Temperate mangrove forests have been poorly studied compared to their tropical counterparts, because they constitute just 1.4% of the global mangrove forest area. Research from tropical mangrove forests suggests they are open systems that provide a large array of ecosystem goods and services. For example, tropical mangrove forests frequently support invertebrate communities in adjacent habitats through the production, export, decomposition and uptake of organic matter. However, ecological differences between temperate and tropical mangrove forests means that information collected in tropical regions cannot be readily extrapolated to temperate systems. Therefore, it is unclear how, or if, temperate mangrove forests supply an organic subsidy to estuarine ecosystems. Here I investigate the linkages between mangrove organic matter production and the role that decomposing mangrove detritus (dead, broken down organic matter) plays in structuring estuarine benthic communities. Research was conducted at two sites (encompassing small-scale differences in sediment properties and macrofaunal communities) in Whangamata Harbour, New Zealand. The production of mangrove detritus was quantified by measuring litter production and its decomposition into detritus. A manipulative detritus addition experiment explored the role of exported mangrove detritus in structuring benthic communities of unvegetated intertidal flats.
The temperate Whangamata Harbour mangrove forest produced the equivalent of 3.24 - 5.38 t DW ha-1 yr-1 of litter, which is comparable to forests at similar latitudes and overlaps with the lower range of tropical mangrove productivity. The decomposition rates of litter following summer litterfall were dependent upon the type of litter, as well as the burial state. However, hypotheses that tidal position and site would affect litter degradation rates were not supported. Leaf and wood litter that was buried in the sediment decomposed significantly slower (1.3 - 1.4 times slower) than litter on the sediment surface. Leaf litter decomposition was faster (63 days to decay by 50%) than wood and root material (460 and 316 days, respectively). Decay models predicted that wood and root material will take years to breakdown, which has implications for New Zealand mangrove removal plans, where wood and roots often remain in situ (following clearances). Finally, a manipulative detrital addition experiment found that mangrove detritus created subtle changes in the relative abundances of a few dominant taxa, rather than shifts in whole community species composition. Communities responded similarly to the addition of mangrove detritus, with the same dominant taxa responding at both experimental sites. The subtle benthic community responses to the large amount of detritus added suggests that mangrove detritus plays a relatively minor role in shaping communities on temperate intertidal flats. The studies that comprise this thesis have together shown that as a result of different input and decomposition rates of mangrove litter, temperate estuarine benthic communities are probably less reliant on mangrove productivity than tropical communities
