Abstract
Didymosphenia geminata (Lyngbye) M. Schmidt is a freshwater benthic diatom infamous for its unique ability to bloom in oligotrophic conditions. Two decades ago, there was an abrupt increase in the frequency of these blooms accompanied by rapid range expansion in the Northern Hemisphere. Hypotheses presented to explain this phenomenon included climate change and the existence of a new ‘blooming’ ecotype. In 2004, D. geminata was first recognised in New Zealand. It was considered a threat to biodiversity and recreational, cultural and economic values. The invasion of the Southern Hemisphere renewed research efforts towards understanding this unsightly species. Because the New Zealand occurrence represented an invasion, Biosecurity New Zealand, a government agency responsible for New Zealand’s biosecurity system, lead the response. However, they had very little information about the alga that could be used for strategic planning. Few studies outline the invasion ecology of D. geminata. Meanwhile, it has spread to 37 catchments of the South Island.
Managing D. geminata’s invasion process in New Zealand has relied on reports of the alga’s habitat profile in the Northern Hemisphere. Based on these, an early assessment of New Zealand’s suitability for D. geminata suggested that >70% of New Zealand’s river sections (stream order > 3) were highly suitable for D. geminata establishment. Yet D. geminata is invasive in New Zealand, and invasive species often do not follow traditional habitat preferences when in new ranges. An assessment of D. geminata’s invasion pathway and habitat window in New Zealand was paramount for successful species management.
My original contribution to knowledge is a description of D. geminata’s invasion ecology in New Zealand. I describe the species’ preferences, tolerances, constraints and limiting factors in terms of invasion, colonisation, growth, removal and recovery. My analysis is based on three studies of D. geminata’s invasion ecology in New Zealand. The first study, based on presence / absence data from national delimiting surveys, determined that overland spread occurred at the same rate as spread between connected bodies of water. The parameters defining D. geminata’s habitat window for colonisation were related to substrata, temperature, geological calcium, geological phosphorus and source of flow. The second study used data from a series of region-scale monitoring surveys to examine D. geminata’s preferences, tolerances and constraints regarding biomass accumulation. The development of high biomass was constrained by water velocity at a threshold of 0.61 m s-1. Within this constraint, biomass accumulation correlated positively with stream temperature. The third study used data from a two-year biomass monitoring study conducted in three reaches of a hill-fed coastal river. It assessed the roles of water flow and temperature in D. geminata’s accumulation, removal and recovery. Temperature and the size of the standing crop were associated with all three phases. Accumulation was additionally influenced by flow variables, while removal was also related to the average size of the substrate particles and depth. The latter also influenced accumulation rates after physical removal.
My results can be used to support D. geminata long-term management decisions and responses. Insights gained about D. geminata’s invasion pathway can be applied in the spread of other aquatic pests in New Zealand. The defined habitat parameters can be used to identify high-risk streams in the North Island and potentially mitigate the impacts of D. geminata in infected catchments.