Abstract
Habitat loss has been highlighted as one of the major threats to marine biodiversity globally. Loss of structurally complex marine habitats or biogenic structures provided by macroalgae can lead to substantial changes in species diversity, richness and abundance. Macrophytes such as kelp from hard rocky reefs and seagrass in soft sediment coastal environments are recognised as ecosystem engineers and have been widely studied. Macroalgal beds found in subtidal soft sediments could also fulfil an ecosystem engineer role in sheltered harbours by providing habitat, contributing to biodiversity, sediment stabilisation, nutrient cycling, primary productivity, and energy to food webs. However, the role of soft sediment macroalgal habitats remains poorly studied. The aim of this thesis was to understand more about the role of an endemic, perennial habitat-forming macroalgae; Adamsiella angustifolia, that forms extensive subtidal beds in New Zealand, with a focus on Otago Harbour, New Zealand. The community structure and diversity of macroalgal beds dominated by A. angustifolia was determined, as well as the primary productivity and nitrogen uptake by this habitat forming species. To understand the drivers of these patterns, abiotic factors crucial to the physiology and productivity of macroalgae, such as light, temperature and nitrogen were measured along a temporal and spatial gradient in Otago Harbour. Adamsiella angustifolia contributed 76 % of biomass with Polysiphonia decipiens, Vertebrata sp., Medeiothamnion lyallii, Ulva spp., Undaria pinnatifida and Ceramium. sp. contributed 95% of the biomass in the beds surveyed across the harbour. 48 species were found in these beds and diversity ranged between 25-38 species per 1.25m2. Biomass showed a spatial pattern with the outer harbour having a higher biomass, likely linked to higher light levels and as a result higher productivity rate. Biomass did not show a seasonal pattern similar to primary productivity, suggesting that A. angustifolia has slow turnover over the years due to its perennial nature, which in turn serves to provide stable habitats. Using rbcL gene sequence data it was found that the species of Adamsiella in Otago Harbour was A. angustifolia and not A. chauvinii as previously reported. This thesis also reports novel findings of exotic macroalgae in A. angustifolia beds, including Ulva sp. A described for the first time in New Zealand, and the first report of the gametophyte stages of Bonnemaisonia hamifera. This thesis provides information on the presence of the invasive kelp Undaria pinnatifida living on soft sediment macroalgal habitats. Undaria contributed significantly to community biomass and is a serious threat to the Adamsiella macroalgal assemblages due to being a strong competitor with a superior photosynthetic and nutrient uptake physiology. Adamsiella angustifolia was well acclimated to low levels of photosynthetically active radiation (PAR) which serves as an advantage in the turbid water columns of Otago Harbour. Adamsiella angustifolia, being perennial macroalgae, contributed to primary productivity throughout the year. However, significant light limitation was observed during winter and spring. The light limitation in primary productivity shown in spring was not linked to patterns of surface PAR and was likely driven by turbidity in the water column. Otago Harbour exhibited low nitrate concentrations in the water column during late spring and summer, while ammonium concentrations remained relatively constant. It is suggested that primary productivity rates during spring and summer in A. angustifolia could be supported by localised ammonium sources during periods of low seawater nitrate. A. angustifolia preferred ammonium over nitrate and urea as a source of nitrogen, and applied passive diffusion, supporting its slow-growing, perennial nature. This thesis provides foundational evidence that suggests the role of A. angustifolia is that of an ecosystem engineer in soft sediment subtidal habitats. By utilising key abiotic parameters of light and nitrogen efficiently, A. angustifolia is able to contribute towards primary productivity annually. The primary productivity drives biomass accumulation, which results in habitat provisioning and supporting a high diversity of macroalgae. A. angustifolia creates an ecosystem through its presence and is a key component of subtidal ecosystems in sheltered harbours in New Zealand.