎蘖动态的影响,并建立了相应的模拟模型：T_t=A₁(1+e^a1-b1t₎-A₂(1+e^a2-b2t)+C×[B₁(1+e^a3-b3t)-B₂(1+e^a4-b4t)]+D.模型以时间为驱动因子,描述了水稻茎蘖数随移栽天数的动态变化过程,对常规及CO₂浓度增加条件下水稻茎蘖的变化均有很好的拟合性.通过不同年份试验数据对模型的检验,预测根均方差（RMSE）最大为44.27个·m^-2,最小为13.96个·m^-2,且相关系数均达到了极显著水平.表明模型的预测程度较高,具有很好的适用性.     

盐、碱胁迫下小冰麦体内的pH及离子平衡

通过混合两种中性盐（NaCl和Na₂SO₄）和两种碱性盐（NaHCO₃和Na₂CO₃）分别模拟出不同强度的盐、碱胁迫条件,对小冰麦苗进行12 d胁迫处理,测定茎叶组织液的pH值及Na⁺、K⁺、Ca²⁺、Cl^-、SO₄^2-、NO₃^-、H₂PO₄-和有机酸等溶质的浓度,以探讨盐、碱两种胁迫下小冰麦体内的pH及离子平衡特点.结果表明：盐、碱胁迫下小冰麦茎叶内的pH值均稳定不变;随胁迫强度的增加,盐胁迫下小冰麦茎叶内有机酸浓度没有明显变化,Cl^-浓度大幅度增加,而碱胁迫下有机酸浓度大幅度增加,Cl^-浓度没有明显变化.盐、碱胁迫下小冰麦茎叶中的阳离子均以Na⁺和K⁺为主,但阴离子的来源明显不同.盐胁迫下无机阴离子对负电荷的贡献起主导作用,其贡献率达61.3%～66.7%;而碱胁迫下,随胁迫强度的增大,有机酸对负离子的贡献率从38.35%上升到61.60%,逐渐成为主导成分.实验结果表明,有机酸积累是小冰麦在碱胁迫下保持体内离子平衡和pH稳定的关键生理响应.     

热带季节雨林冠层树种绒毛番龙眼的光合生理生态特性

采用Li-6400便携式光合作用测定仪,对西双版纳热带季节雨林冠层树种绒毛番龙眼成树树冠上、中、下3层叶片进行了测定,分析西双版纳热带季节雨林冠层树木的光合作用.结果表明,绒毛番龙眼成树具有喜光的光合特性,光饱和点较高(1 000～1 500 μmol·m^-2·s^-1),而光补偿点较低(7.7～15.3 μmol·m^-2·s^-1),对光环境有较强的适应和调节能力,光合有效辐射是影响绒毛番龙眼光合日进程的关键因子;12月,叶片处于成熟期,生长良好,光合能力较强,树冠上层净光合速率（P_n）日变化为单峰型,最大净光合速率（A_max）约为8.9 μmol CO₂·m^-2·s^-1;4月处于新老树叶更替期,光合能力下降,树冠上层P_n日变化为双峰型,中午出现“午休”现象,树冠上层A_max约为4.3 μmol CO₂·m^-2·s^-1;7月上、中层叶片P_n为单峰型,下层出现“午休”.如人为使CO₂浓度在短期内迅速升高,则绒毛番龙眼的P_n会增加,而气孔导度和蒸腾速率降低;CO₂浓度从400 μmol·mol^-1升高到800 μmol·mol^-1时,干季水分利用效率（WUE）提高约50%～100％,雨季WUE较低.     

荔枝的光合特性

在低光强(0至0.15m mol m^-2s^-1)下,荔枝叶子的光合速率随光入射量子通量的增高而增大。在光强0.7m mol m^-2s^-1时光合速率为1.76μmol m^-2s^-1。光合作用的光补偿点约为0.02 m mol m^-2s^-1光量子。荔技叶子具有低的气孔对水分传导率。气孔对水分传导率和蒸腾速率在低光强时随入射量子通量增高而增大:而细胞间CO₂浓度随光强增高而下降。在光强高于0.2m mol m^-2 s^-1光量子时,细胞间CO₂浓度变化较少。在低光强时,叶子的水分利用效率（光合/蒸腾）随光强增高而增大。在光强高于0.2m mol m^-2 s^-1光量子时,水分利用效率明显降低。荔枝叶子的最适光合作用叶温为22-26℃。可能表明在华南夏季中午的高温限制荔枝的田间光合作用。外界CO₂浓度增高相应增高细胞间CO₂浓度。当细胞间CO₂浓度约低于230μ1.1^-1时,光合速率随细胞间CO₂浓度增高而增大。在更高的细胞间CO₂浓度对,光合速率变化则较少。荔枝叶子光合速率对叶子/空气水蒸汽压陡度的变化响应不敏感。气孔对水分传导率和细胞间CO₂浓度随叶子/空气水蒸汽压陡度增大略有降低。     

