Northern New Zealand rhodoliths: assessing faunal and floral diversity in physically contrasting beds

Rhodolith beds are recognised internationally as unique ecosystems, harbouring a high diversity and abundance of marine biota. Beds typically occur in environments of moderate exposure and coarse sandy sediments, in which individual rhodoliths are not moved away from the beds, and do not get buried by fine sediments. For the first time in New Zealand, we have physically and biologically characterised selected rhodolith beds in order to document the marine biota within and beneath them. Three beds at two locations in the Bay of Islands, northern New Zealand, were sampled in February and September 2010; two beds were located in a typical rhodolith habitat (a sandy channel), and one in an atypical habitat (a muddy bay). In each bed, rhodolith species and abundance were analysed along with environmental characteristics (sediment granulometry, light environment, temperature, current speed and direction). Characterisation of biological diversity associated with the rhodolith beds included assessment of macroalgae, invertebrates (surface and subsurface) and fish. Sampling within the beds produced a total of 238 taxa: 197 invertebrates, 37 algal and four fish taxa, indicating that a high diversity of biota occur within these northern New Zealand rhodolith beds. We found significant differences in the abundance, taxon richness, and community structure of the biota among rhodolith beds, and surprisingly, the highest abundance and taxon richness occurred in a bed in a highly sedimented environment. In addition, two adjacent and physically similar beds were significantly different from each other in terms of community structure. This finding points to the importance of sampling individual rhodolith beds and suggests that it would be risky to extrapolate findings to other beds, even those in close proximity.