A community and functional comparison of coral and reef fish assemblages between four decades of coastal urbanisation and thermal stress.

Urbanised coral reefs experience anthropogenic disturbances caused by
coastal development, pollution and nutrient runoff, resulting in turbid,
marginal conditions in which only certain species can persist. Mortality
effects are exacerbated by increasingly regular thermal stress events,
leading to shifts towards novel communities dominated by habitat
generalists and species with low structural complexity. There is limited
data on the turnover processes that occur due to this convergence of
anthropogenic stressors, and how novel urban ecosystems are structured
both at the community and functional level. As such, it is unclear how
they will respond to future disturbance events. Here, we examine the
patterns of coral reef community change, and determine whether ecosystem
functions provided by specialist species are lost post-disturbance. We
present a comparison of community and functional trait-based changes for
scleractinian coral genera and reef fish species assemblages subject to
coastal development, coastal modification, and mass bleaching between two
time periods, 1975-6 and 2018, in Nakagusuku Bay, Okinawa, Japan. We
observed an increase in fish habitat generalists, a dominance shift from
branching to massive/ sub-massive corals and increasing site-based coral
genera richness between years. Fish and coral communities significantly
reassembled, but functional trait-based multivariate space remained
constant, indicating a turnover of species with similar traits. A
compression of coral habitat occurred, with shallow (<5m) and deep
(>8m) coral genera shifting towards the mid-depths (5-8m). We show
that although reef species assemblages altered post disturbance, new
communities retained similar ecosystem functions. This result could be
linked to the stressors experienced by urban reefs, which reflect those
that will occur at an increasing frequency globally in the near future.
Yet, even after shifts to disturbed communities, these fully functioning
reef systems may maintain high conservation value.