Rubén J. Lara

River or mangrove? Tracing major organic matter sources in tropical Brazilian coastal waters

The influence of mangrove-fringed tropical estuaries on coastal carbon budgets has been widely recognised. However, a quantitative differentiation between riverine and mangrove-derived inputs to the dissolved (DOM) and microparticulate organic matter (POM) pool of these environments has been hitherto not possible. Based on lignin-derived phenols and stable carbon isotopes a chemical signature for mangrove, terrestrial and marine-derived organic matter was established for a mangrove estuary in North Brazil. A mixing model was applied to calculate the contribution of each of the three sources to the DOM and POM pool in the estuary throughout 18 tidal cycles in the course of one year. Best source assignment for POM was reached with the yield of lignin phenols and d13C as paired indicators, while the origin of DOM was best identified by the yield of lignin phenols and the acid to aldehyde ratio of vanillyl phenols. Although only about 6 % of the fluvial catchment area is covered by mangroves, their contribution to the estuarine DOM and POM pool generally exceeded several times the terrigenic input from the hinterland. This outwelling of mangrove-derived organic matter was enhanced during the rainy season. DOM and POM were exported from the mangrove to the estuary in similar proportions. Most mangrove-POM was rapidly removed from the water column, while mangrove-DOM behaved conservatively. In contrast, terrestrial DOM was almost entirely removed in the outer part of the estuary, which was accompanied by a concomitant increase in terrestrial POM. This seems to be the result of a geochemical barrier zone for this type of DOM in the estuary. Generally a high proportion of mangrove-DOM was present in the outer part of the estuary, even at high tide. This indicates DOM outwelling from mangroves in adjacent bays or estuaries and points to similar driving forces controlling this process on a regional scale. Mangroves probably play a more important role than rivers for marine carbon budgets along the North Brazilian coast south of the Amazon estuary.

Molecular evidence for lignin degradation in sulfate-reducing mangrove sediments (Amazonia, Brazil)

Abstract —Molecular lignin analyses have become a powerful quantitative approach for estimating flux and fate of vascular plant organic matter in coastal and marine environments. The use of a specific molecular biomarker requires detailed knowledge of its decomposition rates relative to the associated organic matter and its structural diagenetic changes. To gain insight into the poorly known processes of anaerobic lignin diagenesis, molecular analyses were performed in the sulfate-reducing sediment of a north Brazilian mangrove. Organic matter in samples representing different diagenetic stages (i.e., fresh litter, a sediment core, and percolating water) was characterized by alkaline CuO oxidation for lignin composition, element (C, N), and stable carbon isotope analyses. On the basis of these results and on a balance model, long-term in situ decomposition rates of lignin in sulfate-reducing sediments were estimated for the first time. The half-life (T1/2) of lignin derived from mangrove leaf litter (mainlyRhizophora mangle) was ∼150 yr in the upper 1.5 m of the sediment. Associated organic carbon from leaf tissue was depleted to ∼75% within weeks, followed by a slow mineralization in the sediment (T1/2 ≈ 300 yr). Unlike the known pathways of lignin diagenesis, even highly degraded lignin did not show any alterations of the propyl or methoxyl side chains, as evident from stable acid to aldehyde ratios and the proportion of methoxylated phenols (vanillyl and syringyl phenols). Aromatic ring cleavage is probably the principal mechanism for lignin decay in the studied environment. Cinnamyl phenols were highly abundant in mangrove leaves and were rapidly depleted during early diagenesis. Thus, the cinnamyl to vanillyl ratio could be used as a tracer for early diagenesis even under the sulfate-reducing conditions. Syringyl phenols were removed from dissolved organic matter in interstitial water, probably by sorption onto the sediment. Suspended organic matter in a mangrove creek showed a different lignin signature than its source, namely sedimentary organic matter or mangrove litter, with clear evidence for propyl side chain oxidation. This was probably attributable to erosion of aerated thin sediment surface layers during mangrove inundation. Although particulate and dissolved organic matter in the mangrove creek have a common source, their compositional patterns were different, because of different pathways of release, degradation, and transport to the creek.