供氮和增温对倍增二氧化碳浓度下荫香叶片光合作用的影响

供给0～0.6 mg N的盆栽荫香(Cinnamomum burmannii)幼树分别生长在倍增CO ₂(+CO₂,731 μmol·mol^-1)和正常空气CO ₂浓度(CO ₂,365 μmol·mol^-1)的生长箱内,昼夜温度分别为25/23 ℃和32/25 ℃,自然光照下生长30 d.以生长在CO₂和25/23 ℃下的植株为对照研究增温和氮对+CO₂叶片光合作用的影响.结果表明,在+CO₂和25/23 ℃下无氮和氮处理植株的平均光合速率(Pnsat)较+CO₂和32/25 ℃下的叶片高5.1%,温度增高降低叶片Pnsat;而Pnsat随供氮而增高.在+CO₂条件下,生长在32/25 ℃下的叶片Rubisco最大羧化速率(Vcmax)和最大电子传递速率(Jmax)较25/23 ℃下的低(P＜0.05),温度增高降低+CO₂下叶片的Vcmax和Jmax在+CO2下叶片光合呼吸速率(Rp)较低,生长温度增高提升Rp.在CO₂下生长温度从25/23 ℃增至32/25 ℃,叶片的Rubisco含量(NR)和Rubisco活化中心浓度(M)降低,而供氮能增高NR和M.供氮能减缓温度增高对倍增CO₂下荫香叶片光合作用的限制.     

森林和沼泽对溪流水化学特征的影响

以小兴安岭北部公别拉河上游为研究区,于2004年7～9月对森林溪流和沼泽溪流水样进行水化学特征对比分析.结果表明,森林和沼泽溪流水化学类型均为重碳酸盐类钙组Ⅰ型水(C^Ca_Ⅰ).森林溪流水的pH、矿化度、总硬度、HCO₃^-、SO₄^2-、Ca²⁺、Mg²⁺、Fe均低于沼泽溪流,而总氮、总磷、Cl^-、K⁺、Na⁺则高于沼泽溪流.森林溪流和沼泽溪流中重金属元素Fe、Mn、Cu、Zn、Cd、Hg和Pb含量较低,均未超过我国Ⅰ类地表水环境质量标准.森林溪流中总氮含量为（0.27±0.04） mg·L^-1、总磷含量为（0.040±0.005） mg·L^-1,明显高于沼泽溪流中总氮含量(（0.21±0.02） mg·L^-1)和总磷含量(（0.025±0.004） mg·L^-1),沼泽湿地对N、P有较强的储存和吸附能力,且对NH₄⁺-N的吸附作用远大于对NO₃^--N的吸附.沼泽溪流中Fe含量为（0.26±0.05） mg·L^-1,显著高于森林溪流Fe含量,沼泽湿地对Fe起到还原释放作用.     

太湖地区长期施肥条件下水稻-油菜轮作生态系统净碳汇效应及收益评估

以太湖地区水稻-油菜轮作系统长期施肥处理试验田为研究对象,利用历年作物产量、凋落物固碳和农田CO₂排放等实测资料,以及生态系统的物质投入和管理投入等调查资料,估算了该系统的年碳平衡和经济收益.结果表明：不同施肥处理的年碳汇量在0.9～7.5 t C·hm^-2·a^-1,有机无机肥配施的净碳汇量是单施化肥的3倍.系统物质投入的碳成本在0.37～1.13 t C·hm^-2·a^-1,人工管理的碳成本在1.69～1.83 t C·hm^-2·a^-1,年度经济收益在5.8×10³～16.5×10³ CNY·hm^-2·a^-1,有机无机肥配施下的经济效益是单施化肥下的1.1倍.与单施化肥相比,有机无机肥配施单位碳汇的边际成本为217.1 CNY·t^-1 C,与欧盟碳交易市场的碳价格每吨20欧元相近.与单施化肥相比,有机无机肥配施下生态系统不仅生产力较高,而且表现出更高的碳汇效应和经济收益.     

