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Cross-boundary detrital subsidies: Detrital export and effects on receiving intertidal soft-sediment ecosystems

Ecosystems are often connected by the transfer of resource ‘subsidies’ across their boundaries. In temperate estuaries, marine macrophyte leaf litter represents an obvious and visible detrital subsidy to nearby intertidal areas, where it can accumulate in temporally and spatially variable patches. This thesis investigates the ecology of macrophyte detrital subsidies, from their production and export (from the donor ecosystem), to the ecosystem effects of their decay and accumulation on recipient intertidal flats. To quantify estuarine detrital subsidies, the fluxes of macrophyte detritus, and other sources of primary production and nutrients (dissolved and particulates), were measured at the mouth of a mixed habitat temperate estuary. This study demonstrates that the estuary (typical of a North Island, New Zealand estuary) acts as a net exporter of detritus, total nitrogen (N), and phosphorus (P) to the wider coastal environment. While macrophyte detrital subsidies contributed relatively little to the total N and P export, they were transported in large and visible quantities. This study provides empirical data on the supply of detrital subsidies in temperate estuaries, and reveals that they are transported in pulses that vary temporally, in both their source and supply. To explore how detrital deposition and decay in intertidal soft-sediments alters ecosystem function (benthic primary production, metabolism, and nutrient cycling), an in situ experiment manipulated the supply of three detrital sources (mangrove, macroalgae, and seagrass) to experimental plots on a sandflat. Benthic chambers were used to measure sediment-water solute fluxes as proxies for ecosystem function. Detrital enrichment had no significant effects on nutrient cycling, benthic metabolism, or macrofaunal community structure. However, detrital addition instigated transient and source-dependent effects on benthic gross primary production (GPP), where macroalgae and mangrove detritus initially (4 d) decreased GPP, but after 17 d, GPP was slightly enhanced in these detrital treatments. Another field experiment aimed to determine the effects of detrital deposition on benthic ecosystem function in the presence of bioturbating crabs, as well as at different intertidal sites (characterised by different sediment properties). The presence of crabs and seagrass detritus were manipulated in cages on an intertidal sand and muddy-sand flat, and the resulting effects on ecosystem function were measured. Detrital enrichment instigated short-term negative effects on GPP in sand (regardless of the presence of crabs), and nutrient cycling in muddy-sand (but only in the presence of crabs). However, at the site characterised by muddy-sand, detrital enrichment also enhanced benthic metabolism and modified macrofaunal community structure (regardless of the presence/absence of crabs). These results emphasise that the effects of detrital subsidies on ecosystem function are context-dependent. While detrital enrichment did not result in large shifts in benthic community structure or function, subtle and transient effects on some functions were found. In these productive intertidal sediments, detritus is unlikely to be an important primary food source to benthic communities. However, by physically altering the structure and function of receiving sediments, seasonal pulses in the supply of detritus may add to the heterogeneous nature of intertidal flats in both time and space. As benthic ecosystem responses to detrital deposition vary with detrital species, anthropogenic changes to the supply, quality and timing of detrital subsidies (e.g. decline in seagrass, and proliferation of macroalgae blooms) could have flow-on effects to the structure and functioning of receiving soft-sediment communities. 
Type of thesis
Gladstone-Gallagher, R. V. (2017). Cross-boundary detrital subsidies: Detrital export and effects on receiving intertidal soft-sediment ecosystems (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10925
University of Waikato
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