Dissolved organic matter and nutrients in the Lena River, Siberian Arctic : Characteristics and distribution

Dissolved organic carbon (DOC) and nitrogen (DON), amino acids, carbohydrates and inorganic nutrients were measured on samples taken in July 1994 at 18 stations between Yakutsk and the Lena delta, East Siberia. There were no obvious gradients or features along the river, except in the tributaries, the Aldan and Vilyuy rivers, where significantly higher concentrations of several parameters were measured. Concentrations of DOC varied between 300 and 1000 μM C, with most values varying between 500 and 700 μM C (mean 570 μM C). DON concentrations ranged between 9 and 28 μM N (mean 13 μM N). The C/N ratios of bulk dissolved organic matter (DOM) varied from 30 to 58, with 75% of the values being between 45 and 55 (mean 48). Total dissolved amino acids (TDAA) ranged between 1.6 and 5.4 μM, averaged about 3.5 μM, mostly in the combined form, and represented about 28% of the DON. Free amino acids were only about 2% of TDAA. Glycine, aspartic acid and glutamic acid predominated, accounting for about 41% of TDAA. Total dissolved carbohydrates ranged from 190 to 470 μg glucose equivalents l−1 and averaged 299 μg l−1, forming only 1.2 to 2.5% of the DOC pool. The following ranges of inorganic nutrients were measured: nitrate, 0.01 to 1.4 μM N (mean 0.6 μM N); nitrite, 0.03 to 0.1 μM N (mean 0.07 μM N); ammonium, 0.01 to 0.3 μM N (mean 0.13 μM N); phosphate, 0.2 to 1 μM P (mean 0.5 μM P); silicate, 59 to 87 μM Si (mean 66 μM Si). Carbon isotope data of the suspended organic material suggest that the low inorganic nitrogen values are not due to algal uptake, but rather an inherent characteristic of the river and the catchment area. This, together with positive correlations between silicate, DOC and DON and high C/N values, suggests that the composition of DOM in the Lena River is mainly determined by the input of soil-derived, recalcitrant material and not by autochthonous sources.

Tracing dissolved organic substances and nutrients from the Lena River through Laptev Sea (Arctic)

Abstract Dissolved organic substances and inorganic nutrients were determined in the central and northern Laptev Sea north of the Lena delta region in August 1995. These results are discussed with data from the Lena River from July 1994. Enhanced concentrations of dissolved organic matter (DOM) and silicate in the Lena River generate a concentration gradient extending towards the oceanic regions of the Laptev Sea. Several parameters show higher concentrations in a plume of low salinity water ( a concentrations. While inorganic nutrients increased with depth, all components of DOM decreased. Of these, carbohydrates decreased most rapidly. The distribution of DOC is largely controlled by conservative mixing. A correlation between salinity and silicate indicates its riverine input. Most of the silicate values are below the theoretical dilution line between river and ocean reflecting biological uptake. The distributions of carbohydrates and amino acids were quite variable, with the higher concentrations generally associated to low salinity and high chlorophyll values. Lignin phenols, which are unequivocal tracers for terrestrially derived organic matter, were about 23 nM in the Lena delta region, 7 nM in the lower salinity water and about 2 nM in areas less influenced by river runoff. From these values we estimate that the contribution of riverine derived freshwater to the water budget in the Laptev Sea is about 30 to 10%, decreasing towards the Eurasian Basin. Calculation based on this proportion and on the concentrations of DOC in the Lena River and the Laptev Sea indicate that about 60% of the DOC in the surface layer of the Laptev Sea is of terrigenous origin.

