Relationships between estuarine modification and leaf litter decomposition vary with latitude

Decomposition of leaf litter plays a major role in carbon and nutrient cycling and in fuelling food webs. Environmental conditions may influence decomposition rates directly by influencing rates of biological reactions, and indirectly by influencing consumer communities. This study assessed how climate influences the relationship between estuarine nutrient enrichment and decomposition. Eelgrass (.Zostera muelleri) and mangrove (.Avicennia marina) leaves from a common source were deployed within two estuaries with anthropogenically enhanced and two with largely unmodified nutrient loadings, at each of two temperate latitudes of New South Wales, Australia. The latitudes were separated by ~1000 km and differed in mean water temperature by ~2 °C. Mass loss of litter was assessed after 13, 27 and 65 days, and the invertebrate communities colonizing the bags were censused at 13 and 65 days. At the end of the experiment, mass loss of seagrass blades was greater in nutrient enriched estuaries of the lower latitude than in higher latitude estuaries or those that were relatively unmodified. Mass loss of mangrove leaves was greater in the low latitude or nutrient enriched estuaries than in the largely unmodified estuaries of the higher latitude. Aspects of the abiotic environment, in particular temperature and phosphorous loading, were key correlates of litter mass loss, with only weak relationships between mass loss and macroinvertebrate communities. These results suggest that the sensitivity of decomposition processes to estuarine modification may be dependent on climatic setting, and the dominant litter resources present. Manipulative experiments are now needed to confirm the environmental conditions under which nutrient impacts on decomposition are exacerbated.