Elsevier

Marine Chemistry

Volume 73, Issues 3–4, March 2001, Pages 253-271
Marine Chemistry

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

https://doi.org/10.1016/S0304-4203(00)00110-9Get rights and content

Abstract

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 δ13C 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.

Introduction

Mangrove forests are highly productive ecosystems, and they fringe about 60–75% of the tropical coasts MacGill, 1958, Clough, 1998. Plant litter, mainly leaves, represents about one-third of production, and up to half this quantity can be exported by mangrove creeks to adjacent waters (Robertson et al., 1992). The export of these large amounts of organic material has a recognisable effect on the food webs in coastal waters (e.g. Odum and Heald, 1975, Alongi et al., 1989, Alongi, 1990).

There is also evidence for a substantial net export of dissolved organic matter (DOM), reaching the same order of magnitude as litter export in some mangrove areas (Twilley, 1985). However, no general consensus has hitherto been reached about the role mangroves play for the dissolved and microparticulate carbon budgets. There are very few mangrove areas where quantitative long-term export/import balances exist. These are Hinchinbrook Island (Australia) Boto and Bunt, 1981, Boto and Wellington, 1988, Alongi, 1996, Alongi et al., 1998, Ayukai et al., 1998, Rookery Bay (FL, USA) (Twilley, 1985) and Bragança (North Brazil) Dittmar, 1999, Dittmar and Lara, in press-a. Many inconsistencies amongst the published data may have resulted from methodological differences and from the difficulties in accurately determining material fluxes in mangroves, which is due in large and apparently random tidal-dependent oscillations (Boto and Wellington, 1988). Furthermore, differences among the studied ecosystems such as tidal-range, topography, sediment chemistry or community structure are possible reasons for inconsistencies amongst the export balances (Ayukai et al., 1998).

Chemical tracers, such as lignin-derived phenols and stable carbon isotopes, have been applied in coastal environments to identify source and fate of DOM and particulate organic matter (POM). Lignin is a unique tracer for vascular plant material, even suitable to distinguish vegetation types, e.g. between woody angiosperms, gymnosperms or non-woody vascular plants (Hedges and Mann, 1979). Therefore, it has been widely used to trace the fate and transport of terrestrial organic matter (OM) in rivers and marine environments (e.g. Hedges and Ertel, 1982, Ertel et al., 1984, Ertel et al., 1986, Hedges et al., 1986, Moran et al., 1991a, Moran and Hodson, 1994, Kattner et al., 1999). Benner et al. (1990) found that lignin-derived phenols are leached in considerable amount from mangrove leaves (Rhizophora mangle) during early diagenesis. A high percentage of lignin might therefore be present in mangrove-derived DOM. Moran et al. (1991b) traced DOM from a mangrove swamp ecosystem at the Berry Islands (Bahamas) by analysis of dissolved lignin-derived humic substances and naturally fluorescing compounds.

Stable carbon isotope measurements were applied to trace mangrove-derived detritus in coastal food webs or to study the dynamics of particulate organic carbon in mangrove environments Rodelli et al., 1984, Zieman et al., 1984, Lin et al., 1991, Hemminga et al., 1994, Honculada Primavera, 1996, Marguillier et al., 1997. Rezende et al. (1990) proposed a high contribution of marine-derived OM to total outwelling of POM from mangroves in Sepetiba Bay (Rio de Janeiro, Brazil) and suggested that outwelling may be much less significant than expected by simple mass balance studies.

In all these studies no differentiation was made between mangroves and other terrestrial plants. Therefore, a clear chemical assignment of the OM to these sources was hitherto not possible. The objective of the present investigation was to establish a chemical pattern for mangrove-derived OM, with special emphasis on the differentiation between marine, mangrove and other OM of terrestrial origin. The use of only one source indicator is not appropriate to distinguish between three sources. Therefore, a combination of several indicators, stable carbon isotopes, lignin and its parameters, was used to study origin, fate and flux of OM in a north Brazilian mangrove estuary.

Section snippets

The study area

North Brazilian mangroves are among the most extensive in the world. The research area at the Caeté River, near Bragança, is located approximately 150 km to the south-east of the Amazon Estuary (Fig. 1). These mangroves are characterised by well-developed forests with tree heights reaching 20 m and more. The dominant species are Rhizophora mangle, Avicennia germinans and Laguncularia racemosa. The catchment area of the Caeté River comprises about 3000 km2, from which about 6% (186 km2) is

Source indicators

The combined phenolic CuO oxidation products accounted for less than 1% of the DOC and POC. Highest phenol yields were found in OM at Station 1 in the Caeté Estuary. OM samples taken at the stations in the mangrove, in the middle and the mouth of the estuary exhibited generally less than half of these yields (Table 2). The HbA-fraction contained about 75% of dissolved phenols. The HbN-fraction provided more than half of syringaldehyde, less than one third of vanillyl phenols and only ca. 10% of

Conclusions

Based on lignin-derived phenols and stable carbon isotopes a chemical signature for mangrove, terrestrial and marine-derived OM was established. Outwelling of DOM and POM from the mangrove was evident from chemical tracers and generally exceeded the terrigenous input from the river's catchment area by several times. DOM and POM were exported from the mangrove to the estuary in similar proportions. A considerable amount of mangrove-POM was rapidly removed from the water column, while

Acknowledgements

We thank J. Lobbes for the introduction to the lignin analysis. We appreciate the excellent technical assistance of K.-U. Richter and M. Birkicht. We are also grateful to T. Eglinton and three anonymous reviewers for detailed and constructive comments on the manuscript. This study was carried out as part of the Brazilian–German co-operation project MADAM and was supported by the Brazilian National Research Council (CNPq) and the German Ministry for Education and Research (BMBF) under project

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