Role
of Herbivory in Controlling the Coral-Algal Phase
Shift on Nutrient Impacted Reefs
Jessica
Fry
Tropical
Coastal Management MSc, University of
Newcastle-Upon-Tyne
May-June
2004
Aim
and rationale:
A
recent study of the water quality around Tobago has shown that sewage derived
nutrients are impacting all the reef systems (Lapointe et al., 2002) and results seem to
indicate that this pollution is the cause of increased algal growth
(“top-down”). However as yet, no one as
quantified the value of herbivory as a factor in
regulating algal cover (“bottom-up”) and therefore one of the importance’s of
the no-fishing zone in the Buccoo Reef Marine
Park. This project evaluates the generality of
the association between algal cover and herbivore biomass on Caribbean reefs
(Williams & Polunin, 2001), in an area of high
nutrient input, aiding the ongoing debate between ‘bottom-up’ versus ‘top-down’
controls on algal population growth on coral reefs.
Methods:
Seven
sites around the Buccoo Reef Complex were surveyed
using SCUBA, with a minimum of 5 replicas at each site. Three sites were on
inshore reefs and four were on outer reefs.
Each
survey consisted of a stationary underwater visual census of all herbivorous
fish (parrotfish and surgeonfish) within a cylinder of 5m radius, for a duration of 5 minutes, the species were recorded and their
size was estimated to the nearest 5cm. The survey area was thoroughly searched
for all species of sea urchins.
Each
survey also involved an estimation of the benthic community composition; this
involved estimating the % cover of different categories of benthic organisms
within a 25cm2 quadrat at eight specified
locations within the survey area. For any coral within the quadrat the height, diameter, water depth, % recently dead
and long dead was also noted. All macro-algae was
harvested from two randomly selected quadrats per
survey, when present,
and its dry weight calculated.
Photos
1.
Underwater survey methods (photo taken by Richard Langton, 2004)
2.
School of herbivorous striped parrotfish (Scarus iserti)
(photo taken by Richard Langton,
2004)
Initial
findings (full report to follow later)
The
outer reef sites had a significantly higher cover of hard coral and proportion
of the total algae in a cropped state (sum of turf algae, bare substrate and
crustose-coralline algae), and a lower cover of macroalgae and calcareous algae, the fish and coral
diversity was also significantly higher on the outer reef sites. The reef was
generally healthier further offshore and limited water nutrient testing revealed
a nutrient gradient, with high phosphate levels close to Buccoo village and lower readings offshore. The biomass of
all herbivorous fish did vary significantly between survey sites and did not
correlate significantly with the benthic community.
This
therefore suggests “bottom-up” control of the benthic community, whereby an
increase in
growth-promoting nutrients, especially phosphorus, in nearshore waters from land-based pollution fuels rapid,
explosive macroalgal growth.
However,
the parrotfish biomass and urchin abundance did correlate strongly with the
benthic community; positively with the proportion of the total algae in a
cropped state and negatively with the macroalgae (%
cover and biomass), suggesting that parrotfish and urchin communities play an
important role in structuring the benthic community, more so than
surgeonfish.
Although
correlation doesn’t imply causality these findings are consistent with other
reports in the Caribbean and do suggests “top-down” control of the algae
community, where grazing by both herbivorous fish and sea urchins can influence
the algal community compositions.
Management
implications
In
conclusion, both nutrient and herbivory play essential
roles in determining the benthic community composition on the reef, and
therefore both aspects must be managed carefully. It is important to keep the
herbivore population protected, since low populations, especially of parrotfish,
lead to high macroalgae levels and lower coral cover,
which in turn reduces the spatial complexity of reef, leading to a lack of
refuge and further decreasing the fish populations, and hence there could be
definite benefits of the proposed Reef Balls programme.
The
research will become an additional layer for the Buccoo Reef Trusts Geographic Information System database
and will provide baseline information for future monitoring efforts. This is
particularly important since the Tobago House of Assembly and the Central
Government of Trinidad and
Possible
future areas of work
·
How
the nutrient status of the water affects corals susceptibility or immunisation
against diseases?
·
Are
the Diadema antillarum
populations are recovering?
·
Are
the elevated nutrient concentrations causing a community switch to zooanthid and fire coral dominance?
·
What
is the influence of reef topography on herbivore
populations?
Acknowledgements:
I
would like to thank all of the following for their help, support and advice;
All
the staff of The Buccoo Reef Trust, especially Dr
Richard Langton and Dr Owen Day; all the staff of R
& Sea Divers, Crown Point, Tony Melville, Jackie Wilson, Tracey Kearns, John
Austin, Wendy Austin, Leslie Peters and Michelle Alleyne ; the boat captains, Julian Johnson, Karen Johnson,
Dunston Johnson and Leron
Lezama; Dive Masters Dan Wurzbacher and Chris Cole; The staff of the University of
Newcastle Department of Coastal Management, particularly Martin LeTissier, Jeremy Hills, John Bythell, Nick Polunin; Erol Caesar, the Fisheries Officer of the Department of
Marine Resources and Fisheries; and Garvin Nichols for his support in
Trinidad.
This
project was approved and supported by the Royal Geographical Society (with the
Institute of British Geographers); endorsed and supported by the Newcastle
University Expeditions Committee on behalf of the University Scholarships
Committee; supported (in part) by funds from the Project AWARE Foundation; and
supported by the National Environmental Research Council and the Cambridge Round
Table.