Dissolved organic matter in Arctic multi-year sea ice during winter: major components and relationship to ice characteristics

Ice cores were collected between 10.03.93 and 15.03.93 along a 200 m profile on a large ice floe in Fram Strait. The ice was typical of Arctic multi-year ice, having a mean thickness along the profile of 2.56 ±0.53 m. It consisted mostly of columnar ice (83%) grown through congelation of seawater at the ice bottom, and the salinity profiles were characterized by a linear increase from 0 psu at the top to values ranging between 3 and 5 psu at depth. Distributions of dissolved organic carbon (DOC) and nitrogen (DON) and major nutrients were compared with ice texture, salinity and chlorophyll a. DOC, DON, dissolved inorganic nitrogen (DIN), NH4+ and NO2− were present in concentrations in excess of that predicted by dilution curves derived from Arctic surface water values. Only NO3− was depleted, although not exhausted. High DOC and DON values in conjunction with high NH4+ levels indicated that a significant proportion of the dissolved organic matter (DOM) was a result of decomposition/grazing of ice algae and/or detritus. The combination of high NH4+ and NO2− points to regeneration of nitrogen compounds. There was no significant correlation between DOC and Chl a in contrast to DON, which had a positively significant correlation with both salinity and Chl a, and the distribution of DOM in the cores might best be described as a combination of both physical and biological processes. There was no correlation between DOC and DON suggesting an uncoupling of DOC and DON dynamics in multi year ice.

Temporal changes of mangrove vegetation boundaries in Amazonia: Application of GIS and remote sensing techniques

The present work analyses a 25-year timeseries of radar and satellite images, andidentifies areas with losses and gains ofvegetation coverage along ∼166 km of Parascoastline (North Brazil) and in the innerparts of the Bragança peninsula. Thegeomorphology of this region hassignificantly changed in recent years. Aresult of these changes has been a retreatof the mangrove vegetation along thecoastline, mainly due to landward sandmigration, which covers the mudflat andasphyxiates the vegetation. Image analysissuggests that the loss of vegetationcoverage has been the dominating process inthe last 25 years, occurring on ∼ 42% ofthe Bragança coastline and adjacentareas. Vegetation has remained stable along∼ 39% of the coastline, while mudsedimentation has allowed mangrovedevelopment along ∼ 19% of it. On the otherhand, during this period of time, mangroveshave invaded 3.4 km2 (∼ 38%) of theelevated herbaceous flats in the highestsectors of the Bragança peninsula.Despite other possible causes for mangrovedeath along the coastline, such as littoraldrift currents or cyclical changes incoastal sediment dynamics, the invasion ofmangrove into herbaceous elevated flats inthe central peninsula cannot be attributedto these factors. The current dynamics ofvegetation coverage change seem to becompatible with a long-term trend relatedto the predicted rates of sea-level rise.

Amino acid composition of seawater and dissolved humic substances in the Greenland Sea

The amino acid composition of filtered seawater and dissolved organic matter fractionated with XAD-2 resin was measured in samples from the Greenland Sea. Glycine, aspartic acid, glutamic acid, alanine, and serine made up ca. 80% of the total dissolved amino acids (TDAA) in the unfractionated seawater samples and XAD-fractions. Glycine was predominant, accounting for 28% of TDAA in seawater, 41% in the hydrophobic acid (HbA) fraction and 45% in the hydrophobic neutral (HbN) fraction. TDAA concentrations in the hydrophylic (Hl) fraction correlated significantly with chlorophyll a, whereas no significant relationship was found between chlorophyll and TDAA or the individual amino acid composition in HbA and HbN. TDAA values of 150–250 nM are assumed to be present as part of refractory compounds throughout the water column. At depths of about 1000–1500 m almost 100% of TDAA in seawater is found in the “humic” fractions HbA and HbN. On the average TDAA accounted for ca. 24% of the organic nitrogen content in HbN and about 7% of the nitrogen in HbA and Hl fractions, respectively.

