Colonisation, Fragment Recovery, and Disturbance in Zostera muelleri Beds, Raglan

Abstract

Zostera muelleri is a seagrass currently on the decline in New Zealand. Potential conservation methods rely on accurate information tailored to local sites. This thesis considers the Z. muelleri beds at Raglan harbour, New Zealand. It focuses on their mechanisms for acquiring genetic diversity, and their ability to prove resilient to small-scale disturbance.

Extensive sediment sampling at four different study sites within the Raglan harbour has yielded no evidence of a seed bank. Z. muelleri reproduction at this site is likely therefore vegetative. Without sexual reproduction to increase the genetic diversity (and therefore the resilience) of the beds, this diversity can be increased by the natural mechanism of seagrass fragments or the deliberate transplantation of seeds, seedlings, or adult plants. This, however, raises conservation questions regarding the tension between “original” and “resilient” environmental states.

Genetic diversity within the seagrass beds may be introduced via the natural dispersal of seagrass fragments. Fragments collected in austral autumn (April) and spring (September) were tested to determine if length of dispersal (floating) time impacted their ability to grow either rhizome length or new shoots. Fragments were randomly assigned to one of five treatments (T0-T4) and left to float for between 0-28 days before planting (T0 = 0 days; T1 = 7 days; T2 = 14 days; T3 = 21 days; T4 = 28 days). After a six-week planting there was no statistically significant difference in rhizome or shoot growth between treatments or between collection times.

The ability of Z. muelleri to respond to small-scale disturbance was also assessed. The presence of Canada geese (Branta canadensis) has been noted feeding on New Zealand seagrass for the first time, and an experiment mimicking their feeding patterns performed. Z. muelleri proved highly resilient to single-event, small scale disturbance at all tidal levels. However, the rapidly increasing population of geese in the Waikato region is expected to increase the level of disturbance to the Raglan beds, and their ability to respond to repetitive, large-scale disturbance may be crucial to their continued survival.