INTERIM REPORT ON THE ARTIFICIAL REEF
CONSTRUCTED IN THE LONG BAY-OKURA MARINE RESERVE, AUCKLAND
29th July 2002
PERMIT AO1/1 26th APRIL 2001
Preamble:
The aim of the research
was to construct two small patterns of artificial reef comprising different sizes
of pierced concrete hemispherical shells. These were constructed utilising the
American system called Reefballs. Colonisation of these is now being
monitored. It is hoped to extend the study to include work on sedimentation
rates and the impact on the benthic population. It is anticipated that this
activity will fulfil a range of positive outcomes:
(i)
increase conservation awareness from the use of the Reserve as an
educational resource and recreational dive site.
(ii)
permit future informed decisions to be made on artificial reef
construction in New Zealand.
(iii)
monitor the problems of run off which are faced by the reserve.
Reef 1 Zig Zag Reef:
The reef is located at 360
40 52 S, 1740 45 14.8 E in 5.5 m at minimum spring low
tides. It comprises twelve concrete hemispherical shells made of 60 MPa
concrete with microsilca added. There are nine Bay Balls [mass approximately
300 kg, height 0.7 m, base diameter 1.0 m with about 10 piercings] in a N S
line. At the Northern end, there is a Pallet Ball [mass approximately 1200 kg
height 0.9 m; base diameter 1.2m with about 15 piercings]. ]. At the Southern
end, there are two further Pallet Balls; one with a 30x30 cm slate plate
attached to a vertical surface and the other with a plate attached to close the
top hole of the unit; i.e. in a horizontal position. These slates are the same
as the ones utilised by current E. O. S. intertidal studies. Placement took
approximately 6 weeks because of tide/weather/logistical constraints. All 12
units lie on a N S line parallel to the beach with an angle of approximately
450 between this line and the zigzags and about 1.0 metre between
each.
Reef 2 Octagon Reef.
The reef is located at 360
40 51.1 S, 1740 45 19.6 E in 6.2 m at minimum spring low tides.
This also comprises twelve units with four of the smaller Bay Ball units
planted as the central core. There are four Bay Balls around the perimeter; a
Pallet Ball at the Eastern point of the octagon; and one further Pallet Ball on
the Southern margin. There are slate plates attached to a vertical surface of
both these Pallet Balls. Another Pallet Ball is placed on the western side. The
last Pallet Ball forms a tail to the south since inclement weather prevented it
being placed in geometrical perfection. All the spacings are again about 1.0
metre.
Colonisation sequence:
Hard surfaces were
colonised firstly by planktonic organisms, e.g. pelagic phytoplankton. The
first settlement to be observed (after a period of about 6-7 days) was a
microfilamentous brown algae (unidentified, but common in the Gulf). The next
colonising organisms were vagrant (and opportunistic) benthos, such as the
cushion star: Patiriella regularis,
and the hermit crab: Pagurus sp. c.f.
P. novaezelandiae and herbivorous
gastropods like the whelk: Cominella
adspersa (egg cases of the latter were recorded on a number of the
reefballs). Concurrent with this, nektonic organisms like the
spotty: Notolabrus celidotus and
triplefins: Fosterygium varium and Fosterygium sp. feed on small invertebrates
and graze on the algae. The next wave of colonisation observed was that of
sciaphilic invertebrates such as barnacles and polychaete worms. Two
settlements of the rapidly colonising barnacle Austrominius modestus were recorded; the first occurring about ten
days after the placement of the reefballs. A second, more numerous settlement
event of A. modestus occurred about
four weeks later. Of the annelids, the first was the spiny tubeworm: Spirobranchus cariniferus which settled
about two-three weeks after reefball placement. After a month, these reached
lengths of up to 12 mm.
Both artificial reefs are
close to being fully colonised. Biodiversity on the two reefs is still somewhat
low but new species continue to colonise the reef. Most noticeable was the new
settlement of the ascidian (Sea squirt) Asterocarpa
coerulea around the sides of numerous of the Reef Balls. Larger
macro-algae, brown seaweed Carpophyllum
maschalocarpum (still at early stage approximately 5cm long) is
establishing itself around the top of some of the Reef Balls. Also identified
on the zigzag reef were two Large Trophon (Xymene
ambiguus), these are typically sandy shore species and are probably
resident in the area. A new fish
species, a juvenile Sweep (Scorpis lineolatus)
was present at the Octagon Reef. Juvenile sweep are often found on shallow
reefs, close to shelter (provided by the Reef Balls?).
The unidentified
filamentous brown algae still dominates coverage on the reef balls, however the
barnacle A.modestus are now densely
populated particularly on the zigzag reef, with several settlements of
barnacles evident (small and large live shells, and older empty shells). A.modestus is also dominating percentage
coverage on the slate tiles over the spiny tubeworm Spirobranchus cariniferus (which was formally dominant). Numbers of
Cominella adspersa and the starfish Patiriella regularis appear down. What
appeared to be snapper divots were observed around the surrounding reefs. There
appears to be an abnormally high sedimentation rate on the two Artificial
Reefs, which may contribute to the absence of some organisms. Further studies
on sedimentation rates will hopefully follow.
