The legacies of land-clearance and trophic downgrading accumulate to affect structure and function of kelp forests

Aotearoa New Zealand is the last major landmass settled by people, and
therefore provides a recent record of ecological legacy effects in the
coastal zone. Large-scale land clearances of forests accelerated over the
last century, affecting the concentration of suspended sediments, light
environment and nutrient composition on rocky reefs, and consequently the
distribution, abundance and composition of algal forests. Environmental
effects were compounded in many places by overfishing and long-term
declines of large predatory species, often leading to proliferation and
extensive grazing by sea urchins. Here we examine these processes in three
biogeographic regions that have been differentially affected by ecological
legacy effects. The study was based on the depth-specific associations
between sea urchins (Evechinus chloroticus) and the common kelp (Ecklonia
radiata) from multiple sites within each region, some of which were
sampled over two decades within no-take marine reserves and in actively
fished areas. Satellite-derived estimates of water column properties were
used as proxies for the relative effects of coastal sedimentation on kelp
forest habitats. We then used an information-theoretic framework to assess
the relative factor weightings of marine reserve effects on urchin
density, total suspended solids and regional urchin density on the
depth-specific density of E. radiata. Fishing effects were significant
within and outside of reserves in the Northern and Central regions, but
the effect-sizes were by far strongest in the Northern region. In the
Central region characterised by extensive land clearance and forestry
combined with high coastal retention of water, the concentration of total
suspended solids had a major influence on the depth distribution of kelp,
confining it to shallow water (<10m depth) in small patches where
urchins did not overgraze. These patterns are in sharp contrast to the
small marine reserve effects and deep distribution of E. radiata in the
Southern region, which has intact native forested catchments,
comparatively low fishing pressure and a large regional network of marine
reserves. The results highlight important differences in how centuries-old
legacies of land clearance and exploitation affect regional-scale dynamics
of sea urchins and kelp, and define the appropriate spatial scale of
ecosystem-based management of kelp forest habitats.