11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 10: Ecological Processes on Today's Reef Ecosystems
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Ecological Processes in halimeda Kanaloana Meadows in Hawaii From 10 To 90
M Depths
Heather SPALDING* 1
1 Botany Department, University of Hawaii at Manoa, Honolulu, HI
Hawaii’s coral reef ecosystem includes expansive Halimeda kanaloana meadows,
forming a continuous assemblage over soft sediments to 90 m. These meadows form
habitat for cryptic organisms, and are a hunting ground for large, predatory fish. We used
technical diving, ROV surveys, and submersibles to describe spatial and temporal
variation in distribution, abundance, demography, and growth of Halimeda kanaloana,
and to determine the role of herbivory, competition, and recruitment in structuring these
meadows across a depth gradient. Sexual reproduction occurred intermittently throughout
the year; 0.02 – 0.76% of the population was reproductive at any given time.
Fragmentation and clonal propagation allowed large portions of the meadow to recover
from wave induced burial and sand scour. Clearing experiments showed Halimeda could
quickly regrow from the intact holdfast, but was slow (> 20 months) to recolonize areas
cleared of both holdfast and thallus. Individuals were long-lived (> 27 months), but
fluctuated greatly in segment number and height over time, suggesting plant size is not an
appropriate age indicator. Densities peaked at 20 m (342 ± 13 SE individuals per m2), but
varied seasonally and between locations at similar depths. Halimeda growth was rapid
(9.8% ± 1.4% SE new growth per plant per week) and generally decreased with
increasing depth. Herbivores (e.g., Tripneustes gratilla) did not appear to significantly
influence Halimeda densities. Episodic blooms of green algae (e.g., Caulerpa filicoides,
Cladophora sericea) and cyanobacteria (Lyngbya majuscula) overgrew Halimeda, and
most likely competed for light and nutrients with increasing depths. The perennial nature,
rapid growth rates, and asexual reproduction of Halimeda kanaloana appear to contribute
toward the success and persistence of this species in Hawaii. Disturbance removing entire
individuals over a large area, e.g. repeated cruise ship anchoring, would require years for
recovery.
10-70
Space Competition Between acropora Coral And Algae -Different Reaction Of
Coral To Two Types Of Algae With Different Growth Form
Reiko TAMAI* 1 , Kazuhiko SAKAI 1
1 Tropical Biosphere Research Center Sesoko Station, University of the Ryukyus,
Okinawa, Japan
Dominance of reef-building corals on coral reefs is achieved by herbivory and
oligotrophic sea water which restricts algal blooms. Since algae are superior space
competitor to corals, reduced herbivory and increased nutrient may result in phase shift
from dominance by corals to dominance by algae. Although some previous studies
reported interplay among herbivory, algae and corals, studies that focused on the genus
Acropora, which is dominant in coral communities of Indo-Western Pacific, are limited.
We examined the effect of herbivory on algal biomass, and the effect of algae on survival
and growth of Acropora tenuis by conducting field experiments at Iriomote Island, where
human population density is low and land development is localized.
We set up grazer-exclusion cages and transplanted small (≈2 cm in length) coral
fragments of A. tenuis inside and outside the cages. Algal biomass, and survival and
growth of the coral fragments were monthly monitored.
Herbivorous fishes were excluded effectively by the cages, and algal biomass was
significantly higher inside than outside the cages. Although growth rate of the fragments
was significantly lower inside the cages as predicted, the growth rate changed drastically
after algal species composition changed inside the cages at the middle of the
experimental period. Before seasonal change, turf algae dominated inside the cages and
the fragments could not enlarge the attachment area on substrata. Frondose algae
increased after disappearance of turf algae in autumn, and the fragments grew rapidly
afterward.
We conclude that the effects of algae on survival and growth of small corals may be
variable depending not only on algal biomass but also on their growth form.
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