May
- June 2002
Zigzag
Reef:
The zigzag reef is now
entirely colonised. The make up of organisms on the reef has markedly changed
over the past few months. Both biodiversity and biomass of organisms on and
immediately around the reefballs has increased, with the appearance of a host
of new organisms and an increase in numbers of some already existing organisms
(personal observations). Most noticeable was the number of fish, a few new
species of which may now be resident. The zigzag reef has seen the arrival of a
number of juvenile fish from the summer spawning season, seeking shelter and
food, swimming in and around the reef. Schools of small Snapper (less than 90mm
long in schools >50), Trevally (less than 100mm long in schools >50),
Blue Maomao, and Sweep (less than 70mm), were observed swimming around the
reef, with dozens of juvenile Goatfish (Upeneichthys
lineatus), (less than 90mm in length), pale in colour swimming along the
bottom feeding on small invertebrates in and amongst the reefballs. There were
also numerous adult Parore (Girella
tricuspidata) swimming in amongst the reefballs, some large up to 300-350mm
long, some adult Red Pig Fish. The Spotties, Triplefins, and adult Sweep are
still present in similar numbers. The triple fins have increased in size. There
were many snapper divots around the reef, and a few larger snapper (up to around
250mm) were seen on the outskirts of the reef, at the edge of our level of
visibility.
The once dominant
filamentous algae has been greatly reduced in percent coverage to a few percent
only. It has been replaced with a dense covering of barnacles (A.modestus), tube worms (including Spirobranchus cariniferus), and rock
oysters (Saccostrea glomerata). These
are already creating second and third layers in places colonising on top of
dead and empty shells already attached to the reefballs.
There are now at least
three different tubeworms including Chaetopteros
sp, and Spirobranchus cariniferus. The third species (unidentified) has a
large surface coverage on the outside of the reefballs (approx 20%). There have
been at least two different settlements of S.glomerata
with an older smaller settlement of oysters up to 40mm long (1-2% coverage)
and a more numerous recent settlement of oysters up to 20mm long (approx 10%
coverage). The oysters are more prominent on the inside cavity walls and hole
walls of the reefballs than the outside.
There is a small number of
juvenile green-lipped mussels (Perna
caniculus), approximately 2 cm long, establishing themselves in the base of
the side hole walls. New gastropods to the zigzag reef include the Siphon Whelk
(Penion sulcatus), the Spotted Whelk
(Cominella maculosa), and the Oyster
Borer (Lepsiella scobina) which are
all carnivorous and were located on the outside of the reefballs.
Brown macro-algae up to
10cm long (possibly Halopteris
novae-zelandiae) is establishing itself within the holes of the reefballs
with at least two species of sponges also are colonising the outside of
reefballs. Dozens of small shrimps are to be seen seeking shelter in small
cracks underneath the reefballs.
The population of cushion
stars is noticeably down from previous dives, with an increase of a larger
starfish, Coscinasterias calamaria
(up to 200mm) resting in the holes of the reefballs and on inside cavity walls.
Colonisation
on the octagon reef is not yet complete at around 80-90%. The colonisation makeup is very similar to
that of the zigzag reef with a few differences, most notably the difference in
coverage of barnacles and oysters. Very few oysters and barnacles have colonised
the octagon reef in comparison to the zigzag reef. The filamentous algae is
still present though in increasingly smaller coverage but the tubeworms have a
much greater dominance (S,cariniferis,
and the unidentified species). There
are similar numbers and species of juvenile fish present, with various schools
sheltering within the octagon.
Overall change in biota:
New organisms are present
in the area. These have either migrated from other parts of the marine reserve,
or have settled from the moving water body. (A combination of both is likely).
The latter group probably would not have colonised the area if the reefballs
had not been present. It is very
encouraging to observe such large numbers of juveniles present.
Comparison of surfaces:
No significant differences
in the colonisation of the smooth areas of concrete surface, rough aggregate
surfaces nor the slate plates have been noted.
Bottom Conditions:
Sediment: The sites selected have bottom sediments classified as grey-brown shelly muddy fine sand. Sediments around the reefballs contain a larger amount of fine silt than could be expected in this environment. The origin of this is probably urban run-off. There are large numbers of empty bivalve shells on/near the surface.
Two trial sedimentation
collectors (made from a PVC pipe 100mm in diameter and 700mm long, staked to
the seafloor) have been tested around the octagon reef in attempt to study the
rates of sedimentation.
Settlement: The reefballs
still show no appreciable settlement into the sediment. The holes in the base
of the units were left open and no additional thickening measures taken in
manufacture. There have been many storms over the period with onshore winds
gusting up to 100 kmh-1 however the site selected appears to be well
protected, with the artificial reef remaining stable with no significant
scouring even after storm periods.
Concrete pH Tests
Tests of the surface pH of the well-weathered units stored at Gibbons
Crib Walls show that the surface pH lies in the range 8 9. Technical
difficulties with the AUT Datasonde have prevented any meaningful results being
taken from the reefballs in the water as yet. The lack of difference in
colonisation between the available surfaces tends to indicate that the pH range
of the concrete substrate is satisfactory.
Project team:-
|
Supervisor:- Professor John Buckeridge
|
Jonathan Jaffrey
Trent Taylor |
Earth
and Oceanic Science Research Centre Faculty of Science and Engineering
|
|
Paul Murphy |
Auckland
University of Technology
|