First record of Chaetomorpha ligustica (Cladophoraceae, Cladophorales) smothering the seagrass Zostera muelleri in a New Zealand estuary

The filamentous green alga Chaetomorpha ligustica (Cladophoraceae, Cladophorales) was recorded covering seagrass meadows at Pauatahanui Inlet, New Zealand. Species of the genus Chaetomorpha are difficult to identify by their morphologically, and the identification of C. ligustica was confirmed by sequencing the 28S rRNA large subunit providing a high level of confidence in the species designation. In November 2019, we found C. ligustica intertwined with Ulva spp. forming dense, heavy and entangled structures. Here we report, for the first time, negative impacts of this species upon meadows of the New Zealand seagrass Zostera muelleri. We observed a significant loss of seagrass cover and evidence of anoxia under C. ligustica mats two weeks from the first sighting. Chaetomorpha ligustica can easily be misidentified in the field. This may lead to over- and under-reporting of species, and we recommend the need for more careful identification of macroalgal blooms in the future and further research on growth requirements and origins of strains.