Non‐Redfield carbon and nitrogen cycling in the Arctic: Effects of ecosystem structure and dynamics

The C:N ratio is a critical parameter used in both global ocean carbon models and field studies to understand carbon and nutrient cycling as well as to estimate exported carbon from the euphotic zone. The so-called Redfield ratio (C:N = 6.6 by atoms) [Redfield et al., 1963] is widely used for such calculations. Here we present data from the NE Greenland continental shelf that show that most of the C:N ratios for particulate (autotrophic and heterotrophic) and dissolved pools and rates of transformation among them exceed Redfield proportions from June to August, owing to species composition, size, and biological interactions. The ecosystem components that likely comprised sinking particles and had relatively high C:N ratios (geometric means) included (1) the particulate organic matter (C:N = 8.9) dominated by nutrient-deficient diatoms, resulting from low initial nitrate concentrations (approximately 4 μM) in Arctic surface waters; (2) the dominant zooplankton, herbivorous copepods (C:N = 9.6), having lipid storage typical of Arctic copepods; and (3) copepod fecal pellets (C:N = 33.2). Relatively high dissolved organic carbon concentrations (median 105 μM) were approximately 25 to 45 μM higher than reported for other systems and may be broadly characteristic of Arctic waters. A carbon-rich dissolved organic carbon pool also was generated during summer. Since the magnitude of carbon and nitrogen uncoupling in the surface mixed layer appeared to be greater than in other regions and occurred throughout the productive season, the C:N ratio of particulate organic matter may be a better conversion factor than the Redfield ratio to estimate carbon export for broad application in northern high-latitude systems.

Sediment porewater salinity, inundation frequency and mangrove vegetation height in Bragança, North Brazil: an ecohydrology-based empirical model

An empirical model based on an ecohydrological approach was developed, which allows the integration of hydrographical, topographical and physicochemical information with vegetation characteristics of mangroves and marshes of the Bragança Peninsula, North Brazil. A synoptic distribution of sediment porewater salinity was produced with isolines derived from predicted porewater salinity values from a GIS-based, multiple regression model. It includes a non-linear function of calculated inundation frequencies, the measured salinity distribution in the adjacent estuaries and sediment porewater salinities determined in the dry season. An excellent fit between measured and predicted porewater salinities was obtained. Highly significant correlations were found between vegetation height, inundation frequency and predicted porewater salinities. The relationship between vegetation height and predicted porewater salinities indicated the existence of at least three data ‘clusters’, which probably respond to different height to diameter-in-breast-height proportions according to age and environmental stress conditions. The porewater salinities produced by the model proved to be a solid proxy for the average stress situation of mangrove vegetation. The model offers good potential as a tool for analysing wetland dynamics, particularly at a synoptic, basin level.

Evaluation of triterpenols and sterols as organic matter biomarkers in a mangrove ecosystem in northern Brazil

Mangrove leaves, sediment, and excrementfrom the mangrove crab Ucidescordatus from the coastal areas of theBragança peninsula in North Brazil wereanalysed to determine suitable biomarkersfor mangrove-derived organic matter. Leavesof Rhizophora mangle (red mangrove),the dominant species in the area, werecharacterised by high amounts ofβ-amyrin, germanicol, taraxerol, andlupeol. Avicennia germinans (blackmangrove) mainly contained betulin, lupeol,and β-sitosterol, whereas significantquantities of β-sitosterol and lupeolwere typical of Laguncularia racemosa(white mangrove), the locally leastabundant species. Except for betulin, theexcrement of U. cordatus containedall of the above substances, but moststrongly reflected the triterpenolsignature of R. mangle leaves, thepredominant diet of this crab. Surfacesediments from various mangrove locationshad relatively uniform compositions thatpossibly reflect tidal mixing. Sedimentextracts were dominated by taraxerol andcontained smaller amounts ofβ-amyrin, germanicol, and lupeol.Only sediments in a marsh area, dominatedby Sporobolus virginicus (seashoredropseed) and Eleocharis sp. (spikerush), revealed a differentbiomarker distribution. Core samples ofsubrecent sediment (up to 4000 14C yrBP), for which previous pollen analysisindicated vegetation dominated bymangroves, had compositions similar to thatof the surface sediment. Taraxerol was themain component in the examined mangrovesediments and may be a marker for mangrovematter in this region, although analysis ofplant material did not unequivocallysupport this. Germanicol is suggested to bea biomarker for organic matter from R.mangle in North Brazil. It was detected inolder sediments, and was not significantlyaffected by ingestion by land crabs.