Patterns of distribution and abundance of large brown algae and invertebrate herbivores in subtidal regions of northern New Zealand

The distribution and abundance of subtidal laminarian and fucoid algae, echinoids, and four species of herbivorous gastropods were assessed. Line transects and stratified random quadrats were used, encompassing different depths and seven localities ranging over 1000km in northern New Zealand. This sampling was carried out to form a quantitative basis on which to pose hypotheses for later experimental work. Several patterns were apparent, both within and between localities. Fucoid algae were characteristic of depths between 0–6 m, while laminarian algae had peaks of abundance at 3–6 m and from 10–17 m. Intermediate depths were characterized by areas bare of large brown algae, with high densities of echinoids and gastropods. This pattern of a bimodal depth distribution of algae and abundant grazers at intermediate depth was a consistent feature of the northern coastal sites and the semi-exposed offshore islands. Size—frequency distributions for algae at these sites showed that the largest fucoids generally occurred in very shallow water, while the dominant laminarian alga tended to have larger individuals at depths of 10–14 m. In all cases, the biomass distribution with depth of the algal species mirrored their numerical abundance.

Differences between localities were associated both with environmental factors and the unique algal floras of some offshore islands. Echinoids and gastropods were rare at the two most exposed localities, which were also at the extremes of the geographic range. At these sites, depths between 6–10 m were occupied by large brown algae. Deep-water fucoid algae occurred at two offshore localities but the predominant species at one of them was peculiar to that island group.

Because of the general consistency between localities, and the few exceptions to these patterns, hypotheses are considered which could be tested on a local scale to account for echinoid and algal abundances and depth distributions. Our results indicate that the chief questions concerning the structuring of subtidal algal communities are demographic in nature and that an assessment of the status of individual organisms, rather than biomass or productivity, is the appropriate approach to resolve these questions.