Significance of ocean carbonate budgets for the global carbon cycle

Changes in the trace gas composition of the atmosphere over glacial–interglacial cycles are linked to changes in the oceanic carbon cycle. This paper examines the role of biologically driven fluxes of organic and inorganic carbon in modifying the carbon dioxide chemistry of the oceans, and the corresponding implications for the partitioning of CO₂ between the atmosphere and ocean. Relevant details of the marine carbon system are presented together with an assessment of the significance of remineralization and dissolution processes. Recent estimates of the marine carbonate fluxes show significant uncertainties and inconsistencies which must be resolved in order to assess fully the role of the oceans' biota in the marine carbon system. Various types of ocean carbon cycle models have been developed in order to interpret the changes in past atmospheric carbon dioxide. Some take account of the role of the oceans' biota, focussing in the main on the cycling of organic matter. Relatively few have considered the role of the carbonate pump and the subtle interactions between organic and inorganic carbon cycling. The significance of carbonate formation and dissolution, and of the effects of global change on the marine carbonate system, for air–sea fluxes of CO₂ are discussed. Finally some recommendations for future research are made in order to improve our understanding of how spatial and temporal variation in marine carbonate fluxes, in conjunction with processes determining the oxidation and burial of organic matter in the oceans, affect levels of CO₂ in the atmosphere.     

Can export of organic matter from estuaries support zooplankton in nearshore,marine plumes?

Marine and terrestrial ecosystems are connected via transfers of nutrients and organic matter in river discharges. In coastal seas, such freshwater outflows create prominent turbidity plumes. These plumes are areas of high biological activity in the pelagos, of which zooplankton is a key element. Conceptually, the increased biomass of zooplankton consumers in plumes can be supported by two alternative trophic pathways—consumption of fresh marine phytoplankton production stimulated by riverine nutrients, or direct trophic subsidies through the uptake of terrestrial and estuarine organic matter flushed to sea. The relative importance of these two pathways has not been established previously. Isotopic tracing (carbon and nitrogen) was used to measure the extent of incorporation of marine versus terrestrial matter into mesozooplankton consumers in the plumes off a small estuary in eastern Australia. Replicate zooplankton samples were taken during baseflow conditions with minimal freshwater influence to the sea, and during pulsed discharge events that generated turbidity plumes in coastal waters. Food sources utilized by zooplankton differed among locations and with the strength of freshwater flow. Terrestrial and estuarine carbon only made a sizeable contribution (47%) to the carbon demands of zooplankton in the lower estuary during pulsed freshwater flows. By contrast, in plumes that developed in nearshore marine waters, phytoplankton supplied up to 90% of the dietary carbon of zooplankton feeding in the plumes. Overall, it was “fresh” carbon, fixed by marine phytoplankton, the growth of which became stimulated by fluvial nutrient exports, that dominated energy flows in plume regions. The trophic role of terrestrial and estuarine organic exports was comparatively minor. The trophic dynamics of plankton in small coastal plumes is closely linked to variations in freshwater flow, but this coupling operates mainly through the enhancement of in-situ phytoplankton production rather than cross-boundary transfers of organic matter to marine food webs in the pelagos.     

Anaerobic mineralization of marine sediment organic matter: Rates and the role of anaerobic processes in the oceanic carbon economy

Abstract

This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy?

Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments.

A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the anoxic zone.

A model approach was taken in estimating the importance of anaerobic processes in the global marine sediment economy, instead of extrapolating measured rates as done previously. The result, 150 Tg C yr^?1, is two to nine times lower than previous estimates. This rate is about 9% of the global aerobic carbon oxidation rate and is about equal to the rate of long‐term carbon burial. The importance of anaerobic processes in marine sediments lies in their role in determining the amount of carbon preserved, not in the amount of carbon remineralized overall.     

