Ecological Effects of Undaria pinnatifida (Harvey) Suringar and Nutrient-Enrichmenton Intertidal Assemblages in the Wellington Region of New Zealand

摘要

The introduction of non-native species and the alteration of seawater nutrient regimes dueto anthropogenic impacts are two important threats to marine environments. Moreover,these disturbances may interact in such a way that promotes the success of invasivespecies in coastal habitats. This thesis contributes to current gaps in knowledge in theseareas for low-intertidal communities.Algal community dynamics and ecological effects of the invasive kelp Undariapinnatifida on low shores in the Wellington region, New Zealand, were examined, usingfield surveys and experiments. In addition, the role of variability in nutrientconcentrations in coastal waters in mediating algal community structure and diversity,and the success of U. pinnatifida reproduction were investigated.Algal surveys were used in two locations thought to differ in nutrient regimes, theWellington Harbour and the Wellington south coast, to explore the structure anddynamics of algal assemblages. Results showed high variability of low-intertidal algalcommunities among sites, but no consistent differences in algal community compositionwere found between the two locations, despite higher U. pinnatifida cover in the harbour.Over the duration of the study, nutrient regimes did not differ greatly between thelocations.The response of rocky intertidal algal assemblages to chronic exposure to high nutrienteffluent was investigated using two nearshore sewage outfalls in the Wellington region.The Titahi Bay outfall showed a stronger relationship between nutrients and algalcommunity composition. Variation in algal assemblage structure and diversity was bestexplained by phosphate concentrations. By contrast, at the more wave-exposedPencarrow outfall, patterns of change in the algal community were less clear and therewas a much weaker relationship with seawater nutrients.Because removal of native algal canopy species may facilitate the establishment ofinvasive macroalgae, the invasion process of U. pinnatifida in disturbed patches in arocky low-intertidal habitat was investigated. In a site where U. pinnatifida had not yetestablished, patches were scraped clear of native algal cover at two different times ofyear, and recruitment of U. pinnatifida was monitored. While U. pinnatifida invaded thesite, it recruited in control plots at a similar rate as cleared plots, suggesting that physicaldisturbance of the native algal assemblage is not a key requirement for this kelp to invadeand establish in new areas in the low intertidal zone.The response of native algal assemblages to removal of U. pinnatifida individuals wasinvestigated at intertidal sites in the Wellington Harbour and on the south coast. Nosignificant effect of U. pinnatifida on community composition, diversity, and speciesrichness was detected. Removal of this invader did not change native intertidalassemblage structure in either harbour or south coast sites.Lastly, effects of different nutrient regimes and light intensities on early development andreproduction of U. pinnatifida were studied using a laboratory experiment. Under lowlight conditions U. pinnatifida gametophyte growth and reproduction stalled and was notincreased by the addition of nutrients. However, at medium and high light levels,gametophyte growth and reproduction, and particularly early stage sporophyte growthrates increased when exposed to higher nutrient concentrations.These effects could haveimplications for U. pinnatifida population dynamics in intertidal habitats where light isnot often a limiting resource.This research contributed to a better understanding of factors that underlie invasiondynamics, distribution, and ecological effects of U. pinnatifida and seawater nutrientregimes on low-intertidal assemblages in the Wellington region. The outcomes can assistin setting up strategic environmental protection and conservation plans.