Primary production and macro-detritus dynamics in a European salt marsh: carbon and nitrogen budgets

Primary production and the detritus pathway (i.e., detritus production, litter fall, export to coastal waters, and decomposition) were studied over a I-year period in three salt marsh levels (low, middle and high marshes). Carbon and nitrogen content in biomass and macro-detritus were estimated at each step of the detrital pathway. Several lines of evidence suggested that low, middle and high marshes had different properties which affected their interaction with adjoining waters. More than species diversity, the salt marsh zonation across a 1.8 km transect resulted in a spatial and temporal heterogeneity in the distribution of organic matter and nutrients within the salt marsh. The low marsh was the less productive level (1080 g dry weight m-(2) per year) and was a source of organic matter for the two other marsh levels. Almost 89% of the organic matter produced in the low marsh was flushed away by the tide and redistributed inside the marsh, whereas the remainder decomposed quickly (0.023 per day) at the production site. The export of macro-detritus to coastal waters was probably insignificant to the carbon and nitrogen balance of the salt marsh. In the middle and high marshes, most of the production fell as litter directly at the production site. In the high marsh, vegetation stands were very productive (1990 g dry weight m(-2) per year) and litter decomposed slowly (0.0047 per day), The middle marsh was as productive as the high marsh (1910 g dry weight m(-2) per day), but the decay rate (0.028 per day) was similar to that estimated for the low marsh. This study demonstrates that the mineralization of halophyte-derived organic matter takes place almost completely within the marsh itself. Data on carbon and nitrogen decay indicated that only a small part of the high marsh production was rapidly available to fuel food webs of the ecosystem or to be exported to coastal waters. In contrast, the high amount of biomass produced in the middle marsh was rapidly available for food webs, Export of organic matter may be controlled by macro-detritus production (amount and timing), intensity and time distribution of tide events, and, mostly, decay rates. Because for most of the year the marsh is exposed to the atmosphere, we suggest that exchanges of gaseous carbon and nitrogen with the atmosphere should be investigated. (C) 2000 Published by Elsevier Science B.V. All rights reserved.