Carbon pump dynamics and limited organic carbon burial during OAE1a

Oceanic Anoxic Events (OAEs) are conspicuous intervals in the geologic record that are associated with the deposition of organic carbon (OC)-rich marine sediment, linked to extreme biogeochemical perturbations, and characterized by widespread ocean deoxygenation. Mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs, however, remain poorly constrained. In this work we reconstructed the BCP in the western Tethys Ocean across OAE1a (~120 Mya) using sediment geochemistry and OC mass accumulation rates (OC_Acc). We find that OC_Acc were between 0.006 and 3.3 gC m⁻² yr⁻¹, with a mean value of 0.79 ± 0.78 SD gC m⁻² yr⁻¹—these rates are low and comparable to oligotrophic regions in the modern oceans. This challenges longstanding assumptions that oceanic anoxic events are intervals of strongly elevated organic carbon burial. Numerical modelling of the BCP, furthermore, reveals that such low OC fluxes are only possible with either or both low to moderate OC export fluxes from ocean surface waters, with rates similar to oligotrophic (nutrient-poor, <30 gC m⁻² yr⁻¹) and mesotrophic (moderate-nutrients, ~50–100 gC m⁻² yr⁻¹) regions in the modern ocean, and stronger than modern vertical OC attenuation. The low OC fluxes thus reflect a relatively weak BCP. Low to moderate productivity is further supported by palaeoecological and geochemical evidence and was likely maintained through nutrient limitation that developed in response to the burial and sequestration of phosphorus in association with iron minerals under ferruginous (anoxic iron-rich) ocean conditions. Without persistently high productivity, ocean deoxygenation during OAE1a was more likely driven by other physicochemical and biological factors including ocean warming, changes in marine primary producer community composition, and fundamental shifts in the efficiency of the BCP with associated effects and feedbacks.     

Carbon burial by shallow lakes on the Yangtze floodplain and its relevance to regional carbon sequestration

Floodplain lakes may play an important role in the cycling of organic matter at the landscape scale. For those lakes on the middle and lower reaches of the Yangtze (MLY) floodplain which are subjected to intense anthropogenic disturbance, carbon burial rates should, theoretically, be substantial due to the high nutrient input, increased primary production and high sediment accumulation rates. There are more than 600 lakes >1 km² on the Yangtze floodplain including 18 lakes >100 km² and most are shallow and eutrophic. ²¹⁰Pb‐dated cores were combined with total organic carbon (TOC) analyses to determine annual C accumulation rates (C AR; g C m^?2 yr^?1) and the total C stock (since ~1850). The sediment TOC content is relatively low with an average <2% in most lakes. C AR ranged from ~5 to 373 g C m^?2 yr^?1, resulting in C standing stocks of 0.60–15.3 kg C m^?2 (mean: ~5 kg C m^?2) since ~1850. A multicore study of Chaohu lake (770 km²) indicated that spatial variability of C burial was not a significant problem for regional upscaling. The possible effect of changes in lake size and catchment land use on C burial was examined at Taibai lake and indicated that lake shrinkage and declining arable agriculture had limited effects on C AR. The organic C standing stock in individual lakes is, however, significantly dependent on lake size, allowing a simple linear scaling for all the MLY lakes. Total regional C sequestration was ~80 Tg C since ~1850, equivalent to ~11% of C sequestration by soils, but in ~3% of the land area. Shallow lakes from MLY are a substantial regional C sink, although strong mineralization occurs due to their shallow nature and their role as C sinks is threatened due to lake drainage.     

在无有机碳源和缺氧条件下自养菌和异养菌脱氨氮作用

在无有机碳源和缺氧条件下,应用^15N示踪考察4株具有氨氧化作用的菌株在膜反应器中的氨氧化产气情况。当溶氧浓度（DO）〈0．5mg／L的缺氧条件下,自养菌Nitrosomonas sp．单株培养,能将6．3％的氨氮转化为氮气,^15N2产生量占氨氮消耗量的21．86％。自养菌株与异养菌株混合培养,可将30．86％的氨氮转化为氮气,^15N2产生量占氨氮消耗茸的80．38％。企无有机碳的条件下,自养菌和异养菌的协同代时作用,可大大提高缺氧氩氧化产氮气的效率。     

