The arrival and spread of the bloom-forming, freshwater diatom, Didymosphenia geminata, in New Zealand

Abstract

Blooms of the stalked diatom Didymosphenia geminata (Lyngbye) M. Schmidt, 1899 have been increasingly reported worldwide since the 1990s. In 2004, the appearance of blooms in New Zealand, for the first time, highlighted the invasive nature of this organism. In the absence of previous reliable records for this species, D. geminata was assumed to be non-indigenous and a nationally coordinated biosecurity response was initiated in an attempt to contain it. We examined the spread of D. geminata over five years using presence/absence data from national and regional delimiting surveys, combined with information from local agencies on potential vectors and data from national recreational angling surveys. Use of a rigorous sampling method raised confidence in the accuracy of negative results. Survey results supported the 2004 assumption that D. geminata was a recent arrival. Incremental but rapid spread to many South Island rivers from two main foci (the Mararoa and Buller Rivers) suggested mass dispersal of an invasive organism via human vectors. The distribution of affected rivers relative to angler usage data for 2007-08 was consistent with angler-mediated dispersal from catchment to catchment. This conclusion was supported by local information about likely vectors of D. geminata to individual sites. Other potential vectors were considered important (e.g., kayaking and power boats) but no river usage datasets were available to verify the patterns. At the time of writing D. geminata had not been detected in the North Island. Although D. geminata has been detected in a high proportion of South Island rivers, blooms have a more restricted distribution. The extent, drivers and impacts of these blooms are the subject of ongoing studies.