Marine biodiversification in response to evolving phytoplankton stoichiometry

Diversification of the marine biosphere is intimately linked to the evolution of the biogeochemical cycles of carbon, nutrients, and primary productivity. A meta-analysis of the ratio of carbon-to-phosphorus buried in sedimentary rocks during the past 3 billion years indicates that both food quantity and, critically, food quality increased through time as a result of the evolving stoichiometry (nutrient content) of eukaryotic phytoplankton. Evolving food quantity and quality was primarily a function of broad tectonic cycles that influenced not just carbon burial, but also nutrient availability and primary productivity. Increasing nutrient availability during the middle-to-Late Proterozoic culminated in the production of food (phytoplankton biomass and fresh dead organic matter) with C:P Redfield ratios sufficient to finally promote geologically-rapid biodiversification during the Proterozoic–Phanerozoic transition. This resulted in further, massive nutrient sequestration into biomass that triggered positive feedback via nutrient recycling (bioturbation, mesozooplankton grazing) on phytoplankton productivity. Increasing rates and depths of bioturbation through the Phanerozoic suggest that nutrient recycling continued to increase. Increasing bioturbation and nutrient cycling appear to have been necessary to sustain the primary productivity and “energetics” (biomass, metabolic rates, and physical activity such as predation) of the marine biosphere because of the geologically-slow input of macronutrients like phosphorus from land and the continued sequestration of nutrients into marine and terrestrial biomass.     

The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown

Enormous quantities of the free-floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene, as was demonstrated by microscopic analysis of microlaminated sediments recovered from the Lomonosov Ridge during Integrated Ocean Drilling Program (IODP) Expedition 302. The timing of the Azolla phase (~48.5 Ma) coincides with the earliest signs of onset of the transition from a greenhouse towards the modern icehouse Earth. The sustained growth of Azolla , currently ranking among the fastest growing plants on Earth, in a major anoxic oceanic basin may have contributed to decreasing atmospheric p CO₂ levels via burial of Azolla -derived organic matter. The consequences of these enormous Azolla blooms for regional and global nutrient and carbon cycles are still largely unknown. Cultivation experiments have been set up to investigate the influence of elevated p CO₂ on Azolla growth, showing a marked increase in Azolla productivity under elevated (760 and 1910 ppm) p CO₂ conditions. The combined results of organic carbon, sulphur, nitrogen content and ¹⁵N and ¹³C measurements of sediments from the Azolla interval illustrate the potential contribution of nitrogen fixation in a euxinic stratified Eocene Arctic. Flux calculations were used to quantitatively reconstruct the potential storage of carbon (0.9–3.5 10¹⁸ gC) in the Arctic during the Azolla interval. It is estimated that storing 0.9 10¹⁸ to 3.5 10¹⁸ g carbon would result in a 55 to 470 ppm drawdown of p CO₂ under Eocene conditions, indicating that the Arctic Azolla blooms may have had a significant effect on global atmospheric p CO₂ levels through enhanced burial of organic matter.     

Degrading seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) ecosystems: a source of dissolved matter in the Mediterranean

Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP, DIP) flux across the sediment–water interface was assessed in fish farm impacted and pristine seagrass (Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of P. oceanica community health.     

How light and nutrients affect the relationship between autotrophic and heterotrophic biomass in a tropical black water periphyton community

This study examines how nutrients and light affect the relationship between autotrophic biomass and non-autotrophic periphyton organic matter in a tropical black water lake biofilm community. We hypothesized that there is no positive correlation between autotrophic and non-autotrophic organic matter in the periphytic community of a black water humic lake, where non-algal components of periphyton can rely on carbon sources external to the periphyton matrix and where nutrient availability is low. Second, we sought to test our hypothesis that non-autotrophic periphyton organic matter will benefit from nutrient enhancement in a lake where the availability of DOC is high. We performed a field experiment using in situ lake mesocosms to manipulate nutrient concentrations and light availability in a 2 × 2 factorial design. Control treatments (no nutrient added) and nutrient treatments (N + P) were compared in different light conditions: high light (near surface water) and low light (near bottom). No positive correlation was found between autotrophic biomass and non-autotrophic periphyton organic matter, but a negative correlation was observed in high nutrient and light conditions. The low C:P and N:P ratios revealed that the non-autotrophic organic matter mostly comprised a heterotrophic microbial biofilm. High levels of light and nutrients together caused significant changes in periphyton community properties. The non-autotrophic periphyton organic matter was negatively affected by nutrient addition, whereas autotrophic biomass was positively affected, especially in high light conditions. Our results strongly suggest that non-autotrophic periphyton organic matter in a humic lake is primarily comprised of a bacterial biofilm that directly competes for nutrients with autotrophs in the periphytic community. We also observed no effect of nutrient addition on periphyton growing in light-limited conditions. These results suggest that heterotrophic periphytic organisms might experience carbon limitation despite the high availability, but usually low quality, of dissolved carbon in the water column of humic lakes.